| 1. |
- Hong, Beichuan, Ph.D. student, 1989-
(författare)
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Exergy Evaluation of Engine Operations : Combustion Process to Exhaust Flow
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transitioning the transport sector to clean energy sources is crucial for mitigating greenhouse gas emissions and achieving carbon neutrality. A collaborative solution, combining both electric vehicles and combustion engines using renewable fuels, may prove more effective than competitive ones. This necessitates a focus on developing sustainable combustion engines by improving their efficiency through renewable energy sources and innovative technologies.This thesis uses exergy analysis to evaluate engine efficiency, losses, and irreversibilities, as well as the work potential of exhaust flows. Particular emphasis is placed on the implications of these exergy analyses in relation to engine operations, especially concerning combustion processes and exhaust pulsations. Exergy analysis quantifies the maximum work extractable from an energy source, enabling the identification and quantification of losses and inefficiencies in thermal processes. A dual-fuel marine engine with two-stage turbocharging and an ethanol-fueled heavy-duty spark-ignition (SI) engine using lean burn are examined with validated one-dimensional engine models to analyze engine performance and losses from an exergy perspective. In the tested marine engine, irreversibilities are quantified and categorized into three types, with combustion irreversibility being the most significant, followed by losses through gas exchange and heat dissipation. In the ethanol-fueled SI engine, the effect of lean-burn combustion at high load is investigated through the excess air ratio up to 1.8, assessing its impact on thermal efficiency, combustion phasing, as well as energy and exergy distributions. Results indicate that employing lean burn improves engine efficiency with advanced combustion phasing but also leads to more exergy destruction. The importance of maintaining high exergy recovery through turbocharging for diluted operation is also highlighted.Additionally, high-frequency exhaust pulsations resulting from valve motion pose challenges in accurately resolving exhaust energy and exergy. To address this, this thesis investigates methods for exhaust pulse characterization and measurement under unsteady flow conditions. Sensitivity analyses, based on a heavy-duty engine simulation, highlight the importance of time-resolved mass flow measurements in quantifying the energy and exergy of exhaust pulsations. Subsequently, this research implements a Pitot tube-based approach to measure crank angle-resolved engine exhaust mass flow rates and to further analyze the effect of attenuated temperature measurements on resolving instantaneous mass flows. The findings indicate that temperature variations pertaining to exhaust flow conditions have only a relatively small impact on mass flow measurements. Based on the exhaust flow measurements, the mass flow characteristics of exhaust pulsations are also discussed with regard to the blow-down and scavenge phases.
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| 2. |
- 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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| 3. |
- Ahn, MyeongHwan, et al.
(författare)
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Effects of Temperature on the Characteristics of Twin Square Jets by Large Eddy Simulations
- 2022
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Ingår i: AIAA Science and Technology Forum and Exposition, AIAA SciTech Forum 2022. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA).
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Konferensbidrag (refereegranskat)abstract
- In this study, we investigate the effects of temperature on the aerodynamic and aeroacoustics characteristics of twin square jets. Implicit Large Eddy Simulations (ILES) are performed for twin jets with a fixed nozzle pressure ratio (NPR) of 3.0 and temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0. A second-order central scheme is used to resolve acoustic waves, and an artificial dissipation model is applied to capture shock waves and to suppress non-physical oscillations. In addition, the variation of a specific heat ratio as function of temperature is considered under the chemical equilibrium assumption. The numerical results show that the length of potential core is reduced with the increase of temperature due to the enhanced mixing in jet shear layers which can be estimated by turbulent kinetic energy (TKE). Meanwhile, the fluctuations of the transverse velocity show different trends between the cases within the corresponding potential core length, which can be associated with the screeching phenomena of the twin-jet. As temperature increases, the convection Mach number in the jet shear layers is also increased so that the Mach wave is generated for TR of 2.0, 4.0, and 7.0. However, a crackle noise is only observed for TR of 4.0 and 7.0, whose generation is identified by the skewness and kurtosis factors. Relatively low temperature jets (TR of 1.0 and 2.0) are screeching so that peaks are observed in the spectra obtained upstream. On the other hand, broadband component is gradually increased when the jets are heated, and the largest increase is observed at the location exposed to the Mach wave radiation.
