Study On Heat Transfer Of Building Envelope Under Two Outdoor Periodic Waves

Over the past century, many professors have studied on heat transfer through building envelope which experienced steady-state calculation, unsteady-state calculation, unsteady-state calculation under periodic influence, dynamic load calculation and radiation time series method. The calculating method is more and more accurate, and the calculating contents are more and more detailed. It is notable that exept for improving heat transfer calculation methods, we should improve the accuracy of boundary conditions. Accurate outdoor meteorological parameters are necessary condition for the accurate calculation of building energy consumption and the accurate assessment of building thermal environment. The parameters provided by literature are either constant values or discrete values. The constant values cannot reflect the changes of weather in one day. The discrete values are used for numerical calculation input, the quantity of which is large, and you cannot analyze the variation of meteorological parameters. In this situation, after fitting the meteorological parameters'function in different cities and different weather, the auther studied the heat transfer of building envelope under outdoor air temperature wave and solar radiation wave by periodic heat transfer mechanism.To fit outdoor meteorological parameters, I obtain typical meteorological year (TMY) from the national meteorological data from 1976 to 2005. After selecting Xi'an, Xining and Yinchuan data about outdoor air temperature, direct solar radiation of normal and diffuse radiation of horizontal in overcast, cloudy, mist and sunny days, I calculate their mean values in heating season and Xi'an's mean values in summer respectively. Then I fit the functions of outdoor air temperature, direct solar radiation of normal and diffuse radiation of horizontal in the four types of weather by Origin7.5. It shows that the relativity between these functions and observed data is well-matched, and the result can reflect actual law of outdoor air temperature and solar radiation.Harmonic response method is used to analyze periodic heat transfer characteristics of building envelope in this paper. At first, the boundary conditions should be decomposed into sine function with Fourier series, of which the fundamental frequency is 2Ï€/T. Then to comput the transfer matrix of the building envelope, the time variable and the space variable in heat conduction equation of the building envelope should be made Laplace transform and inverse Laplace transform respectively, and then the two functional expressions between boundary conditions and temperature, boundary conditions and heat flux in arbitrary section will be obtained. The two functional expressions will make transfer matrix of the building envelope. At last, after calculating the damping factor and detention period, we can calculate the air temperature and solar radiation waves at the inner surface of the building envelope, and then superimpose the two waves into one inner surface temperature fuction. On the premise of constant indoor air temperature, the heat transfer capacity of the building envelope will be calculated by convective heat exchange between inner surface and indoor air.On the premise of constant inner air temperature and sol-air temperature just made of outdoor air temperature and solar radiation, we can conclude that the inner surface temperatures calculated on the condition of two outdoor waves and of sol-air temperature, have the same result. So the temperature wave with the same frequency and different initial phase can add directly, the amplitude attenuation and phase delay of building envelope has no effect on such waves. Moreover, either in winter or in summer, the average temperature and amplitude calculated on the condition of two outdoor waves is bigger than that calculated on the condition of air temperature. The trough calculated by double-wave method appears earlier than that calculated by air temperature wave.The fitted outdoor air temperature function, normal direct solar radiation function and horizontal diffuse radiation function have higher accuracy, and in different cities and seasons, can reflect temperature and radiation's changing laws in four kinds of weather. The method of heat transfer in building envelope under two outdoor periodic waves can calculate periodic heat transfer capacity and inner surface temperature of the building envelop. It is easier to observe physical meanings in the continuity equations.