I'm conduction a scale model experiment of thrust production in combustion chamber by using different types of fuel source.

Comparing the thrust produced by using hydrogen gas, kerosene and petrol.

Specific energy:

Hydrogen gas = 142 MJ/kg

Kerosene = 42.8 MJ/kg

Petrol = 46.4 MJ/kg

What are the instrument to be use for measuring thrust output of the combustion chamber through combustion of the fuels?

What values do we measure? (eg. airspeed at output of combustion chamber/ temperature/ thrust)

  • 1
    $\begingroup$ Does your chamber have a nozzle? The nozzle of jet engine is what produces most of the thrust. A combustion chamber by itself will just raise the temperature and have little effect on pressure and velocity $\endgroup$
    – Daniel K
    Apr 24 '19 at 21:58
  • $\begingroup$ @DanielKiracofe planning on doing a can type combustion chamber. the design would be similar. but size may differ a bit. i dont know what instrument to use for output thrust measurement. $\endgroup$
    – cat
    Apr 25 '19 at 4:48
  • $\begingroup$ What is the purpose of this exercise? While thrust is ultimately generated because energy is added to the stream in the combustion chamber, how much will be generated depends mainly on the temperatures (inlet, peak and exhaust), design of the other components (nozzle, turbine and compressor), ratio of the heat capacities of the working fluid, and also the flow stream (accelerating already fast stream takes more energy for the same momentum). Most don't depend on the fuel (temperature is usually material-limited) and what does (the energy) is easy to derive. $\endgroup$
    – Jan Hudec
    Apr 25 '19 at 20:41
  • $\begingroup$ @cat you may want to look into LNG. It is cheap and falls in between kerosene and hydrogen in specific energy with reasonably good energy density. Space X seems to like it. $\endgroup$ Apr 26 '19 at 12:11
  • $\begingroup$ I suppose you'll have better luck finding an answer in an engineering forum. $\endgroup$ Apr 28 '19 at 3:52

Option 1: Thrust is a force, and Force = mass * acceleration, and acceleration is change in velocity. So you need to measure the mass flow, the velocity upstream and the velocity downstream. It is probably sufficient to just measure the air mass flow upstream, and measure the fuel flow, and not measure the air mass flow downstream.

Option 2: instead of measuring the air, measure the reaction force. i.e. at some point, you have to support your combustion chamber. It will be bolted down to the floor somewhere. Where ever that is, get a load cell. That is a thing that directly measures force.

But again, with either of these two options, I would expect that your measurement thrust measurement will be pretty close to zero. A combustion chamber will not generate a large thrust on its own. You need a nozzle in order to generate a significant amount of thrust.

  • $\begingroup$ Always go fro option 2 I reckon, no substitute for direct measurement of the variable. $\endgroup$
    – Koyovis
    May 26 '19 at 12:30
  • $\begingroup$ options 2 seems to be more suitable for my experiment. and i had made a few tweaks for the set up. instead of going for GTE combustion chamber, i made a pulsejet and use the same 3 diff fuels as variable $\endgroup$
    – cat
    May 26 '19 at 12:37

First, there is a real danger of an explosion here if the fuel air mixture is too lean. Long ago my science teacher filled a coffee can with natural gas. The can had a little hole on top and one near the bottom (natural gas is lighter than air). He lit the top and sure enough the flame burned down to blue and then the mixture in the can (mildly) blew up.

You may find thrust will be dependent on how much AIR you can ram into the container. All three of those fuels will generate thrust. Much of your question is already known and can be sourced through rocket databases under specific impulse. Not surprisingly, hydrogen/LOX (liquid oxygen) gives the highest value, but other liquid fuels give a higher energy density (per volume). Many 1st stages on rockets use kerosene.

  • $\begingroup$ Since this doesn't answer the question (How to measure thrust produced from combustion chamber of Gas Turbine Engine?), it might be better suited as a comment. $\endgroup$ Apr 26 '19 at 7:09
  • $\begingroup$ On a small, lab, experimental scale you simply see how much weight it lifts, just like James Watt's horse. $\endgroup$ Apr 26 '19 at 8:02
  • $\begingroup$ And now you have the more answer-like part as a comment :) If you were to elaborate on that in the body, you'd have a credible answer. $\endgroup$ Apr 26 '19 at 8:05
  • $\begingroup$ @AEhere the point is before one goes into their garage and tries something dangerous a little background research can be very helpful. One need only look at the specific energy data already provided, along with specific impulse data from rocketry. For airbreathing engines (from cars right on up to jets) getting as much air in is as possible the name of the game. Rather than measuring thrust from compound X, one need only measure its caloric (heat) output. Burn rate may be significant too, as well as boiling point. And for thrust, see how hard it pushes a piston!!! $\endgroup$ Apr 26 '19 at 11:57
  • $\begingroup$ I don't disagree, but: This site is all about getting answers. It's not a discussion forum. There's no chit-chat. This is an excerpt from the site's tour (aviation.stackexchange.com/tour). $\endgroup$ Apr 26 '19 at 12:13

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