I have a 20 cm long glass tube of outer diameter
6.4 mm and inner diameter
4.3 mm. Air at
Ti = 20 °C is pushed through the glass tube at
2 litres per minute. The glass tube has a wire wound resistance heater on it which we can assume is perfectly thermally insulated from the ambient air.
My aim is to find out the power needed to heat the air flowing through the tube to
Te=650 °C at the tube exit.
After doing the calculation using
Where is the power supplied by the heater (
is the mass flow rate (
is the density of the air
kg/m^3 at the bulk temperature ,
I get the power required as being
12.5 W. This seems to me to be too low. Unfortunately I do not know where I am going wrong. Does anybody have any idea where I am making a mistake in my calculation?