Effects of jet-to-target spacing on primary and secondary peak heat transfer for an obliquely impinging jet

Abstract

This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Numax, where Numax is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°.