Q. The local Nusselt number for a laminar vertical flat plate boundary layer with a uniform wall temperature is given by:
(for laminar flow)
Nu = 0.332 * (Pr^(1/3)) * (Ra^(1/2))
where Pr is the Prandt number, Ra is the Raleigh number given by
Ra = (y * (Tw - T)) / (v * e)
where y is the distance from the leading edge, Tw is the wall temperature, T is the free stream temperature, v is the kinematic viscosity, e is the thermal diffusivity, k is the thermal diffusivity, C is the specific heat, and h is the local heat transfer coefficient.
Complete the following:
a) Show that the Nusselt number based on the mean heat transfer coefficient over length L is given by
Nu = (hm * L) / k = 0.677 * (Ra * Pr)^(1/2) + 0.952
b) The coefficient of volumetric expansion, α, is defined as
α = (1 / V) * (dV / dt)
where V is the specific volume. Show that the volumetric expansion for an ideal gas is given by
α = (1 / T) * (dT / dt)
c) A vertical flat plate has a height of 0.5m and a width of 0.71m and reaches a uniform temperature of 30°C. If the room temperature is 18°C, show that the correlation given in part (a) can be used to calculate the heat transfer rate from the plate to the room. The critical Raleigh number for laminar to turbulent flow transition is given by Ra_crit = 10.
d) Estimate the convection heat transfer rate from the flat plate to the room.
e) What would the heat transfer rate be if the plate is kept horizontal with the hot surface facing up? Identify whether the flow is laminar or turbulent. The relevant correlations are given by
Nu = 0.54 * Ra^(1/4) for 10 < Ra < Ra_crit
Nu = 2 * 10^(1/4) for Ra > Ra_crit
(turbulent flow)
f) Explain why the flow will always be laminar if the plate is kept horizontal with the hot surface facing downwards.