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Heat flux analysis with transient heat conduction

  1. Jan 10, 2017 #1
    1. The problem statement, all variables and given/known data
    I have one dimensional heating system. In the center is heating source which is heating two PVC elements located on both sides of the heating source. Heat distribution in other dimensions is negligible because of insulation. Thickness of one PVC element is 0,051 m and surface area 9,19 * 10-3 m2. During heating I measured exiting heat flux of one PVC element with heat flux sensor. I got following results: physics_forum.png
    Before red line the heating power is 14,4 W and after red line 1,6 W. The heat is divided in half between the elements, so initially there's 7,2 W heat flowing to one PVC element. The reason I changed heating power was that I wanted to measure heat flux with steady state and power of 1,6 W (which was successful as the heat flux reaches value of about 80 W/m2 compared to the theoretical value of 86 W/m2 if heating is approximated to be one dimensional), and by heating the elements at first with 14,4 W I could make the experiment be quicker. I didn't have equipment to measure temperature on the hot and cold side of the element, but the room temperature was 18 Celsius.

    The question is: from the results during transient heating before system reaches steady state can I determine any physical properties of the heated material such as thermal resistance, or if the physical properties of the material were known could I determine for example the heating power of the heating source? Before the red line heat flux on the cold surface of PVC element is rising steadily at constant rate for over 3000 seconds so I was thinking maybe I could solve something out of the derivative constant (which is 0,0138 W/m2s during the 3000s period between t = 2000s & t = 5000s) since it most likely is related one way or another to the thermal resistance of material, but after checking through transient heating theory and equations I couldn't think of any way to use them on these results, so I must humbly ask for your advice on this matter since my knowledge is hitting wall and it's about time to use ladder to help get past it.

    2. Relevant equations

    kaavat.png

    3. The attempt at a solution
    The constant heat flux increase in time is already calculated above. Heat power on the cold surface at given time can be calculated by multiplying measured heat flux with surface area. I was thinking that I could also calculate surface temperature from the equation of convection, if I knew the convection coefficient of the room in which this experiment was made. My guess is that radiation is negligible, since the subject was covered with aluminium foil, so all the heat flux measured by heat flux sensor is taken away by convection.
     
  2. jcsd
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