Osamu Moriue

DETAILED NUMERICAL SIMULATIONS OF THE MULTISTAGE SELF- IGNITION PROCESS OF n-HEPTANE ISOLATED DROPLETS AND THEIR VERIFICATION BY COMPARISON WITH MICROGRAVITY EXPERIMENTS

Detailed understanding of the basic physical and chemical processes of self-ignition phenomena for technical fuel sprays is required for many technical combustion applications. Because single droplets as the basic elements of fuel sprays allow the study of fundamental ignition behavior, this work focuses on the multistage self-ignition behavior of Φ 0.7 mm single n -heptane droplets in air. The investigated ambient conditions are temperatures from 580 to 1000 K and pressures from 0.3 to 1 MPa. A substantial physical and chemical model is developed for the detailed numerical simulation. The chemical reaction mechanism consists of a 62-step kinetic model with special consideration of the low-temperature reaction branch. The program was validated by comparison with results from microgravity experiments at Drop Tower Bremen, which allow clarification of the ignition process free from natural convection. Cool flame and hot flame appearances were obtained from non-intrusive interferometric measurement in a well-tested experimental setup. The calculated ignition delays resulting from temperature and temperature gradient criteria, respectively, were compared with these experimental results. Furthermore, the cool flame temperature was measured with a K-type thermocouple of Φ 25 μ m at the place of its appearance and was also compared with the numerical simulations. A quantitative good agreement for first and total ignition delays as well as for the cool flame temperature could be achieved. With this detailed numerical model, the multistage ignition behavior was analyzed, and the ignition criteria employed in interferometric measurement, which are temperature and temperature gradient, respectively, were confirmed.

Pressure effects on combustion of methanol and methanol/dodecanol single droplets and droplet pairs in microgravity

Abstract This paper presents the results of an experimental investigation on the combustion of single droplets and two-droplet arrays of pure methanol and methanol/dodecanol mixtures in air under microgravity conditions. The initial droplet diameters, d 0 , were nominally 0.9 mm. The independent experimental variables were the ambient pressure (0.1–9.0 MPa), fuel mixture ratio (methanol/dodecanol: 100/0–15/85), and interdroplet separation distance l ( l / d 0 = 2.3–8.0). For pure methanol, the results show that the droplet lifetime decreases with increasing interdroplet separation distances at low pressures. At higher pressures (3.0 MPa and above) the droplet lifetime was independent of separation distance. The flame extinguished at a finite droplet size only for pure methanol at 0.1 MPa, in qualitative agreement with theoretical predictions. The extinction droplet diameter was nearly independent of the droplet spacing. Methanol/dodecanol–mixture droplets exhibited microexplosion for both single droplets and droplet arrays. The paper presents maps of the disruption regime for both single droplets and droplet pairs. The difference between the disruptive behavior of single droplets and droplet pairs is explained by differences in liquid-phase circulation induced by the gas-phase asymmetry of the droplet pair. The paper also presents results of the dependence of the onset of disruption (in terms of both volume and time) on the pressure and initial fuel mixture ratio.

Effects of dilution by aromatic hydrocarbons on staged ignition behavior of n-decane droplets

Spontaneous ignition of isolated two-component fuel droplets has been experimentally studied. The components were n-decane (ND)/1-methylnaphthalene (MN), or ND/1,2,4-trimethylbenzene (TMB). Both are n-alkane/aromatic mixtures, and therefore are candidates for model fuels of multicomponent commercial fuels refined from crude petroleum. ND showed two-stage ignition behavior, while the other two fuels (aromatic hydrocarbons) did not and were less reactive. A suspended droplet was suddenly brought to a high temperature for ignition. Observation by a Michelson interferometer detected cool-flame appearance as well as hot-flame appearance. The ambient gas was air, and the droplet diameter was 0.7 mm. The experimental conditions that were varied were ambient temperature, Ta, (500–1000K), ambient pressure, Pa, (0.1–2.0 MPa), and the initial mole fraction of ND in the liquid phase, Z (0–1). In the case Z≠1, Pa was fixed to 0.3 MPa. Ignition delays for cool-flame and hot-flame appearance (t1 and ttot) and the difference between them (t2) were measured, and ignition types (no ignition, cool-flame only, singlestage ignition, and two-stage ignition) were classified on a “Z-Ta” map. It is common to ND/MN and ND/TMB that two-stage ignition region is narrowed and finally vanishes as Z decreases, and that t1, t2, and ttot all increase monotonically as Z decreases. However, TMB was affected more than MN because of its higher volatility. (Normal boiling point: ND 447.3 K, MN 517.9 K, TMB 442.5 K) Numerical simulation with a fully transient one-dimensional model was employed to help the interpretation.

