Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Jamil Khan

Abstract

Spray cooling has gained increasing interest over the last three decades due to its high thermal performance as a direct cooling technique. Researchers are investigating spray cooling in thermal management systems for electronic applications, particularly focusing on the use of dielectric fluids including FC-72, FC-87, ammonia, and refrigerants such as R113 and R134a. Their studies indicate that the usage of dielectric fluids as a working fluid in spray cooling systems is promising and has significant thermal performance enhancement, which is suitable for a wide range of electronic applications. However, several deficiencies have been identified with the working fluids studied thus far, such as flammability, toxicity, corrosivity, and negative impact on the environment. In this dissertation, the performance of spray cooling combined with a modified refrigeration system was experimentally investigated. Two objectives were explored in detail. The first was to design, build, and test an experimental facility that can be used to study a spray cooling system working with refrigerants. The second objective was to investigate the performance of the new low global warming potential (GWP) refrigerants R450a and R513a in spray cooling as an alternative to the refrigerant R134a, which was selected as the baseline for the performance comparison. A spray cooling system, combined with a modified refrigeration system, was designed and built to operate as a prototype for electronic cooling applications. The system was designed to perform multi-tasks, including the operation as an independent spray cooling system, an independent refrigeration system, or an integrated spray cooling refrigeration system. Furthermore, the performance of each of the refrigerants R134a, R450a, and R513a during spray cooling was experimentally evaluated by investigating the influence of various operating conditions. Under the same operating pressure and volumetric flow rate, R450a demonstrated enhanced performance compared to that of R134a and R513a. The overall cooling performance of R450a was higher than that of R134a by ~20%, 25%, and 23% at the chamber pressure of 0.5 MPa and nozzle inlet pressures of 0.7, 0.8, and 0.9 MPa, respectively. On the other hand, under the same operating pressures, the overall cooling performance of R513a was lower than that of R134a by approximately 27%, 25%, and 14%. The maximum heat transfer coefficient (HTC) obtained for R134a, R450a, and R513a were 37.51, 58.94, and 35.95 kW·m-2.K-1, respectively. Furthermore, the critical heat flux (CHF) of R134a, R450a, and R513a were 148.11, 154.83, and 131.88 W.cm-2, respectively. The CHF of R450a was approximately 17% more than that of R134a at the lower volumetric flow rate (2.88 × 10-6 m3·s-1), and 4.3% more at the highest volumetric flow rate (3.68 × 10-6 m3·s-1). Meanwhile, the CHF of R513a was lower than that of R134a by approximately 22% and by11% under the same two flow rate conditions, respectively. A slight variation was seen in the performance of the three working fluids based on the saturation temperature in terms of CHF and HTC. Finally, the thermal performance of the new combined spray-refrigeration system was evaluated in terms of the coefficient of performance (COP) while using R450a and R513a as working fluids. The results indicated a slight enhancement of the COP by 17.8% for R450a and 16% for R513a. In conclusion, R450a can be considered a reasonably good substitute for R134a in spray cooling and refrigeration systems. Although R513A had a lower heat transfer performance than that of R134a, it may still be used in spray cooling applications.

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