Experimental Study On Heat Transfer Characteristics Of Falling Film Evaporation On Horizontal Tubes With Different Tube Diameters

Under the environment of fresh water shortage,horizontal tube falling film evaporation,as the core technology of low temperature multi-effect seawater desalination devices,has attracted many scholars to carry out research with advantages of small heat transfer temperature difference and excellent heat transfer performance.As there are many factors affecting the heat transfer process of horizontal tube falling film evaporation and scholars’ research conditions and conclusions differ obviously,there is a lack of unified research within the scope of practical application conditions and comparisons of heat transfer characteristics of horizontal tubes with different tube diameters commonly used in engineering at present.In addition,there is a lack of heat transfer correlations that can guide the actual operation and design of low temperature multi-effect desalination plants.Therefore,further research is carried out on heat transfer of horizontal tube falling film evaporation in this paper to address the shortcomings of current research.An experimental platform for heat transfer of falling film evaporation on horizontal tubes with variable diameters was set up,32 mm tube diameter and 38 mm tube diameter HAL77-2aluminum and brass tubes were taken as research objects,and the local wall temperature was measured at different angles along the circumference of the tube by arranging thermocouples on the surface of the experimental tube and then rotating the tube around the central axis.The distribution of the local heat transfer coefficient along the circumference of the tube and the average heat transfer coefficient outside the tube were acquired by calculating,the influence of spray density,evaporation temperature,tube diameter,tube spacing and the salinity of working mass on the heat transfer coefficient were revealed by combining the experimental data of horizontal tubes with 19 mm tube diameter and 25 mm tube diameter as well.At the same time,for freshwater and seawater in different conditions of tube diameters,falling film evaporation heat transfer correlations which are more suitable for engineering design and application were proposed.The conclusions of this study are as follows:(1)Within experimental flow range,the average heat transfer coefficient increases continuously first as spray density increases and then tends to level off or decrease slightly when spray density exceeds the critical value;the increase of tube diameter enhances the critical spray density value.As evaporation temperature increases from 50 ℃ to 70 ℃,the average heat transfer coefficient increases for freshwater and decreases for 3% salinity seawater.As tube spacing increases,the average heat transfer coefficient tends to increase;the gain in average heat transfer coefficient for freshwater is greater than that for seawater by the increase of tube spacing.In same experimental conditions,the larger the tube diameter,the smaller the average heat transfer coefficient;the average heat transfer coefficient of 3% salinity seawater is smaller than that of freshwater.(2)Along the circumference of the heat transfer tube,the local heat transfer coefficient has a maximum value near 0 °,then decreases and has a minimum value near 135 °,after 135 ° the local heat transfer coefficient tends to level off or increase slightly.With the change of spray density,evaporation temperature,tube diameter,tube spacing and the salinity of working material,the variation law of the local heat transfer coefficient is similar to the average heat transfer coefficient.The effect on the local heat transfer coefficient caused by the change of tube spacing and the salinity of working material is mainly reflected in 0 °-135 °.(3)Heat transfer correlations of falling film evaporation are fitted by multiple linear regression analysis for horizontal tubes with 19 mm,25 mm,32 mm and 38 mm tube diameters respectively.For horizontal tubes with 19 mm and 25 mm tube diameters,the calculating correlations for the Nusselt number include variables of constant factor,Reynolds number and Prandtl number for different tube spacing to tube diameter ratio conditions when working materials are fresh water and seawater;for horizontal tubes with 32 mm and 38 mm tube diameters,the calculating correlations for the Nusselt number include variables of constant factor,Reynolds number,Prandtl number and tube spacing to tube diameter ratio when working materials are fresh water and seawater.