Split Flow Injector for Solid Fuel Ramjets

The present invention relates to ramjets, and more particularly to split flow injector for a solid fuel ramjet.

Two important problem areas have been identified from studies of the fundamental operating characteristics of solid fuel ramjets (SFRJ):

1. Airflow uniformity requirements together with the weight, volume and pressure drop required for flow straighteners; and

2. Injector sizing requirements and the limits that are placed on fuel loading and performance.

Solid fuel ramjets require an injector step to create a recirculation region to sustain combustion. The size of the recirculation region may be defined in terms of the fuel port area to injector port area ratio, A.sub.p /A.sub.i. The area ratio limits fuel loading for reasonable pressure drops across the injector. Furthermore, even relatively small non-uniformities in the flow field can significantly increase the fuel port to injector port area ratio required to sustain combustion. Thus, some trade-off is required between lower fuel loading (reduced range) and the use of upstream flow straighteners to reduce the non-uniformities in the flow field.

The present invention provides a split flow injector by using an inner tube in the injector port to split the airstream into two streams. The annular area between the tube and the injector port injects the primary airflow into the recirculation region to stabilize combustion. The secondary airflow through the tube is injected downstream of the recirculation region.

FIG. 1 is a cross-sectional view of a prior art injector port for a solid fuel ramjet;

FIG. 2 is a graph illustrating the effect on non-uniform flow on ramjet combustion;

FIG. 3 is a cross-sectional view of a split flow injector port for a solid fuel ramjet;

FIG. 4 is a graph illustrating the effect of the present invention on ramjet combustion; and

FIG. 5 is a cross-sectional view of another embodiment of the present invention.

Referring now to FIG. 1, a single injector ramjet having a ramjet casing 10, an injector port 12 of area A.sub.i, a fuel port 14 of area A.sub.p, solid fuel 16 lining the casing and delineating the fuel port, and a nozzle (not shown) with throat area A.sub.t downstream from the fuel port has a recirculation region 18, indicated by dotted lines, downstream from and adjacent to the injector port. The arrows represent the flow field through the injector port 12 into the recirculation region 18 and fuel port 14.

Since fuel loading, and thus range, is a function of the area of the fuel port, A.sub.p, referring to the graph of FIG. 2, for a given size ramjet and injector port a larger A.sub.p is required to sustain combustion for the case of non-uniform flow, which large A.sub.p is attained at the cost of reduced fuel loading.

FIG. 3 shows a split flow injector port 12' with an inner tube 20 in the injector port which splits the airflow into two streams, a primary stream and a secondary stream. The primary stream is that part of the airflow which travels in the annular region between the inner tube 20 and the injector port 12', and is injected into the recirculation region 18 to stabilize combustion. The primary stream consists of approximately 20 percent of the total airflow. The secondary stream is injected into the fuel port 14 downstream of the recirculation region 18 and has the effect of smoothing the airflow.

The graph of FIG. 4 illustrates the advantage of the split flow injector over the single injector. For a 2.5" diameter casing an injection distance, L, for the secondary stream 0.5" downstream of the injector port 12' sufficed to avoid distubing the recirculation region. With a primary area ration, A.sub.p /A.sub.t, of roughly 3.0 and with the primary stream flowing through an annular region of area equal to 20% of A.sub.i, a strong enough recirculation region 18 to sustain combustion at a fuel port to injector port area ratio of only 1.4 was created. The downstream injection distance scales up to 1.5" and 3.0" for 8" and 15" diameter hardware, respectively. Thus, the reduction of A.sub.p /A.sub.i from approximately 2.0 to 1.4 by using the split flow injector significantly improves allowable fuel loading.

The split flow injector also offers significant reductions in flow straightener. The inner tube 20 creates a longer L/D injector, where L= injection distance downstream from injector port, with a much smaller hydraulic radius, promoting a less turbulent, more axial airflow. This effect is illustrated in FIG. 4 by comparing the effect upon the primary area ratio, A.sub.p /A.sub.t, for L= 2.0" and L = 0.5".

Additionally, the split flow injector allows the recirculation region 18 to be designed with large area ratios to overcome non-uniform flow effects without penalizing the entire injector. Therefore, the split flow injector allows a longer solid fuel grain length for a given ramjet length than a single injector by reducing the flow straightening length upstream of the injector port.

As illustrated in FIG. 5 the split flow injector may be used to break frangible injector port covers and may be integrated easily into other chamber sealing approaches during boost. The inner tube 20' is constructed with a chisel edge 22 and is driven through a frangible glass injector port cover 24 by any suitable means to initiate the sustain phase. Since both primary and secondary airstreams come from a single source, no seals are required on the translating inner tube 20'.

Thus, the split flow injector reduces the critical area ratio parameters for non-axisymmetric inlets upstream of the injector port with an optimum primary stream fraction of approximately 20%. Larger primary stream fractions allow the non-symmetrical flow effects to continue through the injector port, while smaller fractions (10% or less of the total flow) result in insufficient flow to maintain a good recirculation region.