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Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration mitigation in oil free screw compressors

Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration... International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration mitigation in oil free screw compressors 1 2 2 1 J Willie , S W Yonkers , P X Huang and R B Ganatra CVS Engineering GmbH Grossmattstr. 14, D-79618 Rheinfelden, Germany Hi-Bar Blowers, Inc. Fayetteville, GA, USA E-mail: [email protected] Abstract. Noise and vibration are problems that are inherent in screw compressors and other Positive Displacement (PD)Machines. This problem is driven partly by the lobe passing frequencies inside these machines that are generated due to the meshing between the gate and main rotors teeth. Because these compressors are operated over a wide speed range and at various loads they usually run at off-design conditions leading to either over or under- compression. In both scenarios, the pressure within the compressor always changes rapidly to match the discharge or process pressure when the discharge opens thus leading to pulsation and noise. To ensure that the compressor is operated without over and or under compression, which also negatively affects the compressor efficiency Hi- Bar Blowers, Inc developed a Shunt Pulsation Trap (SPT) that will contain and reduce the pulsation inside the compressor cavities thus eliminating the use of a silencer at the inlet and the discharge. During the SPT development the CVS Silo King (SKL 1100) compressor was selected for benchmarking this technology. CFD was used extensively to simulate the SKL 1100 without and with the SPT integrated, which allowed the SPT design to be optimized. This paper will present the results obtained to demonstrate that this technology has the potential to eliminate over and under compression while at the same time leading to energy savings and reducing the compressor footprint and the noise and vibration that are commonplace in screw compressors. 1. Introduction The issue of noise in gas compression is commonplace and very little has been done to address this issue apart from the use of silencers. Even with the use of these silencers, noise levels of up to 100 dBA and above are possible and this is very uncomfortable and can be damaging to the ear of human beings. To address this issue CVS has teamed up with Hi Bar Blowers to investigate the root-cause of pulsation inside positive displacement (PD) compressors with internal compression. In this case, the pilot project is investigating the Silo King (SKL 1100) screw compressor developed by CVS Engineering. A Shunt Pulsation Trap (SPT) has been designed by Hi-Bar Blowers and its purpose is to trap the pressure wave that is responsible for the pulsation inside the screw housing and allow only the non-pulsating flow to leave the compressor. The pulsation suppression will be done over the load range 0-2.5bar(g) with the following goals: • Discharge pulsation level (< 0.1bar) and better noise (< 92dBA) than the design with the reactive silencer; Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 • Smaller footprint. The pulsation trap would be about 25-50 % of the existing reactive silencer ; • Energy savings. About 5-10% energy savings expected. Torque measurements are expected to be lower due to the lower power consumption by eliminating over and under compression. In addition, the back pressure due to the silencer will be eliminated and for the worst case of maximum speed and maximum discharge pressure this amounts to 132 mbar in the Silo King SKL 1100 compressor If successful, SPT would address the noise pollution coming from these compressors. It would address not only the loudness but the noise quality as well. It would also reduce the footprint and the weight of these compressors, which is very important for this application. By reducing or eliminating over and under compression, we expect the pressure drop to decrease, thereby reducing the power consumption of the compressor. The elimination of a silencer means that we would also save the pressure drop introduced by it and eliminate the power consumption that goes with this. Figure 1. Schematic showing under and over-compression The authors are of the opinion that the root cause of high pulsations in screw compressors and other PD machines like blowers is not well understood. Based on the work done by Hi Bar Blowers [5, 6, 7, 8] it is believed that apart from the lobe passing frequencies due to the rotors lobes and the noise generated by gears flow induced noise is predominantly due to high pressure waves when the compressor is operated at off design conditions that lead to over and under compression. The screw compressor is designed for a specific internal volume compression or pressure ratio but in application it is seldom operated at this design condition due to process requirements. Operating at off design condition leads to either over or under compression as shown in figure 1. The plot on the left is showing the case when the compressor is operated at the design point. In this case the pressure inside the pump housing at the discharge p is equal to the discharge pressure p and the process is isentropic. The sketch in the middle is showing the case where we have over-compression and just before the discharge opens the chamber pressure is greater than the discharge pressure. In this case when the discharge opens the chamber pressure drops suddenly from p to p and this can lead to pulsation or pressure fluctuation and noise i D generation. The shaded area corresponds to the extra work that must be done because of the over-compression. In the sketch on the right the chamber pressure p is less than the discharge pressure before the discharge port is opened and it is referred to as under-compression. When the discharge opens a rapid flow is generated from the discharge tube to the chamber until we have an equalization of pressure. The extra work done as a result of the under-compression is shaded in the p-V diagram. If a silencer is used, which is the case in practice, an additional loss is introduced due to the back-pressure of the silencer as depicted in the under-compression case shown in figure 2. The main goal of the SPT is to ensure that there is no pressure drop when the discharge opens 2 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 and so it means using the SPT will lead to additional savings in energy thus optimizing the temperature of the compressor and preventing the use of oil or air cooler, which will lead to additional cost savings. Figure 2. Schematic of the energy saving potential of the SPT technology In the work by Willie [1] it was shown that for most screw compressors there is gas that is trapped between the rotors and the discharge axial gap just before the discharge opens and this can lead to over-compression and to gas pulsations in screw compressors. In [2, 3, 4], other sources of noise inside screw compressors were presented and discussed. In previous work by Hi Bar Blowers the theory behind the development of the SPT technology has been explained [5, 6, 7, 8]. It has been tested for root blowers, which do not have internal compression, and the results showed upto 10 fold reduction in gas pulsations. However, this technology is yet to be tested for screw compressors, which have internal compression, hence the motivation for this work. In this work Computational Fluid Dynamics (CFD) is used to simulate the flow inside the Silo King SKL 1100 screw compressor using CONVERGE CFD. The pre-processor of this software (CONVERGE STUDIO) has advantages over others because of the way it handles moving meshes using adaptive and automatic meshing technique including the cut cell technique in order to mesh complicated moving geometries. The paper begins with the description of the measurement setup and the measurement points used to benchmark the Silo King SKL 1100 compressor