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Ramjet diffuser question

  1. Mar 22, 2005 #1
    According to the information I have gathered on ramjets, a ramjet uses a diffuser to slow down the air thus increasing the pressure. The nozzle is used to maintain pressure in the combustion chamber. So my question is, why can't a ramjet (operating at low speeds) simply not have a diffuser at all?

    If the goal were to make a ramjet that works at the lowest possible speeds and the larger the diffuser to combustion chamber ratio (cross sectional area), the higher the required operating speeds. wouldnt it make the most sense to eliminate the diffuser altogether?

    Also I have not been able to come across any actual equations for calculating diffuser to combustion chamber ratios, nozzle to combustion chamber ratios, etc.
  2. jcsd
  3. Mar 23, 2005 #2


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    If you removed the diffuser, you would have no way of first, stabilizing the airflow entering the burner. Also by removing it you would eliminate the ability to compress the air prior to combustion. The nozzle is also your main thrust producing element, so it does have more of a role than just helping to maintain a combustion pressure.

    I am assuming you are talking about the diffuser inlet CS area. I am not following your reasoning here. You are saying that as the degree of enlargement on the diffuser lessens (i.e. lower burner pressure), the higher the operating speed required. Then you say to get rid of the diffuser which would result in no compression except that due to ram effects, which would require even higher speeds. Perhaps I am misunderstanding you on this point.

    A quick google cam up with the following:



    A good one here...


  4. Mar 23, 2005 #3


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    To add something to what Fred has just answered you, I could say nowadays there is no option to remove the diffuser. Think of the unsteady process of acceleration of such an engine. What you are talking concludes that surely the flow would be supersonic inside the combustion chamber. That is a strong difficulty. The burning process becomes instable at such speeds. The flame stabilization plays a major role in Scramjet Engines. Such engines are designed to burn supersonically. They don't require such a diffusion an the flow remains mainly supersonic after the converging entrance to the chamber. Great efforts have been made at stabilizing the flame, mainly by means of shock wave interaction. A small obstacle is located on the combustor entrance, provoking a shock wave which collides with the flame body at the injector. Such "compression" stabilizes the flame otherwise would be blown away.

    So that, Ramjet engines are designed to operate in a subsonic regime, because all the geometry is prepared to do so. Removing the diffusor and making a straight combustor probably you'll get supersonic flow inside the chamber, and that's another complete different mode of working.

    I don't know how the Scramjet engine of the X-project was started in flight. I think that an airplane thrusted it till some velocity of releasing. I don't believe it would be possible to achieve the Supersonic Combustion mode without varying the geometry of a Ramjet to transform it into a Scramjet.

    I can say little about it. I know how to make numbers with Supersonic flow using Compressible Theory and Gas Dynamics. But this is not the place to resume all these equations. Take a book of Gas Dynamics or so.
  5. Mar 23, 2005 #4


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    I agree with everything you guys said, but, I thought you could acheive supersonic flow through a converging nozzle?
  6. Mar 23, 2005 #5


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    I mean that the entrance to the combustion chamber can be assimilated to a convergent-divergent nozzle. The flow enters by a converging section and then at the combustion chamber either remains sonic (if the nozzle is well designed and there are sonic conditions at the smaller section), subsonic or supersonic. The physics behind it is the same of a converging-diverging nozzle.
  7. Mar 23, 2005 #6


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    Till some time ago, my senior year project was the numerical simulation of a RAM accelerator, similar to a RAMjet but with a premixed burning. I got some AIAA papers directly from one of the discoverers of the technology at U. Washington. The two figures attached correspond to a Ramjet (subsonic combustion), and a RAM accelerator working at Supersonic Premixed Combustion or Superdetonative mode. There you can view the influence of the diffusing after entering the combustion chamber.

    Due to the difficulties of setting up an edible code of Reactive Euler Equations, I have returned to boundary layer theory and axisimmetric jets, which are involved in my actual senior project.

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    Last edited: Mar 23, 2005
  8. Mar 23, 2005 #7
  9. Mar 26, 2005 #8


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    The fact is didn't know that object of the photograph was called "ramjet". Here it is called "stato-reactor", but it has little to do with Ramjet technology.

