Research On The Mechanism Of Cavitation Enhanced Heat Transfer In Micro-channels

With the rapid development of integrated microelectronic technology and laser technology, the integration level of laser diode arrays and electronic chips highly increases, and the power also continuously rises. Timely elimination of the waste heat in the limited space has become a major bottleneck in the development of high-power integrated circuit technology and laser technology. With the rising heat flux in integrated microelectronic devices, the traditional micro-channel cooling technology faces enormous challenges. The research of related heat transfer enhancement methods is important to improve the cooling technology, and has important scientific significance and practical value.Based on the above background, hydrodynamic cavitation enhanced heat transfer in the micro-channel heat sink has been investigated. In the present work, by combining the numerical method and visualized experiments, studies are conducted to reveal the bubble dynamics and its effects on the heat transfer. Also, the effects of flow pattern and several cavitation parameters on the heat transfer enhancement are studied. The results lay a solid foundation of scientific theory and provide important technical support for its practical application.In this paper, the cavitation bubble dynamics near a heated wall is firstly studied based on the VOF model and the Navier-Stokes equations. The change of bubble profiles happened within microseconds and has been successfully captured. The corresponding velocity fields and temperature fields were also obtained. Through an in-depth analysis about the intrinsic relationship among the bubble profiles, liquid velocity fields and temperature layer, the impinging of micro-jet in the bubble collapse is found to be the main reason for the heat transfer enhancement. On this basis, effects of non-dimensional distance between the bubble center and the wall, initial bubble radius and bubble pressure on bubble behavior and heat transfer are analyzed.Further, the bubble dynamics between parallel heated walls and its effects on heat transfer are studied. The results show that due to the limitation of the two walls the bubble behaviors show a totally different characteristic. The bubble between the two parallel walls will experience two collapse processes. In the first collapse, the bubble shrinks mainly occurred in the center direction of the double walls. Therefore, a "dumbbell" shaped bubble will be formed, and finally the bubble is split into two sub-bubbles. In the second collapse, the sub-bubbles continuously shrink and are finally penetrated by micro-jets. Because of the differences of dynamic behavior, effects of the bubble behaviors on the heat transfer are totally different. Thus, parameters of the bubble initial radius, initial pressure and initial position on the cavitation bubble motion and heat transfer are analyzed and discussed in details.On the basis of the two-dimensional simulation, a three-dimensional dynamic model of the cavitation bubble is established for the study of bubble dynamics in the micro-channel and its impact on the heat transfer. The results show that the cavitation bubble growth and collapse behaviors is different from its process near the single wall and between the two parallel walls. Due to the limitation of channel walls, the cavitation bubble grows primarily towards the channel exit direction and forms an ellipsoid. In the collapse process, cavitation bubble firstly shrinks in the opposite direction as the growth process. When the bubble contracts to a flat shape, it shrinks from the bottom and top channel wall surfaces to the center and forms an "olive" shape. Due to the thermal resistance induced by the bubble itself, in the bubble faced region heat transfer conditions are deteriorated. However, in the region of vortex liquid jet flow, heat transfer is strengthened. On this basis, the bubble initial radius, the initial bubble position and other factors on cavitation bubble behaviors and its effects on the heat transfer are further investigated.The AutoCAD and Inventor are employed for the design of visualized experiment platform. The cooper-based micro-channels contain two kinds of flow restricted structures, a rectangular shaped and a two diverging shaped respectively. The flow characteristics and heat transfer performance under the two restricted structures are studied. The effects of Reynolds number, cavitation number and heat flux on the flow patterns and heat transfer are investigated, which lay the theoretical foundation for optimization designs. On the basis of experimental studies, the paper further explores the application of cavitation model in the simulation of micro-channel cavitating flow with surface heat transfer. The flow pattern obtained by the calculation is in accordance with the experimental results. It indicates that the present model is generally applicable for the cavitation flow in micro-channels. However, subject to the imperfection of the model, heat transfer simulation results are not satisfactory. The improvement advices has been proposed based on the further analysis.