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Heat Transfer problem

  1. Jul 23, 2012 #1
    I have a quick question that maybe someone can help me out with. Let’s say I have an object that has a heat output of 48.7 KJ that is constant and it is surround by hydrogen that it flowing around it. Like a rock in a stream. If I know the starting temp of the hydrogen how would I determine how much heat is transferred from the object to the surroundings. What I wanted to do is use hydrogen as a heat exchange. I think this is forced convection but haven’t covered heat transfer in school yet. Would using forced heat convection equations help me determine the heat of the hydrogen after passing the object? Is this a heat transfer and fluids problem?
    Basically I wanted to crunch the numbers on a sterling engine that used hydrogen as the working fluid and is heated from the waste heat of another process. I tried just using the Q=ncp(Th-Tc) but with I get huge numbers for the new temp and so I don’t think I am doing this right
    If we take q=48700 J Tc=293K n=1 mole cp=28.834 J mol-1 K-1
    Th=1981.98 K which seems to be a huge number

    So I am looking for some help in how to model this to be somewhat realistic. I know the velocity of the hydrogen as it passes the heat source is important but I don’t know that and I think it would change with time and temp. If anyone can give me a direction into where I need to look to model this that would be awesome.

    This is not homework it just spare time ideas
     
  2. jcsd
  3. Jul 24, 2012 #2

    Mech_Engineer

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    You have to know something about the geometry of the heater, and how much Hydrogen is flowing around it (pressure and speed/mass flow rate).

    Keep in mind 48 KJ is a measure of energy not power, over what period of time is the energy being extracted?
     
  4. Jul 24, 2012 #3
    The problem with mass flow is that it’s not a steady state problem. At least I don’t think it is. I am using hydrogen as the medium of the stirling engine because its high R value will help in when calculating work out. I am also hydrogen as the coolant for because of its thermal properties. I might be wrong but because it’s a stirling engine the volume and velocity of the gas in the chamber change as the engine warms up. In theory I wanted to use the heat from the other thermo process to power the sterling engine. I kind of wanted to avoid fluid dynamics because I have little exposure to it. Now I am thinking that I might have to solve a PDE in order to model this particular process.
     
  5. Jul 25, 2012 #4

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  6. Jul 25, 2012 #5
    Yeah i think i determined the work output to be W=n*R*Ln (V2/V1)*(Th-Tc) the problem is that I don’t know how hot the hydrogen gas gets. I was thinking that I can use Q=C(∆T) so I can figure how hot my heat exchange box gets from the original thermo processes. Then use how hot it gets along with newton’s law of cooling to determine the amount of heat transferred per unit time from the box to the hydrogen gas. Can I now take that Q and use it to measure how hot the hydrogen gets without having to worry about boundary layers and fluid dynamics.

    I know the 48 KJ I used earlier are not really in the correct formula since its really supposed to be DQ/DT for the thermo process but i just used one mole of working gas to determine the amount of waste heat. I figure i can change the rate of the process to get better numbers out of my sterling engine. I just mainly wanted to know if i was one the right track for modeling heat transfer from a heat exchange to a gas. Also lets just say the heat exchange is 1 m^3 so i can say my cross section surface area is 1 m^2
     
    Last edited: Jul 25, 2012
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