Optimal selection of annulus radius ratio to enhance heat transfer with minimum entropy generation in developing laminar forced convection of water-Al2O3 nanofluid flow

Heat transfer and entropy generation of developing laminar forced convection flow of water-Al_2O_3 nanofluid in a concentric annulus with constant heat flux on the walls is investigated numerically. In order to determine entropy generation of fully developed flow, two approaches are employed and it is shown that only one of these methods can provide appropriate results for flow inside annuli. The effects of concentration of nanoparticles, Reynolds number and thermal boundaries on heat transfer enhancement and entropy generation of developing laminar flow inside annuli with different radius ratios and same cross sectional areas are studied. The results show that radius ratio is a very important decision parameter of an annular heat exchanger such that in each Re, there is an optimum radius ratio to maximize Nu and minimize entropy generation. Moreover, the effect of nanoparticles concentration on heat transfer enhancement and minimizing entropy generation is stronger at higher Reynolds.

Thermal protection mechanism of heat pipe in leading edge under hypersonic conditions

Sharp local structure, like the leading edge of hypersonic aircraft, confronts a severe aerodynamic heating environment at a Mach number greater than 5. To eliminate the danger of a material failure, a semi-active thermal protection system is proposed by integrating a metallic heat pipe into the structure of the leading edge. An analytical heat-balance model is established from traditional aerodynamic theories, and then thermal and mechanical characteristics of the structure are studied at Mach number 6–8 for three refractory alloys, Inconel 625, C-103, and T-111. The feasibility of this simple analytical method as an initial design tool for hypersonic aircraft is assessed through numerical simulations using a finite element method. The results indicate that both the isothermal and the maximum temperatures fall but the von Mises stress increases with a longer design length of the leading edge. These two temperatures and the stress rise remarkably at a higher Mach number. Under all investigated hypersonic conditions, with a 3 mm leading edge radius and a0.15 m design length, the maximum stress exceeds the yield strength of Inconel 625 at Mach numbers greater than 6, which means a material failure. Moreover, both C-103 and T-111 meet all requirements at Mach number 6–8.

Behavior of microbubbles on spatially controlled golden nanoparticles

Incipient plasmonic bubble formation is observed around gold nanopillars with different inter-nanopillar separations. The experimental measurements and theoretical analysis show that the nanobubble formation is due to the enhanced plasmonic resonance rather than from the laser heating. Inter-nanopillar distribution may lead to threshold fluence variations. The lifetime of plasmonic bubbles can reach several minutes. Furthermore, both the radius and the growth rate of the plasmonic nanobubble increase as the inter-nanopillar distribution decreases.Smaller-spacing distributed arrays produced larger bubbles. The maximum growth rate of the bubbles can be reached at about 883.5 × 10^-6m∕s on 1 μm nanopillars, but it is only 56.9 × 10^-6m∕s on 4 μm nanopillars.