BASIC ANALYSIS ON 1+3/2 AND 1+1/2 COUNTERROTATING TURBINES
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Gear ratio
Overall pressure ratio
Tip-speed ratio
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Overall pressure ratio
Pressure coefficient
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The detailed design and overall performances of four inlet stages for an advanced core compressor are presented. These four stages represent two levels of design total pressure ratio (1.82 and 2.05), two levels of rotor aspect ratio (1.19 and 1.63), and two levels of stator aspect ratio (1.26 and 1.78). The individual stages were tested over the stable operating flow range at 70, 90, and 100 percent of design speeds. The performances of the low aspect ratio configurations were substantially better than those of the high aspect ratio configurations. The two low aspect ratio configurations achieved peak efficiencies of 0.876 and 0.872 and corresponding stage efficiencies of 0.845 and 0.840. The high aspect ratio configurations achieved peak ratio efficiencies of 0.851 and 0.849 and corresponding stage efficiencies of 0.821 and 0.831.
Aspect ratio (aeronautics)
Overall pressure ratio
Diameter ratio
Expansion ratio
Surface-area-to-volume ratio
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The present work for the design of nozzle-less radial inflow turbine begins with power requirement of 20 kW. Based on the available parameters like temperature, pressure and mass flow rate required for the design are obtained from cycle analysis initially preliminary design of rotor was developed and from the available loss models the efficiency of the turbine was found. After completion of the preliminary design of turbine, it was felt necessary to optimized the result for best efficiency accordingly an analytical study was undertaken to study the influence of different parameters like inlet absolute Mach number, relative exit Mach number, solidity, relative velocity ratio, hub to shroud radius ratio and rotational speed on efficiency. VISUAL BASIC program is developed to study the effect of different parameters on efficiency, for different speed conditions it can be observed that for same solidity and higher speed gives the compact size with less variation in losses and efficiency. The results obtained from analysis also suggest the value of higher solidity but in practical situation that will restrict the flow through runner that by increasing the losses and reducing the efficiency.
Solidity
Overall pressure ratio
Inflow
Shroud
Windage
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Tip-speed ratio
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A multistage vaneless counter-rotating turbine (MVCRT) eliminates vanes between rotors, which reduces the weight and size of the turbine and avoids viscous losses associated with vanes pronouncedly. An aircraft engine employing such a turbine would have greater thrust to weight ratio and smaller specific fuel consumption. This paper presents the aerodynamic design philosophy and performance analysis of the MVCRTs for gas turbine engines by a case study. The case is about a 1/2*4 turbine, which consists of a rotating frame and four rotors without any vanes between them. The first rotor and the third rotor are connected by a shaft to drive a compressor with a pressure ratio of 11.8, and the second rotor and the fourth rotor are connected by the rotating frame to deliver a total shaft power of around 2 MW. The stage loading of each rotor and flow axial acceleration of each duct are controlled to provide sufficient inlet swirls for their subsequent rotors. The stage work coefficients of each rotor are 0.95, 2.9, 1.4, and 1.0, respectively. Nonuniform radial circulation distributions are also used to maximize the turbine power output. Centrifugal forces in the outer rotor of the turbine are captured by carrying out a finite element analysis (FEA) to validate the aerodynamic design results. Three-dimensional viscous numerical results show that an adiabatic total-to-total efficiency of 91.47% with a pressure ratio of 9.8 at design condition is obtained and achieves the initial design objective very well. Entropy creation associated with the tip leakage and secondary flow is also illustrated for understanding the origins and effects of losses in the turbine. Pressure ratios and efficiency at the speed combinations of the 80% to 100% inner and outer rotor design speeds are discussed to reveal the turbine characteristics at off-design conditions.
Degree of reaction
Overall pressure ratio
Axial Compressor
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Two types of contra-rotating stages are considered; the first uses guide vanes and the second is vaneless. The wheels of the first type use bladings which are mirror images of each other and they operate with inlet and outlet swirl. The second type uses dissimilar bladings in each of the two wheels with axial inlet velocity to the first wheel and axial outlet velocity for the second wheel. An analysis of their performance indicates that both types can reach stage loading coefficients comparable or larger than conventional turbines with the same number of wheels. A comparison of the contra-rotating stages with conventional ones indicate a significant stage efficiency advantage of the contra-rotating over the conventional single rotation stages due mainly to the elimination of stationary vanes. The off-design performance indicates that relative wheel speed must be controlled. The attributes of contra-rotating turbines suggest their potential use in high performance aircraft engines, in dynamic space power systems and in low speed industrial gas turbines.
Axial Compressor
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Water turbine
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Most radial turbines have a peak efficiency at around U/Cis (velocity ratio or jet speed ratio) of 0.7. It is a well-known fact that it is beneficial for radial turbocharger turbines to have a higher efficiency at low U/Cis region, since the pulsating engine exhaust gas at low U/Cis region (with high pressure and temperature) carries more energy compared to that at high U/Cis region (with low pressure and temperature). The improvement of Total to Static efficiency at low U/Cis region will help the turbine extract more energy from the exhaust gas and thereby increase the turbine cycle-averaged Total to Static efficiency. In the past there has been some attempts to move the peak U/Cis to lower values by using conventional or direct design approach on mixed flow impellers. But this approach usually results in reduction of stage performance. In this paper, a methodology is presented to control and move the radial turbine peak efficiency U/Cis to lower values by using a 3D inverse design method. The stage performance is measured by using steady CFD analysis. Furthermore detailed stress and vibration analysis are presented on the mechanical performance of the new design.
Overall pressure ratio
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One-dimensional mean-section flow and blade specific losses proportional to average specific kinetic energy are assumed in the analysis. Range of the work-speed parameter lambda considered includes low to moderate blade speeds with high specific work outputs, where critical turbojet, turbopump, and accessory-drive turbines are encountered. A diffusion factor of 0.5 limits the loading on the downstream stators. Turbine efficiences considered are total or aerodynamic, rating, and static. Efficiences of velocity-diagram types at impulse and that corresponding to values of maximum efficiency are presented and compared to indicate in what range of lambda downstream stators are beneficial as well as the attending improvements in efficiency.
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