An application of a laser technique to the velocity measurement of the liquid-fuel film within intake pipe of internal combustion engines

A new experimental technique is developed by combining a flash photolysis method with a laser-photodiode system to measure the velocity of the flow of thin liquid films. The technique is applied to the measurement of the velocity of the liquid-fuel film within the intake pipe of an internal combustion engine operated under firing condition. The results show that the velocity of the fuel film is of the order of 1/100 of the mean air velocity inside the intake pipe.     

On the Evolution of the Flow Field in a Spark Ignition Engine

The development of the turbulent flow field inside a spark ignition engine is examined by large-eddy simulation (LES), from the intake flow to the tumble break-down. Ten consecutive cold flow engine cycles on a coarse and twenty cycles on a fine grid are simulated and compared to experiments of the same engine. The turbulent subgrid scales are modeled by the standard Smagorinsky and by the recently developed Sigma model. A comparison of the intake flow is made against Particle Image Velocimetry (PIV) measurements along horizontal and vertical lines and to an LES simulation performed by the Darmstadt group. Furthermore, we show the first LES comparison to Magnetic Resonance Velocimetry (MRV conducted by Freudenhammer et al.) measurements, which provided the 3D flow field inside a full scale dummy of the entire upper cylinder head including the valve seat region, at a time which mimics inflow conditions of the corresponding engine. Our LES is in good qualitative and quantitative agreement with the simulation and the experiments, with the notable exception of the measured in-cylinder pressure, which is discussed in detail and compared to 0D simulations and simulations from other groups. A criterion is proposed for estimating the number of cycles needed in a simulation, if experimental data is available. We put emphasis on the flow in the valve seat region, where turbulence is generated, and discuss the formation of the large scale tumble motion, including a comparison of the radial velocity fields on rolled-up planes around the valve seat. Here, spots of high velocities were found in the under flow region, which cannot been seen by the ensemble averaged MRV measurement. Within the compression stroke, a 2D vortex center identification algorithm is applied on slices inside the combustion chamber, yielding a 3D visualization of the tumble vortex, which is found to have a “croissant-like” shape. The tumble vortex trajectory is plotted on the symmetry plane and compared to measurements. Finally, we consider a modified definition of the (turbulent) integral length scale that provided further insight to the tumble break-down process.     

Numerical Analysis of Laminar-Turbulent Transition in a Circular Pipe with Periodic Inflow Perturbations

The problem of the spatio-temporal evolution of perturbations introduced into the inlet cross-section of a circular pipe is solved numerically. The case of time-periodic inflow perturbations is considered for Re = 4000. It is shown that for relatively small inflow perturbations periodic flow regimes and for greater perturbations chaotic regimes are established.Periodic regimes the flow is a superposition of steady flow and a damped wave propagating downstream. The velocity profile of the steady component differs essentially from both the parabolic Poiseuille and developed turbulent flows and is strongly inhomogeneous in the angular direction. The angular distortion of the velocity profile is caused by longitudinal vortices developing as a result of the nonlinear interaction of inflow perturbations.Chaotic flow regimes develop when the amplitude of the inflow perturbations exceeds a certain threshold level. Stochastic high-frequency pulsations appear after the formation of longitudinal vortices in the regions of maximum angular gradient of the axial velocity. In the downstream part of the flow, remote from the transition region, the developed turbulent regime is formed. The distributions of all the statistical moments along the pipe level off and approach the values measured experimentally and calculated numerically for developed turbulent flows.     

Vortical patterns in turbulent flow downstream a 90° curved pipe at high Womersley numbers

The present experimental work focuses on highly pulsatile, i.e. inertia dominated, turbulent flow downstream a curved pipe and aims at investigating the vortical characteristics of such a flow. The flow parameters (Dean and Womersley number) investigated are of the same order as those met in the internal combustion engine environment. The technique employed is time-resolved stereoscopic particle image velocimetry at different cross-sections downstream the pipe bend. These measurements allow the large-scale structures that are formed to be analyzed by means of proper orthogonal decomposition. The flow field changes drastically during a pulsatile cycle, varying from a uniform flow direction across the pipe section from the inside to the outside of the bend to vortical patterns consisting of two counter-rotating cells. This study characterizes and describes pulsatile curved pipe flow at Womersley numbers much higher than previously reported in the literature. Furthermore, the oscillatory behaviour of the Dean cells for the steady flow – the so-called ‘swirl switching’ – is investigated for different downstream stations from the bend exit and it is shown that this motion does not appear in the immediate vicinity of the bend, but only further downstream.     

