| 1. |
- Yu, Rixin, et al.
(författare)
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Effect of Turbulence on HCCI Combustion
- 2007
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Ingår i: Session: Homogeneous Charge Compression Ignition (HCCI) (Part 4 of 8) Combustion Modeling / Optical Diagnostics. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- This paper presents large eddy simulation (LES) and experimental studies of the combustion process of ethanol/air mixture in an experimental optical HCCI engine. The fuel is injected to the intake port manifolds to generate uniform fuel/air mixture in the cylinder. Two different piston shapes, one with a flat disc and one with a square bowl, were employed to generate different in-cylinder turbulence and temperature field prior to auto-ignition. The aim of this study was to scrutinize the effect of in-cylinder turbulence on the temperature field and on the combustion process. The fuel tracer, acetone, is measured using laser induced fluorescence (LIF) to characterize the reaction fronts, and chemiluminescence images were recorded using a high speed camera, with a 0.25 crank angle degree resolution, to further illustrate the combustion process. Pressure in the cylinder is recorded in the experiments. Spatial and temporal resolved LES was used to gain information on the turbulence mixing, heat transfer and combustion process. It was shown that gas temperature in the piston bowl is generally higher than that in the squish, leading to an earlier ignition in the bowl. Compared to the disc engine, the square bowl engine has a higher temperature inhomogeneity owing to the turbulence wall heat transfer. The experimentally observed higher combustion duration and slower pressure rise rate in the square bowl engine as compared to the disc engine can be explained by the higher temperature inhomogeneity in the square bowl engine.
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| 2. |
- Orbay, Raik, 1974, et al.
(författare)
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Swirling turbulent flows in a combustion chamber with and without heat release
- 2013
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361 .- 1873-7153. ; 104, s. 133-146
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents experiments and numerical simulations of swirling turbulent flows with and without combustion in a laboratory gas turbine combustor. Three approaches, particle image velocimetry (PIV), laser Doppler velocimetry (LDV) and large eddy simulation (LES) are employed to minimize the uncertainties of the results. The aim is to characterize the main flow structures and turbulence in a combustor that is relevant to gas turbines. Isothermal flows with different outlet geometry and lean premixed preheated natural gas/air flames with equivalence ratio of 0.47 are considered to demonstrate the effect of heat release and combustor geometry on the flows. At the combustor inlet the swirl numbers are about 1.4 and the Reynolds numbers are about 20,000 for all cases. The PIV, LDV and LES show consistent agreement on the mean flow field, whereas for the velocity variances the results from LDV and LES agree each other but the PIV data show considerably lower turbulence level. The mean flow field is characterized by three large-scale recirculation zones resulted from sudden expansion of the combustor geometry and vortex breakdown due to inflow swirl. Combustor outlet geometry exhibits great impact on the vortex breakdown structure. Heat release enhances the production of turbulence near the axis of the combustor, but it does not alter the fundamental vortex breakdown structure. A mechanistic explanation of the underlying flow physics is presented to describe the effect of swirl, heat release and outlet contraction on the flow field.
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| 3. |
- Joelsson, Tobias, et al.
(författare)
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Large Eddy Simulation and Experiments of the Auto-Ignition Process of Lean Ethanol / Air Mixture in HCCI Engines
- 2008
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Ingår i: SAE International Journal of Fuels and Lubricants. - : SAE International. - 1946-3952 .- 1946-3960. ; 1:1, s. 1110-1119
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Tidskriftsartikel (refereegranskat)abstract
- Recent experiments and numerical studies have showed that piston geometry has a significant effect on the homogeneous charge compression ignition (HCCI) process. There are two effects generated by the combustor geometry: the geometry affects the flow/turbulence in the cylinder; the geometry also affects the temperature stratification. The temperature stratification is more directly responsible for the observed alteration of the auto-ignition process. To clarify this issue further we present in this paper a study of two engines with the same geometry but difference ways of cooling. Measurement of the two engines~a metal engine and quartz piston engine, both with the same piston bowl geometry~is carried out. Large eddy simulation (LES) is used to simulate the flow, the temperature field and the auto-ignition process in the two engines. The fuel is ethanol with a relative air/fuel ratio of 3.3. It is found that lower temperature stratification is established in the metal engine under similar conditions as the optical quartz engine due to the more effective cooling of the piston in the metal engine configuration. The combustion phasing in the two engines is the same by controlling the intake temperature. Both measurements and LES show a more rapid auto-ignition in the metal engine than in the optical engine with the same piston geometry. This confirms the conclusion that large temperature stratification can decrease the pressure-rise-rate and thereby increase the load of HCCI engines. The dependence of temperature stratification on the wall temperature and intake temperature is systematically studied using LES.
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| 4. |
- Yu, Rixin, et al.
