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978-3-8439-3153-3, Reihe Thermodynamik
Heat Transfer Enhancement in Impingement Systems with Surface Enlargements
148 Seiten, Dissertation Universität Stuttgart (2017), Softcover, A5
Jet impingement cooling is widely used in gas turbine industry due to its high performance and low geometric restrictions. The presented work is an experimental study of a generic enlarged multiple jet impingement system. The focus of the investigation is to study the influence of geometric parameters and surface turbulators on the heat transfer distribution in the system. Narrow band thermochromic liquid crystals (TLC) and thermocouples are applied to measure the local heat transfer rate. Pressure taps are used to evaluate the pressure loss. Computational fluid dynamics (CFD) simulations are used to help to guide the development of experiment.
A jet-to-jet distance of 10D impingement configuration is experimentally studied. The heat transfer performance is compared to the one for the jet-to-jet distance of 5D impingement configuration, to learn the influence of the open area. It is found that with the same Reynolds number, the large open area configuration achieves a higher heat transfer. While with the same mass flow rate, the small open area configuration provides a better performance.
Three kinds of surface turbulators are experimentally studied in the multiple jet impingement system. The cubic micro pin fin turbulator is effective due to the pin fin path channeling effect and the surface enlarging effect. Compared to the smooth target plate, the cubic micro pin fin turbulator roughened target plate provides an increased or decreased Nusselt number, depending on the flow condition. Considering the surface area enlargement, the heat flux on the target plate should be increased even with a Nusselt number decrease. This is confirmed by the CFD simulations. The detached ribs are effective because the gaps under the ribs accelerate the wall jet flow. Also, detached ribs eliminate the flow recirculation caused by the attached ribs. Local Nusselt number increase can be seen due to the presence of the detached ribs. The V-ribs on the impingement plate and on the target plate are effective because the vortex pair intensity is enhanced by the ribs. Nearly all tested V-rib roughened cases achieve a Nusselt number increase. Taking the area increase into consideration, all cases obtain a heat flux increase, as confirmed by the CFD simulations. The pressure evaluation shows only small differences between the ribbed cases and the smooth case. This indicates a minor rib effect on the additional pressure loss in the channel.