Study On Vapor-Liquid Two-phase Flow And Heat Transfer In Micro-grooved Heat Pipe

The advantages of compact structure,no-pump and high heat transfer performance make the micro-grooved heat pipes as an important heat transfer element for high heat flux equipment in limited space.The micro-grooved heat pipes are widely used in microelectromechanical systems(MEMS),aerospace thermal control systems,energy thermal conversion.Unlike conventional wicked heat pipes,the circulation of the vapor-liquid in micro-grooved heat pipes relys on the gravity and the capillary force of the micro groove,and surface tension has an important effect on vapor-liquid two-phase flow and phase change heat transfer characteristics in micro groove.Therefore,the study on vapor-liquid two-phase flow and phase change heat transfer characteristics of micro-grooved heat pipes have important scientific significance for revealing vapor-liquid two-phase flow dynamic and phase change heat transfer of microscale,and the study will also provide theoretical guidance and technical support for the application promotion of the micro-grooved heat pipes.With the miniaturization of the gravity heat pipe,the vapor-liquid interfacial tension generated by the micro-grooved structure will have an important impact on the vapor-liquid two-phase flow and phase change heat transfer in the gravity heat pipe.While the influence mechanism of the micro scale on complex vapor-liquid two-phase flow,heat and mass transfer behavior of the gravity heat pipe have not been fully revealed.Particularly,the working mode and nonlinear heat transfer regimes of the micro-grooved gravity heat pipe operates in oscillation state are not fully understand.In addition,the study on heat transfer characteristics of evaporation phase change in axial micro-grooved heat pipe under coupling effects of gravity and capillary force need to be carried out urgently.Moreover,the existing visualization experiments of capillary flow in micro groove flat heat pipe is still in the extraction of two-dimensional flow pattern images,and the quantitative description on reconstruction of three-dimensional vapor-liquid interface in micro-grooved flat heat pipe is still lacking.Furthermore,the simulation study on vapor-liquid phase change heat transfer of micro flat gravity heat pipe is also scarce.Therefore,the two-phase flow mechanical characteristics of micro-grooved gravity heat pipe are studied experimentally in this paper,and the temperature oscillation behavior of the oscillation flow state is analyzed non-linearly.The evaporation heat transfer characteristics of the gravity assisted axial micro-grooved heat pipe are studied experimentally.In addition,three-dimensional reconstruction of vapor-liquid interface in micro-grooved flat heat pipe is carried out by means of optical layered microscopic imaging.Based on lattice Boltzmann method,a theoretical model of vapor-liquid phase change heat transfer process in micro flat gravity heat pipe is established,the heat transfer characteristics of boiling-condensation coupling phase change in a confined confined micro-space are studied numerically.The results and conclusions are summarized as follows:(1)Experimental study on dynamic characteristics of two-phase flow in micro-grooved gravity heat pipeVisualization experiments of the micro-grooved gravity heat pipe which operates in vertical mode with methanol and ethanol as working medium were carried out,micro-grooved gravity heat pipe have aluminum grooves with a depth of 1mm and 7 different widths(0.5,1,1.5,2,2.5,3,4mm).Three kinds of typical flow regimes of vapor-liquid two-phase when heat pipe operated in vertical mode were observed and defined.The effects of heat load and groove dimension on the thermo-hydrodynamic behaviors are examined and further quantitatively recognized by a state diagram depending on Weber number and Bond number.The results indicate that differing from the conventional two-phase thermosyphon,the state of working fluid in a micro-grooved gravity heat pipe is no longer only in a fashion of pool and annular-flow states,but also in a fashion of oscillation state.The oscillation state is the characteristic two-phase flow regime of micro-grooved gravity heat pipe and possesses the feature of random formation and oscillatory motion of liquid plug in minigrooves.The pool state occurs at small Weber number,the annular-flow state is observed at large Weber number,and the oscillation state takes place at medium Weber number when Bond number is approximately smaller than 0.7.The thermal performance of oscillation state is better than that of pool state but is less powerful than that of annular-flow state.The heat transfer regimes are