| 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. |
- Trigell, Emelie
(författare)
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Operating conditions impact on flow and acoustics in turbocharger compressors
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fluid machines are an integral part in energy conversion with applications from pumps, fans, propellers, compressors and turbines. In the automotive industry, turbochargers are commonly employed to counteract the effect of engine downsizing. However, designing efficient compressors with wide operating ranges and reduced noise emissions consitute a challenge.This thesis investigates flow instabilities and sound generation in turbocharger compressors, utilizing compressible Large Eddy Simulations (LES). The numerical approach is validated through sensitivity studies and comparison with measurement data. Three different compressor designs used in both light-duty and heavy-duty applications are examined with the aim of enhancing the understanding of rotating stall mechanism in real-world configurations and their impact on aerodynamically generated noise.The analysis employs compressible Navier-Stokes equations with a scale-resolving model, evaluating its robustness in comparison to other computational methods under various operating conditions. The system's response to time-varying boundary conditions is assessed, and the effect of pulse amplitude is quantified.Subsequently, the mechanism for aerodynamically generated noise, focusing on the broadband components are explored through analysis of the recirculation region. Resolving the Taylor micro-scale in the recirculation region enhances the understanding of the dynamics in this zone. It is demonstrated that an inlet recirculation zone develops near surge conditions, which is highly sensitive to the choice of boundary conditions and turbulence formulation. Passive flow control, such as the ported-shroud, are considered to illustrate their influence on performance, stability and noise.Finally, the system is studied using a two-port method, accounting for rotational effects. This provide insights into the transmission poperties at low frequencies (< 3 kHz) and the mechanism of sound generation. It is demonstrated that the use of Computational Fluid Dynamics can improve the understanding of flow-acoustic interaction in complex geometries. Additionally, the developed numerical simulation and post-processing methods have potential application in a range of turbochargr systems, from hybrids to fuel cell application.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Golliard, Thomas, et al.
(författare)
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Computational Aeroacoustics for a Cold, Non-Ideally Expanded Aerospike Nozzle
- 2023
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Ingår i: Proceedings of ASME Turbo Expo 2023. - : American Society of Mechanical Engineers (ASME).
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Konferensbidrag (refereegranskat)abstract
- In supersonic aerospace applications, aerospike nozzles have been subject of growing interest. These devices lead to enhanced thrust performance compared to conventional nozzles due to continuous altitude adaption and improved thrust vector control. However, supersonic non-ideally expanded jets are known to generate high levels of noise. The aeroacoustic behaviour of circular and rectangular nozzles has been largely discussed whereas data on the aeroacoustic behaviour of aerospike nozzles is scarce. For further industrial development, the identification of the noise generation mechanisms in such configurations is necessary. This study sheds light on the main noise components of a cold jet exhausting an aerospike nozzle. Implicit Large Eddy Simulations (ILES) are deployed to simulate the flow of the cold aerospike at a Nozzle Pressure Ratio (NPR) = 3. For far-field acoustic computation, the Ffowcs-Williams Hawkings (FWH) equation is applied. A mesh sensitivity study is first performed. Then, the configuration is analyzed in terms of near-field instantaneous and time-averaged flow characteristics. It is of crucial interest to characterize the features of the shock-cell structures. The annular shock structure near the aerospike bluff body displays two non-attached shock-cells of length L/Dj ∼ 0.43. The annular jet is then reattaching and this reattachment leads to longer shock-cells of length L/Dj ∼ 0.77. Downstream of the bluff body, a second expansion process takes place and leads to the emergence of a circular shock-cell structure with a first shock-cell length of L/Dj ∼ 1.20. The interaction between the vortical flow structures in the shear layers and the shocks generates Broadband Shock-Associated Noise (BBSAN). In order to enhance understanding of the noise generation mechanism for this configuration, several analyses are performed. Two-point cross-correlations of data acquired in monitoring points located along axial lines in the circular shear layers are used for quantifying the upstream propagating waves associated to a strong tonal component at a Strouhal number St = 0.51. This strong tonal component is known as screech. It is generated by a feedback mechanism between the coherent fluid flow structures propagating downstream in the jet shear layer and the upstream propagating acoustic waves generated at the same frequency by vortex-shock interactions, waves that are interacting with the nozzle lip and excite shear layer instabilities at the frequency of screech. Power spectral density of the radial velocity at monitoring points in the annular jet structure displays three main peaks at St = 0.68, St = 1.21 and St = 2.59. These frequencies correspond to the oscillation modes of the annular shock-cell structure in radial direction. Furthermore, a vortex sheet model is adapted to predict the length of the annular shock-cells. A good agreement is reached between the analytically derived shock-cell length and the simulation results. The shock-cell length is used to predict the central frequency of BBSAN as a function of observation angles. The far-field spectra show mixing noise as well as Broadband Shock-Associated Noise, related to the interaction between the convected vortices in the shear layers and the shock-cell structure. High sound pressure levels (SPL) are detected in agreement with the BBSAN central frequencies which were computed using the annular and circular shock-cell length. Finally, high SPL are obtained at the radial oscillation frequencies for the annular shock-cell structure.
