On the Aerodynamic Design of the Boxprop

Economic factors and environmental awareness are driving the evolution of aircraft engines towards increasingly lower fuel consumption and emissions. The Counter-Rotating Open Rotor (CROR) is actively being researched around the world, promising a significantly increased propulsion efficiency relative to existing turbofans by employing two, unducted, counter-rotating propeller blade rows, thereby increasing the bypass ratio of the engine and decreasing nacelle drag. Historically, these engines have been plagued by high noise levels, mainly due to the impingement of the front rotor tip vortices on the rear rotor. In modern designs, the noise levels have been decreased by clipping the rear, counter-rotating propeller. This comes at a cost of decreased efficiency.An alternative, potential solution lies with the Boxprop, which was invented by Richard Avell\ue1n and Anders Lundbladh. The Boxprop consists of blade pairs joined at the tip, and is conceptually similar to a box wing. This type of propeller could weaken or eliminate the tip vortex found in conventional blades, thereby reducing the acoustic signature.This thesis summarizes advances done in the research regarding the aerodynamics of the Boxprop. Aerodynamic optimization of the Boxprop has shown that it features higher propeller efficiency than conventional propellers with the same number of blades, but lower propeller efficiency than conventional propellers with twice as many blades. A key design feature of optimal Boxprop designs is the sweeping of the blade halves in opposite directions. This reduces the interference between the blades and allows the Boxprop to achieve aerodynamic loading where it is most efficient - close to the tip.A Wake Analysis Method (WAM) is presented in this work which provides a detailed breakdown and quantification of the aerodynamic losses in the flow. It also has the ability to distinguish and quantify the kinetic energy of the tip vortices and wakes. The Wake Analysis Method has been used to analyse both Boxprop blades and conventional propeller blades, and insights from it led to a geometric parametrization and an optimization effort which increased the Boxprop propeller efficiency by 7 percentage points. Early Boxprop blades did not feature a tip vortex since aerodynamic loading near the tip was relatively low. The optimized Boxprop blades have increased the aerodynamic loading near the tip and this has resulted in a vortex-like structure downstream of the Boxprop at cruise conditions. This vortex is significantly weaker and of different origin than the tip vortex of a conventional propeller.A CROR featuring the Boxprop as its front rotor (BPOR) has been designed and its performance at cruise is competitive with other published CRORs, paving the way for future work regarding take-off performance and acoustics

An Optimization Platform for High Speed Propellers

To improve the efficiency by which current power plants translate jet energy into useful thrust the use of turboprop and in particular open rotor aircraft are being revisited. One challenge in association with developing new powerplants for such aircraft is high speed propeller design in general and noise prediction in particular. The Boxprop was invented in 2009 by GKN Aerospace in order to mitigate the effects of the tip vortex on noise and to improve upon the aerodynamics of a conventional propeller blade. The Boxprop is composed of a double-bladed propeller joined at the tips, and the design has the potential to eliminate the tip vortex, and thereby decrease that particular noise source. The complex and highly three-dimensional shape of an advanced propeller blade is challenging to model with classical propeller design methods, requiring instead more sophisticated optimization methods. This paper presents an optimization platform developed for high speed propellers, and illustrates its use by performing a reduced aerodynamic optimization of the Boxprop. The optimization process starts by performing a Latin Hypercube Sampling of the design space, and analyzes the resulting geometries using CFD. A meta-model employing radial basis functions is then used to interpolate on the obtained CFD results, which the GA uses to find optimal candidates along the obtained Pareto front. These designs are then evaluated using CFD, and their data added to the meta-model. The process iterates until the meta-model converges. The results of this paper demonstrate the capability of the presented optimization platform, and applying it on the Boxprop has resulted in valuable design improvements and insights. The obtained designs show less blade interference, more efficiently loaded blades, and less produced swirl. The methodology for geometry generation, meshing and optimizing is fast, robust, and readily extendable to other types of optimization problems, and paves the way for future collaborative research in the area of turbomachinery

Effect of heat exchanger integration in aerodynamic optimization of an aggressive S-duct

