Single-element Lean Direct Injection Research Articles

Experimental Study on OH* Chemiluminescence and Emissions in the Single-Element Lean Direct Injection Combustor

ABSTRACT OH* chemiluminescence and emissions of the convergent swirler module and the Venturi swirler module are studied in a single-element lean direct injection combustor. The OH* chemiluminescence images are captured by a CCD camera with an image intensifier, and the emissions are measured by a gas analysis system. The flame of the Venturi swirler module is closer to the inlet plane of the combustor compared to that of the convergent swirler module. At the equivalence ratio (ϕ) of 0.55, 0.65, and 0.75, the EINOx of the convergent swirler module is 1.02 g/kg, 1.44 g/kg, and 1.98 g/kg, and the EICO is 0.32 g/kg, 0.54 g/kg, and 3.65 g/kg separately. The EINOx of the Venturi swirler module is 0.95 g/kg, 1.37 g/kg, and 2.05 g/kg and the EICO is 0.32 g/kg, 0.93 g/kg, and 5.86 g/kg. The NOx emissions of the two swirler modules are similar. But the CO emission of the convergent swirler module is lower at the ϕ of 0.65 and 0.75. The convergent swirler module has an advantage in reducing emissions. In addition, the influence of the swirl number (S) on OH* Chemiluminescence and emissions of the convergent swirler module is studied. At the ϕ of 0.55, the EINOx is between 0.88 g/kg and 1.17 g/kg and the EICO is between 0.13 g/kg and 0.44 g/kg with different S. With the increase of the S, the NOx emission increases and the CO emission decreases. But at the ϕ of 0.65 and 0.75, with the increase of the S, the NOx emission first increases and then decreases and the CO emission first decreases and then increases.

Numerical Investigation of Combustion Instabilities in a Single-Element Lean Direct Inject Combustor Using Flamelet Based Approaches

Abstract In this paper, high-fidelity large eddy simulations (LES) along with flamelet-based combustion models are assessed to predict combustion dynamics in low-emission gas turbine combustor. A model configuration of a single-element lean direct injection (LDI) combustor from Purdue University (Huang et al., 2014, “Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor,” 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, July 28–30.) is used for the validation of simulation results. Two combustion models based on the flamelet concept, i.e., steady diffusion flamelet (SDF) model and flamelet generated manifold (FGM) model are employed to predict combustion instabilities. Simulations are carried out for two equivalence ratios of φ = 0.6, and 0.4. The results in the form of mode shapes, peak to peak pressure amplitude and power spectrum density (PSD) are compared with the experimental data of Huang et al. (2014, “Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor,” 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, July 28–30.). The effect of variation in the time-step size hence acoustic courant number is studied. Further, two numerical solver options, i.e., pressure-based segregated solver and pressure-based coupled solver, are used to understand their effect on the solution convergence regarding the number of time-steps required to reach the limit cycle of the pressure oscillations. A truncated (half) domain simulation is performed by applying an appropriate acoustic impedance boundary condition at the truncated location. Overall, the simulation results compare well with the experimental data and trends are captured accurately in all simulations. It builds confidence in flamelet-based combustion models for the use in combustion instability modeling which is traditionally done using finite rate chemistry models based on reduced kinetics.

Experimental investigation of lean-dome high-airflow airblast pilot mixers' operability, emissions, and dynamics

In order to further improve operability and low-power combustion efficiency of single-venturi twin concentric swirlers Lean Direct Injection (LDI)-based lean dome combustion systems, the authors' previous work investigated the effect of outer swirler vane angle on LDI reacting flow characteristics. A typical single-element LDI injector comprises of a fuel atomization device (pressure injector or airblast), surrounded by high airflow twin swirlers with converging passages feeding a single venturi followed by a single diverging passage called flare. LDI injectors without flare can be classified as high airflow airblast or air-assist pressure atomizers, in contrast with conventional air atomizing devices where air-to-fuel ratio rarely exceeds 3 or 4. In the present investigation, the impact of the flare section on the reacting flow field and flame structure is systematically explored. Specifically, the lean blowout limits, flame shapes, reacting flow dynamics, and NOx emissions from swirl-venturi LDI and airblast injectors are experimentally characterized using a suite of optical diagnostics, including time-resolved particle image velocimetry, OH⁎/CH⁎/NO2⁎ chemiluminescence, and broadband flame imaging. Results reveal that the inclusion of a flare significantly increases the overall swirl strength of the injector. Conversely, by removing the flare, swirl strength is reduced, leading to flow pattern changes (such as a transition from recirculating to swirling jet flow), higher lean blowout limits, and higher peak oscillation frequency of the flow field, while NOx emissions decrease due to reduced characteristic residence times. Such results demonstrate an inherent design tradeoff between lowering NOx emissions and maintaining/enhancing operability for a single swirl injector.

