Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Jamil A. Khan

Abstract

Today, dissipating high heat flux safely is one of the greatest challenges for thermal engineers in thermal management systems, and it becomes a critical barrier to technological developments for many engineering applications. Due to technological advances and aggressive micro-miniaturization of electronic components, the surface area of most devices has shrunk while the computational power increased exponentially. Therefore, the amount of heat dissipated from surfaces has increased significantly. Numerous cooling techniques have been introduced to replace the traditional air cooling systems and to maintain the efficiency and reliability of electronic components. Microelectronics work efficiently and safely at surface temperatures of < 100 ℃ and 125 ℃ for general and defense applications, respectively. One of the proposed alternative schemes is spray cooling, which is considered one of the most advanced cooling methods. It is used for high and ultra-high heat flux dissipation, as it can dissipate 150-200 W/cm2 while maintaining the surface temperature within this range. Also, spray cooling removes a large amount of energy at a lower liquid flow rate compared to other cooling techniques, such as jet impingement and microchannel heat sink. The thermal performance of spray cooling systems can be enhanced either actively or passively. Active enhancement is a very efficient technique; however, it adds more pumping power. The present work focuses on three main objectives: evaluating and analyzing spray cooling performance, developing a three-dimensional numerical multi-phase model for heat transfer process in spray cooling and enhancing the thermal performance of spray cooling passively.

First, to evaluate and analyze the spray cooling performance of a flat surface, an experimental investigation was conducted in a closed-loop spray cooling system, utilizing deionized water as a cooling medium. A flat copper surface with a diameter of 15 mm was tested at volumetric flow rates that ranged 115 - 180 mL/min, a nozzle-to-surface distance of 8-12 mm, coolant inlet temperature, surface temperature, and chamber pressure, ~22℃, < 100℃ , and atmospheric pressure, respectively. However, the results showed that increasing volumetric flow rate enhances the thermal performance of the spray cooling system at all nozzle-to surface distances, but at the same time increases the required pumping power.

Therefore, a new criterion, “ Performance Evaluation Criterion of SprayCooling” (PECSC), was introduced to evaluate the spray cooling performance precisely, based on the combination of the amount of heat removed and the corresponding pumping power consumed. Using this criterion showed that increasing the nozzle differential pressure minimizes the overall spray cooling performance, and the maximum PECSC was 2022, which was achieved at a volumetric flow rate and nozzle-to-surface distance of 115 mL/min and 10 mm, respectively. Moreover, enhancing the thermal performance of a spray cooling system by increasing nozzle differential pressure is not an economical enhancement option.

Second, a three-dimensional multi-phase numerical model was developed to simulate the heat transfer process and understand the underlying physics in the spray cooling system. STAR-CCM+, 12.04.010-R8 was utilized as a computational fluid dynamics solver. A Lagrangian-Eulerian and Eulerian - Eulerian modeling approaches were adopted to simulate the fluid flow and heat transfer in spray cooling. The predicted and experimental heat transfer coefficients were compared at same operating conditions.

The comparison showed a satisfactory agreement; the maximum absolute deviation was < 15%. The results illustrated that spraying parameters, such as volumetric flow rate, nozzle-to-surface distance, and surface temperature have a significant effect on liquid film characteristics, such as spatial heat transfer coefficient, liquid film thickness, and liquid film velocity. Results showed that the heat transfer coefficient in the spray impingement zone is highly affected by the volumetric flow rate and nozzle-to-surface distance, compared to the film zone. Also, more insights are provided about the heat flow mechanisms that are involved on the target surface. The volumetric flow rate has a dominant effect on the spatial distribution of heat transfer coefficient, liquid film thickness, and surface temperature. Moreover, decreasing the distance between the nozzle and the target surface increases the heat transfer coefficient in the spray impingement zone meaningfully.

Third, inorder to focus on enhancing the spray cooling system passively, the target surface was modified geometrically to increase the surface contact area and change the flow pattern on the surface. Geometrical surface modification is one of the most stable and durable among other enhancement methods. Three surfaces were modified with circular and radial grooves and examined at different operating conditions. The first surface M1 was modified with four circular grooves, each having 0.5 mm width, 0.5 mm depth with 1.5 mm pitch, to increase the surface contact area and the turbulence on the surface. The data analysis of (M1) showed that it had good thermal performance at a high volumetric flow rate but had low thermal performance at a low nozzle differential pressure. At low volumetric flow rate, the water replacement rate is low, and some of the water stagnates in channels and consequently increases the thermal resistance and negatively affects the heat transfer process. In other words, the performance of this surface depends on the pumping power, because the water replacement rate increases with increasing the nozzle differential pressure. Therefore, the second surface (M2) was modified with four radial grooves having widths and heights of 0.5 mm, in addition to the circular grooves, to increase the water replacement rate. The results showed that M2 had better heat transfer performance than M1 due to the decrease in water thermal resistance and the activation of the radial momentum. This means radial flow has a significant effect on the spray cooling heat transfer performance. For further passive heat transfer enhancement, a third surface (M3) was modified with eight radial grooves in addition to the four circular grooves to take advantage of the flow in the radial direction, by increasing the wet surface area and accelarating the drainage rate. The results indicated that M3 had the highest heat transfer performance when compared to the other surfaces, at both low and high volumetric flow rates. The experimental results demonstrated that volumetric flow rate has a significant effect on the spray cooling thermal performance for all surfaces; thus, increasing the volumetric flow rate enhances the thermal performance of enhanced surfaces with different enhancement ratios. The effect of nozzle-to-surface distance depends mainly on both surface geometry, volumetric flow rate, and surface temperature. Furthermore, M3 has the highest heat transfer enhancement ratio at all operating conditions, followed by M2 and M1, where the maximum heat transfer enhancements were 80%, 36.3%, and 28.7%, respectively. Thus, using surfaces modified with a combination of circular and radial grooves can enhance spray cooling heat transfer performance significantly. Moreover, M3 has better thermal performance than a surface modified with only straight grooves by 34% at the same operating conditions.

Rights

© 2019, Azzam Saadi Salman

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