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| 4. |
- Ahn, Myeonghwan, et al.
(författare)
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Flow and Near-field Pressure Fluctuations of Twin Square Jets
- 2021
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Ingår i: AIAA Propulsion and Energy Forum, 2021. - Reston, Virginia : American Institute of Aeronautics and Astronautics Inc, AIAA.
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Konferensbidrag (refereegranskat)abstract
- We aim to investigate the aerodynamic and acoustics characteristics of a twin square jet using an implicit Large Eddy Simulation (ILES). A screeching cold jet condition, a nozzle pressure ratio (NPR) of 3.0, is considered to simulate a coupled twin-jet. A second-order central scheme with a modified version of Jameson’s artificial dissipation is adopted to damp numerical oscillations and to mimic the effect of small-scale turbulence without an explicit subgrid-scale (SGS) model. Numerical results show that the overall trends of time-averaged streamwise velocity profiles are similar to the experimental data, with the largest differences observed at locations associated with the presence of the shock-cell structures. A detailed investigation of the flow fluctuations in jet shear layers is performed. The amplitude of the velocity fluctuations is highly dependent on the location of the shear layers with respect to the twin-jet configuration (upper, lateral, or inner). The coupling mode of twin jets associated with the screech tone is determined as a symmetrical flapping mode be a two-points spacetime cross-correlation analysis. The overall trends of near-field pressure fluctuation spectra by LES agree well with the experimental results in both upstream and downstream regions. Near-field pressure fluctuation spectra by ILES agree well with the experimentally obtained spectra at different locations in the nozzle exit plane as well as at several downstream locations in the near-field acoustic region. The highest screech tone is observed at the inter-nozzle region where superposition of in-phase waves and standing waves are found. Fourier phase and amplitude fields at the fundamental frequency also confirm the symmetrical flapping mode of the twin jets by showing in-phase relations of hydrodynamic/acoustic waves and noise directivities.
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| 5. |
- Ahn, MyeongHwan, et al.
(författare)
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Large-eddy simulations of flow and aeroacoustics of twin square jets including turbulence tripping
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the flow and aeroacoustics of twin square (i.e., aspect ratio of 1.0) jets by implicit large-eddy simulations (LESs) under a nozzle pressure ratio of 3.0 and a temperature ratio of 1.0 conditions. A second-order central scheme coupled with a modified Jameson's artificial dissipation is used to resolve acoustics as well as to capture discontinuous solutions, e.g., shock waves. The flow boundary layer inside of the nozzle is tripped, using a small step in the convergent section of the nozzle. The time-averaged axial velocity and turbulent kinetic energy of LES with boundary layer tripping approaches better to particle image velocimetry experimental data than the LES without turbulence tripping case. A two-point space–time cross-correlation analysis suggests that the twin jets are screeching and are coupled to each other in a symmetrical flapping mode. Intense pressure fluctuations and standing waves are observed between the jets. Spectral proper orthogonal decomposition (SPOD) confirms the determined mode and the relevant wave propagation. The upstream propagating mode associated with the shock-cell structures is confined inside jets. Far-field noise obtained by solving Ffowcs Williams and Hawkings equation is in good agreement with the measured acoustic data. The symmetrical flapping mode of twin jets yields different levels of the screech tone depending on observation planes. The tonalities—the fundamental tone, second and third harmonics—appear clearly in the far-field, showing different contributions at angles corresponding to the directivities revealed by SPOD.
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| 6. |
- Ceci, Alessandro, et al.
