Numerical Investigations of Opposing Mixed Convection Heat Transfer in a Vertical Flat Channel. 2. Vortex Flow in the Case of Symmetrical Heating

We present the results of numerical investigation of the local opposing mixed convection heat transfer in a vertical flat channel with symmetrical heating at low Reynolds numbers. Numerical two-dimensional simulations have been performed for the same channel and for the same conditions as in the experiment using the FLUENT 6.1 code. The transient flow investigations were performed in airflow for the experimental conditions at the Reynolds number equal to 2.1.10(3) and Grashof number 6.2.10(8). Steady state flow investigations were performed for two Reynolds numbers (2.1.10(3) and 4.3.10(3)) and Grashof number up to 3.1.10(9) in order to clarify the effect of the influence of buoyancy forces. In both transient and steady-state modeling cases the results demonstrated that under the high buoyancy effect the staggered circular flow takes place near the heated walls. This makes velocity profiles asymmetrical and causes pulsations of the wall temperature. The wall temperature has a sinusoidal character, however, the resulting averaged values correlate rather well with experimental data for transient and steady-state cases for Re-in = 2.1.10(3). For Rein = 4.3.10(3) the resulting averaged values for x/d(e) > 25 correlated rather well with experimental data. When x/d(e) > 25, the difference between experimental and modeled wall temperature increases, which demonstrates that the model of a laminar flow cannot fully reflect the vortex flow at higher Re numbers.