Question

Could you give me a basic introduction about “Validation of Reynolds analogy concept “ ?

Answer 1

1.....Reynolds analogy holds better for gases since the kinematic viscosity and the thermal diffusivity are of about the same order. (Making a once again a direct proportionality between shearing stresses ad heat transfer).

2.....The Reynolds Analogy holds true for flows in which the temperature profile and the velocity profile are “similar” (including boundary conditions) and thus the heat and momentum are transported at the same rate. These conditions are met if: 1 the pressure gradient in the direction of the flow is zero, 2 the coefficient of turbulent exchange thermal is equal to the coefficient of turbulent exchange for the momentum (turbulent Prandtl number approximately equal to 1) or 3 if the kinematic viscosity is approximately equal to the thermal diffusivity (Prandtl number is approximately equal to 1). Therefore, for external flow (Laminar or turbulent), the Reynolds analogy holds true since the pressure gradient in the direction of the flow is usually zero. For internal flows, this is not the case, (dp/dx does not equals zero). However, for internal fully developed turbulent flows the pressure and temperature gradient along the flow direction is approximately constant.