The Influence of Attached Boundary Layers in Determining the Optimum Loading for Blades in Cascade

1965 ◽  
Vol 69 (650) ◽  
pp. 133-135
Author(s):  
B. S. Stratford
Keyword(s):  
Mach Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
High Efficiency ◽  
Present Note ◽  
Maximum Efficiency ◽  
Critical Mach Number ◽  
Maximum Loading ◽  
Very High

In the design of cascade blading for compressors and turbines it is often assumed that the maximum efficiency will be attained at the maximum loading that just avoids separation and critical Mach number effects. In a turbine, the overall pressure gradient is favourable so that it should be possible to achieve very high loadings and yet avoid separation. The question then arises as to whether or not such high loadings would produce a high efficiency. The present note suggests that in fact there is an optimum loading, beyond which there would be no improvement in efficiency even though separation and critical Mach number effects were avoided.

Modeling of External Pressure Gradient Influence on the Stability of Disturbances in the Boundary Layers of Compressed Gas

2013 ◽  
Vol 8 (4) ◽  
pp. 64-75
Author(s):  
Sergey Gaponov ◽  
Natalya Terekhova
Keyword(s):  
Mach Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Leading Edge ◽  
External Pressure ◽  
Transition To Turbulence ◽  
Compressed Gas ◽  
High Mach ◽  
The Stability ◽  
Very High

This work continues the research on modeling of passive methods of management of flow regimes in the boundary layers of compressed gas. Authors consider the influence of pressure gradient on the evolution of perturbations of different nature. For low Mach number M = 2 increase in pressure contributes to an earlier transition of laminar to turbulent flow, and, on the contrary, drop in the pressure leads to a prolongation of the transition to turbulence. For high Mach number M = 5.35 found that the acoustic disturbances exhibit a very high dependence on the sign and magnitude of the external gradient, with a favorable gradient of the critical Reynolds number becomes smaller than the vortex disturbances, and at worst – boundary layer is destabilized directly on the leading edge

Design and Analysis of a High Pitch to Chord Ratio Cascade Representative of Ducted Propfans

1991 ◽  
Author(s):  
A. Weber ◽  
W. Steinert ◽  
H. Starken
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
High Efficiency ◽  
Pressure Ratio ◽  
Flow Analysis ◽  
High Pitch ◽  
Integral Boundary ◽  
Pressure Loss Coefficient ◽  
Distribution Boundary ◽  
Very High

Efforts to reduce the specific fuel consumption of a modern aero engine focus in particular on increasing the by-pass ratio beyond the current level of around 5. One concept is the counterrotating shrouded propfan operating at low overall pressure ratio and having only very few fan blades of extremely high pitch/chord ratios. The relative inlet Mach numbers cover a range from 0.7 at the hub to 1.1 at the tip section of the first rotor. A propfan cascade was designed by taking into account two characteristic features of a propfan blade-blade section: • a very high pitch/chord ratio of s/c = 2.25 • an inlet Mach number of M1 = 0.90 which leads to transonic flow conditions inside the blade passage In the design process a profile generator and a quasi-3D Euler solver were used iteratively to optimize the profile Mach number distribution. Boundary layer behavior was checked with an integral boundary layer code. The cascade design was verified experimentally in the transonic cascade wind tunnel of DLR at Cologne. The extensive experimental results confirm the design goal of roughly 5 degree flow turning. A total pressure loss coefficient of less than 1.5% was measured at design conditions. This validates the very high efficiency level the propfan concept is calling for. A 2D Navier-Stokes flow analysis code yields good results in comparison to the experimental ones.

21.—On Expansive Free Interactions in Boundary Layers

1976 ◽  
Vol 74 ◽  
pp. 271-283 ◽  
Author(s):  
S. N. Brown ◽  
K. Stewartson ◽  
P. G. Williams ◽  
W. D. Collins
Keyword(s):  
Mach Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Dependent Parameter ◽  
Free Interaction

SynopsisWhen a Mach number dependent parameter σ ≠ 0 is accounted forin an expansive free interaction analogous to that of Stewartson [7], the solution terminates in a singularity with a finite pressure, but with a pressure gradient varying as the inverse three-fifths power of the distance from the singularity, apart from a subtle logarithmic modification.

scholarly journals GaN-Based DC-DC Resonant Boost Converter with Very High Efficiency and Voltage Gain Control

Energies ◽  
2020 ◽  
Vol 13 (23) ◽  
pp. 6403
Author(s):  
Zbigniew Waradzyn ◽  
Robert Stala ◽  
Andrzej Mondzik ◽  
Aleksander Skała ◽  
Adam Penczek
Keyword(s):  
Output Voltage ◽  
High Efficiency ◽  
Voltage Regulation ◽  
Current Control ◽  
Maximum Efficiency ◽  
Switching Frequency ◽  
Switched Capacitor ◽  
Switching Patterns ◽  
Switched Capacitor Converter ◽  
Very High

