Modeling and Validation of a Two-Phase Flow Valve for Expanders in Waste Heat Recovery

Abstract

Waste heat recovery is a promising method to reduce fuel consumption and CO2 emissions in heavy-duty vehicles. An organic Rankine cycle (ORC) is used to convert the thermal energy of the exhaust gases into useable energy to support the power train. A key component of the ORC is the expansion machine where the conversion of thermal into mechanical energy takes place. In the case of volumetric expansion machines such as axial piston expanders, lubrication oil is mixed in with the working fluid to reduce friction and increase the component durability. However, the presence of oil also affects both the efficiency and the fluid dynamical behavior inside the expander. To implement a one-dimensional simulation model that considers the oil influence, a continuous flow approach is selected. Particular attention is dedicated to the inlet and outlet valve modeling, as these have to account for two-phase flow and multicomponent fluid mixtures. A valve model is built up in the simulation environment dymola based on the homogeneous nonequilibrium (HNE) approach. A virtual one-cylinder test bench is set up to calibrate and validate the model. The simulation results show good correspondence with the measurement data.