In the early 's some of the original ideas concerning ramjet propulsion were first developed in Europe. Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust.
To maintain the flow through the nozzle, the combustion must occur at a pressure that is higher than the pressure at the nozzle exit. In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid , much like a turbojet engine. In a turbojet engine, the high pressure in the combustor is generated by a piece of machinery called a compressor.
But there are no compressors in a ramjet. Therefore, ramjets are lighter and simpler than a turbojet. Ramjets produce thrust only when the vehicle is already moving; ramjets cannot produce thrust when the engine is stationary or static. Since a ramjet cannot produce static thrust, some other propulsion system must be used to accelerate the vehicle to a speed where the ramjet begins to produce thrust.
The higher the speed of the vehicle, the better a ramjet works until aerodynamic losses become a dominant factor. The combustion that produces thrust in the ramjet occurs at a subsonic speed in the combustor. For a vehicle traveling supersonically , the air entering the engine must be slowed to subsonic speeds by the aircraft inlet. Shock waves present in the inlet cause performance losses for the propulsion system. This is calculated and accounted for by the script. Mass conservation can be used to relate any two stations in the engine.
Finally, another assumption that is not taken is the perfect efficiency of compression and expansion. This means that the real Brayton cycle is used and these efficiencies are calculated as indicated by the lecture notes [2]. Additionally, we are assuming that the flow is expanding to atmospheric pressure without any losses perfectly expanded nozzle , in the 1D scenario it is a valid assumption since the lack of any geometry impedes us from more accurately calculating if this is the case.
Finally, we are also assuming that the exit pressure is the same as the free stream pressure. It could be close to the engine this might not be the case, due to how the flow surrounding the engine behaves, but since engines are designed to be as aerodynamic as possible, that case is unfeasible.
It relies on the function Takeinput. After receiving all the required values, Inputs. In addition to this, the function Tester. These codes are presented in the following subsections. Ranjan Date: Please try running the code again with different inputs. Iliev; A. Hirjanu Date: It is worth mentioning that using H2 will require pressurized or cryogenic fuel tanks. It is important to note that the specific thrust decreases substantially with Mach number.
While the increase of the propulsive efficiency does not peak the increase of thermodynamic efficiency does at about Mach 5. The initial increase in efficiency is caused by the increase compression ratio, at higher speeds one must compress the flow more to slow it down.
The decrease after 5 could be due to the loss of stagnation pressure which outweighs the gains at high speeds. This is as expected, the free stream pressure allows us to perform dimensional engine analysis calculating initial conditions, the speed of sound, and other factors , but it does not have a dimensional effect on efficiency. In fact, the thrust is often used normalized for free stream pressure and area.
This can be directly related to the definition of propulsive efficiency seen below, as the intake temperature increases, given everything else is constant propulsive efficiency will increase.
It is important to note that the slope seems to be increasing which indicates higher losses at higher shocks. The effects on propulsive efficiency, on the other hand, are quite negligible; an increase in Mach from 1 to 2 causes an increase in efficiency of 0. This is unexpected since stronger shocks reduce the thrust, perhaps they reduce the jet momentum power by slightly more. This is as expected since though it substantially increases the thrust produced it also increases the jet momentum lowering the efficiency.
Theoretically, an engine at given conditions can produce any amount of thrust, by for example increasing the intake area. The propulsive efficiency measures do depend on the thrust, but it is normalized by pressure and intake area, such that it measures how well it is produced, not how much. The nominal thrust also has no effect on the thermodynamic efficiency which is primarily a measure of compression efficiency. Brief note on conical intakes An engine which only has a converging-diverging diffuser still has quite substantial stagnation pressure losses.
A way to substantially reduce the losses is to add a conical intake. It works by creating oblique an oblique shock before the flow enters the intake; this reduces the speed substantially with lower losses. The normal shock that then occurs is much weaker leading to much lower losses.
The increase in propulsive efficiency that this would add would be somewhat penalized by the decreased thermodynamic efficiency, caused by more entropy due to the shocks. However, this seemed like an appealing option to have in our code especially is the user inputted a higher Mach number.
We attempted to solve the equations in the lecture notes [2], however without a knowledge of the required intake area given a conical intake numerical results cannot be produced. This attempt is included in the Annex for reference. Conclusion The relationships produced by our code correlated closely with the relationships explained in the lecture notes [2] and with other empirical data.
The code was able to closely reflect the effects that varying each parameter would have on efficiency. However calculating trend lines is not the same as designing an engine, the model is conceived as a 1D problem, and so though it may give a good approximation, it will not give entirely accurate results.
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