Investigation Of Enhancing Flow Boiling Heat Transfer In Minichannels With Wire Coil Inserts And Nanofluids Under Electric Field

In order to further improve heat transfer performance of the micro/minichannel heat exchanger,many researchers have applied the traditional heat transfer enhancement techniques to the micro/minichannel heat exchanger and studied on them.However,in practical applications,due to limitation of the special structure of the heat exchange system and requirements for high load heat transfer intensity,applying single heat transfer enhancement technique has been unable to meet the actual demand in micro/minichannel heat exchanger,so it is necessary to adapt compound heat transfer enhancement techniques to the micro/minichannel heat exchanger.In view of this,the main objective of this study is to apply electric field heat transfer enhancement technique,wire coil inserts heat transfer enhancement technique and nanofluid heat transfer enhancement technique to minichannel and to study their heat transfer performance,especially to study their heat transfer enhancement effect when combined with each other.According to this objective,the main research work finished is as follows:?1?Study on the flow boiling in minichannels under electric field.A minichannel heat sink with wire electrode structure is proposed.Both the width and height of single minichannel are 2mm.The wire electrode,as the positive pole,is placed in the center of the channel to create inhomogeneous electric field.The flow boiling characteristics of R141b in a vertical minichannel heat sink with and without electric field are studied experimentally,and flow visualization is performed using a high-speed video camera,then to analyze the characteristics of vapor-liquid interface change in flow boiling with and without electric field.In addition,the mechanism of the interface change is preliminarily discussed from a physical viewpoint.The results show that more vapors are produced under electric field,so that the transition of flow patterns happens ahead of time and the electric field changes the motion behaviors of bubble,thereby enhancing heat transfer.The enhancement effect of electric field on heat transfer is remarkable in the bubbly flow and slug flow regions,but weak or not obvious in the churn flow and annular flow regions.Under the present experimental conditions,the maximal heat transfer coefficient with electric field is 5.57 kW/?m²·K?,and the heat transfer enhancement ratio with electric field is between 1.0².48.The electric field has a certain effect on the total pressure drop and the two-phase frictional pressure drop per unit length,but these effects are not significant and the laws are not obvious.The pressure drop fluctuation increases with the increase of the electric field intensity at high and low heat fluxes,but at the medium heat fluxes,the pressure drop fluctuation declines with the increase of the electric field intensity.The Hurst exponent analysis shows that the fluctuation process of pressure drop has different degree of aperiodic long-range correlation,which may lead to chaos in the system.In addition,according to the experimental data and based on Sun–Mishima heat transfer correlation and Li–Wu?2011?pressure drop correlation,two new correlations for prediction of two-phase heat transfer coefficient and pressure drop in minichannel under electric field are proposed.The mean absolute errors of both correlations are 8.5%and 6.4%,respectively.?2?Study on the flow boiling in minichannels with wire coil inserts under electric field.The numerical study for the flow and heat transfer of a single-phase liquid in minichannel with wire coil inserts shows that the wire coil generates vortex in the channel and then enhances the convection heat transfer.This result provides some reference for the flow boiling analysis.The flow boiling characteristics of minichannel with wire coil inserts in the absence/presence of electric field are investigated experimentally.The results show that the wire coil inserts can provide stable nucleation site in the nucleate boiling.In the bubbly and slug flow regions,the wire coil inserts makes the motion behavior of the small bubble and the change of the vapor-liquid interface shape more random,especially more random when applying electric field.In the annular flow regions,the wire coil inserts brings more abundant liquid in the liquid film,which is conducive to the formation of stable nucleation site in the liquid film.No matter whether electric field is applied,the heat transfer coefficient,total pressure drop,two-phase total pressure drop and two-phase frictional pressure drop per unit length of minichannel with wire coil inserts are higher than those of minichannel without wire coil inserts?smooth minichannel?,and the heat transfer enhancement effect of the wire coil inserts is significant in the bubbly and slug flow regions.In most of the working conditions with electric field,the heat transfer enhancement ratio of the wire coil inserted minichannel is lower than that of the smooth minichannel.For the wire coil inserted minichannel,under the present experimental conditions,the maximal heat transfer coefficient without and with electric field are 5.60 kW/?m²·K?and 10.43 kW/?m²·K?,respectively,the heat transfer enhancement ratio with electric field is between 1.0¹.86.Furthermore,no matter whether electric field is applied,the wire coil inserts can repress the flow instability of flow boiling in minichannel to a certain extent.?3?Study on the flow boiling of nanofluids in minichannels under electric field.The flow boiling characteristics of TiO₂/R141b and TiO₂/Span80/R141b nanofluids in smooth minichannels with and without electric field are investigated experimentally.The results show that for electric field-free condition,the heat transfer coefficients of TiO₂/R141b and TiO₂/Span80/R141b increase evenly by 20%and 10%,respectively,compared to that of R141b.For electric field condition,the ranges of the heat transfer enhancement ratio of TiO₂/R141b and TiO₂/Span80/R141b are 1.01¹.58 and 1.01¹.85,respectively.The electric field has a certain effect on the total pressure drop and two-phase frictional pressure drop per unit length of nanofluids,but the effect is not significant and the law is not obvious.Under electric free-field condition,the effect of nanofluids on the pressure drop is not obvious at low heat fluxes,and at high heat fluxes the pressure drop for nanofluid is slightly higher than that for the pure working fluid,and the pressure drop for the TiO₂/Span80/R141b is slightly higher than that for TiO₂/R141b.Moreover,nanoparticles without modification of Span80 would be adsorbed to the positive electrode under electric field,resulting in the decreasing in the stability of nanofluids.This indicates that such nanofluids are not suitable for combining with electric field heat transfer enhancement technique.Therefore,only flow boiling characteristics of TiO₂/Span80/R141b in minichannel with wire coil inserts are studied.The heat transfer coefficient of nanofluids for electric free-field condition is 10%higher than that of the pure working fluid.The heat transfer enhancement ratio of nanofluids for electric field condition is1.01¹.61.For electric field-free condition at low heat fluxes,total pressure drop and two-phase frictional pressure drop per unit length of nanofluids are closer to these of pure working fluid in most of working condition,but in high heat fluxes,the total pressure drop and two-phase frictional pressure drop per unit length are greater than these of pure working fluid.The total pressure drop and two-phase frictional pressure drop per unit length increase respectively 8.4%and 10.6%.The total pressure drop and two-phase frictional pressure drop per unit length of nanofluids with electric condition are 6.8%and 8.2%higher than these without electric field.?4?Overall performance evaluation of electric field,wire coil inserts,nanofluid,and their mutual composite on flow boiling.The evaluation results show that most enhancement techniques embody the excellent overall performance in bubble flow and slug flow,and most enhancement techniques embody the obvious difference in the churn or annular flow.At the medium and high heat fluxes?i.e.churn flow and annular flow?,the overall performance of 3kinds of enhancement techniques,namely nanofluid,electric field and nanofluid/electric field,is not ideal or invalid,and the 4 enhancement techniques,namely,the wire coil inserts,wire coil inserts/electric field,wire coil inserts/nanofluid,and wire coil inserts/nanofluid/electric field have the opposite effect on overall performance.