Jet Interaction in a Small-Bore Optical Diesel Engine
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The shortening of lift-off length associated with jet-wall and jet- jet interaction in a small-bore optical diesel engine. Alvin M. Ruslya, Minh K. Lea, Sanghoon Kook*, a, Evatt R. Hawkesa, b, aSchool of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney NSW 2052, Australia bSchool of Photovoltaic and Renewable Engineering, The University of New South Wales, Sydney NSW 2052, Australia * Corresponding Author: Phone: +61 2 9385 4091, Fax: +61 2 9663 1222, E-mail: [email protected] Abstract Jet-wall and jet-jet interactions are important diesel combustion phenomena that impact fuel- air mixing, flame lift-off and pollutant formation. Previous studies to visualise a wall- interacting jet in heavy-duty diesel engines suggested that the shortening of lift-off length could occur due to the recirculated hot combustion products that are entrained back into incoming diesel jet. The significance of this effect, known as re-entrainment, can be higher in small-bore engines due to shorter nozzle-to-wall distance and increased wall curvature. In this study, we performed hydroxyl chemiluminescence imaging using an intensified CCD camera and high-speed imaging of natural soot luminosity using a CMOS camera in an automotive-size optical diesel engine. To provide detailed understanding of the reacting jet under the influence of re-entrainment as well as jet-jet interaction, various jet trajectories were investigated using one and two-hole injectors coupled with a modified piston that allows 1) the identification of the shortening of lift-off length, 2) the measurements of lift-off lengths for varying degrees of jet-wall interactions and 3) the clarification on inter-jet spacing effects on the lift-off lengths. Findings from the measurements support the re-entrainment theory because the shortening of lift-off length occurs only before the end of injection when the strong jet momentum induces the ambient gas entrainment. The shortening also shows good correspondence with the penetration of the recirculated jet head back towards the centre of the combustion chamber. Simultaneous imaging of the bowl-wall-interacting jet and cylinder-liner-wall-interacting jet depicts a shorter lift-off length for the bowl-wall- interacting jet, which further supports the importance of re-entrainment on diesel combustion. Inter-jet spacing effects on the lift-off length are also studied utilising two-hole injectors with two inter-jet spacing angles (51.4o and 102.8o). A narrower spacing between the jets results in the shorter lift-off length of the primary jet due to the higher jet-jet interaction even if short injection duration is used to suppress the re-entrainment. For tested conditions of this study, the shortening of lift-off length appears to be more sensitive to jet-jet interaction than to the re-entrainment caused by the jet-wall interaction. Highlights • Effect of re-entrainment is studied in a realistic small-bore diesel environment. • Both jet-wall and jet-jet interaction induce shortening of lift-off length. • Upstream lift-off movement correspond well with end-of-injection timing. • Wider inter-jet angle caused longer lift-off due to reduced re-entrainment. • Weaker re-entrainment may suppress downstream soot formation. Keywords: Small-Bore Diesel, Jet-Wall, Jet-Jet, Re-Entrainment. 1. Introduction The interaction between a reacting diesel jet and bowl (or cylinder liner) wall has been of great interest due to its influences on mixing and pollutants formation [[1]-[4]]. It is understood that limited mixing and hence locally rich mixtures in the near-wall central region can increase soot formation [[5]-[7]]. On the other hand, planar laser-induced exciplex fluorescence (LIEF) measurements of high-pressure diesel jets suggest that the wall-impinged jet has higher overall fuel-air mixing rate than the free jet [[5]]. This is because a turbulent vortex is formed at the jet head as the reacting jet impinges on the wall and propagates along the wall [[2],[5]]. The increased mixing in the jet head region is believed to outperform the limited mixing near the impingement point. However, increased soot due to very high soot formation near the impingement point cannot be fully resolved [[6],[8]]. In addition, soot deposition on the wall and valve surfaces originates from the jet-wall impingement [[7],[9],[10]]. Deposited soot on the wall can then make its way to the exhaust or deteriorate the quality of engine oil. In addition to these direct influences of jet-wall interaction on pollutants formation, it is recognised to have significant influence on the flame lift-off length, which in turn impacts downstream combustion and pollutant formation. In a constant-volume combustion chamber, Pickett and Lopez [[8]] attempted to simulate jet-wall and jet-jet interactions by confining a jet