Numerical investigation on the effects of different heating systems and coil tube arrangements on the free convection heat transfer in a confined space filled with high viscosity fluid

Laminar free convection heat transfer in a confined space filled with high viscosity fluid is numerically investigated for different heating systems and coil tube arrangements at Rayleigh (Ra) numbers in the range of 10³ ≤ Ra ≤ 10⁷. In the heating system where the hot coil tubes are immersed in the high viscosity fluid, the effects of the horizontal and vertical distances between the coil tubes and the positions of the coil tubes along the vertical axis on the heat transfer characteristics are studied. The heating system where the coil tubes are placed outside the confined space is also considered. Two-dimensional unsteady conservation equations of the continuity, momentum and energy are discretized and solved by using the finite volume method. The results show that the average Nusselt numbers on all coil tubes except the lower coil tube at Ra = 10⁷ increase as the horizontal and vertical distances between the coil tubes increase. It is found that the horizontally aligned coil tube arrangement is more effective in terms of overall Nusselt number on the coil tube than the vertically aligned coil tube arrangement. The heating system where the coil tubes are outside the confined space at Ra = 10⁷ provides an improvement of 3.25% in average Nusselt values compared to the heating system with horizontally aligned coil tube arrangement. The horizontal distance between the coil tubes, the positions of the coil tubes along the vertical axis and different arrangements of coil tubes have considerable effects on the total Nusselt number. However, the effect of the vertical distance between the coil tubes on the total Nusselt number is negligible at high Ra numbers, 10⁵ ≤ Ra ≤ 10⁷. The numerical method and solution procedure are validated by comparing the results with the available data in the literature.