Instabilities Of Vapor-liquid Phase Change Heat Transfer And Heat Transfer Enhancement In Closed Spaces At Sub-atmospheric Pressures

With the rapid development of the micro-electronic technology,it is urgent to deal with heat dissipation of the electronic equipments with high heat flux.A flat heat pipe is regarded as the effective technology to solve the problem of the heat dissipation.For the flat heat pipe,the distance between the evaporator and condenser is short,and the boiling is coupled with the condensation.So,the heat transfer mechanism of phase change inside the heat pipe is very complicated.Also,the evaporation heat resistance among the total thermal resistances is the most important component.As a result,it is necessary to further find out inner heat transfer mechanisms and investigate the methods of enhanced heat transfer.They are helpful to improve the heat transfer efficiency of the flat heat pipes.In view of the above-mentioned reasons,investigations of the heat transfer mechanism of phase changes and the critical parameters affecting factors on unstable phase changes,copper foam covers and free particles to enhance heat transfer are carried out.The main contents are included in five parts.First,a visual experimental platform was established to investigate the effects of three working fluids on the unstable phase changes.The experimental results show that the instability of the phase change is associated with the heat transfer modes when the de-ionized water is used as the working fluid.With the increase of the heat transfer flux,natural convection,intermittent boiling and fully developed nucleate boiling appear successively.The maximum instability happened during the period of the intermittent boiling.For the ethanol and acetone working fluids,the stable heat transfer of phase change occurs in the periods of natural convection and fully developed nucleate boiling.The conversions between activation and inactivation of nucleation sites result in the instabilities of the heat transfer.Additionally,the filling ratios and the temperature of the cooling water can influence the instabilities.Second,the effects of filling ratios on onset of nucleate boiling,heat transfer performance and deterioration were investigated.According to the experimental data,there is no wall temperature overshoot for the de-ionized water at the filling ratio of 15%.The incipient boiling heat flux and the temperature fluctuations were very small and phase change heat transfer was rather steady.The wall temperature overshoot occurs with the filling ratio increasing.The temperature variations of heating surface temperature were big and the instability of phase change heat transfer was strengthended.While used acetone as working fluid,the effects of filling ratio on the instability can be ignored.The instabilities became more volatile if the average temperature of heating surface was close to that of incipient boiling.The experimental results indicate that there is a coupled effect between the boiling and condensation.Also,the coupled effects are related to the properties of the working fluids.The effect for the de-ionized water is bigger and that for the acetone is smaller.The boiling caused by the little bubbles due to liquid stirring was initially observed.The optimum filling ratio approximate 50% was determined.Additionally,the mechanisms of heat transfer deterioration vary with filling ratios.Third,different structural copper foams were adopted to improve the boiling heat transfer.The copper foam covers can increase nucleation sites density and decrease wall superheat.For the de-ionized water and acetone,the wall superheats decreased by 13.4? and 27.1?,respectively.It has been found that the grooved cooper foam covers can effectively increase the boiling heat transfer coefficient and the maximum heat flux.Compared to the smooth surface,the coefficients of boiling heat transfer for the water and acetone can increase to 2.2 and 4.2 times,respectively,when the grooved structure was used.Next,for the de-ionized water as the base fluid,the influence of free particles of the nanometer,micron and millimeter on the phase change heat transfer performance were conducted.At first,the research of multi-walled carbon nanotube nanofluids and surface surfactants were carried out.After that,the particle diameters influences were investigated.Because the nanoparticles depositions increase the surface roughness benefical to more nucleation sites,the nanofluids can improve the capability of heat transfer for the smooth heating surface.Hence,the biggest superheat of incipient boiling decrease by 15.4?;The heat transfer coefficient increase by 1.45 times of the smooth heating surface.The heat transfer coefficient of heating surface increases with the volume concentrations of the nanofluids and remains stable.However,when the surface surfactant was put into the nanofluids,the heat transfer was deteriorated to aggravate the instabilities of boiling.Meanwhile,the levels of deterioration depend on the concentrations of the surfactant.Moreover,nanoparticles deposition,fluidization and sharp angles formed between particles and heating surface are the main reasons for heat transfer enhancement.Finally,the factors influencing the temperature characteristics of a radial eccentric heat pipe in natural and forced convections were investigated.The results showed that the temperature stability and uniformity of the condenser section are significantly improved by phase change heat transfer and its thermal capacity.In a word,the instabilities of heat transfer and the methods to improve heat transfer were studied in the two phase closed space at sub-atmospheric pressures.These studies provide a new idea for the plat heat pipes to tackle the heat dissipation of the electronic devices.