I have a tube, at the bottom of the tube is burning wood and an air inlet. How can i increase the speed of the exhaust gas coming out of the tube? I thought about "bottle-necking" the exhaust pipe to make it go faster (Bernoulli's principle) but is there any other way?

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    $\begingroup$ What do you need the higher exhaust speed for? $\endgroup$ – costrom Apr 19 '16 at 22:36

The speed of exhaust gases from a stack is governed (approximately) by:

$V=C\sqrt{2gH\frac{T_i-T_o}{T_i}}$, where:

$C$ is the discharge coefficient (typically between 0.65-0.7, but can vary based on the inlet and outlet geometry)

$g$ is the acceleration due to gravity, approximately $9.81 m/s^2$ or $32.2 ft/s^2$

$H$ is the height of the chimney

$T_i$ is the average temperature of the gas in the stack (in $K$ or $^{\circ}R$)

$T_o$ is the temperature of the air outside

You have several ways to increase the velocity of the gases, the easiest of which should be increasing the length of your chimney.


  1. Flue-gas stack

  2. Stack effect

  3. Chimney (draft)

  • $\begingroup$ Thank you so much, this is exactly what i have been looking for! Do you know the name of this equation, or what to search for in Google so i can find it? $\endgroup$ – Mohdak Apr 19 '16 at 22:52
  • $\begingroup$ @Mohdak I took the equations from the wiki pages I mentioned above, but used the relation $V=\frac{Q}{A}$, assuming incompressible flow in the stack $\endgroup$ – costrom Apr 19 '16 at 23:10
  • $\begingroup$ You just saved the life of a final year electrical engineering student. thank you so much! $\endgroup$ – Mohdak Apr 19 '16 at 23:16
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    $\begingroup$ Keep in mind increasing the length may result in additional heat loss, reducing the average temperature, offsetting some of your gains. Appropriate insulation should mitigate the effect. $\endgroup$ – wwarriner Apr 19 '16 at 23:36

If the tube is acting as a chimney, then i would first try a longer tube, and add lots of insulation to the outside of it too.

The hotter gases inside it would be less dense and want to rise faster, the longer tube gives the gases more time to accelerate.

Basically stuff to increase the draft of the chimney


i am not sure about bottelnecking what does it means but the only natural way to increase the exhaust velocities is to place a NOZZLE at the outlet as it converts the pressure energy into kinetic energy(From Steady Flow Energy Equation) and to have more kinetic energy you should have high rate of combustion that will produce high pressure resulting into high velocities through a nozzle. for the non natural processes you can use a centrifugal/reciprocating pump to achieve higher rate of discharge


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