熱傳學期末考
Closed book
1 (25%) Consider flow in a circular tube. Within the test section length (between 1
and 2) a constant heat flux q”S is maintained.
(a). For the following two cases, sketch the surface temperature TS(x) and the fluid
mean temperature Tm(x) as a function of distance along the test section x In case A
flow is hydrodynamically and thermally fully developed. In case B flow is not
developed.
(b). Assuming that the surface flux q”S and the inlet mean temperature Tm,1 are
identical for both cases, will the exit mean temperature Tm,2 for case A be greater than,
equal to or less than Tm,2 for case B ? Briefly explain why
2. (25%) Atmospheric air at 25C and a velocity of 0.5m/s flows over a 50-W
incandescent bulb whose surface temperature is at 140C. The bulb whose surface
temperature is at 140C. The bulb may be approximated as a sphere of 50-m
diameter. What is the rate of heat loss by convection of the air?
3. (10 %). Define the following no-dimensional parameters and their physical
interpretation, Re, Pr, Gr, Nu. For forced convection over a flat plate, what is the
critical parameter and value to characterize the laminar or turbulent convection?
For natural convection over a vertical flat plate, what is the critical parameter and
value to characterize the laminar or turbulent convection?
4. (20%) In two phase flow problem, The Nukiyama first identify different regimes of
pool boiling using power-controlled heating apparatus for demonstrating the boiling
curve as below.
E
C
q”
A
D
Te
Please describe the four different boiling regimes and their physical mechanisms
, at -A, A-C, C-E, E- , and describe physical meaning of the A, C , D point
5. (20%) For flow of a liquid metal through a circular tube, the velocity and
temperature profile at a particular axial location may be approximated as being
uniform and parabolic, respectively. That is, u(r) =C1 and T(r) -TS =C2 [ 1-(r/r0)2],
where C1 and C2 are constants. What is the values of the Nusselt number NuD at this
location ?