The heat transfer coefficients and the pressure distributions were measured downstream of the backward facing step in which the heat transfer was actively or passively enhanced by following procedure. (1)The jet was ejected perpendicularly to the main flow from the portion on the opposite wall of the step. (2) The air flow was sucked from the recirculating zone. (3) A slat was inserted with an inclination to the main flow. These cause the main flow to curve sharply, and then the recirculating zone to shorten, so it is easy to control the lengths of the reattachment point, and consequently, the heat transfer. The thermal performance, η defined as the ratio of the heat transfer coefficient at the recirculating region with these modifications to that of a normal region, was calculated for equivalent pumping power, and it was found that η was larger than unity for the enhanced systems, except in the case of the discharge jet. It is effective to apply the enhancement of the heat transfer by active or passive methods to the suddenly expanding tunnel.
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