Numerical Investigations On Unsteady Flow Of A Scroll Expander For Organic Rankine Cycle

As a key component of micro and small scale waste heat recovery systems, scroll expander has gradually become a hot topic in the research field of waste heat ultilizations, because of its smooth operation, low noise, high efficiency, compact structure and good relability, etc.. Aiming a scroll expander applied in an organic Rankine cycle(ORC) system for vehicle engine waste heat recovery, a study has been conducted to investigate its internal unsteady flow and the factors affecting its performance via three dimensional computational fluid dynamic(CFD) numerical simulations. The main research contents and findings are summarized as follow:The suitable simulation approach for the scroll expander, in regard to meshing and control method and moving boundary control method, has been discussed. A meshing method that the deforming computational domain could be divided by wedge grid stretched from trianlge combing with local grid encryption, was proposed. Dynamic mesh smoothing and local remeshing were adopted for updating the deforming domain grid to ensure the stability and convergence of the solutions. By analyzing the sensitivities of the mass flow rate, torque, and efficiency to the mesh density, the grid independent verification of the numerical model was completed. The validity of the numerical model was verified according to the mass flow rate, the expansion ratio and the output power.Effects of the changes of the suction port location and the scroll top profiles on the scroll expander unsteady performance were analyzed. Both the suction and discharging flow pulsations and the flow behaviors in the suction chamber and the expansion chamber, closely aossciated with the dynamic blocking effect of the orbiting scroll tip on the suction port, were studied. And comparisons of the flow field among different suction structures were also carried out. The results show that the adjustment of the suction port location or scroll top profiles to reduce the blocking degree of the orbiting scroll tip could not only effectivly improve the expander performance but also weaken the pulsations of the suction/discharging flow and the gas forces. As the scroll tip blocking degree reduces, the intensity of the vortex in the plenum chamber becomes weaker and the reverse pass rate of the flow pulsation at the suction port towards the expander inlet decreases. The non-uniform flow field of the suction chamber changes with the crankshaft angle because of the suction port jet flow, the suction swirling flow and the wall constraint. Compared with double circular arc and straight line modification, single circular arc modification could result in a longer duration of the sution chamber asymmetric flow field and a lower pressure distortion degree in the suction chamber.Asymmetric pressure distributions between symmetric working chambers and the formation mechanisms were studied. A novel partial analysis approach on the scroll driving moment was proposed to analyze the relationship between the working chamber aerodynamic asymmetry and the driving moment. The results indicate that the suction chamber presents an asymmetric flow phenomena with the reduction of the flow path between two top scrolls, named aerodynamic separation, before geometrical seperation of the suction chamber, due to the suction swirling flow and the viscous effect of the boundary layer. Pseduo leakage flow through the suction port increases the pressure asymmetric degree of symmetric expansion chambers. Because of the orbiting scroll rotation and flow resistance in the backpressure chamber, flow pulsation between symmetric discharge chambers has a pulsating phase and the pulsation intensity decays gradually. The dominant factor affecting the driving moment pulsation is different in each stage of the suction process. In the early stage, the reduction of the driving moment is mainly caused by the pressure drop of the expansion chamber, the inside wall of which is the outer wall of the fixed scroll wrap. Asymmetric pressure distribution between symmetric chambers will lead to the driving moment pulsation. The driving moment increases firstly and then decreases with the change of the suction chamber pressure during the mid and late stages of the suction process. In the mid state, the curvature reduction of the wall profile results in the increase of the driving moment with a slow decrease of the suction chamber pressure. In the late stage, the decrease of the driving moment is attributed to the aerodynamic seperation of the suction chamber.A bilateral symmetric discharging structure layout was proposed. Effects of discharging structure adjustment on the driving moment and the unsteady flows in the discharge chamber and back pressure chamber were investigated. The results show that bilateral symmetry structure reduces the flow resistances in the discharge chamber and the back pressure chamber, weakens the internal vortex secondary flow losses, and makes the discharging flow more uniform. In addition, the total driving moment increases by 6.38%. Driving moments of the scroll segement of suction chamber and those between symmetric working chambers increase to varying degrees. Meanwhile, the driving moments of the scroll segements between asymmetric working chambers decrease differently.Unsteady flows of the working chambers which exist fluid over-expansion and the flank leakage flow were depicted. The effect of fluid over-expansion on the driving moment was analyzed. Before the discharging process, the expansion chamber pressure rises in advance due to the flank leakage and the volume change of the working chamber. The discharge chamber pressure gradually rebounded from downstream to upstream, closly associated with the working fluid backflow. A backflow vortex constrainting the discharging flow is formed at the aerodynamic discharge port due to the squeeze of the upstream discharging flow, the backflow shear and wall constraint. In the early stage of over-expansion some working fluid in expansion chamber still overcomes the adverse pressure gradient and the viscous force of the boundary layer, and flow into the discharge chamber. As the over-expansion degree increases, reverse leakage flow forms in the radial clearance at the downstream of expansion chamber. Meanwhile, the reverse flow speed increases and the reverse flow region expands, which is attributed to adverse pressure gradient, boundary layer viscous and wall movement. The movement of the orbiting scroll wall has an “absorption” effect on the low speed gas at the upstream boundary position of the reverse flow region where the gas is shifted down decreases, and has a “squeeze” effect on the low speed gas at the downstream boundary position of the reverse flow region where the gas is accelerated. Compared with under-expansion, over-expansion leads to significant drop of the driving moment. The increase of the driving moment pulsation is mainly caused by pressure variation in the expansion chamber. In addition, the increases of the discharge chamber pressure, pressure pulsation of the discharge chamber and flow resistance in the back pressure chamber also contribute to the driving moment pulsation.