A microgravity experiment to elucidate the cool flame dynamics in a multi-droplet system was conducted using the TEXUS-60 sounding rocket. The cool flame position was successfully observed through formaldehyde chemiluminescence. This allowed the study of cool flame propagation speed along n-decane droplet arrays, with droplet spacings of 8 mm (nine droplets) and 16 mm (five droplets). The spread speed was analyzed at an ambient temperature of 570 K and a pressure of 0.1 MPa, using air as the ambient gas. For both droplet configurations, the cool flame occurred from nearly the same position 4.7 s after the droplet was inserted and settled at the combustion position. The cool flame spread speeds decreased from around 250 mm/s to approximately 100 mm/s after the spontaneous ignition, and then increased. The maximum speed reached around 600 mm/s for the 16 mm droplet spacing case. Under these experimental conditions, the results suggest that in the first stage the cool flame spread follows the premixed propagation mode, while in the latter stage the acceleration may be driven by the sequential spontaneous ignition.

Fires in human space flight must be avoided as much as possible. While materials are tested under terrestrial conditions for fire safety in space, fire behavior may be different in the absence of gravity. The usual academic approach to microgravity fire experiments is to use opposed or concurrent flow configurations. This study utilizes a more practical configuration. Samples are ignited at point sources rather than planarly, enabling the multi-directional investigation of flame spread. Furthermore, flame interaction between two ignition points is also studied. Drop tower experiments with one or two point ignition are carried out using 0.2 mm thick polymethyl methacrylate sheets with 40 mm width and 60 mm length. Pressure and flow are maintained at 75 kPa and 10 cm/s respectively, while the oxygen concentration is varied between 21 vol% and 35.4 vol%. Flame spread and sample heating are evaluated by infrared and visual image analysis. Previous experiments in opposed flow configuration are used for reference. After ignition, the flames spread in a circular manner from the ignition point, resulting in varying preheating behavior of the samples. Flame spread rates vary with time and flame size. Especially close to ignition, there is a transient phase in which the flame spread rate increases, whereas the flame spread rate approaches a steady phase with increasing flame size. Lateral flame spread is in some cases faster than opposed flame spread. Two point ignition results in the flames interacting with each other. Flame spread accelerates as the flames approach each other, especially with a flame distance smaller than 10 mm. These experiments show that non-planar ignition and multiple ignition sources can have a significant impact on multi-directional flame propagation in microgravity, which is important for real-world spacecraft fire scenarios.

In human spaceflight, a fire on board a spacecraft is one of the most devastating scenarios. To improve fire safety on board of a spacecraft, material tests are performed to characterize the fire behavior of materials. In real-world scenarios, however, not only continuous materials may be affected, but also discrete, spatially separated fuels. To better understand the flame propagation of discrete fuels in microgravity, a series of drop tower experiments was conducted with thin poly(methyl methacrylate) sheets. Material samples were placed in a 96 mm x 96 mm x 264 mm wind tunnel and the desired flow and gas mixture was initiated using three mass flow controllers. The flame propagation was observed by two visual cameras (front and side view) and an infrared camera (front view). The gap distance between fuel strips and the atmospheric conditions were varied. The atmospheres were selected along the normoxic curve with oxygen mole fractions O2 ranging from 0.21 to 0.354, and respective pressures between 101.3 kPa and 60 kPa. The opposed flow was held constant at 150 mm/s. Depending on the gap width and atmospheric conditions, gaps either led to an increase in flame spread rates or stopped the propagation entirely. Limiting gap widths were found to lie between 5 mm for xO2 = 0.21 and close to 7 mm for xO2 = 0.354 and are approximately linearly dependent on the oxygen mole fraction in the observed parameter regime. If a gap was jumped by the flame, the apparent propagation speed was recorded to be between 1.5 and 3 times faster than on full fuel samples. With respect to the whole sample, the mean propagation rates could be seen to increase with decreasing fuel coverage.

