Numerical simulation of mixed convection of the nanofluid in heat exchangers using a Buongiorno model

• Mixed convection of nanofluids in a cavity is studied.
• By decreasing Ri and diameter of the nanoparticles, the heat transfer rate increases.
• At low Ri, the distribution of the solid particles remains almost uniform.

A numerical study of mixed convection heat transfer of nanofluid (Al2O3–water) in a lid driven cavity flow is carried out by using a Buongiorno model. Several pairs of heater and cooler (HACs) with isothermal walls of Th and Tc (Th > Tc) are located inside the cavity. Two-dimensional Navier–Stokes, energy and volume fraction equations are solved using the finite volume method. The effects of Brownian and thermophoresis diffusion, which cause non-homogeneity, are considered. The effects of volume fraction (0 ≤ φ ≤ 0.05) and nanoparticles' diameter (25 nm ≤ dp ≤ 145 nm) with the location, orientation and number of HACs on flow structure and heat transfer rate are examined in different Richardson numbers (0.01 ≤ Ri ≤ 100).The simulation results indicate that there is an optimal volume fraction of the nano-particles at each Richardson number for which the maximum heat transfer rate can be obtained. Moreover, it is found that for a constant surface area of the HAC at the entire range of Richardson number, the rate of heat transfer is increased by changing the orientation of the HAC from horizontal to vertical. Results also indicate that at low Ri, the distribution of the solid particles remains almost uniform.

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