A model for devolatilization and ignition of an axisymmetric coal particle

A two-dimensional numerical model is developed to predict shape-dependent devolatilization and ignition of single coal particles exposed to radiant heat flux in an ambience at initially constant temperature. Intraparticle temperature gradients and radiation absorption are taken into account as well as the local release of volatiles and therefore the reduction of the raw coal. During pyrolysis, internal pressure gradients occur, which cause a viscous flow of gas through the pores described by Darcys law. The model considers intrinsic char combustion depending on the local oxygen concentration and internal and external volatile oxidation. Ignition is identified either by an increase of gas-phase temperature or the inflection condition of the particle surface temperature for the homogeneous and the heterogeneous ignition mode, respectively. Comparison with a published one-dimensional model, which assumes an isothermal particle, provides reasonable agreement for the ignition delays of particles with diameters of 50–1000 μm inserted into hot air. In contrast to the published data, the calculated results show a different primary ignition phase for particles larger and equal to 300 μm due to intraparticle temperature gradients. Former experiments on coal particle ignition by radiation in cold air under microgravity conditions show different homogeneous ignition delays for spherical, cylindrical, and flat particles. These experimental results are confirmed by the present model: in addition, heterogeneous ignition points are detected in the calculations. Although the homogeneous ignition delays increase in the order of slabs, cylinders, and spheres, which is the order where specific surface decreases, ignition delays can not be sufficiently described in terms of particle-specific surface, especially not for the calculated heterogeneous ignition.

Detailed numerical simulations for the multi-stage self-ignition process of n-decane single droplets with complex chemistry

The knowledge of droplet self-ignition processes as the basic element of spray ignition is necessary for many combustion applications. Detailed numerical analysis for the self-ignition process of n-decane single droplets showing the multistage self-ignition behavior of kerosene, are carried out for this study. A substantial chemical reaction model for n-decane with 603 reactions including 67 species is implemented in a former validated detailed one dimensional numerical simulation model for droplet ignition. This reaction mechanism pays special attention on the low temperature reaction path and the balance between high temperature and low temperature reactions. In the compared experiments the staged ignition process is detected by observing the temperature gradient with a Michelson interferometer, which is the numerical tracer for cool flame and hot flame appearance as well. The comparison between the results from the simulations and the experiments under microgravity conditions carried out at Drop Tower Bremen shows good agreement. Furthermore the numerical observation of important species like OH or formaldehyde is possible due to the implementation of detailed chemistry. This gives hints for other experimental methods to detect the multi-stage self-ignition behavior like formaldehyde-PLIF, which is meanwhile an approved tracer in the experiments, so that there is vice versa a further possibility to validate the present numerical model.

A study on the direct control of the start of homogeneous charge compression ignition combustion by pulsed flame jet with rapid compression expansion machine

Aiming at direct ignition-timing control of homogeneous charge compression ignition in low-load conditions, pulsed flame jet, which is the jet of burning gas issuing from a small cavity facing a combustion chamber, was utilized. The characteristics of homogeneous charge compression ignition combustion initiated by pulsed flame jet were investigated using rapid compression expansion machine, which realizes single compression and expansion strokes and thus the measurement of indicated work. Higher indicated mean effective pressure was obtained using pulsed flame jet without increasing NOx emission drastically, and it was shown that pulsed flame jet has potential to control the onset of homogeneous charge compression ignition combustion appropriately. The validity of pulsed flame jet utilization was shown through pressure and NOx measurements.