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

•Natural convection of nanofluids in a cavity is studied using a Buongiorno model.•By reducing the diameter of the nanoparticles the heat transfer rate increases.•At high Ra, the distribution of the solid particles remains almost uniform.•There is an optimum volume fraction of nanoparticles for maximum Nusselt number.

A numerical study is carried out concerning natural convection heat transfer of nanofluid in a two-dimensional square cavity containing several pairs of heater and coolers (HACs). Walls of the cavity are insulated and several pairs of heater and coolers (HACs) with isothermal walls of Th and Tc (Th > Tc  ) are placed inside the cavity. Two-dimensional Navier–Stokes, energy and volume fraction equations are solved using finite volume discretization method. The effects of various design parameters on the heat transfer rate and distribution of nanoparticles such as Rayleigh number (104⩽Ra⩽107104⩽Ra⩽107), volume fraction (0⩽φ⩽0.050⩽φ⩽0.05) and size of nanoparticles (25nm⩽dp⩽145nm), type of the nanoparticles (Cu, Al2O3 and TiO2), nanofluid average temperature (294K⩽Tave⩽324K), number of the cooler, location of the heater and arrangement of the HAC are investigated. The simulation results are indicated that, HACs location has the most significant influence on the heat transfer rate. It is also found that at low Rayleigh numbers, the particle distribution is fairly non-uniform while at high Ra, particle distribution remains almost uniform. Moreover, it is found that there is an optimal volume fraction of the nano-particles at each Rayleigh number in which the maximum heat transfer rate can be obtained.