Supersonic enginesare commonly used in military aircraft. They have not used in civil aviation or for transport due toseveral operational reasons. There are different concepts, which are beingproposed for supersonic civil transport.
One of the concepts that presented inthis paper is Variable Cycle Engine (Adaptive Cycle Engine). Variable cycleengines would be able to vary their bypass ratios, for optimum efficiency atany combination of speed and altitude within the aircraft’s operating range, despitetraditional engines with fixed airflow.Asupersonic highly loaded high pressure (HP) compressor was designed for a VCEin the conditions of both single and double bypass modes in accordance with thesimilarity principle. The blade profiles were designed by means of NACAairfoil. Then, 3D numerical simulations were performed on the HP compressor ofboth working conditions with different thermodynamic cycle parameters toconfirm the design methods and results. The one equation turbulence model ofSpalart-Allmaras was applied to solve Reynolds’s averaged Navier-Stokes equations.The results of simulation indicate that the compressor performances aresatisfactory in both working conditions with high efficiency.
Further researchreveals, wave structures in the supersonic compressor, behaviour of tipclearance flow and the phenomenon of transition flow in boundary layer.A Variable Cycle Engine (VCE) can be defined as onethat operates with two or more thermodynamic cycles. It is a type of aeroengine whose thermodynamic cycle can be adjusted by changing some components’shape, size or position, and the cycle parameters, such as pressure ratio, massflow, bypass ratio and thrust. It can be varied between those of a turbojet anda turbofan, making it to combine the advantages of both.
These measures mayenable the engine to obtain the optimal thermodynamic cycle, and to acquire thegood adaptability to various flight envelopes.The engine can work as the turbojet when theaircraft requires high specific thrust, such as take-off, acceleration andsupersonic cruise. It also can work as the turbofan when the aircraft requireslow fuel consumption, such as standbyand subsonic cruise.
The most important advantage expected from using VCE infuture supersonic transport is a substantial range improvements as compared toa conventional engine. These range improvements are mainly achieved by reducingthe subsonic specific fuel consumption by around 15% (relative to a Turbojet)and improving the fuel consumption at off-design by the extensive use ofvariable geometry. The future VCE will have a low emission combustor andafterburner. The noise level at take-off will be met by FAR part 36 requirement.
In other words, the future VCE will be environmentally accepted 2.Thedisadvantages are mainly an increase in the engine weight and a more complexcontrol system, therefore the reliability of the engine will be affected. Theperformance of any VCE depends critically on the attainment of the predictedtechnology level improvements. The purpose of research on VCE is to improveoff-design performances, in order to satisfy the needs of broad flight envelope,large combat radius and long cruise duration 3.
Thework mode of VCE discussed in this article is presented in Figure 2:Single bypass mode: The selector valveis closed and all air goes through the Core Drive Fan Stage (CDFS). The fanbypass flow bypasses the core engine through the inner bypass duct and remixeswith the core flow downstream of the low pressure turbine. The nozzle is fullopen to shift the loading to the HP shaft to cope with the added work of theCDFS. At the same time, the expansion ratio and the flow rate rise to increasethe specific thrust with low bypass ratio under supersonic and accelerationcondition.Double bypass mode: The selector valveis full open and the nozzle is now closed to unload the HP turbine and load theLP turbine. The bypass ratio increases for best specific fuel consumption forsubsonic cruise and best exhaust velocity conditions for improved noisesuppression on take-off 4.Axial flow compressor is one of the most importantparts of Gas turbine engine.
Axial-flow compressors are used in medium to largethrust gas turbine and jet engines. The compressor rotates at very high speeds,adding energy to the airflow while at the same time compressing it into asmaller space. The design of axial flow compressors is a great challenge, bothaerodynamically and mechanically.
The aerodynamic compressor design process basicallyconsists of mean line prediction calculation, through flow calculation, andblading procedures. The mean line prediction is the first step withincompressor design. It is a simple one dimensional calculation of flowparameters along the mid height line of the compressor where global parametersas the annulus geometry, the number of stages, and the stage pressure ratiosare scaled 5.
It is necessary to design axial flow compressor at preliminarylevel and require parameters can be checked at initial level so furtherimprovement can be made at primary level before start a Detailed design.It is a challenging job to design appropriatecompressor to meet the demands of VCE, which is the compressor should implementperformance adjustment of the engine and ensure the efficiency being maintainedwithin a higher range. As described by similarity principle, multipleconditions can be converted to the same working condition according to the ruleof equality in reduced wheel speed, reduced mass flow and Mach number alongcircumferential direction and so on. Then, the compressor performances underdifferent working conditions are approximately equal to each other.
In thisresearch, a high loaded high-pressure compressor with high compression ratioand large enthalpy rise was designed for VCE in two operating modes of lowbypass ratio (single bypass mode) and high bypass ratio (double bypass mode)according to this principle 3.Thecomplexities of a supersonic flow in compressor have been summarized: wavestructures such as expansive waves and compressive waves (even the shocks)exist in supersonic regions. Due to this, flow parameter changes drastically inthe channel. Compressive waves may be formed by disturbed flow in that regions,so influence on aerodynamic performances caused by compressive wave-boundarylayer interactions must be considered. The blade boundary layer, influenced byblade’s geometrical parameters and main-flow aerodynamic parameters, usuallydevelops from laminar to turbulence. As a result, to capture the boundary layerdevelopment, and estimating the transition position is significant for theinvestigation of flow performances in the compressor. Besides, variations ofblade profile, blade stacking and end-wall effects often lead to 3Dcharacteristics in the flow fields, where secondary flows, separated flows andcomplicated vortex structures exist.
Indeed, it is important to obtain accurateflow information and aerodynamic performances during the design process, whichis crucial for supersonic compressor.