| 1. |
- Ahn, Myeonghwan, et al.
(författare)
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A numerical study on near-field pressure fluctuations of symmetrical and anti-symmetrical flapping modes of twin-jet using a high-resolution shock-capturing scheme
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier. - 1270-9638 .- 1626-3219. ; , s. 107147-107147
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Tidskriftsartikel (refereegranskat)abstract
- Screeching supersonic jets appears at off-design operating conditions and is perceived as an intense tonal noise. In a twin nozzle configuration, mutual interactions between the two jet plumes may occur with various coupling modes developing depending on the operating conditions and lateral distance between the jets. The investigation of the detailed flow behaviors and near-field pressure fluctuations with relevance to the twin jets system, the analysis of the developed instabilities, will enhance understanding of fundamental features associated with jets located close to each other.In the present study, the single jet is considered first to assess the large eddy simulation (LES) approach used and the near-field pressure fluctuation predictions. Based on the validated solver, twin jets are simulated. Two different twin-nozzle configurations having different separation distance or nozzle-to-nozzle centerline spacing are scrutinized for the same Mach number of 1.358. Notably, the twin jets are screeching by the coupling mode for both set-ups; however, the case of closer inter-nozzle distance presents a symmetrical dominant flapping mode, while the other case shows an anti-symmetrical flapping mode. The strength of the pressure fluctuation at the fundamental frequency changes depending on the location of the observer point (upstream or downstream) and the reference plane (twin-jet and normal to the twin-jet plane). The screech tones of the two cases, observable in the upstream region, are significantly different in the normal to the twin-jet plane direction because of the phase difference of fluctuating pressure. However, the first harmonic component remains strong, regardless of the flapping mode. It is also observed that, at the fundamental frequency, the amplitude of the pressure fluctuation at downstream locations is found to be strong in the normal to the twin-jet plane when the symmetrical flapping mode occurs. This feature is also observed in the twin-jet plane in the case of the opposite mode. By analyzing the developed vertical structures and performing correlation analyses of pressure fluctuations along jet shear layers, the periodicity of the flow in the downstream region with relevance to the fundamental frequency is revealed.
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| 2. |
- Chen, Song, et al.
(författare)
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Flow and aeroacoustic attributes of highly-heated transitional rectangular supersonic jets
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638 .- 1626-3219. ; 114:106747
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Tidskriftsartikel (refereegranskat)abstract
- Heated transitional supersonic jets exhausting from a rectangular nozzle at over-expanded conditions are investigated by Large Eddy Simulations and Ffowcs-Williams and Hawkings acoustic analogy. Four cases with a fixed nozzle pressure ratio but different temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0 are analyzed. Numerical results show that with the increasing temperature the jet velocity significantly increases, whereas its Reynolds number decreases by about one order of magnitude, which leads to a 30% decrease in the jet potential core length and reduction in the number of shock cells. The increasing temperatures also result in supersonic shear layer convection Mach numbers and consequently Mach wave radiations in the acoustic fields. Pressure skewness and kurtosis factors indicate crackle noise and non-linear propagation effects in high temperatures. For the most heated jet TR 7.0, the Mach wave radiation is identified radiating noise at about 120 degrees, while the large turbulence structure noise at about 150 degrees. Furthermore, the vortex sheet model analysis and the LES data detect the existence of upstream-propagating neutral waves inside jet TR 7.0. The observed screech frequency falls within the range of antisymmetric mode indicating that the highly-heated jet is characterized by an antisymmetric oscillation mode at the screech frequency.
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| 3. |
- Chen, Yuqian, et al.
