Natural convection around a pair of hot and cold horizontal microtubes at low Rayleigh numbers

Natural convection around a pair of hot and cold horizontal microtubes in a relative large square enclosure was numerically studied using multi-block Lattice Boltzmann method (LBM). Five typical arrangements of the tube pair were made by placing the hot tube at various positions relative to the cold tube, such as side-by-side with, above or under. The hot and cold horizontal microtubes were put at the middle of a horizontal square enclosure. No slip flow conditions were given for all of the tube and the enclosure walls. Constant temperature conditions were given for the hot and cold tubes, respectively, and the adiabatic condition for the square enclosure. The computational code was validated by performing a benchmark case of a single hot isothermal horizontal microtube in the closed cold square cavity. A simple experimental loop was set up in order to make a comparison with the obtained numerical results. Two stainless microtubes with outer diameter of 1.00 mm were used for heat transfer test with a counter-current arrangement. The inlet water temperatures through the hot and cold microtubes were separately controlled by two water baths. The numerical simulated range of Rayleigh number is Ra < 1500. A correlation of Nusselt number with Rayleigh number was proposed based on the numerical results, which is in general agreement with the obtained experimental data.