| The phenomenon of condensation in tubes widely exists in the process of industrial heat exchange.The research shows that the condensation liquid film is the main thermal resistance in the process of condensation heat transfer.Therefore,it is of great significance to explore a fast and effective measurement method for improving the performance of heat transfer in tubes and promoting the effective utilization of energy.After analyzing and comparing the current mainstream liquid film measurement methods,laser interferometry is used to measure the liquid film thickness.Based on the principle of optical interference,the dynamic measurement of liquid film thickness is realized by designing and building an experimental system.By studying and analyzing the measured results,the distribution and variation characteristics of liquid film under the measured conditions are obtained.At the same time of the experimental measurement,ANSYS and other numerical simulation tools were used to simulate and supplement the experimental conditions to verify the correctness of the experimental measurement method.The specific work and conclusion of this paper are as follows:(1)Formulas are derived based on the interference principle of light,and calculation methods for the thickness of glass tube and condensate film are obtained.The derivation process is summarized and sorted out,and the corresponding program is written for iterative calculation.In order to realize multi-dimensional dynamic measurement,the existing experimental facilities are improved.The optical path layout is changed,and the electric lifting platform is added.The spectrometer,reflector,CCD camera and other equipment are placed on the lifting platform to achieve the scanning effect of the whole lifting.The improved measurement system can satisfy the implementation of two acquisition methods and improve the efficiency of interference fringe image acquisition.(2)The fringe morphology formed by different interference media is analyzed,and the fringe width is inversely proportional to the wall thickness and liquid film thickness.Based on the error analysis of measurement Angle,the reasonable range of measurement Angle should be less than 27.5°.In the process of steam flow,the liquid film has fluctuation phenomenon and sudden fracture.Through observation,it is found that the liquid film is in a state of constant fluctuation.The fluctuation state is more obvious when the time scale is increased.At the same time,the influence of contact Angle and surface tension on liquid film formation cannot be ignored.(3)By comparing the measured data of two fringe image acquisition methods(fixed point time averaging method and instantaneous scanning method),the advantages and disadvantages of these two methods are compared respectively on the time scale of 1 s and 0.1 s.The data measured by instantaneous scanning method is close to the same state,but the data has randomness and chance.The data obtained by fixed point time averaging method is the time average of a large number of data in a period of time,which can reflect the general rule,but cannot reflect the liquid film state at a certain time.Therefore,in order to reflect the general law,fixed point time average method can be selected;In order to explore the liquid film state at a certain moment,instantaneous scanning method is selected for image acquisition.(4)The experimental data measured under different working conditions were analyzed,and the influence law of pipe diameter,heating power and experimental medium on liquid film thickness was obtained.On the whole,with the increase of pipe diameter,liquid film thickness also increases.The thickness of liquid film increases first and then decreases with the increase of heating power.It is found that the liquid film produced by pure water is thicker than that of tap water.It can be seen that pure water without impurities is more conducive to the occurrence of condensation.(5)In order to verify the correctness of the experimental measurement method,the simulation software ANSYS is used for numerical simulation to supplement and improve the experimental conditions which are inconvenient to implement and measure.Through simulation,it is found that the flow patterns in the pipe all present annular flow,but the steam flow process is unstable and the liquid film will fluctuate,which is consistent with the experiment.(6)According to the numerical simulation results,it can be seen that the liquid film is thinner when the diameter ratio is small at the same steam velocity;With the increase of pipe diameter,the fluctuation of liquid film becomes more drastic.When the pipe diameter increases to a certain extent,the liquid film becomes smooth and its thickness also increases.Under the same diameter condition,when the flow rate is low,the liquid film is thicker,and the liquid film surface fluctuates slightly but not violently.With the increase of flow velocity,the condensation phenomenon is accelerated,and the fluctuation of liquid film surface is larger.At the same time,the liquid film accumulation and downward flow can be seen.When the steam velocity increases to a certain extent,the liquid film surface becomes stable and the thickness of the liquid film decreases.When other conditions remain unchanged,the tube diameter has an effect on the heat transfer coefficient.With the increase of the tube diameter,the liquid film thickness increases gradually,and the heat transfer coefficient decreases gradually.The thickness of liquid film decreases and the heat transfer coefficient increases with the increase of steam velocity.By comparing the simulation results with the experimental measurements,the conclusions are basically consistent,which can further verify the correctness and feasibility of the laser interferometry method. |
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