Heat transfer rate

1. Oct 4, 2009

bhdrsc

l have a tube. and air enters the tube at a certain temparature and pressure. the air is threated as an ideal gas. Also, we know the exit pressure of the gas. In additon to those, we know radious of the tube.. How do we find the heat transfer rate.?

2. Oct 4, 2009

CFDFEAGURU

There is a simple equation that could get you in the ball park.

Q = mCp(Delta T)

Q is the heat rate in Btu/hr

m is the mass flow rate in lbs/hr. The radius and the delta p (the pressure drop) will give you the velocity in ft/s

Cp is the specific heat of the air in Btu/lb-F. Simply assume that it is constant unless you know that it is changing and that change needs to be accounted for.

Delta T is the temperature difference between the inlet and outlet in F.

Hope that helps.

Thanks
Matt

3. Oct 5, 2009

bhdrsc

thank u for answering.. in the problem, there is no mass for air. There is just enter velocity of air to the tube. Can we convert from velocity to the mass flow rate.?

4. Oct 5, 2009

CFDFEAGURU

Look up the equation for a mass flow rate and use the properties of air in that equation to calculate the mass flow rate. If you want to you can calculate a volumetric flow rate and convert that to a mass flow rate.

Always, think, if I am not given the values that I need, how can I derive them from what I have.

Also, "thank u for answering", do not use the instant messenger speak on here. In this forum u is you.

Thanks
Matt

5. Oct 5, 2009

Q_Goest

The equation provided by CFDFEAGURU gives the heat transfer rate if you already know the mass flow rate and temperature difference. If you are trying to find those things, then first you need to determine mass flow rate from standard fluid flow calculations (ie: Darcy-Weisbach or equal) and then determine heat transfer from the surroundings by determining convective heat transfer on the OD of the tube, ID of the tube and conduction through the tube wall.

Note that if there is significant pressure drop or heat transfer, you may need to solve numerically by breaking up the tube into sections and determining pressure drop/flow rate/heat transfer in each section.

6. Oct 5, 2009

bhdrsc

entering velocity 15 m/s and exit velocity 23 m/s. Also entering pressure 1,25 and exit pressure 1. Also, we have 15 *C temperature. Thus, we need to break up system sections, dont we.?
And we can do mass and energy balances??

7. Oct 5, 2009

Q_Goest

If you have cross sectional area, you might assume velocity is constant across that area and determine mass flow from that. No need to break it up into sections if you have that, just apply the above equation directly.