I'm currently trying to figure out the airflow of a gas turbine in m3/min.

I thought that i would be able to measure the windspeed over a known cross section on the intakte of the turbine and then air in would be the same as air out.

I was then told that this is not true for gas turbines due to compression of some sort.
If air mass in is not equal to air mass out, how do i then calculate/measure the airflow.
For further info i have the following specs on the turbine:

Thrust in N
EGT in °C
Massflow in kg/s
Exhaust gas velocity in km/h
Power output in kW
Fuel consumption in ml/min

  • $\begingroup$ You say you have the Exhaust gas velocity, massflow so all you need now is the area of the exit then it is simple... $\endgroup$
    – Solar Mike
    Commented Jul 12, 2019 at 7:56
  • $\begingroup$ Could you elaborate in an answer? $\endgroup$
    – Claudi
    Commented Jul 12, 2019 at 7:59
  • $\begingroup$ This is standard mass flow, see grc.nasa.gov/www/k-12/airplane/thrsteq.html But you need the area, which you have not given... $\endgroup$
    – Solar Mike
    Commented Jul 12, 2019 at 8:01
  • $\begingroup$ I have the diameter of the outer point of the jet nozzle, is that the area needed to calculate? $\endgroup$
    – Claudi
    Commented Jul 12, 2019 at 8:07

1 Answer 1


The intake and exhaust are linked by the mass conservation equation, so the mass flow at the exhaust will be the same as at the intake plus any added fuel. Fuel mass is generally negligible as turbines tend to run very lean, but if you have it then you can use it.

If you have the mass flow at the exhaust and want the volumetric flow all you need is to link the mass and the volume, i.e. you need the density.

If this turbine is dealing with a fluid that behaves according to the ideal gas law, you could simply obtain the density from the temperature, which you also have. However this disregards any compressibility effects of the fluid, effectively assuming the Mach number at the exhaust is close to zero: $M_8 = 0$. This can also be solved, but requires more data than you seem to have; see equations 10.16 and 10.21 of the linked document, I'll type them in and elaborate once I have a minute.

Alternatively, since you know the thrust and the mass flow, you could use Tsiolkovsky's equation, or more specifically the fact that: $$F_{thrust} = v_e \cdot \dot{m}$$ where $v_e$ is the effective exhaust velocity. Note that this is not the actual, measurable, exhaust velocity except for the case where the engine exhausts into a vacuum.


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