Lab 3 Flashcards
measuring convective heat transfer coefficients
ex. measure the heat transfer rate from an airplane wing with fuel inside
parameters: Uinfinity
free stream velocity
parameters: L
characteristic wing length
parameters: μ
fluid viscosity
parameters: p (rho)
density
parameter: k
thermal conductivity
parameter: Cp
specific heat
parameters: relevant temperature
ambient and surface
dependeent variable
convective heat transfer coefficanet h-> q
reynolds number
Re = pUinfL/u = Uinf L / v = inertial forces/vicous forces
critical reynolds number
Re ~5x10^5 for a flat plate
transition to turbulent flow
xc/L = Rexc/ReL
ReL > Rexc
trip
this influences the flow to transition to turbulent flow
momentum (velocity) boundary layer
region of flow characterized buy velocity gradient and shear stress
laminar flow over a flat plane
thickness = 5x/sqrt(Rex)
turbulent flow over a flat plane
thickness = 0.37xRex^-1/5
thermal boundary layer thickness
flow characterized by temperature gradient and heat flux
prandtl number
viscosity/thermal diffusivity
pr»_space; 1 thicker momentum BL
pr ~ 1 about equal (gas)
Pr «_space;thicker thermal BL
nusselt number
Nu = hL/kf
laminar flow subject to constant surface temperature
Nux = hx/k = 0.322Re^1/2Pr^1/3
laminar flow subject to constant surface temperature with average boundary layer results
Nu = 0.664 Re^1/2Pr^1/3
laminar flow subject to constant surface temperature for constant heat flux
Nu = 0.453Re^1/2Pr^1/3
pr>0.6
how to measure thermal boundary layer
vertical thermal couple
how to measure velocity boudnary layer
preston probe
bernoulli’s equation to get velocity measurment
ptotal = pu^2/2 + pstatic
pu^2/2 = pdynamic
u = velocity
Local heat transfer coefficient measurement
htop = hbottom = h (symetric consfiguration)
heat disipated by two heaters
qheater = V^2front/Rfront + V^2back/Rback
local convection heat flux leaving top and bottom surface
q’’ = qheater /2LW
local convection heat flux leaving top and bottom surface + radiation
