| Frosting phenomenon of evaporator in winter is one of the main problems restricting the popularization and application of ASHP unit.Low freezing point solution spray-type evaporators show good performance and great application prospect in terms of realizing the frost-free continuous operation of ASHP unit,which has attracted extensive attention of many experts and scholars.In this paper,the characteristics of gas-liquid flow and heat-mass transfer on the air-side of spray-type finned-tube evaporator have been systematically investigated by using the methods of theoretical analysis,numerical simulation and experiment.The effects of operating parameters and spraying modes on the frost-free operation performance of the evaporator are analyzed.A visualized experimental system for gas-liquid(i.e.air-water)falling film flow on the surface of finned-tube was built.The liquid film distribution and flow pattern on the surface of finned-tube were photographed by using the high-speed camera system.The contact angle of fin surface was determined using a droplet shape analyzer.The effects of wall contact angle and gas-liquid flow parameters on the flow characteristics of liquid film on the surface of finned-tube were investigated.It is found that the liquid film on the surface of the fin below the tube is thicker than that on other positions,and the smaller the contact angle of the fin surface is,the more conducive to the formation of full film flow.For the fin with contact angle of 60°,when the liquid flow rate is less than or equal to 45 L/h,the liquid film is difficult to wet the whole finned-tube surface.When the liquid flow rate is more than or equal to 60 L/h,the liquid film wets basically the whole finned-tube surface.At the same time,when the liquid flow rate is more than 60 L/h,the drop flow appears below the pipe.When the liquid flow rate is equal to 100 L/h,the sheet flow appears below the pipe.This shows that the flow pattern of liquid film on the surface of finned-tube is more complex than that on the surface of vertical plate,indicating that the assumption of uniform distribution of liquid film is not suitable for the finned-tube surface.In addition,under the gas-liquid counter-/co-current conditions,when the gas phase flow rate and liquid phase flow rate increase to a certain range,the liquid flow into the test section is decreased significantly and the liquid film thickness is reduced due to the increase of gas-liquid interface friction force.Based on the VOF model,a numerical model of gas-liquid(i.e.air-water)falling film flow on the air-side of finned-tube heat exchanger with gas-liquid interface friction force as a momentum source term was established.The maximum deviation between the calculated average water film thickness and the experimental value in the literature is less than 5.0%,which proves that this model can be used for the theoretical analysis of such problems.The effects of wall contact angle,fin pitch and Reynolds number on the formation and flow characteristics of liquid film were studied using the numerical model,and the formation conditions of full film flow on the finned-tube surface was obtained.The results show that when the Reynolds number of liquid film is small,the wetting area decreases rapidly with increasing the contact angle.On the contrary,when the Reynolds number of liquid film is large,the wetting area decreases slowly and then decreases rapidly.For the fin with contact angle of 10°,when the fin pitch is in the range of 4.8?6.0 mm,it is easy to form full film flow.Based on the numerical results,the dimensionless prediction correlation of the average water film thickness on the surface of finned-tube is established.This average film thickness is 18.3%?32.5%higher than the Nusselt theoretical solution,so the correlation of the film thickness on the surface of flat plate is not suitable for the surface of finned-tube.The above correlation obtained in this paper is closer to the predicted value of the experimental correlation in the literature.Through the numerical calculation of the above model,it is found that under the gas-liquid counter-current condition,when the Reynolds number of liquid film is small,the air Reynolds number should not be greater than 1629.3,otherwise it is difficult to form full film flow on the finned-tube surface.Under the gas-liquid co-current flow condition,when the air Reynolds number is less than or equal to 1303.5,the gas-liquid interface friction force is small,and the change of interface fluctuation shape is small.When the air Reynolds number is greater than 1303.5,the gas-liquid interface friction force increases,and the interface wave pattern changes from independent rolling wave to small independent wave and capillary waves.Meanwhile,the liquid film thickness decreases obviously at high counter-/co-current air Reynolds number,which is consistent with the visualized experimental results in this paper.For gas-liquid countercurrent flow,there is a close cubic curve relationship between the average interface friction force and air Reynolds number,and a strong convex quadratic curve relationship exists between the average wall shear stress and air Reynolds number.For gas-liquid co-current flow,there is a close quadratic curve relationship between the average interface friction force and air Reynolds number and a strong concave quadratic curve relationship exists between the average wall shear stress and air Reynolds number.Three-dimensional model of gas-liquid(i.e.moist air-LiCl aqueous solution)falling film heat and mass transfer on the air-side of finned-tube evaporator was established based on two-film theory and permeation theory.The