GT2017-65172

DESIGN OF A WIDE RANGE CENTRIFUGAL COMPRESSOR STAGE FOR SUPERCRITICAL CO2 POWER CYCLES Robert Pelton Hanwha Techwin Houston Texas USA Tim Allison Southwest Research Institute San Antonio Texas USA Sewoong Jung Hanwha Techwin Houston Texas USA Natalie Smith Southwest Research Institute San Antonio Texas USA ABSTRACT Supercritical carbon dioxide sCO2 power cycles require high compressor efficiency at both the design point and over a wide operating range Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part load operating conditions For sCO2 cycles operating with compressor inlets near the critical point large variations in fluid properties are possible with small changes in temperature or pressure This leads to particular challenges for air cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events Design and off design operating requirements for a wide range compressor impeller are presented where the impeller is implemented on an integrally geared compressor expander IGC concept for a high temperature sCO2 recompression cycle In order to satisfy the range and efficiency requirements of the cycle a novel compressor stage design incorporating a semi open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range These advanced impeller design features are enabled through the use of direct metal laser sintering DMLS manufacturing The resulting design increases the range from 45 to 73 relative to a conventional closed impeller design while maintaining high design point efficiency INTRODUCTION A supercritical carbon dioxide sCO2 power cycle based on recuperated Brayton cycle with re compression has been developed for a 10MWe application The cycle utilizes a multistage main compressor with compressor suction conditions near the critical point to reduce the compression work Therefore the 1st stage main compressor must be designed to manage potential variations in the inlet state of the gas associated with operation near the critical point Near the critical point slight changes in either the temperature or pressure of the gas have large effects on the aerodynamic performance of the compressor For example in the case of a compressor inlet operating at 37 C and 85barA a 2 C variation in inlet temperature is associated with a 45 change in inlet density Therefore to maintain a constant mass flow in the cycle the compressor has to maintain the required head rise across a wide range of flow coefficients Supercritical carbon dioxide sCO2 power cycles demand high off design efficiency from the compressor in addition to the design point Variations in compressor aerodynamic performance may adversely affect the cycle performance and make the turbine difficult to control Even in cases where the design suction conditions are above the saturation line there is still a risk for large gradients in fluid properties within the stage In particular as the flow accelerates though the aerodynamic throat in the inducer the static pressure and temperature will drop Care must also be taken to ensure that the flow does not transition below the saturation line and increase the potential for condensate formation Therefore developing a 1st stage main compressor that can stably operate over a wide range helps ensure a robust operating system Expanding the stable operating range of a compressor has been a goal of many classes of turbomachinery and in many industries including axial and radial compressors turbochargers and aero engines Numerous technologies have been proposed to widen operating range between surge and Proceedings of ASME Turbo Expo 2017 Turbomachinery Technical Conference and Exposition GT2017 June 26 30 2017 Charlotte NC USA GT2017 65172 1Copyright 2017 ASME choke for different types of applications In this paper a novel design is developed for a high pressure sCO2 compressor NOMENCLATURE a00 Inlet sonic velocity d2 Impeller diameter h Enthalpy MU2 U2 a00 Q Volume Flow Rate Range U2 Impeller Tip Speed 4 2 2 2 2 2 2 CYCLE CONSIDERATIONS One advantage of many sCO2 power generation cycles is that the compressor or pump inlet condition can be operated near the critical point resulting in low compression work Although this approach does improve the cycle efficiency it also increases the operability risk since small changes in temperature or pressure will have large effects on the inlet density and volume flow rate A detailed cycle optimization study for an sCO2 recompression cycle was performed for concentrating solar power CSP applications The study which is described more fully in 1 and 2 was performed iteratively with turbomachinery aerodynamic design calculations resulting in a 4 stage reheated expander 2 stage main compressor and 2 stage recompressor The turbomachinery was incorporated into a single integrally geared machine with a main compressor first stage design point inlet condition of 37 C and 85barA The integrally geared core is coupled to a fixed speed generator Based on this configuration an off design cycle optimization study was performed that incorporated turbomachinery performance maps and off design heat exchanger performance calculations in order to determine the best combination of cycle controls that would optimize cycle efficiency over a range of compressor inlet temperatures CIT relevant to daily and annual temperature variations The off design optimization included a combination of inventory mass flow control variation in flow split between the main compressor and recompressor incorporation of variable inlet guide vanes at the compressor inlet changes in cycle pressures and variations in cooling and heating duty required to achieve heat balance for compressor inlet temperatures ranging from 32 55 C The optimal off design control scheme results in changing the compressor operating point for each stage as indicated in Figure 1 assuming constant shaft