the film evaporation and condensation combined with thermal conduction in the pool state,the forced evaporation and condensation in the oscillation state,and the film evaporation and condensation in the annular-flow state,respectively.The heat transfer regime under low heat load in micro-grooved gravity heat pipe mainly corresponds to thermal conduction of the wall,so the filling rate has little effect on temperature responses and axial temperature distribution of the wall.And the filling ratio play a significant role on the two-phase flow and heat transfer characteristics under high load condition.For medium filling ratio,the wall temperature oscillation is caused by the alternative scour of the vapor and liquid plug in each sections.(2)Nonlinear analysis of heat transfer characteristics on oscillation state in micro-grooved gravity heat pipeVisualization experiment and temperature measurement experiment on oscillation state of the micro-grooved gravity heat pipe with groove size of 1mm×1mm which operates in vertical mode were carried out.Two typical oscillation regimes of the liquid plug in oscillation state of the micro-grooved gravity heat pipe are observed and defined.And the modern autoregressive(AR)power spectrum analysis was used to analyze the oscillation characteristics of the wall temperature of the heat pipe operates in oscillation state quantitatively.The results indicate that two typical oscillation regimes of the liquid plugs in oscillation state of the micro-grooved gravity heat pipe are independent-oscillation in single groove and interferential-oscillation in adjacent grooves which corresponding to the condition of medium heat load and high heat load respectively.The independent-oscillation in single groove is characterized by the random formation and oscillation of the liquid plug in single groove and the wall temperature possesses a main frequency which caused by the alternating scour effect of the liquid plug and steam with high temperature on the inner wall of the heat pipe.Differing from the independent-oscillation in single groove,the interaction of liquid slug and the random formation of new liquid plug are the typical characteristics of the interferential-oscillation in adjacent grooves,and the oscillation of the wall temperature presents two main frequencies of low frequency and high frequency.(3)Experimental study on evaporation heat transfer characteristics of gravity assisted heat pipe with axialy micro-groovesVisualization experiments of vapor-liquid two-phase flow and heat transfer are conduc using high-speed digital microscope and thermal infrared imager to investigate the corner-film evaporation regime of gravity assisted axial micro-grooved heat pipe at small tilt angles.The effects of the heat load and tilt angle on these evaporation regimes are analyzed.The evaporation regimes are further quantitatively characterized with a regime diagram developed based on the Bond number and Weber number.In addition,two typical characteristics of the two-phase flow instability and the overshoot start-up of the wall temperature in corner-film evaporation regime are discussed.The results indicate that unlike the horizontal condition,the evaporation regime of a grooved heat pipe with a small tilt angle(θ = 8°、θ = 12°)can include corner-film evaporation induced by the combination of gravity and capillary force in addition to pool-surface evaporation and fin-film evaporation.Corner-film evaporation is the characteristic evaporation regime of gravity assisted axial micro-grooved heat pipe with a small tilt angle,and it exhibits random oscillatory motion of the vapor-liquid interface.Compared with pool-surface evaporation,the heat transfer capacity of phase change in fin-film evaporation is significantly improved.The expansion of the thin-film region makes heat transfer performance of the evaporator in corner-film evaporation superior to that in thin-film evaporation.Under effect of small tilt angle,overshoot of the wall temperature in evaporator section is caused by the evaporation regimes shift from fin-film evaporation to corner-film evaporation during start-up process of the gravity assisted axial micro-grooved heat pipe.The instable pulsation of the vapor-liquid interface is result of the uneven carry effect of vapor flow on condensate in sharp corners and at bottom of the rectangular groove.