Nonlinear subscale turbulent models for very large eddy simulation of turbulent flows

A brief introduction of the time-filtered Navier–Stokes (TFNS) equations for very large eddy simulation (VLES) and its distinct features is presented. A set of nonlinear subscale models and their advantages over the linear subscale eddy viscosity models are described. A guideline for conducting a TFNS/VLES simulation is also provided. In this paper, we present simulations for three turbulent flows. The first one is the turbulent pipe flow at both low and high Reynolds numbers to illustrate the basic features of TFNS/VLES; the second one is the swirling turbulent flow in an LM6000 single injector to further demonstrate the differences between the results from nonlinear models versus linear viscosity models; the third one is a more complex turbulent flow generated in a single-element lean direct injection combustor, which demonstrates that the current TFNS/VLES approach is capable of predicting dynamically important, unsteady turbulent structures even with a relatively coarse mesh grid.

Parametric investigation of combustion instabilities in a single-element lean direct injection combustor

Self-excited combustion dynamics in a liquid-fueled lean direct injection combustor at high pressure (1 MPa) are described. Studied variables include combustor and air plenum length, inlet air temperature, equivalence ratio, fuel nozzle location, and fuel composition. Measured pressure oscillations were dependent on combustor geometry and ranged from about 1% of mean chamber pressure at low equivalence ratio, up to 20% at high equivalence ratio. In the most unstable cases, strong pressure modes were measured throughout the frequency spectrum including a band around 1.2–1.5 kHz representing the 4th longitudinal mode, and another band around 7 kHz. The oscillation amplitudes have a non-monotonic dependency on air temperature, and are affected by the placement of the fuel nozzle relative to the throat of the subsonic swirling air flow. The parametric survey provides a rich dataset suitable for validating high-fidelity simulations and their subsequent use in analyzing and interpreting the complex combustion dynamics.

Simulation of combustion characteristics in a hydrogen fuelled lean single-element direct injection combustor

Combustion characteristics in a hydrogen fuelled single-element lean direct injection (LDI) combustor for low emission gas turbine engine are computationally investigated. A single-element LDI combustor was produced and simulated with steady-state Reynolds-averaged Navier–Stokes reacting computations. Reacting computations of the single-element LDI combustor were performed with an augmented reduced 23-step reaction mechanism for hydrogen/air. A realizable k − ε model was used to obtain turbulence closure. For the single-element LDI combustor with different equivalence ratio, the effective area, axial velocity, total pressure drop coefficient, total temperature, mass fraction of OH, emission index of pollutant NO (EINO) were achieved and discussed.

Spray Combustion Modeling in Lean Direct Injection Combustors, Part II: Multi-Point LDI

The multi-point lean direct injection (MPLDI) strategy, which incorporates several lean direct injection (LDI) modules, has been developed for low-emission aero-propulsion systems. There is a lack of information on MPLDI flow physics and spray characteristics. In extending previous work, this article investigates the reacting spray characteristics of an MPLDI system and makes an effort to highlight the salient features of this system, which enable low NOx combustion. The simulation approach and the numerical models applied and validated for the single-element LDI are used to compute the combusting spray in MPLDI. Simulations are performed under different inlet flow conditions to study the spray properties and combustion due to multi-swirler injection. The results illustrate small-sized drops and uniform distribution of drop diameter with increasing swirl, resulting in rapid fuel vaporization and mixing. In comparison with the single-element LDI, MPLDI creates smaller reaction zones and accomplishes a more uniform drop diameter distribution and a thorough mixing of fully vaporized fuel and air at a shorter distance from the fuel nozzles. Overall, the numerical results are able to explain the underlying features of the MPLDI technique, which primarily influence its NOx reduction performance.

Spray Combustion Modeling in Lean Direct Injection Combustors, Part I: Single-Element LDI

Lean direct injection (LDI) is one of the most promising low NOx combustion concepts for aero-engines. This work reports turbulent reacting spray modeling in a single-element LDI combustor to determine a validated numerical framework that can be used to model such complex combustion systems involving highly swirling flows, droplet breakup, and combustion. Different sub-models are employed within a numerical framework to characterize the spray processes involved in the primary atomization and secondary breakup regimes. The sensitivity of the associated breakup model parameters to the predicted behavior is investigated, thereby fine tuning the sub-models for turbulent spray combustion in LDI. The two-way coupling accomplishes the interactions between the liquid and gas phases and the respective phase properties are analyzed. The size and distribution of drops are computed at various locations and the results are compared with the measurements. The work demonstrates that modeling of primary atomization improves the overall predictions of both gas phase and spray. The validation with the measurements reflects that the numerical framework implemented in this study is able to characterize the effects of highly swirling flows with strong turbulence on spray velocity and size distribution, and strong momentum exchange between the gas and liquid phases.