Inside Pulsejet Engines

Hello everyone, after a while I have decided to post an instructable which gives the reader an detailed insight about the physics that is going on inside a valveless pulsejet engine. In this instructable I will be covering the following;

1. Working of pulsejet engines (Animated step by step)

2. Computational fluid dynamics simulation of a pulsejet engine (This gives you an detailed insight about your design and how each parameters affects the designing of a valveless pulsejet)

3. Noise generated from the engine wall and the noise generated when the mic placed at a distance of 1feet from the exhaust pipe

4. Applications of a pulsejet engines

5. Extension of pulsejets

Please take a look at the video to know the cyclic behaviour of pulsejet engines.

Why is it called as pulsejet?

The major reason for calling it as pulsejet is it produces thrust in alternating manner or in the form of pulses. Basically pulsejet engine does not produce continuous thrust. This can be described in cycles per second or the number of pulses per second.

Physical features:

Unlike conventional Gas turbine engines, pulsejet engines are constant volume combustors and it has few or zero moving parts. No moving parts such as compressor blades required for compression this is happened naturally inside the CC (Combustion chamber) of a pulsejet engine.

Valved engine: They have moving valves which has higher compression ratio compared to valveless engine.

Valveless engine: Absolutely zero moving parts i.e a hollow pipe.

Working:

Stage-1: Initially air/fuel will be blown from inlet with ignition on.

Stage-2: Fuel/air ignites and the hot gases move towards outlet and part towards inlet. This process creates a low pressure region inside combustion chamber.

Stage-3: Due to low pressure regime, a part of hot gas from exhaust and fresh fuel/air from inlet is being sucked into Combustion chamber such that ignition source is not required once the engine sustains.

Stage-4: The hot gas from exhaust is used to ignite fresh fuel/air mixture to continue the second cycle.

Real time experiments:

1. Ensure the weld does not contain any blow holes because even a small hole may lead failure of the engine i.e engine cannot self sustain due to leak of pressure. It is very important to note that the compression pressure in valveless engine is pretty low so leak test is mandatory.

2. Adjust your fuel supply valve to sustain the roar.

CFD:

This is a CFD analysis done for a small pulsejet engine with max dia 76mm. There are three plots first is the pressure values extracted by placing a point probe inside CC (Combustion chamber), second is the exit temperature plot and third is the exit velocity plot.

When we take a look into the pressure plot or the first image, we can see the pressure oscillation inside the combustion chamber (oscillating from higher to lower than atmospheric pressure). This is a good proof that due to this low pressure part of exhaust gas from outlet and fresh air/fuel from inlet is automatically being sucked by CC to sustain the process.

The chamber pressure seems to be just 1.2 to 1.22 bar. This engine operates at 110Hz. This engine has been built and tested.

Models:

Unsteady, Eddy breakup, 2d axisymmetric, acoustics, RANS.

CFD Advantages:

1. Through CFD one can gain an idea whether the designed model is feasible or not

2. Easy design exploration to create new engines as well as better understanding of the working

3. Try playing with the inlet to out dia ratio you will be surprised to see the output

There are totally three sound files

Mic1- Sound received from impermeable surface (Wall)

Mic2- Sound received from 1feet distance away from outlet

Mic3- Sound received when mic placed above pulsejet

Please take a look into the picture for more info on the position of three mics.

Applications:

Currently the major applications is

1. To propel RC (Radio controlled) small airplanes

Future applications:

1. UAV (unmanned aerial vehicle)

2. Propulsive device for missiles & many more

Hope this article will be useful for many people. See you all in next instructable.