David L. Reuss

Vorticity generation and attenuation as vortices convect through a premixed flame

Abstract A sequence of PIV images shows the time history of both the vorticity field and the velocity field as vortices of different strength convect through a premixed flame. The vortices represent individual eddies in turbulent flow; the goal is to understand how each eddy wrinkles the flame and how the flame also may alter the eddy. It is found that weak vortices are completely attenuated primarily due to volume expansion. Strong vortices do survive flame passage, but only if they can weaken the flame due to stretch effects. Intense flame-generated vorticity is measured which has a magnitude that exceeds that of the incident vortex in some cases. The flame-generated vorticity in the products induces a velocity field that tends to reduce the amplitude of flame wrinkling; thus it acts as an additional flame-stabilizing mechanism. This mechanism affects the wrinkling process and should be included in models. A new nondimensional vorticity enhancement parameter ( E ) is suggested as a way to estimate the effect of vortex size, strength, Reynolds number, and Froude number on vorticity attenuation and production. Measurements are made for E approximately equal to 0, −1, and −2, corresponding to no change in vorticity, total attenuation of the vortex, and flame-generated vorticity, respectively. Buoyancy forces are important in one case that is considered, but not in other cases. The results can be used to quantify the size of the small eddies that can be neglected in large eddy simulations; the role of small eddies is estimated in one example.

Temperature measurements in a radially symmetric flame using holographic interferometry

Abstract Holographic interferometry was used to measure the temperature distribution of a lean, laminar, radially symmetric, propane-air flame (φ = 0.55). The following steps were involved: A plane wave interferogram of the flame was created using double-exposure holographic interferometry; a scanning microphotometer was used to measure the position of the fringes in the interferogram; and the path-integrated refractive index distribution in the flame was calculated from the fringe position measurements. The temperature distribution in the flame was then calculated from the refractive index distribution assuming pressure and molecular weight of the gas mixture to be the same throughout the flame. In addition, a temperature profile of the flame was made using a thermocouple in order to obtain results for comparison with those obtained using holographic interferometry. Several findings resulted from this study. Frist, in the region of the flame where the thermocouple was relatively nonperturbing, the interferometrically determined burned gas temperature was withn 50K (3.5%) of that measured by the thermocouple. Second, the refractive index of the reference field must be known to within 2% of the true value to achieve that 50K accuracy. Third, the position of each fringe minimum should be known to within 1% of the local fringe spacing in regions where the fringes are widely spaced; however, the temperature distribution is less sensitive to fringe position errors when the fringes are closely spaced.

A practical guide for using proper orthogonal decomposition in engine research

Proper orthogonal decomposition has been utilized for well over a decade to study turbulence and cyclic variation of flow and combustion properties in internal combustion engines. In addition, proper orthogonal decomposition is useful to quantitatively compare multi-cycle in-cylinder measurements with numerical simulations (large-eddy simulations). However, the application can be daunting, and physical interpretation of proper orthogonal decomposition can be ambiguous. In this paper, the mathematical procedure of proper orthogonal decomposition is described conceptually, and a compact MATLAB® code is provided. However, the major purpose is to empirically illustrate the properties of the proper orthogonal decomposition analysis and to propose practical procedures for application to internal combustion engine flows. Two measured velocity data sets from a motored internal combustion engine are employed, one a highly directed flow (each cycle resembles the ensemble average), and the other an undirected flow (no cycle resembles the average). These data are used to illustrate the degree to which proper orthogonal decomposition can quantitatively distinguish between internal combustion engine flows with these two extreme flow properties. In each flow, proper orthogonal decomposition mode 1 is an excellent estimate of ensemble average, and this study illustrates how it is thus possible to unambiguously quantify the cyclic variability of Reynolds-averaged Navier–Stokes ensemble average and turbulence. In addition, this study demonstrates the benefits of comparing two different samples of cycles using a common proper orthogonal decomposition mode set derived by combining the two samples, the effect of spatial resolution, and a method to evaluate the number of snapshots required to achieve convergence.

