Date of Award

2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

CFD simulation, Crossflow heat exchanger, Three-dimensional analysis, Transient heat transfer

Supervisor

Amir Fartaj

Rights

info:eu-repo/semantics/openAccess

Abstract

Heat exchangers are the key components in automotive, residential and industrial applications. These are employed in vehicle thermal management systems, thermal regulation process as well as heating, ventilation, air conditioning and refrigeration systems. It is important to assess their performance and behavior in transient situation, especially when a sudden change in their operating conditions takes place. This research advances steady state and transient heat transfer in minichannel heat exchanger (MICHX) with the aim of improving heat transfer and exploring transient response. This study uses three-dimensional (3D) computational models to resolve flow and heat transfer in air-to-liquid crossflow MICHX. A finite volume method based ANSYS FLUENT computational fluid dynamics (CFD) code is used to perform the numerical simulations. Models are verified and validated with both the steady state and the transient experimental data and results available in scientific literatures to represent the real world applications. Very good agreements in numerical predictions have been achieved for all models. The current research consists of five stages. In Stage I, the fundamental laminar heat transfer and flow features are focused and modeled in MICHX and conventional flat tube heat exchanger (FTHX) as a benchmark. For a given Reynolds number within the laminar flow regime, MICHX displays significant enhancement of heat transfer coefficient than that of FTHX. In Stage II, computations of 3D flow and heat transfer are carried out in a 5-pass 3-loop air-to-liquid crossflow MICHX, which is equivalent in size of an automotive radiator. The distributions of fluid flow, liquid-side temperature drop, and heat transfer rate are predicted fairly uniform in each loop of the MICHX. The numerical prediction suggests that a single-loop can be applied to represent the multi-loop for further investigations on similar crossflow MICHX. In Stage III, Aluminum oxide (Al_2 O_3) nanoparticles are incorporated with base fluids to improve heat transfer. Significantly improved thermal performance is predicted for inclusion of the Al_2 O_3 nanoparticles. In Stage IV, two distinct heat exchanger modules, sequential and simultaneous, are modeled to improve their thermal performance. For an identical module size, such as similar frontal area and volume, the simultaneous module provides superior thermal performance compared to the conventional sequential module as a benchmark. Finally, in Stage V, transient behaviour of crossflow MICHX for perturbations in hot fluid inlet temperature and mass flow rate are investigated. Faster response time has been observed for higher step variations. After considering the step variations and detailed regression analyses with important variables and parameters, new correlations for transient temperature and Nusselt number have been developed in the current study. The simultaneous module of MICHX can significantly reduce the energy requirements in automotive applications. The correlations for transient heat transfer can be useful tools for the engineers to design control devices in transient situations.

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