Convergent Subsonic Ramjet Utilizing Shockwave Compression

[Newb_Aero_Ninja]

Hi,

I am working on investigating an idea I proposed regarding a ramjet that operates in subsonic flow (of a fixed speed) with a convergent intake. That utilizes the pressure immediately behind a standing shock-wave for compression.

I have posted a link to my initial report here and I now need to undertake the detailed investigation. I am interested to see what opinion there is surrounding this idea and whether it holds any merit and if anyone has investigated convergent subsonic ramjets before?

Thanks for all the advice.

 

[boneh3ad]

Well the first thing I see is that you won't get a shock where $M=1$. I'd suggest you do some reading on compressible gas dynamics to familiarize yourself with converging-diverging nozzles and how a gas is expected to react under such conditions. In particular, there are exact relations that may be used to size your nozzle for a given inlet velocity and the only optimization required will instead be to account for the effects of the boundary layer. Essentially, though, you will have to achieve $M>1$ in order to see a shock form, and in that case, you are going to have to prove that this is somehow more efficient than compressing the gas with a compressor.

Also, Bernoulli's equations is not valid for compressible flows, and stagnation (total) pressure is certainly not constant across a shock. That's one of the defining characteristics of shocks.

 

[bigfooted]

I think what you want to investigate can be investigated completely in an analytic way in 1D, without resorting to 2D and 3D CFD computations. Things like turbulence and viscosity can be important, but are negligible in this stage of the research. The reason people are not investigating subsonic ramjets is because of low specific impulse at subsonic speeds:

http://en.wikipedia.org/wiki/Scramjet#mediaviewer/File:Specific-impulse-kk-20090105.png
This has been investigated in the 1950's and if you google on it you will find reports from that period up to now explaining why ramjets should be operated at around M=2-4.

I suggest reading Liepmann and Roshko - Elements of Gasdynamics. It's a good book and it's cheap. It explains all the remarks boneh3ad made, and gives all the equations that you need to do the analysis in 1D, like the Bernoulli equation for compressible flow (which is quite different from the standard Bernoulli equation that everyone is usually referring to).

 

[Newb_Aero_Ninja]

Hi thanks for the replies, I am aware that shocks from where M>1 and I am also aware that Bernoulli's is only applicable to non-compressible flow and I am aware that stagnation pressure changes across shocks. I did not post a question to physics forums to find answers that are readily available in a variety of sources. I will clarify the report to show that the shockwave is just as M increases past 1.

The aim of this engine would be for combustion to occur within the shock-wave itself, the idea coming from the practical definition of the shock-wave. I agree that in the early studies all subsonic ramjets had a low specific impulse however all the research I can find for subsonic ramjets is for divergent intake jets (like the one found on the Hiller in the report). I cannot find any research on convergent intake subsonic ramjets if you have any sources on this I would appreciate that a lot :).

If you could explain how you would write 1D analysis to take into account the combustion occurring at high pressure at the wave that would be great.

 

[AtomRamjet]

One reason why ramjets operate so reliably at high speed is because of the fact that at supersonic speeds the free-stream stagnation pressure is large enough with respect to the static or "back" pressure to the point where the nozzle is capable of choking and expanding the exhaust flow to a reasonable mach number (and by extension exhaust velocity) in order to obtain sufficient thrust to sustain the aircraft's motion. The key challenge with ramjet design is of course how to maximize that stagnation pressure recovery in order to maximize thrust and minimize SFC and the like.

The problem with your design is that it appears to fail to address the problem of not having access to a high stagnation pressure to static pressure ratio by nature of the design mach number being subsonic. Ramjets don't have the ability to do work on the gas flowing through it like a gas turbine does. As such the ramjet won't be able to accelerate the air to that high of an exhaust velocity. Also, the nozzle is strictly divergent. Does your design happen to rely on the heat addition from the fuel as the primary means to accelerate the flow?

 

[cjl]

Hi thanks for the replies, I am aware that shocks from where M>1 and I am also aware that Bernoulli's is only applicable to non-compressible flow and I am aware that stagnation pressure changes across shocks. I did not post a question to physics forums to find answers that are readily available in a variety of sources. I will clarify the report to show that the shockwave is just as M increases past 1.

