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# 7 Actions

2017
Aug
31
comment Problem finding temperature gain of air through heated glass tube
Thanks. I realised that the issue was that I should have used the logarithmic mean temperature difference to relate the power required to the temperature since the inlet and exit temperatures differ by a large margin. In this case Q=hAdTln where Q is the power required, h is the convection heat transfer coefficient, A is the area of the tube inner surface, and dTln is the logarithmic temperature difference between the inlet and the outlet of the glass tube.
Aug
31
awarded  Scholar
Aug
31
accepted Problem finding temperature gain of air through heated glass tube
Aug
30
comment Problem finding temperature gain of air through heated glass tube
Thanks for your answer. Yes, I used 0.566 kg/m3 for the density. I guess you are right about the small quantities. My ultimate aim is to find out how much length of 1 mm outer diameter electrically insulated nichrome wire I would need to wind around the 20cm tube to get the 12 W using a 120V power suply. The nichrome has a resistance of 12 Ohms/m.
Aug
30
comment Problem finding temperature gain of air through heated glass tube
@Mark I am using quartz glass with a 1200°C maximum operating temperature. I calculated the temperature of the inner wall of the tube and found it to be ´766.6 °C´ at the exit. I made the assumption that the thermal resistance of quartz is negligible for the low wall thickness in this case. I need glass because I want to thermally dessociate certain nitrates in air samples. The nitrates would react with or stick to the walls of other materials which are not chemically inert.
Aug
29
asked Problem finding temperature gain of air through heated glass tube
Aug
29
awarded  Autobiographer