without and with a silencer. This is followed by the case setup description of the SPT design used to numerically validate the potential of this technology and after this the CFD setup is presented and this followed by the results and discussions and the conclusions and future work. 2. Measurements A picture showing the setup of the measurement test stand is presented in figure 3. The stand is used for determining the performance map of the benchmark compressor for testing the SPT technology. The following are measured: Volume flow rate, inlet/discharge temperature and pressure, compressor power, and the silencer pressure drop (∆p). Noise measurement is done using microphone that is located 1 m away from the compressor and similar measurements will be done with the SPT integrated into the compressor for comparison. The operating point simulated is shown in table 1 with an input speed of 3200rpm. This point is close to the design point that has an internal volume compression of 2.1 and an input 3 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 3. Picture of measurement setup in the laboratory speed of 3000rpm. The maximum input speed of this compressor is 3600rpm. The discharge pressure p is taken before the silencer directly at the outlet of the compressor and close to the discharge contour. Table 1. Silo king screw compressor operating point for CFD validation of the SPT technology Parameter Measured value Inlet air pressure (p ) 1.00 bar(a) Inlet air temperature (T ) 28 C Discharge air pressure (p ) 2.5bar(a) Discharge air temperature (T ) 150.9 C Volume flow rate (V ) 807.37 m /h Female rotor speed (n ) 10732.3 rpm Male rotor speed (n ) 13415.4 rpm Compressor power (P) 44 kW Silener pressure drop (∆p) 132 mbar 3. CFD Case Setup The setup used in the CFD simulation of the baseline SKL 1100 screw compressor with and without the use of conventional silencer is presented followed by the description of the setup with the SPT included. Due to the need to capture any shocks in the flow a very fine mesh was used and the settings used is similar to the settings used for modeling the NASA supersonic channel benchmark geometry for capturing shock waves [9]. 4 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) CFD setup of the baseline geometry: Left=3D CAD, Right: Neg. vol. (b) CFD setup: Baseline with monitoring points Figure 4. CAD geom. + neg. volume of baseline geom. including monitoring points for CFD 3.1. CFD setup of the baseline geometry without silencer The CAD geometry of the SKL 1100 is first cleaned for CFD simulation and the negative volume is extracted without the discharge silencer as shown in figure 4(a). Included in the geometry are the suction or inlet or inflow, the discharge or outflow and the rotors. The negative volume is exported as an STL file into CONVERGE STUDIO for the definition of the boundary conditions and the solver conditions and the mesh settings. The speed of rotation of the rotors are also defined. The working fluid is air and the inlet and outlet are given pressure boundaries as measured. Using pressure boundaries will allow the solver to compute the mass flow rate from which the volume flow rate can be computed using the computed density. The computed volume flow rate can then be validated with the volume flow measured. Due to the use of cold gaps the leakages are higher than in the reality where due to the thermal expansion of the rotors the hot gaps are smaller. In order to tune the cold gaps to be able to mimic the hot gaps proximity boundaries and porous media are used in CONVERGE [10]. Doing this will allow the solver to accurately predict the volume flow rate inside the machine. Tw is taken to be the in suction temperature and to determine Tw , the wall temperature at the outlet, the relations out 5 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 ( ) κ−1 T = T , where κ is the polytropic coefficient determined from measurements to be 2 1 1.7. This gives Tw =439 K. For the meshing a base grid of 16mm is used and adaptive mesh out refinement (AMR) is used for all regions at level 4 (1mm) on velocity with Sub-Grid-Scale (SGS) 1.0. At the discharge, fixed embedding is used at level 4 (1mm) and finally, grid scaling from -1 rd to 0 is used at the end of the 3 cycle. Mesh sensitivity analyses is used to determine that the 16mm base grid will be able to resolve the flow as needed. To monitor the flow and enable comparison of the various setups and also for validation of the CFD results with measurement data after the modified SPT is tested, four (4) monitoring points are defined as shown in figure 4(b). The pressure and the temperature are monitored at each point for comparison. 3.2. CFD setup of the baseline geometry with the silencer The setup of the baseline geometry with the discharge silencer included is presented here. The CAD geometry and the negative volume used for the CFD setup are shown in figure 5. The outflow is now located at the discharge of the silencer and the inflow is the same as in the baseline case in figure 4. To determine the pressure at the discharge of the silencer the discharge pressure in table 1, which is measured close to the rotors exit is added to the silencer pressure drop of 132 mbar. When the SPT design is used the silencer pressure drop will be avoided. The settings of the CFD used are similar to those used in the baseline case setup in section 3.1. Figure 5. CFD setup of the baseline geom. + silencer: Left=CAD geom., Right=Neg. vol. 3.3. CFD setup of the baseline geometry with the SPT concepts The setup of the SKL 1100 screw compressor with the SPT Design I and Design II concepts integrated is presented. A two stage SPT design was used in Design I and a one stage design was used in Design II, with each stage having 2 nozzles as shown in figure 6. The purpose of this concept is to ensure that there is no pressure drop at the discharge ensuring that irrespective of the operating point there will be minimum pressure fluctuation or noise and hence optimum operation with no under or over-compression inside the compressor [5, 6, 7, 8, 11, 12]. Based on the results of Design I optimization was done and Design II, which is a single stage design, 6 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 was proposed. The negative volume used in both cases for the CFD simulations are shown in figure 6(b), with the left plot corresponding to the two stage design and the right corresponding to the one stage design. (a) CAD geometry of SKL 1100 with SPT (b) Neg. volume with SPT, left=Design I (two stage STP) & right=Design II Figure 6. CAD model and negative volume of SKL 1100 with SPT integrated In total, four setups and mesh strategies are used to ensure that the pressure waves and the flow is being adequately resolved, namely, A, B, C & D. The setup associated with each case is summarized in table 2. A comparison of the averaged results for the 4 setups shows similar results as depicted in table 3. For comparison with the baseline cases, setup C is used because it was able to mimic the flow in the screw compressor and capture the pressure fluctuation as expected. Also, grid sensitivity analyses showed that this setup was not numerically expensive. These settings are also used to simulate the SPT Design II for comparison with the SPT Design I and the baseline SKL 1100 compressor. 