Numerical Simulation of Transient Development of Flame,Temperature and Velocity under Reduced Gravity in a Methane Air Diffusion Flame

A methane air co flow diffusion flame has been numerically simulated with the help of an in-house developed code at normal gravity, 0.5 G, and 0.0001 G (microgravity) for the study of transient behavior of the flame in terms of flame shape, temperature profile and velocity (streamlines). The study indicates that lower is the gravity level, the higher is the time of early transience. The flame developments during transience are marked by the formation of a secondary flamelet at different heights above the primary flame at all gravity levels. The development of temperature profile at microgravity takes a much longer time to stabilize than the flame development. At normal gravity and 0.5 G gravity level, streamlines, during transience, show intermediate vortices which are finally replaced by recirculation of ambient air from the exit plane. At microgravity, neither any vortex nor any recirculation at any stage is observed. Centerline temperature plots, at all gravity levels during transience, demonstrate a secondary peak at some instants as a consequence of the secondary flamelet formation. The centerline velocity at microgravity decreases gradually during transience, unlike at other two gravity levels where the fall is very sharp and is indicative of negligible buoyancy at microgravity.     

Effects of Gravity on Soot Formation in a Coflow Laminar Methane/Air Diffusion Flame

Simulations of a laminar coflow methane/air diffusion flame at atmospheric pressure are conducted to gain better understanding of the effects of gravity on soot formation by using detailed gas-phase chemistry, complex thermal and transport properties coupled with a semiempirical two-equation soot model and a nongray radiation model. Soot oxidation by O₂, OH and O was considered. Thermal radiation was calculated using the discrete ordinate method coupled with a statistical narrow-band correlated-K model. The spectral absorption coefficient of soot was obtained by Rayleigh’s theory for small particles. The results show that the peak temperature decreases with the decrease of the gravity level. The peak soot volume fraction in microgravity is about twice of that in normal gravity under the present conditions. The numerical results agree very well with available experimental results. The predicted results also show that gravity affects the location and intensity for soot nucleation and surface growth.     

Experimental Study of Subcooled Flow Boiling Heat Transfer on a Smooth Surface in Short-Term Microgravity

The flow boiling heat transfer characteristics of subcooled air-dissolved FC-72 on a smooth surface (chip S) were studied in microgravity by utilizing the drop tower facility in Beijing. The heater, with dimensions of 40 × 10 × 0.5 mm³ (length × width × thickness), was combined with two silicon chips with the dimensions of 20 × 10 × 0.5 mm³. High-speed visualization was used to supplement observation in the heat transfer and vapor-liquid two-phase flow characteristics. In the low and moderate heat fluxes region, the flow boiling of chip S at inlet velocity V =?0.5 m/s shows almost the same regulations as that in pool boiling. All the wall temperatures at different positions along the heater in microgravity are slightly lower than that in normal gravity, which indicates slight heat transfer enhancement. However, in the high heat flux region, the pool boiling of chip S shows much evident deterioration of heat transfer compared with that of flow boiling in microgravity. Moreover, the bubbles of flow boiling in microgravity become larger than that in normal gravity due to the lack of buoyancy Although the difference of the void fraction in x-y plain becomes larger with increasing heat flux under different gravity levels, it shows nearly no effect on heat transfer performance except for critical heat flux (CHF). Once the void fraction in y-z plain at the end of the heater equals 1, the vapor blanket will be formed quickly and transmit from downstream to upstream along the heater, and CHF occurs. Thus, the height of channel is an important parameter to determine CHF in microgravity at a fixed velocity. The flow boiling of chip S at inlet velocity V =?0.5 m/s shows higher CHF than that of pool boiling because of the inertia force, and the CHF under microgravity is about 78–92% of that in normal gravity.     