to a box structure. The results of this experiment showed continual shortening of the lift- off length caused by wall-redirected combustion gases and thereby increased soot luminosity. In later work focused on free-jets [[11]], a conceptual picture was drawn to depict the existence of a high-temperature product reservoir, which is suggested to play a role in stabilisation of diesel flame base. This high-temperature, low-density product reservoir, located on the edge of the vapour boundary, may explain and further empathise importance of jet-wall interaction. This is because the existence of the bowl-wall would potentially enable hot combustion products at the periphery of fuel-air mixture to be redirected back towards the centre of the combustion chamber and possibly entrained into the incoming diesel jet. This entrainment of hot combustion products would cause shortening of the lift-off length, thereby increasing soot formation. This phenomenon, named as re-entrainment, was also observed in heavy-duty diesel engines [[12],[13]] under the influence of realistic jet-wall and jet-jet interactions. The impact of re-entrainment on small-bore diesel engine combustion would be of higher significance than that on heavy-duty diesel engines. This is due to enhanced jet-wall interaction [[14]] associated with a shorter distance between the injector and the bowl wall as well as increased wall curvature. However, the details of the re-entrainment and its effect on the lift-off length in small-bore diesel engines are not well understood. Moreover, the small- bore design causes increased jet-jet interaction as neighbouring jets and their combustion products interact with each other at closer distance from the nozzle. It is well known that narrower inter-jet spacing causes decreased lift-off length [[15],[16]]; however, correspondence between jet-jet interaction and re-entrainment has not been discussed in details, particularly in small-bore diesel engines. In the present study, jet-wall and jet-jet interaction is investigated with great emphasis on the re-entrainment and lift-off length in a small-bore diesel engine. To allow measureable jet- wall interaction, a long injection duration of 2.36 ms (actual) was implemented using a single-hole injector. We also used a modified piston where a section of the bowl-rim was removed to allow a different jet configuration. This achieved two different jet targeting approaches: one aiming at the bowl wall to create a near-axisymmetric wall-interacting jet and the other travelling towards the edge of the bowl-rim cut-out so that a half of the jet impinges on the bowl wall while the other half penetrates further into the bowl-rim cut-out region. The latter arrangement was for simultaneous visualisation of a liner-wall-interacting jet (“free-jet”) and bowl-wall-interacting jet (“wall-jet”), enabling a direct comparison of jets with a different level of wall-interaction. Moreover, two two-hole nozzles of different inter- jet spacing (51.4o and 102.8o) were used to investigate the effect of jet-jet interaction and its influence on re-entrainment and lift-off length. Imaging of OH* chemiluminescence was performed for various conditions mentioned above as well as for various times after the start of injection so that transient behaviour of the flame base could be captured during and after the injection event. High-speed imaging of hot soot luminosity was also performed to ensure that the OH* chemiluminescence signal was not interfered by the soot incandescence signal as well as to observe trends in downstream soot concentration. 2. Experimental Setup 2.1 Optical diesel engine A single-cylinder automotive-size diesel engine with optical access via a quartz piston window was used in this study as shown in Fig. 1. The bottom-view imaging of the combustion chamber was performed using a 45o angled mirror at the void of the extended piston assembly. The design of this engine using a hydraulic-ram-operated drop-down liner allows the cleaning of the chamber between engine runs, which is needed to minimise signal attenuation due to soot deposition on the surface of the quartz piston window. 2.2 Operating conditions and engine specifications Specifications of facilities shown in Fig. 1 and selected operating conditions are listed in Table 1. The single-cylinder optical engine was run by an AC motor at a fixed engine speed of 1200 rpm and was skip-fired at every 10th cycle to minimise thermal loadings on optical pieces and to remove residual gases for minimal cycle-to-cycle fluctuations. The base engine has 83 mm bore and 92 mm stroke with a geometric compression ratio of 17.7. As previously mentioned, however, a portion of the bowl rim was removed resulting in a lower compression ratio of 15.2 for both jet targeting configurations used in this study. The conventional engine head was equipped with an intake port throttle for variable swirl ratios.