4. Results and discussions The CFD simulation results of the baseline SKL 1100 compressor without and with the reactive silencer are first validated with measurement data shown in table 1 to ensure that the results have been able to capture the physics of the flow and are not non-physical due to numerical errors. This is followed by the presentation of the of the SPT results. The ASCII plots of the average volume flow rate and the compressor power for the baseline case without the silencer are shown in figure 7. The averaging is done considering periods of 7 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Table 2. Averaged results of the various setups of SKL 1100 + SPT Design I Setup Base grid AMR regions Embedding Grid scaling Solver A 16mm All, level 4 (1mm) Fixed at outlet, 1 cycle -2; Std. RNG k-eps 2 cycles -1; then 0 Nozzles level 5 (0.5mm) Vel. level 4 (1mm) SGS 1.0 B 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; Std. RNG k-eps 2 cycles -1; then 0 Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) (from map file) SGS 1.0 Fixed at outlet, level 4 (1mm) C 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; CFL=1 nozzles, Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) 2 cycles -1; then 0 Total energy SGS 1.0 Fixed at outlet, (from map file) Stricter PISO level 4 (1mm) RNG k-eps D 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; CFL=1 nozzles, Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) 2 cycles -1; then 0 Total energy Stricter SGS 1.0 Fixed at outlet, (from map file) PISO MUSCL, RNG level 4 (1mm) k-eps Table 3. Averaged results of the various setups of SKL 1100 + SPT Design I Setup Inflow mass(kg/s) Inflow vel. (m/s) Outflow mass (kg/s) Outflow vel. (m/s) Power(kW) A 0.207 35.9 0.177 21.1 40.7 B 0.207 35.9 0.176 20.8 40.7 C 0.189 32.9 0.182 21.2 41.0 D 0.190 33.18 0.180 21.5 41.0 rotation of 360 . In the setup used time advancement has been modified and replaced by the number of degrees that the male (main) rotor is rotating. Thus 360 represents a full rotation of the male rotor. So a time average over a period equal to a complete revolution of the male rotor. For statistical convergence at least three revolutions or Crank Angle 1080 are required. The volume flow rate measured is at the inlet of the compressor and the compressor power is computed from CFD using the main (male) and gate (female) rotors torque. For the setup without the use of proximity boundaries (denoted as original in the plot) the inlet mass flow rate at CA 1700deg is 0.234 kg/s and that at the outlet is 0.228 kg/s, giving a percentage error of 2.6%, which is good for mass balance. The average inlet volume flow rate in this case is 733 m /h. One reason for the disparity between the measured and the simulated volume flow is due to leakages inside the machine and the fact that the CONVERGE solver used does not resolve the flow inside the gaps but uses proximity boundaries to model the flow inside the gaps of the compressor. The model is tuned by using proximity boundaries (denoted as update in the plot) to mimic the flow inside the gaps. This gives an inlet mass flow rate of 0.257 kg/s and hence a volume flow rate of 805 m /hr, which is very close to the measured value. After the baseline simulations and tuning were completed and the results validated with measurement data, the simulations were extended to include the baseline geometry plus the silencer and the baseline geometry plus the SPT Design I. Based on the SPT Design I results optimization was done, which resulted in the SPT Design II concept that was also simulated and compared with the SPT Design I results. 8 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Volume flow rate (b) Compressor power Figure 7. ASCII plots of the volume flow and compressor power predicted, baseline geometry, no silencer The ASCII results of the average mass flow rate and hence the volume flow rate and the compressor power of the baseline geometry plus the silencer are shown figure 8. The mass flow rate at the inlet is 0.2535 kg/s at CA 1600deg and that at the outlet is 0.222 kg/s, giving a percentage error of 10%. The moving average value was changing slightly at 1600 deg. CA and this could be due to the lager geometry from the addition of the silencer. The average compressor power has increased to 41.1kW at 1600 deg CA due to the addition of the silencer and the back pressure it introduces. This value is closer to that measured in table 1. Also noticed is the increase in the average discharge velocity from 41.4 m/s in the case without silencer to 53.25 m/s in the case with silencer (6.2%) as shown in figure 9. The increase is due to the fact that the silencer inlet and outlet have smaller diameters when compared to the outlet of the compressor. The simulation is extended to the baseline geometry with the SPT Design I that has two stage nozzles integrated and after that to the SPT Design II with a single stage nozzle and the results of the mass flow compared to the baseline geometry without and with silencer are shown in figure 10. The average mass flow rate at the inlet of the SPT Design I is reduced compared to the baseline geometry without and with silencer and the baseline geometry plus SPT Design II (See table 4). This could be due to the positioning of the first stage nozzles leading to more leakages inside the compressor. The corresponding power consumption of the compressor with silencer and with the SPT Designs I and II are shown in figure 11. The power consumption is almost identical in the case of the baseline plus silencer compared to the baseline plus SPT Design I and in the comparison between the baseline plus silencer and the baseline plus SPT Design II there a power savings of about 5%. The pressure contour plot over the rotors in figure 12 shows the distribution of pressure 9 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Mass flow rate (b) Compressor power Figure 8. ASCII plots of the mass flow and compressor power predicted, baseline geometry, with silencer over the rotors in the baseline case without silencer and the baseline with SPT Design II. The pressure distribution is more uniform in the design with the SPT Design II than in the baseline. Another observation is that the timing of the opening of the first stage nozzles are not the same as highlighted in the two rectangular boxes in the figure. This has been corrected in the modified SPT design [12]. An important goal of the SPT design is to size the nozzles such that they are choked (Mach number=1). Doing this would ensure that the ∆p at the SPT will match the ∆p at the discharge, thus mitigating the effect of over and under compression and hence the pulsation inside the compressor, thus eliminating the need for a silencer. To check this monitoring points are defined at the nozzles to determine the pressure and the temperature as depicted in figure 4(b). Cut planes through the main (male) and gate (female) rotors are used to monitor the Mach number inside the nozzles as shown in figure 13. Based on this result it can be seen that most of the work is being done by the first nozzle with Mach number between 0.6 and 0.8 while the Mach number in the second nozzle is very low. Based on this result and in additional to the high flow loses in the SPT Design I the SPT design was modified by using a single stage nozzle with the nozzle repositioned in order to minimize or eliminate the high flow loses in the SPT Design I. 10 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Discharge velocity, no silencer (b) Discharge velocity, with silencer Figure 9. ASCII plots of the discharge velocity without and with silencer Figure 10. Mass flow rate of the baseline geometry compared to baseline plus silencer and SPT Design I & Design II 11 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Comp. power, silencer + SPT Design I (b) Comp. power, silencer + SPT Design II Figure 11. ASCII plots of the compressor power compared to silencer plus SPT Design I and silencer plus SPT Design II This is denoted as SPT Design II (See figure 6(b)) and it is simulated using the same settings used in the other setups and the results compared. A summary of the average mass flow rate, volume flow rate, compressor power and the total axial loads on the rotors are presented in table 4 for all the cases considered at CA 1600 deg. Using SPT Design II improves the volume flow rate and reduces the compressor power when compared to SPT Design I and the baseline plus silencer cases. Due to the lack of noise measurement at this stage the improvement in noise or pulsation level is examined by considering the dynamic