Investigation of combustion characterization of single-point and two-point laser ignition of LPG-air mixture in a constant volume combustion chamber

Abstract

In this article, single and two-point laser-induced spark ignition (LISI) of liquified petroleum gas (LPG) - air mixture is investigated. The experiments were tested in a constant volume combustion chamber (CVCC) for an equivalence ratio (ϕ) of 0.6 to 1.2. The CVCC has four optical windows, which are used simultaneously for optical diagnostics and laser ignition. All experiments for both single and two-point laser ignition (LI) were tested at an initial chamber pressure of 6 bar and temperature of 298 K. Initially, the effect of distance between two plasms at focal points of 5 mm, 25 mm, and 45 mm apart on pressure time history was studied and observed a higher peak pressure and minimum combustion duration for 45 mm plasma spacing. It has been observed a peak pressure of 42.17 bar, 42.65 bar, and 43.92 bar for 5 mm, 25 mm, and 45 mm apart plasma spacing respectively for stoichiometric mixture, however, the combustion duration of 84.2 ms, 78.6 ms, and 73.2 ms were observed for 5 mm, 25 mm, and 45 mm apart plasma spacing respectively. The pressure-time (p-t) history for single and two-point LI was recorded, and a comparative study between the single and two-point LI of the LPG-air mixture analysis results in higher pressure with two-point LI of the LPG-air mixture compared to single-point LI for all equivalence ratios. The probability of laser ignition was investigated for single and two-point LI of LPG-air mixtures and observed a higher probability of ignition for two-point LI compared to single-point LI. With two-point LI the LPG-air mixture can be ignited up to an equivalence ratio of 0.52 however with single-point LI the mixture can be ignited up to an equivalence ratio of 0.56. Furthermore, the net heat release (NHR) was calculated for single and two-point LI and observed higher NHR for two-point LI compared to single-point LI. The NHR was observed to be 7872.75 J and 8111.88 J for single-point and two-point LI respectively at an equivalence ratio of 1.0. Lastly, the initial phases of the combustion progress for two-point LI were visualized using the Shadowgraphy technique with a high-speed camera. The growth of flame kernel for the LPG-air mixture of different ϕ (0.6, 0.8, 1.0, and 1.2) was visualized and analyzed and it has been observed the development of two flame kernels one on the upper side and another on the lower side of the chamber. Further, both flame kernel images show two toroidal rings with one front lobe propagating toward the incoming laser source. The flame kernel growth images obtained were used to measure the flame kernel propagation in different directions and it has been observed faster flame kernel propagation at an equivalence ratio of 1.0 in all directions.