Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Dr. Kamran Siddiqui

Abstract

The present thesis is based on an industry-sponsor project involving a novel waste heat-to-electricity conversion system. This proprietary system utilizes thermal energy from a low temperature heat sources to produce torque that drives an electric generator to produce electricity. The system needs to be studied through scientific research to help with optimizing the product development, component design, and overall system performance. The main objectives of this study are to develop simulation tools (numerical model) that will allow to simulate the thermo-fluid processes in various system components and to use this numerical model to study the heat transfer and phase-change processes along with the work interactions.

In the first part of this study, a novel numerical model was developed using the commercial CFD software Fluent. The novelty of this model was its capability to simultaneously simulate the phase-change and moving boundary processes. To the best of our knowledge, this is the first time such integrated model has been developed. The model coupled the Mixture model and the Dynamic Mesh model via developed user-defined functions. However, due to the limitations of the CFD software, the investigations were restricted to the fundamental heat transfer and phase-change processes in the lower vessel and the associated heat exchanger coil which undergoes the boiling process, along with the piston movement in the vessel.

The heat exchanger coil is one of the key components of the system where the primary phase change process takes place and its design has a direct impact on the overall system performance. Thus, the main focus on the second part of this study was on a detailed investigation of the phase-change process in the heat exchanger coil and its design improvement as well as the type of the working fluid. The investigation of the working fluid involved four volatile substances: methanol, ethanol, pentane and butane. Due to the low boiling point and low latent heat pentane was recommended as the best substance among the four. Its low boiling point also allows the system to extract heat at lower waste heat temperatures.

The study of the heat exchange and phase-change processes in the heat exchanger and the vessel involved a detailed investigation of these processes, which include the spatio-temporal variations of the flow patterns and the mixture quality. Various heat exchanger geometries and configurations were considered. A common feature observed in all these configurations was the presence of a cyclic process inside the heat exchanger tube where the low quality mixture (heavy fluid) near the bottom of the vessel enters from one end of the heat exchanger and the high quality mixture (light fluid) escapes in the form of jet into the vessel from the other end of the tube. It was found that the speed of the jet increased with an increase in the surface area of the heat exchanger coil. The impact of heat exchanger geometry and configuration on the flow patterns and the mixture quality as well as the piston movement is presented and discussed in detail. These results will be utilized by Dyverga to improve the heat exchanger design.

The novel model developed in this study will serve as a valuable tool for Dyverga to further study various thermo-fluid processes in all other system components and for the system optimization.


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