| At present, a lot of light transparent material including different kinds of glass and organic polymer films is gradually widely used in a variety of protective structure skins of civil buildings(such as windows, transparent curtain wall and epidermal membrane structure etc) and shelters of agricultural buildings. Their basic functions and effects are obtaining natural lighting and good visual appearance, and promoting farm crop growth. At the same time, thermal insulation characteristics of these skins and shelters are also important. So it is essential to study the heat transfer mechanism of these kinds of light transparent material and constructions in actual building thermal environment.This paper presents and sets up the steady calculation models of entire wavelength radiation heat transfer on single and double hollow transparent building skin between indoor and outdoor thermal environments, and solves the models by the numerical iteration method. Considering various heat transfer ways including environmental long-wave radiation, solar(short-wave) radiation, convection near the skin’s surfaces, conduction of the skin, and conduction and convection of hollow space gas of double skin, the models provide important basis for engineering heat transfer calculation of transparent building skin. Different from opaque building skin, there exist selective transmittance characteristics to transparent building skin in radiation heat transfer process. That is to say, transparent building skin has different long-wave radiation transmittance and solar radiation transmittance, which will greatly add complexity of heat transfer process. This is the key innovation point of the paper. “Transparent building skin” is the expression of civil and architecture subject, actually it is expressed as “long-wave transparent diffuse gray body” in heat transfer subject when long-wave radiation transmittance is not changed with wavelength and direction changing. Through academic calculation and proofing, the models are suitable for heat transfer calculation of building protective structure skin consisted of both long-wave transparent diffuse gray body(glass and organic polymer films) and non-transparent diffuse gray body(brick, stone and concrete etc). In other words, when long-wave radiation transmittance of material is near to zero, the models for long-wave transparent diffuse gray body are equivalent to the classic models for non-transparent diffuse gray body.Based on the above models, nine different kinds of single and double hollow transparent building skin cases consisted of ordinary clear glass, low emissivity glass, polyvinyl chloride(PVC) film or ethylene-tetrafluoro-ethylene(ETFE) film are chosen. According to thermal parameters and transmittance characteristics of above four transparent materials, five thermal characteristics indexes including overall heat flux, overall heat transfer coefficient, solar heat gain coefficient, effective long-wave transmission coefficient and surface radiation heat transfer coefficient are calculated and analyzed for every one of above nine cases. Effective long-wave transmission coefficient defined in the paper is an important thermal characteristics index to describe the effects on environmental transparent long-wave radiation through transparent building skin. The basic mechanisms of the five thermal characteristics indexes are as follow:(1) if there is not solar radiation, more long-wave radiation transmittance of transparent material is, more overall heat flux, overall heat transfer coefficient, effective long-wave transmission coefficient and surface radiation heat transfer coefficient are. That is to say, transparent long-wave radiation through transparent building skin plays a positive role of increasing system’s heat transfer quantity.(2) If there exists solar radiation, the circumstances are complicated. Under winter condition, transparent long-wave radiation through transparent building skin plays a negative role of decreasing system’s heat transfer quantity(in favor of heat preservation and increasing heat gain). Otherwise, under summer condition, transparent long-wave radiation through transparent building skin plays a positive role of increasing system’s heat transfer quantity(to the disadvantage of heat insulation and decreasing heat gain).(3) The value of solar heat gain coefficient is mainly related to solar transmittance and absorbance of transparent material. More solar transmittance is, more solar heat gain coefficient is(to the disadvantage of sun-shading). More solar absorbance is, less solar heat gain coefficient is(to the advantage of sun-shading).Furthermore, three boundary conditions in the models are taken as sensitivity factors to analyze. They are(1) the ratio of the area of the transparent building skin to the area of other indoor surfaces;(2) convective heat transfer coefficients of inside and outside surfaces of the transparent building skin;(3) the difference between surface temperature and air temperature of both indoor and outdoor environments. Analysis objects are four representative transparent building skin cases including single ordinary clear glass, single ETFE film, ordinary double hollow glazing, and double hollow ETFE films. Analyzed thermal characteristics indexes are overall heat flux, effective long-wave transmission coefficient and solar heat gain coefficient. The results show that solar heat gain coefficient is not related to the three sensitivity factors. To overall heat flux and effective long-wave transmission coefficient, the influence degrees of the three sensitivity factors are as follow.(1) The influence of the ratio of the area of the transparent building skin to the area of other indoor surfaces is very little, which can be ignored in engineering calculation.(2) The influence of convective heat transfer coefficients of inside and outside surfaces of the transparent building skin is minor, which can be ignored in engineering calculation if deviation of calculating results of thermal characteristics indexes is less than 5%.(3) The influence of the difference between surface temperature and air temperature of both indoor and outdoor environments is major. When the transparent building skin is not exposed under solar radiation, the effects of the small temperature difference between surface temperature and air temperature of both indoor and outdoor environments on thermal characteristics indexes are relatively minor. However, when the transparent building skin is exposed under solar radiation, the effects of the large temperature difference between surface temperature and air temperature of outdoor environments on thermal characteristics indexes are relatively major. In engineering calculation, the influencing factor can be ignored if deviation of calculating results of chosen indexes is less than 5%. Otherwise it can not be ignored.Generally, according to greenhouse effect, long-wave radiation transmittance of ordinary soda-lime silicate glass is little, and transparent long-wave radiation can be ignored. Therefore, based on the theoretical models presented in the paper, an experiment is performed to validate the existence of transparent long-wave radiation for single layer ordinary soda-lime silicate glass.Finally, preliminary research is carried out on heat transfer model when surface radiation characteristics(temperature and emissivity etc.) are heterogeneous. A two-dimensional radiation heat transfer problem between two closed rooms isolated by an isothermal board is solved by numerical simulation based on Monte Carlo method. This work establishes some foundation in order to further study the subject of radiation heat transfer between two isolated environments through a long-wave transparent diffuse gray body from the standpoint of Heat Transfer Theory in future. |
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