The Research On The Heat Transfer Enhancement Mechanism Of Micro Heat Exchanger Based On Viscoelastic Fluid

The micro-heat exchangers and micro heat sink are ultra-compact heat exchangers with small size and large heat transfer area in unit volume, they are widely used in the areas of microelectronics, aerospace, medical, chemical and biological engineering, materials science, the cooling of high-temperature super conductors, the thermal control of thin film deposition, the cooling of high power laser mirror, and some other occasion that have special requirements in size and weight of heat exchangers. Microchannel heat exchanger is one of the most common types of micro heat exchanger, the flow channel is generally processed in thin silicon wafers, metal or other material sheets. Studies have shown that, in macro conditions, as a heat transfer medium of heat exchanger, the viscoelastic fluid has significant heat transfer enhancement effect. But in the micro conditions, the flow and heat transfer characteristics of the viscoelastic fluid are still unclear. Combining the viscoelastic fluid with the microchannel heat transfer technology can create a kind of new high effective microchannel heat exchanger with better heat transfer function. Therefore, the flow and heat transfer processes of viscoelastic fluid flow and heat transfer process in microchannels are studied by numerical simulation. By the contrast with the effects of heat transfer of pure viscous water, the viscoelastic fluid heat transfer enhancement effect is studied. And based on the theory of rheology and the characteristics of the secondary flow, the mechanism of heat transfer enhancement of the viscoelastic fluid in micro channel is explained. In addition, in the condition of the viscoelastic fluid in the micro-rectangular channel with electric heating, the pulsating flow is added to the viscoelastic fluid to study the coupled heat transfer enhancement of pulsating flow and viscoelastic fluid by means of numerical simulation. Besides the impact laws and mechanism of the influence of the process parameters on the coupled heat transfer enhancement process are investigated. The main results are as following:(1) Based on a reasonable simplification, assumptions and boundary conditions, the full three-dimensional theoretical model of fluid flow and heat transfer in microchannel was established.(2)The study has shown that, the newly developed micro channel heat exchanger with highly effective heat enhancement function in this study has significant heat transfer enhancement compared with the ordinary micro channel heat exchanger, the maximum heat transfer enhancement effect can be more than400%. With the increase of the velocity of the heat transfer medium fluid, the heat transfer enhancement of the newly developed micro channel heat exchanger with highly effective heat enhancement function is more and more obvious, compared with the ordinary micro channel heat exchanger. Thus increasing the flow rate of the viscoelastic fluid can improve the heat transfer enhancement effect. The heat transfer enhancement effect of the viscoelastic fluid depends on the secondary flow intensity.(3) The study has shown that the volume flow weighted average exit temperature of the pulsating viscoelastic fluid in the micro channel are higher than the exit temperature of the steady state, so the pulsating flow can further enhance the heat transfer enhancement. And the heat transfer enhancement effect of pulsating flow on viscoelastic fluid depends on the frequency and amplitude of the inlet pulsating flow. The heat transfer effect increase with the frequency and amplitude. Besides, with the same flow resistance characteristics, heat transfer parameters, frequency and amplitude of pulsating flow, the heat transfer effect of the pulsating viscoelastic fluid is higher than the heat transfer effect of the pure viscous fluid. Besides, the viscoelastic fluid has better coupled heat transfer enhancement effect than viscous fluid, and the coupled heat transfer enhancement effect increase with the frequency and amplitude.