Date of Award

2022

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Dynamic performance, Empirical correlation, Latent heat thermal energy storage, Meso crossflow heat exchanger, PCM heat exchanger, Transient heat transfer

Supervisor

A.Fartaj

Supervisor

X.Nie

Rights

info:eu-repo/semantics/openAccess

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

The increase in energy consumption and the demand for effective thermal management systems necessitate the search for enhanced heat exchangers with high thermal performance, lower weight, and compact sizes. Crossflow heat exchangers are a key component in the thermal management system of many industries such as HVAC, automotive, etc. Generally, heat exchangers are characterized based on their steady state operating conditions. However, a transient analysis helps predict the behavior of heat exchangers when they experience variations in their operational conditions in terms of fluid flow rate and temperature.

Thermal management and thermal comfort can be achieved through active and passive cooling or heating. The transient analysis of active air heating and cooling provides an insight on the response of the heat exchanger to the imposed variations and the time duration in reaching final steady state conditions. On the other hand, passive air cooling represented by a latent heat thermal energy storage offers a solution to provide thermal comfort to occupants for short periods of time. The increasing interest in the thermal energy storage is due to the rising demands for energy, increasing cost, and environmental and economic concerns which need an efficient utilization of existing energy sources and a reduction in energy consumption. A phase change material PCM is introduced as a latent heat thermal energy storage offering a clean compact thermal storage technology that provides extended thermal comfort with possible environmental and energy savings.

The dynamic response of a cross flow heat exchanger subjected to airside mass velocity changes is studied. The transient air heating and cooling performance of the heat exchanger due to step changes in airside mass velocity ratios is examined using dimensional and dimensionless parameters. Results are shown in terms of fluids heat transfer rates, dimensionless outlet temperatures, heat transfer coefficient, Colburn j-factor, and Nusselt number. A generalized empirical correlation to predict the airside Nusselt number as a function of air mass velocity ratio was derived for each of the cases. The results demonstrate that both fluids response is not instantaneous to the step imposed and the magnitude of the step has a dissimilar effect on each fluid’s response time. The increase in the step change results in higher fluids response time.

Passive air cooling is studied due to the implementation of the start-stop function in many vehicles for fuel savings and the increasing demand for electric and hybrid vehicles which require a solution to provide extended passenger thermal comfort. An experimental investigation of an innovative PCM-air-liquid meso heat exchanger to extend the thermal comfort in a vehicle during short periods of engine shutdown is presented. Extended surfaces (fins) are placed inside the PCM medium and integrated throughout the whole heat exchanger to enable and improve the transfer of thermal energy between the PCM and the air. The effect of air mass flow rate variation on the PCM discharging time is discussed. A comparison of heat transfer when using PCM to no PCM case revealed a 120 kJ released from the PCM during the discharging process. Furthermore, doubling the air mass flow rate resulted in a 29% decrease in the cumulative energy transferred to the air. The current experimental results indicate that the use of PCM extends the airside cooling time by more than 4 minutes, thus provides additional thermal comfort. The presented dynamic results along with the PCM efficient thermal energy storing and releasing processes offer a productive thermal management, energy, and environmental savings.

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