Intercooling the core flow in the compression process using bypass air can potentially reduce fuel consumption in commercial aviation. However, one of the critical challenges with intercooling is the installation and weight penalty due to complex ducting and large surface area for air-to-air heat exchangers (HEX). The recent interest in cryogenic hydrogen (LH2) as a potentially carbon-neutral fuel for commercial aviation expands the propulsivesystem’s design space due to the vastly different fuel properties between classical Jet-A and LH2. Regarding intercooling, LH2 adds a formidable heat sink with a high specific heat capacity and low storage temperature at 20K and, if utilised in the intercooling process, should allow for increased cooling power density with less installation penalties than an air-to-air HEX. Furthermore, the heat is transferred to the fuel instead of ejectedinto the bypass air which has potential thermodynamical benefits. The HEX can further be synergistically used to radial turn the core flow in the ICD.This paper presents the integration of a compact air-to-LH2 heat exchanger inside the gas path of the intermediate compressor duct (ICD) as the shape of a truncated cone. Axisymmetric numerical simulations areutilised to evaluate the duct performance and optimise hub and shroud lines for minimal pressure drop andoutlet uniformity. The HEX sizing was based on a preliminary system model of an LH2 commercial aviation engine with 70,000 lbs of thrust

Numerical modeling of laminar-turbulent transition in an interconnecting compressor duct

With the purpose of meeting the ambitious environmental targets set by the European Union (EU) in 2019, after the European Green Deal, new sustainable fuels need to be adapted by the aviation industry. Hydrogen stands out to be a promising candidate due to its CO2-free combustion, and higher energy density compared to kerosene. The main disadvantages of LH2 are its lower density compared to kerosene and the required cryogenic storage temperature, which affects propellant feed system size, mass, and insulation requirements. Nevertheless, the cryogenic temperatures coupled with its high specific heat capacity makes LH2 a formidable coolant, of which engine precooling, intercooling, and recuperation are potentially beneficial applications for aero engines. The focus of this paper is on how to model the vane surfaces of an Intermediate Compressor Duct (ICD) using CFD for the purpose of intercooling to support and prepare for future validation work using the Chalmers low pressure compressor rig. This study will analyze the behavior of different CFD transition models in the prediction of laminar-turbulent transition, mesh dependency, the impact of wall temperature, and the effect of conduction in the vane material. CFD simulations using the Gamma-Theta and Intermittency transition models showed very similar results and highlighted the need of well-refined computational grids to reach mesh independence for pressure loss, heat flow, and transition onset and length. A parametric study where the vane wall temperatures were decreased showed that transition was delayed for decreasing wall temperatures and that the length of the transition zone decreased as well. The results of a conjugated CFD model of a cryogenically cooled ICD vane showed that using only the surface of the vane for exchanging heat led to a relatively small decrease in core air total temperature. Therefore, the merit of using the existing aerodynamic surfaces of the ICD for heat transfer needs to be investigated further by including the hub and shroud surfaces as well, or increasing surface area further by using fins

Wake Analysis of an Aerodynamically Optimized Boxprop High Speed Propeller

The Boxprop is a novel, double-bladed, tip-joined propeller for high-speed flight. The concept draws inspiration from the box wing concept and could potentially decrease tip vortex strength compared with conventional propeller blades. Early Boxprop designs experienced significant amounts of blade interference. By performing a wake analysis and quantifying the various losses of the flow, it could be seen that these Boxprop designs produced 45% more swirl than a conventional reference blade. The reason for this was the proximity of the Boxprop blade halves to each other, which prevented the Boxprop from achieving the required aerodynamic loading on the outer parts of the blade. This paper presents an aerodynamic optimization of a 6-bladed Boxprop aiming at maximizing efficiency and thrust at cruise. A geometric parametrization has been adopted which decreases interference by allowing the blade halves to be swept in opposite directions. Compared with an earlier equal-thrust Boxprop design, the optimized design features a 7% percentage point increase in propeller efficiency and a lower amount of swirl and entropy generation. A vortex-like structure has also appeared downstream of the optimized Boxprop, but with two key differences relative to conventional propellers. (1) Its formation differs from a traditional tip vortex and (2) it is 46% weaker than the tip vortex of an optimized 12-bladed conventional propeller

