Abstract: The two-dimensional unsteady numerical model of backward-facing step flow is established to study heat transfer characteristics on the bottom wall of the backward-facing step in the transition flow range (550Re1 500). The time averaged Nusselt number distributed on the bottom wall is simulated with the increase of Reynolds number and the distribution characteristics are analyzed combined with the development of the flow field structure. The results show that with the increase of Reynolds number, vortex pairs that rotate in the opposite direction appear in the middle area of flow field downstream of the step. The vortex attached to the bottom wall appears successively downstream of the primary recirculation zone and inside the primary recirculation zone. The main peak and the minor peak of the time averaged Nusselt number distributed on the bottom wall appear along the flow direction. The vortex attached to the bottom wall downstream of the primary recirculation zone promotes a rapid increase for the time averaged Nusselt number downstream of the main peak position. While the vortex attached to the bottom wall inside of the primary recirculation zone promotes a rapid increase for the time averaged Nusselt number upstream of the main peak position. The values of the main peak and the minor peak increase with the increase of Reynolds number. The positions of these two peaks first move in the opposite direction, then in the same direction. In short, in the transition flow region, improving Reynolds number can improve the time averaged Nusselt number on the bottom wall surface, thus enhancing the heat transfer effect. When the Reynolds number is higher than 900, the heat exchange efficiency is obviously enhanced in the primary recirculation zone.

Key words: transition flow;backward-facing step;numerical simulation;heat transfer characteristics