How does a ramjet work ?

Ramjet cross-section [3]

Understanding the characteristic of compressible flows

Since a ramjet works under the condition of supersonic flow, where the flow is compressible, first of all we better understand the characteristic of compressible flows. To begin with, I want to tell you the main difference of supersonic flow from subsonic flow. At subsonic flows, velocity of the fluid increases when it passes through a shrinking passage. But when it comes with supersonic flow it is the opposite. As fluid passes through a widening passage, its velocity increases. And the velocity of the fluid can be expressed by the following equation where A is the cross-sectional area, Ma is the Mach number and V is the velocity of the fluid.

If we interpret this equation we end up with two main different conclusion. One for subsonic flow and the other is for supersonic flow.

For subsonic flow where the Mach number is lower than 1, dA/dV is negative which means that the positive increase in area results with a decrease in velocity. As for the supersonic flow where the Mach number is greater than 1, dA/dV is positive, which means that a positive increase in area results with a positive increase at velocity.

How does a ramjet work ?

As we all know, usual turbomachines uses moving parts to compress the fluid. But ramjets use geometry to compress the fluid rather than mechanical compression. Since they don't have any moving part, they are much lighter and simle than turbojets. So how do they actually work? Upcoming fluid which is air, hits to spike and goes into the compression section. Mach number of the air at the inlet is greater than 1 so it is supersonic. With the aid of decreasing area at the inlet air gets compressed and Mach number becomes lower than one so that the air becomes ready for the combustion. At the combustion section hydrogen mixes with air and gives its combustion energy to the fluid then the combustion gases wants to expand and it goes to the section of convergence-divergence nozzle. At the convergence part velocity of the fluid starts to increase and at the throat it reaches to Mach number of 1. After the throat its velocity keeps increasing in the divergence section to the supersonic flow and the high level of velocity at the exhaust part creates the thrust.

Ramjet cross-section [3]

Flame holder

Inside the combustion chamber there is a crucial part called flame holder. Flame holder has two main tasks. One is to mix the air with the hydrogen in a proper way. To make this its design includes a number of holes on it. These holes occupies the mission of creating vortex. Upcoming air in the axial direction goes through these holes and since it changes its direction, it creates vortex and this helps two fluids to mix better and as a result get a better combustion. Second mission of the flame holder is to lower the temperature in the combustion chamber. Some of the air goes directly without going through these holes and it's used to cool the combustion chamber.

Flame holder [2]

Application areas of ramjets

Ramjets are generally used in the defence industry. Some of the fields includes missiles and plane propulsion systems. NASA X-15 is one example for ramjet-rocket powered airplane. Since a ramjet doesn't have any moving part, first motion to the airplane is given by a technique called assisted take-off. In this method airplane is attached to the body of another airplane and when it reaches to required speed it's released and ramjets supply the thrust needed.