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The role of thermal boundary layers on homogeneous charge compression ignited engine combustion was directly evaluated by measuring the spatial profile of the formaldehyde concentration near the combustion chamber wall. Formaldehyde forms naturally as an intermediate species of the combustion process. A through-the-wall imaging scheme, in which the laser propagated at an angle through a window and the fluorescence signal was collected through the same window, was employed to avoid the effect of optical vignetting. Intensity profiles acquired in the wall-normal direction from the mean image show that during the first stage of combustion (cool flame) there is a substantial region of lower intensity, and presumably lower formaldehyde concentration, near the wall. This region extends more than 5 mm. After the completion of the first stage of combustion, the formaldehyde is found adjacent to the wall within the measurement limits of the system. During the second stage of combustion, the intensity near the wall and the intensity gradient in the normal direction remain constant, but the intensity is found to decrease preferentially away from the wall at a distance comparable to that observed in the first-stage combustion. Both of these findings are consistent with a thermally affected region near the wall that delays combustion. Individual cycle images, however, show that the true spatial gradients in the flow field are much steeper than in the mean data, and that regions of high temperature fluid can be found in close proximity to the wall. No appreciable thickening of the thermally affected region was seen at low equivalence ratios where there was incomplete combustion. |
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