|M.Sc Student||Novikov Alina|
|Subject||The Effect of Flow Bleed on the Starting Process of a|
Ramjet Engine Intake
|Department||Department of Aerospace Engineering||Supervisors||Professor Benveniste Natan|
|Professor Rimon Arieli|
The research here presented is a numerical investigation to evaluate the feasibility of early start of a supersonic air-inlet for a ramjet engine. When an air-breathing vehicle enters supersonic flow regime, a normal shock wave appears in front of the air-intake entrance. This kind of flow conditions is termed “unstart”, where part of the mass flow rate spills around the air-intake, and the flow suffers significant total pressure loses. In order to swallow the shock wave, for a fixed geometry intake, the configuration must be accelerated to a very high Mach number (above the designed Mach number). This process requires large amount of energy. When the shock wave is swallowed, the intake is considered “started” and the vehicle can decelerate to the design Mach number. Another way to cause the inlet to start is by enabling a variable intake geometry, i.e., to increase the throat area until swallowing the shock wave is achieved and then returning to the original geometry. The disadvantage of this mechanism is its complexity during the design and operation. In the present research, a simpler mechanism is investigated to start the air-intake. The basic idea is to suck part of the incoming air, and after the shock passes the throat, shutting down the sucking. A generic two-dimensional, mixed compression, supersonic inlet geometry was chosen. The geometry had six holes on the lower wall of the inlet entry, in front of the throat. The purpose of these holes was to remove the excess amount of the air that was spilled out during unstart.
Numerical investigation to validate above assumptions is based on flow simulations with ANSYS Fluent?. The numerical solver chosen was based on inviscid, compressible flow. The solver algorithm was density-based and the integration of the equations was of second order implicit accuracy. The boundary conditions in front of the inlet were chosen to be "pressure far field". The exit of the inlet was defined as "pressure outlet". The walls of the inlet were defined as "wall" with non-penetrate condition, i.e. the flow component perpendicular to the wall is zero.
In the present research, four Mach numbers were tested. In each simulation, the holes were opened gradually until the inlet started. After starting the air-intake, the holes were closed gradually, until reaching its original geometry (with no holes). The purpose of the last act was to check if the inlet remains started even after the bleed through the holes is stopped. The present numerical study reaffirms the original hypothesis that supersonic inlets of mixed compression can “start” without significant over-speeding, but with temporal reduced mass flow rate. It is concluded that a fixed geometry, ramjet inlet that flies at its design Mach number or at a slightly higher Mach number, can “start” without over-speed acceleration if a temporary suction of the order of 20% of the nominal mass flow rate is correctly applied, thus saving a significant amount of energy by restraining from the acceleration.