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| 6. |
- Hong, Beichuan, Ph.D. student, 1989-, et al.
(författare)
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Crank angle-resolved mass flow characterization of engine exhaust pulsations using a Pitot tube and thin-wire thermocouples
- 2023
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; , s. 121725-121725
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Tidskriftsartikel (refereegranskat)abstract
- Characterizing pulsating flow in high-temperature, high-pressure engine exhaust gas is crucial for the development and optimization of exhaust energy recovery systems. However, the experimental investigation of engine exhaust pulses is challenging due to the difficulties in conducting crank angle-resolved measurements under these unsteady flow conditions. This study contributes to characterizing mass flow pulses from an isolated cylinder exhaust of a heavy-duty diesel engine using a single-pipe measurement system, developed for pulsating flow measurement. A Pitot tube-based approach is adopted to measure exhaust mass flow pulsations, complemented by fast temperature measurements obtained using customized unsheathed thin-wire thermocouples. The on-engine experiment is performed by isolating the in-cylinder trapped mass and the valve opening speed to produce different exhaust pulse waveforms. The adopted approach’s sensitivity in resolving instantaneous mass flows is evaluated analytically and experimentally, considering attenuated temperature measurement effects. Based on exhaust flow measurements, mass flow pulses are analyzed with regard to blow-down and scavenge phases. Under the load sweep, the main waveform change occurs during the blow-down phase, with pulse magnitude increasing with the load. In contrast, as the engine speeds up with a comparable trapped mass, the exhaust mass distribution in the blow-down phase decreases from 75.5% at 700 rpm to 41.9% at 1900 rpm. Additionally, it is observed that cycle-to-cycle variations in mass flow pulses align with combustion stability during the blow-down phase and are predominantly influenced by gas-exchange processes during the scavenge phase.
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| 7. |
- Hong, Beichuan, Ph.D. student, 1989-, et al.
(författare)
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Energy and exergy characteristics of an ethanol-fueled heavy-duty SI engine at high-load operation using lean-burn combustion
- 2023
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; , s. 120063-120063
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Tidskriftsartikel (refereegranskat)abstract
- Ethanol, as the most produced renewable biofuel, is considered a promising low-carbon alternative to petroleum-based fuels in the transport sector due to its high energy density and auto-ignition resistance. The lean-burn combustion in spark-ignition (SI) engines has the potential to further improve thermal efficiency in regard to knock mitigation and the reduction of combustion temperature. However, the characteristics of lean-burn combustion in an ethanol-fueled engine in relation to the combustion losses and the gas-exchange process remain unclear, especially for high-load operation. This study contributes with a deeper understanding of the high-load performance of an ethanol-fueled heavy-duty SI engine using lean-burn combustion. Based on the experimental results from a single-cylinder engine test, a 6-cylinder engine model is built by integrating a validated predictive combustion model to characterize the lean-burn combustion process. The engine’s thermal efficiency and combustion phasing are evaluated for knock limited operation and then compared to the theoretical optimum which is regardless of knock. The energy and exergy balances are applied to evaluate the effect of dilution with excess air ratios up to 1.8. Losses through heat transfer, exhaust flow, and incomplete combustion are quantified. In addition, entropy generated through combustion is discussed to identify the relationship between exergy destruction and different operating conditions. In the context of lean-burn combustion, the thermal efficiency at high-load operation incrementally increases from 40.4% at stoichiometric condition to 47.3% at an excess air ratio of 1.8. At the same time, the exergy destruction through combustion increases by 3.3 percentage points across the selected dilution range. Furthermore, the challenging requirements to realize lean-burn combustion with lower exhaust gas temperatures and higher intake boost pressures is assessed through an exergy analysis of the turbocharging system.