pressure monitored for all the cases considered in this work as shown in figure 14. There is a direct correlation between the amplitude of the pressure fluctuation and flow induced noise inside the compressor. Using this argument it can be argued that the case with the SPT Design I has the least fluctuation with the lowest pressure amplitude followed by SPT Design II and the baseline without the silencer having the highest fluctuation as expected. It is clear that SPT Design I has better performance in terms of attenuating the noise but the flow losses are higher compared to SPT Design II due to positioning and the timing 12 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 12. Normalized contour plots of the pressure over the rotors for the baseline geom. without silencer & baseline geom. + SPT Design I Table 4. Summary of the ASCII plots of average flow and compressor power and axial load on rotors for CFD cases considered Case Inflow mass (kg/s) Volume flow (m /h) Power(kW) Total axial load (N) Baseline 0.257 805 38.5 1529 Baseline + silencer 0.2535 794 41.1 1722 SPT Design I 0.229 717 41 1701 SPT Design II 0.2556 800 39.1 1631 of the nozzles. In Design II with the single nozzle the flow losses have reduced but the noise attenuation is slightly reduced compared to Design I but better than the baseline. 5. Conclusions and future work The flow inside the SKL 1100 oil free screw compressor is successfully simulated using CONVERGE CFD and validated with measurement data. To enable the compressor to be run at other operating points without experiencing increase in pulsation and power consumption due to over and under compression Hi Bar Blowers has developed a Shunt Pulsation Trap (STP) that will trap the pulsations inside the compressor and therefore eliminate the need for a silencer and by so doing reduce power consumption due to the silencer back pressure and reduce the footprint of the compressor. As a first step, CFD was used to simulate the compressor with and without the reactive silencer that is used in the CVS Silo King (SKL 1100) oil free screw compressor followed by the simulation of the Hi-Bar Blowers SPT concepts. The following conclusions can be drawn: • The SPT concepts are effective in reducing the outflow fluctuations • The mass flow rate in SPT Design I is about 10% lower than in the baseline without and with silencer whereas for SPT Design II the mass flow is comparable to baseline without and with silencer. The compressor power for the case with SPT Design I and baseline plus silencer are the same whereas SPT Design II has slightly lower power consumption when compared to SPT Design I and baseline plus silencer. 13 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Cut plane, Gate rotor + SPT Design I (b) Cut plane, Main rotor + SPT Design I (c) Mach number, Gate rotor + SPT Design I (d) Mach number, Main rotor + SPT Design I Figure 13. Cut planes through and Mach Number showing flow through 1st & 2nd stage nozzles • The SPT Design I nozzles are not choked with the maximum Mach number of about 0.8 in nozzle 1 and that in nozzle 2 negligible. • The SPT Design II attenuation of the fluctuations is slightly less effective compared to Design I but it improves the volume flow rate significantly and it has less power requirement relative to Design I • The SPT Design II nozzles are not choked with maximum Mach number of 0.4 and the female (gate) rotor nozzle contribution is negligible The results of this work was used by Hi-Bar Blowers in the modification of the SPT design concept leading to the development of the Shunt Enhanced Decompression and Pulsation 14 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 14. Comparison of the pressure fluctuation at the screw region exist for the various designs Trap (SEDAPT) [11] and the Shunt Enhanced Compression and Pulsation Trap (SECAPT) technologies [12]. In the new concepts modification were made to the SPT to ensure that the opening and closing of the nozzles are correctly timed leading to reduction in the flow losses associated with the noise attenuation. As future work, the modified SPT concept designed will be fabricated and tested on the CVS Silo King (SKL 1100) oil free screw compressor. For the sake of time one design point was investigated in this work. When the modified SPT concept is tested other operating points, especially minimum speed and maximum load and minimum speed and no load conditions as well as other input speeds and loading conditions will be simulated and validated with measurement data. Dynamic pressure and vibration measurements will be carried out and used to validate the CFD results. 15 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Nomenclature SPT Shunt Pulsation Trap [-] SEDAPT Shunt Enhanced Decompression and Pulsation Trap [-] SECAPT Shunt Enhanced Compression and Pulsation Trap [-] p Pressure before opening of the outlet [Pa] p Discharge pressure [Pa] V Suction volume flow rate [m /s] p Suction pressure [Pa] ∆p Pressure drop [mbar] p Pressure [Pa] T Temperature [K] V Volume flow rate [m /s] n Rotor speed [rpm] P Compressor power [kW] AMR Adaptive mesh refinement [-] SGS Sub-grid scale [-] PISO Pressure-implicit with splitting of operators [-] MUSCL Monotonic upstream-centered scheme for conservation laws [-] CFL Courant-Friedrichs-Lewy [-] RNG Re-Normalisation Group [-] CA Crank angle [degree] Acknowledgment The authors would like to thank Pietro Scienza of Convergent Science and Riccardo Gretter for their invaluable support with the CFD setup. References [1] Willie J 2017 Use of CFD to Predict Trapped Gas Excitation as Source of Vibration and Noise in Screw Compressors (IOP Conf. Ser.: Mater. Sci. Eng.) 232 012021 [2] Willie J, Asal W and Sachs R 2014 Analysis of the Noise and Vibration of a Dry Screw Compressor (In.: 9th Int. Conf. on Screw Machines 2014, VDI-Berichte 2228 (2014), p. 107-123) [3] Willie J and Sachs R 2015 Structural and Torsional Vibration Analysis of a Dry Screw Compressor (IOP Conf. Ser.: Mater. Sci. Eng.) 90 012005 [4] Willie J and Sachs R 2016 Structural and Torsional Vibration and Noise Analysis of a Dry Screw Compressor (Proc IMechE Part E. J Process Mechanical Engineering) 0, DOI:10.1177/0954408916648989 [5] Huang P X 2012 Gas Pulsations: A Shock Tube Mechanism. (International Compressor Engineering Conference). Paper 2092 https://docs.lib.purdue.edu/icec/2092 [6] Huang P X, Yonkers S, Honkey D and Olenick D 2013 Screw pulsation generation and control: A shock tube mechanism (8th International Conference oon Compressors and their Systems, 9-10th September 2013, City Universtiy London) p 113 [7] Huang P X, Yonkers S and Hokey D 2014 Gas Pulsation Control Using a Shunt Pulsation Trap. (International Compressor Engineering Conference) Paper 2258 http://docs.lib.purdue.edu/icec/2258 [8] Huang P X 2018 A model for the transient pulsation generation at the discharge of a screw compressor by a shock tube analogy (IOP Conf. Ser.: Mater. Sci. Eng.) 425 012022 [9] Hall N 2021 Oblique shock wave. https://www.grc.nasa.gov/www/k-12/airplane/oblique.html [10] Richards J K, Senecal K P and Pomraning E 2021 CONVERGE 3.0 Manual. (Convergent Science, Madison, WI) [11] Huang P X, Yonkers S and Willie J 2022 A Novel Screw Compressor with a Shunt Enhanced Decomposition and Pulsation Trap (SEDAPT). (2022 Purdue International Compressor Engineering Conference) [12] Huang P X, Yonkers S and Willie J 2022 A Novel Screw Compressor with a Shunt Enhanced Compression and Pulsation Trap (SECAPT). (Int. Conf. on Screw Machines 2022, TU Dortmund University, Dortmund, Germany, September 7-8 2022) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png IOP Conference Series: Materials Science and Engineering IOP Publishing

Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration mitigation in oil free screw compressors

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IOP Publishing
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Published under licence by IOP Publishing Ltd