Effects of Reduced Gravity Conditions on Bubble Dispersion Characteristics in the Bubble Column

Bubble-liquid turbulent flow has an excellent heat and mass transfer behaviors than single gas or liquid flow. In order to analyze the effects of normal and reduced gravity on cold bubble-liquid two-phase turbulent flow in bubble column a second-order moment cold bubble-liquid two-phase turbulent model was developed to disclose the bubble dispersion characteristics. Under the reduced gravity condition, volume fraction caused by the decrease of buoyance force is larger than normal gravity level due to bigger bubble solid volume. In addition, bubble frequency is also decreased by in decrease of buoyance force. Normal and shear stresses have strongly anisotropic characteristics at every directions and have larger values under normal gravity than reduced gravity. The liquid turbulent kinetic energy has the two-peak bimodal distribution and weaker than bubble turbulent kinetic energy with one peak unimodal, which is caused by vigorous wake fluctuations. The correlation of fluctuation velocities between bubble and liquid has clearly anisotropic behaviors Under reduced gravity, the bubble motion has a little impact on liquid turbulent flow caused by slight buoyancy force, however, it will greatly reduce the liquid turbulent intensity due to energy cascade transport, which was transformed into bubbles or dissipated by interface friction. Bubble formation and detachment mechanisms affected by gravity conditions lead to the different levels of bubble dispersion distributions.     

Evaluation of the extinction factor in a laminar flame established over a PMMA plate in microgravity

A methodology for estimating the extinction factor at λ=530 nm in diffusion flames is presented. All experiments have been in microgravity and have as their objective the production of quantitative data that can serve to evaluate the soot volume fraction. A better understanding of soot formation and radiative heat transfer is of extreme importance to many practical combustion related processes such as spacecraft fire safety. The experimental methodology implements non-axisymmetric configurations that provide a laminar diffusion flame at atmospheric pressure. PMMA is used as fuel. The oxidizer flows parallel to its surface. Optical measurements are performed at the 4.74 s ZARM drop tower.     

Laser-induced incandescence: excitation intensity

Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).     

Morphology Change in the Optical Microscopic Microstructure of Titanium by Nitriding Reaction and Combustion Synthesis Under Normal and Microgravity

Experiments on heating titanium wire specimens 0.5 mm in diameter by electric current in 0.5 absolute atmospheric pressure (0.5 kg/cm²) of a pure nitrogen gas environment under normal gravity (g = 9.8 m/s² = 1 G) and microgravity (g = G) using a drop shaft that enables 10 s of a microgravitational condition were carried out to investigate fundamentally the influence of microgravity on the reaction between Ti and nitrogen gas. The influence of gravitational acceleration on the morphological changes in optical microscopic microstructures of the specimens was studied. Under normal gravity, the thickness of the TiN layer resulting from the reaction of Ti and nitrogen gas at the outermost surface of the specimen increased with the heating time, but the tendency for the increase was reduced. A Widmanstätten structure, which is made by rapid heating and cooling of the specimen, was observed. However, the typical characteristics of the Widmanstätten structure vanished as the heating time proceeded. The progress of the nitriding reaction of the specimen was reduced, because heating by the electrical current and cooling by the convection of nitrogen gas balance each other out, and the temperature of the specimen became constant. Under microgravity, on the other hand, the thickness of the TiN layer of the outermost surface of the specimen increased with the heating time and was always thicker than that under normal gravity. The influence of microgravity, which suppresses the cooling effect of the thermal convection of nitrogen gas, on the nitriding reaction between titanium and nitrogen gas was significant. A dendritic structure caused by electrical heating was observed. Since the combustion synthesis of TiN was initiated at an elevated temperature after a certain heating time, huge dendritic structures were observed. The TiN tube was made on a final stage by leaching the titanium from the specimen. The influence of microgravity on the microstructure of the specimens that underwent nitriding was notable. Various results of the present study are explained by gravitational acceleration effects. The nitriding reaction became active by a temperature increase in the specimens with heating time due to the suppression of thermal convection and the cooling effect of the nitrogen gas on the specimens was reduced.     

Colloidal hard-sphere crystallization kinetics in microgravity and normal gravity

The hard-sphere disorder-order transition serves as the paradigm for crystallization. We used time-resolved Bragg light scattering from the close-packed planes to measure the kinetics of nucleation and growth of colloidal hard-sphere crystals. The effects of gravity are revealed by comparison of the experiments in microgravity and normal gravity. Crystallites grow faster and larger in microgravity, and the coarsening between crystallites is suppressed by gravity. The face-centered-cubic structure was strongly indicated as being the stable structure for hard-sphere crystals. For a sample with a volume fraction of 0.552, the classic nucleation and growth picture is followed.     

