Experimental Study On Evaporation Process Of Typical Porous Building Materials And Determination Of Evaporation Calculation Model

In south China,solar radiation is strong and rich of precipetation.The method of sprinkling water on the wall and roof can effectively reduce the surface temperature,reduce building energy consumption and improve indoor thermal environment and building urban heat island effect.Because the previous research is basically aimed at the experimental study of cooling effect,the lack of research on the correlation between the water absorption property of the material itself and the cooling effect of evaporation,and the lack of calculation model of evaporation,it is impossible to use the energy consumption simulation analysis software to carry out comprehensive research on a long time scale.In this paper,the performance of the material was tested,the evaporation process of the material under intermittent sprinkler was studied,the influencing factors of the evaporation process were analyzed,and the time-by-time evaporation calculation model was established.First,the material performance parameters test and water absorption experiment are carried out.It is found that the key parameter that affects the water absorption rate is the pore size rather than the porosity.Under the premise of no large pores,the average equivalent pore size material has shorter water absorption time and more water absorption.Evaporation experiments under steady meteorological parameters show that radiation intensity is the main factor affecting total evaporation,and air temperature,relative humidity and wind speed have little influence.The intensity of solar radiation affects the temperature of the material,than determines the evaporation rate at the initial stage,the pore size of the material determines the evaporation rate in the middle and later stages.Through correlation analysis,it is found that the water content of has greater influence on the evaporation rate than temperature.Assuming that the evaporation process is carried out layer by layer,the corresponding depth of the material surface temperature during the period from stable change to sharp change is defined as the "effective evaporation depth"."Effective evaporation depth" in the material,which is related to the average pore size of the material,but has no relationship with the environmental parameters.The greater the effective evaporation depth,the slower the temperature rises,and the longer time evaporation cooling.The index of "effective evaporation depth" comprehensively reflects the water absorption and storage capacity of the material and the speed of moisture migration in the specimen under the influence of solar radiation absorption coefficient.The experiments under dynamic meteorological parameters show that material has larger average equivalent aperture size,the evaporation rate is easy to change.The change of solar radiation also affects evaporation.In a certain period of time,when the total radiation is constant,the solar radiation decreases from big to small,and the evaporation is larger than that of steady solar radiation.The experiment shows that the total day water evaporation takes about 50%~60% of the total solar radiation,and the average temperature of the upper surface can be reduced by about 13 degrees,the effect of evaporation cooling is obvious in the 2 hour interval water dripping.Monte Carlo stochastic simulation is used to analyze the phenomena in evaporation process.During the migration of water in porous media,the smaller the pore size is,the more obvious the effect of the impact of water on the medium,the greater the evaporation resistance is.The pore diameter of the dominant number will be "matched" or "unmatched" with the migration velocity of water in a certain temperature range."matched" will cause the evaporation speed to be accelerated,and the "unmatched" will cause the slow change of evaporation rate.In practice,evaporation will not increase linearly with the increase of radiation,but an inflection point.The throat has a great influence on the evaporation.A sub step method to establish the time by time evaporation of porous materials is proposed.The evaporation process is simplified,the temperature term is regarded as the influencing factor of the evaporation power,and the moisture content is used as the resistance factor.Based on the experimental data of the evaporation test under the steady state environment parameters,the evaporation model of the initial time is set up first,and then the time by time evaporation model is established according to the temperature and water content,the calculation model of the initial evaporation is corrected by the experimental data under the outdoor dynamic environment parameters,and the final evaporation amount model is established at the end.It is verified that the relative error rate of total evaporation is less than 3% throughout the whole day,the calculation model of evaporation is available.The research results laid the foundation for quantitative analysis of passive evaporative cooling of using energy analysis software.