A cooling system for high heat flux applications is examined using microchannel evaporators with water as the working fluid and boiling as the heat transfer mechanism. Experimental studies are performed using single channel microevaporators allowing for better control of the flow mechanics unlike other investigations where multiple, parallel, flow channels can result in a non-uniform distribution of the working fluid. High-speed flow visualizations are performed in conjunction with heat transfer and pressure drop measurements to support the quantitative experimental data. Flow patterns associated with a range of boundary conditions are characterized and then presented in the form of novel flow regime maps that intrinsically reflect the physical mechanisms controlling two-phase pressure distributions and heat transfer behavior. Given the complexity associated with modeling of boiling heat transfer and the lack of a universal model that provides accurate predictions across a broad spectrum of flow conditions, flow regime maps serve as a valuable modeling aid to assist in targeted modeling over specific flow regimes. This work represents a novel and original contribution to the understanding of boiling mechanisms for water in microchannels. The flow patterns in this work are found to be closely coupled with mass flux, heat flux, and channel size; where re-wetting and pressure oscillations play a crucial role, and are likely responsible for its development and evolution. Reversed flow, typically attributed to a non-uniform fluid distribution in multiple channel microevaporators by other researchers, is shown to be a result of the upstream expansion of confined bubbles. During flow boiling, the pressure drop in the microchannel increases with the heat flux for a constant flow rate due to the significant acceleration effects associated with smaller channels, unlike in single-phase flow where the pressure drop is constant. Water flow boiling in rectangular microchannels, although not extensively explored in the published literature, provides an extremely high cooling capacity, with dissipation rates approaching 440 W/cm², making this an ideal candidate for cooling of next generation electronic systems. Single-phase flow studies revealed that pressure and heat transfer coefficient macroscale models are transferable to microchannels with hydraulic diameters down to 200 µm, when the entrance effects and minor losses are properly considered. These studies include laminar developing flow conditions not commonly considered in the literature and fully developed flow. Since the applicability of macroscale theories to microchannels is often questioned, this investigation helps clarify this issue for microchannels within the range of experimental conditions explored in this work. Finally, new correlations for the hydrodynamic entrance length are proposed for rectangular microchannels and good agreement is found when compared with published experimental data over a wide range of Reynolds number. These correlations are more accurate, and original in the sense that they incorporate the effects of channel aspect ratio, and include creeping flow conditions which are currently unavailable for rectangular microchannels. This work represents a major advance in the development of new cooling systems for high heat flux applications requiring dissipation rates in excess of 100 W/cm².

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Subcooled pool boiling on nanotextured surfaces is explored in this study. The experiments are performed in an enclosed viewing chamber. Two silicon wafers are coated with Multiwalled Carbon Nanotubes (MWCNT), 9 microns (Type-A) and 25 microns (Type-B) in height. A third bare silicon wafer is used for control experiments. The test fluid is PF-5060, a fluoroinert with a boiling point of 56°C (Manufacturer: 3M Co.). The apparatus is of the constant heat flux type. Pool boiling experiments in nucleate and film boiling regimes are reported in this study. Experiments are carried out under low subcooling (5 °C and 10 °C) and high subcooling conditions (20°C to ~ 38°C). At approximately 38°C, a non-departing bubble configuration is obtained on a bare silicon wafer. Increase in subcooling is found to enhance the critical heat flux (CHF) and the CHF is found to shift towards higher wall superheats. Presence of MWCNT on the test surface led to an enhancement in heat flux. Potential factors responsible for boiling heat transfer enhancement on heater surfaces coated with MWCNT are identified as follows: a. Enhanced surface area or nano - fin effect b. Higher thermal conductivity of MWCNT than the substrate c. Disruption of vapor-liquid vapor interface in film boiling, and of the “microlayer” region in nucleate boiling d. Enhanced transient heat transfer caused by local quasi-periodic transient liquid-solid contacts due to presence of the “hair like” protrusion of the MWCNT e. Enhancement in the size of cold spots f. Pinning of contact line, leading to enhanced surface area underneath the bubble leading to enhanced heat transfer Presence of MWCNT is found to enhance the phase change heat transfer by approximately 400% in nucleate boiling for conditions of low subcooling. The heat transfer enhancement is found to be independent of the height of MWCNT in nucleate boiling regime in the low subcooling cases. About 75%-120% enhancement in heat transfer is observed for surfaces coated with MWCNT under conditions of high subcooling in the nucleate boiling regime. Surfaces coated with Type-B MWCNT show a 75% enhancement in heat transfer in the film boiling regime under conditions of low subcooling.

