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Question

A fluid of density 978 kg/m3 is heated in pipe, enters a 20 mm diameter tube with a velocity of 2 m/s at 10 C. Convective heat transfer coefficient at outside surface of tube is equal to 10 kW/m2. The length of tube is 5 meter and properties of water at mean temperature is approximated as following,
k=0.332 WmK, μ=4×104 kg/m-s, Pr=2.54
the value of overall heat transfer coefficient is

A
4.73 kW/m2K
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B
8.73 kW/m2K
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C
3.53 kW/m2K
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D
1.73 kW/m2K
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Solution

The correct option is C 3.53 kW/m2K
D=20 mm,
ρ=978 kg/m3,
μ=4×104 kg/m,
Pr=2.54

Re=ρVDμ=978×2×0.024×104Re=97800

Re>2300 flow is turbulent

Using Dittus Boltier equation,

Nu=0.023(Re)0.8(Pr)0.4

hi=328.04×0.3320.02=5445.5 W/m2K

1U=1hi+1h0=15445.5+110000

U=3525.6 W/m2K

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