Measurement And Analysis Of Temperature And Humidity Field In The Air Transferring Heat And Mass With Water Using Dual-wavelength Digital Holographic Interferometry

The core problem of improving the efficiency of evaporative cooling,material drying,and seawater desalination is to understand the mechanism of heat and mass transfer of water evaporating in the air.With the physical model of heat and mass transfer process between the hot water falling film on the solid wall and the air,the T-H(representing temperature and humidity)distribution in the air adjacent to the air-water interface are measured by dual-wavelength digital holographic interference.This paper studied and analyzed the T-H distribution,the relationship between boundary layer thickness,comprehensive heat and mass transfer coefficient and the experimental controled variables.In this paper,two experimental devices for researching water film evaporation by holographic interference are developed.One device is mainly composed of an evaporating channel and a contrast channel,which are completely symmetrical.Some are mainly used to observe and compare the T-H field differences between the horizontal evaporating channel and contrast channel,and other is used to observe and compare the T-H field between the vertical evaporating channel and contrast channel.In the evaporation channel,the water film evaporates in the air and forms convection with the air;in the contrast channel,the air flows across the flat plate heated by the water film,and the water film and air are completely separated by the plate.First,through the direct observation of optical real-time holographic interference,it is found that the interference fringes caused by the change of air temperature and humidity in the horizontal evaporating channel and the contrast channel are obviously different.It is also found that the change of the water film temperature and the flow of air in the channel with evaporation could cause notably change to the interference fringes,confirming that the influence of evaporation and parameters of water and air on the T-H distribution is measurable.Then the dual-wavelength digital holographic interference with 532 nm and671nm laser was used to record the air T-H field in the vertical evaporating channel and the contrast channel,and the optical phase distribution of the two wavelengths caused by the change of T-H field was reconstructed from the hologram.During the experiment,the stability of the standard wavefront and the measured wavefront under each working condition was analyzed by the information entropy of the phase difference,so the experimental working condition was proofed steady.It was found that the airflow could not cauase fringe change without heat and mass transfer.Based on the improved Edlén formula,combined with the relationship between the steam partial pressure,humidity ratio and relative humidity in wet air,the Edlén formula with the parameters of air temperature and humidity is derived for inverting T-H field.With the phase differences of two wavelengths,a set of equations about T-H distribution is established.Comparing the errors of the linear solution and the partial solution of the equations,the linear method is selected to solve the temperature distribution,while the partial method is used to solve the humidity distribution.Additionally,the PSO(particle swarm optimization)algorithm is introduced to reduce the measurement error.Finally,the two-dimensional distribution of temperature and humidity in the experimental device was obtained.In order to establish the relationship between the T-H distribution and the experimental parameters,the two-dimensional fitting and correlation fitting were applied.The two-dimensional fitting factors,two-dimensional fitting weights,correlation fitting factors and correlation fitting weights were obtained and the purpose of getting the twodimensional T-H distribution by certain parameters within the experimental scope is achived.Based on the perspective of the relationship between the correlation fitting factors and the correlation fitting weights,the three factors that have the greatest impact on the T-H field are: water film inlet temperature,inlet air humidity and inlet air temperature,indicating that the inlet approach is the most important factor affectting the water film evaporation.On the basis of the T-H fitting results,this paper analyzed the T-H boundary layer thickness and the comprehensive heat and mass transfer coefficient in the evaporating channel and the contrast channel,and calculated the cooling number and mass transfer coefficient of the experimental device.In the experimental scope,the boundary layer thicknesses of temperature and humidity are about 1.5mm?4mm.Within the boundary layer,the T-H field have a large distribution gradient.In the case of large inlet approach,the thickness of the temperature boundary layer remains basically constant discard the change of inlet approach and wind speed.In the case of high wind speed,the thickness of the humidity boundary layer remains basically constant discard the change of inlet approach and wind speed.Comparing the evaporating channel and the contrast channel,the thickness of the T-H boundary layer in the evaporating channel is fluctuated smaller along the water film.When the inlet approach and wind speed increase,the comprehensive heat and mass transfer coefficient also increases,and it has a linear relationship with the inlet approach.The air-water ratio is a very important parameter indicated by the calculated cooling number and the mass transfer coefficient based on the evaporation area.When designing different types of evaporative cooling equipments,it is necessary to ensure that they are in the optimal range,so as to obtain a higher cooling number and improve the operating efficiency of equipments.