Resistance of a circular pipe with pulsatory variation of the flow rate

The results of an experimental investigation of the hydraulic resistance of a circular pipe for turbulent flow with periodic flow rate fluctuations are presented. The presence of resonance phenomena in the pipe is revealed. It is established that, for hydrodynamic nonstationarity, the pipe resistance is a nonmonotonous function of the frequency of the imposed flow rate fluctuations and differs from the pipe resistance in the stationary flow regime. Under the conditions considered, to find the pipe resistance it is necessary to take into account the variation of the flow kinetic energy with respect to the phase of the imposed flow rate fluctuations due to the deformation of the velocity profile.     

The Effect of Fuel-Injection Timing on In-cylinder Flow and Combustion Performance in a Spark-Ignition Direct-Injection (SIDI) Engine Using Particle Image Velocimetry (PIV)

This study applies particle image velocimetry (PIV) to an optical spark-ignition direct-injection engine in order to investigate the effects of fuel-injection on in-cylinder flow. Five injection timing combinations, each employing a stoichiometric 1:1 split ratio double-injection strategy, were analysed at an engine speed of 1200 RPM and an intake pressure of 100 kPa. Timings ranged from two injections in the intake stroke to two injections in the compression stroke, resulting in a variety of in-cylinder environments from well-mixed to highly turbulent. PIV images were acquired at a sampling frequency of 5 kHz on a selected swirl plane. The flow fields were decomposed into mean and fluctuating components via two spatial filtering approaches — one using a fixed 8 mm cut-off length, and the other using a mean flow speed scaled cut-off length which was tuned in order to match the turbulent kinetic energy (TKE) profile of a 300 Hz temporal filter. From engine performance tests, the in-cylinder pressure traces, indicated mean effective pressure (IMEP), and combustion phasing data showed very high sensitivity to injection timing variations. To explain the observed trend, correspondence between the measured flow and these performance parameters was evaluated. An expected global trend of increasing turbulence with retarded injection timing was clearly observed; however, relationships between TKE and burn rate were not as obvious as anticipated, suggesting that turbulence is not the predominant factor associated with injection timing variations which impacts engine performance. Stronger links were observed between bulk flow velocity and burn rate, particularly during the early stages of flame development. Injection timing was also found to have a significant impact on combustion stability, where it was observed that low-frequency flow fluctuation intensity revealed strong similarities with the coefficient of variance (CoV) of IMEP, suggesting that these fluctuations are a suitable measure of cycle-to-cycle variation — likely due to the influence of bulk flow on flame kernel development.     

Laminar/turbulent oscillating flow in circular pipes

A two-dimensional oscillating flow analysis was conducted simulating the gas flow inside Stirling engine heat exchangers. Both laminar and turbulent oscillating pipe flow were investigated numerically for Re_max = 1,920 (Va = 80), 10,800 (Va = 272), 19,300 (Va = 272), and 60,800 (Va = 126). The results are here compared with experimental results of previous investigators. Predictions of the flow regime on present oscillating flow conditions are also checked by comparing velocity amplitudes and phase difference with those from laminar theory and quasi-steady profile. A high Reynolds number k-ε turbulence model was used for turbulent oscillating pipe flow. Finally, the performance of the k-ε model was evaluated to explore the applicability of quasi-steady turbulent models to unsteady oscillating flow analysis.     

