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The closed Brayton cycle operating with supercritical carbon dioxide is an attractive option for geothermal, nuclear and solar energy conversion. As one of the key component, the performance of the supercritical carbon dioxide compressor attracts lots of attention. Up to now, most studies focused on the centrifugal compressor and the design method of the axial compressor operating with supercritical carbon dioxide is far to matured. The effects of the tip clearance size on the tip leakage flow in an axial compressor are numerically investigated in the present work, with different tip clearance size and operating points. To fully take advantages of the matured design method of the air compressor, a similar air compressor is simulated as a comparison group. The results show that the flow characteristics especially the static pressure coefficient and tip leakage vortex structure is almost the same between S-CO2 compressor and the similar air compressor with small tip clearance size. This is also suitable for the large tip clearance size at low and middle span, while obvious discrepancy arises at high span. The tip leakage vortex of S-CO2 compressor is stronger and more downstream than the air compressor. Understanding these effects is helpful to extend the abundant experience of the air compressor to design S-CO2 compressor.

The compressor is one of the key components in the closed supercritical carbon dioxide (S-CO2) Brayton cycle, but its design method is far from mature. It is naturally expected that the well-established design method of the air compressor can provide favorable guidelines, on the basis of further understanding the effects of the physical property on the compressor flow field. Considering that isentropic compression is one of the core physical processes in the compressor, the physical property effects on this process were mainly investigated in this work. Similarity criterion was considered, and the change rate discrepancy of the main variables in this process between S-CO2 and the ideal air was fully analyzed. Results show that S-CO2 is compressed faster than the ideal air in most cases, along with generating smaller Mach number and larger pressure rise ratio. It is noted the important parameter of the static pressure coefficient distribution with S-CO2 in the compression process is almost the same as that with the ideal air at low Mach number, which is conductive to the extension of the air compressor research experience, but it is quite different at high Mach number. The simulation cases about compressor cascade are further applied and prove the suitability of the revealed physical property effects in the compressor passage. Understanding these effects on the compression process is helpful to improve the design method of the S-CO2 compressor.

There always appear unsteady characteristics during start-up periods of pumps, which can lead to instability of the entire system. However, the lack of a method for quickly and accurately predicting pump start-up performance makes it difficult to analyze the performance of the overall system during the period. To this end, a theoretical model to predict pump transient performance under fast start-up conditions is established in the present study. The theoretical prediction model for pump steady performance is built based on loss modeling first. Then, the balance between pump transient head and pipeline system is considered to determine transient pump performance. A time stepping algorithm is proposed to solve transient pump performance during start-up periods. The established theoretical model and the corresponding time stepping algorithm are applied to a mixed flow pump under fast start-up conditions with various acceleration time. The predicted evolution of pump head shows good agreement with experimental measurements, and average relative errors are within 10% for both steady and transient conditions. In addition, the theoretical model is applied to analyze the transient performance of oscillating curves and impact head. The mechanism for predicted oscillating results and the relation between peak impact head and the acceleration of rotation speed are revealed.

With the development of computational power, large eddy simulation (LES) method is increasingly used in simulating complex flow. However, there still exist many factors affecting the LES quality and appropriate mesh resolution is among one of them. This work aims to develop an automatic procedure to refine the LES mesh by combining adaptive mesh refinement (AMR) and LES quality criteria. An LES refinement criterion is developed by estimating the proper grid length scale which meets the accuracy requirement of LES method. With this criterion, the baseline mesh is automatically refined with the AMR method. In this work, an efficient one-shot refinement strategy is also proposed to reduce the overall simulation time. Current AMR-based LES method is verified with the typical LES test case about the flow past circular cylinder at Re D = 3900. Results show that the automatically refined mesh provides systematically better agreement with experimental results and with current method the balance between ac...

