I have a vertical exhaust pipe with an air inlet at the bottom. Biofuel is burnt also at the bottom of the exhaust pipe.

This means air is drafted from the bottom of the pipe through the inlet, burning the biofuel, then out from the top of the pipe.

How can I determine the air speed?

Should I use Bernoulli's equation?

  • 2
    $\begingroup$ Briefly, no Bernoulli's eqn. won't quite work here, it assumes conservation of energy. In the end the flow rate will be determined by a balance of buoyancy and flow resistance (pipe friction + inlet and exhaust losses). $\endgroup$
    – Dan
    Feb 28, 2016 at 23:03
  • $\begingroup$ Are you trying to comply with 1065.230? It has a good listing if raw exhaust flow meters in 1065.205 table 1. $\endgroup$
    – morristtu
    Feb 29, 2016 at 0:39
  • 1
    $\begingroup$ Are you designing a system? Design errors in exhaust systems can kill. Do you have a system in place to measure? What is your exact problem? What do you mean with biofuel? $\endgroup$
    – mart
    Feb 29, 2016 at 13:11
  • $\begingroup$ Downvote in lieu of inability to place a "close vote" $\endgroup$ Feb 29, 2016 at 15:49
  • $\begingroup$ Calculating the exhaust velocity would best be done with CFD software (e.g. Fluent, StarCCM+, etc.) Bernoulli's equation won't work here, what you need to do is solve the Navier-Stokes equations for your exhaust pipe. The solution of these equations can be complicated, even for seemingly-simple cases, hence the use of specialized software. $\endgroup$
    – Carlton
    Feb 29, 2016 at 16:26

1 Answer 1


I would suggest measuring the flow directly using a flowmeter.

There are generally two types of flowmeter: mass flow, and volumetric flowmeter.

As you are only interested in the velocity, you probably don't want a thermal mass or Coriolis flowmeter. Suitable volumetric flow meter technologies, in ascending order of price and accuracy, are as follows:

  1. Variable area/rotometer
  2. Turbine
  3. Vortex
  4. Ultrasonic

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.