(författare)
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Computational analysis of the indirect combustion noise generation mechanism in a nozzle guided vane in transonic operating conditions
- 2021
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Ingår i: Journal of Sound and Vibration. - : Elsevier. - 0022-460X .- 1095-8568. ; 496
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Tidskriftsartikel (refereegranskat)abstract
- The combustion noise in modern engines is mainly originating from two types of mechanisms. First, chemical reactions in the combustion chamber leads to an unsteady heat release which is responsible of the direct combustion noise. Second, hot and cold blobs of air coming from the combustion chamber are advected and accelerated through turbine stages, giving rise to entropy noise (or indirect combustion noise). In the present work, numerical characterization of indirect combustion noise of a Nozzle Guide Vane passage was assessed using three-dimensional Large Eddy Simulations. The present work offers an overview to the analytical, computational and experimental studies of the topic. Numerical simulations are conducted to reproduce the effects of incoming planar entropy waves from the combustion chamber and to characterize the generated acoustic power. The dynamic features of the flow are addressed by the means of frequency domain and modal analyses techniques such as Fourier Decomposition and Proper Orthogonal Decomposition. Finally, the predicted entropy noise from numerical calculations is compared with the analytical results of an actuator disk model for a stator stage. The present paper proves that the generated indirect combustion noise can be significant for transonic operating conditions. The blade acoustic response is characterized by the excitation of a latent dynamics at the forcing frequency of the planar entropy waves, and it increases as the amplitude of the incoming disturbances increases.
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| 7. |
- Chen, Song, et al.
(författare)
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Effect of an Adjacent Flat Plate on a Highly-Heated Rectangular Supersonic Jet
- 2020
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Ingår i: AIAA Scitech 2020 Forum. - AIAA : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Solid surfaces located in the vicinity of a supersonic jet may affect its flow dynamics and greatly change the aeroacoustic characteristics. Large-eddy simulations (LES) are performed to investigate the plate effects on a highly-heated rectangular supersonic jet. The rectangular nozzle has an aspect ratio of 2.0 and is operated at the over-expanded condition with a nozzle pressure ratio of 3.0 and a nozzle temperature ratio of 7.0. Four cases, JetL0 to JetL3 with a plate-to-nozzle distance ranging from 0 to 3 times of the jet equivalent nozzle diameter are investigated. The large-scale implicit LES computations are performed by a well-validated in-house finite-volume based CFD code, which uses an artificial dissipation mechanism to represent the effect of small-scale turbulence and to damp the numerical oscillation near shocks. The temperature-dependent thermal properties of air in the highly-heated jets are considered by the chemical equilibrium assumption. Numerical results show that among the four cases, JetL0 with the plate directly attached at the nozzle lip shows significant different flow and acoustic fields from the others. It exhibits a longer jet potential core length but without forming a series of well-structured shock diamonds. The other cases show similar shock/expansion wave structures as observed in the free jet but their jet plumes bend towards the plate. This bending of jet leads to JetL1 scrubs over the plate in the downstream. The scrubbing effect, together with the unaffected shock-shear layer interactions and high plate pressure loading, makes JetL1 have a stronger OASPL in the near acoustic fields than the other cases. The spectrum analysis in the nozzle upstream direction shows that the plate removes or mitigates the screech tone observed in the free jet and slightly amplifies the acoustic amplitudes in the low-frequency range.
