CFD Simulation Of CO₂ Power Cycle Rolling Rotor Expander For Waste Heat Recovery Of Engines

Facing with the current increasingly serious energy and environmental issues,the energy saving and emission reduction for internal combustion engine are of great significance,and the engine waste heat recovery is an important way to achieve it.CO₂ power cycle has been regarded as one of the promising technologies in the field of engine waste heat recovery with better adaptability.The expander is the key component of the CO₂ power cycle system for the waste heat recovery of the internal combustion engine.Rolling rotor expander has the following advantages:compact volume,light weight,few moving parts,high stability and long service life,which is very suitable for exhaust heat recovery system of internal combustion engine tail gas.But the current of CO₂ rolling rotor expander little research,few studies through experiments,and the lack of the CO₂ rolling rotor expansion machine on the expansion mechanism,the paper designed the CO₂ rolling piston expander,CFD simulation of the internal flow field,through the analysis of the distribution of pressure field,velocity field and turbulent kinetic energy.Exploration of CO₂ rolling rotor expansion process,and carries on the optimization of structure parameters.It will provide guidance for the design and optimization of the rolling rotor expander in the future.In this study,FLUENT software was used to simulate the internal flow field of the rolling rotor expander,and the internal pressure field,velocity vector distribution and turbulent kinetic energy distribution of supercritical CO₂ rolling expander were obtained.The cause of local vortex in expansion process was analyzed.Under different inlet pressure in the expander?9MPa,10MPa,11MPa?,to observe the changes of the internal pressure and the distribution curve of P-V,the average power is about 1716W,2215W,3043W,found that the inlet pressure range increased,work capacity increased.However,if the inlet pressure keeps increasing and exceeds a reasonable limit,the local vortex in the suction process is enhanced and the pressure loss is increased and the degree of the turbulence is strengthened,which causes a certain impact on the expansion and expander's service life.The initial expansion angle is adjusted and the other conditions are constant,and the initial expansion angles of 217,221,225,229 and 233 degrees are calculated.The results show that when the initial expansion angle is 217 degrees under different working conditions,the internal flow field distribution of the expander is relatively small,and the stability of the expander is better.During the expansion process,comparing 225 degrees and 229 degrees,the pressure turbulence of 217 degrees is small,especially in the vicinity of the gap between the rolling rotor and the cylinder.Under the initial expansion angle of 217 degrees,the other calculation conditions remain unchanged,the average power is about 1916W,2418W and 3233W under different inlet pressures.Compared with the initial expansion angle of the design condition,the working capacity of the expander is increased.The rotational speed of the rolling rotor expander is changed,and 1300r/min,1500r/min and 1700r/min are simulated respectively.It is found that the reduction of a small speed range can make the rolling rotor expander work more smoothly.The results of turbulent kinetic energy analysis are echoed with the results of pressure analysis.