On the use and interpretation of proper orthogonal decomposition of in-cylinder engine flows

The proper orthogonal decomposition (POD) has found increasing application for the comparison of measured and computed data as well as the identification of instantaneous and time varying flow structures, particularly cyclic variability in reciprocating internal combustion engines. The patterns observed in the basis functions or modes are sometimes interpreted as coherent structures, though justification of this is not obvious from the mathematical derivations. Similarly, there is no consensus about whether or not the ensemble mean should be subtracted prior to performing POD on a data set. Synthetic flow fields are used here to reveal POD properties otherwise ambiguous in real stochastic flow data. In particular, each POD mode includes elements of all flow structures from all input snapshots and in general, several modes are needed to reconstruct physical flow structures. POD analysis of two experimental in-cylinder engine data is done: one flow condition where every cycle resembles the ensemble-averaged flow pattern, and the other with large cyclic variability such that no cycles resemble the ensemble average. The energy and flow patterns of the POD modes, derived with and without first subtracting the mean, are compared to each other and to the Reynolds decomposed flow to reveal properties of the POD modes.

Particle Image Velocimetry Measurements in a High-Swirl Engine Used for Evaluation of Computational Fluid Dynamics Calculations

Two-dimensional in-cylinder velocity distributions measured with Particle Image Velocimetry were compared with computed results from Computational Fluid Dynamics codes. A high-swirl, two-valve, four-stroke transparent-combustion-chamber research engine was used. Comparisons were made of mean-flow velocity distributions, swirl-ratio evolution during the intake and compression strokes, and turbulence distributions at top-dead-center compression. This comparison with the measured flows led to more accurate calculations by identifying code improvements including swirl in the residual gas, modeling of the gas exchange during the valve overlap, and improved numerical accuracy. 14 refs., 14 figs.

Effects of refraction on axisymmetric flame temperatures measured by holographic interferometry.

Neglect of refraction can produce errors when temperature distributions in axisymmetric flames are reconstructed by Abel inversion of interferometric fringe data. This study quantifies these errors and their reduction by imaging during interferogram readout. Rays were traced through analytic temperature distributions characteristic of real flames at different equivalence ratios to determine the fringe patterns that would be observed interferometrically with and without imaging. The Abel inversion was applied to each computed fringe pattern to reconstruct the temperature distribution. Reconstructed and analytic distributions were compared to determine the error caused by using the Abel inversion. Our results indicate that proper imaging will generally be necessary to reduce reconstruction errors to below 5% in real axisymmetric flames.

Effects of unsteady stretch on the strength of a freely-propagating flame wrinkled by a vortex

The objective of this study is to experimentally examine the magnitude and rate at which local flame chemistry responds to unsteady changes in imposed stretch rate. To achieve this objective, a time series of velocity field images was obtained using particle image velocimetry (PIV) diagnostics during the unsteady stretching, wrinkling, and local extinction of a laminar premixed flame by a counter-propagating toroidal vortex. The ∼10,000 velocity vectors per image in both the burned and unburned gases enable the measurement of local flame stretch rates and dilatation rate fields. Dilatation rates peak in the flamefront where the gas expands as its temperature increases. A new method is employed wherein the measured peak dilatation rate at each flame segment is used as an indicator of the local flame strength. It is found that the flame requires a relatively long time to be weakened by positive stretch, yet it is rapidly strengthened by negative stretch. Specifically, in regions where positive stretch rates are several times greater than that required to extinguish the steady flame, flame strength remains above 90% of its unstretched value for 1.2 laminar flame times, after which it drops quickly as extinction occurs. Interestingly, no such time lag is observed in regions of negative stretch, even though negative stretch magnitudes are always smaller. Flame strength is found to increase as soon as negative strain is applied, rising linearly to 240% of the unstretched value within one laminar flame time. This result indicates a significant dependence of flame chemistry on negative strain, a phenomenon that has not previously been experimentally quantified, and that may help explain observed increases in turbulent burning velocity above those produced by flame surface density increases alone.