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| 8. |
- Jacob, Stefan, Dr. 1980-, et al.
(författare)
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Acoustic scattering in a small centrifugal compressor based on the use of linearized equations in a rotating frame
- 2023
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Ingår i: Journal of Sound and Vibration. - : Elsevier. - 0022-460X .- 1095-8568. ; 544, s. 117315-117315
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Tidskriftsartikel (refereegranskat)abstract
- Numerical solutions of acoustic wave scattering are often used to describe sound propagation through complex geometries. For cases with flow, various forms of the convected equation have been used. A better alternative that includes vortex-sound interaction is instead to use the linearized and harmonic forms of the unsteady fluid flow governing equations. In this paper, a formulation of the linearized equations that include rotational effects, in an acoustic computation using a rotating frame of reference in a stationary geometry, is presented. We demonstrate that rotational effects can be important, e.g., when computing the transmission loss through high-speed compressors. The implementation of the proposed addition to the existing schemes is both simple and numerically inexpensive. The results are expected to have an impact on the research and development related to noise control of high-performance turbo-machinery, e.g., used in automotive or aviation applications at operating conditions that can be represented by steady background flows.
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| 9. |
- Kraxberger, Florian, et al.
(författare)
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On the Alignment of Acoustic and Coupled Mechanic-Acoustic Eigenmodes in Phonation by Supraglottal Duct Variations
- 2023
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Ingår i: Bioengineering. - : MDPI AG. - 2306-5354. ; 10:12
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Tidskriftsartikel (refereegranskat)abstract
- Sound generation in human phonation and the underlying fluid–structure–acoustic interaction that describes the sound production mechanism are not fully understood. A previous experimental study, with a silicone made vocal fold model connected to a straight vocal tract pipe of fixed length, showed that vibroacoustic coupling can cause a deviation in the vocal fold vibration frequency. This occurred when the fundamental frequency of the vocal fold motion was close to the lowest acoustic resonance frequency of the pipe. What is not fully understood is how the vibroacoustic coupling is influenced by a varying vocal tract length. Presuming that this effect is a pure coupling of the acoustical effects, a numerical simulation model is established based on the computation of the mechanical-acoustic eigenvalue. With varying pipe lengths, the lowest acoustic resonance frequency was adjusted in the experiments and so in the simulation setup. In doing so, the evolution of the vocal folds’ coupled eigenvalues and eigenmodes is investigated, which confirms the experimental findings. Finally, it was shown that for normal phonation conditions, the mechanical mode is the most efficient vibration pattern whenever the acoustic resonance of the pipe (lowest formant) is far away from the vocal folds’ vibration frequency. Whenever the lowest formant is slightly lower than the mechanical vocal fold eigenfrequency, the coupled vocal fold motion pattern at the formant frequency dominates.
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| 10. |
- Laudato, Marco, et al.
(författare)
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Analysis of the contact critical pressure of collapsible tubes for biomedical applications
- 2023
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Ingår i: Continuum Mechanics and Thermodynamics. - : Springer Nature. - 0935-1175 .- 1432-0959. ; 36:1, s. 217-228
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Tidskriftsartikel (refereegranskat)abstract
- The onset of self-excited oscillations in airways and blood vessels is a common phenomenon in the human body, connected to both normal and pathological conditions. A recent experimental investigation has shown that the onset of self-excited oscillations happens for values of the intramural pressure close to the contact critical pressure. The goal of this work is to analyse the dependence of the contact critical pressure on the vessel’s geometric parameters. The methodology is based on the implementation of an experimentally validated computational model of a collapsible tube. The results confirm the correlation between the contact critical pressure and the onset of self-excited oscillations in collapsible tubes. Moreover, a set of general equations to compute the contact critical pressure and the corresponding areas of collapsible tubes with arbitrary geometries has been derived.
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