The Lattice Boltzmann method is used to simulate the flow patterns of natural convection in horizontal cylindrical annulus for aspect ratios in the range of 0.4≤A≤10 and for Prandtl numbers varying from 0.1 to 0.7. At Pr = 0.3 and A = 2, flow patterns on the whole range of Rayleigh number are mapped indicating the lower and upper critical values for transitions. At Pr = 0.7, the influence of aspect ratio on flow pattern is analyzed acquiring the result that the oscillation flow never happens at A≤3. At A = 2, several Prandtl numbers are calculated at certain Rayleigh number and the conclusion is that the steady upward flow keeps when 0.5≤Pr. The results are found in good agreement with existed studies.

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