What is the purpose of ignition in a rocket combustion chamber?

[snookrun]

Hi Everyone.

I am trying to simulate a rocket combustion chamber. The question I have is this. Is the ignition (by any means) in the rocket combustion chamber only used for increasing the temperature of the cryogenic propellant to a state where it can ignite and burn? If you are using an external ignition source (like detonation charges, sparks etc) how does this affect the chemical composition within the rocket combustion chamber, assume you have a stoichmetric mass flow rate of fuel and oxidizer to keep the composition in the rocket combustion at the stoichmetric level.

Cheers
K

 

[yangshi]

I'm not sure about liquid rockets but, for car and airplane engines, ignition increases fuel temperature enough to react with air (oxidizer). If you're using liquid propellants, I'm guessing you don't need continuous ignition since the fuel and oxidizer combust at low temps, unless you're going for variable control. In other words, I think the additional gasses released from ignition can be considered negligible. You could (spitballing) probably even try a glow plug, which helps diesel engines start up in cold weather.
If the ignition source releases non-negligible amount of products, then it depends on the ignition chemicals, whether they react with the fuel, oxidizer, or combustion products. Rocket combustion chamber is difficult to simulate... are you trying to find the pressure in the chamber? If not, there are probably easier ways to find necessary state variables.

 

[snookrun]

Yes, the ignition I am assuming is impulse detonation, so it is not continuous ignition. Right now, I am just assuming there is a sudden increase in the concentration of OH radicals to define the ignition with a short Gaussian impulse. So technically, you want to ignite and increase the Temperature of the fuel and oxidizer such that it reaches the ignition temperature and goes to autoignition and self sustain, am I right to say that?

I am simulating the condition in a Rocket Combustion Chamber at a specified Pressure and pre-defined mass flow rate to keep the composition fairly constant. Right now, I am using the mixture ratio of 4 (O/F) (working with methane/LOX combustion) but I saw some lecture that this type of engine uses 3.2-3.4 as the optimum mixture ratio. What I am interested in is how the chemistry in the rocket chamber interact during combustion. So, defining the condition is the most crucial part of the problem right now.

Thanks for the tips on the negligible by products of ignition source. I will take note. I think I have a grasp of what to model so that is good!

Cheers

K

 

[yangshi]

I believe you are right about ignition; you may look up "fire point".