Design of Inlet for Boundary Layer Ingestion in a Blended Wing Body Aircraft

Abstract

Boundary Layer Ingestion (BLI) is a concept in which the fuselage boundary layer is ingested by the engine to produce benefits like improved fuel efficiency, reduction of ram drag and lower structural weight of the configuration. Blended Wing Body (BWB) concept has been researched on and studied in various forms over the years as an efficient alternative to the conventional transport configurations. Past studies have concluded that of the podded and embedded engine configurations, the BWB architecture is particularly suited to flush mounted embedded engines, as the balance requirements already place them near the aft of the airframe. Despite the benefits, effect of BLI on engine performance is also known to be detrimental because BLI increases pressure distortion and reduces total pressure recovery at the engine fan face. Most of these drawbacks are caused by secondary flow losses (vortices created due to boundary layer separation) due to an adverse pressure gradient in the S-Duct and a non-uniform mass flow ratio. An improved inlet design becomes necessary to reduce these limitations. The aim of this research is to design an inlet embedded on a BWB that ingests significant amount of fuselage boundary layer and produces minimum pressure loss and distortion in the process. Two major consequences of BLI are vital in this regard namely, loss of total pressure recovery and increased total pressure distortion at the Aerodynamic Interface Plane (AIP) or the engine fan-face. Hence the inlet performance is measured by the total Pressure Recovery Factor (PRF) and Distortion Coefficient (DC60). Therefore, this research work aims to design an embedded inlet on a BWB that produces maximum value of PRF and minimum DC60. An extensive literature study was carried out in order to understand the effects of BLI on inlet performance and research work conducted in the past to minimize the losses associated with BLI. Many of these studies focus on S-Ducts ingesting boundary layer and minimization of the losses using flow control techniques. Few studies have focussed on design of a novel inlet configuration that produces best results in terms of PRF and DC60. This thesis has focussed on the design of the inlet based on computational analysis of different inlet configurations to achieve an optimum design. The framework of this report first follows description of criteria and parameters for embedded inlet design. This is followed by an elaboration on the numerical methodology and approach to be used for the Computational Fluid Dynamics (CFD) simulations. The CFD simulations and analyses conducted in this thesis are divided into 2 main stages. The first stage deals with the computational analysis of a BWB in clean configuration (without engines) to obtain velocity profiles over aft fuselage, where the inlets will be embedded. The second stage comprises of the main inlet design. Three main geometrical parameters are chosen for the geometrical design of the inlet, namely inlet aspect ratio (ratio of inlet ellipse major axis length and semi-minor axis length), duct length and duct height. A number of tests are conducted to find out the influence of these parameters on the inlet performance. Few other inlet configurations are investigated, which can produce improved results and finally the design of the internal nacelle lip concludes the design of the inlet. Since the BWB in this research operates at cruise conditions (at M=0.82), initially pressure losses were high. Testing of different inlet aspect ratios with constant length and height of duct showed that an aspect ratio of 1.75 performs best with a PRF of 97.01% and a DC60 value of 41.59%, which was quite high. Further tests regarding variation in duct height showed most optimum results for the lowest height of duct due to reduced secondary flow losses. The duct with a height of 0.3m performed best with PRF=97.7% and DC60=28.45%. Finally length of duct was varied keeping previously obtained aspect ratio and height and the shortest duct length (4.85m) performed best with PRF=97.7% and DC60=28.45% (previous variations in duct height and aspect ratios were conducted using the same length value, hence the results for duct height are similar). Therefore, the inlet obtained from the testing concluded the design as an S-Duct inlet with AR=1.75, L=4.85m and H=0.3m with PRF=97.7%, DC60=28.45% and Mach number at the AIP as 0.6. After investigating other configurations like the reverse s-duct, double gradient duct, zero-gradient duct and zero-height duct, the zero-gradient duct (duct with flat bottom wall for boundary layer and no separation inside the inlet, L=4.85m, AR=1.75, H=1.615m) showed best results with PRF=98.04% and DC60=20.55%. The internal nacelle was designed using a contraction ratio of 1.04 and a lip major-to-minor axis ratio (m/n) of 2. The final design of inlet was a zero-gradient duct with L=4.85m, AR=1.75, H=1.615m, CR=1.04, m/n=2, PRF=98.04%, DC60=20.55% and Mach number at the AIP = 0.538. The same final design model was also tested using ParaPy (a high level Python language) and results showed a zero-gradient duct with L=4.85m, AR=1.75, H=1.615m, CR=1.04, m/n=2, PRF=98.3%, DC60=20.14% and Mach number at the AIP = 0.539. A podded inlet configuration of L=6.46m and area-ratio=1.37 with same mass flow rate and fan-face Mach number as that of the embedded engine was also analysed to compare BLI case with no-BLI. The results showed a pressure loss of nearly 10% for the podded case and a larger wetted area. But the DC60 value was significantly lower (2.6%) in comparison to embedded inlet. Follow up studies can be conducted to improve the results using flow control techniques.