Lean premixed combustors have a narrow stable combustion range compared with diffusion combustors and have flashback. Due to production constraints, the combustor does not usually have a uniform cross-sectional shape. It is important to analyze the propagation behaviors of the lean premixed flame in the swirling flow in the tapered tube where the cross-sectional area changes and the acceleration or deceleration flow exists. In this study, we presented characteristics of the unsteady flame behavior of a lean premixed burner with a tapered circular tube. One of the glass tubes expanded toward the tube outlet, and the other narrowed toward the tube outlet. We analyzed characteristics of flame behaviors with relatively weak swirl flow whose geometrical swirl number in the inlet condition was from 0.24 to 0.37 by using high-speed PIV measurements in the horizontal and vertical planes. The reverse flows appeared along the tube's center axis in the tube, which expanded toward the outlet. On the other hand, the reverse flows did not appear in the tube, which narrowed toward the outlet, and the flow was accelerated. The flame propagated along the reverse flow caused in the center axis in the expanding tube. In the narrowing tube, a reverse flow field was not observed under unburned conditions, but a reverse flow region occurred upstream of the flame tip. In the narrowing tube, the flame propagation made a large difference in the flow fields compared to the unburned condition. Based on the flame propagation behaviors and the PIV results of horizontal planes, it was found that for the same swirl number at the inlet condition, the flame speed in the case of the narrowing tube at the measurement cross-section was almost twice that of the case of the expanding tube under all experimental conditions.
Wearable cooling vest with electric fans has become increasingly popular given the rising risk of heat stress in outdoor environments. The fluidic motion of moist air driven by two cooling fans and the heat transfer in both human tissue and moist air were analyzed by using direct numerical simulation. A new semi-microscopic model originally developed in the cloud microphysics was introduced. It was found that as the fan wind velocity increased from 16 to 160 cm/s the relative humidity just above the skin approached that of the outer air and the skin temperature slowly decreased over time. On the other hand, the heat flux from body compartments to the skin responded quickly to increased airflow, attaining an equilibrium value that was determined by the latent heat released via sweat evaporation. For the relative humidity of 40% and 80 & micro;m sweat drop radius the skin temperature decreased by about 1 degrees C being comparable to the outside temperature. Visualization of fields found that the temperature and heat flux near the skin mirrored streaks exhibited by strong airflow vortices near the skin. The dependency of heat transfer on the Reynolds number, the relative humidity of the outer air and the sweat drop size was analyzed. The airflow was essential to transfer the outside air with low humidity to the skin and to remove the evaporated water vapor from the skin. Both body heat dissipation and water mass loss increased as the Reynolds number rose and the ambient humidity fell.
In this study, the auto-ignition and flame propagation under high temperature and pressure conditions are experimentally investigated. n-C7H16/air mixture is ignited by electric spark at the center of a constant volume vessel of 20-mm inner diameter and 80-mm long under various conditions of temperature (400-500K) and pressure (0.5-1.0MPa), and the flame propagation and auto-ignition behavior near the closed end are observed. In the experiment, distinct end-gas auto-ignition occurs at 500K and 1.0MPa, accompanied by the propagation of both cool and hot flames in the end-gas region. Following the end-gas auto-ignition, amplification of the pressure wave by acoustic resonance is observed, indicating that the propagation and reflections of the pressure wave within the chamber govern the maximum pressure attained.
This study investigates the flame spreading mechanisms on the surface of methane hydrate using numerical simulations with the Fire Dynamics Simulator and Smokeview (FDS-SMV). Methane hydrate, a promising alternative for natural gas transportation, can pose a fire hazard during marine transport. Experimental studies have revealed two distinct flame spreading behaviors depending on surface temperature-low-speed and highspeed spreading-but the mechanisms underlying these differences remain unclear. To address this, simulations were conducted over a temperature range from 193 K to 213 K, encompassing the hydrate dissociation and selfpreservation thresholds. The simulation incorporates two key reactions: methane hydrate dissociation and methane-air combustion. Results show that when the surface temperature exceeds the dissociation threshold, a thin uniform methane-air premixed layer forms prior to ignition, enabling rapid flame propagation at approximately 500 mm/s. Conversely, at lower temperatures, the non-premixed type is formed, and the flame spreading is governed by localized dissociation driven by heat conduction from the leading flame edge, resulting in a significantly reduced flame speed and extended spreading time. Temperature distributions within the hydrate were analyzed and compared with experimental data, revealing consistent trends and confirming the influence of surface temperature on flame behavior. The study concludes that the transition between premixed type and non-premixed type flame spreading modes is governed by whether pre-dissociation occurs, offering valuable insights into fire safety assessments for methane hydrate transport and storage.
