Investigation On Heat Transfer Characteristics Of Ultra-thin Flat Looped Heat Pipe

Along with the development of faster processor speeds and higher levels of integration in electronics,super-cooling schemes capable of dissipating high heat fluxes within small temperature budget and limited spaces become the serious bottleneck in many applications,such as power electronics,advanced batteries,laser diode three-dimensional,integrated chip architectures and so on.As a promising technology for efficient cooling,flat looped heat pipes(FLHPs)display some unique merits,such as high heat transfer capacities,no external power consumption,long heat transfer distances and flexibility for installation.Hence,the miniaturization and thinning design of flat loop heat pipe has become the focus of research at present,and the investigations on high-efficient miniature heat pipes(MHPs)receive intensive attentions.In fact,the further breakthrough in developing MHPs is subject to the better utilization of micro-/nanoscaled two-phase flow and heat transfer process,primarily the boiling phenomenon inside the evaporator.First of all,a series of visualization experiments of flow boiling in microchannels with low aspect ratio has been conducted to investigate the effects of the operating conditions and structural parameters on the bubble behaviors and flow patterns.The characteristics of confined bubble behaviors including the bubble growth,coalescence,and elongation under various mass flow rates,heat fluxes and aspect ratios are explored.Meanwhile,the flow boiling heat transfer characteristics including the heat transfer coefficient and critical heat flux in confined microchannels are disclosed.The experimental results indicate that in the confined elongated bubble flow is the main flow pattern in the microchannel with such low aspect ratio.Besides,it is denoted that the heat transfer capability under the flow pattern can stay stable and barely varies with the vapor quality.This feature provides a good application value for this kind of ultra-shallow microchannels heat sink in heat dissipation.Secondly,with the visualization results of flow boiling in microchannels,the theoretical analysis on the flow and heat transfer mechanism has been conducted.In view of the effect of the aspect ratio,a model of two-phase flow pressure drop during the flow boiling in confined microchannels is established.In addition,on the basis of the different mechanisms under the various flow patterns,two corresponding models in predicting heat transfer coefficient are developed respectively for confined elongated bubble flow and annular flow.Considering the premature critical heat flux(PCHF)occurred in the confined microchannel caused by the rapid vaporization in the thin liquid film zone,a semi-empirical correlation for CHF is proposed for such shallow microchannel.Beyond the fundamental research on the flow boiling,systematic investigation on the thermal performances of the ultra-thin flat looped heat pipe(UFLHP)including the start-up characteristics,operating temperature,thermal resistance and the system instabilities are also conducted with experimental method.Three types of groove configuration applied in the evaporator are developed,and the effect of the adjustment in groove configuration is compared.The experimental results demonstrate that the operating temperature of the UFLHP with the parallelogram configuration can be decreased by 11 ~oC,the start-up response becomes accelerated and the system runs smoothly without temperature oscillation.In addition,the thermal resistance reaches as low as 0.08 K/W,which is reduced by nearly 50%compared to that of the UFLHP with the rectangular groove configuration.Last but not least,mathematical models to describe the heat transfer performance of the ultra-thin flat looped heat pipe are founded.Based on the dynamic cycle process of the liquid slug over the heat pipe system,the one-dimensional model can well predict the effect of the orientations,the length of the liquid/vapor line on the operating temperatures.Besides,in order to further depict the flow and heat transfer process in each channel inside the evaporator,a deepening model is established on the basis of the flow resistance network model.With this deepening model,the thermal performances of the UFLHPs with different groove configurations are be effectively predicted.Overall speaking,the present work focuses on the heat transfer performance of ultra-thin flat looped heat pipe,with both the experimental and theoretical studies carried out.The aim of present work is to clarify the fundamental mechanism in shallow microchannels flow boiling,and disclose the operating characteristics of the ultra-thin flat looped heat pipe,,so as to put forward some effective methods for the development of ultra-thin heat pipes.