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The infrared susceptibility of the propulsion system of an aircraft is significantly affected by nozzle shapes and atmospheric conditions. To examine the effects of nozzle shapes and atmospheric conditions, various nozzle shapes were selected by considering a representative low-observable unmanned aerial vehicle and its propulsion system. Then, usi...
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... a total of four different curvatures were considered by applying the hyperbolic tangent function to the four points: the upper-most line of the minor axis, and the points 1/4, 1/2, and 3/4 from the upper-most line of the minor axis. Figure 2 shows the various nozzle configurations designed in this manner. ...
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... Air Force Research Laboratory presents the three-stream ACE configuration in the project of Adaptive Versatile Engine Technology whose the multi-stream supersonic nozzle has the multiple advantages over the conventional nozzle [4]. The stealth performance heightens utilizing the serpentine configuration [5]. The jet noise is signally reduced by shielding the high-speed main flow from the low-speed third flow [6]. ...
The serpentine multi-stream supersonic nozzle (SMSN) is adopted for the multi-stream exhausted system of the Adaptive Cycle Engine to enhance the stealth performance of next-generation fighter. In this paper, the effects of the nozzle pressure ratio (NPR) on the flow characteristics of the SMSN were studied using the numerical simulation method validated by experimental data. The serpentine configuration leads to the nonuniform pressure distribution. At the mixing position, the expansion and shock waves are generated due to the pressure difference. As the NPR increases, the flow separation and shock wave in the mixing section gradually weaken and disappear. The thrust coefficient rises first and then drops. Due to the flow separation under the design condition, the thrust coefficient is the largest at MPR=6 and TPR=2.272. As the MPR increases at TPR=1.893, the compression effect of the main flow is enhanced on the upper third flow. The thrust coefficient rises first and then drops, and reaches the maximum at MPR=6. As the TPR increases at MPR=5, the compression effect of the main flow is weakened on the upper third flow. The thrust coefficient rises first and then drops, and reaches the maximum at TPR=2.272.
... On average, the IR radiation intensity can be reduced by at least 70% compared to a circular nozzle [12]. Typically, for the serpentine nozzle, there is often a significant reduction in the IR signature in the axial direction, while relatively higher signature levels are observed on the sides and below the nozzle when the aspect ratio of the nozzle increases [13]. ...
... Then, the Pareto frontier can be defined as (13) and the set of Pareto-optimal designs can be denoted as ...
The use of traditional optimization methods in engineering design problems, specifically in aerodynamic and infrared stealth optimization for engine nozzles, requires a large number of objective function evaluations, therefore introducing a considerable challenge in terms of time constraints. In this paper, this limitation is addressed by using a sample-efficient multi-objective Bayesian optimization that takes Kriging as a surrogate model and Expected Hypervolume Improvement as the infill criterion. Using this approach, the probabilistic model is continuously established and updated, and the approximate Pareto front is obtained at a relatively small computational budget. The objective of this work is to evaluate the applicability of employing a multi-objective Bayesian optimization framework for the aerodynamic-infrared shape optimization of an elliptical double serpentine nozzle at 6 km flight condition, where the objective functions are evaluated by means of high-fidelity computational fluid dynamics and reversed Monte Carlo ray tracing simulations. We achieve good results in both infrared radiation signature reduction and aerodynamic performance improvement with a reasonable number of evaluations, indicating that the proposed method is effective and efficient for tackling the computationally intensive optimization challenges in the aircraft design.
... 항공기의 기능 및 임무 요구사항이 더욱 복잡해짐에 따라 전기 부품을 한정된 공간 안에 사용하게 되고, 발열이 높은 소자를 사용하게 되면서 항공기 전자장비 내부의 열 부 하량이 증가하고 있다 [3][4][5][6]. 동시에, 현대 항공기는 장비 자체 의 성능뿐만 아니라 레이더 관측 가능성과 적외선 신호 감지 를 낮추어야 하므로 열관리 시스템은 필수적이다 [7][8][9][10][11]. 이러 한 열관리 시스템은 항공기의 안전과 성능 유지에 중요한 역 할을 한다. ...