In this study, two types of fuel injection methods were applied to a scramjet model combustor equipped with a dual-cavity flameholder and a burned-gas injection to achieve operation at a low flight speed corresponding to a flight Mach number of approximately 3. The first method was single-stage fuel injection, in which hydrogen gas as fuel was injected from a location 60 mm upstream of the combustor inlet. The second method was two-stage fuel injection, in which hydrogen gas as fuel was injected from the downstream cavity floor in addition to the single-stage fuel injection. Flameholding limits, pressure profiles, and high-speed OH chemiluminescence images in the scramjet model combustor for both injection methods were experimentally investigated. The experimental results showed that the two-stage fuel injection significantly improved the flameholding limits compared with those of the single-stage fuel injection. Furthermore, observations of the high-speed OH chemiluminescence images revealed four types of ignition processes in the two-stage fuel injection cases. Among these, the ignition process initiated by the second-stage fuel was unique to the two-stage fuel injection configuration. The flame of the second-stage fuel ignited even when the first-stage fuel could not. Subsequently, the flame of the second-stage fuel propagated upstream and ignited the first-stage fuel. The flame was initially formed from the second-stage fuel as a result of the impingement between the high-temperature burned-gas jet and the second-stage fuel plume.
Heavy fuel oil (HFO) is the primary fuel used in marine transport. However, the development of a chemical reaction mechanism of HFO combustion in marine engines is relatively slow. To address the limitations of existing mechanisms, enhance simulation capability, and promote the optimization of marine engine design, this study developed a practical surrogate model and its corresponding reduced chemical reaction mechanism. In this study, n-hexadecane, n-tetradecane, toluene and 1- methylnaphthalene were used to represent HFO and a reduced chemical reaction mechanism was developed. Firstly, the composition of the surrogate fuel was determined by the chemical composition of HFO. Secondly, the surrogate fuel was optimized by adjusting the cetane number, low heating value, density at 20 degrees C and hydrogen/carbon ratio. Thirdly, based on the multi-level mechanism simplification strategy and the mechanism of 'decoupling method' simplification, a reduced HFO mechanism was constructed with 108 species and 360 reactions. Finally, the four components in the mechanism were verified by the ignition delay time of the shock tube and the component concentration of the jet stirrer reactor. In addition, the experimental data of HFO combustion in a two-stroke low speed marine diesel engine were also used to further evaluate the developed mechanism. The results show that the mechanism has good stability and reliability and can be applied to simulate HFO combustion under marine engine combustion conditions.
Crosswind-induced fire whirl generation and development are investigated in this study, with particular emphasis on the effects of crosswind velocity, wind direction, and fuel mass loss rate on flame height and heat release behavior. Time-resolved flame heights and fire-whirl unsteadiness were quantified using frame-by-frame image analysis, supported by numerical simulations under both steady and time-varying mass loss conditions. The experiments show that crosswind-generated fire whirls differ qualitatively from normal stable whirls. A critical crosswind velocity of approximately 1 m/s was identified, above which a transition occurs from a buoyancy-controlled regime-in which flame height increases with heat release rate-to a wind-controlled regime, where enhanced air entrainment suppresses vertical flame growth despite higher heat release. Wind direction had a strong impact on vortex formation: wall-blocked flows produced tall, turbulent flames, while direct winds generated shorter, more stable flames with reduced angular momentum and limited whirl formation. Increased mass loss rates intensified flames under low-wind conditions, but in the dynamic regime, their influence lagged the flow structures induced by the wind. Computational modeling reproduced the overall experimental trends but consistently underpredicted flame heights due to the idealized representation of wind profiles. These findings underscore the nonlinear, regime-dependent relationship between flame height and heat release in crosswind fire whirls, providing insights essential for fire modeling, urban fire safety, and wind-informed architectural design.