High-resolution turbulent scalar field measurements in an optically accessible internal combustion engine

High-resolution planar laser-induced fluorescence (PLIF) measurements were performed in an optically accessible internal combustion engine to investigate the evolution of the turbulent mixing process during the intake and compression strokes. The PLIF measurements were used to analyze the important turbulent length scales, scalar energy and dissipation spectra, and mean scalar gradients. The fluorescence images had sufficient spatial resolution and integrity to resolve all of the fine-scale features of the flow, allowing for direct determination of the Batchelor length scale. The integral and Taylor scales were also determined from two-point spatial correlations of the fluctuating scalar field using an appropriately defined mean scalar value. The general morphology of the scalar field and the measured integral, Taylor and Batchelor length scales were found to be largely independent of engine speed and intake pressure, but increased as the engine cycle progressed through the intake and compression strokes. The measured Batchelor scales ranged from 22 to 54 μm; the integral scales ranged from 1.8 to 3.5 mm; and the Taylor microscales ranged from 0.6 to 1.2 mm. The Taylor and integral scale values were comparable to values reported in the literature from in-cylinder velocity measurements. The mean scalar gradient, a measure of the fine-scale mixing rate, monotonically decreased as the engine cycle advanced. High-resolution measurements of this type are important in the development and validation of future engine combustion models used in computer simulations.     

Evaluation of the non-steady flow produced by intake ports of direct injection Diesel engines

The most usual way to characterize a D.I. Diesel engine cylinder head is based on steady flow tests with fixed pressure drop across the valve and at different valve lifts. A discharge coefficient and a swirl number are defined, which are representative of the breathing capacity and angular velocity generation of the intake system. A question arising is the validity of such parameters in non-steady conditions, with time scales similar to those of the firing engine, where the valve is moving and the pressure drop across the valve is time dependent. Experimental tests were conducted both in steady and non-steady flow test rigs in order to assess the quasi-steady assumption in terms of the mass flow rate across the valve, as well as swirl produced by the intake port. Time resolved laser-Dopplervelocimetry was used, together with an extension of a conventional test flow rig to non-steady operation.The authors wish to thank the Conselleria de Educación y Cultura de la Generalidad de Valencia for the support in the acquisition of the LDA instrumentation. The authors acknowledge that part of the research work has been performed in the frame of the JOULE project JOUE-CT93.     

Evolution of turbulence characteristics from straight to curved pipes

Fully developed, statistically steady turbulent flow in straight and curved pipes at moderate Reynolds numbers is studied in detail using direct numerical simulations (DNS) based on a spectral element discretisation. After the validation of data and setup against existing DNS results, a comparative study of turbulent characteristics at different bulk Reynolds numbers Re_b = 5300 and 11,700, and various curvature parameters κ = 0, 0.01, 0.1 is presented. In particular, complete Reynolds-stress budgets are reported for the first time. Instantaneous visualisations reveal partial relaminarisation along the inner surface of the curved pipe at the highest curvature, whereas developed turbulence is always maintained at the outer side. The mean flow shows asymmetry in the axial velocity profile and distinct Dean vortices as secondary motions. For strong curvature a distinct bulge appears close to the pipe centre, which has previously been observed in laminar and transitional curved pipes at lower Re_b only. On the other hand, mild curvature allows the interesting observation of a friction factor which is lower than in a straight pipe for the same flow rate.All statistical data, including mean profile, fluctuations and the Reynolds-stress budgets, is available for development and validation of turbulence models in curved geometries.     

Stereoscopic multi-planar PIV measurements of in-cylinder tumbling flow

The non-reacting flow field within the combustion chamber of a motored direct-injection spark-ignition engine with tumble intake port is measured. The three-dimensionality of the flow necessitates the measurement of all three velocity components via stereoscopic particle-image velocimetry in multiple planes. Phase-locked stereoscopic PIV is applied at 15 crank angles during the intake and compression strokes, showing the temporal evolution of the flow field. The flow fields are obtained within a set of 14 axial planes, covering nearly the complete cylinder volume. The stereoscopic PIV setup applied to engine in-cylinder flow and the arising problems and solutions are discussed in detail. The three-dimensional flow field is reconstructed and analyzed using vortex criteria. The tumble vortex is the dominant flow structure, and this vortex varies significantly regarding shape, strength, and position throughout the two strokes. The tumble vortex center moves clockwise through the combustion chamber. At first, the tumble has a c-shape which turns into an almost straight tube at the end of the compression. Small-scale structures are analyzed by the distribution of the turbulent kinetic energy. It is evident that the symmetry plane only represents the 3D flow field after 100 CAD. For earlier crank angles, both kinetic energy (KE) and turbulent kinetic energy (TKE) in the combustion chamber are well below the KE and TKE in the symmetry plane. This should be taken into account when the injection and breakup of the three-dimensional fuel jet are studied. The mean kinetic energy is conserved until late compression by the tumble motion. This conservation ensures through the excited air motion an enhancement of the initial air-fuel mixture which is of interest for direct-injection gasoline engines.     

