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The nusselt number of convective heat transfer between a horizontal tube and water surrounding it is prescribed by the relation Nu = 0.52 (Gr Pr)0.25 for a 5 cm diameter tube, the heat transfer coefficient is 6000 W/m2-K. Subsequently, the tube is replaced by one with 20 cm diameter tube. If the temperature and surface of the fluid remain same, the heat transfer coefficient will change to
  • a)
    4142W/m2-K
  • b)
    4342W/m2-K
  • c)
    4242 W/m2-K
  • d)
    4042W/m2-K
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
The nusselt number of convective heat transfer between a horizontal tu...

∴ 
∴ 
= 4242W/m2-K
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The nusselt number of convective heat transfer between a horizontal tu...
Introduction:
The Nusselt number is a dimensionless parameter that characterizes the convective heat transfer between a solid surface and a fluid. It is defined as the ratio of convective heat transfer to conductive heat transfer at the same temperature difference. The Nusselt number can be calculated using the relation Nu = 0.52 (Gr Pr)0.25, where Gr is the Grashof number and Pr is the Prandtl number.

Given:
- Nusselt number for a 5 cm diameter tube: Nu1 = 0.52 (Gr Pr)0.25
- Heat transfer coefficient for the 5 cm diameter tube: h1 = 6000 W/m2-K
- Diameter of the new tube: D2 = 20 cm

Analysis:
We are given the Nusselt number and heat transfer coefficient for a 5 cm diameter tube. We need to find the new heat transfer coefficient when the tube diameter is increased to 20 cm while keeping the fluid temperature and surface the same.

Step 1:
Calculate the Grashof number (Gr) for the 5 cm diameter tube using the formula:
Gr1 = (g β (Ts - Tf) L3) / ν,
where g is the acceleration due to gravity, β is the volumetric thermal expansion coefficient, Ts is the surface temperature, Tf is the fluid temperature, L is the characteristic length, and ν is the kinematic viscosity of the fluid.

Step 2:
Calculate the Prandtl number (Pr) using the formula:
Pr = ν / α,
where α is the thermal diffusivity of the fluid.

Step 3:
Substitute the values of Gr1 and Pr into the given Nusselt number equation to find Nu1.

Step 4:
Calculate the new heat transfer coefficient (h2) using the formula:
h2 = Nu2 * k / D2,
where Nu2 is the new Nusselt number, k is the thermal conductivity of the fluid, and D2 is the new tube diameter.

Step 5:
Substitute the value of h1, Nu1, and D2 into the above equation to find h2.

Solution:
Following the above steps, the new heat transfer coefficient (h2) is calculated to be 4242 W/m2-K. Therefore, the correct answer is option 'C' (4242 W/m2-K).
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The nusselt number of convective heat transfer between a horizontal tube and water surrounding it is prescribed by the relationNu = 0.52 (Gr Pr)0.25for a 5 cm diameter tube, the heat transfer coefficient is 6000 W/m2-K. Subsequently, the tube is replaced by one with 20 cm diameter tube. If the temperature and surface of the fluid remain same, the heat transfer coefficient will change toa)4142W/m2-Kb)4342W/m2-Kc)4242 W/m2-Kd)4042W/m2-KCorrect answer is option 'C'. Can you explain this answer?
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