Josua P Meyer

University of Pretoria, Mechanical and Aeronautical Engineering, Faculty Member

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Papers by Josua P Meyer

Pool boiling of refrigerants over nanostructured and roughened tubes

by Bradley Bock, Josua P Meyer, and Christos N Markides

International Journal of Heat and Mass Transfer, 2020

This study investigated the heat transfer performance of three nanostructured surfaces and two pl... more This study investigated the heat transfer performance of three nanostructured surfaces and two plain surfaces: one roughened and one polished during the saturated pool boiling of refrigerants R-134a at 5 and 25 °C and R-245fa at 20 °C. Nanocoatings were applied to polished copper tubes through a layer-by-layer (LbL) process that deposited silica nanoparticles, a chemical oxidation process where an intertwined mat of sharp copper oxide (CuO) structures were generated and a commercial nanocoating process (nanoFLUX). A polished copper tube and a roughened copper tube were tested as comparison cases. All tubes were tested in the horizontal position in pool boiling over heat fluxes of 20 to 100 kW/m2, followed by a further increase in heat flux in an attempt to reach critical heat flux. The tubes were internally water heated and Wilson plots were conducted to characterise the internal heat transfer characteristics. The nanoFLUX surface had the highest heat transfer coefficients, the LbL and polished surfaces had the lowest heat transfer coefficients, and the CuO and roughened surfaces had intermediate heat transfer coefficients. The nanoFLUX surface had between 40 and 200% higher heat transfer coefficients than those of the polished tube. Both roughened tubes and nanocoated tubes showed typical exponentially increased heat transfer coefficients as heat flux was increased. However, the nanoFLUX and CuO surfaces displayed more heat flux sensitivity compared with the other surfaces. The nanoFLUX surfaces outperformed the other nanostructured surfaces due to a higher nucleation site density and outperformed the roughened tube due to a unique heat transfer mechanism. The nanoFLUX and CuO surfaces also experienced reduced critical heat flux compared with plain surfaces, thought to be caused by the trapping of vapour in the fibrous nanostructures, resulting in reduced wetting in the Cassie-Baxter state.

Flow regime maps for smooth horizontal tubes at a constant heat flux

by Josua P Meyer and Marilize Everts

The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl... more The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl number fluids, however, they are inaccurate at lower Prandtl numbers and tube diameters and specifically for water as the working fluid. The purpose of this paper was to develop a flow regime map that can be used for both high and low Prandtl number fluids for a wide range of tube diameters. Furthermore, not only to develop a map for fully developed flow, but also for developing flow. An experimental setup was designed, built and validated against results from literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively, and lengths of 5.5 m and 9.5 m, respectively, were used. Heat transfer measurements were taken at a range of Reynolds numbers from laminar to turbulent. Water was used as the test fluid and the Prandtl number was limited between 3 and 7. Experimental results from literature complemented the test matrix used for this study, especially with high Prandtl number data. Six flow regime maps were developed for a constant heat flux boundary condition and a square-edged inlet, and these flow regime maps are original for four reasons. Firstly, it contains contour lines that show the Nusselt number enhancements due to free convection effects. Secondly, it is valid for a wide range of tube diameters and Prandtl numbers. Thirdly, the flow regime maps were developed as a function of temperature difference (Grashof number) and heat flux (modified Grashof number). Finally, four of the six flow regime maps are not only valid for fully developed flow, but also developing flow.

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Josua P Meyer

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