Research On Thermal Fluid In Partially Open Enclosure By Large Eddy Simulation And Experiments

Thermal stratification is a complicated fluid flow phenomenon due to density difference driven by buoyancy. This kind of fluid flow is commonly encountered in building fire, room ventilation, hydraulic system, machinery, chemicals and environmental protection systems. And it is used widely in the project applications. The stratified fluid is a kind of fluid due to different densities driven by buoyancy, which makes the thermal fluid appear complex behaviors. Therefore, it adds difficulties to research on the thermal stratified fluid. The objective of this thesis is to study thermal stratified fluid in an enclosure with openings.In consideration of the widespread popularity and importance of the thermal stratified fluid in an enclosure with openings, researches on typical cases of the thermal stratified fluid in an enclosure with openings were conducted. A mathematical model was set up by combining the associated physical phenomena. Derivation and simplification of the key equations concerned were presented. Discretization in numerical analysis was then outlined. Flow in an enclosure with openings with low Mach number and multi-species were introduced. Turbulence was simulated by large eddy simulation with the model equations filtered. Subgrid-scale models were applied with the numerical solution and convergence criteria described.The advanced Computational Fluid Dynamics software Fire Dynamics Simulator (FDS) released by the National Institute of Standards and Technology in the U.S.A. was modified with such analysis. Vreman subgrid-scale model was applied in simulating the thermal stratified flow in an enclosure with openings. This subgrid-scale model was justified by several fire scenarios of compartment with a vertical vent. The numerical values and experimental data were compared using functional analysis. In the numerical simulation of thermal stratified fluid in an enclosure with openings, the corresponding Vreman subgrid-scale constant Cv for the filter width of 0.05 m was fitted to be 0.1.Based on the fire experiment on compartment with a vent and the associated numerical simulations, Smagorinsky and Vreman subgrid-scale models for simulating the thermally-induced flow field were compared. In addition to comparing with fire experiments, predicted results in a double compartment, a ventilation displacement in an office and laminar flow in clean room environment, predicted results were closer to experiment in using this improved model developed from FDS without disturbed by additive momentum of the supplying ventilations.Scale model experiments and numerical simulations for thermally-induced flow in a long tunnel with two open-ends were carried out. The adaptability of Vreman subgrid-scale model for the thermal flow field in this long tunnel was analyzed. Based on the experimental data and numerical simulation results for different gap widths and inclined angles, the shift characteristics of the higher temperature zone in the tunnel, the variation of flow speed and the height of central plane at openings were analyzed. The results can provide sufficient data for designing smoke management system for an inclined tunnel.Large-scaled fire experiment and numerical simulation were carried out for vertical shaft with a single corner gap and top open. The effects of the different gap widths on the rotation of fire and flow field, the correlations of buoyancy and the Coriolis force, the shift characteristics of the fire whirl was analyzed. The flow field in the vertical shaft was studied using the numerical simulation when the corner gap width of above 4 m was set at 0.15 m. Small-scaled fire experiments were also conducted in 2 m tall vertical shaft with dual corner gaps. The results showed that fire whirls can be generated under a certain heat release rate and corner gap width. A fire whirl would be generated with appropriate corner gaps.Results of this thesis provided the basics for better understanding the thermal flow driven by buoyancy. The study of the thermal flow field driven by buoyancy may offer some reference and help for the research on application in engineering. At the same time, this dissertation paved a way for the further study.