| Liquid film flow on solid surfaces is a common phenomenon in life,such as rainwater flow on glass windows and film flow in externally circular tubes,which are driven by gravity,surface tension and electromagnetic forces.The problem involves various fields such as fluid dynamics,interface science,heat and mass transfer.In this paper,liquid film（falling film）flow and heat transfer driven by gravity are studied.Numerical simulation is an important method to study the heat transfer problem of the falling film flow and plays an important role in revealing the physical mechanism.Surface tension models have a significant impact on the numerical simulation results of heat transfer in falling film flow.Currently,the Continuum surface force model（CSF）is commonly used in numerical studies in the literature to calculate surface tension,and the applicability of other surface tension models has not been investigated and lacks a mechanistic explanation.Therefore,it is instructive to investigate the influence of surface tension models on the numerical calculation results of the falling film flow for future research in this field.In this paper,the falling film flow and heat transfer characteristics of smooth vertical flat plate,horizontal circular tube,horizontal elliptical tube,and horizontal circular tube pulsating falling film flow under different surface tension models are investigated.The main findings of the study are as follows:（1）On a smooth vertical flat plate,the gravity-driven thin liquid film flow is affected by surface tension,and if the surface tension is neglected,the liquid film tends to splash rather than flow continuously and steadily.In the entrance section,both models can obtain a free spreading complete liquid film,but the liquid film flow under the CSF model is slightly faster,and the liquid film layer shows regular fluctuations with high frequency and low amplitude.In the development section,the liquid film under both models gradually changed from smooth surface to fluctuating surface and showed signs of isolated waves,but no stable isolated waves were formed,and the morphology of the liquid film under both models began to differ greatly,with irregular fluctuations of low frequency and high amplitude in the liquid film layer.In the stable section,the liquid film under both models showed a sinusoidal shape with a steep front end and a prolonged tail.The morphology of the liquid film under the two models is relatively similar.The liquid film layer has complex fluctuations with higher amplitude at lower frequencies and small waves superimposed on large waves.（2）In horizontal circular tube falling film flow and heat transfer.Surface tension has an important effect on the gravity-driven thin liquid film flow on the horizontal tube,it can limit the diffusion of the liquid film and promote the deformation of the liquid film surface,if the surface tension is neglected,the liquid film will splash rather than flow steadily and continuously.Near the upper and lower stagnation points of the liquid film,both CSF and Continuum surface stress（CSS）models were able to simulate similar liquid film profiles,while the absence of surface tension would make the liquid film severely distorted.Both CSF and CSS models were able to reflect the flow characteristics of the horizontal tube falling film,including suspended drops,column necking or necking break,film thickness,vortices,and liquid film separation,and velocity components;there is no significant difference between the time-averaged film thickness on the horizontal pipe under CSF and CSS models.In terms of heat transfer characteristics,it is found that the local heat transfer coefficient increases with the increase of liquid film flow,and the effect is more significant on the upper part of the pipe.In addition,the calculation ignoring the surface tension will overestimate the heat transfer coefficient,especially when the liquid film flow is minimal.Both CSF and CSS models can better calculate the heat transfer in falling film flow,especially at high liquid film flow rates(Г=0.284 kg·m-1·s-1).（3）In horizontal elliptical tube falling film flow and heat transfer.The liquid film thickness gradually becomes thinner from the inlet,and the absence of surface tension will make the liquid film thickness larger.The liquid film thickness calculated by CSF and CSS models have some differences,but both can reflect the influence of surface tension on the falling film flow process.When the ratio of long and short axis of elliptical tube increases,the liquid film stays in the upper and lower stagnation zone for a shorter time,which leads to a shorter liquid film accumulation time and a reduced liquid film thickness,thus making the local heat transfer coefficient increases,especially nearθ=20-180°.Both CSF and CSS models can reflect the wall heat transfer of elliptical tube,but there are some differences.（4）In pulsating falling film flow and heat transfer in a horizontal circular tube.Neglecting the surface tension leads to a reduction of the liquid film thickness over the whole circumference,especially near the upper hysteresis region.Both the CSF and CSS models reflect the liquid film flow characteristics under pulsating falling film flow outside the tube when the liquid film flow rate is small.With the increase of the liquid film flow rate,the calculated liquid film thickness values of pulsating and steady falling film flow under CSF model are in good agreement,while the liquid film thickness values under steady falling film flow are high in the case of CSS model.In terms of falling film heat transfer characteristics,the calculated results of CSF model are better than those of CSS model at low liquid film flow rate,but there is no difference between them at higher liquid film flow rate. |
