Past exams Flashcards
What is the Formula for the crictical radius
rc = k/h
What is the critical radius of insulation
The radius below which a cylindrical layer of insulation increases the HT instead of decreasing it
What is contact resistance
The thermal resistance caused by non smoothness of surfaces in contact
What is the Formula for the Reynolds Analogy
Cf/2 * Re = Nu
Does the therm. BL hight increase or decrease with increasing Pr number?
decreases
How was the Nusselt correlation obtained initially
purely mathematiclly
for internal convection, how do the mean temperature to length graphs look like for constant heat flux / constant Temperatur
Const HF: linear below Ts
Const. T: logarithmic below Ts
For the Rayleigh Number, what Temperatur do we use for beta, also give the formula
beta = 1/T
T is the mean film Temperature
Is T-x gradient higher for larger k or smaller k?
It is smaller for for larger k
How does the T-x for convection look like?
Logarithmic
If a pipe flow is heated from the inlet onward,
the thermal entry length is at least as long as the
hydrodynamic entry length?
Yes
For a heated and fully developed turbulent pipe
flow, doubling the thermal conductivity leads to
a 1.8-fold increase of the heat transfer
coefficient?
No, it leads to a 1.52-fold increase (1.5 is also
fine)
The Grashof number represents the ratio
between bouyancy and inertial forces?
No, it represent ratio of bouyancy and viscous
forces
Both natural and forced convection must be
considered if Gr/Re^2 =1?
Yes
The largest heat transfer coefficient for phase
change applications can be achieved in the film
boiling regime?
No, in nucleate boiling regime
The total, hemispherical emissivity and
absorptivity of a surface are equal, if the surface
is diffuse ?
No, if diffuse and gray
Wien’s law can be obtained by integrating
Planck’s law with respect to the wavelength?
No, by differentiating wrt to wavelength
There is no net radiative heat transfer between
surfaces if all participating surfaces are in
thermal equlibrium?
Yes
Is the Biot number h made up of just h_convec, or both h_convec and h_rad?
Both
