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Infrared thermographic images depicting the ignition sequence of an n-dodecane droplet using the plasma arc ignition system with a duration set to 100 ms.
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This experimental study investigated droplet burning properties of ten common fuels including
two alcohols, four simple hydrocarbons and four kerosene blends. Specifically, ignition
delays and burning rate constants were quantified for each of these fuels using a combination
of high-speed IR and visible imaging. Ignition delays measured using a sho...
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Citations
... The tip of the fiber was fused into a sphere to hold the droplet, as shown in Figure 28d. This basic method of a vertical or horizontal suspender fiber has been used in many experimental studies, e.g., [81,133,137]. In the case of droplet vaporization, it was found that there is a transient heating phase that precedes the steady-state. ...
... For the experiments, they used two electrical spark ignition sources that were placed symmetrically around the droplet and retracted after ignition [143]. The spark ignition method was also used in many other studies, e.g., [136] or very similar methods such as arc discharge ignition [101] or plasma arc ignition [137]. In an experimental study, Yates et al. [144] investigated the spark ignition of hydrogen in oxygen and nitrogen mixtures at 100 K between 0.03 to 0.28 MPa. ...
The combustion of hydrogen and oxygen is important in the field of liquid rocket propulsion and in the near future also in aeronautical and automotive propulsion as well. In a rocket engine, a spray combustion process with gaseous hydrogen and liquid oxygen is formed as a result of the vaporization of the liquid hydrogen before it enters the combustion chamber. To optimize engine performance by improving combustion efficiency and aiming for smaller combustion chambers as well as to avoid combustion instabilities, a fundamental understanding of the vaporization and combustion processes at relevant ambient pressures is necessary.
In order to study these fundamental processes focusing on the most central element of the technical spray combustion process, the single droplet, an experimental apparatus was developed which allows the combustion of single liquid oxygen droplets in a hydrogen atmosphere. It is shown that the dimensions of a single droplet in a technical spray (ร5-150 ฮผm) can be scaled up in reduced gravity conditions. The experimental setup was integrated into a drop capsule to perform experiments under microgravity conditions at the drop tower in Bremen to allow for the application of sophisticated diagnostics on large stationary and mostly spherical droplets. The main component of the setup is the cryogenic combustion chamber, which is surrounded by a liquid nitrogen jacket. The temperature of 77 K allowed gaseous oxygen to condense and to generate a liquid oxygen droplet at the tip of a quartz suspender. The droplet with a diameter of about 0.7 mm was ignited by a laser-induced plasma breakdown at different ambient pressures in the sub- and supercritical regime (0.1 to 5.7 MPa), and the combustion during the free-fall phase was investigated by shadowgraph imaging and ๐๐ป chemiluminescence diagnostics (hydroxyl radical).
Despite a wide flammability range of the hydrogen/oxygen system, the ignition had to take place very close to the droplet surface due to the high diffusion rate of hydrogen. As a consequence of the ignition, the burning droplet detached from the suspender in all experiments and burned free-floating next to the suspender. This was initially seen as a problem but the detachment meant that the droplet was no longer affected by the suspender. At low pressures (< 0.8 MPa), it was observed that the shape of the droplet changed significantly during combustion, which is attributed to the formation of a (water) ice crust in close vicinity to the droplet surface. In a parametric study, it was found that the burning rate in the subcritical regime increases significantly with increasing pressure, whereas the flame standoff ratio decreases only slightly. In the supercritical regime, the measured data indicate a slight decrease in the burning rate. However, due to the vanishing of the surface tension, no defined detachment of the droplet from the suspender occurred, so that the subcritical experiments are not directly comparable with the experiments under supercritical conditions. An alternative evaluation method indicates a continuous increase of the burning rate in the transition to the supercritical regime.
So far, only numerical simulations on single liquid oxygen droplet combustion have been performed and reported in the literature. By means of the developed experimental apparatus it has become possible to provide a first experimental database (exploiting microgravity conditions). These data provide key combustion research parameters, such as the flame standoff ratio or the burning rate constant, which can be used for validation and future development of numerical models. These fundamental models form the basis for advanced simulations that also include the interactions in a spray and can lead to an improvement in the volumetric heat release and thus of the efficiency of an engine.
This experimental study explores the effects of acoustic excitation on burning droplets of liquid ethanol loaded with reactive aluminum nanoparticles (nAl). Continuously fed fuel droplet combustion and flame extinction (blowout) experiments were conducted in the vicinity of a pressure node (or velocity antinode) created in a closed acoustic waveguide, with a range of applied resonant forcing frequencies, pressure or velocity perturbation amplitudes, and particle loading concentrations. Simultaneous phase-locked OH* chemiluminescence and visible images were taken to quantify the influence of nanoparticle concentration on burning rate constant (K) and flame-acoustic coupling dynamics. In the presence of velocity perturbations, nAl-laden droplets were observed to burn for longer periods of time than they burned in the absence of acoustics, likely resulting from suppression of particle agglomerate formation and expulsion. It was found that at higher forcing frequencies, there were relatively greater enhancements in K values of nAl-loaded droplets than for lower frequencies. Interestingly, while at higher forcing amplitudes, the burning rate constant increased for a given loading concentration, that increase tended to be reduced at higher loading concentrations. Phase-locked imaging demonstrated that the presence of nAl increased the Rayleigh index (G) as compared with neat fuels under the same forcing conditions. Higher amplitude excitation leading to extinction showed that the addition of nAl could significantly increase the mean extinction strain rate, in some cases by up to 44%.
The effect of nanoscale energetic aluminum (nAl) and inert silicon dioxide (nSiO2) particulate additives on ethanol droplet combustion was studied under atmospheric conditions. Three different types of droplet experiments were performed to study the influence of the experiment itself on combustion behavior. Simultaneous visible and intensified ultraviolet (UV) images were taken to determine the burning rate constant (K) as well as flame dynamics via OH* chemiluminescence imaging. The addition of nAl appeared to yield a systematic increase in K, by up to 13%, and increasing loading concentrations led to changes in droplet combustion dynamics. Flow instabilities, including liquid jetting and altered droplet deformation, were observed, creating unsteady combustion when the nAl-laden droplet was continuously fed via a quartz capillary. In contrast, the addition of nSiO2 showed relatively small changes in K, possibly only as large an increase as 5%, with a lack of consistent trends for increasing nSiO2 concentration for different fuel delivery methods, in part due to the formation of large residual shell-like structures in the later stages of combustion. A simple droplet combustion model suggests that possible enhancement mechanisms for K are related to alterations in thermal conductivity as well as flame temperature with the nAl additive. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of particulate residue revealed further differences in morphology and residue constituents after combustion.
Two new methods for igniting suspended droplets, namely Plasma Arc Ignition (PAI) and Photoignition (PI), were developed which offer relatively short ignition initiation times within 65โ110 ms range for all fuels except hexadecane that is about 260 ms. The fuels included ethanol, methanol, n-heptane, n-dodecane, toluene, RP-2, JP-8, Fischer-Tropsch (FT), diesel #2, and hexadecane. The resulting average droplet ignition delays were up to an order of magnitude smaller than other methods for ignition of fuel droplets reported in the literature such as heated furnaces and ignition coils. The significant reduction in the ignition delay is most likely due to the substantially hotter ignition temperatures (โฅ2000 K) of these methods, which makes them more suitable for relatively well-defined ignition events and capable of igniting less volatile fuels such as diesel and hexadecane. To the best of our knowledge, this is the first report on the use of these techniques for the ignition of suspended fuel droplets.
