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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?

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    $\begingroup$ 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 '16 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 '16 at 0:39
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    $\begingroup$ 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 '16 at 13:11
  • $\begingroup$ Downvote in lieu of inability to place a "close vote" $\endgroup$ – Carl Witthoft Feb 29 '16 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 '16 at 16:26
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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
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