Effect Of Gravity On The Gas Phase Generation Caused By Temperature Gradient And Concentration Gradient

The heat and mass transfer under microgravity is a combination of microgravity science and heat and mass transfer, which becomes the focus of present research and front subject. In the pool boiling process, bubble behavior is relatively complicated, it involves the nucleation of bubble on the heating surface, the growth of bubble, the departure of bubble and interphase heat and mass transfer process. The growth process of hydrogen bubble of photo-biohydrogen production involves complicated biochemical reactions. Few scholars study on the phase change problem during this process, the hydrogen bubble is ignored, which is seen as a component dissolved in solution. In this paper, taking account of the mass and energy exchange at the two-phase interface, the VOF method was adopted to study the basic bubble dynamics of pool boiling and the growth process of hydrogen bubble under different gravity conditions. A single-bubble model of vapor bubble(hydrogen bubble) was established to simulate and analyse the bubble dynamics, temperature field(concentration field), flow field distributions in space and terrestrial conditions.The study of the influence of gravity on pool boiling process found that the temperature distribution and streamline profiles around the bubble, and the velocity component in the z-direction and y-direction under microgravity conditions are different from those of the terrestrial condition, which can be explained as the Marangoni flow causing eddies near the interface of the two phases. The bubble does not detach from the heating wall directly under the microgravity conditions. The heat transfer of the gas-liquid interface is strengthened as well. The Marangoni effects caused by the surface tension gradient are more significant at lower microgravity than higher one. The bubble growth under microgravity is more complicated than that of terrestrial condition. At microgravity conditions, lower microgravity will result in a higher bubble growth rate. When the microgravity smaller enough(at 0.00005 g), the bubble diameter is very similar to that of 1 g(slightly larger than that of 1 g).The study also found that in the process of pool boiling under microgravity, with the increase of system pressure, the diameter of bubble decreases; With the increase of bulk flow subcooling degree, the diameter of bubble decreases. With the increase of system pressure, the wall heat transfer coefficient is reduced, and the fluctuation amplitude decreases.With the increase of bulk flow subcooling degree, the wall heat transfer coefficient is reduced, and the amplitude of fluctuation is reduced.The study of the influence of gravity on growth process of hydrogen bubble of photo-biohydrogen production found that using Fluent software to obtain the initial concentration field of substrate solution through a porous particle immobilized photosynthetic bacteria-cell for photo-biohydrogen production is feasible. In terrestrial condition, the location of hydrogen bubble along the horizontal direction and vertical direction are changing greatly during the growth process. In microgravity condition, the location of the hydrogen bubble has some changes taken place in the horizontal direction, and change in vertical direction is not obvious. The study also found that gravity has great influence on the hydrogen components of porous particle and channel, velocity field around hydrogen bubble and bulk solution. With the decrease of the gravity, the quality of the source term of hydrogen bubble interface is more and more uniform.