Comparative studies on air, water and nanofluids based Rayleigh–Benard natural convection using lattice Boltzmann method: CFD and exergy analysis

Abstract

The present study incorporates laminar natural convection and entropy generation in Rayleigh–Benard (R–B) convection with air, water and alumina–water nanofluid as working fluids. The fluid flow and energy equations are solved using D2Q9 and D2Q5 LBM models, respectively. The effects of Rayleigh numbers (Ra = 5 × 103, 104, 105) and volume fractions ( $$\emptyset$$ = 0 to 0.08) of nanoparticles on heat transfer and irreversibility are investigated. Results show that the heat transfer evaluated based on Nusselt number is enhanced due to addition of nanoparticles in the base fluid. The maximum enhancement in Nusselt number is found to be 13.93% at Ra = 105 with 8% of nanoparticle in base fluid. The various irreversibilities considered in this study are thermal, fluid flow and total irreversibility, where fluid flow and total irreversibilities in the study depend on irreversibility ratio. The irreversibility ratios taken into account are 10–2, 10–3, 10–4 and 10–5. One facet of study shows the deviation in onset of critical Rayleigh number for air is 1.58%. The other facet indicates dimensionless heat transfer, fluid flow and total irreversibility decrease with the increase in volume fraction of nanoparticles in the base fluid. The analyzed results of irreversibilities are presented in normalized form. In addition, dimensionless entropy generation maps and Bejan number contours are also plotted.