Central receiver heat transfer enhancement using jet impingement with passive velocity excitation

Abstract

A new central receiver concept is being developed that attempts to trap solar rays and limit the re-radiation view factor. It has an array of hexagonal pyramid-like structures that form the absorbing surface facing the concentrated solar irradiation. Internal to these structures are concave surfaces where the heat transfer fluid impinges after emanating from jet nozzles. It therefore uses jet impingement heat transfer enhancement like the receivers proposed by Garbrecht et al and Lubkoll et al. The current paper extends previous work by the author by adding passive velocity excitation to the jet in the form of different obstructions in the pipe nozzle, as originally investigated using a cylindrical obstruction for a jet impacting on a flat plate by Uddin. Large Eddy Simulation is used to evaluate the temporal distribution of Nusselt number and velocity variation. The results of a jet with no obstruction is first compared to the experimental work of Lee et al as validation and then to those when using a cylindrical, triangular and airfoil-shaped obstruction to investigate the compromise between enhanced heat transfer and associated pressure drop. It is found that the triangular insert has the highest peak Nusselt number (4.6-fold increase over no obstruction) and 2.1 times the average Nusselt number coming at the expensive of a 60- fold increase in pressure drop. The cylindrical insert has a 2.4 times increase in maximum Nusselt number with an associated 42-fold increase in pressure drop, and although its effect is localized, the average Nusselt number increases 1.8 times. The airfoil insert considered was too streamlined to have a significant effect; its average Nusselt number being 33% higher at the expense of a 2.3 times higher pressure drop. There is therefore scope for optimizing the shape of the insert to balance the increase in convective heat transfer with the increase in pressure drop. Other methods of disturbing the flow (like introducing swirl) can also be investigated by extending the current method.