The research activity refers to the assessment of more detailed physical information on ignition and early development of combustion of a turbulent charge in a closed vessel. The objectives of the research work refer to the investigations of three main topics : i) the features and the operation of different kinds of ignition sources, ii) the structure and the intensity of the flow-field of the gas mixture in the chamber, iii) the interaction of the ignition kernel (early flame) with the flow-field of the fresh reacting mixture. More general objective refers to the perspective of a more direct control of the early combustion phase in i.c. spark-ignited engines, leading to the reduction of cycle-to-cycle variations and HJC emissions. The first stage work has covered the generation and the characterization of the flow field in the constant-volume combustion chamber. This has been accomplished by means of a transient, fast filling of the chamber through one or two inlet ducts. In the second stage of the sub-project, different ignition sources have been developed and characterized, in terms of 1) their electrical features, 2) their capability of triggering ignition in reference and well-assessed conditions of 4-bars, stoichiometric propane/air mixtures, 3) the dependence of their behaviour on the boundary conditions (wall- and electrode- effects). Three ignition sources were selected: a) a conventional ignition-coil system, featuring essentially glow-discharge, b) a fast H.V. pulser, operating mostly in the break-down mode, c) a peculiar spark igniter (micro-plasma-jet). First experimental results indicate that, under the given operating conditions and within the early phase (t<5 ms), the growth rate of the ignition kernel depends poorly on the turbulence intensity and more markedly on the rate of ignition energy release. At the end of the second year of the sub-programme, the work has been focused on the experimental characterization of the ignition kernel, developing from a conventional coil-based igniter in different fluid dynamic conditions and equivalence ratios. Experimental results provide the sizes and shapes of the ignition kernels in a propane/air charge at 4 bars as functions of the time from the spark onset, the level of the turbulence intensity in the chamber and the equivalence ratio. The experimental data confirm in greater details previously observed features: there is weak dependence, if any, of the time-law of the kernel growth on the turbulent intensities, at least in the first few milliseconds from the spark onset and in the explored conditions. A quantitative visualization/evaluation of the flow field has been achieved, by means of a new velocimetric technique, named Soot Incandescence Velocimetry (S.I.V.), which has been developed and demonstrated. It works in a fashion similar to the well-known P.I,V., being basically a iasersheet technique, but uses instead optically absorbing seeding particles and quite different Iaserpulse shape and duration.. An image processing procedure has been developed, which extracts from the raw velocimetric data reported previously some relevant fluid dynamic data, For each test case, referring to given operating conditions, results of the processing are : a 2-D map of the vectorial velocity field in equally-spaced grid points and the spectrum of the scalar velocities of each SIV image. The evaluation of other 2-D fluid dynamic quantities can be obtained easily from the above results.
Experimental study of the growth of the ignition kernel in a turbulent flow-field - Programme JOU2-CT92-0162 Project FC.2 - FINAL REPORT 01.01.1993 to 30.06.1996
A Borghese;V Moccia
1996
Abstract
The research activity refers to the assessment of more detailed physical information on ignition and early development of combustion of a turbulent charge in a closed vessel. The objectives of the research work refer to the investigations of three main topics : i) the features and the operation of different kinds of ignition sources, ii) the structure and the intensity of the flow-field of the gas mixture in the chamber, iii) the interaction of the ignition kernel (early flame) with the flow-field of the fresh reacting mixture. More general objective refers to the perspective of a more direct control of the early combustion phase in i.c. spark-ignited engines, leading to the reduction of cycle-to-cycle variations and HJC emissions. The first stage work has covered the generation and the characterization of the flow field in the constant-volume combustion chamber. This has been accomplished by means of a transient, fast filling of the chamber through one or two inlet ducts. In the second stage of the sub-project, different ignition sources have been developed and characterized, in terms of 1) their electrical features, 2) their capability of triggering ignition in reference and well-assessed conditions of 4-bars, stoichiometric propane/air mixtures, 3) the dependence of their behaviour on the boundary conditions (wall- and electrode- effects). Three ignition sources were selected: a) a conventional ignition-coil system, featuring essentially glow-discharge, b) a fast H.V. pulser, operating mostly in the break-down mode, c) a peculiar spark igniter (micro-plasma-jet). First experimental results indicate that, under the given operating conditions and within the early phase (t<5 ms), the growth rate of the ignition kernel depends poorly on the turbulence intensity and more markedly on the rate of ignition energy release. At the end of the second year of the sub-programme, the work has been focused on the experimental characterization of the ignition kernel, developing from a conventional coil-based igniter in different fluid dynamic conditions and equivalence ratios. Experimental results provide the sizes and shapes of the ignition kernels in a propane/air charge at 4 bars as functions of the time from the spark onset, the level of the turbulence intensity in the chamber and the equivalence ratio. The experimental data confirm in greater details previously observed features: there is weak dependence, if any, of the time-law of the kernel growth on the turbulent intensities, at least in the first few milliseconds from the spark onset and in the explored conditions. A quantitative visualization/evaluation of the flow field has been achieved, by means of a new velocimetric technique, named Soot Incandescence Velocimetry (S.I.V.), which has been developed and demonstrated. It works in a fashion similar to the well-known P.I,V., being basically a iasersheet technique, but uses instead optically absorbing seeding particles and quite different Iaserpulse shape and duration.. An image processing procedure has been developed, which extracts from the raw velocimetric data reported previously some relevant fluid dynamic data, For each test case, referring to given operating conditions, results of the processing are : a 2-D map of the vectorial velocity field in equally-spaced grid points and the spectrum of the scalar velocities of each SIV image. The evaluation of other 2-D fluid dynamic quantities can be obtained easily from the above results.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
