# Homework Help: Calculating heat transfer

1. Feb 8, 2012

### gomerpyle

1. The problem statement, all variables and given/known data

<< Oversize image deleted by Moderator >>

From this problem statement I am to find the heat transferred to the oxygen stream and the hydrogen stream.

2. Relevant equations

q''_0 = q''_oxygen + q"_hydrogen

3. The attempt at a solution

I first drew a thermal resistance circuit shown here:

<< Oversize image deleted by Moderator >>

From that, I did an energy balanced on the whole system:

<< Oversize image deleted by Moderator >>

I figured I could just calculate the temperature at the heat generating interface, then plug it back into the equations for q''_oxygen and q''_hydrogen, multiply by the area and I'll get the heat transferred in watts. The thing that confused me was when I calculated the temperature at that interface (T1) it ended up being lower than the ambient temperature in the oxygen stream as you can see in the pic. If this is true, that would mean heat would flow the other direction, but the problem statement implies this should not be the case (heat transfer TO not FROM the streams).

Either I'm doing something totally wrong, or there is not enough heat being generated at that interface to cause a heat transfer towards the oxygen stream. Can somebody please help shed some light on this?

Last edited by a moderator: Feb 8, 2012
2. Feb 8, 2012

### LawrenceC

Is your heat generation term correct? Your theory is correct. Heat generation is 3.5X10^5 W/cm^2. That is not 35 W/m^2.

Last edited: Feb 8, 2012
3. Feb 8, 2012

### gomerpyle

I converted 3.5*10^5 w/cm^2 to 35 w/m^2 because all of the thermal conductivities and heat transfer coefficients have the units of meters in them.

4. Feb 8, 2012

### LawrenceC

When you go from watts/cm^2 to watts/m^2, how can you possibly come up with a smaller number?

5. Feb 8, 2012

### Staff: Mentor

Please take care not to post oversize images that blow up the forum display window. I've had to delete your images to restore the forum display window -- please reply with a re-post of the images, after they have been re-sized small enough to fit in the standard display window. Thanks.