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• Homogeneous nuclei manifest when stack power exceeds 31 kW. • The average median diameter of heterogeneous droplets increases by 26.5 %. • A short residence time yields minimal droplets of 1.79 mg/s at 100 kW. • Condensation induces a maximum 4.18 % reduction in the entrainment ratio. The hydrogen recirculation ejector plays a pivotal role in proton exchange membrane fuel cell systems. Nevertheless, a comprehensive grasp of the intricate two-phase flow characteristics within hydrogen recirculation ejectors remains elusive. This investigation delves into the condensation and droplet behaviors of an ejector designed for a 100 kW fuel cell stack. The analysis relies on a two-phase flow model that takes into account both homogeneous and heterogeneous condensation. The findings reveal that homogeneous condensation nuclei manifest within the mixing chamber when the stack power surpasses 31 kW. The average median diameter of homogeneous droplets at the ejector outlet measures 0.367 μm, whereas that of heterogeneous droplets stands at 1.265 μm, signifying a 26.5 % augmentation compared to their initial size. Additionally, two crucial factors impact droplet size: residence time and subcooling degree. At the 100 kW condition, the droplet residence time is a mere 0.272 ms, yielding a meager droplet flow rate of merely 1.79 mg/s. In contrast, at the 53 kW condition, the downstream of the mixing chamber exhibits the maximum subcooling degree, resulting in the maximum droplet flow rate of 22.12 mg/s. A temperature elevation of 2.68 K at the ejector outlet is observed due to the latent heat of condensation. Furthermore, condensation has a marginal impact on the entrainment ratio, with the most significant reduction being 4.18 %, averaging at 1.42 %. [ABSTRACT FROM AUTHOR]

• Effects of wind on the pool fire flame morphology and flow field were revealed. • Flame half-width of pool fires under wind were quantified using scaling analysis. • The crosswise profiles of heat flux and temperature show a similar trend. • The downstream crosswise heat flux was described by the Boltzmann function. This paper reveals the crosswise profiles of heat flux and temperature over the downstream surface (ground) of a pool fire in relative strong wind, which was not well addressed in the past. Two square gaseous burners (25 cm, 30 cm) are used with propane as the fuel producing different heat release rates to simulate pool fires. The wind speed ranges from 1.5 m/s to 4 m/s (with 0.5 m/s intervals for various cases), concerning relatively strong wind conditions (Fr > 0.87). The crosswise heat flux profile first shows to be nearly stable (fluctuation less than 20 %) in the continuous flame region, then a sharp decrease in the intermittent flame region and finally goes to near zero. The temperature profile shows a similar trend as heat flux. For the near stable region, the flame boundary layer goes into the counter-rotating streamwise vortex pairs to form the streak like flames. And the counter-rotating pair (CVP) on the flame makes the streaks on the sides of the burner to be even larger, as verified by the CFD simulation. The heat flux is different under the streaks and troughs of the flame (higher under trough than in streaks), which interprets the fluctuation in the nearly stable region. Then the sharp decrease happens in the intermittent flame region. The flame half-width is obtained, which sits at about the middle of the sharp decrease region. And finally, for the position further away in the crosswise direction, the profile turns to be nearly zero. The crosswise profiles can be well represented by a Boltzmann function. The obtained experimental results and the proposed functions on the crosswise profiles of heat flux and temperature provide an essential base to estimate the heat feedback to the downstream surface of a wind-driven fire. [ABSTRACT FROM AUTHOR]

• Burning rates of two tandem hydrocarbon pool fires under wind were investigated. • LPF's burning rate is mainly affected by air entrainment and external radiation. • A function was derived for the LPF's burning rate when air entrainment is sufficient. • Ratio of the FPF burning rate to that of the SPF fell into three phases. • Burning rates of two pool fires can be well described based on stagnant film model. The intention of this experimental work is to investigate the burning rates of two tandem-placed free-burning square pool fires (15 cm), separated by different separation distances (S/D = 1∼6) under the action of crossflow (0∼6.0 m/s). Two representative hydrocarbon liquid fuels with various sootiness, n-heptane and ethanol, were employed. A total of 204 conditions were considered comprehensively. Results showed that the burning rates of the leading pool fire (LPF) and the following pool fire (FPF) were found to be markedly different from that of the wind-blown single pool fire (SPF) and were attributable to the change of heat feedback mechanisms. The combined effects of the constraint air entrainment and external radiation from the FPF are conducive to the change of the LPF burning rates when tandem pool fires merge. The increment of LPF burning rate compared with SPF burning rate of n-heptane pool fires at a fixed crossflow velocity was deduced to be a function of the external radiation from the FPF when air entrainment is sufficient at a relatively larger separation distance (i.e., non-merging cases). Apart from the external heat transfer to fuel surface and the air entrainment restriction, the sheltering effect of the leading fire on the effective crossflow velocity also dramatically influences the FPF burning rates. The ratio of FPF burning rate to that of a SPF in crossflow, was observed to fall into three phases in terms of a defined Fr ⁎ for various separation distances and crossflow velocities, of which phase I with a rising branch, phase II with a falling branch and phase III reached a plateau. Finally, a stagnant film model was employed to correlate well the burning rates of convection-dominated SPF, LPF and FPF under crossflow, while the modified effective crossflow velocity was applied to the FPF. These results provide a basis for better understanding the underlying physics of multiple fires mass burning fluxes caused by complex flame-flow interactions. [ABSTRACT FROM AUTHOR]

