Study Of Steam-liquid Stratified Flow Condensation Heat Transfer Process In Vacuum Horizontal Tube

Two-phase flow and steam condensation in horizontal tube is widely used in desalination,air conditioning,power plant air cooler and other fields.It is of important scientific significance and practical value to study the process of the steam-liquid two-phase flow and condensation heat transfer in horizontal tube.In the evaporator of low temperature multi-effect evaporation desalination,the steam condensation process in horizontal tube has the characteristics of high vacuum,low mass flow rate and small heat transfer temperature difference,little research has been done on this process.Experimental and numerical study of small temperature difference condensation heat transfer process when the mass flow rate is 1～10 kg/(m2·s)and steam saturation temperature is 50～75 ℃ has been carried out in this paper.An experimental system of steam condensation flow in vacuum horizontal tube was built up to investigate the effects of steam mass flow rate,saturation temperature,heat transfer temperature difference,vapor quality and non-condensable gas mass fraction on steam-liquid flow and heat transfer process.The flow pattern in the tube is stratified flow under experimental conditions.The Biberg’s correlation and Rouhani’s void fraction correlation are used together to predict the experimental value of wetted angle accurately.The tube wall is divided into filmwise condensation section and liquid accumulation section by the stratified interface.Based on the experimental data,new correlations for pressure drop per tube length and local condensation heat transfer coefficient were proposed.The correlation for liquid accumulation section reflects the effect of the liquid level height and tube wall location on local condensation heat transfer coefficient.The distributed parameter model of flow and heat transfer inside and outside the horizontal tube in desalination evaporator was proposed.The calculation of steam stratified flow condensation in the horizontal tube and seawater falling film evaporation out the horizontal tube was carried out.The effects of steam mass flow rate,saturation temperature and heat transfer temperature difference on the circumferential and axial distributions of tube wall temperature,the heat transfer temperature difference inside and outside the tube,the heat transfer coefficients inside and outside the tube,the heat flux and total heat transfer coefficient were analyzed.The proportion of heat transfer rate in liquid accumulation section in the total cross-section heat transfer could reach more than 10%,which can not be ignored.The total heat transfer rate and effective heat transfer length increase first and then decrease with the increase of steam mass flow rate.The small temperature difference condensation heat transfer process could be divided into three sections:high efficiency heat transfer section,insufficient heat transfer temperature difference section and insufficient mass flow rate section.The steam mass flow rate,heat transfer temperature difference and heat transfer tube length are matched in the high efficiency heat transfer section,so the heat transfer area is fully utilized and the total heat transfer rate is large.The effects of saturation temperature and heat transfer tube length on the three sections are studied,and this provides specific control indicators for design and optimization of condensation heat transfer process in desalination evaporator.Based on the mixed steam condensation model proposed by Caruso and Maio,a theoretical calculation model is established for the condensation heat transfer of mixed steam with non-condensable gas in a vacuum horizontal tube.Combining the calculation model with the distributed parameter model,the condensation heat transfer process of mixed steam with non-condensable gas is calculated.The effects of mass flow rate,total heat transfer temperature difference,saturation temperature and inlet non-condensable gas mass fraction on the saturation temperature depressions caused by flow resistance and non-condensable gas mass fraction respectively and the total saturation temperature depression were studied.The steam mass flow rate corresponding to the maximum heat transfer rate of the mixed steam increases with the total temperature difference and decreases with the non-condensable gas mass fraction.