speed These results indicate that the required flow range for steady off design operation of the main compressor stages is approximately 20 assuming variable IGVs on the first stage and conventional stage design see baseline The various line colors in the stage 1 chart correspond to different IGV positions ranging from 10 deg orange line to 30 deg blue line and different compressor operating points that may be encountered during normal plant operation are marked with a blue x Although these results indicate that only moderate range is required to accomplish off design operation stages with wider a high efficiency operating range would improve off design compressor and cycle efficiency Figure 1 Optimal main compressor operating points The off design results shown in Figure 1 assume steady off design operation but the effect of transients must also be considered to determine compressor range requirements It is estimated that in a production application there may be 2 C variation in the compressor inlet temperature potentially due to solar transients control system response rates etc In the case of a compressor operating at 37 C and 85barA 2 C change in inlet temperature is associated with a 45 change in inlet density Figure 2 This is much greater than the density variation for compressor operating in an air Brayton cycle with an inlet near 1bar where a 2 C variation in inlet temperature will only have a 1 4 change in density Figure 2 Compressor inlet gas density sensitivity to changes in temperature 2Copyright 2017 ASME Consequently to permit stable operation of sCO2 machinery during such transients a range of approximately 65 is desired This accounts for variations in gas density plus 10 choke and surge margins BASELINE COMPRESSOR DESIGN A meanline compressor sizing and initial performance estimates were developed to match the cycle specification The meanline sizing and non dimensional performance of the 1st stage main compressor are shown in Table 1 The head and flow coefficients were set at 0 98 and 0 061 respectively A higher flow coefficient stage would typically be used in an integrally geared machine but was not achievable in this case due to mechanical limitations of the gears and bearings associated with the high power density in an integrally geared machine Table 1 Main compressor 1st stage design and performance Parameter Symbol Units Value Inlet Pressure p00 BarA 85 2 Inlet Temperature T00 C 37 Flow Coeff 0 0612 Isen Head Coeff TT s 0 977 Stage Efficiency TT s 83 98 Stage Pressure Ratio PRTT 1 91 Machine Mach No MU2 0 838 A preliminary 3D flowpath was then generated to match the meanline design A shrouded impeller is applied since the tight tip clearances required for an efficient open impeller could not be maintained at these stage pressures The stage design included an axial inlet backswept blades and a vaned diffuser Figure 3 Figure 3 2D cross section and 3D model of the baseline compressor design Several iterations of the design were made based on CFD assessment to refine the flowpath in order to maximize efficiency and range The CFD analyses were performed using the commercial CFD solver Fine Turbo1 A fully structured single passage hexahedral mesh with matching periodic boundaries was created as the computational domain The total mesh size of the main passage and the casing treatment was approximately 2 8 million The one equation turbulence model Spalart Allmaras with wall functions was applied for the computations The boundary layer was resolved to an average y value less than 200 for the analysis Wall functions were used to avoid solving a much finer mesh which would be required to resolve the boundary layer The inlet boundary condition was defined by total pressure and temperature with a given velocity direction At the outlet boundary condition a fixed static pressure was applied at high flow and the mass flow rate was specified at low flow Non slip and adiabatic wall boundary conditions were applied as well The computation was considered converged when the root mean square RMS residual value was less than 10 6 or the oscillation of inlet and outlet mass flow rate was less than 0 1 Steady state CFD was conducted for this evaluation since is it is economical to run which enables a wide range of designs to be considered in detail Accurately identifying the stall limit is challenging with a simple steady state CFD analysis Although the exact stall point may be difficult to model steady CFD can be used to assess the relative improvement in the range of one design relative to another In this paper the stall flow rate identified in the CFD analysis is defined as the flow rate when either the static pressure no longer increases with lower mass flow rate or a stable solution could not be achieved Fluid properties used in the analysis were based on real gas properties calculated using NIST REFPROP For the baseline design the range was calculated to be 43 and an efficiency of 87 was estimated following the procedure detailed above Figure 4 shows predicted streamlines in the impeller at the lowest flow point where a steady CFD solution would converge The CFD solution shows clearly that stall develops at the inducer shroud Therefore the most effective range extension techniques for this case should address inducer stall 1 Fine Turbo is block structured Navier Stokes CFD software product by Numeca specialized to the simulation of internal multi stage rotating and turbomachinery flows 3Copyright 2017 ASME Figure 4 Streamlines with relative Mach number contours in the baseline compressor model for the lowest stable flow solution RANGE EXTENSION CONCEPT OVERVIEW A broad spectrum of range extension concepts were considered for this application including variable inlet guide vanes IGVs variable diffusers splitters partially shrouded impellers and casing