To: SRR99
It is true, that a ram jet is open at both ends. However when combustion takes place the hot gasses can't be expelled from both ends as to do so there would be no propulsive force. It is the pressure entering the front end that acts as a forward wall during combustion which provides a "forward wall" to react against. That is why it is called a "ram jet".
This ram effect is the same as the louvers in the pulse jet but in this case, there are no moving parts. There is also a device called a flame holder which assists in maintaing the flame front.
The combustion cycle is not continuous. As each combustion cycle takes place the flame front (pressure pulse) passes down the tube and in doing so reduces the combustion pressure in front of the flame holder and the pressure drops allowing ram air pressure to enter the tube and the whole thing starts over again. The flame holder also maintains a part of the fire providing an ignition source for the next combustion cycle.
Regarding the combuster cans, they are full of holes which it would seem would allow the combustion gasses to escape and flow forward as well as back. What keeps this from happening is the compressed air that is being forced into the combusters by the compressor thus creating a closed container.
If this wall of pressure is ever disturbed due to an aerodynamic problem in the compressor section the flame would shoot out of the front of the engine.
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The Cat