When the gas is ignited in a jet engine, what keeps from the expanding fuel from going out the intake way?

Question

Since gas expands in all directions, why, in a jet engine, does it create forward thrust, what keeps it from pushing air though the intake?

Answer 1

The pressure of the inrushing air from the compressor and, while in motion, ram pressure from oncoming air. Remember, force is pressure times area. The incoming air is restricted so that, even though this pressure is higher than that of the exhaust, the force on the engine tending to slow the plane is less than the engine thrust.

Answer 2

I'm guessing you are referring to an axial flow turbine. There are fan blades in front of the ignition chamber. They are angled so they act as compressors, compressing the air as it is drawn into the ignition chamber. It combines with the fuel and burns, the heat slams against those fan blades and prevent it from exiting the chamber in the wrong direction. There are more fan blade aft of the chamber that literally draw the exhaust gases out of the chamber. All of the fan blades are connected to an axle. You turn the axle, drawing in air, add the fuel and ignite it. The expanding heat takes over and then drives the axle. The more fuel you feed it, the faster the blades turn. Pour in a bit more fuel aft of the aft blades and suddenly you see a spike in both exhaust gas temperatures and power output. Call it "after burners."
Lastly, add to the end of the exhaust tube, a variable diameter tube that can open wide, narrow down or point left, right up, down or a combination of those directions and you can increase (or decrease) the power output and control the direction of the exhaust gases being driven out the back of your engine. Picture the diagram of a balloon with the arrows pushing on all sides but one, the nosel. There, the arrows are all rushing out of that hole. The compression blades compact the air so tightly that it seals off that end of the "balloon."