Numerical Simulation Of Falling Film Evaporation Flow And Heat Transfer Characteristics In Multi Row Of Horizontal Tubes

Horizontal tube falling film evaporation technology has been widely applied in engineering field due to its outstanding advantages.The flow and heat transfer characteristics of liquid film outside the tube are the most important research contents in the technology.In view of the shortcomings of the existing research results,a three-dimensional model of falling film evaporation in multi row tubes is established in this paper.Using the fixed wall temperature condition and the VOF method,the numerical simulation is carried out.The flow and heat transfer characteristics of the liquid film outside the multi row tube are obtained,and the different parameters(the spray density,the height of the cloth and the structure of the tube)are further investigated.The response rule of flow and heat transfer characteristic parameters under the change of feed temperature and wall temperature was studied.The main contents and conclusions of this paper are as follows:(1)A three-dimensional model of falling film evaporation in three tube rows is established.The effect of steam on the flow of liquid film is fully considered.The reliability of the calculation method and model is completed,and the distribution of the flow and heat transfer characteristics outside the tube is obtained.The results show that:(1)The distribution of liquid film thickness outside the tube shows significant fluctuation,and the minimum value of liquid film thickness appears in the range of 100°-110°.(2)The larger the number of tube rows,the smaller the thickness o f liquid film and the greater the liquid velocity.(3)There will be steam swirl between tubes,and the swirling speed will gradually decrease with the increase of tube row numbers.(4)The heat flux and heat transfer difference decrease with the increase of tube row number,but the heat transfer coefficient distribution hardly changes.(5)The development of thermal boundary layer accelerates with the increase of tube row number.(2)The influence of different factors on liquid film flow is studied.The results show that:(1)The increase of spray density will significantly increase the thickness of liquid film and the liquid film velocity.When the spray density is too small(Re=450)or too large(Re=1350),the liquid film thickness will be too small and the flow velocity too large,which will lead to the separation of liquid film from the tube wall.(2)The increase of the spray height will lead to the increase of the liquid film velocity and the decrease of the thickness outside the first row tube,but not to the liquid film flow outside the two or three row tube.(3)The increase of tube spacing will increase the flow velocity and reduce the thickness of the liquid film when the spray height is constant.The liquid will be stacked between the tubes when the tube spacing is small,and the liquid film velocity will increase rapidly when the tube spacing is large,which both will lead to the separation of liquid film at the bottom of the third row.(4)When the feeding water is unsaturated,the liquid film flow will be stable.There will be obvious fluctuation at the bottom of the third row and even the liquid film will fall off when it is saturated,and the liquid film fluctuation is more intense with greater heat transfer temperature difference.(3)The influence of different factors on heat transfer characteristics is studied.The results show that:(1)The heat flux and heat transfer temperature difference increase with the increase of spray density,and the heat transfer coefficient will also increase slightly.(2)The heat flux density increases significantly with the increase of theoretical heat transfer temperature difference and it has nothing to do with the feeding status.The distribution of heat transfer coefficient is more unstable when the feeding water is saturated.(3)The increase of tube spacing will significantly lead to the increase of heat flux and heat transfer coefficient outside the tube.(4)The liquid flow among tubes is more complex when the tubes are staggered,and the heat transfer in the local area can reach relatively high value,but the dry-out area will be large at the same time.The liquid flow outside the tube is relatively stable when the tubes are in line,and the heat flux outside the tube decreases steadily with the increase of the tube row number.The average heat flux outside the tube is higher under in line arrangement than that of staggered tubes.