Experimental Study And Numerical Simulation Of Falling Film Flow Process Outside The Outer Corrugated Tube

Horizontal tube falling film evaporator has high heat transfer efficiency and can operate at low temperature difference.They are widely used in seawater desalination,food processing,petroleum refining and chemical engineering.The thickness and the velocity distribution of the liquid film outside the tube significantly affect the heat transfer coefficient of the process.Therefore,it is important to study the distribution characteristics of the liquid film outside the smooth tubes and the outer corrugated tubes under the cold condition.Experimental and numerical simulation were used to investigate the falling film outside the outer corrugated tubes.Firstly,the variation of the liquid film thickness and the velocity distribution characteristics with the liquid spray density and the tube spacing outside the falling film tube were experimentally studied.The results show that with the increasement of Reynolds number,the five flow patterns are presented in turn.They are droplet flow,droplet/column flow,column flow,column/sheet flow and sheet flow,respectively.The transition of the flow pattern between the outer corrugated tubes from droplet flow to column flow occurs at a smaller Reynolds number than the smooth tubes.The thickness of the liquid film outside the smooth tube decreased from the top to the bottom.The thinnest film thickness is between 90-120°.After the outer corrugated tubes removes the liquid in the inter-corrugated groove,the thickness and the circumferential distribution of the film thickness outside the corrugation are similar to the smooth tubes with the same diameter.Secondly,the experimental values are compared with the simulation results to verify the accuracy of the mathematical model.Finally,the effects of different contact angles,tube diameters and corrugation size on the distribution of liquid film thickness outside the falling film tube were simulated.The results show that with the increase of the contact angle,the liquid film outside the outer corrugated tubes can completely cover the entire tube wall,avoiding the phenomenon of “drywall”.the average liquid film thickness outside the corrugation in the circumferential direction increases with the increase of the liquid spray density,while it decreases with the distance increasing of tube spacing and the tube diameter.The uniformity of the distribution of the liquid film in the circumferential direction and the velocity are related to the liquid film thickness.The nonuniformity of the circumferential distribution of the corrugated liquid film increases with the increase of the liquid film thickness.The distribution law of the liquid velocity at the air-liquid interface in the circumferential direction is opposite to the distribution law of the liquid film thickness.The distribution of liquid velocity at the gas-liquid interface in the circumferential directionis opposite to the distribution law of liquid film thickness.Between the two adjacent peaks,the liquid in the groove has existing local circulation.With the increase of the corrugated diameter,the distribution of the liquid film outside the outer corrugated tubes is serrated and the fluctuation is gradually increased.When the radius of the corrugation increases,the circulation phenomenon at the trough is more pronounced.