Invited Review: Combustion instability in spray-guided stratified-charge engines: A review

This article reviews systematic research on combustion instabilities (principally rare, random misfires and partial burns) in spray-guided stratified-charge (SGSC) engines operated at part load with highly stratified fuel -air -residual mixtures. Results from high-speed optical imaging diagnostics and numerical simulation provide a conceptual framework and quantify the sensitivity of ignition and flame propagation to strong, cyclically varying temporal and spatial gradients in the flow field and in the fuel -air -residual distribution. For SGSC engines using multi-hole injectors, spark stretching and locally rich ignition are beneficial. Combustion instability is dominated by convective flow fluctuations that impede motion of the spark or flame kernel toward the bulk of the fuel, coupled with low flame speeds due to locally lean mixtures surrounding the kernel. In SGSC engines using outwardly opening piezo-electric injectors, ignition and early flame growth are strongly influenced by the sprays characteristic recirculation vortex. For both injection systems, the spray and the intake/compression-generated flow field influence each other. Factors underlying the benefits of multi-pulse injection are identified. Unresolved questions include (1) the extent to which piezo-SGSC misfires are caused by failure to form a flame kernel rather than by flame-kernel extinction (as in multi-hole SGSC engines); (2) the relative contributions of partially premixed flame propagation and mixing-controlled combustion under the exceptionally late-injection conditions that permit SGSC operation on E85-like fuels with very low NOx and soot emissions; and (3) the effects of flow-field variability on later combustion, where fuel-air-residual mixing within the piston bowl becomes important.

Experimental metrics for identifying origins of combustion variability during spark-assisted compression ignition

Abstract Spark-assisted compression ignition, SACI, can be used to control the combustion phasing of compression-ignition gasoline engines. However, implementation of this technique can be confounded by cyclic variability. The purpose of this paper is to define experimental metrics that describe the SACI process and to demonstrate the use of these metrics for identifying the source(s) of cyclic variability during the SACI process. This study focused on a light load condition (7 mg/cycle, 200 kPa i.m.e.p.), where spray-guided direct fuel injection with spark ignition and an exhaust-rebreathing strategy was employed to achieve flame propagation, which led to compression ignition. This study employed a combination of measurements including pressure-based heat-release analysis, spark-discharge voltage/current measurements, and cycle-resolved combustion imaging. Based on these measurements, four distinct combustion periods were identified; namely, the spark discharge, the early kernel growth (EKG), flame propagation, and the compression ignition periods. Metrics were defined to characterize each period and used to identify the contribution of each period to the cyclic variability of the main heat release. For the light load condition studied here, the EKG period had the largest effect on the crank angle (CA) position of 50 per cent mass burned, CA50. The spark-discharge event may affect CA50 indirectly through its influence on EKG. However, this could not be definitively assessed here since the camera was incapable of recording both the spark-discharge event and the flame images during cycles of the same tests.

Measurement of digital particle image velocimetry precision using electro-optically created particle-image displacements

The displacement (velocity) precision achieved with digital particle image velocimetry (PIV) was measured. The purpose of this work was to determine the precision and sensitivity of digital PIV using real rather than theoretical images at 1 and 2 mm spatial resolution. The displacement measurement precision was determined by measuring the RMS noise from 60 identical displacement distributions. This work is unique in that it uses electro-optical image shifting to create a repeatable image displacement distribution of random particle fields. The displacement variance between images is caused by the shot-to-shot variation in: (1) the particle-image fields, (2) the camera noise and (3) the variance in the correlation peak detection. In addition to magnification variations, the particle-number density, imaging-lens f-stop and image-plane position errors were varied to determine the best configuration. The results indicate that both the ensemble-mean and the RMS fluctuations of the image displacements are affected by these parameters and comparisons with results found in the literature are presented. The extents of these variations are quantified. This variance does not, of course, include errors due to random gradients and out-of-plane pairing losses, which exist in real turbulent flows.