On the numerical simulation of flow nucleate boiling using the volume of fluid method

The numerical simulation of flow boiling in a two-dimensional channel using the Volume of Fluid (VOF) method is investigated. The simulations were performed for a 500 x 38.1 mm two-dimensional channel with a constant temperature boundary condition at the heated walls. The inlet and outlet boundary conditions were set as mass flow and pressure outlet, respectively. The mass source terms due to phase change is incorporated via a User Defined Function (UDF) using the model of Lee [18] and the turbulence is modeled using the SST k-w model. The effect of the solution methodology, time step of the simulation and the coefficient in the mass source term on the bubble and boiling dynamics is investigated. The mass source coefficient was changed from 0.1s^-1 to 10s^-1 and the simulations were performed for time steps in the range 0.00001s to 0.01s. The results show that the implicit method is not suitable to simulate bubble generation, growth and departure from the wall. The explicit method with geo-construct, however, can capture the bubble growth and departure and is more suitable to simulate nucleate flow boiling in channels. For time steps above 0.01s, the bubble generation cannot be tracked and the simulations diverge. The solution is sensitive to the coefficient in the mass source term, with the bubble diameter increasing with the phase change coefficient. When the coefficient is greater than 5s^-1, the vapor extends along the wall and may not be physically correct. The heat transfer coefficient decreases with the phase change coefficient. The highest temperature is found to be at the liquid-vapor interface and not in the central region of the vapor bubbles.