Heat Transfer Enhancement in a Double Sequential Impingement Channel

Abstract

Sequential impingement channels can reduce the adverse effect of crossflow in narrow impingement channels, as well as increase the cooling efficiency. In this work, sequential impingement channels are experimentally investigated using the transient liquid crystal technique to assess their thermal performances. A low heat transfer region is identified in the downstream part of the first channel where the flow is discharged into the second plenum. Various means of increasing the heat transfer at this location are investigated. Ribs on the target plate allow for an increase of the average heat transfer coefficient with small losses in pressure. Reducing the channel cross-section increases the mean flow velocity and, combined with the ribs, allows for a further increase of the heat transfer. Additionally, the geometrical changes of the channel caused by the addition of a ramp with a rounded corner, allow to decrease the pressure losses associated with the discharge into the second plenum, which is not optimal in the baseline configuration due to the sharp corner of the purge hole. Further reducing the cross-section to increase the heat transfer, however, increases the pressure losses due to the small open area in the transition zone.

Accuracy improvement of the transient heater foil technique for heat transfer tests: preliminary results

“…The transient liquid crystal technique previously applied for impingement studies at GTT used mainly the heater mesh technique [7,8]. A variation in form of using the heater foil was developed and applied in various contexts [4,6,9]. At time t=0, instead of causing a step change in fluid temperature an electric current is passed through the heater foil, which generates a power Q that provides a constant local heat flux q=Q/A, where A is the foil area.…”

“…In the case of complex geometries, where the flow can take different paths, this method is not well suited, because the temperature evolution will not be a perfect step, and different flows will be at different temperatures, making it difficult to establish the reference flow temperature at each point. In this scenario, for example in sequential impingement arrays with bypasses between channels [6], heating the surface directly can be an advantage: the upstream flow remains at ambient temperature, so that the fact that several flows mix is not a problem.…”

“…A step heater foil technique has been developed to address these complex configurations [6]. Its accuracy, however, is not satisfactory for cases where the surface under consideration experiences very different heat transfer coefficients at different locations, for example in jet impingement cases: when the applied heat flux is constant, the temperature evolution will reach an asymptotic value which depends on the heat transfer coefficient, with higher heat transfer coefficients leading to lower temperatures.…”