Experimental Investigation of Novel High Shear Injector at Elevated Pressure Conditions: Hydrodynamics, Spray and Combustion Characterization

Abstract

The stringent pollution and emission norms due to the present climate change and global warming have pushed industries to meet these norms and cut down their emission levels. In the same pipeline, the aviation industries do not remain untouched. The emissions from aircraft engines burning fossil fuel, NOx, CO, unburnt hydrocarbon, etc., affect the atmosphere's upper layer temperature and air quality. These increasing number of air-transport demands and strict emission norms have forced the aviation industry to develop a next-generation aero-engine that can burn fossil fuel more efficiently and meet the emission norms. Liquid fuel is a major power source for most power-generating units, such as land-based and air-based gas turbine combustors, rocket engines, industrial burners etc. The high energy density per unit volume for liquid fuel makes it a better candidate than gaseous fuel for an air-breathing engine/combustor. The extraction of power from the liquid fuel involves various stages such as fuel injection, its atomization in smaller droplets, oxidizer and fuel droplet mixing and then ignition of this fuel-oxidizer mixture. The fuel injection process and technique are key to enhancing the gas turbine combustion performance and reducing the emission levels to meet the pollution norms in upcoming eras of aviation transport. However, optimization of the fuel injection system remains a key challenge, especially in liquid-powered gas turbine engines. Modern-day aircraft combustors utilize high shear fuel injectors that consist of multiple arrangements of swirlers along with a concentric fuel nozzle that generates coflowing swirling air to enhance atomization quality and get a homogeneous level of fuel-air reactant mixture prior to combustion. The key observations of the current work are discussed in four parts. In the first part, we have designed, developed and characterized the performance of a new class of high shear injector (HSI). The benchmark to evaluate the performance of HSI are Spray flow field, Droplet size distribution, and droplet dispersion across a wide range of air-to-liquid mass ratios (ALR; 4-14). In the first part of the study, the influence of the injector’s geometrical features over the time-averaged and dynamical spray characteristics are examined using high fidelity laser-diagnostics technique (high-speed PIV). These features are swirl numbers (SN_Prim), airflow split-ratio(γ), area ratio(Δ), flare angle(θ) and relative flow orientation of primary and secondary swirlers (co and counter-rotation). A Simplex pressure-swirl fuel nozzle is mounted at the center of the injector to deliver the liquid. The non-dimensional length scales (radial; W/Df and axial; L/Df) are used to distinguish the test cases. W/Df and L/Df are governed by nearby swirl number SN5 for both counter and co-rotation swirl configuration. Here, SN5 is the experimental swirl number calculated 5mm from the exit. The length scales, W/Df and L/Df, are more sensitive to the split-ratio of primary and secondary swirlers, flare angle, and it’s mixing length. The spray droplet size and spatial uniformity are insensitive to the test variables. Further, dominant dynamic spatial modes changes with a change in W/Df of the recirculation zone and the oscillation frequency of the most dominating modes shifts to a lower value with increasing W/Df. Simplex pressure-swirl fuel nozzle and dual orifice fuel nozzle are commonly used for fuel delivery at the center of the fuel injector/atomizer in the present-day gas turbine combustor. However, these fuel-nozzle have limitations such as hollow-cone liquid sheets collapsing at higher pressure, prone to plugging of narrow passages with contaminations over time, and high delivery pressure requirement. The second part of the work addresses these issues by replacing the same with a discrete liquid-jet fuel nozzle with a simple orifice design and low injection pressure. The performance of a high shear injector with a discrete liquid – jet mounted at the center is evaluated and compared to the performance achieved with a high shear injector using a simplex pressure-swirl fuel nozzle. The comparison shows the potential of a discrete liquid -jet fuel nozzle to replace the simplex pressure-swirl fuel nozzle with the proper design of high shear injector. The injectors have excellent atomization capability along with superior azimuthal distribution of spray. The Sauter-Mean Diameters (SMD) across all the test cases are in the range of 9-30µm,15-37µm, 15-50µm and 23-75µm at ALR 14.1, 9.44, 7.08, and 4.72 respectively. Further, the Std. Deviation of azimuthal spatial uniformity in an azimuthal plane is below 6 percent of the mean. A high shear injector consists of multiple swirler that produce swirl flow, and swirl flow is generally characterized by swirl number (SN). Above particular SN, the swirl flow creates a negative axial pressure gradient at the central axis which manifests a vortex breakdown bubble (VBB), also called the recirculation zone or central toroidal recirculation zone (CTRZ). The CTRZ help to stabilize the flame inside the gas turbine combustor. However, the onset of the vortex breakdown bubble is associated with a self-excited instability known as precessing vortex core oscillation. The PVC oscillation in a swirl flow-based combustor aids the thermoacoustic instability, resulting in severe hardware damage and poor emission characteristic of the engine. The third part of the work addresses the suppression of PVC oscillation to avoid the thermoacoustic -instability by modifying the fuel nozzle mounted at the center of the injector. A dummy cylindrical post is attached to the fuel nozzle that acts as the centerbody. The work shows the intermittent or absolute suppression of PVC oscillation with proper design of centerbody and variation of flow Reynold number. The diameters of the centerbody considered are Dc = 7;9 and 11mm. The results further demonstrate the suppression of loud whistle-like acoustic sound with the suppression of PVC oscillations in the flow. Considering the current global warming scenario and emission norms, the fourth part of the work addresses the soot formation study using Laser-induced Incandesce Imaging (LII) in a turbulent non-premixed ethylene swirl flame at a constant global equivalence ratio, ∅global=0.55 in a high shear injector. First, the impact of the split-ratio of primary and secondary swirler on soot formation is estimated at the given Reynolds number and pressure conditions for constant ∅global, which shows that a 60/40 swirl cup produces lower soot than a 40/60 swirl cup. Further, at constant pressure and ∅global=0.55, soot volume fraction reduces from ~4 ppm to ~0.8 ppm by increasing the Reynolds number from Re~5000 to Re~ 15000, and at Re ~20000, no soot is observed. At constant exit bulk velocity and constant ∅global=0.55, the soot volume fraction scales-up with pressure as p2.1 on log-log plots. Further, pressure increment increases the soot formation at a constant Re number. Overall, it is observed that pressure endorses soot formation. In addition, a large formation of soot particles is majorly observed in the annual jet’s region of the swirl flow field.