This paper presents a concept for the operation of a resonant DC–DC switched-capacitor converter with very high efficiency and output voltage regulation. In its basic concept, such a converter operates as a switched-capacitor voltage doubler (SCVD) in the Zero Current Switching (ZCS) mode with a constant output voltage. The proposed methods of switching allow for the switched-capacitor (SC) converter output voltage regulation, and improve its efficiency by the operation with Zero Voltage Switching (ZVS). In this paper, various switching patterns are proposed to achieve high efficiency and the output voltage control by frequency or duty cycle regulation. Some examples of the application of the proposed switching patterns are presented: in current control at the start-up of the converter, in a bi-directional converter, and in a modular cascaded system. The paper also presents an analytical model as well as the relationships between the switching frequency, voltage ratio and efficiency. Further, it demonstrates the experimental verification of the waveforms, voltage ratios, as well as efficiency. The proposed experimental setup achieved a maximum efficiency of 99.228%. The implementation of the proposed switching patterns with the ZVS operation along with the GaN-based (Gallium Nitride) design, with a planar choke, leads to a high-efficiency and low-volume solution for the SCVD converter and is competitive with the switch-mode step-up converters.

ABOUT CONSTRUCTION OF FLAT BODIES WITH INCREASED CRITICAL MACH NUMBER

2016 ◽  
Vol 47 (6) ◽  
pp. 563-579
Author(s):  
Sergey Alexandrovich Takovitskii
Keyword(s):  
Mach Number ◽  
Critical Mach Number

scholarly journals ZVS Auxiliary Circuit for a 10 kW Unregulated LLC Full-Bridge Operating at Resonant Frequency for Aircraft Application

Energies ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1850 ◽  
Author(s):  
Yann E. Bouvier ◽  
Diego Serrano ◽  
Uroš Borović ◽  
Gonzalo Moreno ◽  
Miroslav Vasić ◽  
...  
Keyword(s):  
Resonant Frequency ◽  
Power Density ◽  
High Efficiency ◽  
Harmonic Approximation ◽  
Maximum Efficiency ◽  
Voltage Gain ◽  
Load Range ◽  
Transition Analysis ◽  
Aircraft Application ◽  
Auxiliary Circuit

In modern aircraft designs, following the More Electrical Aircraft (MEA) philosophy, there is a growing need for new high-power converters. In this context, innovative solutions to provide high efficiency and power density are required. This paper proposes an unregulated LLC full-bridge operating at resonant frequency to obtain a constant gain at all loads. The first harmonic approximation (FHA) model is not accurate enough to estimate the voltage gain in converters with high parasitic resistance. A modified FHA model is proposed for voltage gain analysis, and time-based models are used to calculate the instantaneous current required for the ZVS transition analysis. A method using charge instead of current is proposed and used for this ZVS analysis. Using this method, an auxiliary circuit is proposed to achieve complete ZVS within the whole load range, avoiding a gapped transformer design and increasing the efficiency and power density. A 28 Vdc output voltage prototype, with 10 kW peak output power, has been developed to validate the theoretical analysis and the proposed auxiliary circuit. The maximum efficiency (96.3%) is achieved at the nominal power of 5 kW.

scholarly journals Design of Grating Al2O3 Passivation Structure Optimized for High-Efficiency Cu(In,Ga)Se2 Solar Cells

Sensors ◽  
2021 ◽  
Vol 21 (14) ◽  
pp. 4849
Author(s):  
Chan Hyeon Park ◽  
Jun Yong Kim ◽  
Shi-Joon Sung ◽  
Dae-Hwan Kim ◽  
Yun Seon Do
Keyword(s):  
Solar Cells ◽  
High Efficiency ◽  
Surface Passivation ◽  
Rear Surface ◽  
Maximum Efficiency ◽  
Structural Factors ◽  
Carrier Recombination ◽  
Cigs Solar Cells ◽  
Effective Carrier ◽  
Opening Width

In this paper, we propose an optimized structure of thin Cu(In,Ga)Se2 (CIGS) solar cells with a grating aluminum oxide (Al2O3) passivation layer (GAPL) providing nano-sized contact openings in order to improve power conversion efficiency using optoelectrical simulations. Al2O3 is used as a rear surface passivation material to reduce carrier recombination and improve reflectivity at a rear surface for high efficiency in thin CIGS solar cells. To realize high efficiency for thin CIGS solar cells, the optimized structure was designed by manipulating two structural factors: the contact opening width (COW) and the pitch of the GAPL. Compared with an unpassivated thin CIGS solar cell, the efficiency was improved up to 20.38% when the pitch of the GAPL was 7.5–12.5 μm. Furthermore, the efficiency was improved as the COW of the GAPL was decreased. The maximum efficiency value occurred when the COW was 100 nm because of the effective carrier recombination inhibition and high reflectivity of the Al2O3 insulator passivation with local contacts. These results indicate that the designed structure has optimized structural points for high-efficiency thin CIGS solar cells. Therefore, the photovoltaic (PV) generator and sensor designers can achieve the higher performance of photosensitive thin CIGS solar cells by considering these results.

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