| 1. |
- Andrae, Johan, 1973-, et al.
(författare)
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A Numerical Study of Sidewall Quenching with Propane/Air Flames
- 2002
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Ingår i: Proceedings of the Combustion Institute. - 0082-0784 .- 1878-027X. ; 29, s. 789-795
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Tidskriftsartikel (refereegranskat)abstract
- The head-on (i.e., stagnation) configuration has generally been used to numerically and experimentally characterize the flame-wall interaction with complex chemistry and multicomponent transport. Other studies have treated the transient case of a flame propagating toward a wall, and combustion in a boundary layer has also been dealt with. In this paper, a two-dimensional stationary model has been used to study the sidewall quenching,of laminar propane/air flames in a boundary-layer flow. This geometry may be described as a flame parallel to the wall that is swept away with a laminar boundary-layer flow while propagating toward and interacting with the wall. The main purpose has been to examine the extent to which the flame can propagate toward the cooled wall for lean flames compared to stoichiometric flames. A detailed kinetic model is used to examine the oxidation of both the fuel and the intermediate hydrocarbons (IHCs). For stoichiometric and near stoichiometric mixtures, the thermal coupling between the flame and the wall is small but significant. However, for very lean flames, the thermal coupling between the flame and the wall is found to be very significant. The intermediate hydrocarbons are the dominant emissions for stoichiometric and near-stoichiometric flames in contrast to the leaner flames in which the fuel becomes more significant. This implies that the IHCs are very important for the overall hydrocarbon emissions from flame quenching; as a result detailed kinetics of complex fuels should be used when determining the unburned hydrocarbon emissions.
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| 2. |
- Andrae, Johan, et al.
(författare)
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Kinetic and Transport Effects of Pressurized Methane Flames in a Boundary Layer
- 2003
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Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 133:4, s. 503-506
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Tidskriftsartikel (refereegranskat)abstract
- We have recently modeled combustion of lean methane-air [1] mixtures in a boundary layer flow using the program CRESLAF [2, 3 and 4]. A uniform fuel-air mixture above the auto-ignition temperature was introduced at the inlet edge of the wall, which gave an arrested flame zone, propagating in the upstream direction with a local flame speed that is equal and opposite to the local flow velocity. We compared the interaction of this flame with three model wall materials representing three idealized cases from a chemical point of view, a completely inert wall, a radical recombining wall, and a wall supporting catalytic combustion. Here we report on an analogous study of a flame geometry that may be considered a combination of a one-dimensional flame propagating towards a wall and the combustion of a uniform fuel-air mixture in a boundary layer flow. In contrast to our previous work [1] where we had a uniform inlet flow composition consisting of unburnt gas, here there is only unburnt gas close to the walls while there is burnt gas in the center of the channel. The present study concerns lean pressurized methane flames propagating toward hot isothermal walls where chemistry on the wall is considered important. The main purpose is to compare the results with those obtained in Ref. [1], which enables us, for the same flow field (boundary layer flow), to compare the effect of flame geometry on the wall effects. We have found and have been able to explain theoretically that such subtle changes of the flame geometry, which would be rather difficult to study experimentally, may have surprisingly significant effects on the combustion process.
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| 3. |
- Andrae, Johan, 1973-, et al.
(författare)
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Numerical studies of wall effects with laminar methane flames
- 2002
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Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 128:1-2, s. 165-180
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Tidskriftsartikel (refereegranskat)abstract
- Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (phi = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (phi = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.
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| 4. |
- Edsberg, Lennart
(författare)
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Design and characterisation of a close-concentric annular reactor for kinetic studies at high temperatures
- 2002
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 57:5, s. 749-762
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Tidskriftsartikel (refereegranskat)abstract
- A novel annular reactor for kinetic studies at high temperature and flow conditions has been designed to keep eccentricity tolerances below 10%. In a previous work, we have shown that it is very important to keep such low eccentricity values in order to collect reliable kinetic data from this type of reactors. As proposed in this study, a modified reactor with the use of a spacer could guarantee an annular duct with low levels of eccentricity. Manufacturing tolerances or deformation effects giving rise to eccentricity can be significantly minimised when using this apparatus. The reactor has been both experimentally and theoretically characterised. Carbon monoxide oxidation was used as a model reaction under mass-transfer limited conditions revealing an eccentricity of similar to5%. With such small eccentricity levels, a concentric annular form can be assumed in the reactor analysis. Simple 1D or 2D models can therefore be inexpensively used in the evaluation of the kinetic data. Also, prior to the design of the annular reactor, a numerical investigation was carried out to clarify the effects of eccentricity, physical properties of the carrier gas and the annular aspect ratio on mass-transfer limitations. Contrary to expectations, a considerable increase in the fuel mass-diffusivity by carrier gas substitution did not change the mass-transfer rates for cases when eccentricity and aspect ratios were high.
