Illustration 4.1
Pressurized air is to be heated by flowing into a pipe of 2.54 cm diameter. The air at 200oC and 2 atm pressure enters in the pipe at 10 m/s. The temperature of the entire pipe is maintained at 220oC. Evaluate the heat transfer coefficient for a unit length of a tube considering the constant heat flux conditions are maintained at the pipe wall. What will be the bulk temperature of the air at the end of 3 m length of the tube?
The following data for the entering air (at 200oC) has been given,
Solution 4.1
Reynolds number can be calculated from the above data,
The value of Reynolds number shows that the flow is in turbulent zone. Thus the Dittus-Boelter equation (eq.4.3) should be used,
Thus h can be calculated for the known values of k, and d, which comes out to be
Energy balance is required to evaluate the increase in bulk temperature in a 3 m length of the tube,
Therefore the temperature of the air leaving the pipe will be at 210.81oC.