Critical Equipment Falling Film Evaporator Optimization Research Of Mechanical Vapor Recompression Heat Pump (MVRHP)

The operating principle of MVRHP was mathematically analyzed and indicated that by improve the falling film evaporator performance not only favor in decrease the temperature difference, but also can reduce the cost of the compressor selection; this paper pointed the advantages of using twisted oval tube as the vertical falling film evaporator tube fittings.The laminar falling film heat transfer process and Nu of twisted oval tube applied to turbulent falling film have been analyzed. It found that the main factor affected the laminar falling film thickness is liquid film flow rate. The best tube diameter being used on laminar falling film heat transfer is believed around35mm. Combined with heat transfer correlation of twisted oval tube turbulent flowing and Prandtl analogies, twisted oval tube turbulent falling film Nu was fitted. Contrasted with circle tube Nu fitted by predecessors, it can obtain turbulence at lower flow rate, the heat transfer performance is better than circle tube, while the advantage is decreasing accompanied by the ascent of Re.The film formation effect in twisted oval tube was numerically simulated by VOF method. The falling film was displayed spiral flow status. The film formation process was distinct in different velocity, it appears liquid splash at high flow rate. Compared with circle tube and oval tube, in twisted oval tube, the falling film is thinner, film appeared fluctuations in the transition section of the long axis and short axis, liquid film flow lines was spiral downward. Overall, excluding the impact of outlet boundary conditions, it appeared well film distribution in twisted oval tube in a certain velocity. The possibility of drywall phenomenon occurring is very small. It proved that twisted oval tube is suitable for falling film evaporation process.Tube-side heat transfer coefficient a of twisted oval tube and circle tube was compared at two conditions:different Re using water as flowing medium; different Pr while Re=8000, proved that twisted oval tube heat transfer performance is better than circle tube. A triangular arranged twisted oval tubes heat exchanger was used in an experiment to analyze the shell-side pressure drop performance. Result demonstrated that the twisted oval tube self-supporting structure pressure drop is significantly lower than ideal circle tube bundle. This feature can surely to reduce the shell volume, improve flow rate, thereby increasing the shell-side heat transfer coefficient and save material.