Summary Adaptive Mesh Refinement (AMR) shows attractive properties in automatically refining the flow region of interest, and with AMR better prediction can be obtained with much less labour work and cost compared to manually re-meshing or the global mesh refinement. Cartesian AMR is well established; however, AMR on hybrid unstructured mesh which is heavily employed in the high-Reynolds number flow simulation, is less matured and existing methods may result in degraded mesh quality, which mostly happens in the boundary layer or near the sharp geometric features. User intervention or additional constraints, such as freezing all boundary layer elements or refining the whole boundary layer, are required to assist the refinement process. In this work, a novel AMR strategy is developed to handle existing difficulties. In the new method, high-order unstructured elements are first generated based on the baseline mesh; then the refinement is conducted in the parametric space; at last the mesh suitable for the solver is output. Generating refined elements in the parametric space with high-order elements is the key of this method and this helps to guarantee both the accuracy and robustness. With the current method, three-dimensional hybrid unstructured mesh of huge size and complex geometry can be automatically refined, without user intervention nor additional constraints. With test cases including the two-dimensional airfoil and three-dimensional full aircraft, the current AMR method proves to be accurate, simple and robust. This article is protected by copyright. All rights reserved.

Shock wave and tip leakage are important flow features at small length scales. These flow phenomena and their interactions play important roles in the performance of modern transonic fans and compressors. In most numerical predictions of these features, mesh convergence studies are conducted using overall performance data as criteria. However, less effort is made in assessing the quality of the predicted small-scale features using a mesh that yields a fairly accurate overall performance. In this work, this problem is addressed using the adaptive mesh refinement (AMR) method, which automatically refines the local mesh and provides very high resolution for the small-scale flow feature, at much less cost compared with globally refining the mesh. An accurate and robust AMR system suitable for turbomachinery applications is developed in this work and the widely studied NASA Rotor-37 case is investigated using the current AMR method. The complex interactions between the shock wave and the boundary layer, as well as those between the shock wave and the tip vortex, are accurately captured by AMR with a very high local grid resolution, and the flow mechanisms are analyzed in detail. The baseline mesh, which is considered to be “acceptable” according to the commonly used mesh convergence study, is unable to capture the detailed interaction between the shock wave and the boundary layer. Moreover, it falsely predicts the tip leakage vortex breakdown, which is a consequence of inadequate resolution in the tip region. Current work highlights the importance of a careful check of the mesh convergence, if small-scale features are the primary concern. The AMR method developed in this work successfully captures the flow details in the transonic compressor in an automatic fashion, and has been verified to be efficient compared with the globally mesh refinement or manually mesh regeneration.

A bleeding system is necessary for gas turbines and may largely affect the compressor performance. In the existing literature the circumferentially uniform bleeding is widely studied; however, it is different from the industrial bleeding geometry and may not fully reflect the effects on the compressor. We conducted a numerical study to investigate the flow mechanism of the real industrial bleeding geometry in a single-stage high-speed axial compressor environment. Results show that bleeding effects increase the adiabatic efficiency and slightly reduce the total-to-static pressure ratio. Furthermore, it is discovered that a locally aperiodic flow field in the rotor passages is generated due to the real bleeding geometry. In the tip region the maximum incident angle difference can reach 1 degree. Locally aperiodic bleeding makes the tip leakage strength and blockage different in various rotor passages.

Using supercritical carbon dioxide (SCO2) as the working fluid of a closed Brayton cycle gas turbine is widely recognized nowadays, because of its compact layout and high efficiency for modest turbine inlet temperature. It is an attractive option for geothermal, nuclear and solar energy conversion. Compressor is one of the key components for the supercritical carbon dioxide Brayton cycle. With established or developing small power supercritical carbon dioxide test loop, centrifugal compressor with small mass flow rate is mainly investigated and manufactured in the literature; however, nuclear energy conversion contains more power, and axial compressor is preferred to provide SCO2 compression with larger mass flow rate which is less studied in the literature. The performance of the axial supercritical carbon dioxide compressor is investigated in the current work. An axial supercritical carbon dioxide compressor with mass flow rate of 1000kg/s is designed. The thermodynamic region of the carbon dioxide is slightly above the vapor-liquid critical point with inlet total temperature 310K and total pressure 9MPa. Numerical simulation is then conducted to assess this axial compressor with look-up table adopted to handle the nonlinear variation property of supercritical carbon dioxide near the critical point. The results show that the performance of the design point of the designed axial compressor matches the primary target. Small corner separation occurs near the hub, and the flow motion of the tip leakage fluid is similar with the well-studied air compressor. Violent property variation near the critical point creates troubles for convergence near the stall condition, and the stall mechanism predictions are more difficult for the axial supercritical carbon dioxide compressor.

