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Flash boiling is an effective way to enhance both atomization and evaporation characteristics of liquid fuel spray by introducing explosion of vapor bubbles, compared with the conventional high pressure injection. However, the evaporation characteristics and mechanism of flash boiling spray, especially, that associated with the explosion of vapor bubbles at high superheat degree, is still unknown. In this study, the vapor and liquid mass distributions in flash boiling sprays were quantified by use of ultraviolet/visible laser absorption/scattering (UV-LAS) imaging technique over a broad range of superheat degree. Based on the results of macro-structure characteristics of liquid/vapor phase spray and total evaporated vapor mass, influence of bubble explosion on detailed evaporation characteristics of flash boiling spray are investigated. And, by associating with our previous work on breakup mechanism of superheated droplet and liquid jet, the effect of two evaporation regimes, namely surface evaporation and flashing evaporation, on evaporation of flash boiling spray has been successfully identified for the first time. These results provide insightful information for understanding the evaporation mechanism of a superheated liquid jet and modeling a flash boiling spray.

This paper examines the changing nature of volatility spillovers among the U.S. and eight East Asian stock markets between two financial crises: the Asian currency crisis and the U.S. subprime credit crisis. Our empirical results suggest that volatility is not always spilled over from the directly affected markets to surrounding markets in crisis periods. The East Asian markets who directly suffered from the Asian currency crisis are the ones to which volatility is spilled over from other markets during the Asian currency crisis period, whereas uni-directional volatility spillovers from the U.S. market to other markets are observed during both crisis periods. This difference can be explained by a pre-determined hierarchy in which volatility spillovers tend to start from the U.S. market regardless of the geographical origin of the crisis. Furthermore, our results reveal that the markets in three major Asian financial hubs, i.e., Japan, Hong Kong and Singapore, are the markets to which volatility is spilled over uni-directionally from several other countries during the subprime credit crisis period, but not during the Asian currency crisis period. We attribute this difference to crisis-specific (currency or credit crisis), market-specific (credit derivatives market participation and foreign currency reserves), and time-specific (more integrated global market) factors.

Flash-boiling atomization is an effective way to enhance fuel jet breakup by introducing explosion of vapor bubbles and thus improve the evaporation of fuel spray, compared with the conventional high pressure injection. However, the break-up mechanism of a superheated jet, especially, that associated with the explosion of vapor bubbles inside the droplets, is still unknown. In this study, a superheated droplet generator was developed for observing the droplet morphology variation and the breakup process resulting from the vapor bubbles inside a superheated droplet by microscopic imaging. It was found that the droplet morphology is mainly influenced by droplet temperature, but micro bubbles formation and the breakup of the superheated droplet are dominated by superheat degree, and the superheat degree of 25 °C is an important critical point at which the droplet breakup occurs resulted from the ever-increasing void fraction exceeding a value of approximately 50% and the breakup mode shifts from aerodynamic mode to thermodynamic mode. The surface tension of superheated droplet was also evaluated by the droplet morphology, and the results show that the maximum reduction in surface tension reaches 70% as superheat degree increases to approximately 25 °C, and this explains the sharp decrease in SMD for a flash boiling spray when the superheat degree approaches this level. These results provide insightful information for understanding the breakup mechanism of superheated droplets and liquid jet and its modeling.

White organic light-emitting diodes (WOLEDs) with non-doped construction have compelling advantages, such as simplifying manufacturing process and reducing costs, thus attracting more attention. To improve the efficiency of non-doped WOLEDs, the selection of suitable interlayer is still a big problem to balance carriers. Here, for the first time, we employed a thermally activated delayed fluorescence (TADF) material with bipolar transport characteristics as the interlayer to modulate the exciton distribution, and fabricated efficient and ultrathin non-doped devices. The optimized monochromatic and white OLED exhibited maximum external quantum efficiencies of 18.1% and 15.6%, current efficiencies of 55.7 cd/A and 46.6 cd/A, as well as power efficiencies of 43.8 lm/W and 41.8 lm/W, respectively. More importantly, the devices adopted an ultrathin non-doped emissive layer architecture, and its thickness was only 3.6 nm, which was beneficial to reduce costs and simplify preparation process. Our results illustrated a new strategy that the bipolar TADF interlayer can suppress triplet-triplet annihilation (TTA) effect and improve the exciton utilization, which presents unprecedented opportunities for fabricating highly efficient, low cost and commercial WOLEDs.

