For turbulent flow over a flat plate, the average value of Nusselt num...
The average heat transfer coefficient reduces with length as 1/5th power of the length.
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For turbulent flow over a flat plate, the average value of Nusselt num...
False Statement: The average heat transfer coefficient increases as 1/5 power of plate length.
Explanation:
The Nusselt number (Nu) is defined as the ratio of convective to conductive heat transfer across a boundary layer. It is used to predict the heat transfer coefficient in convective heat transfer. For turbulent flow over a flat plate, the average value of Nusselt number is given by the relation:
Nu = 0.664 (Re)0.5(Pr)0.33
Where,
Re = Reynolds number
Pr = Prandtl number
Reynolds number (Re) is a dimensionless number that describes the ratio of inertial forces to viscous forces and is given by:
Re = ρ u L / μ
Where,
ρ = density of fluid
u = velocity of fluid
L = characteristic length (plate length in this case)
μ = dynamic viscosity of fluid
Prandtl number (Pr) is a dimensionless number that describes the ratio of momentum diffusivity to thermal diffusivity and is given by:
Pr = μ Cp / k
Where,
Cp = specific heat capacity of fluid
k = thermal conductivity of fluid
Now, let's analyze the given options one by one:
a) 1/5 power of plate length: This statement is false. As we can see from the above equations, the Nusselt number depends on the Reynolds and Prandtl numbers. The plate length (L) only appears in the Reynolds number. Hence, the average heat transfer coefficient does not depend on the plate length to the power of 1/5.
b) 2/3 power of thermal conductivity: This statement is true. As we can see from the Prandtl number equation, the Nusselt number depends on the thermal conductivity (k) to the power of 1/3.
c) 1/3 power of specific heat: This statement is true. As we can see from the Prandtl number equation, the Nusselt number depends on the specific heat capacity (Cp) to the power of 1/3.
d) 4/5 power of a free stream velocity: This statement is false. The free stream velocity (u) appears in the Reynolds number equation. Therefore, the average heat transfer coefficient depends on the free stream velocity to the power of 1/2, not 4/5.
In conclusion, the false statement is option 'A', which states that the average heat transfer coefficient increases as 1/5 power of plate length.