| As an essential guarantee for the reliable operation of electronic devices,the electronic heat dissipation technology is widely used in aerospace,large servers,military equipment,industrial robots,and civil electronic products.The aero-engine,computer chip,and TR module of radar are the core components of the equipment.The electronic radiator plays a vital role in maintaining the reliable operation of these electronic devices.As an efficient cooling technology,microchannel liquid cooling technology has been widely used in high-power electronic devices.Electronic devices are developing in miniature and high power;so,it is urgent to explore more efficient microchannel liquid cooling technology.Based on this,this dissertation investigated the enhanced heat transfer performance of the combinations of pulsating flow,graphene nanofluid,and CNT/paraffin composite PCM in a microchannel with the pin-fins with a unique shape.The specific research contents are as follows:(1)Reference to the existing literature,four kinds of microchannels with staggered arrays of pin-fins with different shapes(cone,cylindrical,square column,and pyramid)were designed.The thermal resistance,Nusselt number,and friction factor of four kinds of microchannels are studied by numerical simulation under different conditions.It is found that the comprehensive thermal performance of cone and pyramid microchannel with pin-fins with variable cross-section is better than pin-fins with constant cross-section,namely square and cylindrical microchannel.The overall thermal performance of the pyramid channel is the best.The optimal design of the inclination angle and end clearance of pyramid pin-fin is carried out on this basis.The optimal scheme is obtained when the inclination angle is 60 °,and the end clearance is 0.5mm.(2)The process design and manufacturing of microchannel with pyramid pin-fins are carried out.The heat transfer experiments under steady flow using pure water as the coolant in the microchannel are carried out,and the flow and heat transfer characteristics in the channel are analyzed.It is found that the pressure drop shows exponential growth with the increase of Reynolds number,which leads to a slowdown in the growth of volume flow rate with pump power.The mechanism of heat transfer enhancement in the channel is explained by analyzing the field synergy characteristic.The energy efficiency factor of pump power is defined and analyzed by fitting the equations of Nussel number on pump power.The enhancement characteristics of heat transfer by pump power are revealed,and heat transfer enhancement efficiency is quantified.The results show that the energy efficiency factor increases first and then decrease.It can be predicted that the enhanced heat transfer efficiency will drop to near 0 when the pump power increase to a certain threshold.It explains that the unsustainability of enhanced heat transfer by pump power.(3)Four kinds of wave voltage(square wave,sawtooth wave,anti sawtooth wave,and sine wave)are selected to drive the micropump to produce pulsating flow.The experiments of flow and heat transfer for pulsating flow in the microchannel are carried out.It is found that the square wave voltage is better than the other three kinds of wave voltage by analyzing the flow characteristics and thermal performance.The experiments of flow and heat transfer for pulsating flow driven by square wave voltage with different duty ratios are carried out on this basis.It is found that there is always an optimal pulsating frequency(5 or 6 Hz)under different duty ratios,which leads to the best heat transfer performance.And square waves voltage with 25% and 30% duty cycles are better than that with other duty cycles.Also,the characteristic waveform parameters of pulsating flows are defined and analyzed.The weight analysis method's influence weights of different characteristic parameters on heat transfer enhancement are qualitatively analyzed.The order of importance weight is: surge rate > amplitude > peak > peak time >trough time.On the other hand,the field synergy characteristic is analyzed by numerical method under the condition of pulsating flow,which further reveals the mechanism of heat transfer enhancement by pulsating flow.(4)Graphene/water nanofluids with a concentration range of 0.05 ? 0.2% are prepared by a two-step method.The heat transfer enhancement of graphene/water nanofluids for pulsating flow driven by the square wave voltage with a duty cycle of 25%is studied.The results show that the optimal pulsating frequency is not affected by the concentration of nanofluids.The heat transfer performance is enhanced by using the graphene/water nanofluids as the coolant under pulsating flow.But the increase of the heat transfer efficiency is lower than that of nanofluids' corresponding thermal conductivity.The nanofluids show higher thermal performance under pulsating flow due to the strong disturbance and mixing of fluid in the microchannel.The enhancement of Nusselt number of nanofluids under pulsating flow increase by 3.7?5% compared to under steady flow.The adsorption and deposition of nanoparticles in the microchannel are quantitatively characterized by the image processing method,which further confirmed that the strong disturbance of pulsating flow inhibited the deposition of nanoparticles.(5)Four kinds of paraffin with different melting points are mixed to form composite paraffin with a wide melting point range.CNT / paraffin composite PCMs with a mass fraction range of 5 ? 15% are prepared by adding carbon nanotubes(CNT)into composite paraffin.The experimental study on heat transfer enhancement of composite PCM under nanofluid pulsating flow is carried out.The results show that the optimal pulsating frequency is 5Hz.When the mass fraction of CNT is 5%,the heat transfer efficiency decreases.When the mass fraction of CNT is 15%,the maximum enhancement rate of the Nusselt number is about 17.3%. |
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