Collaborative investigation of the internal flow and near-nozzle flow of an eight-hole gasoline injector (Engine Combustion Network Spray G)

The internal details of fuel injectors have a profound impact on the emissions from gasoline direct injection engines. However, the impact of injector design features is not currently understood, due to the difficulty in observing and modeling internal injector flows. Gasoline direct injection flows involve moving geometry, flash boiling, and high levels of turbulent two-phase mixing. In order to better simulate these injectors, five different modeling approaches have been employed to study the engine combustion network Spray G injector. These simulation results have been compared to experimental measurements obtained, among other techniques, with X-ray diagnostics, allowing the predictions to be evaluated and critiqued. The ability of the models to predict mass flow rate through the injector is confirmed, but other features of the predictions vary in their accuracy. The prediction of plume width and fuel mass distribution varies widely, with volume-of-fluid tending to overly concentrate the fuel. All the simulations, however, seem to struggle with predicting fuel dispersion and by inference, jet velocity. This shortcoming of the predictions suggests a need to improve Eulerian modeling of dense fuel jets.

Robust-mode analysis of hydrodynamic flows

The emergence of techniques to extract high-frequency high-resolution data introduces a new avenue for modal decomposition to assess the underlying dynamics, especially of complex flows. However, this task requires the differentiation of robust, repeatable flow constituents from noise and other irregular features of a flow. Traditional approaches involving low-pass filtering and principle components analysis have shortcomings. The approach outlined here, referred to as robust-mode analysis, is based on Koopman decomposition. Three applications to (a) a counter-rotating cellular flame state, (b) variations in financial markets, and (c) turbulent injector flows are provided.

Dynamic-mode decomposition based analysis of shear coaxial jets with and without transverse acoustic driving

Modal decompositions of unperturbed and acoustically driven injector flows from shear coaxial jets are implemented using dynamic-mode decomposition, which is a natural approach in the search for collective oscillatory behaviour in nonlinear systems. Previous studies using proper orthogonal decomposition had revealed the most energetic pairs of coherent structures in injector flows. One of the difficulties in extracting lower-energy coherent structures follows from the need to differentiate robust flow constituents from noise and other irregular facets of a flow. The identification of robust features is critical for applications such as flow control as well, since only they can be used for the tasks. A dynamic-mode decomposition based algorithm for this differentiation is introduced and used to identify different classes of robust dynamic modes. They include (1) background modes located outside the injector flow that decay rapidly, (2) injector modes – including those presented in earlier studies – located in the vicinity of the flow, (3) modes that persist under acoustic driving, (4) modes responding linearly to the driving and, most interestingly, (5) a mode whose density exhibits antiphase oscillatory behaviour in the observation plane and that appears only when $J$, the outer-to-inner-jet momentum flux ratio, is sufficiently large; we infer that this is a projection of a mode rotating about the symmetry axis and born via a spontaneous symmetry breaking. Each of these classes of modes is analysed as $J$ is increased, and their consequences for the flow patterns are discussed.

Optical Methods for Studies of Self-Excited Oscillations and the Effect of Dampers in a High Pressure Single Sector Combustor

Self-excited periodic instabilities in a staged lean burn injector could be forced by operating the combustor at off-design conditions. These pressure oscillations were studied in a high pressure single sector combustor with optical access. Two damper configurations were installed and tested with respect to their damping efficiency in relation to the configuration without dampers. For a variety of test conditions, derived from a part load case, time traces of pressure in the combustor were measured, and amplitudes were derived from their Fourier transformation. These measurements were performed for several combinations of the operating parameters, i.e., injector pressure drop, air/fuel ratio (AFR), pilot/main fuel split, and preheat temperature. These tests “ranked” the respective damper configurations and their individual efficiency with respect to the configuration without dampers. Although a general trend could be observed, the ranking was not strictly consistent for all operating conditions. For several test cases, preferably with pronounced self-excited pressure oscillations, phase-resolved planar optical measurement techniques were applied to investigate the change of spatial structures of fuel, reaction zones, and temperature distributions over a period of an oscillation. A pulsating motion was detected for both pilot and main flame, driven by a pulsating transport of the liquid fuel. This pulsation, in turn, is caused by a fluctuating air velocity, in connection with a prefilming airblast type atomizer. A phase shift between pilot and main injector heat release was observed, corresponding to a shift of fuel penetration. Local Rayleigh indices were calculated qualitatively, based on phase-resolved OH chemiluminescence used as marker for heat release, and corresponding pressure values. This identified regions, where a local amplification of pressure oscillations occurred. These regions were largely identical to the reaction regions of pilot and main injector, whereas the recirculation zone between the injector flows was found to exhibit a damping effect.

Optical Methods for Studies of Self-Excited Oscillations and the Effect of Dampers in a High Pressure Single Sector Combustor

Self-excited periodic instabilities in a staged lean burn injector could be forced by operating the combustor at off-design conditions. These pressure oscillations were studied in a high pressure single sector combustor with optical access. Two damper configurations were installed and tested with respect to their damping efficiency in relation to the configuration without dampers. For a variety of test conditions, derived from a part load case, time traces of pressure in the combustor were measured, and amplitudes were derived from their Fourier transformation. These measurements were performed for several combinations of the operating parameters, i.e., injector pressure drop, air/fuel ratio, pilot/main fuel split and preheat temperature. These tests “ranked” the respective damper configurations and their individual efficiency with respect to the configuration without dampers. Although a general trend could be observed, the ranking was not strictly consistent for all operating conditions. For several test cases, preferably with pronounced self-excited pressure oscillations, phase-resolved planar optical measurement techniques were applied to investigate the change of spatial structures of fuel, reaction zones and temperature distributions over a period of an oscillation. A pulsating motion was detected for both pilot and main flame, driven by a pulsating transport of the liquid fuel. This pulsation, in turn, is caused by a fluctuating air velocity, in connection with a prefilming airblast type atomizer. A phase shift between pilot and main injector heat release was observed, corresponding to a shift of fuel penetration. Local Rayleigh indices were calculated qualitatively, based on phase-resolved OH chemiluminescence used as marker for heat release, and corresponding pressure values. This identified regions, where a local amplification of pressure oscillations occurred. These regions were largely identical to the reaction regions of pilot and main injector, whereas the recirculation zone between the injector flows was found to exhibit a damping effect.

A Turbulence Model of Bi-Component Fuel Droplet for Atomizing Sprays

A new approach to account for simultaneously finite thermal conductivity, finite mass diffusivity and turbulence effects within atomizing liquid sprays is presented in this paper. The main contribution of this paper is to incorporate the liquid turbulence effect in modeling the multi-component droplet liquid jet evaporation. For this study, we consider a binary mixture of heptane and decane liquid fuel injected into a hot gas environment. The finite conductivity model is based on a newly developed two-temperature two-layer film theory of Chen et al. [1], where the turbulence characteristics of the droplet are used to estimate the effective thermal conductivity. Fuel droplets inherit turbulence from high Reynolds number issuing liquid injector flows. The present paper extends the formulation of Chen et al. [1] to estimate effective mass transfer diffusivity within the drop. In this model four regions are considered, interior and the surface of the droplet, the liquid gas interface and the surrounding gas phase. An approximate solution to the quasi-steady energy equation was used to derive an explicit expression for the heat flux from the surrounding gas to the droplet–gas interface, with inter-diffusion of fuel vapor and the surrounding gas taken into account. The thermo-transport properties including their dependence on temperature are considered. Validation studies were carried out by comparison with the experimental results.