Fundamental Research On Applications Of Marangoni Effects In Heat Pipes Under Microgravity

The Marangoni convection induced by surface tension gradient is one of main topics in microgravity fluid mechanics, that has important influce to heat transfer, crystal preparation, metallurgy, and so on. For all the pure working fluid in heat pipes, the surface tension is a decreasing function of the temperature. Therefore, surface motions driven by Marangoni effects are directed toward the cold region, which may be unfavourable for the return liquid to the evaporator. Researches exhibit a surface tension increases with the temperature in some binary solutions at certain temperatures and concentrations. As a result, the surface motions are directed toward the hot region, which is called inverse Marangoni convection. One of its applications is the use of these solutions as the working fluid in heat pipes, which can increase the mass flux of return liquid and improve the performances of heat pipes.Laboratory measurements of surface tensions have been conducted for 2-propanol, 1-butanol, 1-pentanol aqueous solutions at different temperatures and concentrations by wilhelmy plate method. It shows a positive and unlinear surface tension gradient with the temperature when the temperature exceeds near about 40℃. In addition, it is more obvious when the the number of Carbon atoms of the solute is increasing. And the experimental observation of the inverse Marangoni convection in small size rectangular pool with a constant temperature difference is investigated. Then through the establishment of physical and mathematical models, the numerical simulation of the two-dimensional inverse Marangoni convection under microgravity condition is carried out, which obtains the temperature and velocity distribution, and the effects of the temperature difference, the size ratio and the temperature coefficient on the flow are analyzed, the flow coupled with the buoyancy convection under gravity condition is also calculated. Results show that the inverse Marangoni convection induces surface motions from the cold region to the hot region, which is enhanced with the increase of the temperature difference and the temperature coefficient, and the interface become instable. Coupled with the buoyancy convection, two flows weaken each other due to the opposite flow direction, and the influence of Bond number to the flow is discussed. With the Bo number increases, the buoyancy convection gradually become dominant in the region, but the Marangoni effects can not be ignored.