Numerical simulation and characterization of jet flows in indoor environments

Zhu Shi, Purdue University

Abstract

Jet flows are prevalent in indoor environment and other engineering applications. Typical examples in indoor environment include the flow discharged from personal ventilation systems, and the jet exhaled through breathing or coughing. When there is density (or temperature) difference between the jet and surroundings, jet flow becomes stratified jet. Due to its complication, stratified jet flow is difficult to model, especially in the developing or transitional region of the flow. Studying stratified jet flows is of great significance for understanding the mixing dynamics of jet and ambient environment. This is particularly important for optimizing indoor environment design, or obtaining accurate boundary conditions in indoor air flow simulations. Various turbulence models have been used to simulate stratified flows. This investigation systematically evaluated the performance of seven turbulence models under different turbulence levels and stratification levels, by comparing simulation results with experimental data. Mean velocity, turbulent kinetic energy and turbulent shear stress were examined in the comparisons. Mean square error values were used to quantify the evaluation. For the weakly stratified jet, all seven models could predict well the mean velocity, but for the strongly stratified jet, the Reynolds stress model and LES overpredicted the velocity in the unstable stratification region. SST k-ω was the overall best model. This investigation also analyzed the computing costs of the models as well as the vorticity and entrainment ratios predicted in the simulation. This study introduced a new dynamic turbulent Schmidt number model which can determine turbulent Schmidt number based on local flow structure. The proposed model can improve the prediction of density distribution especially at downstream locations, although it takes 10% additional computing time. Furthermore, this study developed a CFD model to investigate gasper-induced jet flow. The results indicated that the jet centerline velocity profile could collapse into a universal curve after normalization; meanwhile, the lateral velocity profiles at downstream locations followed self-similarity rule. Based on that, the study proposed two models to predict normalized velocity at jet centerline, and lateral velocity at downstream locations, respectively. A flow rate model was also developed to predict the mainstream flow rates at various downstream locations of gasper-induced jet. The CFD model and developed flow rate model were further used to assess the impact of gasper on air quality in the breathing zones of passengers.

Degree

M.S.M.E.

Advisors

Chen, Purdue University.

Subject Area

Mechanical engineering

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