Article
Thermodynamics
Xiao Zhang, Joseph D. Chung, Elaine S. Oran
Summary: Recent work has investigated the blue whirl, a soot-free hydrocarbon flame, and found that it is composed of a triple flame in a swirling flow undergoing vortex breakdown. This paper presents three-dimensional numerical simulations of triple flames in swirling flows with and without vortex breakdown, and discusses their relation to the blue whirl. The results show that the shape of the triple flame depends on the occurrence and location of vortex breakdown. The findings provide insights into the flow structure and flame shapes of the blue whirl.
COMBUSTION AND FLAME
(2022)
Article
Mechanics
Christopher M. Douglas, Benjamin L. Emerson, Timothy C. Lieuwen
Summary: This paper presents bifurcation analyses characterising the nonlinear dynamics of fully developed laminar annular jets with respect to the centrebody diameter d, Reynolds number Re, and swirl ratio S. The results reveal the non-monotonic evolution of the axisymmetric jet's steady topology under varying swirl ratio S, and the existence of multiple bifurcations and instabilities under different flow conditions.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Benjamin W. Keeton, Jaime Carpio, Keiko K. Nomura, Antonio L. Sanchez, Forman A. Williams
Summary: The transition to bubble and conical vortex breakdown in low-Mach-number laminar axisymmetric variable-density swirling jets is studied using theoretical predictions and numerical simulations. The critical values of swirl number for the onset of bubble and cone are determined as the jet-to-ambient density ratio is varied. Numerical calculations and theoretical predictions show consistent results, and the critical swirl number for the cone is found to depend on viscous effects.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Christopher M. Douglas, Lutz Lesshafft
Summary: This paper explores the effect of axial and radial confinement on the flow topology of laminar swirling jets and finds that confinement significantly influences the flow behavior.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Christopher M. Douglas, Benjamin L. Emerson, Timothy C. Lieuwen
Summary: This paper characterizes the steady and time-periodic behavior of swirling jets using numerical bifurcation analysis. The study reveals a wide array of flow patterns within a certain range of Reynolds number and swirl ratio, showing both stable axisymmetric flows and unstable three-dimensional flow structures. The research highlights the role of unsteady nonlinear interactions on the overall behavior of swirling jets and discusses the similarities and differences between this type of unconfined jet and other classes of swirling flows.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Thermodynamics
Mohamed Y. Hashim, Jonggeun Bae, Jeekeun Lee
Summary: Large Eddy Simulation (LES) is used to study turbulent swirling flow characteristics in a gas turbine combustor. The study focuses on the effect of contraction ratio on vortex breakdown phenomena and turbulence structure. Experimental measurements and LES results show reasonable agreement. Increasing the contraction ratio leads to an increase in pressure drop and mean axial velocity at the contraction part. The stagnation point and the size of the central toroidal recirculation zone stabilize at certain contraction ratios, while the swirl level is enhanced. The presence of contraction ratio successfully suppresses pressure and axial velocity fluctuations in the shear layer zones.
APPLIED THERMAL ENGINEERING
(2023)
Article
Energy & Fuels
Tao Zhou, Kai Zhao, Feng Li, Kefu Wang, Nan Meng, Duo Wang
Summary: This paper investigates the impact of inner and outer swirl flow on the spray combustion characteristics and NOx emission in a concentric staged triple swirler lean direct injection (LDI) combustor. The study utilizes numerical analysis and large eddy simulation (LES) to simulate the flow field, spray dispersion, and combustion process. The results show that increasing the swirling intensity and flow rate of the outer swirler can significantly reduce the NOx emissions in the LDI combustor.
Article
Mechanics
Pradeep Moise, Joseph Mathew
Summary: This computational study examines different forms of vortex breakdown in unconfined swirling jets, including bubble and conical forms, and investigates their flow features, development of spiral coherent structures, and bistable behavior. The study highlights the distinct features of turbulent BVB and CVB, which can potentially be used towards improving designs of swirl-stabilized combustors.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Moritz Sieber, C. Oliver Paschereit, Kilian Oberleithner
Summary: A method is proposed to estimate the properties of global hydrodynamic instability in turbulent flows by analyzing measurement data of limit-cycle oscillations. The flow dynamics are separated into deterministic and stochastic contributions, with models developed to account for the interaction between the two. The methodology is applied to a turbulent swirling jet to identify the supercritical Hopf bifurcation and demonstrate the identification of flow state from stationary measurements.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Toshan Lal Sahu, Ujjwal Chetan, Jagannath Mahato, Prabir Kumar Kar, Prasanta Kumar Das, Rajaram Lakkaraju
Summary: In this study, the breakup morphology of a swirling liquid jet is analyzed through numerical simulations. It is found that viscous forces dominate the flow and suppress the Kelvin-Helmholtz instability at the interface, while centrifugal instability destabilizes the helical rim and leads to the formation of droplets.
