Investigation of atomization mechanisms and flame structure of a twin-fluid injector for different liquid fuels

Abstract

Diminishing fossil fuel resources, ever-increasing energy cost, and the mounting concerns for environmental emissions have precipitated worldwide research on alternative fuels. Biodiesel, a popular renewable energy source, is produced from the transesterification process of source oils such as vegetable oil (VO) requiring processing cost and energy input. However, highly viscous glycerol produced as the waste byproduct also decreases the economically viability of biodiesel. Previous studies show that without fuel preheating or hardware modification, high viscosity fuels such as VO and glycerol cannot be burnt cleanly with the application of the typical air blast (AB) injector due to the high viscosity. However, extremely low emissions of diesel, kerosene, biodiesel, straight VO and glycerol flames at the combustor exit are reported using a novel flow blurring (FB) injector. The PDPA measurements in the FB sprays at least 1.0 cm downstream of the injector exit quantitatively show the superior fuel-flexibility and atomization capability of the FB injector as compared to the AB atomizer. This study seeks to gain insight into the detailed flame structure of both conventional and alternative fuels atomized by the FB injector. The atomization mechanism in the FB injector near field is also investigated using a high speed imaging technique and particle image velocimetry (PIV) to explore the FB spray characteristics in the near field of the injector. First, the combustion of diesel, biodiesel and straight vegetable oil (VO) using a Flow Blurring (FB) injector is investigated. Measurements of gas temperature and CO and NOx concentrations at various axial and radial locations of the combustor are acquired using custom-designed thermocouple and gas sampling probes. Heat loss rate through the combustor is estimated from wall temperatures measured by an infra-red camera. A simple droplet model is used to predict fuel vaporization behaviour in the dark-region near the injector exit. Results show that the FB injector produced low-emission clean blue flames indicating mainly premixed combustion for all three fuels. Matching profiles of heat loss rate and product gas temperature show that the combustion efficiency is fuel independent. Next, a fuel-flexible dual-fuel combustor to simultaneously burn methane and/or straight glycerol without preheating either glycerol or air is investigated by utilizing a FB liquid fuel injector. Product gas temperature, NOX and CO emissions at multiple locations inside the combustor are measured to quantitatively assess the flame structure, related to liquid atomization, droplet evaporation, and fuel-air mixing in the near field. The impact of fuel mix and air to liquid mass ratio (ALR) on combustion performance is investigated. Pure glycerol flame is also investigated to demonstrate the fuel flexibility and ease of switching between gas and liquid fuels in the present system. Results show that the methane combustion can assist glycerol vaporization to results in its rapid oxidation. In spite of the differences in the flame structure, profiles of product gas temperature and emissions at the combustor exit reveal that complete and mainly lean premixed combustion with low emissions is achieved for all of the test cases indicating excellent fuel flexibility of the present combustor using the FB injector. Next, high-speed visualization and time-resolved Particle Image Velocimetry (PIV) techniques are employed to investigate the FB spray in the near field of the injector to delineate the underlying mechanisms of atomization. Experiments are performed using water as the liquid and air as the atomizing gas. Flow visualization at the injector exit focused on field of view with the dimension of 2.3 mm x 1.4 mm, spatial resolution of 7.16 µm per pixel, exposure time of 1 µs, and image acquisition rate of 100 k frames per second (fps). Image sequence illustrates mostly fine droplets indicating that primary breakup by FB atomization occurs within the injector. Few larger droplets appearing at the injector periphery undergo secondary breakup by Rayleigh-Taylor instabilities. Time-resolved PIV technique is applied to quantify the droplet dynamics in the injector near field. Plots of instantaneous, mean, and root-mean-square droplet velocities are presented to reveal the secondary breakup process. Results show that the secondary atomization process to produce fine and stable spray is complete within a short distance of about 5.0 mm from the injector exit. These superior characteristics of the FB injector are desirable to achieve clean combustion of different fuels in practical systems. The impact of ALR shows that the increase in ALR improves both primary FB atomization and secondary atomization in the near field. Next, glycerol atomization in the near field of the FB injector is investigated in detail. Time-resolved PIV with exposure time of 1 ms and laser pulse rate of 15 kHz is utilized to probe the glycerol spray at spatial resolution of 16.83 µm per pixel. PIV results describe the droplet dynamics in terms of the instantaneous, mean, and root-mean-square (RMS) velocities, and space-time analysis and probability distribution profiles of the axial velocity. In addition, high-speed imaging (75 kHz) coupled with backside lighting is applied to reveal the glycerol breakup process at spatial resolution of 7.16 µm per pixel and exposure time of 1 µs. Results show that the primary breakup by FB atomization or bubble explosion within the injector results in a combination of slow-moving streaks and fast-moving droplets at the injector exit. Then, the secondary breakup by Rayleigh-Taylor instability occurs at farther downstream locations where the high-velocity atomizing air stretches the streaks into thin streaks that disintegrate into smaller streaks, and subsequently, into fine droplets. Thus, within a short distance downstream of the injector exit (< 30.0 mm), most of the glycerol is atomized into fine droplets demonstrating excellent atomization performance of the FB injector. Both primary FB atomization and secondary atomization process improve with the increasing ALR.