Modelling multiphase heat and mass transfer using a tuned single phase fluid approximation

Abstract

Modern aircraft engines are becoming more and more dependent on advanced heat exchanger technology to increase their efficiency and decrease their fuel consumption. This is best demonstrated by the SWITCH WET engine, which recovers waste heat from the exhaust to greatly improve the specific fuel consumption and thermal efficiency of the core engine cycle. The goal of this project is to create computational techniques for modelling the mass and heat transfer mechanisms found in heat exchangers. The important objectives of the thesis were to review the existing pressure drop and heat transfer correlations between water boiling and the creation of a precise tuned model that uses single-phase simulations to mimic water vaporization in a heated tube and validate the model with experimental data. This study provided insight into the tuned material model that was developed as well as how the tuned model affected the two-phase boiling simulation. It was discovered that the model had some errors in its wall temperature prediction. Consequently, the near wall effects of the model were thoroughly investigated, and the influence of thermal conductivity was identified as the source of the issue. On the other hand, the bulk quantities of the flow were accurately predicted by the model. These results contribute to the understanding and validation of the CFD simulations using the tuned model while also highlighting areas where further improvements are necessary. The study further revealed that the prediction of pressure drop along the domain was not trustworthy as the pressure drop was inclined towards a single phase pressure drop of the liquid phase. The error was mainly due to the lower velocity increase with respect to the density change while boiling. The imbalance between the density and velocity would be attributed to the lack of turbulent mixing between the phases which may be a reason for the inaccurate predictions of wall temperature.