(författare)
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Effect of Temperature Stratification on the Auto-Ignition of Lean Ethanol/Air Mixture in HCCI Engine
- 2008
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; :2008-01-1669
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Tidskriftsartikel (refereegranskat)abstract
- It has been known from multi-zone simulations that HCCI combustion can be significantly affected by temperature stratification of the in-cylinder gas. With the same combustion timing (i.e., crank angles at 50% heat release, denoted as CA50), large temperature stratification tends to prolong the combustion duration and lower down the in-cylinder pressure-rise-rate. With low pressure-rise-rate HCCI engines can be operated at high load, therefore it is of practical importance to look into more details about how temperature stratification affects the auto-ignition process. It has been realized that multi-zone simulations can not account for the effects of spatial structures of the stratified temperature field, i.e., how the size of the hot and cold spots in the temperature field could affect the auto-ignition process. This question is investigated in the present work by large eddy simulation (LES) method which is capable of resolving the in-cylinder turbulence field in space and time. The initial temperature field for LES is presumed as the superimposition of a mean temperature and a sine-function fluctuating temperature. The engine runs on ethanol with a relative air/fuel ratio of 3.3. The LES results show that the initial shape of hot/cold spots is quickly modified by turbulence. A particular hot/cold spot size on the order of large eddy integral scale is found at which the combustion duration tends to be shorter. This reveals the fact that not only the magnitude of the temperature stratification but also the spatial structure of the stratification could affect the auto-ignition process.
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| 5. |
- Yu, Rixin, et al.
(författare)
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Effect of Turbulence and Initial Temperature Inhomogeneity on Homogeneous Charge Compression Ignition Combustion
- 2006
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; :2006-01-3318
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Tidskriftsartikel (refereegranskat)abstract
- A 0.5-liter optical HCCI engine firing a mixture of n-heptane (50%) and iso-octane (50%) with air/fuel ratio of 3 is studied using large eddy simulation (LES) and laser diagnostics. Formaldehyde and OH LIF and in-cylinder pressure were measured in the experiments to characterize the ignition process. The LES made use of a detailed chemical kinetic mechanism that consists of 233 species and 2019 reactions. The auto-ignition simulation is coupled with LES by the use of a renormalized reaction progress variable. Systematic LES study on the effect of initial temperature inhomogeneity and turbulence intensity has been carried out to delineate their effect on the ignition process. It was shown that the charge under the present experimental condition would not be ignited without initial temperature inhomogeneity. Increasing temperature inhomogeneity leads to earlier ignition whereas increasing turbulence intensity would retard the ignition. This is mostly due to the effect of turbulence on the bulk flow that turbulence tends to decrease the temperature inhomogeneity by enhanced eddy heat transfer. The LES results suggest that desirable ignition timing could be achieved by controlling the turbulence intensity and temperature inhomogeneity.
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| 6. |
- Yu, Rixin, et al.
(författare)
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Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine
- 2006
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; :2006-01-3436
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 x 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 1283 cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid. In the intake phase the large-scale swirling and tumbling flow streams are shown to be responsible for the generation of large-scale vortex pipes which break down to small-scale turbulent eddies. In the later phase of compression turbulence is mainly produced in the engine bowl. The bore wall and the piston bowl wall heat the fluid near the walls. Turbulence and the large-scale coherent vortex shedding due to the Kelvin-Helmholtz instability are responsible for the enhanced heat transfer between the bulk flow and the walls. A temperature inhomogeneity of about 50 - 60 K can be generated in the cylinder.
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| 7. |
- da Rocha, Rodolfo Cavaliere, et al.
(författare)
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Chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission
- 2019
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 246, s. 24-33
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Tidskriftsartikel (refereegranskat)abstract
- This work reports on a study of chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission. A survey of chemical mechanisms available in the literature was first conducted, and the performance of 10 mechanisms was analysed in terms of the prediction of shock tube ignition delay times, laminar flame speeds and NO x concentrations for NH 3 /air and NH 3 /H 2 /air flames for a wide range of operating conditions. Then, three of these mechanisms were reduced and their performance compared against the behaviour of the original mechanisms. The results confirm that pure NH 3 flames have high ignition delay times and rather low flame speeds, and that the addition of H 2 to NH 3 flames increases exponentially the flame speed, and significantly the NO x emission. The currently available chemical kinetic mechanisms predict rather scattered ignition delay times, laminar flame speeds, and NO x concentrations in NH 3 flames, indicating that improvements in the sub-mechanisms of NH 3 and NH 3 /H 2 oxidation are still needed. Sensitivity analysis for NO formation indicates that NO formation in NH 3 flames is mainly produced through the NH 3 /O 2 chemical process, and sensitivity analysis for flame speed reveals that the differences among mechanisms are due to the relative importance of the reactions of the NNH and HNO sub-mechanisms. The reduced mechanisms show high fidelity when compared with the original ones, despite some discrepancies at high pressures.
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| 8. |
- Hesameddin, Fatehi, et al.
