Determination of the minimum ignition energy (MIE) of premixed propane/air
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This Master thesis describes an experimental study aimed at determining the minimum ignition energy (MIE) of premixed propane/air mixtures. Two fundamentally different circuits have been employed for creating the sparks in the experiments: a replica of the American Society for Testing and Materials (ASTM) apparatus (ASTM, 2007) and an improved version of the synchronized spark generator described by Randeberg et al. (2006). The explosion vessel was a 0.20 dm3 cylindrical plastic tube, equipped with two 1.6 mm flat-ended tungsten electrodes. In the present work, the electrodes were flanged with glass plates and the spark gap was set to 2.0 mm. The principle of the capacitive spark circuit of ASTM was to charge an appropriate capacitance connected across the spark gap through a large resistor (1012 Ω) by a high-voltage power supply until the spark occurred. Different spark energies could be obtained mainly by varying the value of the capacitance. Premixed propane/air was used as test gas. About 220 tests were performed with the ASTM circuit to find the boundary between ignition and no-ignition in propane/air mixtures. The investigated propane concentrations were in the range 3.5 to 8.0 vol. %. By following the approach of Moorhouse et al. (1974), the MIE in the present work was determined by drawing the highest possible boundary line through all the experimental points that had no ignition points below it. This resulted in a minimum ignition energy of 0.48 mJ at a propane concentration of 5.2 vol. %. This is twice the value reported by Lewis and von Elbe (1961), but in very good agreement with the result obtained by Moorhouse et al.. The quenching effect of the 2.0 mm spark gap for propane/air mixtures of less than 4.0 vol. % was confirmed. However, the present results do not indicate that 2.0 mm is the quenching distance for any specific concentration on the fuel rich side in the investigated concentration range (3.5-8.0 vol. %). According to the data reported by Lewis and von Elbe, a flanged spark gap of 2.0 mm should correspond to the quenching distance for a propane/air concentration of about 6.0 vol. % on the fuel rich side. The probability of ignition for propane/air mixtures of 5.2 and 5.5 vol. % were also studied by varying the spark energy levels in the range 0.40 to 0.75 mJ for both concentrations. Based on the classical U-shaped curve by Lewis and von Elbe (1961), presenting MIE as function of propane concentration, it was expected that a mixture of 5.2 vol. % propane in air would be more ignitable than a mixture of 5.5 vol. %. However, the present results do not show any significant difference in the ignitability for the two concentrations. By applying a statistical model to the data at each of the two concentrations, MIE for 1% probability of ignition were calculated. They were both significantly higher than the Lewis and von Elbe value of 0.25 mJ. Randeberg et al. (2006) described a spark generator for producing synchronized capacitive spark discharges of low energy, down to below 0.1 mJ. However, Eckhoff et al. (2008) re-examined the discharge circuit of Randeberg et al., and concluded that, due to the hidden subtle energy supply, the real spark energies in the experiments were significantly larger than those quoted in the published paper. The additional energy in the discharge was of the order of 1 mJ. With the aim to minimize the additional energy,...