Airblast Injector Research Articles

Towards the optimization of a pulsatile three-stream coaxial airblast injector

A large-scale parametric air–water test stand (AWTS) study involving more than 40 evaluations was carried out for the purposes of three-stream airblast reactor feed injector characterization and optimization; a subset of seven air stream combinations is discussed here. The role of CFD as a supplement to, or a replacement for, air–water testing is of great industrial interest. To this end a set of CFD simulations was carried out to complement the AWTS study. Pressure responses, spray opening characteristics near the feed injector face, and spray distribution were primary measures for both the AWTS and CFD programs. It was found that, over the range of variables studied, there was somewhat of a match between CFD and AWTS results. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS (frequencies between 75 and 600 Hz), and an interesting transition in spray bursting character occurred at moderate inner air flows. The oscillatory flow pattern mimics prior work in terms of the energy of the fluctuations, but the fact that the present fluctuations occur at an order of magnitude lower frequency is apparently related to the comparatively low gas/liquid momentum ratio in the current study. Overall, it is shown that the CFD method contained herein can be used to supplement, but not replace, air–water testing for said injector configuration.

Emissions Reductions in Diesel and Kerosene Flames Using a Novel Fuel Injector

A SPRAY serves as the heart of almost every type of liquidfueled combustion system. Ideally, to promote combustion withmaximumefficiency andminimumharmful emissions, an injector should deliver a fuel spray that rapidly disperses and evaporates to yield a homogeneousmixture of gaseous fuel and air. Characteristics of an ideal injector include good atomization over a wide range of either steady or transient fuel flow rates, unaffected by flow instabilities, low power requirements, scalability, resistant to blockages, and delivery of a fine spray [1,2]. Reviews by McDonell and Samuelsen [3], Razdan [4],Mansour [5], andNakamura et al. [6] deal with fuel sprays optimized for lean combustion. Pressure atomization, air-assist and air-blast atomization, and effervescent atomization [7] are commonly used techniques for a variety of injector applications. In the current study, we are exploring the so-called flow-blurring injector of Ganan-Calvo [8] in a combustion system. As illustrated in Fig. 1, the flow-blurring (FB) concept consists of a liquid nozzle and an orifice plate situated downstream of the nozzle. The nozzle’s wall is tapered at the outlet, and the sharpened edge of the orifice plate is the same diameter as the inner diameter of the nozzle (d). When the axial distance between the nozzle exit and orifice plate (H) is small (i.e., H=d < 0:25), some of the gas flowing into the lateral cylindrical passageway between the nozzle exit and orifice plane is forced upstream a short distance into the nozzle carrying the liquid. The backflow of gas causes vigorous combination of turbulent mixing and effervescing, or the so-called flow-blurring, which results in a spray with very fine droplets. Simmons et al. [9] conducted cold-flow visualization experiments for a range of geometric and flow conditions to demonstrate the high efficiency of the FB injector concept. To the best of our knowledge, the FB injector concept has never been applied to combustion systems. The objective of this study is to experimentally characterize emissions performance of the FB injector in a liquid-fueled combustor operated at atmospheric pressure. Experiments using diesel and kerosene fuels are performed for fixed fuel and total airflow rates, and the atomizing airflow rate is varied. The FB injector performance (NOx and CO emissions) is compared with that of a commercial air-blast (AB) injector. II. Experimental Setup

Use of Faraday instabilities to enhance fuel pulverisation in air-blast atomisers

The atomization of liquids into a spray is an important process in many industrial applications and particularly in the aero-engine sector. Conventional air-blast injectors in aircraft engines today use aerodynamic shearing effects to atomize the liquid fuel. However, at operating conditions where the air velocity is below 30 m/s (such as ground start and high altitude restart) the atomization quality is poor. Consequently combustion is less efficient with high pollutant emissions. The objective of this study is to validate a new concept of injector which couples the shearing effects with the principle of ultrasonic atomization. The latter consists of using piezoelectric actuators to generate the oscillations of a wall in contact with the liquid film. This excitation perpendicular to the liquid film surface creates Faraday instabilities at the liquid/air interface. Amplitudes higher than a defined threshold value induce the break-up of ligaments and the formation of droplets. To cite this article: M. Boukra et al., C. R. Mecanique 337 (2009).

Operating liquid-fuel airblast injectors in low-pressure test rigs: strategies forscaling down the flow conditions

For reasons of economy, gas turbine atomizers are often tested at lowerpressures and temperatures than exist in the engine. The spray behaviourdiffers in the rig from that in the engine. For good measurement practiceit is necessary to find rig operating conditions at which the aspect ofthe two phase flow being measured (e.g. patternation, droplet size) issimilar in the rig to that in the engine. We describe a number of strategiesfor finding operating conditions which allow good measurement. Theseconditions reduce the air and fuel flow rates below engine values but conservecertain flow quantities. We define an error function to quantify how wellthe rig conditions match key non-dimensional groups.We consider anexample of a typical aero gas turbine airblast atomizer operated in a rig at323 K. To preserve patternation it is desirable to conserve the Reynolds(Re), Mach(Ma) andStokes (Stk)numbers. A perfect match is not possible but we find a numberof acceptable compromise strategies. The match improves asthe rig pressure increases up to ∼600 kPawhere Re andMa are matchedand Stk is correctto an order of magnitude. Further pressure increase give little improvement. It may be possibleto match Stkby using a different fuel.

