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Improving the thermal efficiency of internal combustion engines requires a high compression ratio, a high thermal load, and high mechanical strength in the combustion chambers. However, they will promote excessive temperatures within the combustion chamber and increase the likelihood of knocking. As the piston reciprocates, the engine oil injected into the piston crown oil gallery can oscillate within the gallery to reduce the piston temperature. However, the interaction between the ambient air and the interface of the axial jet liquid column results in unstable fluctuations, significantly affecting the filling rate of the oil gallery. In this study, the fluctuation of the jet interface is analyzed by changing the Reynolds number (Re) and bending radius R of the nozzle. Two-dimensional/two-component and two-dimensional/three-component particle image velocimetry are used to measure the flow fields in the longitudinal section of two bent nozzles with small (R = 15 mm) and large (R = 60 mm) curvature radii and horizontal cross-sections at 4D, 2.5D, and 1D from the exit of each bent nozzle, respectively, to explore the causes of the fluctuating internal flow. By calculating the secondary flow intensity Se on the inner and outer wall sides of the nozzle, the inner and outer jet interface width changes, and the different fragmentation mechanisms of the jet interface, it is shown that the higher frequency of the high-speed core region at the outer wall side of the nozzle interior can drive the waveform on the outside of the jet interface to break and produce small-diameter droplets easily.

The Conventional Laser Absorption Scattering (C-LAS) technique is used to measure the mixture concentration and visualize the vapor phase. The former is determined by the attenuation of visible and ultraviolet light whereas the latter is achieved via light absorption and scattering theory. The C-LAS uses the Nd: YAG pulsed laser and CCD cameras to provide one spray shot at a particular instance which requires time and effort. However, the temporal measurement of a single spray shot is not possible. To record the distribution of the whole vapor phase in an injection event and measure liquid and vapor concentrations inside the spray, a High-Speed Laser Absorption Scattering (HS-LAS) technique was developed. The HS-LAS consists of continuous diode light sources, high-speed video cameras, and an image intensifier for UV light, which can provide the temporal variation of a single-shot spray. In the experiment, a commercial seven-hole injector with a hole diameter of 0.123 mm allowing high injection pressure of up to 100 MPa was used to avoid the potential inconsistencies with a single-hole test injector. The diesel surrogate fuel which consists of 97.5% n-tridecane and 2.5% of volume-based 1-methylnaphthalene was used. The injection amount of 5.0 mg/hole was selected to investigate the structure and mixture formation process of the spray. The findings of the experiments show that this imaging approach is a promising diagnostic technique for concurrently obtaining quantitative information on the quantity of vapor and droplets in a fuel spray. Furthermore, the turbulent/vortex fluid dynamics’ temporal development/variation can be investigated.

This paper investigates the effects of the split-injection strategy with a negative dwell time using a multi-hole diesel injector under evaporating conditions. To compare with negative dwell conditions, the single-injection and positive and zero dwell conditions were adopted. The Laser Absorption Scattering (LAS) technique was employed to measure the temporal and spatial evolution of fuel evaporation and mixture concentration in the experiments. The split injection of 7:3 was selected according to the results of the previous study that proposed a homogeneous mixture after the end of injection. The dwell times of −50 us, 0 us, and 120 us were selected based on the measurement of the injection rate. The blended fuel consisting of 97.5% of n-tridecane and 2.5% of 1-methylnaphthalene in volume was used. The results show that a shorter spray tip penetration was present in the split injection with a negative dwell time. This phenomenon is expected to reduce the possibility of wall impingement and result in fewer nitrogen oxides (NOx) and unburned hydrocarbons (UHCs) under the injection conditions with a negative dwell time, especially for the conditions of a large injection amount and small-bore engines. Moreover, different spray shapes and behavior occurred in the case of different injection strategies. The spray of split-injection strategy with a negative dwell time has large radial dispersion, and it can improve the air utilization inside a cylinder.

This study evaluates the heat dissipation and Nusselt number for an S-shaped double-bend pipe, for which an experimental evaluation is lacking. In terms of the velocity field, the mean velocity and turbulent kinetic energy were measured through particle image velocimetry. Heat transfer characteristics were evaluated in validated conjugate heat transfer simulations, and a k-ω SST turbulence model was used for flow simulation inside the pipe. Heat transfer enhancement was observed at the first bend, as observed in previous studies on single-bend and U-shaped bends, whereas no heat transfer enhancement was observed at the second bend. This result was due to higher turbulent heat flux at the first bend because of higher eddy diffusion on the outside of the bend, whereas eddy diffusion was lower on the outside of the second bend owing to the history of the first bend. The heat transfer characteristics of the S-shaped double-bend pipe elucidated in this study provide valuable insight for devising strategies to reduce heat loss in automotive exhaust pipes with multiple bends. Furthermore, the conjugate heat transfer simulation model used in this study provides a benchmark for heat transfer calculations for multi-bend pipes.

