|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
ramjet/scramjet: what makes expanded gas to go out in the back not the front?
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? |
#2
|
|||
|
|||
peter wrote:
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? The inlet raises the static pressure of the incoming air by slowing it down. In a scramjet the inlet is a carefully designed converging duct. A supersonic flow in a converging duct will slow down although in a scramjet the flow never goes subsonic. In a ramjet the inlet is a (also carefully designed) converging-diverging duct which slows the flow to subsonic speeds. Jim Davis |
#3
|
|||
|
|||
there is a combination of factors.
the combustion gas already has a considerable inerita towards the outlet. plus the gas entering through the inlet creates higher pressure there than at the outlet. The concept of path of least resistance applies here. peter wrote: In looking at a ramjet/scramjet diagram, it seems that the burnt gas would want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? |
#4
|
|||
|
|||
"peter" wrote in message news:eaVmd.6978$m36.1500@trnddc02...
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? Drive a car at 10mph. Stick a lit match out the window. Does the match's flame expand evenly in all directions, or does the wind blow the flame backwards? The ramjet/scramjet have vast amounts of air piling in from the front, a wall of air that the flames cannot blow back against. In fact, it took a while to figure out how to keep the engines lit in the face of that hurricane. When it is lit, the flames don't want to crawl forward against all that incoming air - they want to go backward, in the direction the wind is pushing it. Mike Miller, Materials Engineer |
#5
|
|||
|
|||
"peter" wrote in message news:eaVmd.6978$m36.1500@trnddc02...
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? One way to look at this is: if no fuel was added, you can easily see why the gas would continue through the engine - it would even provide a little bit of thrust (though less than the drag caused by the intake duct). Adding combustion doesn't change the pressure (it can't really, because as you mention there are openings on both ends!), it just raises the temperature - that increase in temperature increases the thrust to useful levels. But the temperature increase does not really change the pressure through the engine - it just makes the pressure more available for work. (To be fair, the pressure is slightly higher in the exhuast because of energy available from the higher temperature, but the effect is minor when considering the direction of mass flow through the engine. |
#6
|
|||
|
|||
So far as I know, ramjets and scramjets are the same as all other
jet engines. The expanded gas goes out the back because that's the direction of pressure drop. In a turbojet, a compressor at the front of the engine raises the pressure of the incoming air. In a ramjet or scramjet, the kinetic energy of the incoming air is (partially) converted into pressure by the shape of the intake. Then, the air is mixed with fuel and burned. The pressure actually *decreases* a bit across the flame, in order to accelerate the airflow to accomodate the increased volume. This burning is totally unlike what happens in a piston engine. Forget about that, and think about the burners on a gas stove. Pressure drops a bit (not a lot), but volume increases a lot due to the temperature increase. The combustion products are then expanded out the rear nozzle, which converts the pressure into kinetic energy. Now the rear nozzle doesn't have as much of a pressure drop as the intake has a pressure rise (because we lost a little pressure in between). So why does the engine have thrust? The answer is that there is a great deal more volume heading out the rear nozzle than entered through the front. Though the exhaust nozzle converts less pressure back into kinetic energy, each unit mass of gas has more volume (or higher temperature, more energy, all the same thing). So at any given pressure, the exhaust gas is moving faster than the intake gas at the same pressure. By the time the nozzle has expanded the exhaust gas back to the intake pressure, the exhaust has much more velocity than the intake air. The difference in velocity multiplied by the mass flow of air through the engine is the thrust generated. |
#7
|
|||
|
|||
"peter" wrote:
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would want to go in both direction: towards both the inlet and the outlet. Since there is no valve to prevent a back flow, why wouldn't this push the air out of the inlet and stall the engine? I don't think _any_ of you guys get it. These are all plausible-sounding excuses that don't really explain anything. You have a presurized chamber. Normally gas in a pressurized chamber is equal in all directions, regardless of flow. If the inlet and exhaust ports were the same size, total inlet and exhaust pressure would be equal, which would seem to imply zero net thrust. First point: so far as I understand, the exhaust port is a lot larger than the inlet port, so net thrust is forward. Second point: forward motion and duct design cause inlet pressure to be greater than chamber pressure, so inlet air does not get blown back out the front. Third point: in hypersonic combusion, air flow is a matter of inertia, not pressure. The air flows too fast for pressure to propagate. These are all conjecture. I have no expertise in this field other than having sketched a idea for a nuclear-powered ramjet engine in grade school. Please explain where I'm wrong. /kenw Ken Wallewein K&M Systems Integration Phone (403)274-7848 Fax (403)275-4535 www.kmsi.net |
#8
|
|||
|
|||
|
#9
|
|||
|
|||
|
#10
|
|||
|
|||
David Summers wrote:
Not typically true. The inlet is normally larger than the exhaust port, Not necessarily. so that the exhaust will be moving much faster that the inlet. Although the exhaust velocity is higher (which, everything else being equal gives a thinner stream), the exhaust gas is also much hotter (and hence takes up more volume) than the inlet air, and contains more mass (the fuel has been added to the airstream), and is the same pressure as the ambient, wheras the inlet has been compressed by the ram effect. I believe that overall, the inlet is usually somewhat smaller than the exhaust- the exhaust usually takes up the full diameter of the engine, the inlet often doesn't. Many jet engines have a facility to reject the excess air that the engine cannot burn, particularly at high speed. Ramjets usually have an inlet shape that rejects part of the flow. David Summers |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Help collimating a "classic" Tasco 11T-R 4.5" reflecting telescope | Fu Manchu | Amateur Astronomy | 6 | August 12th 04 11:44 PM |
Pres. Kerry's NASA | ed kyle | Policy | 354 | March 11th 04 07:05 PM |
UFO Activities from Biblical Times (Long Text) | Kazmer Ujvarosy | UK Astronomy | 3 | December 25th 03 10:41 PM |
UFO Activities from Biblical Times (LONG TEXT) | Kazmer Ujvarosy | SETI | 2 | December 25th 03 07:33 PM |
UFO Activities from Biblical Times | Kazmer Ujvarosy | Astronomy Misc | 0 | December 25th 03 05:21 AM |