Stator Diffusion Enhancement Using a Re-circulating Co-flowing Steady Jet

This paper will outline a steady flow control technique that augments the diffusion process within a stator passage via a continuous co-flowing secondary flow stream along the suction surface. The technique is similar to that used for flow vectoring in nozzles where a secondary flow stream is used to enhance the diffusion and vectoring of high speed jets. Diffusion factors in excess of 0.95 are simulated and the “penalty” for the secondary system is addressed with an availability and simple power analysis. Losses within the secondary flow stream were included in the availability analysis, but it did not account for losses within a delivery system of this secondary flow. This was accomplished through the 1D power analysis which assessed this technique’s impact on the efficiency of an axial compression stage and the sensitivity of this efficiency to the secondary flow system’s efficiency. Also, a system level analysis is presented to assess the merits that may be realized in a notional engine with this type of flow control. Particularly, impacts on specific fuel consumption and thrust-to-weight ratio were addressed. A cascade experiment was performed to demonstrate the concept and was conducted in a blow-down cascade tunnel. Significant improvements in diffusion were qualitatively seen from the DPIV measurements despite limitations in achieving the desired secondary flow conditions. INTRODUCTION The trend to higher stage loading in axial turbomachines continues to put an increasing demand on the stator to provide higher amounts of diffusion. Many researchers are addressing this issue through the use of varying flow control techniques [14]. Many of these techniques involve the use of blowing jets or suction devices or a combination of the two to control the amount of separation and thereby increase the diffusion and overall performance of the blade section. There are others who are showing promise using unsteady techniques with synthetic jets to control boundary layer parameters [5,6]. These techniques are very appealing in that they impart a zero net mass change to the core flow. Still others are using plasmas for boundary layer control [7]. The flow control technique outlined in this paper imparts no net mass flow change to the core flow through the use of a re-circulating co-flowing jet that enhances the diffusion and turning levels through its interaction with the core flow. This technique is similar to those used in jet nozzle flow vectoring which create a secondary flow using a suction collar and pump [8-11]. The technique outlined here spawned from the work performed in fluidic vectoring in nozzles and is an attempt to create a similar condition in the stator passage with a recirculating co-flowing system. It is envisioned that this secondary flow is sustained with a re-circulating secondary flow system as shown in . Figure 2 The “penalty” of this type of arrangement on the overall compression system may not be too great if the required total pressure ratio of the secondary flow system and its efficiency requirement is reasonable. A power and availability analysis was conducted in an attempt to address this issue. Also, a system level analysis using a notional engine configuration is carried out to assess the impact on engine performance. The concern over heating effects caused by the re-circulating flow and the possible need for intercooling the secondary flow was raised in private communications [12]. This technique relies on momentum and heat exchange between the core flow and the secondary flow as it traverses the blade so it may not be