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| 5. |
- Edsberg, Lennart
(författare)
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Effect of eccentricity and interaction between kinetics and mass transfer on the behaviour of catalytic annular reactors : a comparison between lumped and distributed models
- 2001
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 56:16, s. 4863-4878
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Tidskriftsartikel (refereegranskat)abstract
- A numerical investigation has been carried out on an isothermal annular reactor for kinetic studies under laminar flow conditions. The scope was to investigate the effects of eccentricity in the annular reactor and the adequacy of using lumped models when modelling such cases. ID-models were compared with the corresponding 3D-models for different values of eccentricities and for different kinetic rate laws. In the case of linear kinetics, the solutions of the lumped models agreed satisfactorily with the solutions of the distributed models for all eccentricity values investigated. However, for non-linear kinetics, especially for LHHW rate expressions, significant differences in conversion between the models were observed during a mixed chemical diffusion-control regime. The differences became progressively larger as the eccentricity was increased. An enhanced ID-model, with compensation for those non-linear effects, was designed and gave satisfactory agreement with the 3D-models. For conditions when the ID-model could predict multiple steady states, a higher eccentricity produced a wider temperature range over which multiplicity occurred. This was so even in the cases where the multidimensional models did not predict any multiple steady states. It is not in general possible to rely on evaluating kinetic data from an eccentric annular reactor by using a ID-model. In order to be sure to obtain accurate data from this type of reactor, an eccentricity of less than 10% is allowed.
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| 6. |
- Edsberg, Lennart
(författare)
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Natriuresis and the extracellular volume expansion by hypertonic saline
- 2003
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Ingår i: Journal of Surgical Research. - : Elsevier BV. - 0022-4804 .- 1095-8673. ; 113:1, s. 6-12
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Tidskriftsartikel (refereegranskat)abstract
- Background. The mechanisms governing the duration of the extracellular fluid volume (ECF) expansion as a result of intravenous infusion of hypertonic saline solution are poorly understood. We hypothesized that the duration is closely related to the sodium excretion. Materials and methods. Six conscious splenectomized ewes with a mean body weight of 30 kg were given an intravenous infusion of 4 ml kg(-1) of 7.5% saline solution on two occasions, one over a period of 5 min and another over a period of 20 min. Mass balance and volume kinetic calculations of the distribution and elimination of fluid were performed after repeated sampling of the plasma sodium concentration and the urinary excretion of water and sodium during 3 h. Results. On considering the addition of sodium to and its excretion from the body, the plasma sodium concentration indicated a 10% dilution of the extracellular space. The volume expansion decayed at an average rate of 20% of the volume expansion per hour, which, however, varied greatly in the animals, depending on their capacity to excrete sodium. After 1 h, increasing natriuresis promoted translocation. of water into the cells, which amounted to 25-35% of the total elimination. Computer simulations indicated that tripled natriuresis (up to approximately 750 mmol l(-1)) would increase the rate of elimination to 45% of the volume expansion per hour. Conclusion. The sodium excretion was inversely proportional to the duration of the extracellular volume expansion by 7.5% saline.
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| 7. |
- Edsberg, Lennart
(författare)
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Simplified method for effectiveness factor calculations in irregular geometries of washcoats
- 2000
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 55:8, s. 1447-1459
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Tidskriftsartikel (refereegranskat)abstract
- A simplified method to calculate the effectiveness factors in irregular geometries of washcoats is presented. The method is based on sectioning the washcoat into particles and treating each particle in a 1D approach. This simplified method was compared with a rigorous 2D model for different kinds of geometrical shapes and kinetic expressions of the washcoat. For a first- and 0.75th-order reaction, the simplified method showed a good agreement with the 2D model. For a Langmuir-Hinshelwood rate expression, giving effectiveness factors higher than one, the simplified method could give rise to errors up to 14% due to inhibition effects. However, this was only significant at intermediate Values of temperatures and high surface concentrations on the washcoat. In general, the simplified method gives a good a priori estimation of the effectiveness factor that can replace tedious and time-consuming finite element calculations in irregular shapes of washcoats. A comparison of the CPU-time required to solve a 2D-reactor model has been conducted for the different approaches to evaluate the effectiveness factor.
