  Natural convection from a stack of isothermal solid horizontal cylinders has
been investigated numerically in a three dimensional computational domain.
Simulations were conducted in both laminar and turbulent flow regimes of
Rayleigh number (Ra) spanning in the range (10^4 to 10^8) and (10^10 to 10^13),
respectively. In the present study, the length to diameter ratio of the
cylinders has been varied in the range 0.5 to 20. Three different stack
arrangements were considered for the numerical simulations by arranging three,
six and ten number of cylinders in a triangular manner. The present
computational study is able to appraise very interesting thermo-buoyant plume
structures around the stack of cylinders. The average Nusselt number (Nu) shows
a positive dependence on Ra for all L/D. The average Nu for a stack of
three-cylinders is marginally higher than that of six-cylinders followed by
ten-cylinders. Furthermore, at a particular Ra, Nu is significantly higher for
short cylinders (low L/D) and decreases with increase in L/D up to 10 or 15 and
remain constant for long cylinders. In addition, the present numerical results
are also compared with the stack of hollow cylinders. A new Nusselt number
correlation has been developed for different stacks as a function of Ra and
L/D, which would be useful to industrial practitioners and academic
researchers.
