Study On Single-phase Enhanced Heat Transfer And Boling Heat Transfer In Helical Mini-channel

In recent years,the electronic components become more and more miniaturized with the continuous improvement of science and technology.Therefore,the conventional heat dissipation methods are difficult to meet the increasingly heat dissipation requirements.Helical mini-channel heat exchangers have been widely used in aerospace and micro-electromechanical systems due to their large heat exchange area.It is necessary to investigate the enhanced heat transfer in the helical mini-channel to meet the higher heat dissipation requirements.In this thesis,the distilled water is used as the working fluid to investigate the single-phase enhanced heat transfer and boiling heat transfer in the helical mini-channel.The study is divided into three parts:（1）In order to investigate the effects of vortex generator on heat transfer and entropy generation in helical mini-channels with rectangular section,the numerical investigation of the heat transfer and entropy generation in helical mini-channel with 5 types of vortex generator was performed by using RNGκ-εturbulence model.The selected vortex generator structures are a rectangle,a prism,an ellipse and two triangles with different placement which having the same length,width and height.Under the conditions of heat flux of 300 k W/m²and Reynolds number of 4500～12000,the friction factor,Nusselt number,thermal resistance and entropy generation in helical mini-channels equipped with different vortex generators and a smooth helical mini-channel are analyze.The results show that within the range of Reynolds numbers,the Nusselt number and friction factor of the five channels equipped with vortex generator are larger than that of smooth channel,and the thermal resistance is lower than that of smooth channel.When Re≤7500,the entropy generation of 5 channels with vortex generators are lower than that of the smooth channel,while these trends is inversed when 7500<Re<12000.The heat transfer performance is insensitive to the addition of vortex generator.（2）In order to investigate the effects of experimental conditions and structures on two-phase pressure drop and liquid distribution in helical mini-channels with rectangular section,two models of smooth helical mini-channel and helical mini-channel with rectangular vortex generator are established.With air-water as working fluids,the two-phase pressure drop and liquid distribution of two models under the conditions of u_in=0.22～0.32 m/s andα=0.55～0.59 are analyzed.The results,obtained by numerical simulation,show that the liquid is on the outside of the channel and the gas is on the inside due to the effect of centrifugal force.The increase of inlet void fraction will reduce the thickness of liquid film on the outside of channel.The rectangular vortex generator in the helical channel enables the working fluid to generate secondary flows to enhance mixing,which can effectively improve the section void fraction.Moreover,the increase of inlet velocity,the decrease of inlet void fraction and the addition of rectangular vortex generator will increase the two-phase pressure drop in the helical mini-channels with rectangular section.（3）An experimental investigation on boiling heat transfer in helical mini-channel with the distilled water is presented.Tests are performed in the range of mass velocity 115.92～330.77 kg/（m²·s）,electric power 0～600 W,and inlet temperature 55～80℃.The effects of experimental parameters on the boiling heat transfer coefficient and boiling instability of the helical mini-channel are experimentally investigated.In addition,the experimental data of the heat transfer coefficient obtained from the experiment are compared with the existing correlations,and the correlation with the best prediction accuracy is modified.From the experiments,it is found that the heat transfer coefficient is insensitive with the mass velocity during stable flow boiling,the influence of inlet temperature and heat flux on heat transfer coefficient varies with the variation of experimental parameters.Moreover,flow instability increases with the increase of mass velocity and inlet temperature,and first increase then decrease with increasing heat flux.The new modified correlation shows in good prediction with MAE（Mean Absolute Error）of 8.9%.