Study On Oscillating Flow And Heat Transfer Characteristics Of Gas-liquid Two-phase Flow

As one of the classic problems of fluid heat and mass transfer, the oscillating flow and heat transfer phenomena of gas- liquid two-phase flow relates to many engineering problems such as defense and vehicle engineering, and it has great theoretical significance and engineering application background. The oscillating flow process of two-phase flow is a multi-dimensional, multi-phase, and unsteady transient movement, at the same time, because of its higher convective heat transfer coefficient, it can also strengthen heat transfer effect. In order to deepen the understanding of oscillating flow and heat transfer characteristics of two-phase flow, and to reveal its oscillating heat transfer mechanism, the author carried out the studies of experiment and numerical calculation model for two-phase flow oscillating flow and heat transfer process. The main study conclusions are as follows:The three turbulence models of RNG k-ε、Realizable k-ε and SST k-ω were adopted to simulate the two-phase flow oscillating flow and heat transfer process in cylindrical cavity and engine-oil oscillating cooling process in piston cooling gallery. The key points focused on two-phase flow states in each of key positions, the variations of surface heat transfer coefficient with crank angle, and the variations of surface heat transfer coefficient w ith speed, after the comparison of simulation results and experimental results, it was found that the calculating accuracy of SST k-ω model is the highest, but with the increase of Reynolds number, the calculating accuracy would decrease. The study was carried out for multi-phase flow model of two-phase flow oscillating flow and heat transfer process, the Level-Set function was introduced to VOF model to form CLSVOF coupling model, the conclusion was drawn through the study: CLSVOF model could effectively simulate the transient distribution and dynamic variation of two-phase flow, the error amplification of heat transfer coefficient calculation value was about 0.6%, it was obvious that the calculating accuracy of C LSVOF model was significantly higher than VOF model.The oscillating experimental platform for oscillating flow visualization experiment and temperature measurement experiment at the same time was designed and finally established, the heating device for moving part and the experimental piece were also developed. Through the oscillating flow visualization experiment, the influence laws of Reynolds number and filling rate with oscillating flow pattern of two-phase flow were obtained, and it was found that Reynolds number mainly affected the bubble state and turbulence intensity in two-phase flow, while the filling rate mainly affected the flow form and movement rule of two-phase flow. Through the temperature measurement experiment, the variation laws of cavity surface temperature and the surface recycling average heat transfer coefficient with Reynolds number and fill ing rate were achieved, an experimental correlation of rectangular cavity oscillating heat transfer was obtained on the basis of the experimental data in a fitting method, this experimental correlation considered the effects of speed, filling rate and physical parameter of fluid for oscillating heat transfer, so it had a more accurate prediction.The analysis and study for the mechanism of oscillating flow and heat transfer of gas- liquid two-phase flow were carried out with the combination of numerical simulation and experimental study; it was found that the increase o f Reynolds number would lead to the change of vorticity field, on both sides of the interface between gas phase and liquid phase, the velocity gradients of two phases were opposite and very close in size, the gradient changed a little; with the increasing of Reynolds number, the air percentage in liquid raised accordingly, but the bubble sizes decreased, the turbulence pulsation of flow field enhanced, all of which illustrated that Reynolds number was the fundamental factor that influenced the stability of fluid structure. In the range of Reynolds number studied in this paper, the effect of two-phase flow oscillating heat transfer enhancement was dramatically determined by liquid filling rate, while the Reynolds number mainly affected the stability of flow field.