Flame image velocimetry analysis of reacting jet flow fields with a variation of injection pressure in a small-bore diesel engine

Abstract

This study measures in-flame flow fields in a single-cylinder small-bore optical diesel engine using Flame Image Velocimetry (FIV) applied to high-speed soot luminosity movies. Three injection pressures were tested for a two-hole nozzle injector to cause jet-wall interaction and a significant jet-jet interaction within 45° inter-jet spacing. The high-pressure fuel jets were also under the strong influence of a swirl flow. For each test condition, soot luminosity signals were recorded at a high framing rate of 45 kHz with which the time-resolved, two-dimensional FIV post-processing was performed based on the image contrast variations associated with flame structure evolution and internal pattern change. A total of 100 combustion events for each injection pressure were recorded and processed to address the inherent cyclic variations. The ensemble-averaged flow fields were used for detailed flow structure discussion, and Reynolds decomposition using a spatial filtering method was applied to obtain high-frequency fluctuations that were found to be primarily turbulence. The detailed analysis of flow fields suggested that increased injection pressure leads to enhanced jet flow travelling along the bowl wall and higher flow vectors penetrating back towards the nozzle upon the impingement on the wall. Within the jet-jet interaction region, the flow vectors tend to follow the swirl direction, which increases with increasing injection pressure. The FIV also captured a turbulent ring vortex formed in the wall-jet head, which becomes larger and clearer at higher injection pressure. A vortex generated in the centre of combustion chamber was due to the swirl flow with its position being shifted at higher injection pressure. The bulk flow magnitude indicated significant cyclic variations, which increases with injection pressure. The turbulence intensity is also enhanced due to higher injection pressure, which primarily occurs in the wall-jet head region and the jet-jet interaction region.

Flow-Field Analysis of Isobaric Combustion Using Multiple Injectors in an Optical Accessible Diesel Engine

“…However, using the side injectors for both fuel injections leads to fewer vortex structures than in the C+S configuration. The flame-wall interaction effects [31] in S+S configuration are clearly evident at 19.6 °CA aTDC showing the strong flow vectors traveling along the bowl-wall in both up-swirl and downswirl sides of flame.…”

“…There are numerous techniques used in literature to get the quantitative description of the flow velocity in the combustion engines ranging from point measurement such as hot wire anemometry [23], laser doppler velocimetry (LDV) [24] to 2D flow-field measurement like particle image velocimetry (PIV) [22,25]. In contrast to this complex PIV technique, high-speed soot luminosity images have been used to get the details of the flow-field information in combustion engine experiments using a simple technique called flame image velocimetry (FIV) [26][27][28][29][30][31][32]. In FIV, a contrast in soot signal resulting from the change in flame luminosity structure is used as a tracking source and then the displacement vectors are determined based on the standard cross-correlation algorithm.…”

Performance Analysis and In-Cylinder Visualization of Conventional Diesel and Isobaric Combustion in an Optical Diesel Engine

“…Figure 10 shows the flow-field distribution for CDC (top row), IsoL (middle row) and IsoH (bottom row) combustion cases. The FIV analysis is reliant upon the combustion luminosity, which is dominated by incandescence signals of hot soot particles within the high-temperature flames [30,37]. Therefore, the FIV is applied near the peak heat release rate region (Figure 4) where the averaged soot luminosity signal (Figure 9) is higher.…”

“…Not only the FIV measurement can be applied to line-of-sight integrated soot luminosity images, but also the 2D laser sheet visualization of soot particles [26]. Previous studies applied this approach to look in swirl ratio estimation [27], swirlinduced vortex [28], injection-induced vortex [27,29,30], and diesel knock [31].…”

Potential for Shock-Wave Generation at Diesel Engine Conditions and Its Influence on Spray Characteristics

“…Fuel injection systems in modern engines have evolved towards higher injection pressures to achieve a better quality of mixture combustion. With the elevated fuel injection pressures, sprays possess a high level of turbulence, which can enhance the air entrainment and fuel breakup [1][2][3]. On the other hand, the higher injection pressure results in faster spray penetration that can improve air utilization and combustion speed [4].…”