Impingement/effusion cooling with a hollow cylinder structure for additive manufacturing: Effect of channel gap height

The present study is to investigate the effect of gap height on heat/mass transfer for impingement/effusion cooling of gas turbine hot components, using a hollow cylinder structure created by three perforated plates with two channels. The detailed local heat/mass transfer coefficients on all surfaces of the structure are measured in the test section using a naphthalene sublimation method. Flow characteristics in the structure are revealed using a numerical simulation such as jet impingement, wall jets, and toroidal vortices. The hole spacing (P/D) and thickness of plate (t/D) in the present study are 6 and 1, respectively. The various gap height ratios, which mean the ratios between the gap height of the first channel and that of the second channel, are 1:4 (0.2D-0.8D), 1:1 (0.5D-0.5D), and 4:1 (0.8D-0.2D). Depending on the gap height ratios, heat/mass transfer distributions are obtained differently on each surface of the structure. The area-averaged Sherwood number on each surface shows a large variation ranging from 27% reduction in UM of the Case 3 and 42% increase in UB of the Case 3 with different gap heights at the Reynolds number (Re_D) of 7,000. Considering the benefits of heat/mass transfer augmentation and the drawbacks associated with an increase in pressure drop, the structural configuration of the first channel of low gap height and the second channel of high gap height has the highest thermal performance factor, with an improvement of 4.8% at Re_D = 7,000. The scale of gap height and wall thickness of the gas turbine hot components should be selected in terms of the total blade wall thickness, number of multi-layers, and manufacturing tolerance in additive manufacturing. Therefore, as gap height for the channel in the laminated structure is limited, the case of the gap height ratios of 1:4, which is the low gap height for the first channel and the high gap height for the second channel, offers the best cooling performance for gas turbine components compared to its counterpart having a high gap height of the first channel and a low gap height of the second channel.