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The effect of split injection on the fuel spray and combustion processes in a rapid compression and expansion machine was investigated using the visualization process. A two-dimensional piston cavity, designed with the cross section of a reentrant piston, was installed in the combustion chamber to observe the combustion process from the lateral side. Combustion experiments were conducted with injection pressures of 80 MPa, 120 MPa, and 180 MPa and an O2 concentration of 15%. The spray/wall interaction, mixture distribution, and ignition location were investigated using the shadow method. Along with natural flame luminescence, different spray impinging behaviors on combustion process were studied. Furthermore, the combustion characteristics of in-cylinder pressure, apparent heat release rate, and combustion phase were recorded and analyzed simultaneously. The results showed that both high injection pressure and split injection with a longer interval effectively improved the combustion performance. In addition, when the pilot injection was advanced further, the injection interval had a larger influence in reducing soot generation, while the effect of high injection pressure on heat release decreased. Flame separation was found to occur at high injection pressures. It was observed that the flame separation caused by the strong spray momentum was beneficial for reducing soot generation owing to the greater fuel-air interaction area. The spray and combustion processes were investigated in detail, and the significant effects of different injection pressures and injection intervals on combustion performance with the split injection method were highlighted.

Single-hole (SH) injectors were compared with multi-hole (MH) injectors to reveal the differences in nozzle internal flow and spray propagation. The multi-phase flow inside the nozzles was numerica...

We investigate turbulent flow with highly pulsating axial velocity passing through a duct with both first and second bends. The time-dependent velocity fields downstream of the bends were measured using time-resolved stereo particle image velocimetry for the steady case (Reynolds number Re = 36,700) and the pulsatile case (Re = 37,800 and Womersley number α = 59.1). Proper orthogonal decomposition (POD) of the in-plane velocity data isolates the energetic structures of the secondary flow. The modes downstream of the first bend have a Dean motion (mode 0), single swirl (mode 1), and double swirl (mode 2), which agree with those of previous studies on steady turbulent flow. Downstream of the second bend, additional vortices appear in the modes owing to the secondary flow originating in the first bend. The modal structure of the pulsatile case is virtually the same as that of the steady case. To our knowledge, we are the first to find swirl switching in pulsatile flow, whereas the switching has been reported only for steady cases. We further conduct a time-frequency analysis via wavelet transformation onto the POD time coefficient, showing intermittency in energy of the mode associated with swirl switching., This work was partially supported by a research grant from The Hiroshima University Education and Research Support Foundation.

This paper discusses the turbulent flow and heat transfer from a uniform air flow with high temperature to the outside through a 90&deg, curved square pipe. Both conjugate heat transfer (CHT) simulation and experiments of temperature field measurements at cross sections of the pipe are performed. A straight pipe is investigated and compared with the 90&deg, curved pipe. The temperature of the air flow at the inlet of the pipe is set at 402 K, and the corresponding Reynolds number is approximately 6 &times, 104. To obtain the spatial average temperature at each cross section, the temperature fields are measured along the streamwise of the pipes and in the circumferential direction using thermocouples at each cross section from the inlet to the outlet of both the straight and curved pipes. Furthermore, the simulation is performed for turbulent flow and heat transfer inside the pipe wall using the Re-normalization group (RNG) k-&epsilon, turbulence model and CHT method. Both the experimental and numerical results show that the curvature of the pipe result in a deviation and impingement in the high-temperature core and a separation between the wall and air, resulting in a secondary flow pattern of the temperature distribution.

Reducing heat loss is one of the most important development concerns for improving the thermal efficiency of the diesel engine. In order to know heat transfer through the combustion chamber wall mo...

