Hello,

We have some relations among density, viscosity, and heat transfer, the thermal divisibility is a relation of alpha = k/(rho*Cp), where k is the diffusion coefficient. The Prandtl number is the relation between thermal diffusion and the momentum diffusion ( Pr = alpha/ nu and nu is the ratio of mu/rho, where mu is the dynamic viscosity.

The Pradtl number measures the ratio of thermal and momentum boundary layer. And Nusselt number is a Nu= h*L/k =f(Pr, Re) where is the Reynolds Number , Re =U*rho*L/mu.

A higher viscosity does not implicate in a higher transfer of heat. It will depends on the Thermal capacity Cp, condutivity k, viscosity mu, Velocity U, and the convection heat transfer coefficient h.

We do not have packed particles in fluids, it makes sense in solids, but in fluids there is no aggregation in this sense.

Regarding to shear stress, the friction generates heat, and this heat is come from the kinetic energy of the fluid: movement converted in heat. And it is not reversible. So, the viscosity dissipation always generates heat and it always exits, otherwise, the fluid will never stop to flow. The energy lost, pressure drop, etc, are due to viscosity dissipation and this energy is converted in heat. We can accounting for this effect or not. It generates a lot or a little heat? It is a complex source to be evaluated and the impact of the model is not always important compared to other else. It will depends on the case.

I advice you to read Adrian Bejan, Convective Heat Transfer. There are good discussion in the book related to this and can help you to answer your doubts.

best regards!

Hello,

Talking particularly for solids, as the density is higher compared to liquids and gases, the molecules will be closely packed and hence due to intermolecular vibration the Heat transfer due to conduction will be higher.

Whereas in liquids you have viscosity instead of density and heat transfers due to convection rather than condition, and that too increased with increase in viscosity generally.

And in gases as the molecules are very loosely bound, you'll have very low heat transfer, which is also due to convection mainly.

Hi Alexander

The question is too open. Let’s focus the heat transfer associated with forced convection in pipes. Say, a hot fluid flows in a smooth pipe and heat is lost to the surrounding air atmosphere. Take for instance the Dittus and Boelter (1930) correlation to predict h, the convective heat transfer coefficient [say, Btu / (hr ft2 ºF)], at the internal pipe surface (see heat transfer textbooks). The variables you referred to (fluid density and viscosity) are right there as dimensionless groups. Density and viscosity are present in Re (Reynolds number). Viscosity is also present in Pr (Prandtl number). The Nu (Nusselt number) contains h.