Abstract Pool boiling process is frequently encountered in a number of engineering applications. However, it is difficult to exactly predict the heat transfer coefficient. This is because the boiling phenomenon is rather complex and influenced by many factors, such as surface condition, heater size, geometry, material, arrangement of heated rods, and refrigerants, etc. The key boiling parameters (bubble dynamics data) such as bubble departure diameter, frequency and nucleation site density will be varied in such different heated surface resulting in the different effect of heat transfer. The present study is ain at providing the pool boiling data for plasma coating and helical wire wrapped enhanced tubes. Furthermore, more fundamental of the physical phenomenon can be obtained. This study was performed experimentally. R-134a and R-600a were used as refrigerants. The surface condition will be changed with plasma spray coating and helical wire wrapped. It is expected that the surface condition can affect the nucleate boiling heat transfer in certain degree. In addition, boiling visualization was also made to broaden our basic understanding of the bubble diameter and dynamics while growing. Thermal design data of a flooded type evaporator of high performance as well as more and further physical insight of the above-stated nucleate boiling heat transfer can be acquired. The results will hopefully be helpful not only for the academia but for the industry.

Abstract Pool boiling process is frequently encountered in a number of engineering applications. However, it is difficult to exactly predict the heat transfer coefficient. This is because the nucleate pool boiling phenomenon is rather complex and influenced by many factors, such as surface roughness, areas of heater, material, geometry, arrangement of heated rods, and refrigerants, etc. The key boiling parameters (bubble dynamics data) such as bubble departure frequency, diameter and active nucleation site density will be varied in such different heated surface resulting in the different effect of heat transfer. This study was performed experimentally. R-134a was used as refrigerants, and the present study is aim at providing the pool boiling data for smooth and plasma coating enhanced tube bundles. It is expected that the surface condition, amount of test tubes, geometric of bundles and different heat flux can affect the nucleate boiling heat transfer in certain degree. In addition were calculated and developed that heat transfer coefficients and relevant corrections. Furthermore, more fundamental of the physical phenomenon can be obtained. According to the results of experiments, Boiling curves and calculations of the bundle factors and geometry factors were subsequently secured. The enhanced heat transfer coefficients with coated tube bundles are 1.1-2.0 times higher than smooth tube bundles. The 1.5-2.3 and 1.1-3.8 bundle factors obtained from the smooth tube bundles and coated tube bundles respectively. The geometry factors were about 1 for all arrangements studied herein. Thermal design data of a flooded type evaporator of high performance as well as more and further physical insight of the above-stated nucleate boiling heat transfer can be acquired. The results will hopefully be helpful not only for the academia but for the industry.

Abstract Pool boiling process is frequently encountered in a number of engineering applications. It is difficult to exactly predict the heat transfer coefficient. This is because the boiling phenomenon is rather complex and influenced by many factors, such as surface condition, heater size, geometry, material, arrangement of heated rods, and refrigerants, etc. The key boiling parameters (bubble dynamics data) such as bubble departure diameter, frequency, velocity and nucleation site density will be varied in such different heated rod pitches resulting in the different effect of heat transfer. Furthermore, more fundamental of the physical phenomenon can be obtained. Pool boiling heat transfer of R-134a is investigated experimentally on twin tube arrangement. The tube pitch is 1.65 and 2.5. The surface condition was prepared with plasma spray coating. In addition, using the high-speed digital camera and LDV, the bubble diameter and dynamics of R-134a were measured while growing. The boiling curves in different twin-tube pitches were drawn and the influence of bubble velocity on heat transfer coefficients was also examined. Finally, to broaden our basic understanding of different arrangement of heated rods and heat transfer mechanisms, thermal design data of a flooded type evaporator of high performance as well as more and further physical insight of the above-stated nucleate boiling heat transfer can be acquired. The results would hopefully be helpful not only for the academia but also for the industry.

Thesis--Auburn University, 2009. / Abstract. Includes bibliographic references (p.145-152).

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