| Under the background of energy shortage in the world today,it is of great important to seek an energy saving and high-efficiency energy utilization equipment.After years of development and research,falling film evaporation technology of horizontal tube has been proved to have the advantages of high heat transfer efficiency,energy and water saving,easy to realize multi-effect operation and so on.And it has a broad development prospect.However,the process of falling film flow and heat transfer outside horizontal tube is a complex two-phase flow and heat and mass transfer process which is affected by many factors.On the one hand,both the inner and outer sides of the horizontal tube undergo phase transformation.On the other hand,the heat transfer performance is affected by flow or geometric parameters such as spray water flow,tube diameter,tube shape and temperature.The falling film process becomes more complex when air flow is introduced.Therefore,the in-depth study of falling film flow pattern,liquid film thickness,velocity distribution and heat transfer performance under airflow is of great academic value and engineering significance for the design and application of horizontal tube falling film evaporation equipment.On the basis of literature investigation,this paper determines the research direction and content.Two horizontal tubes arranged in vertical direction were taken as the research objects.And a three-dimensional mathematical physical model was established to study the falling film flow and evaporative heat transfer process under the action of current.At the same time,the influence laws of airflow velocity,spray flow rate and tube spacing on the falling film process were analyzed.The flow pattern,film thickness and velocity distribution of falling film outside horizontal tube were simulated and analyzed.The results showed that the shear force at the gas-liquid interface caused by the counter current airflow hinders the flow of the falling film,and the airflow at different velocities will make the liquid film spread out of the horizontal tube appear different degrees of lag.Under the condition of constant Reynolds number,the overall thickness of the liquid film becomes thicker along with counter flow velocity increases.The velocity of the liquid film along the circumferential direction decreases as the airflow speed increases,and the velocity fluctuates obviously in the bottom wake region.For the axial distribution,due to the different properties of liquid film flow,the liquid film velocity increases as the airflow velocity increases when θ=0°,while the liquid film velocity decreases with the increase of airflow velocity when θ=90°.Under the condition of constant airflow velocity,the thickness and velocity of the liquid film increase along with Reynolds number increases.With the increase of tube spacing,the liquid film thickness decreases in the range of θ > 60°,and the film thickness at other locations is almost not affected by tube spacing.The evaporative heat transfer performance of falling film under airflow is simulated and analyzed.The results showed that along the circumferential direction,the wall temperature of the heat exchange tube increases rapidly at first,then slowly increases and finally fluctuates.The wall temperature difference at the same section of the heat exchanger tube increases at first and then decreases with the increase of counter flow velocity.The local heat transfer coefficient is closely related to the liquid film thickness.Along the circumferential direction,both of them decrease rapidly at first and then decrease slowly.After θ = 160°,the local heat transfer coefficient decreases rapidly while the liquid film thickness increases.Countercurrent airflow can aggravate heat transfer.At lower airflow velocity,the heat transfer effect is obviously weakened,while at higher airflow velocity,the heat transfer coefficient is slightly increased but the change range is very small.Under the condition of constant airflow velocity,the local heat transfer coefficient increases with the increase of Re number,and the heat transfer effect of the upper tube is better than that of the lower tube. |