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1757-8981
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1757-899X
DOI
10.1088/1757-899x/1267/1/012017
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Abstract

International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Use of CFD to optimize the design of a shunt pulsation trap (SPT) used for noise and vibration mitigation in oil free screw compressors 1 2 2 1 J Willie , S W Yonkers , P X Huang and R B Ganatra CVS Engineering GmbH Grossmattstr. 14, D-79618 Rheinfelden, Germany Hi-Bar Blowers, Inc. Fayetteville, GA, USA E-mail: [email protected] Abstract. Noise and vibration are problems that are inherent in screw compressors and other Positive Displacement (PD)Machines. This problem is driven partly by the lobe passing frequencies inside these machines that are generated due to the meshing between the gate and main rotors teeth. Because these compressors are operated over a wide speed range and at various loads they usually run at off-design conditions leading to either over or under- compression. In both scenarios, the pressure within the compressor always changes rapidly to match the discharge or process pressure when the discharge opens thus leading to pulsation and noise. To ensure that the compressor is operated without over and or under compression, which also negatively affects the compressor efficiency Hi- Bar Blowers, Inc developed a Shunt Pulsation Trap (SPT) that will contain and reduce the pulsation inside the compressor cavities thus eliminating the use of a silencer at the inlet and the discharge. During the SPT development the CVS Silo King (SKL 1100) compressor was selected for benchmarking this technology. CFD was used extensively to simulate the SKL 1100 without and with the SPT integrated, which allowed the SPT design to be optimized. This paper will present the results obtained to demonstrate that this technology has the potential to eliminate over and under compression while at the same time leading to energy savings and reducing the compressor footprint and the noise and vibration that are commonplace in screw compressors. 1. Introduction The issue of noise in gas compression is commonplace and very little has been done to address this issue apart from the use of silencers. Even with the use of these silencers, noise levels of up to 100 dBA and above are possible and this is very uncomfortable and can be damaging to the ear of human beings. To address this issue CVS has teamed up with Hi Bar Blowers to investigate the root-cause of pulsation inside positive displacement (PD) compressors with internal compression. In this case, the pilot project is investigating the Silo King (SKL 1100) screw compressor developed by CVS Engineering. A Shunt Pulsation Trap (SPT) has been designed by Hi-Bar Blowers and its purpose is to trap the pressure wave that is responsible for the pulsation inside the screw housing and allow only the non-pulsating flow to leave the compressor. The pulsation suppression will be done over the load range 0-2.5bar(g) with the following goals: • Discharge pulsation level (< 0.1bar) and better noise (< 92dBA) than the design with the reactive silencer; Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 • Smaller footprint. The pulsation trap would be about 25-50 % of the existing reactive silencer ; • Energy savings. About 5-10% energy savings expected. Torque measurements are expected to be lower due to the lower power consumption by eliminating over and under compression. In addition, the back pressure due to the silencer will be eliminated and for the worst case of maximum speed and maximum discharge pressure this amounts to 132 mbar in the Silo King SKL 1100 compressor If successful, SPT would address the noise pollution coming from these compressors. It would address not only the loudness but the noise quality as well. It would also reduce the footprint and the weight of these compressors, which is very important for this application. By reducing or eliminating over and under compression, we expect the pressure drop to decrease, thereby reducing the power consumption of the compressor. The elimination of a silencer means that we would also save the pressure drop introduced by it and eliminate the power consumption that goes with this. Figure 1. Schematic showing under and over-compression The authors are of the opinion that the root cause of high pulsations in screw compressors and other PD machines like blowers is not well understood. Based on the work done by Hi Bar Blowers [5, 6, 7, 8] it is believed that apart from the lobe passing frequencies due to the rotors lobes and the noise generated by gears flow induced noise is predominantly due to high pressure waves when the compressor is operated at off design conditions that lead to over and under compression. The screw compressor is designed for a specific internal volume compression or pressure ratio but in application it is seldom operated at this design condition due to process requirements. Operating at off design condition leads to either over or under compression as shown in figure 1. The plot on the left is showing the case when the compressor is operated at the design point. In this case the pressure inside the pump housing at the discharge p is equal to the discharge pressure p and the process is isentropic. The sketch in the middle is showing the case where we have over-compression and just before the discharge opens the chamber pressure is greater than the discharge pressure. In this case when the discharge opens the chamber pressure drops suddenly from p to p and this can lead to pulsation or pressure fluctuation and noise i D generation. The shaded area corresponds to the extra work that must be done because of the over-compression. In the sketch on the right the chamber pressure p is less than the discharge pressure before the discharge port is opened and it is referred to as under-compression. When the discharge opens a rapid flow is generated from the discharge tube to the chamber until we have an equalization of pressure. The extra work done as a result of the under-compression is shaded in the p-V diagram. If a silencer is used, which is the case in practice, an additional loss is introduced due to the back-pressure of the silencer as depicted in the under-compression case shown in figure 2. The main goal of the SPT is to ensure that there is no pressure drop when the discharge opens 2 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 and so it means using the SPT will lead to additional savings in energy thus optimizing the temperature of the compressor and preventing the use of oil or air cooler, which will lead to additional cost savings. Figure 2. Schematic of the energy saving potential of the SPT technology In the work by Willie [1] it was shown that for most screw compressors there is gas that is trapped between the rotors and the discharge axial gap just before the discharge opens and this can lead to over-compression and to gas pulsations in screw compressors. In [2, 3, 4], other sources of noise inside screw compressors were presented and discussed. In previous work by Hi Bar Blowers the theory behind the development of the SPT technology has been explained [5, 6, 7, 8]. It has been tested for root blowers, which do not have internal compression, and the results showed upto 10 fold reduction in gas pulsations. However, this technology is yet to be tested for screw compressors, which have internal compression, hence the motivation for this work. In this work Computational Fluid Dynamics (CFD) is used to simulate the flow inside the Silo King SKL 1100 screw compressor using CONVERGE CFD. The pre-processor of this software (CONVERGE STUDIO) has advantages over others because of the way it handles moving meshes using adaptive and automatic meshing technique including the cut cell technique in order to mesh complicated moving geometries. The paper begins with the description