mikah Posted October 12, 2011 Share Posted October 12, 2011 I'm trying to build a subsonic ramjet to try and get the technique down so I can make a cluster to put on a go-kart, bicycle, or lawnmower. I've looked all over the internet and the only reliable science and math I can find is for supersonic ramjets or from a hobby forum that doesn't really seem to understand math. So basically what I am trying to find out is how narrow my diffuser needs to come down to and the length of diffuser, how that relates to the combustion chamber and nozzle, and the best shape for my fuel injector. If you can give me the equations it would be greatly appreciated. Link to comment Share on other sites More sharing options...
ewmon Posted October 12, 2011 Share Posted October 12, 2011 Ramjets don't work at go-kart speeds. work statically (0 mph) and at low speeds, and you can find various designs on the internet. Both types of engines are extremely loud and, in my opinion, dangerous. Link to comment Share on other sites More sharing options...
mikah Posted October 12, 2011 Author Share Posted October 12, 2011 Well to start it I would use like a leaf blower or compressed air to get the air moving so I can start the reaction so it will sustain, once it sustains it should produce a small amount of thrust, with the cluster this should be enough to get me moving at low speeds, I'll have the diffuser cut to spin the incoming air which stabilizes the compression at low speeds, if what I read was true. But either way I would like make a static ramjet as I already made a valveless pulse jet. Link to comment Share on other sites More sharing options...
ewmon Posted October 12, 2011 Share Posted October 12, 2011 I don't know what you're reading, so it's hard to comment on the particular design. The idea of a "ram" jet is that there's a forward speed that causes compression due to "ram" effect. Even hood scoops on cars produces very little increased engine power due to their ram effect. From my perspective, a "static ramjet" is an oxymoron. 1 Link to comment Share on other sites More sharing options...
CaptainPanic Posted October 12, 2011 Share Posted October 12, 2011 Wikipedia says about ramjets: A ramjet [...] is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. They are serious about the forward speed... but it's the next sentence which should tell you that you're not onto a good idea: Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3. This type of jet can operate up to speeds of Mach 6. Relatively to the speeds at which ramjets work, your lawnmower is probably just not going to go fast enough (I'm not sure how ambitious you are... Mach 3?). I would seriously advise you to read the wiki page, and check out the Brayton cycle (the engine cycle). In the ideal case (real life is always worse, never better), the efficiency is: [math]\eta = 1 - \frac {T_1}{T_2} = 1 - \left(\frac{P_1}{P_2}\right)^{(\gamma-1)/\gamma}[/math] And the ratio P1/P2 will be extremely tiny for your case, because at low speeds you will achieve nearly no compression at all. So, you might not even achieve any significant efficiency at all (meaning your results need to be rounded off to 0). Link to comment Share on other sites More sharing options...
mikah Posted October 13, 2011 Author Share Posted October 13, 2011 Well shouldn't providing the airflow cause compression, once proper compression is achieved shouldn't the reaction sustain? The air is taken in, mixed with fuel, ignited and compressed, then the gas is expelled through the nozzle creating low pressure in the combustion chamber which brings in more air that mixes with more fuel and so on. Either way, I have seen static ramjets made where the air is fed by a leaf blower, and I would like to make one like that to test the ramjet design and see how a realworld model functions. Link to comment Share on other sites More sharing options...
Leader Bee Posted October 13, 2011 Share Posted October 13, 2011 Dont you think youd just be better off with a pulsejet? They're cheap, easy to make, simple and often have no moving parts depending on whether you decide to go for a valved or valveless design. Link to comment Share on other sites More sharing options...
