Effect of holes arrangement on heat transfer in impingement/effusion cooling double wall schemes

In the present work, two different impingement/effusion geometries have been investigated, with staggered hole configuration and equal number of impingement and effusion holes. The first geometry presents impingement hole pitch-to-diameter ratios of 10.5 in both the orthogonal directions and jet-to-target plate spacing of 6.5 hole diameters, with effusion holes inclined of 20° with respect to the target surface. The second geometry shows impingement hole pitch-to-diameter ratios of 3.0 in both the orthogonal directions, jet-to-target plate spacing of 2.5 diameters and normal effusion holes. For each geometry, two relative arrangements between impingement and effusion holes have been investigated, as well as various Reynolds numbers for the sparser geometry. The experimental investigation has been performed by applying a transient technique, using narrow band thermochromic liquid crystals (TLCs) for surface temperature measurement. A CFD analysis has also been performed in order to provide a complete comprehension of the phenomena. Results show unique heat transfer patterns for every investigated geometry. Weak jet-jet interactions have been recorded for the sparser array geometry, while intense secondary peaks and a complex heat transfer pattern are present for the denser one, which is also strongly influenced by the presence and position of effusion holes. For both the geometries effusion holes increase heat transfer with respect to impingement-only, which has been mainly attributed to a reduction in flow recirculation for the sparser geometry and to the suppression of spent coolant flow for the denser one.