Reasearch Of The Theory Of Thermal Stratification In Buildings And Its Applications

Thermal stratification (TS) is of significant importance to reduce building energy consumption, and to improve indoor air quality and thermal comfort. It turns into a crucial factor to achieve both building energy efficiency and improved indoor environment, and thus it has been a hot topic in the area of building energy and environment. However, TS in buildings is a complicated phenomena relating to detailed indoor airflows. To make use of this effect, reliable mathematic models need to be developed and much theoretical work remains to be resolved.To develop mathematic models, this work began with basic theoretical research of the TS phenomena. First, turbulence influenced by the effect of TS was investigated and an improved turbulent viscosity was proposed to replace the traditional fully turbulent ones. Second, relation between cooling load/building energy and TS was studied and a simplified model, so-called zonal model, for predicting cooling load/building energy based on distributed parameters was developed. Third, based on the close relation between these two mathematic models, and their mutual benefit to each other, turbulent model and zonal model were, for the first time, coupled to provide mutually necessary parameters.To begin with, complexity and particularity distinguished the turbulence of indoor TS from the traditional isotropic turbulence. The deficiency of the fully turbulent EVM/EDM(Eddy viscosity model/Eddy diffusivity model) models was emphasized. The corrections of Reynolds stress and turbulent heat flux, turbulent viscosity and turbulent Prandtl due to the TS effect were investigated. The coupled effect of shear stress and thermal buoyancy force on indoor TS was concluded. The turbulence under unstably thermal stratification was also discussed.Due to the rigorous framework of the application of Renormalization group theory to the turbulence, the RNG k-εmodel was studied to check its applicability to the TS effect, and an improved turbulence model was eventually developed in the framework of RNG turbulence. A hybrid wall function was also developed based its applicability to the TS effect. The new turbulence system with the combination of an improved turbulent viscosity, turbulent Prandtl number and wall function was to provide reliable and economical predictions of indoor thermally stratified flows.From the angle of moderate distributed parameters, a zonal model was developed to well represent the effect of TS on cooling load/building energy. This model was highly advantageous to predicting cooling load/building energy reduction due to the TS. In this model, a heat transfer factor due to temperature difference across a horizontal air layer was firstly clearly defined and was accurately related to the local turbulent intensity.One single-scale numerical model was always unable to effectively predict indoor environment and building energy because of the lack of necessary conditions for CFD/NHT (Computational fluid dynamics/Numerical heat transfer) simulation, and that of reliable values of heat transfer coefficients for zonal model. Based on the mutual dependence, the coupling simulation of CFD with zonal model was then proposed. To provide a promising tool for predicting indoor thermally stratified flows and building energy, many problems in the coupling simulation, such as data transfer methods and coupling schemes were extensively investigated.Based on the development of mathematical models above, validation work and applications of these models were carried out.To validate the new turbulence model, both salt-bath scale-down experiment and full-scale air experiment were performed to obtain valuable data. Extensive data of indoor TS by large eddy simulation and experiments in literature were also used to further validate the new turbulence model. Some experimental data and CFD results were combined to validate the zonal model. Two key parameters in the zonal model, surface convection coefficient and heat transfer factor due to temperature difference were numerically resolved, and thus the empirical problem caused by them was effectively overcome.The new turbulence model was applied to investigate the effects of TS on internal gravity wave, pollutant dispersion and cooling load reduction. Both internal gravity wave and low-frequency wave driven by horizontal flows were numerically captured. A simplified thermodynamic criterion was developed to judge the stability of indoor thermally stratified flows. The TS effect was found to be disadvantageous to the improvement indoor air quality because it might drive air pollutants to spread horizontally. The application of CFD-zonal model coupled simulation to high-large enclosures under stratification cooling was also performed. This application was aimed to analyze the impact of ventilation in the upper part of the space on the cooling load and thermal environment in the occupied zone. Base on the calculation of heat shift from the upper space to lower space, it also provided a method to predict the cooling load in the air-conditioned space.This present work carried out many studies on indoor TS effect, including theoretical research of turbulence damped by TS, development and validation of a new turbulence model, establishment and validation of a zonal model, basic theory and method for the CFD-zonal model coupled simulation, and applications of these models. Research methods applied included theoretical, numerical and experimental methods. A significant importance of this work was to relate indoor turbulence influenced by the TS effect to building energy, and then to provide theoretical framework for the effective evaluation of the moden building environment and energy. Based on this present work, it should be realizable to use indoor thermal stratification effect to achieve both building energy efficiency and improved indoor environment.