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Forced convection

Yue-Tzu Yang
Department of Mechanical Engineering, National Cheng Kung University, Tainan, 701, Taiwan

Kuo-Teng Tsai
Department of Mechanical Engineering, National Cheng Kung University, Tainan, 701, Taiwan


This study presents the numerical simulation of a transient mixing process of the high temperature turbulent flow behind a backstep with a low temperature wall mass injection, and compares with the experimental results by Yang et al. [16]. The turbulent governing equations are solved using a Control-Volume-based Finite-Difference Method with power-law scheme. The well-known k − ε turbulence model and its associated wall functions are used to describe the turbulent structure. The velocity and pressure terms of the momentum equations are solved by the SIMPLE (Semi-Implicit Method for Pressure-Linked Equation) method. Orthogonal non-uniform staggered grids are used for the establishment of mesh grids.
The parameters studied include inlet velocity (U0 = 10, 20 and 30 m/s), inlet temperature (T0 = 150, 200 and 300 °C), and the wall injection rate (Q = 0.15, 0.25 and 0.35 m3/min). Heat transfer and temperature have been analyzed for steady and unsteady state with a constant height of the step. The numerical calculations on the flow field indicate that the reattachment length Xr becomes shorter as the wall injection rate increases. The error of the reattachment length is about 7%∼10% compared with the experimental results of Yang et al. [16]. In the recirculation zone, the horizontal velocity is decreased whereas the vertical velocity is increased with a uniform wall injection. The turbulent characteristics show that the maximum turbulent intensity and Reynolds stress on any section happens near the shear layer or the larger gradient of the velocity. In the recirculation zone, the turbulent intensity decreases as the wall injection increases. Downstream of the reattachment point, the turbulent intensity increases as the wall injection increases.

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