Modification of equation of motion of fluid-conveying pipe for laminar and turbulent flow profiles

Considering the non-uniformity of the flow velocity distribution in fluid-conveying pipes caused by the viscosity of real fluids, the centrifugal force term in the equation of motion of the pipe is modified for laminar and turbulent flow profiles. The flow-profile-modification factors are found to be 1.333, 1.015–1.040 and 1.035–1.055 for laminar flow in circular pipes, turbulent flow in smooth-wall circular pipes and turbulent flow in rough-wall circular pipes, respectively. The critical flow velocities for divergence in the above-mentioned three cases are found to be 13.4%, 0.74–1.9% and 1.7–2.6%, respectively, lower than that with plug flow, while those for flutter are even lower, which could reach 36% for the laminar flow profile. By introducing two new concepts of equivalent flow velocity and equivalent mass, fluid-conveying pipe problems with different flow profiles can be solved with the equation of motion for plug flow.     

Local and global bifurcations of valve mechanism

In this paper we study in detail problems of nonlinear oscillations of valve mechanism at internal combustion engine. The practical measurement indicates that stiffness of valve mechanism is not constant but is a function of the rotational angle of the cam. For simplicity of analysis we replace valve mechanism of internal combustion engine with a nonlinear oscillator of single degree of freedom under combined parametric and forcing excitation. We use the method of multiple scales and normal form theory to study local and global bifurcations of valve mechanism at internal combustion engine.     

Turbulent pipe flow downstream a 90° pipe bend with and without superimposed swirl

In the present work, the turbulent flow downstream a 90° pipe bend is investigated by means of stereoscopic particle image velocimetry. In particular, the three dimensional flow field at the exit of the curved pipe is documented for non-swirling and swirling flow conditions, with the latter being generated through a unique axially rotating pipe flow facility. The non-swirling flow was examined through snapshot proper orthogonal decomposition (POD) with the aim to reveal the unsteady behaviour of the Dean vortices under turbulent flow conditions, the so-called “swirl-switching” phenomenon. In respect to the swirling turbulent pipe flow, covering a wide range of swirl strengths, POD has been employed to study the effect of varying strength of swirl on the Dean vortices as well as the interplay of swirling motion and Dean cells. Furthermore, the visualised large scale structures in turbulent swirling flows through the bend are found to incline and tear up with increasing swirl intensity. The present time-resolved, three component, experimental velocity field data will provide a unique and useful database for future studies; in particular for the CFD community.     

Holographic particle image velocimetry applied to the flow within the cylinder of a four-valve internal combustion engine

A holographic particle-image velocimetry (HPIV) system is developed to investigate the in-cylinder air flow in a motored four-valve engine operated at 1,500 rpm. Image aberrations introduced by the optical liner of the engine are optically eliminated. The use of a reference hologram to compensate for errors induced by fine reference beam misalignments is described. The remaining errors are quantitatively discussed in the text. The application of a wavelength selected laser diode for hologram reconstruction is discussed. High-resolution velocity measurements of the in-cylinder flow are made in axial planes during the intake and compression stroke. Prospects and limitations to full three-dimensional extensions of the HPIV system are discussed. The results show with emphasis on large- and small-scale flow structures the HPIV system to be a reliable diagnostic tool for internal combustion engines.     

