Turbulent Flow and Heat Transfer Modelling for Building Ventilation 
Researcher: 
ShiaHui Peng peng@foi.se 
Supervisor: 
Lars Davidson lada@chalmers.se 
Cooperation:  Working Organization and Technology, National Institute for Working Life 
Sponsors:  Working Organization and Technology, National Institute for Working Life 
Publications:  [19] 
Start of project:  spring 1995 
End of project:  spring 1998 
THE PROJECT The project is focused on studies on the assessment of ventilation performance and the development of turbulence models accounting for LowReynoldsnumber (LRN) effects and buoyant convection with heat transfer. Assessment of building ventilation performance is discussed in view of room air distribution and passive contaminant dispersion. Different concepts and methods for analyzing and assessing ventilation flow systems are addressed and reexamined. Several new scales have been developed, including the local purging effectiveness, the expected contaminant dispersion index, and the local specific contaminantaccumulating index. The approach to numerically reveal these scales is presented. The purging flow rate is reformulated in several expressions different from its original definition and the previous description. Some scales defined in terms of this quantity are discussed. Using stochastic theory in conjunction with the compartmental method, a Markov chain model is proposed to determine the transfer probability which is needed to compute the regional purging flow rate. This model contains extra and useful information that is not included in the previous deterministic analyses. The new scales and methods are expected to be applicable for diagnozing problems and optimizing designs of ventilation systems. For simulating recirculating ventilation flows, comparison is made for three types of high Reynoldsnumber twoequation turbulence models, including the ke model, the kw model and the kt model. It is found that both the kw model and the kt model give relatively poor results. Modifications are made for the kw model in which the model constants are re established and the turbulent transport term in the exact wequation is remodelled. Based on these modifications, a new LRN kw model is proposed where the damping functions are re devised and the nearwall asymptotic behaviour is emphasized. The mechanism for simulating transition is preserved in the modified model. The LRN formulation is further extended for analyzing buoyantdriven cavity flows at moderate Rayleigh numbers, where laminar, transitional and fully developed turbulent boundary layers subsequently arise along vertical walls. The model behaviour accounting for transition onset is discussed, and some disciplines are derived and used in LRN formulation. Large eddy simulation (LES) is implemented for turbulent buoyant flows with heat transfer. A modified subgridscale (SGS) buoyancy model is proposed. The modification enables the model to get rid of entailing noreal solution for simulating thermal convection flows as does the original buoyancy model. Furthermore, the proposed model is able to account for energy backscatter for flows where thermal stratification is significant. Comparison is made for several SGS models when applied to stratified and unstratified buoyant flows. The modified model, in general, gives promising results in comparison with DNS and experimental data. For a buoyant cavity flow at a moderate Rayleigh number, it is found that the SGS model performs inappropriately. The failure and success of SGS models for handling this type of flows are analyzed and discussed. The behaviour of SGS models in accounting for energy backscatter is regarded as being an essential ingredient for predicting natural transitional boundary layer flows. REFERENCES  click to view postscript file

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