Impact of Jet Reynolds Number and Feed Channel Geometry on Heat Transfer in a Channel with Inclined Target Surface Cooled by Single Array of Centered Impinging Jets with Outflow in Both Directions
An experimental investigation has been carried out to study the heat transfer characteristics in a channel with heated target plate inclined at an angle cooled by single array of equally spaced centered impinging jets for three different jet Reynolds numbers (Re=9300, 14400 and 18800). Air ejected from an array of orifices impinges on the heated target surface The target plate forms the leading edge of a gas turbine blade cooled by jet impingement technique. The work includes the effect of jet Re and feed channel aspect ratios (H/d = 5, 7, 9 where H=2.5, 3.5, 4.5 cm and d=0.5 cm) on the heat transfer characteristics for a given orifice jet plate configuration with outflow exiting in both directions (with inclined heated target surface). In general, It has been observed that, H/d=9 gives the maximum heat transfer over the entire length of the target surface as compared to all feed channel aspect ratios. H/d=9 gives 3% more heat transfer from the target surface as compared to H/d=5 (for Re=14400). Also, it has been observed that the magnitude of the averaged local Nusselt number increases with an increase in the jet Re for all the feed channel aspect ratios studied.
Numerical investigation of local heat transfer distribution on surfaces with a non-uniform temperature under an array of impinging jets with various nozzle shapes
Numerical calculations of heat transfer characteristics of an impingement cooling system with 
a non-uniform temperature on a cooled surface using ANSYS CFX have been performed. The 
influence of a surface heat flux qw(x) and a nozzle shape on the Nusselt number distribution on the 
cooled surface has been studied. The setup consisted of a cylindrical plenum with an inline array of 
ten impingement jets. Cylindrical, convergent divergent shapes of nozzles and linear temperature 
distribution on the cooled surface have been considered for various heat fluxes qw(x). Results 
indicate that geometry of the cylindrical nozzles resulted in the highest Nusselt numbers along the 
cooled surface. The line of the averaged Nusselt number has a trend to increase in the direction of 
the flow for the cooling system with increasing values of the surface heat flux q(x). This tendency 
can be observed for all presented shapes of jets. On the other hand, for decreasing functions of the 
heat flux qw(x), the Nusselt number distribution is more uniform. It can be observed for all types 
of nozzles. Very similar values of the Nusselt number occur especially for the non-uniform heat flux 
5000-2500W/m2. For constant values of the heat flux q(x) = 5000W/m2, the line of the average 
Nusselt number has a trend to increase slightly in the direction of the flow. Numerical analysis 
of different mesh density results in good convergence of the GCI index, what excludes mesh size 
dependency. The presented study is an extension of the paper (Marzec and Kucaba-Pietal, 2016) 
and aims at answering the question how the Nusselt number distribution on the cooled surface is 
affected by various geometries of nozzles for a non-uniform surface heat flux qw(x).
Intensification of Bubble Disintegration and Dispersion by Mechanical Stirring in Gas Injection Refining
Water model experiments were performed for establishing highly efficient gas injection refining processes. Mechanical stirring was applied to disintegrate the injected bubbles and to disperse them widely in the bath. The bubble disintegration and dispersion were investigated by changing rotation mode (direction of rotation), rotation speed, blade size of the impeller and gas flow rate. Forward rotation of the impeller induced a stable tangential flow and could not disperse bubbles in the bath due to formation of a vortex around the impeller shaft. The tangential flow could be suppressed by forward–interrupt rotation, which could reduce the vortex formation to some degree. However, forward–interrupt rotation could not disperse the bubbles widely in the bath. Forward–reverse rotation could prevent the vortex formation completely and create a strong shear stress field, which intensified the bubble disintegration and dispersion in the bath. Higher impeller rotation speed and larger blade length in forward–reverse rotation could enhance the bubble disintegration and make the dispersed bubbles smaller. The bubble dispersion zone became wider with larger blade length. The bubble size tended to be larger at higher gas flow rates. However, its dependence on the gas flow rate became smaller at higher impeller rotation speed.