Turbulent Flow and Heat Transfer Modelling for Building Ventilation
|Cooperation:||Working Organization and Technology, National Institute for Working Life|
|Sponsors:||Working Organization and Technology, National Institute for Working Life|
|Start of project:||spring 1995|
|End of project:||spring 1998|
The project is focused on studies on the assessment of ventilation performance and the development of turbulence models accounting for Low-Reynolds-number (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 re-examined. Several new scales have been developed, including the local purging effectiveness, the expected contaminant dispersion index, and the local specific contaminant-accumulating index. The approach to numerically reveal these scales is presented. The purging flow rate is re-formulated 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- Reynolds-number two-equation turbulence models, including the k-e model, the k-w model and the k-t model. It is found that both the k-w model and the k-t model give relatively poor results. Modifications are made for the k-w model in which the model constants are re- established and the turbulent transport term in the exact w-equation is re-modelled. Based on these modifications, a new LRN k-w model is proposed where the damping functions are re- devised and the near-wall asymptotic behaviour is emphasized. The mechanism for simulating transition is preserved in the modified model. The LRN formulation is further extended for analyzing buoyant-driven 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 subgrid-scale (SGS) buoyancy model is proposed. The modification enables the model to get rid of entailing no-real 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.
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