Rapid Liquid Fuel Mixing for Lean-Burning Combustors: Low-Power Performance

Designers of advanced gas turbine combustors are considering lean direct injection strategies to achieve low NOx emission levels. In the present study, the performance of a multipoint radial airblast fuel injector Lean Burn injector (LBI) is explored for various conditions that target low-power gas turbine engine operation. Reacting tests were conducted in a model can combustor at 4 and 6.6 atm, and at a dome air preheat temperature of 533 K, using Jet-A as the liquid fuel. Emissions measurements were made at equivalence ratios between 0.37 and 0.65. The pressure drop across the airblast injector holes was maintained at 3 and 7–8 percent. The results indicate that the LBI performance for the conditions considered is not sufficiently predicted by existing emissions correlations. In addition, NOx performance is impacted by atomizing air flows, suggesting that droplet size is critical even at the expense of penetration to the wall opposite the injector. The results provide a baseline from which to optimize the performance of the LBI for low-power operation.

SIMPLE DESCRIPTION OF THE COMBUSTION STRUCTURES IN THE STABILIZATION STAGE OF A SPRAY JET FLAME

The structure of a two-phase flame has been investigated within its stabilization region in the nearfield of a spray jet. The spray is produced by a coaxial air-blast injector fed with liquid methanol. We focus on a specific structure of the two-phase flame, observed experimentally where the flame presents a double structure with a predominant diffusion character for each reaction zone. The analysis is justified from experimental results of phase Doppler anemometry and planar laser-induced fluorescence of OH. The dynamics of the spray obtained from phase Doppler velocimetry are studied in terms of size classes defined from the Stokes number. The size classification shows that, where the flame stabilizes, the spray is composed of two fluids, one with high inertia (high Stokes number), the other characterized by a low Stokes number. The structure of the two-phase flame is analyzed in the low-inertia part of the spray. The emphasis is put on a regime where tau(ch) < tau(vap) < tau(mix); a double structure may develop in what we called the vaporization regime, since droplets carl cross reaction zones. Such a double structure has been predicted by Continillo and Sirignano [25] and by Greenberg and Sarig [26, 27] through numerical modeling of a hue-phase counterflow flame. The present article gives an experimental confirmation of a real occurrence of such a flame structure in turbulent spray jets and proposes a simplified description in the low-inertia part of the spray and for the flame sheet approximation.

Planar Laser Induced Fluorescence Measurement of [ OH] in the Stabilization Stage of a Spray Jet Flame

Planar laser induced fluorescence of OH is quantitatively applied in the lift-off region of a spray jet flame. The spray is produced by a coaxial air-blast injector fed with liquid methanol. OH is laser excited through the (0,1) vibrational band so that the shifted fluorescence emitted via the ( 0,0) and ( 1,1) bands can be spectrally discriminated against elastic scattering by the methanol drops. The lift-off location is determined at each laser shot making it possible to investigate the reactive structures in their own coordinate system. Statistical analysis of the fluorescence images shows that the lifted flame may develop in the remarkable structure of two diverging diffusion-like fronts as was predicted by modeling of spray combustion in a counterflow laminar diffusion flame. Calibration of the fluorescence imaging system into absolute OH concentration is performed in a laminar diffusion flame of methanol vapor where [ OH] is determined in situ by absorption measurement with the same optical device. The OH concentration conditioned on the elevation above the lift-off is found to hold a constant superequilibrium value about four times the equilibrium level.

Flame stabilization and OH fluorescence mapping of the combustion structures in the near field of a spray jet

Phase-doppler velocimetry and two-dimensional laser-induced florescence of OH have been used to investigate flame stabilizating in the near field of a spray jet of methanol above an air-blast injector. Comparison between the size-classified velocity fields and the scatter plot of lift-off locations reveal the essential role played by the large-scale mixing structures, in which the low inertia part of the spray has enough time to vaporize and mix with entrained air before burning. An analysis has been developed within the low Stokes flow to derive large-scale coupling functions in the region of flame stabilization. In this frame the stoichiometric mixture fraction, where combustion is possible, is determined by the curvature of the fuel mass fraction profile which in turn increases with the elevation above the injector. A vaporization regime ( τ ch < τ vap < τ mix) of flame stabilization has been analyzed along with its lower and upper limits, due to atomization and dynamic requirements respectively. In this stabilization regime the two-phase flame develops with the remarkable structure of two diverging fronts. Experimental data on the height of flame liftoff and its statistical distributions along with the structures of the stabilized flames, are in good agreement with predictions. However, this approach does not apply to the main part of the spray, which feeds progressively the combustion at downstream distances and in different regimes.