Article
Thermodynamics
Benjamin W. Keeton, Keiko K. Nomura, Antonio L. Sanchez, Forman A. Williams
Summary: Unsteady axisymmetric numerical simulations were conducted to investigate the transition to bubble and conical vortex breakdown in low-Mach-number laminar swirling Burke-Schumann flames. The critical swirl number for the onset of each transition was determined by varying the fuel jet mass fraction and examining the resulting flow and flame shapes. The study provides insights into the mechanisms governing flame behavior and confirms the validity of theoretical predictions.
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2023)
Article
Mechanics
Junhua Zhang, Xin Hui, Qiang An, Xiao Han, Chuhan Wang
Summary: This paper investigates the instability dynamics of helical vortices in a twin annular swirling flow configuration. It is found that the azimuthal mode with wavenumber m = 1 is absolutely unstable in the central annular inner jet region at low swirl ratios, while the m = 2 mode is absolutely unstable in the surrounding annular outer jet region. With increased swirl ratio, a single-helix vortex is formed near the nozzle exit, which transitions to convective instability downstream and is eventually replaced by a double-helix vortex in the far-field.
Article
Mechanics
Yang Zhang, Maarten Vanierschot
Summary: This paper investigates coherent structures in an annular swirling jet flow undergoing vortex breakdown using numerical simulation and experimental validation, confirming the existence and characteristics of these structures.
Article
Engineering, Mechanical
Arun Pattanshetti, R. Santhosh, Nasir Attar
Summary: Three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations were conducted in a coaxial swirling jet to investigate the flow characteristics under different flow states defined by the modified Rossby number. The study found that the renormalization group oRNGTHORN k - epsilon model is suitable for simulating flow with Rom > 1, while the Reynolds stress model (RSM) is best for simulating flow with Rom <= 1. The results provide insights into fluid mixing phenomena and dominant axial oscillations prevalent in the recirculation zones.
JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME
(2022)
Article
Thermodynamics
Benjamin W. Keeton, Keiko K. Nomura, Antonio L. Sanchez, Forman A. Williams
Summary: Axisymmetric numerical simulations are conducted to investigate the influence of swirl on the stabilization of non-premixed jet flames. The results show that the coupling between swirl and flame propagation velocity as well as air-jet velocity affects the liftoff and blowoff of the flame, and the rotational speed of the swirl also affects the liftoff height and stability of the flame.
COMBUSTION AND FLAME
(2023)
Article
Energy & Fuels
Kai Zhang, Giandomenico Lupo, Christophe Duwig
Summary: Humidified gas turbine (HGT) is a promising technology with advantages of higher electrical efficiency, lower investment costs, and lower emissions compared to traditional thermal power plants. Research shows that variability in wet bio-syngas fuel can significantly impact flame physicochemical behavior, potentially leading to stability issues and reduced cycle performance.
Article
Construction & Building Technology
Parastoo Sadeghian, Christophe Duwig, Olof Skoldenberg, Ann Tammelin, Ardalan Rahimi Hosseini, Sasan Sadrizadeh
Summary: Patient warming is an effective method to prevent hypothermia during surgeries, but concerns about air pollution from warming blankets should be considered. Research shows that using forced-air warming blankets can significantly increase air temperature at the wound area, providing more effective warming during surgery.
ADVANCES IN BUILDING ENERGY RESEARCH
(2022)
Article
Engineering, Environmental
Marc Rovira, Klas Engvall, Christophe Duwig
Summary: This study examines the capabilities of a data-driven workflow for automated key feature identification in reactive flows. The proposed workflow aims to accelerate the analysis of chemical engineering datasets by generating automatic and explainable classification results for regions with distinct physics. The three main steps of the workflow, namely dimensionality reduction, unsupervised clustering, and feature correlation, are discussed. The study demonstrates the theoretical and practical differences between the previous and current algorithms used in the workflow. The updated workflow is shown to have faster, more accurate, and more robust key feature identification capabilities, closer to human intuition than previous methods. The study also serves as a tutorial for researchers interested in applying these algorithms.
CHEMICAL ENGINEERING JOURNAL
(2022)
Article
Energy & Fuels
Yazhou Shen, Kai Zhang, Christophe Duwig
Summary: This study investigates the combustion characteristics of wet ammonia/hydrogen blends and finds that NO emissions can be reduced under low hydrogen content and rich combustion conditions. In addition, the addition of an appropriate amount of steam enhances flame stability and combustion efficiency.
Article
Biophysics
Ephraim Gutmark, Vijay Anand, Aaron Wheeler, Alexander Zahn, Yuval Cavari, Tal Eluk, Maor Hay, David Katoshevski, Iris Gutmark-Little
Summary: Intrapulmonary Percussive Ventilation (IPV) is a high-frequency airway clearance technique that helps in mucus removal for mechanically ventilated and unventilated patients. However, there is a lack of clear evidence on its effectiveness. This article proposes a novel method utilizing a high-frequency acoustic field and traditional IPV to slowly push the mucus simulant towards the mouth.