treatments Variable IGVs are used to change the amount of flow that a stage can accept through the addition of pre swirl at the inlet 1 By incorporating a variable IGV geometry the amount of pre swirl can be optimized for each operating condition and thus the compressor operating range is widened significantly While the literature consists of extensive work an example of a more recent study is that of Mohseni et al 4 who show the potential to improve efficiency with various IGV profiles through reducing loss coefficients by 50 at high setting angles In general vaneless diffusers offer a wider operating range than vaned diffusers 5 but have lower stage efficiency Many authors have investigated these effects on compressor range with changes in vaneless diffuser design including parallel walls curved walls nonparallel walls and the use of hub and or shroud pinch Features like pinch 6 or optimizing solidity can help stabilize the flow through the diffuser and improve range Variable diffusers vanes are another application of variable geometry that can extend the operating range Similar to variable IGVs variable diffusers allow for high compressor efficiency over a wider range of operation It has also been shown 7 that several diffuser variables influence the onset of stall These factors highlight the potential utility of variable diffuser geometry to extend range and improve stage efficiency 8 The major disadvantages to employing a variable geometry design are the complications associated with an active system both from a stand point of controls as well as sealing especially for process gas applications at high pressures and temperatures As design tools and manufacturing capabilities advance the implementation of complex geometries into compressor designs has become more feasible More intricate 3D stacking methods provide designs with greater means to control pressure gradients and the development of secondary flows and losses through a blade row Early works 9 address the use of lean and sweep on impellers while more recent works 10 11 have shown improvements of several points efficiency through the use of 3D stacking and combinations blade lean and sweep A thick bladed impeller concept 12 recommends adding material to the trailing edge of the blade thus filling the region that would otherwise experience excessively thick boundary layers flow separation and loss and can reduce the surge mass flow rate by 70 A more common alternative to achieving the same effect without increasing blade thickness and rotating mass is to reduce diffuser height Since these approaches focus on improving exducer stability it was not evaluated in further detail Splitters have long been used to facilitate range extension for open impellers through reducing blockage at the inducer and solidity near the trailing edge 13 While the use of splitters is widely accepted and commonly utilized for open impellers manufacturing capabilities have eliminated them from use in closed impellers With more advanced manufacturing methods such as 3D printing the implementation of splitter blades has become an option for closed impellers Other impeller range extension concepts that could be realized through direct metal laser sintering DMLS are partially shrouded impellers 14 Shroud designs can include features for flow control both passive and active used to improve compressor operating range typically by delaying the onset of stall Passive designs include various casing or shroud treatments and bleed cavities and active control features that typically involve removing or injecting fluid in the endwall boundary layer Casing treatments have been studied as a means of delaying the onset of stall for many years in both axial and radial machines 15 17 Casing treatments include lateral and skewed slots radial grooves pin holes honeycomb and circumferential grooves In general the increased range at low mass flow rates comes at the cost of reduced efficiency although a few designs have shown negligible effects on efficiency Shroud bleed is an effective passive mechanism used to extend the operating range of open centrifugal compressors It is well documented that shroud bleed can help extend range by 30 50 18 This large improvement in operating range may sacrifice efficiency if not carefully matched to the stage Some authors have shown that implementing vanes in the bleed slot can lessen this efficiency decrement by controlling the amount of swirl 19 Boundary layer control is used extensively across various aerodynamic applications The implementation in compressors requires increased manufacturing and system complexity Several studies have shown potential range improvements through injection at the shroud 20 and hub endwalls 21 but there is little work documenting the effects of this type of control on the blade or vane boundary layers 4Copyright 2017 ASME INTEGRATION OF RANGE EXTENSION CONCEPTS The feasibility and impact of all of the concepts presented above were evaluated Based on the preliminary compressor model analysis each concept was ranked based on expected aerodynamic performance and mechanical feasibility with 0 as the lowest to 10 for the highest as shown in Table 2 Several concepts were eliminated from consideration without a detailed analytical evaluation because of known mechanical challenges or minimal expected range improvements based on information reported in the open literature These include variable diffusers which are expected to produce excellent range but will be difficult to incorporate mechanically Based on a review of published work casing grooves and flow injection are expected to give minimal range extension compared to a shroud bleed and were eliminated from consideration Finally the thick bladed impeller concept was eliminated since it did not offer any range enhancement for the current stage