Wall imprint of turbulent structures and heat transfer in multiple impinging jet arrays

Abstract

Thermal imprint and heat transfer on the target surface in multiple impinging jet arrays have been investigated in conjunction with flow pattern and large-scale eddy structure. Surface temperature was measured with liquid crystal thermography (LCT) in a range of jet configurations with hexagonal and in-line orifice arrangements for different combinations of distances between the orifices s/D m = 2-6 and between the orifice plate and the impingement surface H/D m = 3-10 (where D m is the orifice diameter). The hexagonal arrangement was considered with two different orifice shapes: sharp-edged and contoured. For selected configurations, the distribution of Nusselt number and its peculiarities were analysed in relation with the underlying eddy structure educed by proper orthogonal decomposition (POD) of the snapshots of fluid velocity measured with particle image velocimetry (PIV). Owing to the breakdown of the jets, heat transfer deteriorates with increasing orifice-to-plate distance. The jet interaction and breakdown become more severe as the jets are placed closer to each other. The large-scale eddies originating from the jet-edge shear layers grow as they are convected towards the impingement plate. Eddies of sizes between 0.2 and 0.3 orifice diameters are shown to break up the jets and cause mixing of fresh and spent fluid, lowering the beneficial temperature gradient. In some configurations, an asymmetric flow pattern is generated with embedded weak eddies on only one side of the diagonal symmetry line, which is reflected in an asymmetrical heat transfer distribution on the impingement plate. For H/D m > 4, the Nusselt number shows peak values in and around the jet impingement centres, but relatively uniform distribution of turbulence kinetic energy with local negative energy production close to the impingement surface.

Local Heat Transfer Characteristics of Multi Jet Impingement on High Temperature Plate Surfaces

“…LEON F. G. GEERS 18) focused on both pressure and velocity distribution characteristics, which were similar to the water cooling condition. Kercher and Tabakoff 19) presented the spatially-averaged surface heat transfer coefficients beneath an array of impinging jets with an in-line arrangement, whereas the results demonstrated that the Reynolds number and the streamwise/spanwise hole spacing were the most important parameters which affected all magnitudes of the spatial-averaged heat transfer coefficients.…”

DNS analysis of multiple impinging jets

“…The contours of mean velocity magnitude on the plane1 and 2 are visualized. Compared to the instantaneous flow, for the narrow spacing case, we can clearly mention the notable flow feature due to jet-jet interaction: early breakdown of potential core and excessive reduction outer edge of jet occurs before the jet impingement; large-scale flow has been pointed out as 'fountain' in previous literatures (Geers, Hanjalić, and Tummers, 2006;Weigand and Spring, 2011) are formed around the center of the impinging wall(r = 0), regardless of the jet-to-jet spacing.…”

“…Thus in order to overcome the shortcoming of single impinging jet (SIJ), i.e., the occurrence of both inhomogeneous heat distribution on the wall and the narrow heating area, multiple impinging jets (MIJ) are generally introduced in industrial applications. Thus far it is well-known that each jet of MIJ interacts with each other, producing the complex flow field (Geers, Hanjalić, and Tummers, 2006;Weigand and Spring, 2011). In addition, the heat transfer performance is significantly influenced with their interaction.…”

Experimental Investigation of High-temperature Steel Plate Cooled by Multiple Nozzle Arrays

“…In this case, 7 mm nozzle spacing, and 5.0 mm jet-to-target distance were set by optimizing the cooling parameters. Many studies [7][8][9] had been focused on the flow field and heat transfer of multiple air jet arrays, of which the pressure and velocity distribution characteristics are similar to the water cooling condition. The flow pattern includes stagnation region, decelerated flow, recirculating flow and vortex.…”