What is the physical significance of Prandtl number?

What is the physical significance of Prandtl number?

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 the physical significance of the Prandtl number does the value of the Prandtl number depend on the type of flow or the flow geometry does the Prandtl number of air change with pressure does it change with temperature?

The Prandtl number is a fluid property that does not depend on the type of flow or flow geometry. The values of , cp and k are known to change with temperature so the Prandtl number will change with temperature.

What is the unit of Nusselt number?

NUSSELT NUMBER. 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. Nusselt number is a measure of the ratio between heat transfer by convection (α) and heat transfer by conduction alone (λ/L).

What is the difference between Nusselt number and Biot number?

Both are dimensionless parameters. Biot number uses thermal conductivity of the body (not fluid), whereas Nusselt number uses thermal conductivity of the fluid. The differences between Biot and Nusselt number is in definition of heat transfer coefficient, which is defined as: h=-k (dT/dn)w/(Tw-T0).

Which Prandtl number is lowest?

The Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity to thermal diffusivity

• gases – Pr ranges 0.7 – 1.0.
• dry air.
• water – Pr ranges 1 – 10.
• liquid metals – Pr ranges 0.001 – 0.03.
• oils – Pr ranges 50 – 2000.

What is Biot and Fourier number?

Fourier number is a measure of heat conducted through a body relative to heat stored. The Biot number is the ratio of the internal resistance of a body to heat conduction to its external resistance to heat convection.

What does the Biot number represent?

The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations. This ratio determines whether or not the temperatures inside a body will vary significantly in space, while the body heats or cools over time, from a thermal gradient applied to its surface.

What is the effectiveness of fin?

An effectiveness of indicates that the fin actually acts as insulation, slowing down the heat transfer from the surface. This situation can occur when fins made of low thermal conductivity materials are used. An effectiveness of indicates that the fins are enhancing heat transfer from the surface, as they should.

How do you determine fin effectiveness?

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1. The fin efficiency is defined as the ratio of the heat transfer to the fin to the heat transfer to an ideal fin. ηth =
2. qfin. hAfin(Tb − T∞) , Tf = T∞,and Afin = 2Ac + Atip (Square and Recatngular ) 1.35.
3. Atip = t × W. Fig. 2.4. Rectangular Fin. For cylindrical: Afin = πDL +

What NTU means?

The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference (LMTD).

Which of the following has maximum value of thermal conductivity?

Copper

Which has least value of conductivity?

Which of the following has least value of conductivity?

• A. Glass.
• Water.
• Plastic.
• Air.

What is thermal conductivity unit?

watts per

What is a good thermal conductivity value?

It is measured in Watts per Metre Kelvin (W/mK). To allow you to get a feel of insulating materials – their thermal conductivity varies between about 0.008 W/mK for vacuum insulated panels (so these are the best, but very expensive!) to about 0.061 W/mK for some types of wood fibre.

What does thermal conductivity depend on?

The thermal conductivity of a specific material is highly dependent on a number of factors. These include the temperature gradient, the properties of the material, and the path length that the heat follows.

How is thermal conductivity calculated?

It is generally expressed in terms of power/(length * temperature). These units describe the rate of conduction of heat through a material of unit thickness and for each Kelvin of temperature difference.

Is thermal conductivity the same as U value?

The U-value is expressed in watts per square metre, per degree kelvin, or W/m2K. Start by calculating the thermal resistances of each element (R-values). The R-value is the thickness of the product in metres ÷ Lambda (thermal conductivity). The lower the U-value, the more efficient the construction.

Does density affect heat?

Density often decreases with heat, because molecules spread out and move faster. Well, specific heat is usually specified as a quantity of heat per unit mass, so it doesn’t depend on the density at all. You can think of density as inversely proportional to the volume occupied per unit mass.