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.

Liquid fuel-only injection into combustion air is widely used in gas turbine combustors for power generation, particularly in large capacity type. This combustion air goes by the name of ‘crossflow’. For the liquid fuel-only injection, breakup process, trajectory of liquid jet and droplet sizes have been extensively investigated. However, there remain much space to be devised for improvement in atomization. In the present study, twin-fluid injection is introduced instead of liquid-only injection into a gas turbine combustor, where atomization air is supplied through the side port and mixed with liquid in the mixing port at right angle, and two phases mixture is injected into the crossflow. A simple-structured twin-fluid atomizer is designed. An experimental study was conducted to compare the behaviours between liquid-only and twin-fluid injected transversely into a crossflow. High-speed photography technique is used to clarify the atomization characteristics of jet/spray. One feature of twin-fluid injection is that annular liquid film appears at the exit of the mixing port, and the film easily disintegrates by aerodynamical force of the crossflow. The jet/spray of the twin-fluid injection is not blown away easily by crossflow, and flow across the crossflow. Furthermore, the jet/spray of the twin-fluid spread and droplets disperse more widely in the crossflow. In addition, the twin-fluid injection produces numerous fine droplets and few coarse droplets.

In general, technical methods for improving the thermal efficiency of an engine increase the heat load on peripheral components. Recently, a piston cooling gallery equipped with a flow path has been developed. The engine oil is supplied an oil jet from the nozzle, which is placed under the piston to the piston gallery entrance hall. The nozzle of the oil jet is curved to minimize its size, and the jet interface between ambient air and oil fluctuates near the nozzle exit owing to the shape. Few studies have investigated the behavior of oil jets ejecting from curved pipes. We therefore investigated the flow in two nozzles having a basic bend of 90° with radii of curvature of 15 and 60 mm. Our results clarify the effect of internal flow on the ejecting oil jet behavior. Silicone oil was used as the working fluid. The kinematic viscosity of the silicone oil at 298 K was similar to that of engine oil at 353 K. The behavior of the oil jet was investigated by visualization using background light. A light-emitting-diode displacement meter was installed to measure the jet width. We found that the width of the oil jet increased on the downstream side with large fluctuation of the interface under the condition of a small radius of curvature and large Reynolds number. Furthermore, we time-synchronously measured flow in the nozzle, two-dimensional two-component time-resolved particle image velocimetry, and visualization of the jet. The Reynolds number was set from 1000 to 3000, which is close to that of the engine oil jet. The oil flow velocity in the nozzle fluctuated in the radial direction. The fluctuation became strong under the condition of a small radius and large Reynolds number. The fluctuation propagation speed calculated from the correlation coefficient was as high as the flow speed itself. Furthermore, the jet interface fluctuation speed in the flow direction was as high as the fluctuation propagation speed in the nozzle. Our results demonstrate that the cause of the interface fluctuation is the fluctuation propagation of flow in the nozzle.

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.

This paper discusses the turbulent flow and heat transfer from a uniform air flow with high temperature to the outside through a 90° 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° 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 × 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-ε 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.

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.

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.

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.

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.

The present paper discusses acceleration in the swimming of a small three-dimensional fish with two motions, carangiform and anguilliform. Flow fields generated by fish deformations are investigated numerically by the constrained interpolation profile method in combination with an immersed boundary method. The three-dimensional vortical structure visualized using a second invariant and the pressure field around the fish body show that a fish with anguilliform motion accelerates more rapidly than one with carangiform motion because of a larger thrust due to the strong transverse vortex in the wake of the fish and a large pressure variation around the fish body. It is also found that the time variations of inline swimming speed of a small fish and the fluid force acting on it can be estimated using a free-fall model, and the fluid force can be expressed by a linear function of the fish speed. This function consists of a thrust part that is independent of fish speed and a viscous drag part that is proportional to fish speed. Thus, time histories of swimming speed, swimming distance, and fluid force can be predicted by simple functions from rest to terminal speed.