Calculations of evaporation characteristics (distillation curve, two-phase diagram, and critical points) of surrogates are described in detail. The efficiency of some surrogate blends, represented in literature, in reflecting the evaporation characteristics was analyzed. Based on the analysis, the chemical capabilities of surrogate models are not linked to their abilities to reflect the phase-equilibrium properties of real fuel. It is shown that model design of practical fuels must include the phase-equilibrium and distillation curve calculations. A surrogate mixture was selected, which closely matches the boiling-point curve and two-phase diagram for jet-A. Next, physical properties of reference fuel were taken into consideration: combustion enthalpy, formation enthalpy, molar weight, approximate formula (carbon per hydrogen ratio), sooting tendency index, critical point, two-phase diagram, and distillation curve.
A liquid fuel combustor based on the FLOX® burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. The operability limits of the burner were mapped out for full load conditions at 3.5 bars by varying global lambda (λG) from 1.25–2.00 and bulk jet velocity (vBulk) from 80–140 m/s. Exhaust gas measurements show NOx and CO levels below 5 and 10 ppm respectively (corrected for reference 15% O2) at λG = 1.89. Optical and laser diagnostic measurement techniques have been employed to characterize the spray flames. The flames at stable burner operation points (BOPs) show a predominantly jet like flame shape irrespective of λG and vBulk. Droplets in the size range 2–40 μm have been measured close to the nozzle exit plane. Velocities conditioned on the droplet size show large droplets d > 15 μm transitioning from negative slip velocity at the exit plane to positive slip velocity at downstream location. The positive slip velocities and slow evaporation of large droplets lead to droplets travelling further into the combustion chamber and hence resulting in long flames. A comprehensive data set for the spray characteristic of the new liquid FLOX® burner is made available.
The present paper describes the proposed strategy of fuel model design based on identification of chemical and physical criteria for the selection of initial formula of the reference fuel. The first 8 criteria established and studied in previous papers so far are combustion enthalpy, formation enthalpy, molecular weight, C/H-ratio, sooting tendency index, critical point, two-phase diagram, and distillation curve. With these criteria established, the following candidate formula of the kerosene surrogate blend is defined and optimized to adequately mimic the properties of the real fuel: 10% n-propylcyclohexane, 13% iso-octane, 20% n-dodecane, 23% 1-methylnaphthalene, and 32% n-hexadecane. In this work, the ignition delay time has been studied as the next optimization criterion. To keep the model size small, the core reaction mechanism — the skeletal kinetics of n-heptane and iso-octane combustion including aromatics formation, developed earlier — is extended by n-propylcyclohexane, 1-methylnaphthalene, n-dodecane, and n-hexadecane sub-models. The lumped mechanisms for larger n-alkanes are constructed in a similar way to that for n-decane. The n-propylcyclohexane oxidation sub-model is derived from a skeletal mechanism for the low and high temperature cyclohexane oxidation. Reactions for 1-methylnaphthlene oxidation are included in the sub-mechanism for the formation of aromatics up to 5 ringed molecules. The mechanism includes 189 species and 1125 reactions. The proposed sub-models and overall mechanism are validated against experimental data obtained in shock tubes and in jet stirred reactor.s The simulations of ignition delay data for all hydrocarbons and their mixtures, i.e. for kerosene, are in good agreement with the measured data.
In this work the ongoing development of a jet-stabilized FLOX®(Flameless Oxidation)-type low-emission combustor for liquid fuels is described. The desired application of this concept is a micro gas turbine range extender for next generation car concepts. Diesel DIN EN 590 was used to operate the combustor, which is very similar to other fuels like bio-diesel, light heating oil and kerosene and therefore provides a link to other gas turbine applications in power generation. The investigation of flame stabilization of jet flames as well as fuel atomization, spray dispersion and evaporation is essential for the design of an effective and reliable combustor for liquid fuels. An axisymmetric single-nozzle combustion chamber was chosen for the initial setup. A variety of burner configurations was tested in order to investigate the influence of different design parameters on the flame shape, the flame stability and emissions. Two pressure atomizers and one air-blast atomizer were combined with two types of air nozzles and two different burner front plates (axisymmetric and off-centered jet nozzle). Finally, a twelve nozzle FLOX® combustor with pre-evaporator was designed and characterized. The combustor was operated at atmospheric pressure with preheated air (300° C) and in a range of equivalence ratios φ between 0.5 and 0.95 (λ = 1.05–2). The maximum thermal power was 40 kW. For each combustor configuration and operating condition the flame shape was imaged by OH*-chemiluminescence, together with an analysis of the exhaust gas emissions. Laser sheet imaging was used to identify the spray geometry for all axisymmetric combustors. Wall temperatures were measured for two configurations using temperature sensitive paints, which will be utilized in future CFD modeling. The results show a dependence of NOx emissions on the flame’s lift-off height, which in turn is defined by the spray properties and evaporation conditions. The tendency to soot formation was strongly dependent on the correlation of the recirculation zone to the spray cone geometry. In particular, strong soot formation was observed when unevaporated droplets entered the recirculation zone.
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