Wake Energy Analysis Method Applied to the Boxprop Propeller Concept

Inspired by Prandtl\u27s theory on aircraft wings with minimum induced drag, the authors have introduced a double-bladed propeller, the Boxprop, intended for high-speed flight. The basic idea is to join the propeller blades pairwise at the tip to decrease tip vortex strength and improve mechanical properties compared to a conventional propeller.The present work develops a wake analysis method allowing an energy breakdown of the flow as well as making the irreversibility of the flow explicit by using the entropy lost work concept. The method quantifies the strength of flow features such as tip vortices and wakes in terms of engine power. In contrast to existing work, this method removes assumptions of uniform flow, no radial flow, and constant static pressure in the propeller jet.The results of the wake analysis method can be summarized into three key findings 1) the energy in the tip-vortex of the Boxprop design is comparatively speaking non-existent, 2) the swirl energy level of the Boxprop is higher and this turbomachine is thus more in need of a downstream counter-rotating blade to recover the energy, 3) the Boxprop develops a much larger part of its thrust closer to the hub. Analysis of this aspect of the flow reveals that blade interference approaching the tip, where the blades in a pair are more closely spaced, is quite pronounced. In turn, this indicates that maximum efficiency Boxprop designs are more likely to be obtained by having larger axial separation of the two blades

The heat transfer potential of compressor vanes on a hydrogen fueled turbofan engine

Hydrogen is a promising fuel for future aviation due to its CO2-free combustion. In addition, its excellent cooling properties as it is heated from cryogenic conditions to the appropriate combustion temperatures provides a multitude of opportunities. This paper investigates the heat transfer potential of stator surfaces in a modern high-speed low-pressure compressor by incorporating cooling channels within the stator vane surfaces, where hydrogen is allowed to flow and cool the engine core air. Computational Fluid Dynamics simulations were carried out to assess the aerothermal performance of this cooled compressor and were compared to heat transfer correlations. A core air temperature drop of 9.5\ua0K was observed for this cooling channel design while being relatively insensitive to the thermal conductivity of the vane and cooling channel wall thickness. The thermal resistance was dominated by the air-side convective heat transfer, and more surface area on the air-side would therefore be required in order to increase overall heat flow. While good agreement with established heat transfer correlations was found for both turbulent and transitional flow, the correlation for the transitional case yielded decent accuracy only as long as the flow remains attached, and while transition was dominated by the bypass mode. A system level analysis, indicated a limited but favorable impact at engine performance level, amounting to a specific fuel consumption improvement of up to 0.8\ua0% in cruise and an estimated reduction of 3.6\ua0% in cruise NOx. The results clearly show that, although it is possible to achieve high heat transfer rate per unit area in compressor vanes, the impact on cycle performance is constrained by the limited available wetted area in the low-pressure compressor

Aerodynamic and aeroacoustic comparison of optimized high-speed propeller blades

The Boxprop is a high-speed propeller concept intended for aircraft engines, which features blade pairs connected at the tip in order to decrease tip vortex strength, possibly reducing noise and improving aerodynamic performance relative to conventional high-speed propellers. This paper investigates the aerodynamic and aeroacoustic performance of three aerodynamically optimized high speed propellers; a 6-bladed conventional propeller, a 12-bladed conventional propeller, and a 6-bladed Boxprop. Performance results will be be compared for the three designs, with a focus on sectional performance and wake flow characteristics, and will show that the 6-bladed Boxprop performance lies somewhat in-between its 6 and 12-bladed conventional counterparts. The noise level at various observer positions is presented, and shows that the noise roughly follows the values of efficiency for the three propellers, with the Boxprop noise level being higher than the 12-bladed conventional propeller, but lower than the 6-bladed one. The lower blade loading and higher efficiency of the Boxprop relative to the 6-bladed conventional propeller results in slightly lower levels of noise at cruise

Energy balance analysis of a propeller in open water

This paper proposes a methodology based on control volume analysis of energy, applied on Computational Fluid\ua0Dynamics (CFD) results, for analyzing ship propulsion interaction effects as a complement to the well-established\ua0terminology, including thrust deduction, wake fraction and propulsive efficiency. The method, titled Energy\ua0Balance Analysis, is demonstrated on a propeller operating in open water. Through consideration of a complete\ua0energy balance, including kinetic energy flux, turbulent kinetic energy flux, internal energy flux (originating from\ua0dissipation) and pressure work, all possible hydrodynamic losses are included in the analysis, implying that it\ua0should be possible to avoid sub-optimized solutions. The results for different control volumes and grid refinements\ua0are compared. The deviation of the power obtained from the proposed energy balance analysis relative\ua0to the power based on integrated forces on the propeller is less than 1%. The method is considered promising for\ua0analyzing and understanding propulsor hull interaction for conventional, as well as novel propulsion configurations.\ua0The energy balance analysis is conducted as a post-processing step and could be used in automated\ua0optimization procedures