A. Franke

Lund University, Lund, Skåne, Sweden

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Publications (5)4.13 Total impact

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    ABSTRACT: The ionization sensor is an electrical probe for diagnostics in internal combustion engines. Laser-induced fluorescence (LIF) imaging of fuel, hydroxyl (OH), and nitric oxide (NO) distributions has been employed to extend our knowledge about the governing processes leading to its signal. By monitoring the flame propagation in quiescent and turbulent mixtures, the cycle-to-cycle variations in the early sensor signal was attributed to the stochastic contact between flame front and electrodes. An analysis of the relationship between gas temperature and sensor current in the post-flame gas suggests a dominant role of alkali traces in the ionization process at the conditions under study. Significant cooling of the burned gas in the vicinity of the electrodes was observed in quiescent mixtures. Imaging of the post-flame gas in turbulent combustion revealed moving structures with varying NO and OH concentrations, which were identified as sources of variation in the sensor current.
    Applied Physics B 12/2005; 81(8):1135-1142. DOI:10.1007/s00340-005-1969-0 · 1.86 Impact Factor
  • M. Akyuz · M. Rahman · A. Larsson · V. Cooray · A. Franke ·
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    ABSTRACT: An experiment of laser-triggered electric discharges has been conducted in a uniform background electric field. By focusing the beam of a pulsed Nd-YAG laser (wavelength 532 nm, pulse duration 5 ns) in a narrow (15 mm) plane-parallel air gap exposed to a DC voltage, streamer discharges and disruptive discharges were triggered. The laser energy and the background electric field strength were varied in the ranges 0-180 mJ and 0.8-1.4 MV/m, respectively, and the laser plasma was induced either close to the anode, close to the cathode or mid-gap. From images of the electrode gap and from current measurements, the occurrence of and time to the streamer discharge and the disruptive discharge were determined. No distinct level was found of either the laser energy or the plasma energy for the transition from one discharge case to another, even if the general trend was that the energy required for a certain discharge case was reduced when increasing the background electric field. These and other observations make the method of laser triggering a less suitable method for studies of streamer initiation and streamer propagation, but revealed several interesting features of laser-triggered electric discharges.
    IEEE Transactions on Dielectrics and Electrical Insulation 11/2005; 12(5-12):1060 - 1070. DOI:10.1109/TDEI.2005.1522198 · 1.28 Impact Factor

  • H. Neij · A. Saitzkoff · R. Reinmann · A. Franke · M. Aldén ·
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    ABSTRACT: An experiment is performed under well-controlled conditions to evaluate the prospects for fuel-to-air equivalence ratio determination by ion current measurements. To this end, two-dimensional laser-induced fluorescence (2D LIF) is utilized to acquire the true, cycle-resolved equivalence ratio conditions in the vicinity of the electrode gap. The experiment is conducted in an optically accessible combustion cell of constant volume, where methane doped with a fluorescent tracer, acetone, is injected into quiescent air and ignited by an electrical discharge. Different degrees of fuel homogeneity are achieved by varying the air/fuel mixing time prior to ignition. Apart from the local equivalence ratio, the following parameters are measured: the ion current, the combustion chamber pressure and the global equivalence ratio, which is derived from the oxygen content of the exhaust gases and is used for calibration of the 2D LIF data. After data extraction, the ion current and equivalence ratio parameters are examined for interrelations, both as ensemble averages and as individuals.
    Combustion Science and Technology 12/1998; 140(1-6):295-314. DOI:10.1080/00102209808915775 · 0.99 Impact Factor