THE INTENSITY OF CONVECTION OF FLUIDS WITH DIFFERENT PRANDTL NUMBER IN A VERTICAL CYLINDER OF LARGE ASPECT RATIO
We study the free convection in a long vertical cylinder with constant temperature gradient on the side surface. The wall temperature does not change its value in the circumferential direction, the flow is laminar; the fluid is Newtonian; the change in the density from the temperature used in the Boussinesq approximation, the problem is stationary. The movement is simulated by means of CFD in Ansys CFX. Verification of the used model is made. The natural convection of three fluids (water, gas-saturated oil, degassed oil) is studied for aspect ratio (the ratio of height to diameter) of 60, 80, 100, 200. Types of convective motion depending on the Rayleigh number is classified as “immovable” Ra < 3·103, “spiral” 3·103 < Ra < 1,2·104, “cellular” 1,2·104 < Ra. We define qualitative and quantitative criteria, on which the classification is based. A spiral movement is characterized by a constant average cross-section of vertical velocity. With cellular motion, the average vertical velocity of cross section, experiences a significant change in the height of the cylinder. The intensity of convection is characterized by the vertical speed. The influence of Prandtl number on the intensity of convection is marked. Empirical dependence of the average volume of the vertical speed of the Rayleigh number is received. A single dependence for different fluids is obtained by the use of scale speed ur = (Q0g/ρcpγ)1/3 Q0 - the average value of the modulus of the heat flow through the side wall, calculated by means of Ansys CFX post-processor; g - acceleration of gravity; ρ - density; c p - the heat capacity at constant pressure, γ - the vertical temperature gradient on the wall. We determine the value of aspect ratio (h = 60) at which the cylinder can be considered infinitely long for the considered problem.