JOURNAL OF BIOMECHANICS
(2022)
Article
Chemistry, Physical
Kai Zhang, YaZhou Shen, Rahul Palulli, Ali Ghobadian, Jamshid Nouri, Christophe Duwig
Summary: In line with the decarbonisation of power sector, carbon-free fuels such as green ammonia are being investigated for use in energy-intensive industries. This study examines the combustion characteristics of steam-diluted, decomposed eNH3 in a novel multi-nozzle direct injection burner. Results show that the highest H2% containing, wettest ammonia flame emits the lowest total emissions, and wall heat loss is responsible for N2O formation in distributed flame.
INTERNATIONAL JOURNAL OF HYDROGEN ENERGY
(2023)
Article
Chemistry, Physical
Israfil Soyler, Kai Zhang, Christophe Duwig, Xi Jiang, Nader Karimi
Summary: In this study, a surrogate model is built to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions using the polynomial chaos expansion (PCE) method. The research shows that variation in hydrogen in the fuel blend has the strongest effect on the uncertainty of all investigated physicochemical properties of the flame, and preheating of the reactants can significantly reduce the variability of laminar flame speed. The consequences of these uncertainties on different combustion technologies are discussed, and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.
INTERNATIONAL JOURNAL OF HYDROGEN ENERGY
(2023)
Article
Thermodynamics
Kai Zhang, Alpo Laitinen, Yazhou Shen, Ville Vuorinen, Christophe Duwig
Summary: Low-grade heat recovery is crucial for improving the high energy efficiency of power electronics. This paper investigates the performance of reactive fluid in a practical heat exchanger using a high-fidelity finite rate chemistry method, which is a key step in implementing the Ericsson cycle for low-temperature heat-to-electricity conversion. The use of reactive fluid can significantly boost the electrical efficiency of the Ericsson cycle by at least 260%, while reducing the volume of the primary heat exchanger component and improving thermal performance. The effectiveness of the Ericsson cycle is limited when dealing with high-temperature hot sources, but reactive heat transfer shows promise in enhancing thermal exchange rates.
APPLIED THERMAL ENGINEERING
(2023)
Article
Thermodynamics
Yazhou Shen, Kai Zhang, Yan Zhang, Christophe Duwig
Summary: This paper presents a numerical study on the efficient use of green methanol in a wet Brayton cycle with chemical recuperation. The results show that steam addition affects the oxidation pathway and leads to distributed flames, where autoignition mechanism dominates the ignition process.
Article
Mechanics
Myeonghwan Ahn, Mihai Mihaescu, Aatresh Karnam, Ephraim Gutmark
Summary: In this study, the flow and aeroacoustics of twin square jets with an aspect ratio of 1.0 were investigated using implicit large-eddy simulations (LES) under specific conditions. The study revealed the presence of screeching twin jets in a symmetrical flapping mode, characterized by intense pressure fluctuations and standing waves between the jets. The results from spectral proper orthogonal decomposition (SPOD) and solving Ffowcs Williams and Hawkings equation for far-field noise were in good agreement with experimental data. The study also found different levels of screech tone produced by the twin jets depending on the observation planes.
Article
Thermodynamics
Yazhou Shen, Shareq Mohd Nazir, Kai Zhang, Christophe Duwig
Summary: This study proposes a novel chemical-recuperated, humidified gas turbine concept using e-fuel ammonia to overcome technical challenges and improve cycle performance through waste heat utilization. Thermodynamic analysis shows a highest net electrical efficiency of 56.7%, exceeding that of an ammonia-fueled Brayton cycle by 20.6%-points. The efficiency improvement is attributed to chemical recuperation of fuel and steam injection.
Article
Energy & Fuels
Rahul Palulli, Simeon Dybe, Kai Zhang, Felix Guthe, Panduranga Reddy Alemela, Christian Oliver Paschereit, Christophe Duwig
Summary: The decarbonization of industry requires phasing out fossil fuels and replacing them with green hydrogen. This study focuses on the non-premixed combustion of steam-diluted hydrogen in a swirl-stabilized combustor. The flame characteristics, including shape, stabilization, and effects of flow-field, stratification, and temperature distribution, are analyzed.
Article
Mechanics
A. Karnam, M. Saleem, E. Gutmark
Summary: This study discusses the influence of nozzle exit geometry on the interaction between convective Kelvin-Helmholtz instabilities and the shock structure of supersonic jets. Circular and rectangular nozzles with an aspect ratio of 2 were evaluated using near field acoustics and Schlieren images. The study observed different screech instabilities in both nozzle configurations and found comparable trends in screech behavior. The results also revealed that the oscillation mode of the jets influenced the standing waves in the jet near field.
Article
Mechanics
M. Saleem, A. Karnam, O. Rodriguez, J. Liu, E. Gutmark
Summary: The role of micro-vortex generators (MVGs) in reducing supersonic jet noise is investigated in this study. Experimental and numerical simulation results show that MVGs can interact with the shock cell structure of the jet plume and modify its core velocities, leading to a significant reduction in jet noise. These findings have important implications for the design and development of noise reduction technologies for supersonic jet engines.
