Caleb Riggins

Soot Volume Fraction Measurements from Droplet Combustion Experiments of Sooty Fuels Caleb Riggins

Mentor: Yuhao Xu Department of Mechanical Engineering

Introduction: Observations on sooting dynamics during the combustion of liquid fuel have been an area of interest for aerospace industries due to the growth and motivation to increase fuel efficiency. Earlier experiments conducted in the 1990s were performed to analyze sooting behaviors under microgravity, under validations of one-dimensional theory analysis. The pioneering results of microgravity experimentation provided crucial insights regarding steady-state burning behaviors for calculating theoretical predictions. Still, they did not take into consideration transient states, and often they are not quantitative [1]. With the motivation to improve fuel efficiency, the aerospace industry strives to gain a better understanding of the combustion process of Kerosene. Therefore, free-floating droplet combustion experiments were conducted in the Combustion Integrated Rack (CIR) onboard the International Space Station (ISS) as part of NASA’s Flame Extinguishment Experiment (FLEX) and the subsequent FLEX-2 programs [2]. During the FLEX-2 experiments, a free-floating fuel droplet was ignited using energized Kanthal coils. This study aims to develop and evaluate independent methods of quantitatively extracting soot volume fraction (SVF) from FLEX-2 experimental data. Materials and Methods: The main method includes burning n-heptane droplets under microgravity with different droplet diameters using the full-field light extinction method (FFLEM) apparatus to quantitively extract SVF measurements. Soot volume fraction measurements provide essential information for studying soot growth, radiant transport, and post-flame particulates [3]. Laser-induced incandescence (LII) and full-field light extinction method [1] are favorably considered two nonintrusive approaches for SVF measurements. A disadvantage of LII is that it does not provide absolute SVF, and thus it must be calibrated against other techniques [4]. Therefore, this study concerns the FFLEM technique to quantify soot emissions, which is based on the attenuation of light when a laser beam passes through the sootcontaining region [5]. Results and Discussion: Results for this study were attempted using a mathematical simulation programmed in MATLAB. The code was written to ultimately be able to quantify soot emissions more effectively. Unfortunately, there were some setbacks due to technical complexities within the code data versions and the COVID-19 pandemic. Efforts were furthermore made to obtain more knowledge and understand sooting behaviors through extensive literature review for future project work in improving fuel efficiencies. Some results acquired from the literature review reflect the results we are generating within this project. Results show that as you move away from the droplet, SVF increases and then decreases. Figures 1 and 2 show this observation.

Figure 1: Time sequence of a 1.75 mm n-heptane droplet burning in atmospheric pressure air from .2 to .5 seconds [1]. Figure 2: Corresponding results for Figure 1. Here, SVF is plotted against a nondimensional radius. For all times, soot production was constant and increased as the diameter increased [1].

Summary: This current project is aiming to quantify soot emissions more effectively to improve overall fuel efficiency while ultimately saving the environment as well as improving economic and technological advances in the aerospace industry. Older experiments were able to develop qualitative results but not quantitative. The goal of this project is to develop more effective ways to quantify SVF. Due to COVID19, some efforts were delayed in being able to quantify SVF effectively using MATLAB. Fortunately, there were still efforts made to evaluate qualitative data to ensure a progressive move towards successful work within this project in the future.

References:

[1] M.Y. Choi, L. Kyeong-Okk, Investigation of sooting in microgravity droplet combustion, Symp. (Int.) Combust., 1996, Elsevier, pp. 1243-1249. [2] D.L. Dietrich, V. Nayagam, M.C. Hicks, et al., Droplet Combustion Experiments Aboard the International Space Station, Microgravity Sci. Technol. 26 (2014) 65-76. [3] M. Choi, G.W. Mulholland, A. Hamins, T. Kashiwagi, Comparisons of the soot volume fraction using gravimetric and light extinction techniques, Combust. Flame 102 (1995) 161-169. [4] D.R. Snelling, K.A. Thomson, G.J. Smallwood, Ö.L. Gülder, Two-dimensional imaging of soot volume fraction in laminar diffusion flames, Appl. Opt. 38 (1999) 2478-2485.

[5] K.-O. Lee, S.L. Manzello, M.Y. Choi, The Effects of Initial Diameter on Sooting and Burning Behavior of Isolated Droplets under Microgravity Conditions, Combust. Sci. Technol. 132 (1998) 139156.

Awardee and Student:

Dr. Yuhao Xu is an Assistant Professor with research interests in Droplet Combustion, Multiphase Heat Transfer, Microfluidics, and Biosensors. Caleb Riggins a senior, majoring in Mechanical Engineering.

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