What does a higher Prandtl number mean?

What does a higher Prandtl number mean?

Higher values of Prandtl number strongly affects not only the heat transfer, but also the temperature distribution. Prandtl number defined as (μ. Cp/k) is a fluid property. It acts as a link between the fluid flow and how it affects the temperature profile for visualization think of a simple geometry like flat plate.

Is higher Nusselt number better?

In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. A value between one and 10 is characteristic of slug flow or laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range.

What material do you expect to have a high Prandtl number and why?

Thermal diffusivity dominates in the case of mercury. Engine oils have a high Prandtl number and so the convective component of heat transfer is higher.

Does Prandtl number change with temperature?

It is found that the temperature decreases as the Prandtl number Pr increases. Physically, the increase of Pr means the decrease of thermal conductivity of fluid.

What happens when Prandtl number is less than 1?

Small values of the Prandtl number (less than 1) in a given fluid indicates that thermal diffusion occurs at a greater rate than momentum diffusion and therefore heat conduction is more effective than convection.

Where is Prandtl number used?

The Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity to thermal diffusivity. The Prandtl Number is often used in heat transfer and free and forced convection calculations.

What is L in Nusselt number?

Nusselt number, Nu, is the dimensionless parameter characterizing convective heat transfer. It is defined as. where α is convective heat transfer coefficient, L is representative dimension (e.g., diameter for pipes), and λ is the thermal conductivity of the fluid.

What does Prandtl number depend on?

fluid properties
The Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity to thermal diffusivity. The Prandtl Number is often used in heat transfer and free and forced convection calculations. It depends on the fluid properties.

Why is Prandtl number important?

In heat transfer problems, the Prandtl number controls the relative thickness of the momentum and thermal boundary layers. When Pr is small, it means that the heat diffuses quickly compared to the velocity (momentum).

Is Prandtl number dimensionless?

The Prandtl number is an example of a dimensionless number that is an intrinsic property of a fluid. Fluids with small Prandtl numbers are free-flowing liquids with high thermal conductivity and are therefore a good choice for heat conducting liquids.

What is Prandtl number worth?

Prandtl number is a characteristic of the fluid only. For air at room temperature Pr is 0.71 and most common gases have similar values. The Prandtl number of water at 17°C is 7.56. Liquids in general have high Prandtl numbers, with values as high as 105 for some oils.

What do small values of the Prandtl number mean?

Small values of the Prandtl number, Pr << 1, means the thermal diffusivity dominates. Whereas with large values, Pr >> 1, the momentum diffusivity dominates the behavior. For example, the listed value for liquid mercury indicates that the heat conduction is more significant compared to convection, so thermal diffusivity is dominant.

What kind of liquid has a good Prandtl value?

Tab. 9.3 lists Prandtl values for selected fluids. As can be seen, liquid metals are very good heat transfer liquids. Interestingly, air is a decent heat transfer liquid as well, whereas typical organic solvents are not.

How are Prandtl numbers used in heat transfer?

Heat transfer applications with low-Prandtl number fluids are often in the transition range between conduction and convection dominated regimes. When dealing with Prandtl numbers, we have to define a laminar part of Prandtl number and a turbulent part of Prandtl number.

How is the Prandtl number different from the Reynolds number?

In comparison to Reynolds number, the Prandtl number is not dependent on geometry of an object involved in the problem, but is dependent solely on the fluid and the fluid state. As such, the Prandtl number is often found in property tables alongside other properties such as viscosity and thermal conductivity.