Experimental Study On Capillary Flow And Evaporation

Capillary flow and micro-evaporation behave very different features due to capillary force whose influence is usually neglected at macroscopic scale.Both of them have a wide range of applications in engineering such as chemical industry,refrigeration,MEMS and space technology,thus presenting great values to study.The thsis investigated the flow and evaporation behavior and the underpinning mechanism at micro-scale by conducting a series of experiments in capillary cylindrical tubes and porous media with the assistance of high-speed-camera and microscope.The research results can also be applied to interpret performances of loop heat pipe in terms of capillary flow and micro-evaporation.The main content and conclusions of the thesis can be summarized as follows:1.Review on capillary flow,evaporation in microchannel and experimental study on loop heat pipe with pointing out the flaws in existing studies on the theoretical and engineering works.2.Experimentally study on capillary flow and evaporation behavior in micro-tubes of diameter from 5-500?m and those in porous media of average diameter of 25?m.3.It is found by comparing the experimental data and predictions.of Lucas-Washburn Equation that dynamic contact angle has great effects on capillary flow,especially before a certain time point and Lucas-Washbum Equation should be modified before the point when used.The time point is highly associated with tube diameter.On the other hand,the deviation of experimental data from predictive values in porous media from experimental data is attributed to the increase of viscous drag instead of dynamic contact angle.4.Both forced and natural evaporation were experimentally studied in the thesis.It is found that the forced evaporation occurs in capillary tube in the form of "liquid jetting" during which "liquid bridge" and "liquid slug" appear at the outlet of the tube,greatly affecting the evaporation.The influences of heat load,heating location and tube diameter are involved in the paper as well with corresponding mechanisms behind the different phenomenon.5.The experimental results obtained in the last several chapters are applied to interpret the performance of loop heat pipe,including temperature oscillation in evaporator,start-up under low heat load and configuration optimization of evaporator grooves.Some suggestions on designing loop heat pipe are also provided with a verified experiment presented as the support.