Browsing by Author "Hadef, R"

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    Effects of co- and counter-swirl on the droplet characteristics in a spray flame
    (ELSEVIER, 2008) Hadef, R; Lenze, B
    This paper reports measurements of droplet characteristics and flow field in a spray flame with inner and outer swirling air streams. The spatial distribution of droplet characteristics produced by the burner’s airblast atomizer was measured using dual-phase Doppler anemometry (PDA). The spray flame was operated near the lean blow-out limit at two flow conditions: co-swirling (flow rotation in the same direction) and counter-swirling (flow rotation in opposite directions). In both cases, the flame exhibited a U-shaped form and was marked by a large central recirculation zone. Based on the measurements of the droplet velocity components, differences between both configurations appeared for the counter-rotational setup mainly in the near burner region, where the decrease of total swirl causes deeper penetration of the droplets from the inner duct into the combustion chamber, resulting in a much more homogeneous distribution than the other one. The droplet size in terms of the Sauter mean diameter (SMD) shows little variation in the change of the direction swirl condition. Application of counter-swirl results in more turbulent droplet motion.
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    Investigation of laminar pressurized flames for soot model validation using SV-CARS and LII
    (ELSEVIER, 2005) Hadef, R; Geigle, Klaus Peter; Michael, S
    Quasi-simultaneous measurements of temperature and soot volume fraction in pressurized and atmospheric flames are presented. A dual-flame burner concept yielded stable laminar flames for a variety of equivalence ratios, pressures, and fuels, and permitted the investigation of flames without the influence of soot oxidation. A CARS-based technique (shifted vibrational CARS) for temperature measurements, which offers high accuracy over the entire relevant temperature and soot concentration range, is described. Comparison of temperature measurements in the nonsooting part of a laminar diffusion flame at atmospheric pressure by SV-CARS and conventional N2 Q-branch CARS yielded excellent agreement. This new technique was applied to quasi-1D laminar flames with soot concentrations up to 10 ppm and pressures up to 5 bar. The temperature profiles measured in these flames were combined with soot concentration measurements using LII; calibration and correction for signal trapping yielded quantitative soot volume fraction data. The temperature and soot concentration data were combined to generate a comprehensive dataset for the validation of an improved kinetic soot model for the prediction of soot formation in premixed combustion at elevated pressure.
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    Measurements of droplets characteristics in a swirl-stabilized spray flame
    (ELSEVIER, 2005) Hadef, R; Lenze, B
    Experiments have been performed in a kerosene airblast atomized spray flame where the fuel supply is sandwiched between two coswirling air streams. A phase-Doppler particle sizing system was used to measure fuel droplet size, velocity and turbulent kinetic energy of droplets and gas as well as the fuel volume flux within the combustor for two values of air preheat temperature, 200 C and 400 C. The results reveal that a small liquid fuel exists in the centre of the combustor and due to larger droplets with the swirl effect; the droplets characteristics (velocity and turbulent kinetic energy) did not follow the gas ones. Based upon these measurements, an optimum swirl number will exist with every atomization and burner arrangement of a liquid-fuelled flame associated and will be different from that associated with the corresponding gas-fuelled flame. The investigated atomizer shows a marked influence of the inlet air temperature on the measured droplet size which may be attributed to the design of the internal airflow, promoting prompt atomization at the dominant atomization mode.
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    Modeling laser-induced incandescence of soot
    (Springer-Verlag, 2007) michelsen, A; liu, F; Hadef, R
    We have performed a comparison of ten models that predict the temporal behavior of laser-induced incandescence (LII) of soot. In this paper we present a summary of the models and comparisons of calculated temperatures, diameters, signals, and energy-balance terms. The models were run assuming laser heating at 532 nm at fluences of 0.05 and 0.70 J/cm2 with a laser temporal profile provided. Calculations were performed for a single primary particle with a diameter of 30 nm at an ambient temperature of 1800 K and a pressure of 1 bar. Preliminary calculations were performed with a fully constrained model. The comparison of unconstrained models demonstrates a wide spread in calculated LII signals. Many of the differences can be attributed to the values of a few important parameters, such as the refractive-index function E(m) and thermal and mass accommodation coefficients. Constraining these parameters brings most of the models into much better agreement with each other, particularly for the low-fluence case. Agreement among models is not as good for the high-fluence case, even when selected parameters are constrained. The reason for greater variability in model results at high fluence appears to be related to solution approaches to mass and heat loss by sublimation.