Owing to cavitation inside the nozzle, spray breakup, and complex flow fields in the cylinder of a spark ignition engine, a certain degree of cycle-to-cycle variation (CCV) remains in different spray cycles under the same operating conditions, thereby affecting the spark ignition engine significantly. Therefore, the effect of cross-flow on the cyclic variation of the spray characteristics is investigated in this work. The CCV of fuel spray characteristics is evaluated via the statistical analysis of 30 repeated experiments under atmospheric pressure. First, the coefficient of variation (COV) is used to assess variations in the spray penetration, area, and optical thickness. Moreover, the effects of injection pressure and cross-flow velocity on the statistical characteristics of the spray are analyzed at the end of injection (EOI). Finally, a dimensionless “Ins number”, which is defined as the parameter of any spray characteristic at a certain time divided by the average experimental value, is proposed to evaluate the spray variation characteristics under cross-flow conditions. Therefore, when the absolute value of the Ins number approaches 1, the CCV of the spray characteristic parameters is low. Results show that the COV of the spray penetration is the largest in the initial stage of injection and then decreases gradually. Meanwhile, the COV of the vertical penetration can be categorized into two stages. Furthermore, it is discovered that the variation in the horizontal penetration is enhanced by the cross-flow and injection pressure at the EOI. In addition, in terms of the COV of the optical thickness, the variation on the windward side is greater, and the cross-flow enhanced the variation in the spray tip region. Most of the Ins numbers of the spray characteristics range from 0.9 to 1.1 (±10%) in this study, and they can provide data support for improving the accuracy of empirical prediction equations and models.

Heat loss is one of the main causes of energy losses in modern direct injection diesel engines. This heat loss of the engine occurs during combustion, mainly due to the heat transfer between the impinging spray flame and the piston cavity wall. It is of more critical in small size engines. In order to decrease heat transfer, we need to examine the phenomenon of heat transfer through the combustion chamber walls more fully. To achieve this, we investigated the effects of flame impingement on transient heat flux to the wall. By using a constant volume vessel with a fixed impingement wall, the surface heat flux of the wall at the locations of spray flame impingement was measured with three thin film thermocouple heat flux sensors. The combined effect of impingement distance and injection pressure on the heat transfer was investigated parametrically. The results showed that an increase of injection pressure with longer impinging distance led to an increase in the heat transfer coefficient, which had a dominant effect on local heat flux compared with local temperature distribution. Moreover, we confirmed that the relation between Nusselt number and Reynolds number is a useful measure for describing the heat transfer phenomena in diesel combustion.

The turbulent pulsating flow and heat transfer in straight and 90° curved square pipes are investigated in this study. Both experimental temperature field measurements at the cross-sections of the pipes and conjugate heat transfer (CHT) simulation were performed. The steady turbulent flow was investigated and compared to the pulsating flow under the same time-averaged Reynolds number. The time-averaged Reynolds number of the pulsating flow, as well as the steady flow, was approximately 60,000. The Womersley number of the pulsating flow was 43.1, corresponding to a 30 Hz pulsating frequency. Meanwhile, the Dean number in the curved pipe was approximately 31,000. The results showed that the local heat flux of the pulsating flow was greater than that of the steady flow when the location was closer to the upstream pulsation generator. However, the total heat flux of the pulsating flow was less than that of the steady flow. Moreover, the instantaneous velocity and temperature fields of the simulation were used to demonstrate the heat transfer mechanism of the pulsating flow. The behaviors, such as the obvious separation between the air and pipe wall, the low-temperature core impingement, and the reverse flow, suppress the heat transfer.

It is well known that the liquid adhesion by impingement in the direct-injection spark-ignition (DISI) engine hinders the engine combustion efficiency and increases the particular matter (PM) emissions. Although numerous investigations were done on it, it is still blur for the scholars owing to the complicated droplets-wall dynamics. Due to the dense liquid near the wall, it is difficult to observe the impinging droplets in this region, let alone analyzing it on the micro level. Therefore, the “multiple droplets producer” was applied to cut the spray for clear observation in this study. Tslicer was finally determined at 40 μm to make the multiple droplets impacting on the wall at the near-wall region under various injection pressure from 10 to 30 MPa during the quasi-steady state. Four different locations of (15, 15), (17, 15), (20, 15), (22, 15) were selected along the jet development after impingement. Particle image analysis (PIA) technology was applied to capture the micro behaviors. The diameter and velocity of droplets were calculated and analyzed as well as the Weber number. Results show that droplet behaviors near the wall can be recorded at Tslicer = 40 μm during the injection. Droplet size becomes larger with spray propagation at the near-wall region. Observations demonstrate that the coalescence phenomenon of the secondary droplets as well as the splashing crown structures occurrence leads to the larger droplet and lower velocity. Moreover, non-dimensional parameter We was used to further prove the splashing transition occurring from (17, 15) to (20, 15). Additionally, the experimental results can provide strong evidence and verification basis for computational fluid dynamics (CFD) simulations.

