In the study of combustion characteristics of liquid rocket fuels, it is customary to either study the combustion of liquid fuel droplets or the combustion of fuel sprays. However, the two are closely related to each other, because in a typical rocket combustion chamber, the burning of droplets, droplet clusters, and fuel sprays occur simultaneously.

Schematic layout of suspended droplet experiment test facility consisting of the high-speed imaging configuration and the photoignition system

The study of droplet burning is mostly focused either on single suspended droplets or free droplets, usually in a free fall. In either case, the conditions regarding the combustion of a single fuel droplet deviates from what is expected in typical liquid rocket engines (LREs). This not only concerns the chamber pressure and the temperature that is usually much lower in a typical droplet study, but also the droplet ignition process and the related transient effects such as temperature ramp up that is much slower than what takes place in LREs. As a result, reported values of combustion parameters such as the burning rate constant and the ignition delay time obtained under typical experimental conditions may not entirely represent the combustion processes in LREs.

The burning of a single fuel droplet involves evaporation of the fuel at the surface of the droplet, which leads to a diffusion flame to form around the droplet as the fuel vapor reacts with the surrounding air. While in LREs the flame regime encompasses the entire spray, characterizing the burning of single droplets is required to assist with modeling of such sprays. Specifically, the fundamental processes that occur during the burning of individual droplets are pertinent to understanding combustion dynamics for multiphase reactive systems that are prone to self-excited combustion instabilities.

Another objective of this research was to study possible effects of a faster and hotter ignition approach on the combustion characteristics of suspended hydrocarbon fuel droplets. Photoignition (PI) was selected as an alternative ignition method because it is much faster than any conventional approaches in droplet ignition. Another advantage of PI is the ease of its application for subscale test rocket engines at elevated pressures.

Substantial experimental work has been devoted to the ignition of single droplet and fuel spray, as well as the ignition delay in past decades. However, a systematic study to correlate the stated combustion parameters to the type and the concentration of fuel additives for rocket relevant fuels is yet to be reported. The present experimental study has been focused on the burning characteristics of various hydrocarbon fuels to quantify changes in important combustion metrics related to combustion dynamics. The goal of this research was to investigate the burning rate constant and the ignition delay in different hydrocarbon fuel droplets under ambient conditions.

In order to investigate combustion characteristics of hydrocarbon fuels, the suspended droplet approach was implemented. The fuel droplet with a nominal size of 1.4±0.1 mm was suspended from a 0.15 mm uncoated quartz fiber from Polymicro Technologies Inc. The droplets were first produced using a hypodermic needle and then mechanically transferred to the fiber. To increase the surface tension of the fuel droplet on the fiber, a bead was generated at the end of each fiber through a heating process using an oxy-propane torch after the polymer coating of the fiber was chemically removed. Each quartz fiber was used for a set of 10-15 identical experiments in order to generate statistically reliable data. The fiber was flame treated with a butane torch and mechanically cleaned between tests, as needed.

The burning of the fuel droplet was captured by a high-speed camera, model Phantom v7.1 from Vision Research Inc., imaging at 1,000 fps. In order to provide the proper contrast between the liquid droplet and background for image processing, a cluster of adjustable intensity white light emitting diodes (LED’s) was implemented to provide sufficient backlighting to create shadowgraph images of the droplet burning process. A dual-band bandpass filter, Semrock FF01-433/530-25, was placed in front of the camera in order to limit the spectral spread of light reaching the high-speed camera during the burning of fuel droplets. The LED’s were turned off for the evaluation of the ignition delay time of the droplets in order to observe the appearance of the flame that was indicative of the initiation of droplet combustion.

This work was done by Alireza Badakhshan, Engineering Research Corporation; John W. Bennewitz, UCLA; and Douglas G. Talley for the Air Force Research Laboratory. AFRL-0254

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Combustion Characteristics of Hydrocarbon Droplets Induced by Photoignition of Aluminum Nanoparticles

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This article first appeared in the August, 2017 issue of Aerospace & Defense Technology Magazine.

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