If you thought the radiator stuff sucked, this is 100 times worse. Delete
now if you're not mathematically inclined.
Ok, I didn't have time to run a "hard" number, but from some data, 90
degree ambient on a sunny day, the car will heat up to around 130 deg F
steady state after 30 minutes or so. Some info on data sources and
methodology:
Q_solar = (solar flux) * (alpha) * (surface area) * PAF
The solar flux is from Mil-E-87145, Appendix A, Fig 1-17 (get it online
somewhere). At sea level, it is around 360 BTU/hr-sq. ft at sea level
(increases with altitude). Alpha is the absortivity of the paint (usually
.2 for gray aircraft paint, no idea for a gold car; contact DuPont or PPG).
Surface area is the surface area of the vehicle cabin in sq. ft. PAF is
projected area factor, or how much of the area in question is parallel to
the incidence of sunlight in sq.ft. Now,
Q_windshield = (solar flux) * (tau) * (surface area of windshield)
same solar flux, tau is transmissivity for a windshield (around .66 from
SAE Applied Thermodynamics Manual).
Now you have the cockpit radiating back to the sky:
QRad = epsilon*PAF*surface area*sigma
where: epsilon is paint emissivity
PAF is projected area factor
sigma is Stefan-Boltzmann constant
PAF - Projected Area Factor
As - Total surface area ,ft2
s - Stefan-Boltzman constant
This is getting out of hand, sorry. I'll wrap it up fast - get an
engineer (I'm for hire ;-) ) to figure out the convective heat transfer
from the car back to the ambient, h*A*delta temp (once the car heats up it
loses radiant heat to the ambient while still absorbing heat from the solar
load). Sum these up, and get the overall heat transfer coefficient for the
car, the UA. You need to know the insulation and thickness of head liner,
etc. This is the hard part for us to calculate on an aircraft. A
manufacturer should have this data. Chrysler's thermo department blows
ours away, for instance, and has TONS of data.. Figure out your net heat
transfer, and you can get the inside temp (steady state) based on this
info.
Really tough to email, and the dinner bell is ringing.
Ron
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