(författare)
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Experimental and numerical study of flow and ignition and lean blowout characteristics of jet-cooled wall flameholder in a dual-mode combustor
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 122
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Tidskriftsartikel (refereegranskat)abstract
- The wall flameholder is one of the credible alternatives to realize pilot ignition in augmented/ramjet combustors. To overcome the ablation for a long-term operation, two types of jet cooling, external-inhaled air and pressure-driven jet cooling, are proposed for the wall flameholder. In this work, the flow and combustion process in a laboratory scale rig is studied for different cooling schemes and cooling conditions using experimental and numerical methods. Flow analysis in pressure-driven jet cooling scheme shows that the flow field of flameholder is influenced significantly by the cooling hole angle on the oblique plate α and on the rear plate β. In particular, the cooling jet angle combinations (α=30o, β=30o) and (α=90o, β=150o) are the two schemes with the most different characteristics. To investigate the effects of jet cooling type and cooling jet angle on the flow, ignition, and lean blowout (LBO) characteristics, the two distinctively different angle combinations are applied to form two kinds of jet cooling schemes mentioned above. Results suggest that the jet cooling type has less impact on the flow field but more influence on the flow loss than the cooling jet angle. The ignition performance of cooling schemes with α=30o and β=30o is better than that of those with α=90o and β=150o, but it has a more significant flow loss. The LBO limits of external-inhaled air cooling are lower than that of pressure-driven jet cooling. Moreover, the ignition and LBO limits decrease gradually with the increased mainstream temperature and they are only slightly affected by the mainstream velocity. Notably, the pressure-driven jet cooling scheme can slightly reduce the flow loss but it leads to a deteriorated ignition and LBO performance. The external-inhaled air cooling scheme with α=30o and β=30o has an excellent ignition and LBO performance, and the ignition and LBO limits increase with the increasing cooling air flow rate and the decreasing cooling air temperature.
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| 4. |
- Chen, Yuqian, et al.
(författare)
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Experimental study on combustion and flow resistance characteristics of an afterburner with air-cooled bluff-body flameholder
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- In the afterburner assembled with an air-cooled bluff-body flameholder, cooling air is directly injected into the recirculation zone behind the bluff-body, which can reduce the local temperature and increase the oxygen concentration of the gas mixture in the wake of the bluff-body, thereby affecting the total pressure loss and combustion characteristics. To better understand the flow and combustion process of the system, the exhaust gas temperature, cold and hot total pressure losses in a rectangular premixed combustor are investigated under different cooling air jet conditions. Experimental results show that the added cooling air could improve the combustion efficiency and widen the blowout limit, whereas it could also give rise to an extra total pressure loss. However, when the cooling air flow rate was higher than a critical value, i.e., after the blowing ratio reached 2.5, the recirculation zone could be blown away, resulting in a failed ignition in the afterburner. Notably, the decreased temperature difference between the mainstream and the cooling air could improve the combustion efficiency and reduce the thermal resistance loss but enlarge the cold flow loss and hot total pressure loss. Moreover, since the oxygen content declined and autoignition appeared after the mainstream temperature reached 1100 K, the exhaust gas temperature and combustion efficiency declined rapidly, and the hot total pressure loss also decreased. In addition, with the fuel-gas ratio increasing, the combustion efficiency significantly dropped, the exhaust gas temperature and thermal resistance loss firstly increased to a peak value (at the equivalence ratio of 1.14) and then decreased for excessively fuel-rich combustion.
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| 5. |
- Chen, Youqian, et al.
(författare)
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Influences of accelerating states on supercritical n-decane heat transfer in a horizontal tube applied for scramjet engine cooling
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 109
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Tidskriftsartikel (refereegranskat)abstract
- Supercritical n-decane is applied in the regenerative cooling system of scramjet engines while scramjets are always in various irregular movements. With the desire to explore heat transfer performance of supercritical n-decane under actual flight conditions, numerical simulations and analysis of different states during the accelerated flight process are carried out under three different heat fluxes and eight varying acceleration states. Specific flow and heat transfer mechanisms are explored by analyzing the temperature and flow fields affected by acceleration. Results show that acceleration weakens the intensity of the first abnormal heat transfer state but does not affect its appearance location, and there are almost no effects on the second heat transfer state. Another important conclusion is that the external factors have a great influence on supercritical n-decane heat transfer. The overall average surface heat transfer coefficient could be even up to 27.5% higher than for an ordinary horizontal tube. It is indicated that more attention should be paid to the external factors when investigating supercritical flow and heat transfer in the research and development of scramjet engine cooling.
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| 6. |
- Chen, Yuqian, et al.
