W.X. Tian

Numerical Solution on Spherical Vacuum Bubble Collapse Using MPS Method

Single vacuum bubble collapse in subcooled water has been simulated using the moving particle semi-implicit (MPS) method in the present study. The liquid is described using moving particles, and the bubble-liquid interface was set to be the vacuum pressure boundary without interfacial heat mass transfer. The topological shape of the vacuum bubble is determined according to the location of interfacial particles. The time dependent bubble diameter, interfacial velocity, and bubble collapse time were obtained within a wide parametric range. Comparison with Rayleigh’s prediction indicates a good consistency, which validates the applicability and accuracy of the MPS method. The potential void-induced water hammer pressure pulse was also evaluated, which is instructive for the cavitation erosion study. The present paper discovers fundamental characteristics of vacuum bubble hydrodynamics, and it is also instructive for further applications of the MPS method to complicated bubble dynamics.

Numerical Simulation on Collapse of Vapor Bubble Using Particle Method

In this paper, the collapse of a void bubble filled with vapor content is numerically investigated using a novel moving particle semi-implicit with meshless advection by flow-directional local grid (MPS-MAFL) method. The interfacial velocity, collapse time, bubble shape variation, peak pressure, rebound bubble radius, and other interesting parameters were obtained and are discussed profoundly. The vapor bubble undergoes several cycles of oscillation with reduced amplitude during the whole collapse process, which is similar to cavitation bubble collapse. The computational results show that the bubble collapse time is linearly proportional to the initial bubble size, which agrees with the Rayleigh equation. The minimum rebound bubble radius ratio is less affected by initial bubble size for a large bubble. Comparison work was also conducted against experimental data by Board and Kimpton. The comparison revealed that the MPS method supplied with an adiabatic compression assumption for vapor content is more suitable to evaluate the collapse behaviors of a low-pressure vapor bubble. This work is helpful for further application of the moving particle semi-implicit with meshless advection using flow-directional local grid (MPS-MAFL) method to solving complicated bubble dynamics.

Prediction of Post-Dryout Heat Transfer in Vertical Annular Channels Using Artificial Neural Network Method

Experimental investigations on post-dryout heat transfer in 10×8.1, 10×7 and 10×6mm annular test sections have been carried out under low-pressure and low mass flow rate conditions. An Artificial Neural Network (ANN) was trained successfully based on the experimental data for predicting the average post-dryout Nusselt number. Based on the ANN, the effects of gap size, pressure, steam Reynolds number, Reg , inlet quality, xi , Prandtl number, (Prg )W, and the ratio of heat flux of inner-tube to that of outer-tube, qi /qo , on post-dryout heat transfer were analyzed, respectively. In present study, Nusselt number in annular channels with big gap size is larger than that in annular channels with small gap size. Nusselt number increases significantly in 1.5mm and 2.0mm annular channels while it is almost constant in 0.95mm annular channel with increasing pressure or qi /qo . Nusselt number increases with Reg in case of 0.95mm and 1.5mm gap sizes. However, Nusselt number in 2.0mm annular channel firstly increases and then decreases with increasing Reg . Nusselt number decreases with increasing inlet quality under all three annular channels condition. Nusselt number decreases significantly with increasing (Prg )W when (Prg )W is less than 1.5. The changes of Nusselt number in 1.5mm or 2.0mm annular channels are larger than that in 0.95mm annular channel.Copyright