of the measurement setup and the measurement points used to benchmark the Silo King SKL 1100 compressor without and with a silencer. This is followed by the case setup description of the SPT design used to numerically validate the potential of this technology and after this the CFD setup is presented and this followed by the results and discussions and the conclusions and future work. 2. Measurements A picture showing the setup of the measurement test stand is presented in figure 3. The stand is used for determining the performance map of the benchmark compressor for testing the SPT technology. The following are measured: Volume flow rate, inlet/discharge temperature and pressure, compressor power, and the silencer pressure drop (∆p). Noise measurement is done using microphone that is located 1 m away from the compressor and similar measurements will be done with the SPT integrated into the compressor for comparison. The operating point simulated is shown in table 1 with an input speed of 3200rpm. This point is close to the design point that has an internal volume compression of 2.1 and an input 3 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 3. Picture of measurement setup in the laboratory speed of 3000rpm. The maximum input speed of this compressor is 3600rpm. The discharge pressure p is taken before the silencer directly at the outlet of the compressor and close to the discharge contour. Table 1. Silo king screw compressor operating point for CFD validation of the SPT technology Parameter Measured value Inlet air pressure (p ) 1.00 bar(a) Inlet air temperature (T ) 28 C Discharge air pressure (p ) 2.5bar(a) Discharge air temperature (T ) 150.9 C Volume flow rate (V ) 807.37 m /h Female rotor speed (n ) 10732.3 rpm Male rotor speed (n ) 13415.4 rpm Compressor power (P) 44 kW Silener pressure drop (∆p) 132 mbar 3. CFD Case Setup The setup used in the CFD simulation of the baseline SKL 1100 screw compressor with and without the use of conventional silencer is presented followed by the description of the setup with the SPT included. Due to the need to capture any shocks in the flow a very fine mesh was used and the settings used is similar to the settings used for modeling the NASA supersonic channel benchmark geometry for capturing shock waves [9]. 4 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) CFD setup of the baseline geometry: Left=3D CAD, Right: Neg. vol. (b) CFD setup: Baseline with monitoring points Figure 4. CAD geom. + neg. volume of baseline geom. including monitoring points for CFD 3.1. CFD setup of the baseline geometry without silencer The CAD geometry of the SKL 1100 is first cleaned for CFD simulation and the negative volume is extracted without the discharge silencer as shown in figure 4(a). Included in the geometry are the suction or inlet or inflow, the discharge or outflow and the rotors. The negative volume is exported as an STL file into CONVERGE STUDIO for the definition of the boundary conditions and the solver conditions and the mesh settings. The speed of rotation of the rotors are also defined. The working fluid is air and the inlet and outlet are given pressure boundaries as measured. Using pressure boundaries will allow the solver to compute the mass flow rate from which the volume flow rate can be computed using the computed density. The computed volume flow rate can then be validated with the volume flow measured. Due to the use of cold gaps the leakages are higher than in the reality where due to the thermal expansion of the rotors the hot gaps are smaller. In order to tune the cold gaps to be able to mimic the hot gaps proximity boundaries and porous media are used in CONVERGE [10]. Doing this will allow the solver to accurately predict the volume flow rate inside the machine. Tw is taken to be the in suction temperature and to determine Tw , the wall temperature at the outlet, the relations out 5 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 ( ) κ−1 T = T , where κ is the polytropic coefficient determined from measurements to be 2 1 1.7. This gives Tw =439 K. For the meshing a base grid of 16mm is used and adaptive mesh out refinement (AMR) is used for all regions at level 4 (1mm) on velocity with Sub-Grid-Scale (SGS) 1.0. At the discharge, fixed embedding is used at level 4 (1mm) and finally, grid scaling from -1 rd to 0 is used at the end of the 3 cycle. Mesh sensitivity analyses is used to determine that the 16mm base grid will be able to resolve the flow as needed. To monitor the flow and enable comparison of the various setups and also for validation of the CFD results with measurement data after the modified SPT is tested, four (4) monitoring points are defined as shown in figure 4(b). The pressure and the temperature are monitored at each point for comparison. 3.2. CFD setup of the baseline geometry with the silencer The setup of the baseline geometry with the discharge silencer included is presented here. The CAD geometry and the negative volume used for the CFD setup are shown in figure 5. The outflow is now located at the discharge of the silencer and the inflow is the same as in the baseline case in figure 4. To determine the pressure at the discharge of the silencer the discharge pressure in table 1, which is measured close to the rotors exit is added to the silencer pressure drop of 132 mbar. When the SPT design is used the silencer pressure drop will be avoided. The settings of the CFD used are similar to those used in the baseline case setup in section 3.1. Figure 5. CFD setup of the baseline geom. + silencer: Left=CAD geom., Right=Neg. vol. 3.3. CFD setup of the baseline geometry with the SPT concepts The setup of the SKL 1100 screw compressor with the SPT Design I and Design II concepts integrated is presented. A two stage SPT design was used in Design I and a one stage design was used in Design II, with each stage having 2 nozzles as shown in figure 6. The purpose of this concept is to ensure that there is no pressure drop at the discharge ensuring that irrespective of the operating point there will be minimum pressure fluctuation or noise and hence optimum operation with no under or over-compression inside the compressor [5, 6, 7, 8, 11, 12]. Based on the results of Design I optimization was done and Design II, which is a single stage design, 6 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 was proposed. The negative volume used in both cases for the CFD simulations are shown in figure 6(b), with the left plot corresponding to the two stage design and the right corresponding to the one stage design. (a) CAD geometry of SKL 1100 with SPT (b) Neg. volume with SPT, left=Design I (two stage STP) & right=Design II Figure 6. CAD model and negative volume of SKL 1100 with SPT integrated In total, four setups and mesh strategies are used to ensure that the pressure waves and the flow is being adequately resolved, namely, A, B, C & D. The setup associated with each case is summarized in table 2. A comparison of the averaged results for the 4 setups shows similar results as depicted in table 3. For comparison with the baseline cases, setup C is used because it was able to mimic the flow in the screw compressor and capture the pressure fluctuation as expected. Also, grid sensitivity analyses showed that this setup was not numerically expensive. These settings are also used to simulate the SPT Design II for comparison with the SPT Design I and the baseline SKL 1100 compressor. 