Bubbles surviving from the last injection process play an important role in the early stage of the next injection process, and ultimately affect the quality of diesel spray characteristics. In this paper, a combination of the LES (Large Eddy Simulation) method and VOF (Volume of Fluid) model was applied to study the unsteady liquid jet flow at the early stage of injection coupling the cavitating flow inside orifices, especially to analyze the effects of the residual bubble on the diesel jet. It was found that the instability of the jet surface was intensified by the cavitating flow inside the nozzle. The development of jet instability waves and the cavitation collapse inside the jet aggravated the interactions between air and diesel jet, which caused the liquid core to separate into some large liquid bulks a bit earlier. Then, the ligaments formed from the bulks broke up into droplets. Furthermore, the double mushroom-like shape occurred with the residual bubble, while just a single mushroom head occurred without the residual bubble. The length of liquid core decreased with the increasing of injection pressure, and the phenomenon that the air inside the spray chamber was partly sucked into orifice was observed. [ABSTRACT FROM AUTHOR]

Abstract: Heat and mass transfer in macellular materials signifies an important topic of research for a range of advanced applications such as in thermal, aerospace, geotechnical and scaffold tissue engineering etc. Based on the mathematical similarity of various transport problems, this paper proposes a modified bidirectional evolutionary structural optimization (BESO) method for design of biphasic microstructural composites with desirable transport properties. The cellular materials considered herein comprise periodic base cells and the homogenization technique is adopted to determine their effective (bulk) properties. The key is to optimize the topology of base cell model for minimizing the difference between the effective and target transport properties. Numerical examples agree well with the well-known benchmarking microstructures and some of them are prototyped using biphasic solid free-form fabrication (SFF) technology. To facilitate comprehension of the algorithm, a short MATLAB program is provided in the Appendix. [Copyright &y& Elsevier]

• Concurrent flame spread behaviors over glass fiber reinforced composites were explored. • GFUPR-ATH/MH has a higher flame spread rate than GFUPR under external airflow and radiation. • A theoretical analysis of concurrent flame spread over composites was performed. • Decomposition of ATH/MH may cause a larger total flame heat feedback in preheat zone. This paper reports an experimental investigation of concurrent flame spread over composites, for the first time, with combined flame retardants and external radiation factors. Glass fiber reinforced unsaturated polyester resins without flame retardants (GFUPR) as well as those containing aluminum hydroxide (ATH) and magnesium hydroxide (MH), which were utilized widely in modern high-speed train, were systematically explored. Infrared measurement was employed to track the displacement of pyrolysis front. Because of the dilution of H 2 O generated during the decomposition of ATH and MH, the flame temperature (T f) of GFUPR-ATH/MH is thought to be lower than that of GFUPR. Nonetheless, the most noteworthy finding is that the flame spread rate, flame preheat length and flame height for GFUPR-ATH/MH could be larger than that of GFUPR if concurrent airflow velocity exceeds 0.5 m/s under high external radiation. To interpret this unexpected while practically important phenomena, theoretical analysis based on the heat/mass transfer theory is established to explore the general mechanism. It is concluded that in the transition from natural-convection to forced-convection, the different in the flame convection in burning zone between the sample with high T f (i.e. GFUPR) and the sample with low T f (i.e. GFUPR-ATH/MH), will decrease rapidly. It is primarily because the sample with low T f is earlier to enter in the region where forced-convection is controlled as the airflow velocity increases. The critical parameter (Fr 2 Re)-1 = 1 could be used to characterize this transition. As a result, in forced-convection region, the role of thermal conductivity of the fuel-gas mixture (k mix) may emerge, that the sample with high k mix but low T f (i.e. GFUPR-ATH/MH) may impart more heat to burning zone than the sample with low k mix but high T f (i.e. GFUPR), generating more combustible gases in the burning zone hence larger flame heat feedback to preheat zone. Moreover, this effect will be reinforced by the larger flame preheat length of GFUPR-ATH/MH, due to the temperature dependency of molecular diffusivity. This mechanism analysis is well corroborated by the experimental results. This new finding may facilitate understanding of the effect of flame retardants on the heat/mass transfer mechanism in flame spread under concurrent airflow and external radiation. [ABSTRACT FROM AUTHOR]