Question

For laminar forced convection over a flat plate, if the free stream velocity increases by a factor of 2. the average heat transfer coefficient

• A.
  remains same
• B.
  decreases by a factor of $\sqrt{2}$
• C.
  rises by a factor of $\sqrt{2}$
• D.
  rises by a factor of 4

Answer 1

The correct option is C

rises by a factor of $\sqrt{2}$
For the laminar flow over flat plate,
$Re<3\times {10}^5$
Nusselt number,
$Nu=\frac{hl}{k}=0.664R{e}^{1/2}P{r}^{1/3}$
Where $Re=\frac{Ul}{v}$
$\therefore$ $\frac{hl}{k}=0.664\times {\left(\frac{Ul}{v}\right)}^{1/2}\times P{r}^{1/3}$
$h\propto U^{1/2}$
$\frac{h_2}{h_1}={\left(\frac{U_2}{U_1}\right)}^{1/2}$
$={\left(\frac{2U_2}{U_1}\right)}^{1/2}=\sqrt{2}$
Or $h_2=\sqrt{2}{h}_1$