(författare)
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Investigation of flame characteristics and cooling effectiveness of jet-cooled wall flameholders in vitiated flow
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 127
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and numerical studies were performed to understand the flame characteristics and cooling effect of jet-cooled wall flameholders in a simplified augmented combustor. The time-averaged flame structure and wall temperature were investigated employing an instantaneous flame capturing system with image processing technology and a wall temperature measurement system, respectively. Flow and fuel distribution simulations were carried out to analyze the flame structure and cooling effect of the jet-cooled flameholders in high-temperature and high-velocity vitiated flows. It was found that the jet cooling type and the cooling jet angle on the oblique plate α and on the rear plate β greatly affect the flame features and cooling effect of the flameholder. The external-inhaled air cooling schemes can increase the flame luminosity and temperature and create a flame hollowed-out zone in the near-wall region to reduce flame radiation and heat convection, thereby achieving an excellent cooling effect. The pressure-driven gas jet cooling schemes yielded a weaker flame luminosity and its flame structure in the cavity more sensitive to the cooling jet angle than external-inhaled air cooling schemes. Moreover, the external-inhaled air cooling schemes have significantly better cooling effects than the pressure-driven gas jet cooling schemes under different mainstream conditions and fuel flow rates. Notably, the cooling schemes with α=30∘ and β=30∘ have more uniform wall temperature than that of α=90∘ and β=150∘ on the above two plates. Furthermore, the external-inhaled air cooling scheme with α=90∘ and β=150∘ has the best cooling performance under different mainstream and cooling air conditions and presents great cooling effectiveness in the case of enhancing cooling air temperature.
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| 7. |
- Chen, Yuqian, et al.
(författare)
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Study on ignition process and flame expansion and propagation characteristics in jet-cooled pilot flameholders using image processing techniques
- 2022
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 129
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Tidskriftsartikel (refereegranskat)abstract
- Wall-type pilot flameholder has been used in aero-engines to effectively create a stable ignition source and achieve an excellent ignition performance in high-velocity flows. The ignition delay, flame propagation, and flame boundary are crucial characteristics to evaluate the performance of pilot flameholders. Previous work displayed the significantly changed ignition performance and flow features of the wall flameholder when jet cooling schemes were adopted, indicating that the ignition process and the flame features strongly depend on the cooling jet and the optimal design of jet cooling schemes needs further investigation. Therefore, in this study, two practical image processing algorithms are developed to characterize the flame development of the ignition process, reveal the effect of cooling schemes on the ignition delay time, and quantitatively analyze the flame area and expansion ratio of the wall flameholder during stable combustion. Two image processing methods proposed based on MATLAB algorithms exhibit effectiveness in studying the ignition process and flame expansion characteristics. The ignition process is divided into four phases: kernel generation, kernel propagation, flame growth, and stable combustion. Meanwhile, the ignition delay time is identified by counting the total pixels of the flame projected region during the ignition process. The edge detection and pixel statistics of the flame projected region in the binary luminosity image show the flame boundary and expansion ratio under various working conditions. It was found that the cooling jet angle has a significant impact on the ignition process, ignition delay time, flame propagation, and steady flame features. The pressure-driven gas jet cooling scheme with a smaller jet angle and the external-inhaled air cooling scheme with a larger jet angle present a better capability of ignition and flame expansion and propagation.
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| 8. |
- Fureby, Christer, et al.
(författare)
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Large Eddy Simulation of cavity stabilized ramjet combustion
- 2023
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Ingår i: Aerospace Science and Technology. - 1270-9638. ; 141
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Tidskriftsartikel (refereegranskat)abstract
- In this study we use finite rate chemistry Large Eddy Simulation (LES) to examine flow, injection, mixing, self-ignition and turbulent combustion in a dual-mode ramjet combustor with a cavity flameholder. The target case is the dual-mode ramjet combustor studied experimentally by Micka & Driscoll in a series of publications under different operating conditions characterized by varying air stagnation temperatures, T0, and fueled with hydrogen or hydrogen-ethylene blends. Here, only hydrogen fuel is considered. Three skeletal and comprehensive chemical reaction mechanisms are used to evaluate the influence of the reaction mechanism. Based on comparisons with ignition, laminar flame, and dual-mode ramjet combustor experiments the Z22 mechanism is found superior to the D7 and J20 mechanisms, which is then used throughout this study. For the ramjet experiments, two stabilization modes were reported: cavity-stabilized combustion at low T0 and jet-wake-stabilized combustion at high T0. For in-between values of T0, combustion oscillates between these modes. The LES results using the Z22 reaction mechanism are in good agreement with the experimental data: cavity-stabilized combustion is observed for low T0, and for this case combustion anchors at the leading-edge of the cavity. For high T0 jet-wake-stabilized combustion occurs in the near-wake of the fuel-plume. Between these two modes, combustion oscillates between modes that can be roughly identified as jet-wake and cavity stabilized. The LES predictions also reveal details about high-frequency oscillations (∼1 kHz) that are observed in particular for low and intermediate T0. The H2-air mixture appears to burn as an auto-ignition assisted premixed flame in the cavity-stabilized case, and as a diffusion-flame in the jet-wake stabilized case, whereas for the in-between cases, the burning modes changes accordingly.
