Influence of flow tree-dimensionality on the heat transfer of a narrow channel backward facing step flows

• Published in: International journal of thermal sciences 2018-10, Vol.132, p.234-248
• Main Authors: Juste, G.L., Fajardo, P.
• Format: Article
• Language: English
• Subjects: Backward facing step; Heat transfer; Narrow channels; Three-dimensional instabilities
• ISSN: 1290-0729; 1778-4166
• DOI: 10.1016/j.ijthermalsci.2018.06.005

This manuscript studies the forced-mixed convective flow on a three-dimensional backward facing step with low aspect ratio (AR = 4) and expansion ratio ER = 2 for Reynolds number in the laminar and beginning of transitional regime (Re 100–1200). The analysis is performed using large eddy simulations, with the main objective assessing the effect of sidewalls on heat transfer characteristics on bottom wall in narrow channels with sudden expansion. The numerical model has been validated with experimental and numerical results from the literature and qualitatively with the experimental results obtained through Moiré deflectometry. The bottom surface was kept at constant temperature greater than the flow inlet temperature, while the other walls are considered to be adiabatic. To decouple the three-dimensional flow features due to the sidewalls and the intrinsic three-dimensional instabilities of the separated flow, two different boundary conditions on lateral walls, slip and non-slip, have been used. The results obtained show that when slip sidewalls are considered, the three-dimensional intrinsic structures begging to appear for Re equal to 1200. These structures enhance the heat transfer in the bottom wall. On the contrary, with non-slip sidewalls, the strong three-dimensional structures caused by the sidewalls represented by the upper side recirculation bubble and the wall jets mask the intrinsic three-dimensional instability, decreasing the heat transfer in the lower wall downstream of the step. As a consequence, the surface averaged Nusselt for all Reynolds numbers corresponding to the beginning of the transitional flow is lower for the case of non-slip sidewalls than for the case of slip sidewalls. Thus, the study concludes that sidewalls have a negative effect on heat transfer in narrow channels for flow Reynolds numbers in the early transitional regime. •Large eddy simulation with dynamic SGS model is a suitable formulation to simulate heat transfer in BFS flow at all regimes.•For Reynolds greater than 1000 three-dimensional instabilities inherent to the flow appear.•For forced-mixed convective flow, the heat transfer does not alter the flow topology in BFS with low aspect ratio.•Maximum value of the Nu number appears in the zone where the reattachment length is minimum.•At the early transitional regime, the effect of sidewalls in narrow BFS channels dampens the 3D instability.