Extra-long highway tunnels in cold regions experience frost heave disasters near the entrances, consequently, the designation of insulation layers is crucial in guaranteeing stability and safety. Temperature distribution functions as a pivotal indicator in assessing the onset of frost heave, which is notably influenced by airflow and water/ice phase transitions. In this study, a new coupled model was developed on the foundation of heat transfer, Richards equation, and aerodynamics theories, with the objective of predicting the spatiotemporal distribution of temperature within tunnels. k-epsilon equation was utilized to delineate the airflow movement and deformed geometry method was employed to simulate the excavation process. On-site monitoring of temperature was conducted at selected typical sections to validate the proposed model. Subsequently, the proposed model was utilized to determine the optimal thickness and length of the insulation layers in order to prevent frost heave damage to the lining. The findings indicate that the model introduced in this paper possesses the capability to accurately predict the temperature distribution within the tunnel, both axially and longitudinally. The optimal thickness and length of the insulation layer, as proposed for this specific environment under varying airflow velocities, can serve as pertinent references for the design of cold region tunnels.
Methane-ammonia blended combustion has emerged as a promising strategy to reduce carbon emissions while maximizing the utilization of existing fossil-fuel-based burners, serving as a viable intermediate step towards carbon neutrality. In order to effectively implement methane-ammonia combustion strategies in practical combustors, it is crucial to understand the combustion characteristics of premixed methane-ammonia-air flames, particularly focusing on how turbulence and varying ammonia blending ratios influence flame structure and NO formation. Therefore, in the present study, turbulent premixed methane-ammonia-air jet flames at different ammonia blending ratios were investigated using two-dimensional direct numerical simulation (DNS). Local heat release rates, flame front curvature, tangential strain rates, NO production rates, and their correlations were systematically evaluated. Results indicate that, under turbulent flame interactions, flame front curvature has a dominant influence; specifically, flame elements convex toward the burned side exhibit increased heat release rates. These effects are consistently predicted across different ammonia blending ratios when appropriately normalized with their corresponding laminar flame quantities. However, a comparison between stretched laminar flames and local turbulent flame elements reveals that convex flame elements significantly enhance NO formation, which cannot be fully captured by laminar planar flame analyses alone. Therefore, it is essential to incorporate curvature effects for accurately evaluating NOx emissions in turbulent combustion.
Lean combustion technology has been one of the promising approaches for developing next-generation engines with high thermal efficiency and low environmental impact. However, the practical lean limit of such engines is typically constrained by combustion stability. To address this issue, the present study introduces a newly designed intake adapter that can effectively generate in-cylinder tumble flow during the engine's intake stroke, which is expected to promote ignition and flame propagation. The effect of this adapter was evaluated using a single-cylinder spark-ignition (SI) engine, with a particular focus on thermal efficiency and cycle-to-cycle variation. The results show that, with the adapter, the engine achieved a significant extension of the lean limit compared to the baseline case without flow control. Specifically, under operating conditions of IMEP=1.15 MPa and a compression ratio (CR) of 17, stable combustion was sustained at an air-fuel equivalence ratio of lambda=2.26. Meanwhile, thermal efficiency improved by up to 3% relative to the no-adapter case. Furthermore, under lean conditions, the concentrations of unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust were significantly reduced, indicating that the adapter enables stable combustion even under ultra-lean conditions. Finally, it was found that adjustments to the geometric parameters of the intake adapter can further enhance both thermal efficiency and lean limit performance under high-load and high-compression-ratio operations.
