| 1. |
- Ong, Jiun Cai, et al.
(författare)
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Effects of ambient pressure and nozzle diameter on ignition characteristics in diesel spray combustion
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 290
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Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations are performed to investigate the effects of ambient density (ρam) and nozzle diameter (Dnoz) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρam of 14.8, 30.0, and 58.5 kg/m3 and different Dnoz of 100, 180, and 363 μm are considered. The results show that the predicted ignition delay times are in good agreement with measurements. The current results show that the mixture at the spray central region becomes more fuel-rich as Dnoz increases. This leads to a shift in the high-temperature ignition location from the spray tip towards the spray periphery as Dnoz increases at ρam of 14.8 kg/m3. At higher ρam of 30.0 and 58.5 kg/m3, the ignition locations for all Dnoz cases occur at the spray periphery due to shorter ignition timing and the overly fuel-rich spray central region. The numerical results show that the first ignition location during the high-temperature ignition occurs at the fuel-rich region at ρam⩽30.0 kg/m3 across different Dnoz. At ρam=58.5 kg/m3, the ignition occurs at the fuel-lean region for the 100 and 180 μm cases, but at the fuel-rich region for the 363 μm nozzle case. This distinctive difference in the result at 58.5 kg/m3 is likely due to the relatively longer ignition delay time in the 363 μm nozzle case. Furthermore, the longer ignition delay time as Dnoz increases can be related to the higher local scalar dissipation rate in the large nozzle case.
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| 2. |
- Ong, Jiun Cai, et al.
(författare)
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Large eddy simulation of n-dodecane spray flame : Effects of injection pressure on spray combustion characteristics at low ambient temperature
- 2023
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 39:2, s. 2631-2642
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Tidskriftsartikel (refereegranskat)abstract
- The present study uses large-eddy simulations (LES) to identify the underlying mechanism that governs the ignition and stabilization mechanisms of ECN Spray A flame for different injection pressures (Pinj) and ambient temperatures (Tam). Two Pinj of 50 MPa and 150 MPa, as well as two Tam of 750 K and 900 K are considered. The numerical model is validated against the experimental fuel penetration, radial mixture frac- tion distribution, ignition delay time, and lift-off length. The combustion characteristics of all four spray flames are well predicted, with a maximum relative difference of 15% to the measurements. At 900 K, hightemperature ignition (HTI) occurs in the fuel-rich mixture at the spray head of the high Pinj spray flame, but at the spray periphery of the low Pinj spray flame. This is due to the low Pinj case having fuel-richer mixture in the inner spray region. Nonetheless, the spray flames at both Pinj exhibit double-flame structure. At 750 K, HTI occurs at the fuel-rich and fuel-lean regions for spray flames with Pinj = 50 MPa and 150 MPa, respectively. Reducing the Pinj leads to a lower injection velocity, less turbulent fluctuation, slower mixing, and hence the occurrence of HTI at the fuel-rich mixtures. The spray flame in the low Pinj case at 750 K ex-hibits a triple-flame structure at the lift-off position, while the high Pinj case exhibits a lean premixed reaction zone. This difference is attributed to the distribution of fuel-rich mixtures. Despite differences in the flame structures, auto-ignition process plays a key role to stabilize the lift-off position for all four spray flames. The auto-ignition process is also found to be dependent on the cool-flame products upstream of the lift-off position. In particular for the low Tam cases, the heat transfer effect from the main flame to the fuel-rich regions is suggested to also contribute to the flame stabilization mechanisms.
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| 3. |
- Ong, Jiun Cai, et al.
