Modified SST k-ω-γ Model and Prediction of Laminar to Turbulent Flow Transition in Helical Tube
Helical tube heat exchangers are widely used in various fields due to compactness and good heat transfer performance.Different from the flow in a straight pipe,the fluid will be subjected to centrifugal force when flowing in a helical tube.Due to the existence of centrifugal force,the critical Reynolds number for the transition from laminar to turbulent flow in the helical tube increases with the increasing of helical curvature ratios.In the current investigation,the transition process from laminar to turbulent flow in the helical tube is studied by numerical method.By analyzing the relationship between the resistance coefficient and Reynolds number,the accuracy of simulating the transition process by the shear stress transport(SST)k-ω-γ-Reθ model and SST k-ω-γ model is compared,and the effects of different inlet turbulence intensities(5%and 10%)on the calculation results are analyzed.The SST k-ω-γ-Reθ model is more sensitive to inlet turbulent intensities,while the SST k-ω-γ model is less affected by inlet turbulent intensities.The critical Reynolds number obtained by SST k-ω-γ model is larger than the critical Reynolds number calculated by the empirical correlation in the literature.In this paper,by adjusting the empirical coefficient of γ transport equation in SST k-ω-γ model,it is found that CTU1 has a significant influence on the onset of the transition process.With the same helical curvature ratio,the critical Reynolds number increases with the increasing of CTU1.In the current investigation,based on the existing empirical formula,the CTU1 for different helical curvature ratios(δ=0.02,0.04,0.06,and 0.11)are determined,and the correlation between helical curvature and CTU1 is obtained by fitting.The accuracy of the modified SST k-ω-γ model in simulating the laminar and turbulent friction factors in helical tubes is verified,and the differences of velocity,turbulent kinetic energy and turbulent viscosity in the calculation results of SST k-ω model and modified SST k-ω-γ model are compared.
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