Solving Rate of Combustion Problem

[daimlerpogi]

Rate of combustion??

Homework Statement

1. The broader side of a rectangular fiberglass of dimensions 3m x 5m is brought near the flame. The flame is produced upon combustion of gasoline. If the rate of combustion is 0.002 g/s, determine the minimum thickness of the fiberglass so that the temperature difference between hot and cold sides of the fiberglass is 40 degree Celsius. The latent heat of combustion of gasoline is 46,000 J/g.

k = 0.04 W/m.C

Homework Equations

Fourier's Law of Conduction

H = kA(Th-Tc)/L
where:
H - Heat if conduction (Watts of J/s)
k - Thermal Conductivity
A - Area (meter squared)
Th - Hot side temperature in Celsius
Tc - Cold side temperature in Celsius
L - Thickness

Heat of Combustion
Qc = mLc
m - mass in kg
Lc - Latent heat of combustion

The Attempt at a Solution

I really don't know where to start. The rate of conduction that is given is supposed to be H right? But the unit is g/s instead of J/s. And also, there aren't any Th and Tc given. Using the heat of combustion formula, i supposed you can get the mass of gasoline in the given 0.002 g/s. This is the last problem I'm solving. (I solved the other 4 but can't solve this one)

 

[tiny-tim]

Welcome to PF!

The broader side of a rectangular fiberglass of dimensions 3m x 5m is brought near the flame. The flame is produced upon combustion of gasoline.
If the rate of combustion is 0.002 g/s, determine the minimum thickness of the fiberglass so that the temperature difference between hot and cold sides of the fiberglass is 40 degree Celsius. The latent heat of combustion of gasoline is 46,000 J/g.

k = 0.04 W/m.C
…
I really don't know where to start. The rate of conduction that is given is supposed to be H right? But the unit is g/s instead of J/s. And also, there aren't any Th and Tc given. Using the heat of combustion formula, i supposed you can get the mass of gasoline in the given 0.002 g/s.

Hi daimlerpogi ! Welcome to PF!

You don't need either Th or Tc separately … the formula only needs the combination (Th - Tc), which is 40C.

If you're bothered about the units, just put them side-by-side until you see how to fit them like a jigsaw

In this case, they give you a g/s and a J/g, but you want a J/s.

ok … just multiply … (J/g)(g/s) = J/s !

 

[daimlerpogi]

determine the minimum thickness of the fiberglass so that the temperature difference between hot and cold sides of the fiberglass is 40 degree Celsius.

My bad, didn't notice that! Thanks tiny-tim!

I got it now!

(46000 J/g)(0.002 g/s) = 92 W

Thanks again! :D