4. Results and discussions The CFD simulation results of the baseline SKL 1100 compressor without and with the reactive silencer are first validated with measurement data shown in table 1 to ensure that the results have been able to capture the physics of the flow and are not non-physical due to numerical errors. This is followed by the presentation of the of the SPT results. The ASCII plots of the average volume flow rate and the compressor power for the baseline case without the silencer are shown in figure 7. The averaging is done considering periods of 7 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Table 2. Averaged results of the various setups of SKL 1100 + SPT Design I Setup Base grid AMR regions Embedding Grid scaling Solver A 16mm All, level 4 (1mm) Fixed at outlet, 1 cycle -2; Std. RNG k-eps 2 cycles -1; then 0 Nozzles level 5 (0.5mm) Vel. level 4 (1mm) SGS 1.0 B 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; Std. RNG k-eps 2 cycles -1; then 0 Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) (from map file) SGS 1.0 Fixed at outlet, level 4 (1mm) C 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; CFL=1 nozzles, Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) 2 cycles -1; then 0 Total energy SGS 1.0 Fixed at outlet, (from map file) Stricter PISO level 4 (1mm) RNG k-eps D 16mm All, level 4 (1mm) Fixed at nozzles, 1 cycle -2; CFL=1 nozzles, Nozzles level 5 (0.5mm) Vel. level 5 (0.5mm) 2 cycles -1; then 0 Total energy Stricter SGS 1.0 Fixed at outlet, (from map file) PISO MUSCL, RNG level 4 (1mm) k-eps Table 3. Averaged results of the various setups of SKL 1100 + SPT Design I Setup Inflow mass(kg/s) Inflow vel. (m/s) Outflow mass (kg/s) Outflow vel. (m/s) Power(kW) A 0.207 35.9 0.177 21.1 40.7 B 0.207 35.9 0.176 20.8 40.7 C 0.189 32.9 0.182 21.2 41.0 D 0.190 33.18 0.180 21.5 41.0 rotation of 360 . In the setup used time advancement has been modified and replaced by the number of degrees that the male (main) rotor is rotating. Thus 360 represents a full rotation of the male rotor. So a time average over a period equal to a complete revolution of the male rotor. For statistical convergence at least three revolutions or Crank Angle 1080 are required. The volume flow rate measured is at the inlet of the compressor and the compressor power is computed from CFD using the main (male) and gate (female) rotors torque. For the setup without the use of proximity boundaries (denoted as original in the plot) the inlet mass flow rate at CA 1700deg is 0.234 kg/s and that at the outlet is 0.228 kg/s, giving a percentage error of 2.6%, which is good for mass balance. The average inlet volume flow rate in this case is 733 m /h. One reason for the disparity between the measured and the simulated volume flow is due to leakages inside the machine and the fact that the CONVERGE solver used does not resolve the flow inside the gaps but uses proximity boundaries to model the flow inside the gaps of the compressor. The model is tuned by using proximity boundaries (denoted as update in the plot) to mimic the flow inside the gaps. This gives an inlet mass flow rate of 0.257 kg/s and hence a volume flow rate of 805 m /hr, which is very close to the measured value. After the baseline simulations and tuning were completed and the results validated with measurement data, the simulations were extended to include the baseline geometry plus the silencer and the baseline geometry plus the SPT Design I. Based on the SPT Design I results optimization was done, which resulted in the SPT Design II concept that was also simulated and compared with the SPT Design I results. 8 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Volume flow rate (b) Compressor power Figure 7. ASCII plots of the volume flow and compressor power predicted, baseline geometry, no silencer The ASCII results of the average mass flow rate and hence the volume flow rate and the compressor power of the baseline geometry plus the silencer are shown figure 8. The mass flow rate at the inlet is 0.2535 kg/s at CA 1600deg and that at the outlet is 0.222 kg/s, giving a percentage error of 10%. The moving average value was changing slightly at 1600 deg. CA and this could be due to the lager geometry from the addition of the silencer. The average compressor power has increased to 41.1kW at 1600 deg CA due to the addition of the silencer and the back pressure it introduces. This value is closer to that measured in table 1. Also noticed is the increase in the average discharge velocity from 41.4 m/s in the case without silencer to 53.25 m/s in the case with silencer (6.2%) as shown in figure 9. The increase is due to the fact that the silencer inlet and outlet have smaller diameters when compared to the outlet of the compressor. The simulation is extended to the baseline geometry with the SPT Design I that has two stage nozzles integrated and after that to the SPT Design II with a single stage nozzle and the results of the mass flow compared to the baseline geometry without and with silencer are shown in figure 10. The average mass flow rate at the inlet of the SPT Design I is reduced compared to the baseline geometry without and with silencer and the baseline geometry plus SPT Design II (See table 4). This could be due to the positioning of the first stage nozzles leading to more leakages inside the compressor. The corresponding power consumption of the compressor with silencer and with the SPT Designs I and II are shown in figure 11. The power consumption is almost identical in the case of the baseline plus silencer compared to the baseline plus SPT Design I and in the comparison between the baseline plus silencer and the baseline plus SPT Design II there a power savings of about 5%. The pressure contour plot over the rotors in figure 12 shows the distribution of pressure 9 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Mass flow rate (b) Compressor power Figure 8. ASCII plots of the mass flow and compressor power predicted, baseline geometry, with silencer over the rotors in the baseline case without silencer and the baseline with SPT Design II. The pressure distribution is more uniform in the design with the SPT Design II than in the baseline. Another observation is that the timing of the opening of the first stage nozzles are not the same as highlighted in the two rectangular boxes in the figure. This has been corrected in the modified SPT design [12]. An important goal of the SPT design is to size the nozzles such that they are choked (Mach number=1). Doing this would ensure that the ∆p at the SPT will match the ∆p at the discharge, thus mitigating the effect of over and under compression and hence the pulsation inside the compressor, thus eliminating the need for a silencer. To check this monitoring points are defined at the nozzles to determine the pressure and the temperature as depicted in figure 4(b). Cut planes through the main (male) and gate (female) rotors are used to monitor the Mach number inside the nozzles as shown in figure 13. Based on this result it can be seen that most of the work is being done by the first nozzle with Mach number between 0.6 and 0.8 while the Mach number in the second nozzle is very low. Based on this result and in additional to the high flow loses in the SPT Design I the SPT design was modified by using a single stage nozzle with the nozzle repositioned in order to minimize or eliminate the high flow loses in the SPT Design I. 10 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Discharge velocity, no silencer (b) Discharge velocity, with silencer Figure 9. ASCII plots of the discharge velocity without and with silencer Figure 10. Mass flow rate of the baseline geometry compared to baseline plus silencer and SPT Design I & Design II 11 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Comp. power, silencer + SPT Design I (b) Comp. power, silencer + SPT Design II Figure 11. ASCII plots of the compressor power compared to silencer plus SPT Design I and silencer plus SPT Design II This is denoted as SPT Design II (See figure 6(b)) and it is simulated using the same settings used in the other setups and the results compared. A summary of the average mass flow rate, volume flow rate, compressor power and the total axial loads on the rotors are presented in table 4 for all the cases considered at CA 1600 deg. Using SPT Design II improves the volume flow rate and reduces the compressor power when compared to SPT Design I and the baseline plus silencer cases. Due to the lack of noise measurement