Flow Visualization And Thermodynamic Analysis Of The Heat Transfer Process In Micro Heat Sinks With Complex Structure

A lot of problems of heat dissipation exist in the heat transfer area, which seriously restrict the development of the energy power, aerospace, biotechnology and chemical engineering, nuclear energy and microelectronic technology. For example, heat dissipation of micro devices has the feature of high heat flux and small heat transfer area. With the rapid development of large scale integrated circuit, the cooling ability of microelectronic chip per unit area rises, even as high as 107W/m2. In order to keep the constant temperature of electronic devices, excess heat must be removed timely. With the development of micro processing technology, micro heat sinks become one of the effective methods to solve the problem of high flux, which contain the merits of high surface-to-volume ratio, compact structure and efficient cooling efficiency etc.In this paper, to solve the cooling problem of microelectronic devices with high heat flux, combining working fluid and structure, a novel micro heat sink with complex structure was designed and investigated by experimental, numerical and theoretical methods. Furthermore, thermodynamic model in the heat transfer process and mathematical model of structure optimization of micro heat sinks with complex structure were also established, which provide the theoretical basis of thermal enhancement of micro heat sinks. The main contents include as following:Firstly, in order to solve the weak heat transfer under small Reynolds number in the microchannel with fan-shaped cavities, a more complex structure of microchannel with cavity and rib was designed by adding internal rib into two cavities. Moreover, the comprehensive performance of heat transfer in microchannels with different shapes of ribs and cavities was also numerically investigated. The results show that due to the sudden enlargement area and reduced velocity, the vortices are form in the cavity zones, which help to mix the cold and hot fluid quickly. However, due to the sudden shrinkage area and increased velocity, the fluid can maintain high kinetic energy to slip the cavity zones, thus preventing form the stagnation zone. In a word, microchannel with combined ribs and cavities can obviously enhance the internal disturbance. The microchannel with trapezoidal ribs and triangular cavities presents the best comprehensive performance under small Reynolds number, while the microchannel with triangular ribs and triangular cavities shows the best performance under high Reynolds number.Secondly, according to the first and second thermodynamics, an entropy generation model of flow and heat transfer in the microchannel with complex structure was established. The essence of heat transfer enhancement was analyzed from the view points of thermodynamics and field synergy principle. The results show that with the decrease of net temperature gradient of fluid, the heat transfer performance can be improved. Moreover, the non-dimensional parameters, such as thermal enhancement factor, augment entropy generation number, field synergy number and transport efficiency of thermal energy, can use to evaluate the heat transfer performance in micro heat sinks. However, the emphasis of evaluation is different due to the different definitions.Thirdly, a more advanced system of flow visualization of Micro-Particle Image Velocimetry(Micro-PIV for short) is used to investigate the flow characteristic in the microchannel presented in the Chapter 2. The experimental results show that the centerline velocity varies periodically, where is peak in the constant cross section and valley in the reentrant cavity. Due to the existent of ribs, the fluid in the cavity zones has higher kinetic energy to be taken away under small Reynolds number. The vortex is easy to form in the cavity zones under high Reynolds number, which also helps to enhance heat transfer.Fourthly, Al2O3 nanofluids with various volume fractions were prepared by two-step methods, and its thermal conductivity and dynamic viscosity were measured experimentally. Then, the characteristic of Al2O3 nanofluids flowing through the micro heat sinks with fan-shaped cavity was investigated experimentally. The results show that with the increase of concentration of surfactant SDS, the thermal conductivity decreases. The effect of surfactant on nanofluids is that it can uniformly disperse nanoparticles into based fluid but it is bad for heat transfer. Therefore, the amount of surfactant must tradeoff the aspects of heat transfer and uniformity. The experimental results of heat transfer is not agreement with the numerical results, which indicate the single model is not validated for modeling the flow and heat transfer of nanofluids. Compared with the results for the pure water, it is found that the heat transfer performance of nanofluids is better. With the increase of Reynolds number and volume fraction, both the heat transfer performance and pressure drop increase. According to plot of performance evaluation plot, nanofluids play an important role in the heat transfer enhancement.Fifth, a novel double-layered micro heat sink with complex structure was designed and the model of entropy generation was also established. Then, the overall packaging was designed and the effect of flow manners on heat transfer performance was analyzed. The results show it is unfavorable to use double-layered micro heat sinks to cool under small volume flow, due to the larger irreversible loss. On the contrary, it is better to use its counter flow to cool because of the more uniformity of temperature distribution. It can eradicate the thermal stress effectively. Moreover, the effect of entropy generation of the bottom layer on the total entropy generation is domain. The effect of different flow manners on overall heat transfer performance is obvious.Sixth, according to the model of convective heat transfer of single fluid, a micro heat sink was designed under given size and heat input. Moreover, mathematical model of geometry optimization was established in the base of the multi-objective genetic optimization algorithm. Thus, the geometry of microchannels with good heat transfer performance was determined with the optimization goal of the minimum of thermal resistance and pump power. For a given heat input 100 W and cooling area 10mm×10mm, a micro heat sink with the cooling requirement was designed firstly, then the comprehensive heat transfer performance was evaluated used performance evaluation plot. The results show that the microchannel with medium αc shows the best heat transfer performance. According to the minimum goals of thermal resistance and pumping power, a series of optimal values of the thermal resistance are obtained under the different pumping power.