Investigation On Convective Heat Transfer Enhancement Of Microchannel With Dimples/Protrusions

Marine electric propulsion system has many advantages over the traditional propulsion system,so it has been widely used in many types of ships,and it will be the major warship propulsion system in the future.The increasing power of electric propulsion system makes more high-power electronic equipment should be installed on the ship with limited space.However the temperature has a very important influence on the reliability of electronic equipment,so it is very important for the marine electric propulsion system to solve the problem of removing the heat from electronic equipment with limited space.Micro-channel heat exchangers has been widely used and studied because of its remarkable ability to remove heat from micro-scale devices with high heat flux.The optimization of the microchannel structure and the use of a new type of heat transfer medium can greatly improve the heat transfer performance of micro-channel,which is commonly practiced.Heat transfer surface with Spherical dimple can enhance the heat transfer while reducing the flow loss,so it is a widely used heat transfer structure,and its application to the micro-channel is expected to enhance the convective heat transfer capacity with small pressure loss.Nanofluids can enhance the convective heat transfer of channels,because of its high thermal conductivity.However,due to its high viscosity,the pressure loss increases rapidly.So it is expected that the combination of the dimple and the nanofluids will enhance heat transfer with small pressure loss.For the reasons mentioned above,the flow and heat transfer characteristics of the microchannel with different spherical protrusion height and protrusion pitch are studied,then the flow and heat transfer performance of the spherical protrusion channel with embedded dimple are studied.Finally,the nanofluid is applied to the spherical protrusion channel with embedded dimple.The main work of this theses is as follows:(1)The laminar flow model in fluent software was used to simulate the flow heat transfer characteristics of water in the microchannel with protrusion.By analyzing the velocity field and temperature field of the channel,the mechanism of different protrusion height and different protrusion pitch on the heat transfer characteristics of the channel is revealed,which lays the foundation for the study of newly-designed channel.(2)The flow and heat transfer characteristics of spherical protrusion microchannel with embedded dimple under laminar flow are studied by numerical simulation.The influence of protrusion pitch,dimple diameter and dimple depth on the flow and heat transfer characteristics of channel is revealed by analyzing the velocity field,temperature field and field synergy angle,which provides a theoretical basis for the optimization design of spherical protrusion channel with embedded spherical dimple.The correlations of Fanning fraction factor f,Nusselt number(Nu)and thermal performance(?)of the micro-channel with the spherical concave structure are obtained by the method of Response Surface Methodology.So it can be used to provide reference for the design of the newly-designed channel.According to the above-mentioned correlation formula,the interaction between two structural parameters is analyzed,and the theoretical basis for determining the influence degree of each structural parameter on the flow heat transfer characteristic can be provided.(3)Numerical simulation of the flow and heat transfer characteristics of the alumina-water nanofluids in the spherical protrusion channel with the spherical dimple by using the single-phase flow method.So the effects of the volume fraction of the nanofluids on the flow and heat transfer can be investigated.Which provides theoretical guidance for the use of nanofluids as heat transfer medium for the micro-channels.In order to evaluate the comprehensive performance of the composite heat transfer scheme of the protrusion channel with dimple and the nanofluids,the Nusselt number,friction factor and the thermal property of the medium will be taken into the Entropy generation formula,and the entropy generation characteristics of different medium will be obtained.The rationality of the composite heat transfer scheme and its applying conditions will be gained by comparing the entropy generation of nanofluids with the entropy generation of base fluid.