| The horizontal falling film evaporator has the advantages of compact structure,low refrigerant charge,low cost and high heat transfer efficiency.Therefore,it is widely used in refrigeration,air conditioning and seawater desalination.The main heat transfer principle of it is as follows: falling film evaporation heat transfer outside the tube and steam condensation heat transfer inside the tube.In horizontal tube falling-film evaporator,the flow state and uniformity of liquid film outside the tube directly affect the performance of heat exchanger.In operation,with the increase of the amount of condensate,the dryness of condensation heat transfer in the horizontal falling film tube decreases,resulting in the reduction of condensation heat transfer effect.In addition,the accumulation of condensate leads to the reduction of condensation heat transfer efficiency.Inclined heat exchange tube can effectively solve the problems of slow flow rate of liquid film outside the tube and condensate accumulation in the tube.Therefore,this paper mainly studies the flow state and thickness uniformity of the liquid film outside the inclined falling film tube,so as to provide theoretical guidance for improving the effect of falling film evaporation.Firstly,the measurement method of liquid film thickness of outer tube using laser-induced fluorescence technology was explored.The influence rules of test parameters such as fluorescence agent concentration and laser intensity were studied.The optimal laser power intensity was proposed for measuring liquid film thickness of outer tube.By comparing the test results,it is found that the error is within 8%.It shows that laser induced fluorescence technique is feasible to measure liquid film thickness outside tube.Then,on the basis of theoretical analysis,a test platform for the liquid film thickness of falling film heat exchange tube was built.The liquid film flow and distribution characteristics in inclined tube were studied experimentally.Through image digital processing technology,the distribution characteristics of the influence of spray Reynolds number and tilt angle on the liquid film thickness outside the different tube type are studied.The research results are as follows:（1）The influence of tube type:Under the same inclination angle,the tube type has an effect on the spreading speed of liquid film.Under the same working condition,the time required to complete a complete liquid film spreading is the minimum for semi-elliptical tube and the maximum for circular tube;The flow and distribution of the liquid film in the wake region of the circular tube are inferior to that of the elliptical tube,and even obvious liquid column fracture phenomenon appears in the tail of the tube bundle.The liquid film spreading speed of the semi-elliptical tube is faster,and the diameter of the liquid column leaving the falling film tube is smaller.The tube type has little influence on the thickness distribution.The axial direction of the liquid film is stable at first,then increases,and then becomes stable again.The thickness of the liquid film at the peak is 2.45～4.14 times that of the liquid film at the stationary section.With the increase of flow rate,the greater the extrusion pressure of the fluid on both sides,the steeper the wave crest,and the greater the difference between the liquid film thickness at the wave crest and the stationary section.The larger the difference between the liquid film thickness at the wave peak and the stationary section,the smaller the liquid film thickness at the stationary section.The liquid film thickness decreases first and then increases along the circumferential angle.And the minimum values appear in the range of 90°-120°,160° and 170°,respectively.The average circumferential thickness of the liquid film is round > elliptic > semi-elliptic,and the distribution characteristics is not affected by the tilt Angle and spray flow rate.The average film thickness of elliptic tube and semi-elliptic tube is 4.53% and 6.11% lower than that of circular tube.（2）The influence of tilt angle : The peak value of the liquid film thickness outside the tube gradually decreases with the increase of the tilt angle.Meanwhile,the distribution of the water film is more uniform.With the increase of the tilt angle,the peak value of the liquid film thickness keeps moving downward,and the downward degree is semi-elliptic tube > elliptic tube > round tube.The liquid film coverage increases with the increase of Reynolds number,and circular tube > elliptic tube >semi-elliptic tube.The liquid film coverage increases first and then decreases with the increase of inclination angle.The coverage of circular tube is the largest when Φ = 10-15 °,the coverage of elliptic tube is the largest when Φ =15°,and the coverage of semi-elliptic tube is the largest when Φ =10°.With Re number increases,the liquid film thickness fluctuation variance of three type of pipe increases slightly,the variances of the circular tube is opposite bigger,oval tube and half elliptical tube,with the increase of angle,liquid membrane fluctuation variance,increase with the decrease of the first round and elliptical tube when the angle is 15 ° minimum variance,half elliptical tube in the angle of 10 °minimum variance;The variance of inclined tube is smaller than that of horizontal tube（3）The optimum inclination angle of circular tube and elliptical tube is 15 °,and that of semi elliptical tube is 10 °.Among all kinds of falling film evaporators,the inclined falling film liquid film of semi elliptical tube with an inclined angle of 10°is the most uniform and the thinnest.The research results of this paper can lay a theoretical foundation for the subsequent heat transfer analysis,design and development of inclined falling film tube. |
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