Methods of decreasing the CO2 emissions of the internal combustion engine have been suggested. For example, an engine can be designed with a high compression ratio and/or a downsizing turbocharger. However, these methods generate high combustion temperatures that increase the heat load. The piston cooling gallery has been proposed as a system for cooling the engine piston. The piston cooling gallery is an oil flow path that is set internal to the piston. An oil jet injected from a nozzle placed under the piston flows into the piston cooling gallery through an entrance hall. It may thus be desirable to control the shape of the oil jet such that it is stable and straight. However, the interface of the ambient air and oil jet may have unstable waviness because of Kelvin- Helmholtz instability and/or Rayleigh-Taylor instability. In addition, we investigated the flow and found that the propagation of the flow speed fluctuation of the nozzle internal flow results in the waviness of the oil jet in a previous study. To further clarify the relationship between oil jet interface instability immediately after nozzle exit and flow in nozzle, this paper reports on two types of particle image velocimetry (PIV), namely two-dimensional two-velocity-component PIV and two-dimensional three-velocity-component PIV, in addition to two-component and three-component snapshot proper orthogonal decompositions, and analyzes turbulence propagation adopting a cross-correlation method. We find a characteristic basis vector with large energy that propagates the fluctuation downstream under the condition that the interface between the oil jet and air has strong waviness.

Much attention has recently been given to high-efficiency cooling of pistons in internal combustion engines by cooling channels because of improved thermal efficiency. Cooling a piston efficiently requires a grasp of the gas-liquid multiphase flow state. However, because the magnitude and direction of the inertial force applied to the piston change depending on the crank angle, the flow field in the cooling channel that forms a complex gas-liquid multiphase flow remains a problem. Therefore, we developed a rig test apparatus simulating the reciprocating motion of a piston and visualized internal flows in a clear acrylic channel using a high-speed camera. This paper examines the effects of the Reynolds number of the oil jet and oscillation frequency of the reciprocating motion on flow characteristics in a right circular cylindrical channel. The Reynolds number and oscillation frequency were tested in the ranges 1000 to 2500 and 0 to 8.33 Hz, respectively. We found that the oil flow pattern in the channel forms the complex air-oil multiphase flow via air entrainment caused by the collision between the oil jet and the channel oil at the inlet. The gas phase area ratio in the channel increases with increasing Reynolds number, but its fluctuation is dominated by oscillation frequency. The fluctuation of the gas phase center of gravity becomes larger because of increases in inertial force with increases in oscillation frequency. The average bubble diameter in the channel decreases with increasing Reynolds number and oscillation frequency. We found that bubbles of small diameter are generated because the interfacial fluctuation of the oil jet increases as the jet goes downstream.

A pulsatile turbulent flow within an S-shaped double bend pipe is experimentally and numerically studied to characterize the flow field in conditions resembling an automotive engine environment. Particle image velocimetry (PIV) measurements were carried out to measure streamwise and secondary flow velocities. The flows are accelerated around the inner side walls of both bends. The secondary flow, after passing through the second bend, is directed toward the inner side in the core of the cross section, and, as a result, Lyne-type vortices, which are not consistent with the second bend curvature, are formed. A numerical simulation is performed under the same condition as the experiments with computational fluid dynamics software. The numerical simulation gives qualitative results in comparison with the experimental data though there is some deviation, and shows the cause of the Lyne-type vortex formation in the second bend. After passing through the first bend, the high-speed region appearing around the inner side shifts in accordance with the Dean-type secondary flow formed in the first bend, and thus the non-uniform flow enters the second bend. In the second bend, the low-velocity region in which the centrifugal force is not strong enough to direct the flow toward the outer side, appears in the core of the cross section. Details of the Lyne-type vortex formation are discussed by considering the driving forces of the secondary flow.