... (i) nozzle shape alteration or optical blocking of engine parts heated by combustion gases; 20,21 for all views: ...
... Knowles and Saddington [7] reported that lobed nozzles are efficient in promoting the rapid mixing of exhaust gases with ambient air (by ~200%) compared to circular nozzles. The use of an S-shaped nozzle is reported [8,9,10] to reduce the IR signature of the engine from the rear view as it blocks the direct visibility of engine hot parts. ...
... Ever since their introduction, IR-guided missiles as strike weapons led to over 90% of all air combat losses, even with nonimaging seekers (with point detectors) that were in use in 1970s and 1980s. Non-imaging seekers were able to lock-on to only the aero-engine's inner hot parts and exhaust gases (especially in afterburning mode); both are prominent IR sources in direct rear view [6]. Their effectiveness was evident even though earlier generations of IR-guided missiles did not have all-aspect [7] and target recognition [8] capabilities. ...
Aircraft low observables’ features are crucial in the long-wave infrared (LW-IR) band, due to imaging sensors used in IR search and track and in the latest generation of IR-guided missiles. Earthshine irradiance on the aircraft bottom surface is an important source; hence, it is derived using data for atmospheric transmission. Emission due to skin-friction heating (important at high ${\varepsilon _{\rm{bot}}}$ ε b o t ) and earthshine reflection (important at low ${\varepsilon _{\rm{bot}}}$ ε b o t ) are compared by a dimensionless ratio for different bottom surface emissivities ( ${\varepsilon _{\rm{bot}}}$ ε b o t ). The infrared cross section of aircraft in direct view from below is obtained in the LW-IR band, which shows that aircraft is seen also due to negative contrast.
... A number of studies have been conducted on methods to reduce IR signals generated from aircraft exhaust gas, for example, by reducing the temperature of the exhaust gas, or shielding the high temperature part using bypass air, a serpentine nozzle, or aft-deck. Various studies have also been actively conducted on how the serpentine nozzle design parameters affect thrust, thermal flow field and IR reduction [2][3][4][5][6][7]. It was shown by Sun et al. [2] and Shan et al. [7] that as the ratio of shielding of the hot part of the engine in the nozzle increased, the IR signal decreased, but at the same time the thrust penalty increased. ...
Most existing plume infrared (IR) suppression techniques are difficult to apply to an already established aircraft system and cannot be applied in an active way. As an alternative solution, the present study explores an active technique for shielding IR signals by injecting particles into the engine exhaust plume. A Eulerian–Lagrangian-based discrete phase method was used to calculate a multiphase flow composed of exhaust gas and injected particles. To analyze the characteristics of thermal flow and particle distribution under flight conditions, a cruise condition of Mach number 0.7 at an altitude of 20,000 ft was considered. Water droplets and carbon particles with diameters of 5 and 10 μm were considered, and placed randomly within the particle layer. To analyze the shielding effect according to particle size, material type, and distribution pattern, the transmittance of IR electromagnetic waves of 3–5 μm wavelength was analyzed using a Maxwell multilevel fast multipole method. When the total mass was kept the same, 5 μm water droplets in wavelength bands below 3.5–3.7 μm, and 10 μm water droplets or 5 μm carbon particles in wavelength bands above 3.5–3.7 μm were more effective for shielding. If the single-layer transmittance information obtained in the present study is extended to actual particle layers of several tens of centimeters, it is expected that the shielding effect will be significantly higher.
... Varney [2] used a test to determine an aircraft's IR characteristics to learn to predict the IR level of aircrafts and evaluate their ability to survive a battle. Up to now, researchers have carried out detailed studies of the radiation from the exhaust system, including the analysis and optimization of nozzles from circle to other shapes [3,4], Sshaped [5,6] nozzles, rocket exhaust system [7][8][9][10] and helicopter exhaust system [11]. ...