The flame spread of thin PMMA sheets has been studied both in opposed configuration under microgravity (9.3 s drop tower) and in downward configuration under normal gravity conditions. The position of the flame front has been determined with the help of an infrared camera. Propagation rates have been measured over a range of 30–200 mm/s forced opposed flow (only microgravity), 60–101.3 kPa ambient pressure, and oxygen concentrations of 21–35.4 vol. %, which corresponds to real environmental conditions on spacecraft. In addition to variations along the normoxic curve with constant oxygen partial pressure, the parameters oxygen and pressure were examined independently of each other in order to be able to determine their respective effects. The variation of the flow velocity was carried out at 60 kPa and 35.4 vol. % oxygen. The results for both microgravity and normal gravity are discussed separately on the basis of the respective atmospheric effect. Along the normoxic curve, a significant increase in the flame spread rate was found with increasing oxygen concentration and correspondingly decreasing pressure. This results in a significant increase in the risk of fire with regard to future exploration missions. The effect of the flow velocity cannot be neglected in the investigated velocity range and was linked to the influencing parameters in a correlation. Based on this parameterization, a linear function is given that reflects both the downward and the opposed results well. This function can be used to predict the flame spread in the given parameter space. Under normal gravity, the tested samples show a slightly increased propagation rate in most cases. This can essentially be attributed to the difference in velocity between the buoyant flow and the forced flow in µg.

IR videography has proven useful for tracking the position of the pyrolysis front under a flame propagating upward in 1g. Since PMMA burns with low soot formation in 1g conditions, the radiation density of the gases above the sample is very low and the flame is hardly visible. As a result, the surface can be easily followed and the temperature at the pyrolysis front can be determined. The phenomenological definition of the pyrolysis front remains difficult. In experiments on concurrent propagation along cylindrical PMMA samples under µg conditions during the Sounding Rocket flight TEXUS 57, it was shown that the drastically increased soot production compared to terrestrial conditions leads to a significantly increased radiation fraction by the gas phase above the sample. Here, pyrolysis front temperatures were determined to exceed 450 °C and include a significant radiative fraction of soot formed between the flame and the sample. This also sheds light on the significantly elevated local temperatures under µg conditions, which can lead to self-sustaining burning of some materials in microgravity that are inflammable or extinguishing on ground– this despite the overall reduced mass burn rate observed in microgravity. The paper compares pyrolysis front velocities in upward configuration (1g) along cylindrical PMMA samples with diameters of 5, 20, and 25 mm with concurrent propagation (µg) of the same samples. Both the 1g and µg experiments were performed in hypoxic exploration atmosphere (70 kPa, 26.5% O2). While the 1g experiments were subjected to buoyant convection only with a typical but averaged flow velocity of 20 to 30 cm/s, a forced inflow between 20 and 30 cm/s was set for the µg experiments. Soot formation was also analyzed in the microgravity tests.

The combustion of single liquid oxygen droplets in gaseous hydrogen is investigated experimentally under microgravity conditions to shed light on spray combustion processes in rocket engines. Using a drop tower apparatus, experiments are performed varying the ambient pressure between 0.1-5.7 MPa, which corresponds to a reduced pressure of oxygen p(r) between 0.02-1.12. The combustion is investigated using high-speed shadowgraph imaging to track the droplet shape and OH-chemiluminescence to identify the flame zone. At low pressures (p(r) < 0 . 15 ), the droplet shape is found to change significantly during combustion likely due to the formation of a water ice layer around the droplet. Small jets of oxygen appear to break out of this ice layer, leading to an observed increase in linear and angular momentum of the droplet. At higher pressures, the visible effect of ice formation near the droplet surface decreases. The combustion process at different pressures in the subcritical and the supercritical regime is compared and discussed. The pressure has a limited influence on the flame standoff ratio, whereas it influences the burning rate constant substantially. Specifically, the experimental data suggest a maximum of the burning rate constant near the critical pressure, which is consistent with several experiments on hydrocarbon droplet combustion. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