(författare)
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Large-eddy simulation of n-dodecane spray flame : Effects of nozzle diameters on autoignition at varying ambient temperatures
- 2021
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 38:2, s. 3427-3434
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, large-eddy simulations (LES) are used to identify the underlying mechanism that governs the ignition phenomena of spray flames from different nozzle diameters when the ambient temperature (T am) varies. Two nozzle sizes of 90μm and 186μm are chosen. They correspond to the nozzle sizes used by Spray A and Spray D, respectively, in the Engine Combustion Network. LES studies of both nozzles are performed at three T am of 800K, 900K, and 1000K. The numerical models are validated using the experimental liquid and vapor penetration, mixture fraction (Z) distribution, as well as ignition delay time (IDT). The ignition characteristics of both Spray A and Spray D are well predicted, with a maximum relative difference of 14% as compared to the experiments. The simulations also predict the annular ignition sites for Spray D at T am 900K, which is consistent with the experimental observation. It is found that the mixture with Z 0.2 at the spray periphery is more favorable for ignition to occur than the overly fuel-rich mixture of Z < 0.2 formed in the core of spray. This leads to the annular ignition sites at higher T am. Significantly longer IDT for Spray D is obtained at T am of 800K due to higher scalar dissipation rates (χ) during high temperature (HT) ignition. The maximum χ during HT ignition for Spray D is larger than that in Spray A by approximately a factor of 5. In contrast, at Tam=K, the χ values are similar between Spray A and Spray D. This elucidates the increase in the difference of IDT between Spray D and Spray A as T am decreases. This may explain the contradicting findings on the effects of nozzle diameters on IDT from literature.
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| 4. |
- Ong, Jiun Cai, et al.
(författare)
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Numerical Study of the Influence of Turbulence-Chemistry Interaction on URANS Simulations of Diesel Spray Flame Structures under Marine Engine-like Conditions
- 2021
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Ingår i: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 35:14, s. 11457-11467
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Tidskriftsartikel (refereegranskat)abstract
- The present work performs unsteady Reynolds-averaged Navier-Stokes simulations to study the effect of turbulence-chemistry interaction (TCI) on diesel spray flames. Three nozzle diameters (d0) of 100, 180, and 363 μm are considered in the present study. The Eulerian stochastic fields (ESF) method (with the TCI effect) and well-stirred reactor (WSR) model (without the TCI effect) are considered in the present work. The model evaluation is carried out for ambient gas densities (ρam) of 30.0 and 58.5 kg/m3. The ESF method is demonstrated to be able to reproduce the ignition delay time (IDT) and lift-off length (LOL) with an improved accuracy than that from the WSR method. Furthermore, TCI has relatively more influence on LOL than on IDT. A normalized LOL (LOL*) is introduced, which considers the effect of d0, and its subsequent effect on the fuel-richness in the rich premixed core region is analyzed. The RO2 distribution is less influenced by the TCI effect as ambient density increases. The ESF model generally predicts a longer and wider CH2O distribution. The difference in the spatial distribution of CH2O between the ESF and WSR model diminishes as d0 increases. At ρam = 30.0 kg/m3, the ESF method results in a broader region of OH with lower peak OH values than in the WSR case. However, at ρam = 58.5 kg/m3, the variation of the peak OH value is less susceptible to the increase in d0 and the presence of the TCI model. Furthermore, the influence of TCI on the total OH mass decreases as d0 increases. The total NOx mass qualitatively follows the same trend as the total OH mass. This present work clearly shows that the influence of TCI on the global spray and combustion characteristics becomes less prominent when d0 increases.
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| 5. |
- Zhang, Min, et al.
(författare)
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An investigation on early evolution of soot in n-dodecane spray combustion using large eddy simulation
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 293
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Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ>1.5) and temperature (T>1800K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process.
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| 6. |
- Zhang, Min, et al.
(författare)
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Effects of ambient CO2 and H2O on soot processes in n-dodecane spray combustion using large eddy simulation
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 312
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Tidskriftsartikel (refereegranskat)abstract
- In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO2) and water (H2O) additions on the soot formation and oxidation processes in an n-dodecane spray flame. In the soot model, acetylene (C2H2) is soot precursor and surface growth species, while hydroxyl radical (OH) and oxygen (O2) are soot oxidizers. The effect of ambient CO2 and H2O additions on soot formation/oxidation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO2 and H2O additions increase C2H2 but reduce OH formation by lowering the flame temperature. This leads to a higher soot mass formed. On the contrary to the thermal effects, the ambient CO2 and H2O additions reduce the soot formation due to their chemical effects. The reaction CH2∗+CO2↔CH2O+CO is found to be responsible for reducing C2H2 formation. The ambient H2O addition results in a higher OH but lower the C2H2 mass formed owing to the reverse reactions H2+OH↔H2O+H and OH+OH↔H2O+O. Furthermore, the chemical effects is more significant than the thermal effects under the tested conditions. This leads to a lower soot mass formed when adding ambient CO2 and H2O.
