Heat transfer and surface flow visualization around a 180 deg turn in a rectangular channel

The aim of the present study is to obtain detailed measurements of the convective heat transfer coefficient nearby a 180 deg sharp turn in a rectangular channel, and to prove that the use of infrared thermography nay be appropriate to experimentally study this type of problems. A rectangular two-pass channel, which is 40 mm high and 200 mm wide, is tested for two different geometries of the tip of the partition wall: a square tip and a semicircular one. To perform surface flow visualization and heat transfer measurements, the heated-thin-foil technique is used and results in terms of temperature maps and Nusselt number Nu distributions are obtained. Nu is computed by means of the local bulk temperature which is evaluated by making a one-dimensional energy balance along the channel. Reynolds number, based on average inlet velocity and hydraulic diameter of the channel, is varied between 1.6×104 and 6.4×104. By moving in the streamwise direction, at the beginning of the heated zone a rapid increase of the wall temperature is found due to the development of the thermal boundary layer. Two well distinguishable separation zones are found, one at the first outer corner of the channel and the other attached to the partition wall and just downstream of its tip. The influence of the geometry of the latter seems to be limited mainly to the position of the onset of the second separation zone. A third weak recirculation zone seems to appear in the vicinity of the second outer corner at the highest tested Reynolds number.