Thermo-Structural Analysis of a Roots Expander

Abstract

Roots expander is a waste heat recovery device that utilizes the pressure energy from the engine exhaust to produce useful work. It is mounted downstream of the engine and consists of a housing with a pair of twisted rotors that are separated from each other by fine clearances. As the expander is exposed to high temperature exhaust gases from the engine, there will be deformations primarily caused by thermal expansion. There can be variable deformations of the parts (owing to different materials) resulting in change in clearances that can affect the performance of this device. Thus, it is important to understand the thermo-structural behavior of expander, which is the motivation behind the present work. This paper will detail the methodology that was used to model the thermo-structural behavior of expander using CFD & FE analysis. A co-simulation method was adopted to couple transient CFD analysis with steady state FE thermal simulations to get the temperature distribution on the device. The obtained temperature results are then used in steady state FE structural analysis to determine the deformations. A macro & Microsoft Excel based clearance calculator was developed to determine the clearance change based on the deformation results. Based on the change in clearances, necessary modifications can be made to the design of expander to achieve optimum efficiency.