Mass Flow Rate and Net Thrust

To achieve smooth mode transition, it is vital for CCE system to maintain the cruise conditions of air fbw rate and propulsive thrust. The aforementioned control sequence has been so far discussed from the point of view of ft>w pat­terns relating to variable geometry manipulation, whereas it is also necessary to check the latter consequence against the thermo-cycle estimation with re­spect to total air flow rate and thrust. Concerning the air flow rate, it is obvious

that, as MSV opening area becomes larger, the core engine air fbw rate should be reduced to keep the constant total air fbw rate. In order to evaluate the net thrust, however, further assumptions are needed to handle the flow condi­tions in ram combustor and exhaust nozzle. A simplest 1D ft>w analysis was presently adopted, wherein the ram fuel control sequence was referred to the thermo-cycle simulation and the nozzle was assumed to behave just like an ideal one, with fully matched expansion of the perfectly burned fuel and air mixture.

The results of calculation for CASEs 1-5 are thus plotted in Figure 8, in which the corresponding curves from the thermo-cycle simulation are shown altogether. The upper part of Figure 8 shows the air ft>w rates (total —x—, turbo(fan) —□—, core —o—, and ram—0—, respectively). As ram rating is in­creased, turbo and core flow rates decrease accordingly, so that the total air flow rate is almost kept constant. The lower part of Figure 8 shows the net thrust, in which slightly larger values, due to idealized 1-D nozzle ft>w model­ing, were plotted against the thermo-cycle prediction curve. CFD calculation results were thus found to give surprisingly good agreement with the thermo­cycle simulation results.

2. Conclusions

Turbo-ram combined cycle engine employs several variable geometries to ensure smooth transition from turbo to ram or reverse mode operation. A con­trol sequence for those variable parameters was established based upon tran­sient engine thermo-cycle simulation, so that the corresponding ft>w patterns were examined by CFD visualization within the fan bypass and ram inlet duct, and the rear mixing region.

According to the present control sequence, a smooth mode transition seems feasible, since there appeared neither recirculation nor disruptive difficulty in the fbw pattern changes from the aerodynamic point of view. Nevertheless, careful investigation is further needed for fan bypass jet mixing at a particular instance when the ram air starts to be introduced. It may be also mentioned that there appears strong shear in the radial direction due to presence of the core engine and bypass duct flows, which must be negotiated in the ram combustor.

The total air flow rate and net thrust of the engine showed a surprisingly good agreement between the engine thermo-cycle simulation and CFD model calculation. Since it is very much difficult to perform experiments of CCE as a complete engine system, the present approach of integrating CFD numerical simulation may be advantageously used to figure out the system feasibility during the mode transition. Further improvement to achieve fully unsteady treatment at several component interfaces is definitely needed in future.

References

Miyagi, H., Miyagawa, H. et al (1995a). Combined cycle engine reserch in japanese hypr project. AIAA Paper 95-2751.

Miyagi, H., Miyagawa, H. et al (1995b). Research and development status of combined cy­cle engine demonstrator. In Proceedings of Second International Symposium on Japanese National Project for Super-Hypersonic Transport Propulsion System, pages 237-244.

Takeo, M., Yasushi, N. et al (1999). Research and development of combined cycle engine demonstrator. In Proceedings of Third International Symposium on Japanese National Project for Super-Hypersonic Transport Propulsion System, pages 229-236.