Brayton cycle combustion process

[ehabmozart]

Why does or how does combustion of fuel and air in a combustion chamber in a brayton cycle is idealized by a CONSTANT pressure. I remember that in a diesel cycle, it was idealized by constant pressure because as P increases by ignition but the volume increase offsets this increase. However, in this brayton cycle case, a combustion chamber is fixed and idealizing combustion by constant volume seems more logical.

Thanks in advance

 

[jack action]

The combustion chamber has open ends where air enters from one end and exit from the other with a known flow rate. As soon as the pressure want to increase in the combustion chamber, the gases are accelerated through the exhaust instead, such that the pressure never increases.