Theoretical Research For The Heat Transfer Mechanism Of The Closed Loop PHP On The Stable Operation

Pulsating heat pipe(PHP)with the characteristic of its simple structure,lower cost,high efficiency is considered one of the most potential new high efficiency heat transfer element to solve the small space and high heat flux cooling since the late 1990 s.Meniscus zone of thin liquid film evaporation model is built to study the heat and mass transfer within the thin liquid film between the bubble and wall of the PHP.And a numerical model for the closed loop PHP on stable operation is built to study the flow characteristics and heat transfer characteristics of the vapor plug and liquid slug on the stable operation.In the model for the meniscus zone of thin liquid film evaporation,the two dimensional Navier-Stokes equation is adopted to solve the liquid film flow and a second differential equation for the liquid film thickness is gained and calculated by the fourth order Runge-Kutta.According to the conservation of energy and mass,the connection between the mass transfer and heat transfer is established.The impact to the thin film heat and mass transfer because of the thin liquid film heat transfer resistance and heat pipe diameter and superheat is studied.The results show that the gas-liquid interface thermal resistance plays a main role under the condition of the very thin of thickness of the thin liquid film(μm orders of magnitude).Because the percentage of the thermal conductivity resistance is above 0.9 under the condition that the thickness of the thin liquid film is greater than 1 mm,it plays a main role.In Tv = 323.15 K,d = 2 mm,the increasing of the temperature difference between the tube wall and steam makes the boundary of the thin liquid film evaporation area and intrinsic meniscus region moved forward.The thin liquid film evaporation decreased.While,the heat transfer,mass transfer and the maximum heat flux density of the thin liquid film evaporation zone will increase.In Tv = 323.15 K,ΔT = 5 K,the thickness and length of the liquid film are increased with the increasing of the tube diameter,which makes the differential gas-liquid phase pressure hindering the liquid film evaporation reduced.So,the heat transfer,mass transfer and the maximum heat flux density of the thin liquid film evaporation zone will also increase.In combination with the existing experimental phenomenon study and the theoretical research results,a mathematical and physics model for the closed loop pulsating heat pipe in the stable operation is built.In the model,the stress and thermal of the liquid slug and vapor plug in different section is analyzed.The conservation of mass,momentum,energy equation is established.The new vapor plug formation and the old vapor plug vanishing is also considered.Then the programs are written in the Matlab.The first-order difference method is used to discrete and calculate the differential equations.The results show that the working medium in the PHP has a certain macroscopic flowing direction(forward)accompanying the phenomenon of stagnant and backward.So the PHP in the process of stable will have a circular flow and be accompanied by a local oscillation and pulsation.In the first 9 s simulation time,the maximum temperature of the working medium has been rising.It indicates that the vapor-liquid slug does not leave the heating section and is in the stage of energy accumulation.After 9 s,the PHP enters the stage of stable operation with the temperature shock and fluctuation in the vicinity of 323.15 K.But,the oscillations of the temperature do not have a fixed amplitude and oscillation period from the perspective of the graph.