| Fluid mechanics investigations of single submerged jets issuing from convergent nozzles have shown that coherent flow structures form readily in the free jet and subsequently cause secondary vortices to form along impingement surfaces. Similar coherent flow structures are difficult to achieve in steady jets issuing from orifices, which are almost always used in jet arrays due to simplicity of construction. Advances in automated machining methods now make the construction of arrays of convergent nozzles practical. This study was therefore undertaken to compare the heat transfer performance of arrays of jets issuing from orifices or convergent nozzles. Since coherent flow structures may be modified by flow pulsations, the effect of pulsation was also considered.; An apparatus was designed and fabricated to produce square in-line arrays of nine, circular air jets. The convergent nozzle was conical in design with a linear decrease in diameter from the inlet opening to the discharge opening. The jets impinged normally on a test cell which measured instantaneous and time-averaged, local convective heat transfer coefficients using a heat flux microsensor. Hot film anemometry was used to characterize the flow-fields of the steady and pulsating, impinging jets in terms of instantaneous and time-averaged velocities and turbulence intensities. An ensemble averaging technique was used to separate the periodic components of flow and heat transfer variables from the turbulent components and thereby assess the effect of flow organization separately from turbulence effects.; The flow field of the steady nozzle jets indicated the presence of coherent flow structures in the jet shear layer, unlike for the jets issuing from orifices. Heat transfer results demonstrated that coherent flow structures promoted by the convergent nozzles enhanced convective heat transfer, in terms of equivalent pressure drop, at locations away from the stagnation point. This resulted in a significantly more uniform convective heat transfer distribution. Large amplitude flow pulsations produced a reduction in stagnation point Nusselt number for both the orifice and nozzle jets, due to nonlinear dynamics effects, without affecting the Nusselt number away from the stagnation region, thus further increasing the uniformity of heat transfer distributions. |