Water is fundamental to a range of natural phenomena and is equally crucial in various engineering applications that demand efficient energy utilization, such as energy conversion with chemical reactions. However, the microscopic mechanisms of energy transfer in water remain unclear. This study investigated the instantaneous energy transfer (IET), formulated using forces obtained from ab initio molecular dynamics (AIMD) with a comprehensive analysis of its correlation with intermolecular and intramolecular structures in liquid water. By comparing our findings with those obtained from classical molecular dynamics (CMD), we examined the validity of the AIMD-based method and assessed the performance of the TIP4P/2005f water model. The results of the O-O, O-H, and H-H radial distribution functions (RDFs) indicated that the strongly constrained and appropriately normed (SCAN) functional in AIMD provided a more accurate representation of experimental values compared with the PBE-D3 functional. On the other hand, although the TIP4P/2005f model in CMD accurately reproduced some structural features, the classical force field exhibited some limitations, particularly in reproducing the height and width of the first peak in the O-O RDF. Moreover, we identified correlations between the IET and distance from the target oxygen atom to its nearest oxygen or hydrogen atoms, revealing that the characteristics of IET depend on this distance. Specifically, the mean IET: IET efficiency (IETE) was higher at shorter interatomic distances, indicating that both instantaneous intermolecular and intramolecular structures determine the IETE. It was also shown that variations in OH bond length significantly contributed to IETE. Additionally, our findings revealed that AIMD utilizing the SCAN functional shows higher IETE compared with that of CMD employing the TIP4P/2005f model. Through this study, the proposed method to evaluate IET has been validated, which will give fundamentals to understand transport phenomena in condensed phase in the framework of AIMD.
This study investigates bubble formation, pressure drop (flow rate), and wall temperature variation during fluid heating in a lattice-shaped microchannel under constant heat flux conditions. The microchannel was designed with periodically arranged pillars to trap bubbles in specific regions, enabling simplified and controlled observation of bubble behavior and its influence on neighboring bubble formation. Imaging and infrared thermography were used to visualize bubble dynamics and measure wall temperature distributions. The fluid was supplied under constant driving pressure, and the flow rate during bubble formation was recorded. To complement the experiments, a numerical simulation was performed by coupling two-dimensional flow analysis with a probabilistic element-filling model. Two bubble formation models were considered: a random model, in which bubbles form independently, and a neighbor-driven model, in which the filling probability increases near already filled regions, mimicking bubble propagation. The neighbor-driven model produced clustered bubble patterns and wider flow passages, resulting in lower pressure drops compared to the random case. An analytical model for pressure drop was developed based on the simulation results, incorporating both random and propagative bubble formation mechanisms. This model was applied to the experimental data and showed good agreement with the measured relationship between unfilled ratio and flow rate. Temperature measurement showed that the effect of heat transfer to the bubble propagation was relatively small in this study. The findings underscore the importance of accounting for both stochastic and deterministic effects in bubble formation within microchannels.
Understanding droplet penetration and evaporation in inkjet printing is essential for evaluating the energy requirements during drying and fixation, which are reflected in the droplet lifetime. This study investigates model ink droplets composed of water, propylene glycol, and glycerol deposited on a porous polyimide substrate with pore diameters of 300 nm and 1000 nm. The wetting conditions are either hydrophilic or hydrophobic. An experimental approach was employed by monitoring the geometric evolution of 100 pL droplets on the substrate. Immediately after the droplet impact, all cases exhibited spreading behavior for 5-20 ms. Moving to the next stage, most droplets showed a quasi-pinned contact line. Droplets containing 50 wt% water on hydrophobic surfaces penetrated significantly faster on the smaller pores (300 nm), resulting in a droplet lifetime approximately 30% shorter than on the 1000 nm pores. In contrast, droplets with higher water contents (75 wt% and 95 wt%) exhibited no significant difference in penetration time across hydrophobic surfaces, regardless of pore size. On hydrophilic substrates, all mixtures consistently demonstrated faster penetration. The simultaneous evaporation along the droplet lifetime is significant for higher water content on hydrophobic media. An analytical model based on the surface energy described by Owen-Wendt-Rabel-Kaelble (OWRK) was applied. Contact angle and penetration rate from the Young and Lucas-Washburn (LW) equation are calculated by using OWRK for the cos theta term and compared with the experiment result. The comparison shows Young-OWRK calculation will have an accurate prediction for droplets having the identical size with surface energy measurement (similar to 2 mu L), while a disparity is observed for the smaller droplet size (similar to 100 pL) due to having more dynamics from the nozzle ejection. LW-OWRK equation calculation results on much faster penetration compared to the experiment, implying the need to use a more complex LW equation to represent the porous media characteristics.