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| 8. |
- Edsberg, Lennart
(författare)
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Simplified method of effectiveness factor calculations for irregular geometries of washcoats - A general case in a 3D concentration field
- 2000
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Konferensbidrag (refereegranskat)abstract
- A simplified 3D-model for isothermal conditions has been developed to reduce the calculations of the combined rate of pore diffusion and chemical reaction in washcoats of irregular geometries. The method is based on sectioning the washcoat into particles thus treating the solid-phase according to a corresponding boundary value problem. The simplified 3D-model was compared with a rigorous model for five different geometrical configurations of a square monolith with uneven washcoat coating. For the first and power law (0.7th order) kinetics the solutions of the reduced model were in very good agreement in comparison with the more expensive 3D-model. The computational burden could be even further reduced by using a lumped model in spite of peripherally varying concentration profiles. The agreement using the lumped model was satisfactory whenever proper correlations for the mass-transfer coefficient and the effectiveness factor were used.
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| 9. |
- Edsberg, Lennart, et al.
(författare)
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Volume kinetic analysis of fluid shifts accompanying intravenous infusions of glucose solution
- 2003
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Ingår i: Cell Biochemistry and Biophysics. - 1085-9195 .- 1559-0283. ; 39:3, s. 211-222
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Forskningsöversikt (refereegranskat)abstract
- Volume kinetics is a mathematical tool for macroscopic (whole-body) evaluation of the distribution and elimination of fluid given by intravenous infusion. Although the kinetic system has mostly been applied to crystalloid fluids, such as Ringer's solution, it has more recently been extended to glucose solution, which is characterized by interdependence between glucose and fluid kinetics. The elimination of glucose, as estimated by a one-compartment open model, serves as the driving force for cellular uptake of glucose and, by virtue of osmosis, of water. Key findings include the observation that the infused fluid, besides being accumulated in the cells, occupies a central body fluid space (VI), which is no larger than 3-4 L, and that the cellular hydration has a much longer time-course than the hydration of V-1. This explains the risk of hypovolemia associated with rapid infusion of 5% glucose; the dilution of V-1, which is quite substantial owing to the small size of this space at baseline, stimulates a brisk diuresis while the excess water is being "trapped" in the cells along with the glucose. Model linearity has been demonstrated for 2.5% glucose solution and this allows the construction of nomograms for administration of such fluid during surgery and critical illness.
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| 10. |
- Edsberg, Lennart, et al.
(författare)
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Volume kinetics of glucose solutions given by intravenous infusion
- 2001
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Ingår i: British Journal of Anaesthesia. - : Elsevier BV. - 0007-0912 .- 1471-6771. ; 87:6, s. 834-843
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Tidskriftsartikel (refereegranskat)abstract
- Glucose solutions given by intravenous (i.v.) infusion exert volume effects that are governed by the amount of fluid administered and also by the metabolism of the glucose. To understand better how the body handles glucose solutions, two volume kinetic models were developed in which consideration was given to the osmotic fluid shifts that accompany the metabolism of glucose. These models were fitted to data obtained when 21 volunteers who were given approximately 1 litre of glucose 2.5 or 5% or Ringer's solution (control) over 45 min. The maximum haemodilution was similar for all three fluids, but it decreased more rapidly when glucose had been infused. The volume of distribution for the infused glucose molecules was larger (similar to 12 litres) than for the infused fluid, which amounted to (mean (SEM)) 3.7 (0.3) (glucose 2.5%). 2.8 (0.2) (glucose 5%), and 2.5 (0.2) litres (Ringer). Fluid accumulated in a remote (cellular) body fluid space when glucose had been administered (similar to0.2 and 0.4 litres, respectively), while expansion of an intermediate fluid space (7.1 (13) litres) could be demonstrated in 33% of the Ringer experiments. In conclusion, kinetic models were developed which consider the relationship between the glucose metabolism and the disposition of intravenous fluid. One of them, in which infused fluid expands two instead of three body fluid spaces, was successfully fitted to data on blood glucose and blood haemoglobin obtained during infusions of 2.5 and 5% glucose.
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