(4)Visualization experimental study on vapor-liquid interface of micro-grooved flat heat pipeVisualization experiment on vapor-liquid interface of micro-grooved flat heat pipe working at steady state is carried out,the grooves are milled on quartz plate with dimension of 0.5mm×0.5mm and ethanol is used as working medium.Based on principle of optical delamination microscopy,three-dimensional reconstruction of the two-phase interface of the micro grooves flat heat pipe is realized.Three dimensional distribution characteristics of vapor-liquid and heat transfer regimes of condenser,adiabatic section and evaporator are analysed and discussed.The experimental results indicate that when micro-grooved flat heat pipe works in horizontal quasi-steady state,the effects of gravity on distribution of the liquid layer in direction of axial and radial are weak,therefore the liquid layer distributes in sharp corners of square groove or covers the wall of groove almost symmetrically.Different from the effect of gravity,the effect of surface tension on distribution of liquid phase is significant.Under effect of the surface tension,the vapor-liquid two-phase interface of the condensate liquid plug is symmetrically distributed in central region of the grooves as a shape of bullet and the warhead points to condenser.In the region except the liquid plug,the concave meniscus of liquid phase to vapor phase is formed under effect of surface tension at vapor-liquid interface for different depths.Due to the capillary pressure difference caused by the difference in radius of curvature between condenser and evaporator along axial direction of micro-grooved flat heat pipe,the condensate liquid flows from condenser through adiabatic section to evaporator to supplement the loss of liquid caused by evaporation.(5)Numerical study on heat transfer of boiling-condensation phase changes in micro flat gravity heat pipeBased on the multiphase lattice Boltzmann method combining the finite difference scheme,a theoretical model of vapor-liquid phase change is developed and vapor-liquid two-phase flow dynamics and thermodynamic behaviors of co-existing boiling and condensation phase changes in a confined micro-space is analysed numerically.Particular focus is paid on effects of surface wettability on bubble and droplet hydrodynamic behaviors and heat transfer performance.The results indicate that the interaction between boiling and condensation is mainly manifested in the dripping of condensate droplets.On one hand,the falling droplets can directly contact the growing bubble,accelerating the collapse of the bubble,and facilitating the vapor-liquid heat exchange.On the other hand,droplets fall into the overheated liquid and hence result in a relative motion of the liquid in boiling pool,which can contribute to bubble movement or detachment.The surface wettability plays a significant role in the bubble dynamics and thermodynamics of boiling in a confined space.The boiling phase change usually occurs in the region of the local heat source on the hydrophilic surface while over almost the entirety of the hydrophobic surface.The boiling heat transfer coefficients at local heat source on the hydrophilic surface are higher than that on the hydrophobic surface at both the initial and steady state stages.In addition,the fluctuation of heat transfer coefficient on the hydrophobic surface is more obvious than that on the hydrophilic surface at the steady-state stage.As the contact angle increases,the thermal response time of the hydrophilic surface is shorter than that of hydrophobic surface.Correspondingly,the hydrophobic surface maintains a higher temperature than the hydrophilic surface.In addition,the wall temperature near the onset of nucleate boiling point is affected by surface wettability.Therefore,the temperature of the heating surface decreases once a bubble is generated on the hydrophilic surface owing to the liquid micro-layer while it shows a slight increase tendency on the hydrophobic surface.In summary,the current work systematically reveals the complex vapor-liquid two-phase flow characteristics and heat transfer behaviors and regimes of micro-grooved gravity heat pipe in vertical mode.The study reveals the effect of the extra-gravity on heat transfer regimes of gravity assisted axial micro-grooved heat pipe at small tilt angles.We also gain the three-dimensional distribution characteristics of vapor-liquid two-phase interface and heat transfer regimes of the micro-grooved flat heat pipe preliminary.The interaction mechanism,the vapor-liquid two-phase flow characteristics and thermodynamic behaviors of boiling-condensation phase change in confined micro-space of micro flat gravity heat pipe are also revealed.The correspongding research results provide an effective theoretical support for the design and optimization of micro-grooved heat pipes in practical applications,and the study also offers an important supplement to the characteristics of vapor-liquid two-phase flow and the phase change heat transfer mechanism of microscale.