The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A + is a better characterization of this breakdown than the riblet spacing, with an optimum A + 1/2 ≈ 11. It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y + ≈ 30, with typical streamwise wavelengths l + ≈ 150. They are connected with a Kelvin–Helmholtzlike instability of the mean velocity profile, also found in flows over plant canopies and other surfaces with transpiration. A simplified stability model for the ribbed surface approximately accounts for the scaling of the viscous breakdown with A + .

/pdf/drag-reduction-by-riblets-263jdbm7cw.pdf

Drag reduction by riblets

Global advancement of solar thermal energy technologies for industrial process heat and its future prospects: A review

The interaction of the overlying turbulent flow with a riblet surface and its impact on drag reduction are analysed. The ‘viscous regime’ of vanishing riblet spacing, in which the drag reduction produced by the riblets is proportional to their size, is reasonably well understood, but this paper focuses on the behaviour for spacings s+ ≃ 10–20, expressed in wall units, where the viscous regime breaks down and the reduction eventually becomes an increase. Experimental evidence suggests that the two regimes are largely independent, and, based on a re-evaluation of existing data, it is shown that the optimal rib size is collapsed best by the square root of the groove cross-section, lg+=Ag+1/2. The mechanism of the breakdown is investigated by systematic DNSs with increasing riblet sizes. It is found that the breakdown is caused by the appearance of long spanwise rollers below y+ ≈ 20, with typical streamwise wavelengths λx+ ≈ 150, that develop from a two-dimensional Kelvin–Helmholtz-like instability of the mean streamwise flow, similar to those over plant canopies and porous surfaces. They account for the drag breakdown, both qualitatively and quantitatively. It is shown that a simplified linear instability model explains the scaling of the breakdown spacing with lg+.

/pdf/hydrodynamic-stability-and-breakdown-of-the-viscous-regime-4iu3hz1164.pdf

Hydrodynamic stability and breakdown of the viscous regime over riblets

This article presents the results of an experimental research on the thermal efficiency (TE) of an evacuated tube solar collector (ETSC) working with a nano-suspension of carbon nanotubes dispersed in distillated water. The efficacy of filling ratio (FR) of the heat pipes; the tilt angle (TA) of the collector, and the dispersion mass fraction of the carbon nanotubes within the distillated water on the TE of the collector was investigated. A model was developed based on the response surface methodology (RSM) to optimize the operating conditions in order to maximize the TE of the collector. The accuracy of the RSM model was verified with some additional experiments. It was found that the RSM model is able to optimize the TE of a collector with the accuracy of 1.6%. Also, it was found that the presence of carbon nanotubes inside the evaporator of the heat pipes can promote the nucleate boiling mechanism which in turn increased the TE of the solar collector. Also, the optimum filling ratio value and the installation angle were identified which were 0.6 and 55°, respectively. This was ascribed to a trade-off trend identified between the exist region inside the thermosyphon heat pipe and the amount of the vapor transported between the condenser and evaporator units, and also a trade-off between the residence time of the carrying fluid and the gravity effect, respectively.

Smart optimization of a thermosyphon heat pipe for an evacuated tube solar collector using response surface methodology (RSM)

http://www.pierre-ricco.co.uk/ricco_pierre_publications_files/ricco_quadrio_ijhff_2008.pdf

Wall-oscillation conditions for drag reduction in turbulent channel flow

Drag reduction of turbulent flow over thin rectangular riblets

The objective of this research was to design a low cost parabolic solar dish concentrator with small-to moderate size for direct electricity generation. Such model can be installed in rural areas which are not connected to governmental grid. Three diameters of the dish; 5, 10 and 20 m are investigated and the focal point to dish diameter ratio is set to be 0.3 in all studied cases. Special attention is given to the selection of the appropriate dimensions of the reflecting surfaces to be cut from the available sheets in the market aiming to reduce both cutting cost and sheets cost. The dimensions of the ribs and rings which support the reflecting surface are optimized in order to minimize the entire weight of the dish while providing the minimum possible total deflection and stresses in the beams. The study applies full stress analysis of the frame of the dish using Autodesk Inventor. The study recommends to use landscape orientation for the reflective facets and increase the ribs angle and the distance between the connecting rings. The methodology presented is robust and can be extended to larger dish diameters.

Osama A. El-Samni

Papers

Drag reduction of turbulent flow over thin rectangular riblets

Mechanical design of a low cost parabolic solar dish concentrator

Thermal performance of a heat-pipe evacuated-tube solar collector at high inlet temperatures

Wire fin heat exchanger using aluminium fumarate for adsorption heat pumps

Effect of wave amplitude on turbulent flow in a wavy channel by direct numerical simulation