Numerical and Experimental Investigations on Multiple Air Jets in Counterflow for Generating Aircraft Gas Turbine Engine Inlet Flow Distortion Patterns

摘要

The performance of an aircraft gas turbine engine is adversely affected by thenon-uniform or distorted flow in the inlet duct. Inlet flow distortion lowers the surgemargin of the engine‟s compression system with surge occurring at much lower pressureratios at all engine speeds. The compressor and/or engine are subjected to ground tests inthe presence of inlet distortion to evaluate its performance. The simplest method ofsimulating inlet distortion during these tests is by installing a distortion screen ahead ofthe engine on the test bed. The uniform inlet flow to the compressor becomes nonuniformwith total pressure loss after passing through the distortion screen. Though thedistortion screens offer a number of significant advantages, they have somedisadvantages.The air jet distortion system can alleviate many of the operational disadvantagesencountered with the conventional distortion screens. The system consists of a number ofair jets arranged in a circumferential array in a plane and issuing opposite to the primaryair flow entering the engine. The jets interact with the primary stream and cause a localtotal pressure loss due to momentum exchange. The individual mass flow rates from thejets can be varied to obtain a required total pressure pattern ahead of the compressor atthe Aerodynamic Interface Plane (AIP).A systematic study of the flow field of confined, turbulent, incompressible,axisymmetric jet issuing into counterflow is covered in this research programme. The jetpenetration length and the jet width are reduced compared to unconfined counterflow anda linear relationship between the velocity ratio and the jet length ceases to be valid.The flow field of a circular compressible turbulent jet and then a system of fourjets arranged circumferentially and issuing into a confined counterflow was studiedexperimentally and numerically. For the four jet system the mass flow rates in the fourjets were equal in the first part of the study and in the second part they were unequal. Theloss in total pressure due to the jet(s) interacting with the counterflow was quantified by atotal pressure loss parameter λp0. The total pressure loss increased with increasing massflow ratio. The total pressure loss distribution was evaluated at several locations behindthe jet injector(s). The total pressure non-uniformity quantified by Distortion Index (DI)was found to be highest at a location just downstream of the jet injector and at fardownstream locations low values of DI were observed.From the understanding gained with a single jet and four jets in counterflow amethodology was developed to generate a given total pressure distortion pattern at theAIP. The methodology employs computations to obtain the total pressure distortion at theAIP with quasi-one-dimensional inviscid analysis used as a starting point to estimate themass flow rate in the jets. The inviscid analysis also provides a direction to the iterativeprocedure to vary the mass flow rate in the jets at the end of each computational step. Themethodology is demonstrated to generate a given total pressure distortion pattern usingfour jets and is further extended to a larger number of jets, twelve and later twenty jets.The total pressure distortion patterns typical of use in aircraft gas turbine engine testingare generated accurately with a smaller number of jets than reported in the literature.