The Influence of the Gravity Force on Longwave Marangoni Patterns in Two-Layer Films

An Euler–Euler two-fluid model based on the second-order-moment closure approach and the granular kinetic theory of dense gas-particle flows was presented. Anisotropy of gas-solid two-phase stress and the interaction between two-phase stresses are fully considered by two-phase Reynolds stress model and the transport equation of two-phase stress correlation. Under the microgravity space environments, hydrodynamic characters and particle dispersion behaviors of dense gas-particle turbulence flows are numerically simulated. Simulation results of particle concentration and particle velocity are in good agreement with measurement data under earth gravity environment. Decreased gravity can decrease the particle dispersion and can weaken the particle–particle collision as well as it is in favor of producing isotropic flow structures. Moreover, axial–axial fluctuation velocity correlation of gas and particle in earth gravity is approximately 3.0 times greater than those of microgravity and it is smaller than axial particle velocity fluctuation due to larger particle inertia and the larger particle turbulence diffusions.     

Effects of Gravity on Ignition and Combustion Characteristics of Externally Heated Polyethylene Film

The objective of this research is to investigate the effects of gravity on the ignition and the combustion characteristics of the Polyethylene (PE) film by outer heating. Combustion experiments of PE film were carried out in a normal gravity field and the microgravity field. In the microgravity experiments, it was carried out in 50 m-class drop facility. Here it can be realized 10^??4G microgravity field in about 2.5-3.0 second. The PE film is heated by the inserted high-temperature chamber. In the experiments, the PE was used film type. The chamber temperature was fixed at 900 K and 1000 K. In the case of microgravity field, the ignition delay period has become about 50 percent shorter than that in the case of the normal gravitational field. In the normal gravity field, since the PE surface layer is cooled by natural convection, the ignition delay period is considered to be longer than that in the microgravity field. The combustion time in the normal gravity was about 0.8 sec. In the microgravity field, the combustion time was more than 2 sec, and it could not be measured during the free fall period.     

Optical diagnostics on sooting laminar diffusion flames in microgravity

Laser-induced incandescence was successfully applied to the investigation of soot formation in both buoyant and non-buoyant laminar jet diffusion flames. Microgravity experiments were conducted in the Drop Tower Bremen, Germany. By the use of imaging laser-induced incandescence (LII) it was possible for the first time to obtain simultaneously two-dimensional information on soot concentration and primary particle size under microgravity. Additionally, temperature fields were measured by 2-color emission pyrometry. Results for the fuels propane and ethene show that soot formation and oxidation is drastically altered under microgravity. Maximum temperatures are reduced by roughly 220 K and 120 K, respectively, which in the case of ethene results in a termination of oxidation processes and the emission of soot. The distribution of soot within the non-buoyant flames is always concentrated in relatively small bands. For all non-buoyant flames investigated the maximum primary particle size roughly doubles compared to the buoyant ones.     

Numerical simulation of soot formation in pulverized coal combustion with detailed chemical reaction mechanism

A two-dimensional unsteady numerical simulation with a detailed chemical reaction mechanism that considers 158 species and 1804 reactions is applied to pulverized coal combustion in a mixing layer and the soot formation behavior is investigated in detail. The computational conditions and ignition process are the same as those in our previous work (Muto et al., 2017). The results show that the peak of the mass density of the soot is distributed in the region where the gas temperature is higher than the unburned gas temperature of the mixture of volatile matter and air (1300–1400?K) and lower than the flame temperature (2000?K $～$). This is due to the fact that soot formation from the precursors (C₂H₂ and C₆H₆) is enhanced as the gas temperature increases, whereas the quantities of the precursors and the produced soot are reduced due to oxidation at the higher gas temperature condition that exists close to the flame. The peak value of the mass density of the soot is also distributed in the region between the peak values of the gas temperature and the probability density function of the number of coal particles.     

The effects of gravity on the features of the interfacial waves in annular two-phase flow

A physical model of interfacial waves in annular two-phase flow was studied in both microgravity and normal gravity. The wave structure was obtained for local film thickness and velocity measurements using a conductance probe technique. It was found that the wave height, and not its width, is strongly affected by changing the gravity level. In fact, the wave height in normal gravity is more than twice that in microgravity. Using an analogous approach to a turbulent, single-phase flow in a rough tube, a preliminary mathematical model was proposed to calculate the wave amplitude. The model fits well with the experimental data and shows that the wave height in normal gravity is approximately 1.7 times the combined thickness of the viscous sublayer and transition zones in the turbulent gas stream. The wave height in microgravity was estimated to be approximately 80% of the total thickness.     

