Author: Lulin Jiang Publisher: ISBN: Category : Electronic dissertations Languages : en Pages : 207
Book Description
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 æm per pixel, exposure time of 1 æs, 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 æm 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 æm per pixel and exposure time of 1 æs. 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 (
Author: Nathan James Vardaman Publisher: ISBN: Category : Languages : en Pages : 274
Book Description
Liquid fuel atomization is widely used for combustion in many applications. With the strong emphasis on emissions regulations coupled with the ever increasing drive to improve energy efficiency, all aspects of combustion are being thoroughly researched. One key way to achieve the above goals is further improvement in the liquid fuel atomization process. Better atomization improves mixing of fuel and air, thus results in lower emissions, whereas improved liquid fuel injector designs can improve energy efficiency. The flow-blurring (FB) atomization technique, developed recently and investigated at the University of Alabama, has shown promise in both these areas. Previous research has shown that the FB injector produces smaller droplets and a more desirable droplet distribution than the commercial air-blast injector. In addition, the FB injector is able to successfully atomize a wider range of fuels, and it is much less susceptible to the change in surface tension or viscosity of the liquid fuel. In this study, a computational fluid dynamics (CFD) model is created to mimic the mixing of the fuel and air inside the injector, and thus, understand the underlying physics of the FB atomization process. The 2D model is assumed to be asymmetric and incompressible, and it uses the mixture model for the two-phase flow. A transient solution is found and analyzed revealing a recirculation zone, due to a stagnation point near the exit, is formed within the fuel tube of the injector. The recirculation zone is responsible for the mixing of fuel and air and the formation of bubbles. Prior experimental research conclusions are compared with the model as various operating conditions are implored for verification of the models accuracy. Finally, the model is utilized by simulating and studying the effect of geometric modifications within the wall gap of the FB injector. An inner-slant wall gap provides promising results compared to the original geometry. The geometry modifications are then implemented in an actual injector tested in an atmospheric burner. Emissions measurements, thermal imaging of the combustor surface, and OH* chemiluminescence imaging of the flame are used to first verify proper operation of the combustor and then to characterize the flame structure. Several operating conditions are altered and the influence of these changes is studied. OH* chemiluminescence images reveal the flame is stable and a better distribution of OH* signals represents improved atomization. Finally, the geometric modifications to the injector are tested to determine the performance improvements with respect to the baseline design. Experimental results of the different geometries indicate the injector with inner-slant seems to improve the atomization process. The inner-slant injector has lower emissions for a range of ALR values and a lower pressure drop though the injector compared to the original geometry.
Author: Charles Clark Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Liquid jet in crossflow systems are often used as lightweight and efficient mechanisms of atomizing fuel prior to entertainment in the flame holder and combustion, making them integral components of liquid fueled engines. Unfortunately, such systems are susceptible to deviations in both trajectory and breakup rate, depending primarily on the Weber number and momentum flux ratio of the injected jet. In these studies, the effects of solid obstructions, called pintiles, on the variability of liquid jet in cross flow trajectory and breakup are investigated. Initial investigations looked at the impacts of broad geometric parameters on flow independence, using Mie scatter imaging and phase Doppler particle analysis. The results of that investigation yielded an optimal overarching geometry for pintiles. This knowledge was then refined by looking at specific face characteristics of the obstructions, primarily investigating face angle and concavity. Spray characteristics were spatially resolved using LIF/Mie particle sizing techniques, revealing that modest convex surfaces yielded the most consistent breakup characteristics across space, while simultaneously improving the average breakup distance of the liquid jet. Finally, this progression of pintile characteristics is investigated on the effects pintiles have on overarching flame properties, using C2*/CH* chemiluminescence ratios to determine spatially resolved equivalence ratio distributions across a wide range of Weber numbers and momentum flux ratios encompassing breakup regimes from the enhanced capillary modes through to shear breakup modes. Results from these studies demonstrate significant improvement of combustion properties from the introduction of the pintiles.
Author: John Ernest Stevenson Publisher: ISBN: Category : Fuel Languages : en Pages : 54
Book Description
A new liquid-fuel injector was designed for use in the atmospheric-pressure, model gas turbine combustor in Bucknell University's Combustion Research Laboratory during alternative fuel testing. The current liquid-fuel injector requires a higher-than-desired pressure drop and volumetric flow rate to provide proper atomization of liquid fuels. An air-blast atomizer type of fuel injector was chosen and an experiment utilizing water as the working fluid was performed on a variable-geometry prototype. Visualization of the spray pattern was achieved through photography and the pressure drop was measured as a function of the required operating parameters. Experimental correlations were used to estimate droplet sizes over flow conditions similar to that which would be experienced in the actual combustor. The results of this experiment were used to select the desired geometric parameters for the proposed final injector design and a CAD model was generated. Eventually, the new injector will be fabricated and tested to provide final validation of the design prior to use in the combustion test apparatus.
Author: Lulin Jiang
Author: Oladapo S. Akinyemi
Author: Nathan James Vardaman
Author: A. A. Putnam
Author: Dr. Kuehn
Author: Edmund Giffen
Author: ILASS.
Author: Charles Clark
Author: Md Nayer Nasim
Author: John Ernest Stevenson