Herein, three oscillating fin propulsors connected in a parallel and in a staggered arrangement were investigated to develop an environmental-friendly and energy-saving propulsor. In this study, the flow pattern induced by these fish-like propulsors was achieved with a low Reynolds number and was measured using the particle image velocimetry (PIV) technique. In the case of the staggered fin setting (front: 1, rear: 2), which showed a relatively high propulsion performance, the flow pattern detected by multi-sliced 2D PIV was found to be a jet-like contracting flow pattern. On the other hand, results obtained for the parallel setting and inverted triangle formation (front: 2, rear: 1) showed branching flow patterns in the horizontal direction. The results for the jet-like contracting flow patterns suggested the existence of dominant flow patterns, providing a high thrust force with low power relative to conventional fin propulsors.

The present study discusses an acceleration of a swimming killifish-like small fish by fluid force, using three-dimensional simulations with CIP method in combination with an immersed boundary method. Simulation results have indicated that time variation of a small fish speed in small Reynolds number can be estimated as a free fall model of a sphere, that is, the fluid force can be separated into positive part (thrust) and negative part (viscous drag). Thrust is almost independent of Reynolds number, but a coefficient of viscous drag is a decreasing function of Reynolds number when a swimming motion of a fish with a shape is determined.

The present experimental study deals with a pulsatile turbulent flow simulating the exhaust flow of an automotive engine. In the experiments, a four-cylinder engine is used as an exhaust-flow generator to realize flow conditions close to those in an engine environment. Particle image velocimetry (PIV) measurements visualize the flow field in an S-shaped double-bend duct at a Reynolds number of 48,000 and a Womersley number of 70.9. Stereo PIV, which is classified as a two-dimensional three-component measurement, is conducted in the duct cross sections located downstream of the bends. The stereo PIV system is synchronized with the engine operation to enable phase-locked measurements at particular phases, and the phase-averaged results show the large-scale vortical structures and the duct axial velocity distribution. Downstream of the first bend, the secondary flow consists of vortices that circulate as Dean-type vortices. Downstream of the second bend, by contrast, vortices that circulate in opposite directions to the Dean-type vortices, so-called Lyne-type vortices, form in the core of the cross section. These secondary flows persist without significant changes in their large-scale vortical structures over time. Time-resolved PIV is conducted to track the temporal evolution of the flow in the bend planes. The results show that the flow reverses locally along the inner wall of the bends during flow deceleration. Snapshot proper orthogonal decomposition (POD) is used on the time-resolved PIV data to extract the significant flow structure from the instantaneous field. We propose POD as a good post-processing tool for the instantaneous data of pulsatile cases.

The high efficiency cooling of pistons in internal combustion engines by oil jets has recently received widespread attention. To date, curved circular oil jet nozzles have been commonly employed. However, although liquid jets from straight circular nozzles have been studied, there are few reports regarding the characteristics of liquid jets injected by curved circular nozzles. The present work therefore examined the characteristics of oil jets, such as jet width and interfacial fluctuations, following injection via either a 90° curved circular nozzle or straight circular nozzle into a stationary atmosphere. Oil jet parameters were assessed at Reynolds numbers from 1000 to 3000 and the jet interfaces were visualized using a background illumination method in conjunction with a high-speed camera. The oil jet injected from the curved nozzle exhibits more complex behavior compared with that from the straight nozzle. The cross section of the jet from the curved nozzle perpendicular to the direction of movement gradually expanded from a circular shape to an elliptical shape in the downstream direction. The side corresponding to the long diameter of this elliptical jet shape was also observed to switch depending on the Reynolds number. At the inner and outer side interfaces, the outer side interface became larger than the inner side interface at the Reynolds number of 1500 or less. However, the inner side interface became greater than the outer side interface at the Reynolds number of 2000 or more. The effects of the velocity distribution following the bend in the curved nozzle on oil jet behavior were examined by stereoscopic particle image velocimetry. The nozzle internal flow evidently generated movement from the outer side toward the inner side at the Reynolds number of 2000, while flow toward the inner side was dominant when the Reynolds number was 2500.