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| 7. |
- Zhang, Min, et al.
(författare)
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Large eddy simulation of combustion recession : Effects of ambient temperature and injection pressure
- 2023
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Ingår i: Fuel. - 0016-2361. ; 351
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, large eddy simulation is used to investigate combustion recession for the Engine Combustion Network Spray A flame at two ambient temperatures (850K and 800K) and two injection pressures (100MPa and 50MPa). The present numerical results are able to capture different combustion recession phenomena after the end-of-injection (AEOI). With an injection pressure of 100MPa, the model predicts a “separated” combustion recession at the ambient temperature of 850K and no combustion recession at the ambient temperature of 800K, in which both predictions correspond to the measurements. The combustion recession is mainly controlled by the auto-ignition process at the ambient temperature of 850K. At the ambient temperature of 800K, the local temperature within the fuel-rich region is not high enough to promote the high-temperature ignition process. As time progresses, the mixture within the fuel-rich region rapidly transitions to become an overly fuel-lean mixture, which further hinders high-temperature ignition to occur. Nonetheless, it is shown that lowering the injection pressure to 50MPa causes the combustion recession to occur at the ambient temperature of 800 K. This is likely attributed to the low injection case having a lower air entrainment rate AEOI, which causes the mixtures upstream of the lift-off position to transition slower from fuel-rich to fuel-lean mixtures.
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| 8. |
- Zhang, Min, et al.
(författare)
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Large eddy simulation of soot formation and oxidation for different ambient temperatures and oxygen levels
- 2022
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 306
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the numerical study of soot formation and oxidation processes across different ambient temperatures (900K, 1000K, and 1100K) and oxygen levels (15% and 21% O2) using large eddy simulation coupled with a two-equation soot model. The predicted ignition delay time, lift-off length and soot distribution show good agreements with the corresponding experimental data. A stronger oxidation of the precursor (C2H2) in the 21% O2 cases results in a lower C2H2 formed, as compared to the 15% O2 cases. The increasing ambient temperature leads to the fuel-richer region (roughly equivalence ratio > 1.6) becoming more favorable for C2H2 formation and, consequently, soot formation. This is more apparent in the 15% O2 cases due to a weaker oxidation of C2H2 via O and OH radicals. As a result, the difference in the soot mass between the 15% and 21% O2 cases becomes larger as the ambient temperature increases. The effects of ambient temperature and O2 level on soot sub-processes are investigated. In addition to the flame temperature, OH mass and soot surface area are the dominant parameters in the oxidation processes via OH and O2 at varying O2 levels, respectively.
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| 9. |
- Zhang, Min, et al.
(författare)
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Large eddy simulation of transient combustion and soot recession in the ECN Spray A and D flames
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 329
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the numerical results of combustion and soot recession in the Engine Combustion Network (ECN) Spray A and D flames using large eddy simulations (LES). The nominal injector nozzle diameters for the ECN Spray A and D are 90μm and 186μm. A two-equation soot model is implemented to model the soot formation and oxidation processes. The numerical model is validated by comparing the simulated and measured data in terms of ignition delay time (IDT), lift-off-length (LOL), and temporal evolution of soot mass. The combustion and soot recession processes are analyzed after the end-of-injection (AEOI). The combustion recession processes are first driven by auto-ignition wave propagation followed also by the convective flow in both the Spray A and D flames. A separated high-temperature flame structure is observed in the Spray A flame due to having small favorable mixture regions for the auto-ignition to occur upstream of the quasi-steady lift-off position AEOI. In contrast, a spatially-continuous high-temperature flame structure is formed in the Spray D flame due to having more ignitable mixture regions upstream of the quasi-steady lift-off position and a higher heat release. Soot recession is observed in the Spray D flame but not in the Spray A flame. This is attributed to the mixture upstream of the quasi-steady state soot region becoming favorable to promote soot formation in the Spray D flame, but it becomes fuel-lean AEOI in the Spray A flame.
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