at this stage the improvement in noise or pulsation level is examined by considering the dynamic pressure monitored for all the cases considered in this work as shown in figure 14. There is a direct correlation between the amplitude of the pressure fluctuation and flow induced noise inside the compressor. Using this argument it can be argued that the case with the SPT Design I has the least fluctuation with the lowest pressure amplitude followed by SPT Design II and the baseline without the silencer having the highest fluctuation as expected. It is clear that SPT Design I has better performance in terms of attenuating the noise but the flow losses are higher compared to SPT Design II due to positioning and the timing 12 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 12. Normalized contour plots of the pressure over the rotors for the baseline geom. without silencer & baseline geom. + SPT Design I Table 4. Summary of the ASCII plots of average flow and compressor power and axial load on rotors for CFD cases considered Case Inflow mass (kg/s) Volume flow (m /h) Power(kW) Total axial load (N) Baseline 0.257 805 38.5 1529 Baseline + silencer 0.2535 794 41.1 1722 SPT Design I 0.229 717 41 1701 SPT Design II 0.2556 800 39.1 1631 of the nozzles. In Design II with the single nozzle the flow losses have reduced but the noise attenuation is slightly reduced compared to Design I but better than the baseline. 5. Conclusions and future work The flow inside the SKL 1100 oil free screw compressor is successfully simulated using CONVERGE CFD and validated with measurement data. To enable the compressor to be run at other operating points without experiencing increase in pulsation and power consumption due to over and under compression Hi Bar Blowers has developed a Shunt Pulsation Trap (STP) that will trap the pulsations inside the compressor and therefore eliminate the need for a silencer and by so doing reduce power consumption due to the silencer back pressure and reduce the footprint of the compressor. As a first step, CFD was used to simulate the compressor with and without the reactive silencer that is used in the CVS Silo King (SKL 1100) oil free screw compressor followed by the simulation of the Hi-Bar Blowers SPT concepts. The following conclusions can be drawn: • The SPT concepts are effective in reducing the outflow fluctuations • The mass flow rate in SPT Design I is about 10% lower than in the baseline without and with silencer whereas for SPT Design II the mass flow is comparable to baseline without and with silencer. The compressor power for the case with SPT Design I and baseline plus silencer are the same whereas SPT Design II has slightly lower power consumption when compared to SPT Design I and baseline plus silencer. 13 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 (a) Cut plane, Gate rotor + SPT Design I (b) Cut plane, Main rotor + SPT Design I (c) Mach number, Gate rotor + SPT Design I (d) Mach number, Main rotor + SPT Design I Figure 13. Cut planes through and Mach Number showing flow through 1st & 2nd stage nozzles • The SPT Design I nozzles are not choked with the maximum Mach number of about 0.8 in nozzle 1 and that in nozzle 2 negligible. • The SPT Design II attenuation of the fluctuations is slightly less effective compared to Design I but it improves the volume flow rate significantly and it has less power requirement relative to Design I • The SPT Design II nozzles are not choked with maximum Mach number of 0.4 and the female (gate) rotor nozzle contribution is negligible The results of this work was used by Hi-Bar Blowers in the modification of the SPT design concept leading to the development of the Shunt Enhanced Decompression and Pulsation 14 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Figure 14. Comparison of the pressure fluctuation at the screw region exist for the various designs Trap (SEDAPT) [11] and the Shunt Enhanced Compression and Pulsation Trap (SECAPT) technologies [12]. In the new concepts modification were made to the SPT to ensure that the opening and closing of the nozzles are correctly timed leading to reduction in the flow losses associated with the noise attenuation. As future work, the modified SPT concept designed will be fabricated and tested on the CVS Silo King (SKL 1100) oil free screw compressor. For the sake of time one design point was investigated in this work. When the modified SPT concept is tested other operating points, especially minimum speed and maximum load and minimum speed and no load conditions as well as other input speeds and loading conditions will be simulated and validated with measurement data. Dynamic pressure and vibration measurements will be carried out and used to validate the CFD results. 15 International Conference on Screw Machines 2022 IOP Publishing IOP Conf. Series: Materials Science and Engineering 1267 (2022) 012017 doi:10.1088/1757-899X/1267/1/012017 Nomenclature SPT Shunt Pulsation Trap [-] SEDAPT Shunt Enhanced Decompression and Pulsation Trap [-] SECAPT Shunt Enhanced Compression and Pulsation Trap [-] p Pressure before opening of the outlet [Pa] p Discharge pressure [Pa] V Suction volume flow rate [m /s] p Suction pressure [Pa] ∆p Pressure drop [mbar] p Pressure [Pa] T Temperature [K] V Volume flow rate [m /s] n Rotor speed [rpm] P Compressor power [kW] AMR Adaptive mesh refinement [-] SGS Sub-grid scale [-] PISO Pressure-implicit with splitting of operators [-] MUSCL Monotonic upstream-centered scheme for conservation laws [-] CFL Courant-Friedrichs-Lewy [-] RNG Re-Normalisation Group [-] CA Crank angle [degree] Acknowledgment The authors would like to thank Pietro Scienza of Convergent Science and Riccardo Gretter for their invaluable support with the CFD setup. References [1] Willie J 2017 Use of CFD to Predict Trapped Gas Excitation as Source of Vibration and Noise in Screw Compressors (IOP Conf. Ser.: Mater. Sci. Eng.) 232 012021 [2] Willie J, Asal W and Sachs R 2014 Analysis of the Noise and Vibration of a Dry Screw Compressor (In.: 9th Int. Conf. on Screw Machines 2014, VDI-Berichte 2228 (2014), p. 107-123) [3] Willie J and Sachs R 2015 Structural and Torsional Vibration Analysis of a Dry Screw Compressor (IOP Conf. Ser.: Mater. Sci. Eng.) 90 012005 [4] Willie J and Sachs R 2016 Structural and Torsional Vibration and Noise Analysis of a Dry Screw Compressor (Proc IMechE Part E. J Process Mechanical Engineering) 0, DOI:10.1177/0954408916648989 [5] Huang P X 2012 Gas Pulsations: A Shock Tube Mechanism. (International Compressor Engineering Conference). Paper 2092 https://docs.lib.purdue.edu/icec/2092 [6] Huang P X, Yonkers S, Honkey D and Olenick D 2013 Screw pulsation generation and control: A shock tube mechanism (8th International Conference oon Compressors and their Systems, 9-10th September 2013, City Universtiy London) p 113 [7] Huang P X, Yonkers S and Hokey D 2014 Gas Pulsation Control Using a Shunt Pulsation Trap. (International Compressor Engineering Conference) Paper 2258 http://docs.lib.purdue.edu/icec/2258 [8] Huang P X 2018 A model for the transient pulsation generation at the discharge of a screw compressor by a shock tube analogy (IOP Conf. Ser.: Mater. Sci. Eng.) 425 012022 [9] Hall N 2021 Oblique shock wave. https://www.grc.nasa.gov/www/k-12/airplane/oblique.html [10] Richards J K, Senecal K P and Pomraning E 2021 CONVERGE 3.0 Manual. (Convergent Science, Madison, WI) [11] Huang P X, Yonkers S and Willie J 2022 A Novel Screw Compressor with a Shunt Enhanced Decomposition and Pulsation Trap (SEDAPT). (2022 Purdue International Compressor Engineering Conference) [12] Huang P X, Yonkers S and Willie J 2022 A Novel Screw Compressor with a Shunt Enhanced Compression and Pulsation Trap (SECAPT). (Int. Conf. on Screw Machines 2022, TU Dortmund University, Dortmund, Germany, September 7-8 2022)

Journal

IOP Conference Series: Materials Science and Engineering – IOP Publishing

Published: Nov 1, 2022

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