Water at 20°C flows normal to the axis of a circular tube with a velocity of 1.5 m/sec. The diameter of the tube is 25 mm. Calculate the average heat transfer coefficient if the tube surface is maintained at a uniform temperature of 80°C. Also estimate the heat transfer rate per unit length of the tube. Properties of water at
Tf = $\frac{20+80}{2}$ = 50°C are
Specific heat, cp = 4.1813 kJ/kg-K
Kinematic viscosity,n=0.568´10–6 m2 /sec
Thermal conductivity, k = 0.6395 W/m-K
Prandtl number, Pr = 3.68
Density, r = 990 kg/m3
Dynamic viscosity of water
($\mu _{W }$) at 80°C = 3.5456 ´ 10–4 kg/m-sec
($\mu _{\infty }$) at 20°C = 1.006 ´ 10–3 kg/m-sec.
Use the relation