The aim of this engine would be for combustion to occur within the shock-wave itself, the idea coming from the practical definition of the shock-wave. I agree that in the early studies all subsonic ramjets had a low specific impulse however all the research I can find for subsonic ramjets is for divergent intake jets (like the one found on the Hiller in the report). I cannot find any research on convergent intake subsonic ramjets if you have any sources on this I would appreciate that a lot :).

If you could explain how you would write 1D analysis to take into account the combustion occurring at high pressure at the wave that would be great.

The problem is that in order to get the flow to a mach number of approximately 1, your convergent section will actually be accelerating the flow (since the freestream is subsonic). This makes your inlet a nozzle, and your pressure at the shock will be lower than ambient. If you look at it in energy terms, this has to be the case - the total pressure of the flow cannot increase, so if you're accelerating the flow to mach 1, the static pressure will decrease. This is exactly backwards from what you want in a ramjet - you want to increase the static pressure as much as possible prior to combustion to increase the efficiency.

 

[Newb_Aero_Ninja]

You don't think that having the actual shock itself ignited is possible?

 

[cjl]

What do you mean ignite the shock? You ignite fuel (which will need to be injected somewhere), and ideally, you want the pressure at the combustion region to be as high as possible.

 

[Newb_Aero_Ninja]

Along the shockwave itself, (very thin region) the pressure is high so I mean I want the fuel to flow in then explode when it hits this high pressure region and generate thrust, might be a crazy unfeasible idea but that is what I am trying to figure out XD?

 

[cjl]

What makes you think the pressure is high at the shock? You're looking at an incoming flow of approximately mach 1, which means you'll have an extremely weak shock (if you have one at all). This won't create much of a pressure step, and your post-shock pressure will still be below ambient.

 

[Newb_Aero_Ninja]

Is the actual shockwave itself not a physical pile up of particles that can no longer pass all their energy into the particles when they bump into them?

 

[cjl]

The shockwave is a step change in the flow properties - it can be a small step or a large step, and although the pressure always increases across the shock, if the incoming pressure is low, the outgoing pressure can also be low. Also, the pressure within the shock isn't any higher than the pressure behind the shock. The shock is not a pile of particles, it is where the properties of the flow experience an abrupt change.

 

[Newb_Aero_Ninja]

It is a high energy region, under high pressure as far as I was aware.

 

[AtomRamjet]

It is a high energy region, under high pressure as far as I was aware.

Yes but the pressure even further aft of the shock will be of much higher static pressure thanks to simple subsonic gas dynamics. Combustion needs to occur under as high of static pressure as possible to improve fuel-to-air mixing rates, combustion rates, combustion efficiencies, etc. The shock in the "nozzle" area is not that far aft of the throat and as such will only see a miniscule jump in static pressure and the static pressure forward of the shock isn't that high to begin with since the inlet at the front has converted so much of the available stagnation pressure into velocity.

 

[cjl]

It is a high energy region, under high pressure as far as I was aware.

It's no higher in energy or pressure than the region immediately post-shock. Unfortunately, if you're starting with a subsonic flow, the best pressure recovery you can get is going to simply be an isentropic diffuser. The reason ramjets have poor efficiency in low-speed flows is that there simply isn't that much flow energy available to go into compression, so the pressure ratios are always going to be awful.

 

[RandomGuy88]

The static pressure behind the normal shock (P2) is a function of the Mach number and the static pressure in front of the normal shock (P1). At M = 1 the ratio P2/P1 is only equal to 1 so you get no rise in static pressure (or temperature). So you need your shock to form at a significantly higher Mach number before you get any reasonable compression. But remember as M increase the ratio P2/P1 increases. So even if you have a very high Mach number, you would still have a very low P2 just because P1 is so low. This is the point AtomRamJet made in Post 14. Your static pressure dropped significantly as the flow accelerates through the converging nozzle to the shock. So you start off with a low P1 so even a high M might not be sufficient for you to get large P2.

 

[cjl]

Not to mention that a high mach number isn't achievable, since a converging nozzle can never accelerate a flow beyond sonic.