Natural convection in a narrow horizontal cylindrical annulus is numerically investigated for fluids of Pr less than or equal to 0.3. For Pr less than or equal to 0.2, hydrodynamic instability induces steady or oscillatory flows consisting of multiple like-rotating cells. For Pr = 0.3, thermal instability creates a counter-rotating cell on the top of annulus. For a fluid of Pr approximate to 0, the multiple cells are distributed uniformly in the lower and upper parts of annulus. As PI is increased, the cells are shifted upwards. The like-rotating cells drift downward, as time goes on, and the speed of travel increases with increase of Pi. For Pr = 0.3, the oscillatory flows after secondary instability consist of multiple like-rotating cells in the vertical section of annulus and one or more counter-rotating cells on the top part, and multiple oscillatory flows are found.