Study On Enhanced Flow Boiling Heat Transfer Based On Minichannels With Microwire Clusters Sintered Surface

With the heightened consumption and gradual depletion for non-renewable fossil fuels worldwide,the exploitation and application of energy efficient and environmentally friendly technologies have become a tendency.Furthermore,with the upgrades in technology and manufacturing processes,electronic devices have shown a trend towards increasing miniaturisation in size,while their heat generation power has become more and more aggravated with the upgrading of device performance,which poses a huge challenge to traditional air-cooling,water-cooling,passive phase-change heat transfer and other enhanced heat transfer technologies.The minichannel flow boiling enhanced heat transfer has excellent heat transfer efficiency due to its scale effect,phase-change heat transfer etc.,and has received sustained attention and research from many scholars at domestic and overseas in recent years.In order to meet the heat dissipation needs of the new era for aerospace,supercomputing and other cutting-edge technology fields,this paper develops a microwire cluster sintered surface fine channel through high-temperature atmosphere sintering technology and combines the connectivity structure between channels and EHD effect to achieve composite enhanced heat transfer,providing a theoretical basis and technical guidance for heat transfer of high-performance high heat flux components.Focusing on this objective,this research focuses on the following work aspects:(1)To address the problems of high onset superheat and significant pressure fluctuations in nucleation boiling on conventional smooth surface minichannels,a novel microwire cluster sintered surface minichannel heat sink(the microwire cluster is formed by twisting copper wires with a wire diameter of 30μm)is proposed and its flow boiling characteristics are investigated.The effect of the cavity size on the cavity activation of the heated surface is analysed by the cavity activation theory,and a range of cavity sizes is obtained for the activation of the sintered surface of the microwire clusters,and the surface morphology and structural characteristics of the microwire clusters are analysed by combining SEM images and micrographs.The flow boiling experiments are performed using R141b as the working medium,and the effects of different mass fluxes(161.2 kg/m~2s,255.2 kg/m~2s and 349.2 kg/m~2s),inlet subcooling(16.7°C,13.7°C and 10.7°C)and microwire cluster parameters(Str=24,32 and 48)on the heat transfer characteristics,pressure drop characteristics and flow instability of the flow boiling of minichannels on the sintered surface of microwire clusters are investigated.The behaviour of the vapour-liquid phase is visualised by means of a macro high-speed imaging system,and the bubble growth and detachment characteristics are analysed from the perspective of bubble dynamics to explore the enhanced heat transfer mechanism of minichannels on the sintered surface of the microwire clusters.The results show that compared with the conventional smooth surface minichannel,the microwire cluster sintered surface minichannel improves the density and distribution uniformity of the nucleation site,and the flow-boiling heat transfer performance is greatly enhanced,with the two-phase heat transfer coefficient up to 3.29 times.(2)In order to address the problem of inlet reflux in traditional parallel multi-channels at high heat fluxes,a sintered minichannel with inter-channel staggered expansion connecting structure is proposed based on microwire cluster sintered surface minichannels.The effects of inlet subcooling and mass flow rate on the flow boiling characteristics of the interconnected structure are investigated,and the variation of boiling curves,two-phase heat transfer coefficients,flow pressure drop and flow instability are obtained.The results show that the sintered minichannel with interconnected structure can improve the boiling characteristics and two-phase heat transfer coefficient,and suppress the inlet reflux phenomenon at high heat fluxes.A transient simulation of the confined bubble flow in a interconnected minichannel is conducted by Fluent to analyse the bubble kinetic behaviour and explore the flow boiling enhancement mechanism.The results show that the confined bubble flow into the interconnected region first expands and then contracts when re-entering the channel,enhancing the inter-channel heat and mass transfer.The behaviour of the nucleation site,inter-channel flow pattern and inlet return phenomenon are investigated by visualisation,and the growth and detachment of the nucleation site at the interconnected region are analysed to compare the difference in inter-channel flow pattern with and without the interconnected structure,providing a basis for revealing the enhanced flow boiling heat transfer mechanism of the interconnected structure.In addition,the effects of different connection parameters(connection spacing and connection angle)on the flow boiling characteristics are studied,and the mechanisms of the effect of the connection structure parameters on the heat transfer characteristics and pressure drop characteristics of the minichannel flow boiling are explored.(3)The flow boiling characteristics in minichannels with interconnected structure between minichannels with microwire cluster sintering surface under the action of different electric fields are studied.The dynamics of the bubble detachment process under electric field and the distribution of electric field strength in the channel are studied by applying a DC high voltage in the channel to generate a non-uniform electric field,and the effect of electric field on the detachment frequency and detachment diameter of the bubble is obtained by numerical simulation.The results show that the wire electrode generates a non-uniform electric field in the minichannel,and the bubbles are easily detached at the position where the bottom surface of the minichannel heating is facing the electrode.The enhanced flow boiling characteristics of the electric field under different voltage conditions are investigated in combination with experiments and the behaviour of the bubbles under the electric field is visualised.(4)To evaluate and compare the performance of flow boiling in the minichannel with different enhanced heat transfer methods,quantitative analysis is provided in terms of two dimensions:the heat transfer enhancement factorψand the comprehensive performance evaluation factor PEC.The flow boiling performance of three enhancement methods:microwire cluster surface,microwire cluster surface/connected structure and microwire cluster surface/connected structure/electric field are evaluated,and the flow boiling performance enhancement effect of single enhancement method and multiple enhanced methods are compared and analysed.The results show thatψand PEC are greater than 1 for all three strengthening methods,indicating that the flow boiling performance of fine channels based on the sintered surface of microwire clusters is enhanced.