Date of Award

2-15-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Experimental, Heat exchanger, Heat transfer, Minichannel heat exchanger, Transient

Supervisor

A. Fartaj

Rights

info:eu-repo/semantics/openAccess

Abstract

Heat exchangers are essential components of many systems and their use is extended to include various industrial, chemical, and automotive applications. A dynamic response study of a heat exchanger is essential for better representation of its design, selection, and analysis as it operates in conjunction with other process equipment. This study aims to experimentally investigate the transient performance of compact heat exchangers. A wide-range well prepared experimental setup is designed and assembled to examine the transient behavior of various types of cross-flow liquid to air heat exchangers. This set up is capable of stepping up or down the temperatures and flow rates for both hot and cold fluids covering a broad range of perturbations that occur in transient scenarios. A step function is a close approximation of a transient variation in heat exchangers, therefore, this work examines the transient response of both fluids under hot liquid side step changes in mass flow from 0.5 to 2.5, and inlet temperature from 1.5 to 3.5 while air inlet conditions are kept constant. Results are presented in terms of transient dimensional and non-dimensional outlet responses of fluids temperatures, heat transfer rates, heat balance, normalized outlet temperatures, and effectiveness. The current experimental findings provide an overview of the characteristic behavior of specific parameters such as response time, initial delay and time constant of both fluids. A comparison of the results obtained is made with the limited experimental work found in the literature. It is observed that the response time is faster for both fluids with the increase of the mass and temperature perturbations and it is higher for the hot liquid than the cold air side. An adverse trend is found between the hot and cold fluids’ effectiveness in temperature step changes, while the same trend is found in mass flow steps. Mass flow step changes for positive and negative steps exhibit an asymmetric trend of both fluids. This work also examines the transient effect of using an engineered fluid, such as Al2O3/water nanofluid, in heat exchangers due to their improved thermophysical properties. The nanofluid is analyzed in terms of its particle size distribution, chemical characterization, and agglomeration of suspensions using the Transmission electron microscopy (TEM), Energy- Dispersive Spectrometry (EDS) and Dynamic light scattering (DLS). Experimental results show that the response time of the nanofluid for the temperature and mass flow steps are faster compared to water. An increase of up to 19% in heat transfer rate is observed when using nanofluid. A comparison of the dynamic performance of a minichannel heat exchanger and a conventional radiator using step variations in inlet liquid temperature and mass flow rate is investigated. Results show longer response time for the conventional radiator compared to the minichannel heat exchanger. In addition, the analytical model for transient heat exchanger response is assessed with the non-dimensional outlet temperature response of a traditional tube and fin heat exchanger subjected to mass flow step changes. A more than 20% overestimate prediction is found for the transient temperature responses using the analytical model. The Empirical correlations developed for temperature and mass flow rate steps are found in a good agreement with the experimental data. The conclusion reached in this study provides an insight on the transient behavior of conventional and minichannel heat exchangers under liquid mass flow and temperatures steps. This experimental work is used to further establish and enrich a database for future advances on the dynamic response of heat exchangers subjected to step changes in liquid inlet conditions.

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