In this study, the effects of nozzle size, CO2 and H2O molar fraction, pressure, and temperature on the infrared characteristics of round jets are studied. The backward Monte Carlo method is used to calculate the spectral radiation characteristics and infrared images. The results show that kλ is insensitive to the molar fraction of the species, and it is slightly more sensitive to pressure. Temperature has the greatest effect on the kλ. In addition, the effects of characteristic length of the jet, molar fraction of the species, and pressure on the infrared characteristics are similar. (1) When the characteristic optical thickness of the jet is thick enough, with the increasing of above parameters, the spectral radiance increases at 3∼3.6μm and 4.6∼5μm, while it decreases around 4.3μm. The magnitude of the two radiance peaks near 4.1μm and 4.4μm remain almost unchanged. However, the peak position moves to both sides. (2) Otherwise, there will be only one radiance peak in 4.1∼4.4μm. The height of radiance peak decreases with the decrease of characteristic optical thickness.
... The shield ratio was found to have little effect on the infrared signature of gas outside the nozzle exit since the gas species composition and temperature does not change with the shield ratio. An et al. [14] have estimated the plume IR signatures of various S-Shaped nozzle configurations and found that although infrared signature levels were reduced significantly in the axial direction, relatively higher signature levels were observed on the left and right sides and below the nozzle due to increase in the aspect ratio of the nozzle outlet as well as curvature, which led to a wider distribution of the plumes along a downward slope. The lock-on range associated with the plume IR signature reduced considerably with increased curvature and aspect ratio in the nozzle shape. ...
View Video Presentation: https://doi.org/10.2514/6.2021-3546.vid The hot exposed parts of the engine and the engine plume are a major source of the overall Infrared signature of an aircraft. In order to reduce the Infrared radiation from engine parts and engine plume, 2D nozzles with high aspect ratio cross-sections are employed in stealth aircraft. However, these 2D nozzles are longer than conventional nozzles and are associated with weight and performance penalties. In the present article, the effect of nozzle length and shape on temperature distribution on nozzle surfaces is studied using inputs of a low bypass ratio turbofan engine. It is found that shorter nozzle surfaces are exposed to lower temperature compared to the lengthy nozzle. The shorter nozzles are also found to have lower skin friction losses besides being lighter than longer nozzles.
... Decher (2004) has studied the characteristics of the infrared signal by varying the nozzle shapes. An et al. (2016) have performed an analytical study on exhaust nozzles with large aspect ratios and for an S-shaped nozzle using the CFD-FASTRAN commercial code to study the infrared signal characteristics according to the thermal flow change in the rear fuselage of the aircraft. By measuring the exhaust plume infrared signals from a microturbine engine using a spectroradiometer, Gu et al. (2017) have shown that infrared signatures of the square nozzle are lower than that of the circular nozzle. ...
Schlieren visualization of the plume ejected from the microgas turbine nozzle was conducted to understand infrared signal characteristics of various shapes of the exhaust nozzle. At the same time, the precise temperature distribution and infrared signal measurement were performed and compared. The maximum thrust of the microgas turbine used in the experiment is 230 N, the maximum speed of revolution is 108,500 rpm, and the maximum exhaust gas temperature is 750 °C. Seven nozzles were used for this experiment which included a cone nozzle, five square nozzles with aspect ratios (AR) ranging from 1 to 5 and an S-shaped nozzle with aspect ratio 6. The infrared signal emitted from the exhaust plume decreased as the aspect ratio increased. Schlieren flow visualization images show that cone nozzle had a uniform flow pattern, while square nozzle had a bright triangle pattern in the dispersed plume. As the aspect ratio of square nozzles increased, a bright triangle pattern reduced in size. On comparing Schlieren visualization with the temperature distribution, it can be understood that the triangular shape of core plume plays a major role in the temperature diffusion with the surrounding air. Based on the temperature distribution and the results of the Schlieren visualization, three types of exhaust plume models are suggested. These three models are homogenous plume, intermediate plume and two-dimensional plume with hot core, which correspond to the cone nozzle, the AR2 square nozzle and AR5 square nozzle, respectively.