To investigate the basic combustion processes in a cryogenic rocket engine, the combustion of individual liquid oxygen droplets in a hydrogen atmosphere under microgravity conditions is experimentally and numerically investigated. For this purpose, an experimental setup is used in the ZARM (Center of Applied Space Technology and Microgravity) drop tower. In a combustion chamber cooled with liquid nitrogen, a single oxygen droplet is produced on a quartz suspender and ignited with a laser-induced plasma spark. The subsequent combustion is investigated with various diagnostics. Shadowgraph images are used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame is determined by OH-chemiluminescence. The initial droplet width varies between 0.66-0.93 mm and combustion is investigated at pressures between 1 and 45 bar. The effects of ambient pressure on combustion are discussed by means of the burning rate and the droplet-to-flame ratio. As the pressure increases, the burning rate increases, but the droplet-to-flame diameter ratio decreases slightly. The condensation or freezing of water vapor outside the reaction zone and near the droplet surface is investigated for different pressures. As the pressure increases, the distance between the condensation/freezing zone and the flame decreases. In addition, a numerical model for hydrogen-oxygen combustion is presented. Simulation results predict the emergence of three, distinct flame regions during the ignition transient; two premixed flames and a central, diffusion flame. For the quasi-steady diffusion flame that results, the numerical results point to condensation/freezing outside the reaction zone, qualitatively confirming the experimental results.

To investigate the fundamentals of liquid oxygen droplet combustion in hydrogen under microgravity conditions, a drop tower apparatus has been developed. In a cryogenic combustion chamber cooled with liquid nitrogen, single oxygen droplets with a diameter of 1 mm are suspended at the tip of a thin suspender. Ignition is accomplished after microgravity conditions are reached by a near-infrared laser, which generates a plasma spark positioned in the immediate vicinity of the oxygen droplet. The subsequent combustion is investigated with various optical diagnostics. Shadowgraph imaging is used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame are determined by OH chemiluminescence imaging. The oxygen/hydrogen combustion at two different pressure levels is reported. At a pressure of 0.1 MPa, irregular structures appear to form on the droplet surface during the combustion process, which lasts 137 ms before the droplet is completely consumed. The formation of these irregular structures is consistent with the condensation and freezing of water vapor in the cold-temperature region near the droplet surface. At a higher pressure of 4 MPa (reduced pressure pr,O2 =0.79), the water ice formation is significantly reduced due to the shorter burning time of 66 ms and the closer proximity of the flame to the droplet surface. The measured burning constant is k0.1 = 5.5 mm2/s at 0.1 MPa and k4 = 7.1 mm2/s at 4 MPa, respectively. This increase with increasing pressure is consistent with the smaller flame standoff distance.

Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations-one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.

Florian Meyer, University of Bremen, Center of Applied Space Technology and Microgravity (ZARM), Germany

Laser shock forming is a new high speed forming process based on TEA-CO2-laser induced shock waves. In former publications laser shock forming was already presented as a process which can be used for deep drawing, stretch drawing and cutting of thin copper and aluminum sheets. The process utilizes an initiated plasma shock wave on the target surface, which leads to the sheets forming. Several pulses can be applied at one point in order to achieve a high forming degree without increasing the energy density beyond the ablation limit. During the process, pressure peaks in the range of some MPa can be achieved. In order to classify the process in the framework of high speed forming processes, the temporal varying deformation velocity due to different materials have been identified based on a stretch drawing process by using different pulse energies. Therefore a new high speed measurement system based on the shadowing effects is designed and its suitability is shown. The determined strain rate of 520 s-1 meets one of the criteria for the classification of laser shock stretch drawing as a high-speed forming process.

Christian Eigenbrod, University of Bremen, Center of Applied Space Technology and Microgravity (ZARM), Germany