Characteristic Research Of Steam Condensation In A Horizontal Tube With High Vacuum And Low Mass Flow Rate

With the development of economy, the demand for fresh water has increased, the shortage of water resources has became serious. The "Twelfth Five-Year National Plan for Science and Technology’ requests to accelerate the development of the desalination technology, especially low temperature multi-effect distillation (MED) desalination technology. Condensation inside a horizontal pipe is an important thermal process in low temperature MED desalination. The condensation phenomenon inside a horizontal pipe in MED has some special characteristics:low temperature difference, low mass flow rate, high requirements for flow resistance, and etc. Studying the condensation inside a horizontal pipe and improving the efficiency of heat exchanger, can provide technical basis and theoretical basis for the design of seawater desalination equipment with intellectual property rights.Condensation in a horizontal pipe widely exists in people’s life. Studying the condensation heat transfer in horizontal pipes is of great significance to improve the performance of heat exchangers. In order to study the condensation inside a horizontal tube, experimental studies and theoretical analysis of flow patterns, heat transfer coefficient, and pressure drop have been carried out for condensation inside horizontal tubes. The main contents are as following:(1) The test bench of the steam condensation in a horizontal pipe has been set up. The experimental studies have been taken off with the inlet mass flow rate of0to8kg/(m2·s) and the steam saturation temperature of45to70℃.(2) Based on the observation experiments of the flow pattern, the influences of the mass flow rate, vapor quality and saturation temperature on the liquid level angle have been discussed; the calculation model of void fraction and liquid level angle have been discussed too, and the empirical formulae have been given out.(3) Based on the experimental studies of heat transfer coefficient inside a horizontal pipe, the influences of stratified-flow on the heat transfer have been analyzed, the influences of temperature difference, saturation temperature, and mass flow rate on heat transfer process have been discussed, the relationship between the heat transfer coefficient distribution along the circumferential direction and the liquid level angle has been analyzed, the reasons that the condensation heat transfer coefficient to decreases along the tube length have been explored, and the heat transfer coefficient of the condensate side and the steam side have been analyzed.(4) Based on the experimental studies of pressure drop inside the pipe, the influences of the inlet mass flow rate, the Reynolds number of the steam, the saturation temperature and the temperature difference on the friction coefficient have been discussed, the relationship between the interface surface roughness and the shear force of gas-liquid has been discussed.(5) Based on the experimental studies of flow pattern, heat transfer coefficient and flow resistance, the influences of inlet mass flow rate, temperature difference and saturated temperature on the heat transfer of single tube have been discussed. The optimal vapor quality has been analyzed too.(6) Under the bipolar coordinate system, low Reynolds number model has been used to calculate the liquid level, the pressure drop and the velocity field distribution inside the horizontal tube, and the influences of the physical properties of the gas-phase and the liquid-phase on liquid level and flow resistance have been discussed.