CaptainPanic Posted October 13, 2011 Share Posted October 13, 2011 (edited) Well shouldn't providing the airflow cause compression, once proper compression is achieved shouldn't the reaction sustain? The air is taken in, mixed with fuel, ignited and compressed, then the gas is expelled through the nozzle creating low pressure in the combustion chamber which brings in more air that mixes with more fuel and so on. Either way, I have seen static ramjets made where the air is fed by a leaf blower, and I would like to make one like that to test the ramjet design and see how a realworld model functions. The point is that you must compress before adding the fuel... otherwise you just have a fireplace like in your holiday cottage. A "static ram jet with a leaf blower" is in fact a jet engine. The leaf blower is a compressor and will compress the air a lot more than you can achieve by just going 50-100 km/h in your go kart. Normal jet engines have a turbine at the back end of the engine to power the compressor at the front, but if you wish to do that with a leaf blower, it's still an ordinary jet engine (not a ram jet!). Edited October 13, 2011 by CaptainPanic Link to comment Share on other sites More sharing options...
mikah Posted October 14, 2011 Author Share Posted October 14, 2011 Leader Bee, I already built a valveless pulsejet. CaptainPanic, wouldn't the leaf blower be simulating forward movement though, considering I haven't attached it to my jet. From what I've come to understand, any air entering the inlet is compressed by the geometry of the design, it's mixed with fuel, then ignited, the air expands according to what sort of fuel you have and what kind and how much gas is produced by the reaction, the air will move in the path of least resistance, which is the nozzle which forces a large amount of air through a much smaller hole, and that creates forward thrust. Now most ramjets get the airspeed to start the reaction from another source, some missiles have ramjets and the jets they are attached to get them up to speed, I think cruise missiles have them and they use a rocket of some sort, I don't know what kind but several are often used with ramjets. All I'm really trying to do, is make a ramjet to test my ability in planning, solving equations, and welding according to very specific instructions. The pulse jet was a section of pipe with a plate welded over one end and two smaller diameter pipes attached, one for fuel intake and one for air intake. It fired up quickly and sustained. Now I want a harder challenge, which is the ramjet, after this I'm focusing on a coil gun and a submarine, then I will try and make a turbojet. I just enjoy dicking around with my friends and this is how we do it since our parents stopped buying chemicals to play chemist with. Link to comment Share on other sites More sharing options...
InigoMontoya Posted October 14, 2011 Share Posted October 14, 2011 (edited) Whether or not the leaf blower acts as "forward motion" or as a typical compressor would depend largely upon whether you were running your combustor in what's known as "connected pipe" or "free stream." If connected pipe, no, it wouldn't. If free stream, yes, it would. However, understand that when you are safely subsonic (ie, not even into transonic regimes) and operating free stream you're not going to see any reasonable compression ratio. I mean, you might literally be talking about a 1.001 : 1 compression ratio. Oooo, that's power there! Keep in mind that while subsonic ramjets HAVE been successful, they're typically in the transonic regime - and even then performance sucks. Edited October 14, 2011 by InigoMontoya Link to comment Share on other sites More sharing options...
mikah Posted October 14, 2011 Author Share Posted October 14, 2011 So are you telling me I won't get that ultra impressive sound of compression and expansion that I'm looking for? Cause that's all I really want out of this first engine, no real movement or performance, but a demonstration of principles. Link to comment Share on other sites More sharing options...
InigoMontoya Posted October 14, 2011 Share Posted October 14, 2011 (edited) It won't sound any more impressive than a leaf blower... Right until you turn off the leaf blower. Then it'll sound like fuel pump (assuming you're running one). Edited October 14, 2011 by InigoMontoya Link to comment Share on other sites More sharing options...
mikah Posted October 16, 2011 Author Share Posted October 16, 2011 Oh well, thanks for the info. Link to comment Share on other sites More sharing options...
CaptainPanic Posted October 17, 2011 Share Posted October 17, 2011 If you want to simulate low subsonic speeds, use a computer fan (power of 5 W), not a leaf blower of 1 kW. Link to comment Share on other sites More sharing options...
TonyMcC Posted October 17, 2011 Share Posted October 17, 2011 In the 1960's I worked with Bloodhound Guided Missiles. These used two ramjet engines for most of their flight, but needed four rocket motors to get them around mach 1 before the ramjets became useful. If you want to see one in action follow the link. http://www.britishpathe.com/record.php?id=67192 Link to comment Share on other sites More sharing options...
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