What does a high Grashof number mean?
At higher Grashof numbers, the boundary layer is turbulent; at lower Grashof numbers, the boundary layer is laminar that is in the range 103 < GrL < 106.
How is Grashof number calculated?
The Grashof Number formula for pipes is: GrD=g⋅β(Ts−T∞)D3ν2 G r D = g · β ( T s – T ∞ ) D 3 ν 2 , where: GrD is the Grashof Number for Pipes.
What is the value of Grashof number?
The significance of the Grashof number is that it represents the ratio between the buoyancy force due to spatial variation in fluid density (caused by temperature differences) to the restraining force due to the viscosisty of the fluid.
Can Nusselt number be less than 1?
It is clear from earlier statement that the Nusselt number can never be less than 1 because the convective flux can be zero but the molecular flux term will be similar in the numerator and the denominator. Therefore, the minimum value of Nusselt number is 1 when the convective flux is zero.
Why Prandtl number is 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).
What is the purpose of Nusselt number?
The Nusselt number is a dimensionless number closely related to Péclet number and both numbers are used to describe the ratio of the thermal energy convected to the fluid to the thermal energy conducted within the fluid.
What is the value of prandtl no?
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.
What is the importance of Nusselt number?
A Nusselt number of value one represents heat transfer by pure conduction. 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 is L in Nusselt number?
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. 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?
Biot number uses thermal conductivity of the body (not fluid), whereas Nusselt number uses thermal conductivity of the fluid. Nusselt number can be used for heat exchangers, because fluid convection dominates, calculation of the heat transfer coefficient of various flows such as in pipes.
Is Biot number dimensionless?
The Biot number is a dimensionless group that compares the relative transport resistances, external and internal.
Which metal has highest thermal conductivity?
Diamond is the leading thermally conductive material and has conductivity values measured 5x’s higher than copper, the most manufactured metal in the United States. Diamond atoms are composed of a simple carbon backbone that is an ideal molecular structure for effective heat transfer.
How do you increase fin effectiveness?
Since the fin efficiency is inversely proportional to Λ, it can be improved either by increasing k and B, or by decreasing 〈h〉 and L. If the average heat transfer coefficient, 〈h〉, is increased due to an increase in the air velocity past the fin, the fin efficiency decreases.
What is the effectiveness of fin?
Effectiveness of fin (εfin): It is defined as the ratio of the actual heat transfer that takes place from the fin to the heat that would be dissipated from the same surface area without fin.
Can Fin effectiveness be less than 1?
The fin efficiency will always be less than one, as assuming the temperature throughout the fin is at the base temperature would increase the heat transfer rate.
Under what condition does fin efficiency become 100%?
Under what situations does the fin efficiency becomes 100%. The heat transfer from the fin will be maximum. For the cases of a constant cross-section of very long fins and fins with insulated tips, the fin efficiency can be 100.
What does value of fin effectiveness is 1 mean?
1. For effectiveness εf =1 indicates that the addition of the fin to the surface does not affect heat transfer at all. In other word, the heat transfer to the fin through the base area Aw is equal to the heat transferred from the same area Aw to the surrounding medium.
How can we know fin is infinitely long?
Therefore, a fin whose length is L=m/5 can be considered to be an infinitely long fin.