Research On The Mechanism Of Flow Boiling Heat Transfer In Micro-Porous Structures And The Coupling Effect Of External Flow

With the development of energy utilization,the integration of electronic devices,compact heat exchangers,aerospace vehicles and other components have gradually increased.To solve the heat dissipation problem of high-temperature components,flow boiling is widely used to carry out thermal management for heating surfaces and devices,and the utilization of porous coatings and porous structures can further achieve heat transfer enhancement.However,for flow boiling in porous media,due to the complex pore-throat structure and the large randomness of the fluid flow distribution,the mechanism of interactions among different phases at pore-scale is insufficient and unclear,which is a challenge to avoid dryout,over-temperature accidents,and burning-down of the materials.Besides,understanding the effect of operating conditions and the coupling mechanism of external flow is important and needs to be further studied.This dissertation systematically studies the above difficulties:To reveal the role of trapped liquid and its influence on phase change process in micro-porous structure,a pore-scale investigation with micromodel was built to obtain bubble behaviors and characteristics of heat transfer and pressure drop during flow boiling inside the pores,and quantify the transport mechanism of trapped liquid.The experimental results showed that the liquid bridge and liquid film flow caused by the capillary effect play an important role to supply coolant to the heating surface,and the pore-throat parameters can significantly affect liquid bridge stability and thin liquid film flow rate.Combined with theoretical analysis,a designed porous structure with multiple throat sizes was proposed to enhance the heat transfer coefficient,reduce the flow resistance,and improve the reliability of the cooling system.To study the effect of operating pressures and physical property changes,the traditional phase change model was improved to simulate boiling process in porous media,which improved the model convergence and accuracy.The non-isothermal heat transfer process in the two-phase zone at low pressures was studied,and the difference between the local thermal equilibrium and the local thermal nonequilibrium hypothesis was analyzed.The model was applied to simulate phase-changed transpiration cooling for aircraft,studied the effect of non-uniform heat flux and low pressure on dynamic response of the temperature field,analyzed the reason of cooling failure of thermal protection,and proposed effective methods to remove vapor blockage and enhance cooling efficiency.To understand spontaneous flow process of fluid in porous structures by capillary effect,a pore-scale experiment was carried out with lab-on-a-chip method.It was revealed that the pore-throat structure can cause spontaneous capillary flow of fluid driven by phase change process,and the increase of heat flux and the decrease of pore size can improve capillary flow rate within a certain range.Numerical simulation discussed the effect of porous parameters on the limit of spontaneous capillary flow,and regarded this capillary flow has a promising load adaptability under non-uniform heating conditions,which verifies its feasibility in the field of aerospace thermal removal.To comprehend the interactions between two-phase outflow from porous media and free flow in external region,a pore-scale investigation was carried out to study the coupling effect of internal and external flows,and learn the influence of particle size and velocity ratio on the phase chang process.Based on the experimental results,a boundary condition to study the coupling effect between the porous zone and the external flow field was built,and the phase-changed transpiration cooling with high Mach number was simulated to study the effect of altitude,angle of attack,and Mach number.It was regarded that the external flow can affect the pressure distribution on the porous surface,and change the flow rate inside the porous structures.Besides,the gas film generated by the phase change can effectively reduce the heat flux imposed on the porous surface by the high-temperature external flow.