4. Liquid ammonia is heated as it flows at a mean velocity of 2 m s through a ci

Question

4. Liquid ammonia is heated as it flows at a mean velocity of 2 m s through a circular pipe. The pipe, which has an internal diameter of 75 mm, is at a uniform temperature of 27°C, and the ammonia at a section 1.2 m from the inlet to the pipe has a temperature of -23°C. Use the following information to estimate the local heat transfer flux at l = 1.2 m. Note, the properties of ammonia liquid have been taken at -23°C, except where stated. Liquid ammonia properties Density = 600 kg m-3 Specific heat capacity = 4.86 kJ kg-1 K-1 Dynamic viscosity (at 27°C) 1.19 x 104kg m-1 s- Dynamic viscosity 2.05 × 10-4 kg m-1 s-1 Thermal conductivity = 5.11 × 10-4 kW m-1 s-1 Heat transfer correlations: 0.14 Nu = 186Re-Pr3 for laminar flow Nu = 0.023Re0.8 Pr033 for turbulent flow.

Explanation / Answer

Solution:

Let us go to the basics first.

Re = vd / = 600 x 2 x 0.075 / 2.05 x10^-4 = 4.39 x 10^5 which is > 2000 so turbulent flow

Pr = C / k = 2.05 x 10 ^-4 x 4.86 x 10^3 / (5.11 x 10 ^-4 x 10^3) = 1.949

Nu = 0.023Re^0.8Pr^0.33

=> Nu = 0.023 (4.39 x 10^5)^0.8 x (1.949)^0.33 = 936.132

Also, we know that

Nu = hd/k

=> h = 936.132 x 5.11 x 10^-4 x 10 ^3 / 0.075 =6378.18 w/m^2k

q= h (Ts - To)

=>q = 6378.18 *(27 - (-23)) = 318909 W/m2 or 318.9 KW/m2 (Answer => Heat Flux)

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