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| 8. |
- 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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| 9. |
- Giramondi, Nicola, 1991-
(författare)
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Diffusive Combustion of Ethanol in a Dual-Fuel Direct Injection Compression Ignition Engine
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The impact of climate change due to global warming necessitates rapid and extensive measures to enhance the sustainability of the energy and transport sectors. In this context, there are large environmental and societal benefits to be gained by replacing diesel with renewable fuels for road freight transport. This solution may facilitate and expedite the transition towards fossil-free, carbon-neutral transport, while the electrification process takes shape. Short-chain alcohol fuels have favorable properties for the enhancement of engine performance and the abatement of pollutant emissions, however, they necessitate ignition aid systems in compression ignition engines. The present research investigates a novel concept of dual-fuel combustion for heavy-duty compression ignition engine applications by means of engine tests and three-dimensional combustion simulations. This concept involves the direct injection of pure ethanol as main fuel through a centrally mounted injector, and minimal quantities of diesel as pilot fuel via a separate injector. The objective is to achieve diffusive combustion of ethanol in a process analogous to conventional diesel combustion throughout the entire engine load range, with a higher thermal efficiency and lower pollutant emissions. Single-cylinder engine tests were carried out to evaluate the influence of combustion characteristics and performance with respect to dual-injection strategy, engine load, ethanol ratio and configuration of the diesel pilot injector. The characteristics and performance of ethanol-diesel direct injection compression ignition (DICI) combustion were compared to two sets of baselines, that are conventional diesel combustion and dual-injections of diesel via the main and pilot injector in the same proportion as in the dual-fuel test points. At low load conditions, increasing the separation between the diesel pilot and ethanol main injection enabled the achievement of diffusive combustion of ethanol, avoiding combustion instability and partial misfire thanks to minimal quantities of diesel injected. At high load conditions, a minimum main-pilot separation was instead required to limit the degree of ethanol premixing at ignition. Using a diesel pilot injector having a lower number of sprays with a wider hole diameter promoted a more robust ignition of ethanol, while also causing a reduction of engine performance. Parallel to the experimental work, three-dimensional combustion simulations were carried out in order to investigate the interaction between diesel and ethanol sprays during ignition at various engine operating conditions, from low to full load. At the operating conditions investigated during engine tests, the ignition of a subset of ethanol sprays was locally triggered by the contact with the products of diesel combustion. Subsequently, ignition propagated towards the neighboring ethanol sprays, until reaching the furthest ones from the diesel pilot injector. The coupling between experimental and numerical results highlighted the noteworthy predictive capability of the adopted combustion model with respect to the ethanol combustion characteristics. In conclusion, the present research work provides a solid starting point for future studies on diffusive combustion of alcohol fuels in compression ignition engines. The structured knowledge built in the course of the doctoral project lays the foundation for the development of a fuel-flexible engine for heavy-duty applications.
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| 10. |
- Gojon, Romain, et al.
(författare)
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Impact of an Adjacent Surface on a Rectangular Overexpanded Supersonic Jet
- 2023
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Ingår i: Flow Turbulence and Combustion. - : Springer Nature. - 1386-6184 .- 1573-1987.
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Tidskriftsartikel (refereegranskat)abstract
- Flow and acoustic fields of a rectangular over-expanded supersonic jet interacting with an adjacent parallel plate are investigated using compressible Large Eddy Simulations (LES). The jet exits from a converging diverging rectangular nozzle of aspect ratio 2 with a design Mach number 1.5. Four distances (0 to 3 equivalent diameters) between the plate and the adjacent lip of the rectangular jet in the minor axis plane are studied. The geometry of the nozzle, the positions of the plate, and the exit conditions are identical to the ones of an experimental study. Snapshots and mean velocity fields are presented. Good agreement with the PIV experimental measurements is obtained. Previously, the corresponding free jet has been found to undergo a strong flapping motion in the minor axis plane due to screech. Here, it is shown that the intensity of the screech increases for certain distances from the plate and decreases for others, as compared to the corresponding free jet. Two points space-time cross correlations of the pressure along the jet’s shear-layers show, in two cases, an amplification of the aeroacoustic feedback mechanism leading to screech noise in the jet shear-layer closer to the plate. This amplification is due to acoustic waves impinging on the plate, and generating propagating waves back towards the jet, thus exciting the shear-layer at the screech frequency, around the tenth shock cell. Moreover, when the jet develops as a wall jet on the plate, the screech frequency and its associated flapping motion is canceled but a symmetrical oscillation of the jet at a lower frequency becomes dominant and radiates in the near acoustic field. This oscillation mode, as the ones associated with the screech tones for the other cases studied, can be explained by the use of a vortex sheet model of the ideally expanded equivalent planar jet.
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