Boiling Heat Transfer Enhancement Study For Micro-/nano-structure Modified Surfaces In Rectangular Narrow Microchannels

The traditional single-phase heat transfer enhancement methods are insufficient to meet the compactness and high efficiency requirements of heat exchangers coming from the sharp increase in power density and the miniaturization trend of equipments.The microscale phase change cooling technique using latent heat of evaporation is considered to be one of the most promising methods to solve the thermal management problem of current electronic devices of high heat dissipation.However,there are still many shortcomings for the study of flow boiling heat transfer characteristics in microchannels,and the understanding of corresponding mechanisms is still lacking.Meanwhile the micro-/nano-scale modified structures have a significant effect on the flow boiling process on the heat transfer surface.There is a need for comprehensive and meticulous parametric experimental testing and numerical simulation.In this paper,the flow boiling heat transfer characteristics of micro/nano-structure modified surfaces with various apparent parameters in microchannels were studied.The effects of operating conditions such as heat flux,mass flux and inlet quality and the apparent surface parameters,such as micro-/nano-scale structures and(heterogeneous)wettability distribution on the thermal and hydraulic performance,bubble dynamics and flow patterns during flow boiling in microchannels were investigated through instrument measurements and high-speed camera.By observing and analyzing the bubble dynamics and phase interface motion behaviors,the heat transfer mechanisms of micro-/nano-structure modified surface were explored in depth,which helped to supplement and improve the highly efficient heat transfer enhancement theory utilizing boiling applied to microchannel cooling heat sinks.In this paper,the flow boiling heat transfer in microchannels and the micro-/nanostructure modified surfaces enhancing boiling heat transfer were reviewed and discussed in detail.The test facility and experimental equipment for microchannel flow boiling experiments were introduced.The experimental methods and data processing process as well as the high-speed camera observation system and the corresponding image processing tool were elaborated.Single-phase heat transfer and pressure drop experiments were tested for reliability.A micro-scale three-dimensional porous copper structured surface with superhydrophobic wettability was constructed.The subcooled boiling curve was analyzed and the boiling incipience characteristics was studied by prediction correlations.The effects of mass flux and heat flux on the heat transfer coefficient and pressure drop were discussed.The porous copper surface could promote the nucleate boiling process greatly,and its superhydrophobic wettability made bubbles difficult to depart from the vaporized core cavity after nucleation,so the heat transfer coefficient was not affected by the mass flux but increased with increasing heat flux,showing that nucleate boiling dominated the heat transfer mechanism.Surface fabricated with zinc oxide micro-rods with micron size and diameter was prepared,and then experimentally tested during saturated boiling and subcooled boiling with image processing to explore the initial nucleate boiling characteristics and bubble dynamics.With the increase of heat fluxes during subcooled boiling experiments,dominated flow patterns on the heat transfer surface was transmitted from the intermittent isolated bubble/slug flow to the confined slug/annular flow domain.The wall temperature fluctuation would be greatly suppressed,and the heat transfer characteristics greatly increased with increments of heat fluxes.A nanosilica-particle coated surface with superhydrophilic wettability was prepared.The effects of surface wettability,heat flux,inlet quality and mass flux on the saturated boiling heat transfer characteristics in microchannels were analyzed systematically,and the potential mechanisms were further discussed by subsequent image processing combined with theoretical analysis.The coated surface could maintain stable liquid-vapor interface distribution due to super-hydrophilicity,and performed better for high inlet quality.The influence of inertial force increased correspondingly with increments of mass flux,which greatly reduced the influence of surface wettability and heat flux,thus heat transfer characteristics of test surfaces became similar.By designing a series of fabrication procedures comprised of transferring heterogeneous distribution patterns to the heat transfer substrate,heterogeneous wettable surfaces with hydrophilic/hydrophobic stripe pattern were fabricated.The bubble departure behaviors were promoted and coalesce between nucleatd bubbles of the adjacent vaporized core cavity was prevented because of the heterogeneous wettability pattern.Meanhwile the hydrophilic base could serve to supply the liquid renewal and limit the bubble contact diameter to eliminate local dryout,thus the heat transfer characteristics of heterogeneous wettable surfaces were significantly enhanced.The computational fluid dynamics method was used to study the saturated boiling heat transfer characteristics in the annular flow regime on the superhydrophilic surface in the rectangular narrow microchannel under the open-source software OpenFOAM platform.The heat transfer characteristics obtained by simulation were compared against experimental tests and verified.The effects of mass flux,heat flux and inlet quality on the local velocaity field,phase interface,temperature field and phase change source distribution during the microscale boiling phase change process were discussed by numerical simulation method.