Investigation On Cavitation Mechanism And Suppression Method Close To An Afterburning Fuel Pump Casing Tongue

After-burningfuel pump is one of the important parts of fuel control system inaero-engine, its performance and reliability is critical to the viability andcombatperformance improvement of fighters. Recently, infrequent casing tonguecavitation damage and low flow-rate instability phenomena were found in anafter-burningfuel pump, which is a high-speed centrifugal pump, of a certainaero-engine after a long term operation ofoff-design conditions. A series of numericalinvestigation was conducted to explore the internal flow characteristic underoff-design conditions and consequentcasing tongue cavitation mechanism,which hasgreat meaning to develop optimization design theory and improve the hydraulicperformance and reliability of high-speed centrifugal pump, as well as the promotionof high performance aero-engine development.The block topology and multi-block structured grid have thus been constructed,due to complex geometric structure of the flow passage between the annular casingand straight radial blades of the open impeller. Utilizing Reynolds-AveragedNavier-Stokes equations (RANS)and a phase change cavitation model，the turbulentfield inside the pump was simulated. The flow-rate headcharacteristics predicted byunsteady resultsare consistent with the experimental measurement, then the internalflow were explored by the unsteady calculation. Under large flow-rate condition,there is limited rotational cavitation on the front edge of the blades as well as thebottom of the hub，which will lead to locally cavitation damage. The annular chamberflow near the casing tongue separates mildly. Under small flow-rate condition, nocavitation happens on the impeller surface, while high-speed fuel near the casingtongue comes into serious flow separation, which will probably bring about vortexcavitation in shear turbulence there.Then, in respect of high-speed separation flow cavitation mechanism, RANSturbulence/transition model and Large-Eddy Simulation (LES) dynamic subgrid scalemodel have been adoptedrespectively to simulate the high-speed separated flow in asimple geometric structure, an experimental single-hole fuel nozzle,separation transition was found to play a dominant role in the cavitation inception of fuelseparated flow. In the separation transitional zone, Kelvin-Helmholtz(K-H)vortexbillow and distortion motions lead to transient negative pressure fluctuation whichtriggers cavitation inception. Transient cavitation bubbles moving within the K-Hvortex form an observable cavitation area, while there is also cavitation at theseparation line. Based on the time statistical average of the LES single-phase results,a cavitation inception criterion for high-speed separated flow has been deduced. It canbe considered as a judging method of cavitation inception in the downstreamseparation transitional zonewhether the separation line cavitates. The nozzle inletfilleting with just a small radius obviously suppresses the transition process anddecrease the large-scale fluctuated negative pressure.Finally, with the dynamic subgrid scale model, the separated flow near theafter-burning fuel pump casing tongue under off-design conditions was investigated.The results identify the unsteady process of separation shear layer destabilization aswell as vortex shedding and distortion. It is found that: under small flow-ratecondition, separation transitional cavitation comes into being near the casing tonguewhich induces wall damage hole there. Also, filleted only with a large radius, the flowseparation and cavitation at the casing tongue can be fully avoided. In considerationof structure strength and external dimension restraint, the effect of narrow radialwidth and rectangular cross-section chamber designs on improving pumphydraulicperformance and preventing casing tongue cavitation have been studied andevaluated. The above results provide a good theory basis and engineering guidancefor optimal design of the after-burning fuel pump.