NUMERICAL MODELLING OF THE HYDRODYNAMICS, HEAT TRANSPORT AND EVAPORATION OF ALUMINA NANOPARTICLES-BIODIESEL SURROGATE

Abstract

ABSTRACT

Biodiesel is a renewable fuel that can readily replace petrodiesel in internal combustion engines and furnaces. However, it contains less energy density than petrodiesel, which can be enhanced by adding energetic nanoparticles. This study modelled the flow, heat transfer and evaporation characteristics of an isolated biodiesel surrogate (methyl decanoate) droplet containing Al 2 O 3 nanoparticles sedimenting in air. The effects of nanoparticle volume fraction (φ), Reynolds number (Re) and evaporating temperatures in the range 0 to 0.1; 0.1 to 250; and 523 to 723 K, on flow vigour, Nusselt number (Nu), Sherwood number (Sh) and droplet regression, respectively, were investigated. The influence of internal circulation on the modes of heat transfer during non-evaporative heating was also examined with the initial droplet and ambient temperatures of 300 and 400 K, respectively. The problem governing equations, including the continuity, momentum and energy, were discretized and solved using the finite volume method with ANSYS Fluent 18.1 while a MATLAB program was written for implementing the evaporation model. At the domain inlet, outlet, walls and centreline, the Dirichlet, pressure outlet, Neumann and axisymmetric boundary conditions were imposed, respectively. User-defined functions were written in C++ to prescribe the continuity of tangential velocity and shear stress at the liquid-gas interface. A mesh consisting of 151423 nodes was chosen for the simulation after conducting a grid independence test. The validations of the drag, heat transfer and evaporation rates were in good agreement (±10%) with the experimental and numerical data obtained from literature for Re up to 100. The droplet's internal flow structure was similar to the Hill’s vortex for all Reynolds numbers. At critical Reynolds number, Re = 23.29, lung-shaped vortices were formed behind the droplet and grew in size with the increase in Re. At Re of 0.1, the isotherms inside the droplet were concentric about its centre, signifying pure diffusion. The isotherms within the droplet transformed from concentric circles at low Re to two deformed cells at high Re. There was an increase in Nu by 8.56 and 110.64%; 15.96 and 41.78% when Re increased from 0.1 to 50 and 250; and φ from 0 to 0.02 and 0.1, respectively. The square of the droplet diameter regressed linearly at a faster rate with the increase in Re than φ, obeying the classical D-squared law during evaporation for all the cases considered. An increase in φ from 0 to 0.04 enhanced the heat transfer during evaporation by 0.24 and 0.30% for Re of 100 and 200, respectively. Sherwood numbers increased with increasing φ for non-isothermal droplet evaporation but did not surpass 0.1%. The reduction in evaporation time for φ of 0.04 and 0.1 at Re of 100 were 1.66 and 1.20% respectively. Heat transfer enhancements were observed with the addition of Al 2 O 3 nanoparticles in methyl decanoate, while the changes in flow and mass transfer characteristics were marginal. The modelling and simulation of the evaporation characteristics of an isolated Al 2 O 3 nanoparticles-biodiesel surrogate droplet showed enhanced performance.