This study investigates the frost growth behaviors on a silver iodide (AgI) dot-patterned surface under desublimation conditions. The surface was fabricated using a two-step process combining UV lithography and UV nanoimprinting, resulting in a flat substrate with AgI dots embedded in a UV-curable resin (X433). The main objectives were to capture 3D frost morphologies through replica method and to evaluate whether such localized nucleation sites can induce laterally confined frost growth as observed in previous studies using AgI stripe patterns. Experiments were conducted at ambient temperatures of 2 degrees C and 10 degrees C with 40% relative humidity. Surface temperatures of--15.5 degrees C and--20.5 degrees C were selected to realize desublimation conditions, where the dew point is below freezing point. Frost formations on the AgI-patterned and bare resin surfaces were compared. On the AgI dot-patterned surface, frost nucleated selectively at certain AgI dots and grew horizontally, forming isolated columnar and planar crystals. These structures remained spatially separated for over 3 hours. In contrast, the bare surface exhibited typical frost behavior, i.e. dropwise condensation, droplet freezing, ice bridging, and eventually full coverage. Frost growth rate increased at lower surface temperature on both surfaces. On the AgIpatterned surface, the combination of higher ambient temperature and lower surface temperature further promoted nucleation and growth from the AgI dots, enhancing horizontal crystal development. To reconstruct the 3D frost structures, the replica method was applied. The AgI-patterned surface exhibited clear, laterally extended frost morphologies, while the bare PET surface showed random, interconnected frost growth. These results demonstrate that AgI dot patterns can effectively localize nucleation and preserve isolated and directional frost growth, offering a valuable approach for investigating frost growth mechanisms and developing frost controlling strategy.
The relaxation from intra-to intermolecular vibrations of water is compared in the cases of the infrared heating that emits the radiation to the infrared absorption band of water with a wavelength-selective emitter and the conductive heating with a heater. The wavelength-selective emitters have a metamaterial structure of Au/Cr/Al2O3/Au/Cr/substrate and continuously emit the radiation in the wavelength range about 3.03 or 6.06 mu m, corresponding to the stretching or bending vibrations of water, respectively. The relative reflectance of water is measured by using the attenuated total reflection method and a Fourier transform infrared spectrometer in both cases. The increase in relative reflectance at the wavelength range from 14.5 to 14.9 mu m, corresponding to the intermolecular vibrations of water, is obtained by subtracting the relative reflectance in the conductive heating from the relative reflectance in the infrared heating. The t-test determines that the increase in relative reflectance can be considered statistically positive. It is concluded that the infrared heating that emits the radiation to the infrared absorption band of water can promote the intermolecular vibration of water better than the conductive heating with a heater. This conclusion means that the infrared heating with wavelength-selectivity in the infrared absorption band of water may enhance water evaporation more than the conductive heating.
Direct numerical simulations (DNS) are conducted to investigate the effects of spanwise domain size on stably stratified turbulent shear layers. The focus is on the formation and spatial organization of elongated large-scale structures (ELSS), which emerge following the transition from Kelvin-Helmholtz instability and characterized by streamwise extents far exceeding the shear layer's thickness. Simulations are conducted for a temporally developing shear layer under stable density stratification. The spanwise extent is varied, while the streamwise and vertical domain sizes are fixed. Flow visualizations, one-point statistics, energy spectra, and two-point correlation functions are used to assess the influence of spanwise confinement on the transition process and late-time turbulence characteristics. The results show that when the spanwise domain size is very small, the transition process is altered and ELSS fail to develop properly. For intermediate domain sizes, the streamwise elongation of ELSS is captured, but their meandering and spatial repetition are suppressed. Statistical analysis reveals that while the meandering of ELSS contributes to large-scale structure, the presence of multiple alternating ELSS in the spanwise direction is more critical to the overall flow statistics. These findings emphasize the importance of spanwise configurations of ELSS in the dynamics and energetics of stably stratified shear layers.