    If so, I think all what Fred has said answers your question.
  10. May 12, 2005 #9
    if the ramjet needs to go fast (lets say around 700 miles per hour) so that the air is fast at the inlet and get compresed in the duffuser what if the engine had a venturi to speed up the air to that extremely fast speed? could the ramjet go slower then? These drawings should clear up my question.
  11. May 31, 2005 #10
    Because you have energy loss when a gas is not expanded properly. You need a diffuser because you want to maximize the mach number at the exit. If you dont have a diffuser then your back pressure will be way too low and the shock produced by the supersonic gas will be pushed out of the engine and you will have compression waves. Ideally you want to perfectly expand the gas so that the shock sits at the exit of the engine
  12. Aug 13, 2005 #11
    For a science project I want to make a small working ramjet. The ramjet is a CO2 cartridge. It has an opening diameter of .6 cm, internal diameter of 1.5 cm, and exit diameter of 1 cm. I am using an airblower for a wind tunnel to test the ramjet. I am going to use methane or possibly butane for a fuel. How should I make the flameholder? Can it be an ordinary peice of sheet metal with holes drilled in it? What is the maximum/minimum size of the total cross section of the holes should be? What is the equation used to determined the best cross-section area of the diffuser and exit?
  13. Aug 16, 2005 #12
    Will not work, safe your time

    I predict the flames coming out the front. Don't forget a 747 engine (cf6-80-c2) for instance uses ram effect by using a subsonic diffuser. However, it still needs many stages of compression for the engine to work.
  14. Aug 16, 2005 #13


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    Ram effect and a ramjet are two unrelated topics. A 747 does not utilize a ramjet.

    I would recommend looking at NASA's report database for NACA reports. A quick search on ramjets will provide you a lot of information. I have seen homemade ramjets and the flameholder and spray bar configurations were trial and error. I know it's not much help.

    Here is the report database: http://naca.larc.nasa.gov/

    Also, take a look at this particular report:
  15. Aug 16, 2005 #14


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    Hey, serj;
    You might find it interesting and perhaps informative to snoop around here a bit.
  16. Aug 16, 2005 #15
    How is the flameholder supposed to be set up?According to this site(http://www.pulse-jets.com/ramjet/ramjet.htm) the flameholder cant touch the inside or it will disrupt the boundary layer.

    What equation determines the diameter of the diffuser entrance, combustion chamber, and nozzle exit?Sorry if I asked that again.
  17. Aug 17, 2005 #16

    I understand that Garvin, I just have never heard of a 'successful ramjet' working at low subsonic speeds (40-80 m/s), which is about the max speed out of a blower wind tunnel. I believe that the adverse pressure gradient in the inlet diffuser for one will make the BL separate.

    I gave the Turbofan example because the subsonic ram recovering effect is the same as tried here. I just don't see how it would work at these speeds but maybe you could educate me :tongue2:
  18. Aug 22, 2005 #17
    I've got a few more questions. Were should the fuel injector go and were should the fuel igniter go? Also, How many holes should I drill into the flameholder? And what size should each hole be relativeto the size of the flameholder?
  19. Aug 23, 2005 #18
    I bought two metal 1&1/2" gauge hyperdermic needles as fuel injectors. I wanted to see if it would work so I filled them with alcohol and squirted it into the flame of a lighter (not the safest thing to do but I figured the velocity of the stream of liquid would be fast enough to give me time to react). The alcohol did not ignite. It did burn in prior testing.is the flame not hot enough or is it because the needle does not spray it in a mist but in a stream? For my actual test I will use a gas so will it matter that the needle sprays in a stream?
  20. Aug 23, 2005 #19


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    Your alcohol coming from the syringes likely had little or no atomization. As with gasoline, liquid alcohol won't burn. You have to have enough of it vapourized to initiate burning. Atomizing the fuel gives far more surface area to evaporate, and thus enhances the combustion process.
  21. Aug 23, 2005 #20
    I retried the expiriment again holding the flame further away and using more alcohol. Success! But I would still prefer using butane, is the pressure of liquid butane too much for a plastic to plastic j&b weld (will the rubber tube welded to the plastic fitting on the needle pop off?)?
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