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| 9. |
- Hakima, Houman, et al.
(författare)
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Concurrent attitude and orbit control for deorbiter CubeSat
- 2020
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Ingår i: Aerospace Science and Technology. - : Elsevier. - 1270-9638 .- 1626-3219. ; 97
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Tidskriftsartikel (refereegranskat)abstract
- This paper details a concurrent attitude and orbit control method for a debris-removing nanosatellite, called deobriter CubeSat, during the rendezvous and synchronization maneuver with an uncontrollable tumbling debris object. The CubeSat is designed based on the utilization of an eight-unit form factor and commercially-available components with substantial space heritage, and is intended for the removal of sizable debris objects in low-Earth orbit. In particular, a low-thrust propulsion system is used for orbit control, as well as three reaction wheels allowing for a three-axis attitude control. Since the thruster can only produce force in one direction in the body frame, the spacecraft is considered to be underactuated. The controller employs the reaction wheels and the thruster to simultaneously rendezvous and synchronize the attitude of the CubeSat with the tumbling debris object, allowing for a concurrent attitude and position tracking. Detailed derivation of the concurrent controller is discussed, the effects of high-order derivatives are analyzed, and the stability of the system is proved. Simulation scenarios are created for two different thruster operation modes, namely, unsaturated thrust force and continuously-saturated thrust force, in order to verify the performance of the controller, as well as its robustness against gravity gradient disturbance torque and gravitational perturbation force.
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| 10. |
- Huang, Zhongjie, 1985, et al.
(författare)
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Aeroacoustic analysis of aerodynamically optimized joined-blade propeller for future electric aircraft at cruise and take-off
- 2020
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638. ; 107
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Tidskriftsartikel (refereegranskat)abstract
- A novel propeller with the blade tips joined in pairs, named Boxprop, is designed and optimized for a conceptual electric aircraft using an efficient optimization platform. According to the thrust requirement of the electric aircraft at cruise, the Boxprop with optimal efficiency is down-selected from the Pareto front of thrust coefficient and propeller efficiency. Furthermore, the blade pitch angle is adjusted to meet the thrust requirement at take-off. It is found that the Boxprop is capable of suppressing tip vortices and inducing a wider wake behind blade tip in comparison to a conventional propeller usually shedding a concentrated tip vortex, which could potentially improve the propulsive efficiency. Afterwards, the aeroacoustic analysis performed by the hybrid integral method of Reynolds-Averaged Navier Stokes equations (RANS) and convected Ffowcs Williams and Hawkings (FW-H) equation shows that the tonal noise from the Boxprop with three joined blades operating at cruise is similar to a conventional three-bladed propeller, though being stronger than a conventional six-bladed propeller. Although the tip vortices have been suppressed by the joined-blade tips of the Boxprop, the corresponding tonal noise reduction is not prominent. Next, the Boxprop noise at take-off is studied. Unsteady RANS is used to resolve varying flow structures that become dominant under the take-off condition. Angle of attack (AOA) is found as an important factor influencing the noise generation. The radiated noise upstream and downstream of the propeller significantly intensifies due to increasing AOA. The AOA effects of the Boxprop follow a similar trend to a conventional propeller. The findings for the Boxprop aeroacoustics have enhanced the understanding of tip-vortex suppression techniques in connection with the tonal noise generation, which will be greatly helpful to the aeroacoustic design of Boxprop applied to electric aircraft in the future.
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