Question

For laminar flow over flat plate, the average value of Nusselt Number Nu is prescribed by
$Nu=0.664R{e}^{0.5}P{r}^{0.33}$
Which of the following should be changed independently keeping other parameters same to double the heat transfer coefficient?

• A. Specific heat to be increased 9 times
• B. Density should be increased 8 times
• C. Dynamic viscosity has to be decreased 64 times
• D. Plate length should be decreased 16 times

Answer 1

The correct option is C Dynamic viscosity has to be decreased 64 times

$\frac{\overline{h}l}{k}=0.664{\left[\frac{\rho Vl}{\mu }\right]}^{1/2}{\left[\frac{\mu C_p}{k}\right]}^{1/3}$

$\Rightarrow \overline{h}\propto V^{1/2}{\rho }^{1/2}{k}^{2/3}{c}_P^{1/3}{l}^{-1/2}{\mu }^{-1/6}$

From above expresssion it is clear make $\overline{(}h)$ twice, decrease $\mu$ by 64 times.