(författare)
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Effect of Volatile Reactions on the Thermochemical Conversion of Biomass Particles
- 2017
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Ingår i: 8th International Conference on Applied Energy (ICAE2016). - : Elsevier. ; 105, s. 4648-4654
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Konferensbidrag (refereegranskat)abstract
- A numerical and experimental study on the conversion of a biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the biomass pyrolysis. The numerical domain consists of a particle and its surrounding and the model considers detailed chemical kinetic mechanism for reaction of pyrolysis products. A detailed pyrolysis model is employed which provides the composition of pyrolysis products. The effect of gas phase reaction on the conversion time and temperature of the particle is analyzed and it was shown that the gas phase reactions results in shorter pyrolysis time. H2O mole fraction and temperature above a biomass pellet from wheat straw (WS) and stem wood (SW) were experimentally measured using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The TDLAS data were used to validate the numerical model developed for biomass conversion. The results showed that by considering the gas phase reactions a good agreement between the measurement and the model prediction for mass loss and temperature can be achieved. For H2O mole fraction on top of the particle, on the other hand, some discrepancy between the model prediction and the experimental data was observed. Nevertheless, the difference in H2O mole fraction would be much larger by neglecting the gas phase reaction at the particle boundary.
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| 9. |
- Ibron, Christian, et al.
(författare)
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Large Eddy Simulation of an Ignition Front in a Heavy Duty Partially Premixed Combustion Engine
- 2019
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Ingår i: 14th International Conference on Engines & Vehicles: Technical paper. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
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Konferensbidrag (refereegranskat)abstract
- In partially premixed combustion engines high octane number fuels are injected into the cylinder during the late part of the compression cycle, giving the fuel and oxidizer enough time to mix into a desirable stratified mixture. If ignited by auto-ignition such a gas composition can react in a combustion mode dominated by ignition wave propagation. 3D-CFD modeling of such a combustion mode is challenging as the rate of fuel consumption can be dependent on both mixing history and turbulence acting on the reaction wave. This paper presents a large eddy simulation (LES) study of the effects of stratification in scalar concentration (enthalpy and reactant mass fraction) due to large scale turbulence on the propagation of reaction waves in PPC combustion engines. The studied case is a closed cycle simulation of a single cylinder of a Scania D13 engine running PRF81 (81% iso-octane and 19% n-heptane). Two injection timings are investigated; start of injection at -17 CAD aTDC and -30 CAD aTDC. One-equation transported turbulence sub-grid closure is used for the unresolved momentum and scalar fluxes and the fuel spray is modelled using a Lagrangian particle tracking (LPT) approach. Initial flow conditions (prior to intake valve closing) are generated using a scale forcing method with a prescribed large-scale swirl mean flow motion. Fuel reactivity is modeled using finite rate chemistry based on a skeletal chemical kinetic mechanism (44 species, 140 reactions). The results are compared with optical engine experimental data and satisfactory agreement with the experiments is obtained in terms of the liquid spray length, cylinder pressure trace and ignition location. A majority of the fuel consumption is found to be in ignition fronts where small variations in temperature at low fuel concentrations are observed to cause large stratification in ignition delay time.
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| 10. |
- Klason, Torbern, et al.
(författare)
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Investigation of radiative heat transfer in fixed bed biomass furnaces
- 2008
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Ingår i: Fuel. - : Elsevier BV. - 1873-7153 .- 0016-2361. ; 87:10-11, s. 2141-2153
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an investigation of the radiative heat transfer process in two fixed bed furnaces firing biomass fuels and the performance of several widely used models for calculation of radiative heat transfer in the free-room of fixed bed furnaces. The simple optically thin (OT) model, the spherical harmonic P-1-approximation model, the grey gas model based on finite volume discretization (FGG), and the more accurate but time consuming spectral line weighted-sum-of-grey-gases (SLW) model are investigated. The effective mean grey gas absorption coefficients are calculated using an optimised version of the exponential wide band model (EWBM) based on an optical mean beam length. Fly-ash and char particles are taken into account using Mie scattering. In the investigated updraft small-scale fixed bed furnace radiative transfer carries heat from the bed to the free-room, whereas in the cross-current bed large-scale industry furnace, radiative transfer brings heat from the hot zones in the free-room to the drying zone of the bed. Not all the investigated models can predict these heat transfer trends, and the sensitivity of results to model parameters is fairly different in the two furnaces. In the small-scale furnace, the gas absorption coefficient predicted by using different optical lengths has great impact on the predicted temperature field. In the large-scale furnaces, the predicted temperature field is less sensitive to the optical length. In both furnaces, with the same radiative properties, the low-computational-cost P-1 model predicts a temperature field in the free-room similar to that by the more time consuming SLW model. In general, the radiative heat transfer rates to the fuel bed are not very sensitive to the radiative properties, but they are sensitive to the different radiative heat transfer models. For a realistic prediction of the radiative heat transfer rate to the fuel bed or to the walls, more computationally demanding models such as the FGG or SLW models should be used.
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