The supercritical CO2-based power cycle is very promising for its potentially higher efficiency and compactness compared to steam-based power cycle. Turbine is the critical component in the supercritical CO2-based cycle which delivers the power. Compared to the gas turbine or steam turbine of similar power output, the size of the supercritical CO2 radial turbine is much smaller and the axial force on the impeller is much larger. The load on the thrust bearing could be too heavy for long-term safe operation. Therefore, it is necessary to balance the axial force on the impeller through aerodynamic design to reduce the load on the thrust bearing. The impeller backface design with radial pump-out vanes proves to be an effective design to reduce the axial force on the impeller of radial turbomachinery, which is widely used in the pump industry. This work investigates the impeller backface cavity flow of a supercritical CO2 radial turbine and the application of the pump-out vanes to the impeller through computational fluid dynamics simulations. Design variations of the pump-out vane are presented and their performance variations are discussed from the view of viscous compressible fluid, instead of the commonly assumed inviscid incompressible fluid in the pump industry.

There are local flow phenomena like shock wave/boundary interaction and tip leakage flow which strongly influence the compressor performance and stability. AMR (Adaptive Mesh Refinement) strategy shows attractive property for automatically refining local mesh and predicting higher local phenomenon details. This paper develops the AMR strategy for turbomachinery with unstructured mesh. Curved surface boundary matching is focused in AMR process for achieving high level simulation accuracy. The developed AMR strategy is used to improve shock wave prediction in this paper. Firstly two dimensional RANS simulation of compressor cascade L030-4 is conducted to test the AMR strategy. Refined mesh shows better shock wave details compared with the almost none shock wave structure of baseline mesh. Then quasi-3D DDES (Delayed Detached Eddy Simulation) simulation of this compressor cascade is conducted. Shock wave oscillation phenomenon is clearly shown for this cascade. Local mesh of shock wave oscillation region is automatically refined by AMR. The refined mesh predicts better shock wave details and better turbulence motion comparing to the baseline mesh.

This paper investigates the Runge-Kutta implicit scheme applied to the solution of the time spectral method for periodic unsteady flow simulation. Several explicit and implicit time integration schemes including the Runge-Kutta scheme, Block-Jacobi SSOR (symmetric successive over relaxation)scheme and Block-Jacobi Runge-Kutta/Implicit scheme are implemented into an in-house code and applied to the time marching solution of the time spectral method. The time integration is coupled with Full Approximation Storage (FAS) type multi-grid method for convergence acceleration. The in-house code is based on the finite volume method and solves the RANS (Reynolds Averaged Navier-Stokes) equations on multi-block structured mesh. For spatial discretization the 3rd/5th order WENO (weighted essentially nonoscillatory) upwind scheme is used for reconstruction and the convective flux is computed with Roe approximate Riemann solver. The widely used one-equation Spalart-Allmaras turbulence model is used in the simulations. The time integration schemes for the solution of the time spectral method are tested with two different compressor cascades with periodically oscillating inlet boundary conditions. The first case is a low speed compressor stator with inlet flow angle varying with time. The second case is a high speed compressor rotor with inlet boundary condition profile to simulation the influence of upstream wakes. The results show that for moderate frequencies and wave mode numbers, the Block-Jacobi Runge-Kutta/Implicit scheme shows favorable convergence behavior compared to the other schemes. However, for extremely high frequencies and wave mode numbers such as in the simulation of high rotating speed compressors, the advantage of the Block-Jacobi Runge-Kutta/Implicit scheme over the explicit Runge-Kutta scheme is totally lost.Copyright © 2014 by ASME