Currently, multi-element diffusion flames find applications in flame synthesis, combustion mechanism studies, and aerospace engine design. Therefore, investigating the characteristics of multi-element diffusion flames under both terrestrial gravity and microgravity conditions is crucial. In this study, numerical simulation methods are used to investigate the structural characteristics of ethylene-oxygen multielement laminar diffusion flames and the effects of pressure and oxygen volume fraction on the flame structure under both terrestrial gravity and microgravity conditions.

This study was conducted using ANSYS Fluent software. First, a geometric model of the multi-element combustion chamber was constructed. The selected computational model included solving flow using a laminar model, diffusion using Fick's law, chemical reactions using a finite rate model, and radiation using a discrete ordinates model. Grid independence verification was performed, with 600, 000 grids chosen for calculations eventually. After the computational model was established, three sets of operating conditions were designed to study variations in flame behavior under different gravitational accelerations (0-9.8 m/s²), pressures (50-500 kPa), and oxygen volume fractions (0.25-1.00). The flame height obtained from the numerical simulation differed by less than 10% from the experimental results; thus, our method was considered to provide reliable results.

The results indicated that the flame had a double-layer structure. With decreasing gravity, because of the inhibition of buoyancy, the flame height increased from 7.3 to 12.8 mm, whereas the flame temperature decreased by 300 K. With increasing pressure, both the outer flame height and width decreased. At normal gravity, the temperature increased by 590 K, whereas it increased by only 80 K at microgravity. At 500 kPa pressure, the normal gravity fire separated, changing from a closed-tip flame to an open-tip flame at microgravity. As the volume fraction of oxygen decreased from 1.00, the flame height gradually increased. When it reached 0.50, the flame changed from a single flame to a double-layer one. Under normal gravity conditions, the flame temperature decreased by about 250 K, whereas it decreased by 600 K under microgravity conditions.

The multi-element diffusion flame exhibited a double-layer structure under atmospheric pressure and fuel-rich conditions, with the inner and outer flames generated by the combustion of ethylene and CO, respectively. Meanwhile, modifications in pressure or oxygen volume fractions could change the shape from double-layer fire to separate flames or open-tip flame. The microgravity conditions enhanced the role of radiative heat transfer, leading to a significant decrease in flame temperature and eliminating convective mass transfer caused by buoyancy, thus increasing the flame height and width. Increasing pressure accelerated the reaction rate, increased the flame temperature, and reduced the flame height and width. Under microgravity conditions, increasing pressure enhanced the radiative heat transfer and lowered the flame tip temperature. Reducing the oxygen volume fraction reduced the flame temperature, increased the flame height, and converted the flame from separate flames to a double-layer flame, which was more susceptible to radiative effects and had a particularly low flame temperature in microgravity.

Flame synthesis is a method for the preparation of condensed-phase nanomaterials. It is energy efficient, cost effective and has the potential for large scale industrial applications.

This review examined the control technologies for the morphology, composition and microstructure of flame synthesized condensed-phase nanomaterials from four aspects, i.e., flame stability, flame temperature and species profiles, product particle size and morphology, and product physicochemical properties. The flame stability control part mainly introduced the swirl stabilization method, including both strong swirl stabilization (S_{g, tan} > 5 or S > 0.6) and weak swirl stabilization (S≤0.6). The calculation and design criteria of different swirl numbers for different types of swirlers were evaluated. Methods such as the addition of pilot flame, the increase of active component concentrations at the flame nozzle, the use of high-enthalpy precursor solution, and the addition of sheath gas were also summarized. For the control of flame temperature and species profiles, adjustments might be made to the fuel-air equivalence ratio, oxygen flow rate for atomization, or precursor liquid flow rate and concentration. The installation of the cooling meshes and quenching rings, water addition to precursor liquid and high-enthalpy solvent replacement were also good strategies. The particle size and morphology of the product might be controlled indirectly by adjusting the flame temperature and species profiles which could affect the particle dynamics process. From another perspective, it might also be regulated by substrate materials, droplet microexplosion, precursor ultrafine atomization, inclusion of high boiling point active agent, and plasma discharge, etc. These were realized by controlling the droplet size before particle formation and regulating particle attributes for precipitation, collision, and condensation, thereby changing its size and shape. Four primary approaches for controlling physicochemical properties were summarized, including crystal structure control, element doping, core-shell structure design and post-heat treatment. The crystal structure control mainly included temperature-induced phase transitions and doping-induced phase transitions. The element doping was mainly achieved by regulating precursor liquid with different components and different proportions. The core-shell structure design was mainly realized by using the different precipitation characteristics of different solutes or by the installation of auxiliary devices to stagger the time and space distributions of the two different kinds of materials. The post-heat treatment process primarily consisted of the annealing procedures at different atmospheres, temperatures and time durations to remove unwanted combustion residues and defects, and to induce phase transformation, etc.

The first two aspects of this review mainly focus on the control of the external parameters such as flame temperatures and species profiles, while the latter two on the control of the internal properties of the materials such as particle size and physicochemical properties. In actual experiments or production, the control technologies will need to be comprehensively used according to specific situations. Considering the complexity of the composition of the precursor liquid, the stability, volatility, and precipitation characteristics had better be comprehensively analyzed before flame synthesis, so that the mechanism identification and active regulation of the material nucleation and other processes will be enabled from the initial stage of particle formation.