calculated value of moisture absorption by this model is in good agreement with the experimental value in the literature,which proves that the model can be used to analyze the characteristics of gas-liquid falling film absorption.A dimensionless correlation for predicting the average film thickness of solution on the finned-tube surface is established on the basis of the numerical results.The effects of inlet air temperature,inlet air humidity,and inlet solution parameters on the gas-liquid counter-current falling film heat and mass transfer characteristics were numerically investigated.The results show that at low inlet air temperature,for different air humidity and solution parameters conditons,when the heat and mass transfer between the small droplet surface and the gas phase formed in the spray process is not considered,the sensible heat transfer coefficient is much larger than the latent heat transfer coefficient,indicating that the mass transfer flux in the gas-liquid falling film absorption process is quite small at low ambient temperature.With the increase of solution concentration(30%?39%),the density,dynamic viscosity,surface tension and gas-liquid interface friction force of the solution liquid film increase on the surface of the finned-tube,resulting in that the wave amplitude of gas-liquid interface on the surface of finned-tube is increased significantly,the disturbance degree of gasliquid interface is strengthened,the total thermal resistance is decreased by about 10%,and the heat and mass transfer process near the gas-liquid interface is enhanced,so the surface Nu number is increased.The increase of solution flow strengthens obviously the interface disturbance degree,reduces the thermal resistance,and enchances the gasliquid heat and mass transfer process.Based on the above conclusions,it can be deduced that before the solution enters the falling film zone,the small droplets with surface area more than 10 times larger than the liquid film surface of the fin area are used to fully absorb heat and mass from air in the spray zone,which is expected to realize the efficient and low resistance operation of the spray-type evaporator.The effects of inlet air velocity on the gas-liquid falling flim heat and mass transfer characteristics on the finned-tube surface were studied numerically.The effect of air velocity on the total liquid holding capacity of liquid film on the surface of finned-tube was analyzed.The results show that as the air velocity increases from 0.2 m/s to 1.2 m/s,the total liquid holding capacity of liquid film on the surface of finned-tube first increases and then decreases,the fluctuation amplitude of liquid film first increases and then decreases,the disturbance degree of gas-liquid interface first increases and then decreases,and the total thermal resistance first decrease and then increase.When the air velocity equals to 0.6 m/s within the range of simulated working conditions,the total thermal resistance is the smallest,the interface disturbance intensity is maximum,the heat and mass transfer process near the gas-liquid interface is enhanced,and the average surface Nu number reaches the peak value(more than 990).Besides,the dimensionless prediction correlations of the average heat transfer coefficient,the average surface Nu number and the gas phase average pressure drop are established based on the extreme model and poly model.On the basis of theoretical research and analysis,a novel evaporator structure for frost-free ASHP was proposed,which integrates spray zone without packing,finnedtube falling film zone and horizontal smooth tube falling film zone.Combined with literature research and theoretical analysis,the main structural parameters of this evaporator were designed,and an experimental system for performance test of spraytype evaporator was designed and built.The effects of inlet air twmperature,inlet air flow rate and LiCl solution spraying modes on the evaporator energy efficiency ratio(i.e.the ratio of total heat transfer capacity to the sum of power consumption of fan and solution pump),heat transfer expansion coefficient(i.e.the ratio of total heat transfer capacity to sensible heat transfer capacity)and air pressure drop were studied experimentally.The results show that for the continuous spraying operation mode of LiCl solution with mass concentration of 36%,as the inlet air temperatures increase from minus 10 degrees Celsius to 15 degrees Celsius under different inlet air flow rates,the energy efficiency ratio of the evaporator increases by about 11%,and the heat transfer expansion coefficient decreases by about 3%?6%,indicating that compared with the numerical simulation of pure gas-liquid falling film heat and mass transfer at low ambient temperature,the latent heat transfer capacity accounts for a large proportion in the evaporator experiment due to the existence of small droplets in the spray zone.Under different inlet air temperatures,with the increase of inlet air flow rate(i.e.the fan frequency increases from 10 Hz to 50 Hz),the energy efficiency ratio increases by about 8%?12%,the heat transfer expansion coefficient increases by less than 1.1%.In addition,the effects of no spraying,intermittent spraying and continuous spraying operation modes on the evaporator energy efficiency ratio,heat transfer expansion coefficient,exergy loss and air pressure drop are compared and analyzed by experiments.It is found that the intermittent spraying mode is an economical and reliable scheme to realize frost-free,continuous,efficient and stable operation of ASHP evaporator in low temperature environment. |
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