Cycle resolved multi-planar flow measurements in a four-valve combustion engine

The non-reacting flow in a one-cylinder four-valve combustion engine is measured via cycle resolved two-component/two-dimensional (2C/2D) particle-image velocimetry (PIV). The three-dimensional structure of the velocity field is analyzed based on the flow field measured in eight planar planes within the cylinder for several crank angles during the intake and compression phase. Using the mean and statistical values of the single planes quasi three-dimensional flow fields are reconstructed for crank angles of 80°, 160°, and 240° atdc. This enables the detailed analysis of the spatial distribution of the large and small scale flow structures, e.g., by visualizing large vortical structures and the distribution of the turbulent kinetic energy. It was found that two ring vortices evolving beneath the inlet valves are the dominant large scale structures that seem to be of major concern for the mixing process in the cylinder of a four-valve combustion engine operated at 1500 rpm. Furthermore, the temporal evolution of the flow field within the symmetry plane of the cylinder, measured for crank angles between 40° and 320° atdc in steps of 20°, is discussed. The results give new insight into the complex three-dimensional flow in the combustion chamber of a one-cylinder four-valve combustion engine. That is, the tumble vortex only seems to be of secondary importance for the flow concerning the mixing process at 1500 rpm. This is an essential result for future work considering the fluid mechanics of fuel-air-interaction processes and mixing principles in combustion engines.     

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

Shallow cavity flow tone experiments: onset of locked-on states

Fully turbulent inflow past a shallow cavity is investigated for the configuration of an axisymmetric cavity mounted in a pipe. Emphasis is on conditions giving rise to coherent oscillations, which can lead to locked-on states of flow tones in the pipe–cavity system. Unsteady surface pressure measurements are interpreted using three-dimensional representations of amplitude–frequency, and velocity; these representations are constructed for a range of cavity depth. Assessment of these data involves a variety of approaches. Evaluation of pressure gradients on plan views of the three-dimensional representations allows extraction of the frequencies of the instability (Strouhal) modes of the cavity oscillation. These frequency components are correlated with traditional models originally formulated for cavities in a free-stream. In addition, they are normalized using two length scales: inflow boundary-layer thickness and pipe diameter. These scales are consistent with those employed for the hydrodynamic instability of the separated shear layer, and are linked to the large-scale mode of the shear layer oscillation, which occurs at relatively long cavity length. In fact, a simple scaling based on pipe diameter can correlate the frequencies of the dominant peaks over a range of cavity depth.The foregoing considerations provide evidence that pronounced flow tones can be generated from a fully turbulent inflow at very low Mach number, including the limiting case of fully developed turbulent flow in a pipe. These tones can arise even for the extreme case of a cavity having a length over an order of magnitude longer than its depth. Suppression of tones is generally achieved if the cavity is sufficiently shallow.     

Pulsating turbulent pipe flow in the current dominated regime at high and very-high frequencies

The paper presents Direct Numerical Simulations of sinusoidal pulsating turbulent flow, at low bulk Reynolds numbers, with high frequency, in a straight pipe. Our objective is to study pulsating flow considering it as the superposition of a temporal unsteadiness on a mean current, and from this viewpoint, to decompose the flow in a mean and an oscillating part. Firstly, we examine the time-averaged statistics, which show that the parent flow retains its properties. Then, we analyze the oscillating part of the flow, and confirm the notion that for rapidly pulsating flow, the amplitude of the streamwise velocity and the phase lag at different radial locations follow the solution of the laminar Stokes problem. In addition, we find that the modulation of the turbulent fluctuations follows approximately the sinusoidal form of the imposed pulsation, and that the ratio of the frequency parameter to the amplitude of the streamwise velocity can be used as a scaling factor. We investigate the effects of the amplitude and the frequency of the imposed unsteadiness on the modulation of the time-averaged properties and the turbulence statistics, through a systematic analysis. Finally, we examine the time evolution of the mean velocity and the turbulent fluctuations. These results indicate that a lower limit for the high frequency regime can be identified, based on the level of conformity of the phase-averaged profiles on their steady-state counterparts. For very high frequencies, we find that that the flow behavior does not change, indicating the absence of an upper limit for the high frequency regime.