Iron Burning in Pressurised Oxygen Under Microgravity Conditions

An investigation of cylindrical iron rods burning in pressurised oxygen under microgravity conditions is presented. It has been shown that, under similar experimental conditions, the melting rate of a burning, cylindrical iron rod is higher in microgravity than in normal gravity by a factor of 1.8 ± 0.3. This paper presents microanalysis of quenched samples obtained in a microgravity environment in a 2.0 s duration drop tower facility in Brisbane, Australia. These images indicate that the solid/liquid interface is highly convex in reduced gravity, compared to the planar geometry typically observed in normal gravity, which increases the contact area between liquid and solid phases by a factor of 1.7 ± 0.1. Thus, there is good agreement between the proportional increase in solid/liquid interface surface area and melting rate in microgravity. This indicates that the cause of the increased melting rates for cylindrical iron rods burning in microgravity is altered interfacial geometry at the solid/liquid interface.     

Investigations on Soot Formation in Heptane Jet Diffusion Flames by Optical Techniques

Two-dimensional optical measurements were performed for the investigation of soot formation of n-heptane laminar gas-jet diffusion flames under buoyant and non-buoyant conditions utilizing the Bremen Drop Tower. Techniques employed were laser-induced incandescence for the determination of soot concentration and primary particle sizes and two-color emission pyrometry with a three-point Abel inversion for the measurement of temperature fields. In line with former experiments for other hydrocarbon fuels the investigations revealed drastic differences in the sooting behavior between flames under normal and microgravity. With the lack of buoyancy maximum soot temperatures were reduced by roughly 300 K and maximum primary particle sizes were more than doubled.     

Quantitative investigation of soot distribution by laser-induced incandescence

Strategies employed for quantitative measurement by laser-induced incandescence are detailed. Data are obtained for several laminar diffusion flames formed from blended Diesel fuels of known composition. A tomographic procedure is developed to scale the two-dimensional data to soot volume fraction and to correct for the trapping of signal by the soot field. Scaling is achieved by use of laser extinction along the measurement plane. The findings are used in discussions of measurement issues within turbulent environments. Data are augmented with elastic scattering measurements, allowing particle-size and number-density distributions to be inferred. A degree of axial and radial similarity among various flames suggests that the processes of soot formation and oxidation occur over similar time scales for each fuel.     

Two-color laser-induced incandescence (2C-LII) technique for absolute soot volume fraction measurements in flames

A two-color version of the laser-induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in flames. By using a calibrated tungsten ribbon lamp, soot peak temperatures were measured as a function of fluence at several locations in an ethylene diffusion flame by using a steeply edged laser beam profile. Above a certain fluence threshold, peak temperatures were tightly distributed just above 4000 K independent of the particle size and number density. Radial profiles of soot volume fraction were obtained and compared (not calibrated) with results from the laser extinction technique. Good agreement showed the validity of the 2C-LII technique at a controlled fluence.     

Numerical Simulation of Bubble Dynamics in Microgravity

A numerical method for the simulation of two-phase flows under microgravity conditions is presented in this paper. The level set method is combined with the moving mesh method in a collocated grid to capture the moving interfaces of the two-phase flow, and a SIMPLER-based method is employed to numerically solve the complete incompressible Navier-Stokes equations, and the surface tension force is modeled by a continuum surface force approximation. Based on the numerical results, the coalescence process of two bubbles under microgravity conditions (10^???2×g) is compared to that under normal gravity, and the effect of gravities on the bubbles coalescence dynamics is analyzed. It is showed that the velocity fields inside and around the bubbles under different gravity conditions are quite similar, but the strength of vortices behind the bubbles in the normal gravity is much stronger than that under microgravity conditions. It is also found that under microgravity conditions, the time for two bubbles coalescence is much longer, and the deformation of bubbles is much less, than that under the normal gravity.     

重力对聚合物形成的影响

在聚合物合成过程呼聚合物凝固过程中,聚合物分子量和聚合物颗粒尺寸的大小,分布和聚合物聚集态结构强烈依赖于重力,本文分增力,减重力和微重力三部分阐述了近年来重力对聚合物形成影响研究概况,重点介绍了微重力下聚合物一些研究成果,并指出了今后的发展方向。     

Two-phase Flow in Fuel Cells in Short-term Microgravity Condition

A visual observation of liquid–gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.