Srikrishna Sahu

Understanding the physics of primary liquid breakup process and its correlation with the evolution of spray characteristics in a rotary slinger atomizer is the goal of the present research. Experiments were conducted in a high-speed slinger test rig that houses a static liquid delivery manifold to uniformly supply the liquid to the rotating slinger disc that contains a single row of orifices carved on its peripheral surface for liquid injection.The atomizer was operated for a wide range of conditions by varying the rotational speed and liquid feed rate. The liquid breakup structure at the exit of the slinger orifices was visualized using front light illumination technique, while the droplet size was measured at different radial stations away from the slinger surface by application of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique. The visualization images highlighted strong influence of Coriolis force as the liquid tends to accumulate on one side of the channel (that is opposite to the rotational direction) for all cases. It was observed that while the liquid thickness is smaller for higher rotational speed, it does not vary much with liquid feed rate at the same speed and, instead, in such case the span of the liquid is wider. A theoretical analysis was developed to describe the in-channel liquid behaviour that accounts for the effect of Coriolis and surface tension forces. Interestingly, the theory could explain the above observations. The differences in the predictions in comparison to the analysis by Dahm et al. (2006a) was attributed to the assumption of annular film flow in the latter. The liquid breakup mode (stream, sheet or transition mode) could be described by Coriolis Bond number (Bo) that refers to the ratio of Coriolis to surface tension forces and proportional to the spread parameter (span to thickness ratio), while the liquid breakup regimes were identified on a Bo−q plot, where q is the liquid to air momentum flux ratio. The variation of characteristic droplet sizes with both rotational speed and feed rate were examined, and again some interesting trends are identified. The correlation between liquid breakup mode/regime with the measured droplet size was established using the above non-dimensional numbers.

The ability of the rotary slingers to achieve fuel atomization by employing the rotational speed of the turbine shaft obviates the use of high-pressure fuel pump and makes those a popular choice for small gas turbine engines. Such atomizers contain arrays of orifices on their outer periphery for injection of the liquid into the combustion chambers. In the present work, we experimentally investigate the influence of the choice of orifice size on primary liquid breakup dynamics and spray characteristics in slinger atomizers. Three slinger discs with the same disc size and number of orifices but with different orifice size, do (= 1 mm, 1.5 mm, and 2 mm) are examined in a high-speed slinger test rig for a range of rotational speed (2000–10,000 rpm) and liquid feed rate (0.2–1 lpm). The operating conditions are relevant to part-load and engine relighting conditions for which the selection of geometric parameters of the injector may play a crucial role in achieving the atomizer performance. In the current work, the liquid breakup process in the vicinity of the slinger orifices was visualized using front light illumination technique, whereas the pulse-shadowgraph technique was employed for droplet sizing. It is found that smaller orifice size promotes column mode of liquid breakup which results in larger primary liquid breakup length and subsequently results in larger Sauter mean diameter. The results highlight that the orifice size is an important design parameter as the liquid atomization in slinger atomizers is sensitive to the selection of orifice size especially for lower range of rotational speeds.

A parametric study of cross-stream pulsed spray injection and wall impingement characteristics in a circular channel has been carried out with an objective to investigate performance improvement in selective catalytic reduction (SCR) systems for exhaust gas after-treatment in diesel engines. The spray computations are performed based on a combined Eulerian and Lagrangian based modeling approach for the continuous and the discrete phases respectively. The simulation results are validated using the in-house experimental data for water injection in both open atmospheric and channel flow conditions. The spray structure, penetration, and droplet size are compared and good agreement between the experiments and simulations is demonstrated. Though the focus of the present work is on studying the influence of liquid preheating on post-impingement spray and wall-film characteristics as well as evolution of species mass fraction, the effect of air mass flow rates, and temperature, and the choice of the working liquid (water or Urea Water Solution (UWS)) are also investigated. While the mass flow rate of air and the selection of the above liquids for spray injection are found to have marginal influence on the results, the liquid preheating is shown to strongly affect the droplet evaporation and thus, the spray impact process. Especially, the liquid injection temperature that leads to flash evaporation results in reduced wall filming, higher ammonia generation rate and spatial uniformity of NH3 across the channel. This paper demonstrates preheating as a promising approach for the improvement of SCR performance.