Sorption thermal-energy storage plays a critical role in addressing the mismatch between thermal-energy consumption and supply. However, the previous evaluations of sorption thermal-energy storage materials and cycles were mainly based on the equilibrium results of energy storage density. In this study, we develop evaluation models to provide an optimal working pair selection reference for single-stage sorption/resorption cycles under short- and long-term storage modes, and compare the thermal-energy storage density in the discharging stage, the effective discharging time, the temperature gradient, the exergy output during the discharge phase (ΔEdis), and the exergetic coefficient of performance (ECOP). By considering ΔEdis as a criterion, the best halides are determined to be NH4Cl and CaCl2 for single-stage sorption cycles at heat-source temperatures of 60–80 °C and 90–180 °C, respectively; however multi-halide can be a better choice when the temperature varies over optimal temperature range of single-halide, i.e., from 80 °C to 160 °C. Resorption is also analysed in this study. The results show that multi-halide is not suitable for resorption cycles because a part of the components will fail to sorb during the discharging stage. The effective reaction-temperature ranges of the resorption cycles are wider than those of the corresponding sorption cycles, whereas the performance values decrease mainly because of the larger required sorbent mass.

Spray tip penetration is one of the important characteristics for optimizing an engine combustion system. The conventional penetration models proposed so far have not considered cyclic variations of spray. The spray cyclic variation, however, is an inherent feature of a spray injected into turbulent ambient gas at a high speed. In this work, in order to study the effects of cyclic variations in spray structure on spray tip penetration, the liquid phase distributions of diesel sprays were measured 36 times (cycles) for each condition of injection pressure and ambient density at a constant volume chamber with wide optical windows. The experimental results were analyzed through such statistical methods as Probability Presence Image (PPI) and Intersection over Union (IoU). It was found that the spray cyclic variation gradually increased with time after start of injection and became obviously large at the late stage. This cyclic variation in spray structure might cause variation up to ± 9% in spray tip penetration. A spray tip penetration model was developed by introducing a factor to consider the effect of spray cyclic variation. The factor of cyclic variation (f) and the presence probability (PP) could be correlated through experiments and expressed by a cubic polynomial function.

This investigation is to evaluate and quantify the influence of bubble formation inside the nozzle on breakup characteristics of a superheated liquid jet outside the nozzle. The effect of fuel properties was examined using methanol, ethanol and butanol. A unique optically-transparent slit nozzle with a high-speed micro-imaging system were utilized for quantifying the bubble formation inside and the liquid jet breakup outside the nozzle. Correlation between bubble number density and breakup of superheated liquid jet was obtained for all the fuel. The bubble is demonstrated as the main driving forces enhancing the breakup of superheated liquid jet. By introducing a parameter K , it is easier to correlate the superheated liquid jet breakup to the bubble number density as Δ f = K ⋅ n . K is related with the superheat degree, Reynolds number, Weber number, and the ratio of liquid to ambient gas viscosities, which has different form at the transition stage (0.2 P a / P s ⩽ 0.3) and the flare flashing stage ( P a / P s ⩽ 0.2), implying different breakup regimes.

Flash-boiling atomization is an effective way to enhance fuel jet breakup and evaporation and to improve the quality of fuel spray in an SIDI engine especially at the cold start condition, compared to the conventional high pressure injection. However, what happened inside the nozzle and how it affects the breakup and atomization characteristics of a superheated liquid jet (spray) is still unknown. In this study, a two-dimensional transparent nozzle was developed for quantitatively observing the bubble formation inside the nozzle and liquid jet breakup near the nozzle exit by high-speed microscopic imaging. It was found that the liquid jet breakup characteristics is strongly affected by the number density and size distribution of bubbles inside nozzle. The superheat degree has predominant effect on both characteristics of the jet breakup and the bubble formation. Superheat degree of 30 °C is a critical point at which the bubble formation rate and the atomization characteristics of the superheated jet changes significantly. These results provide insightful information for understanding the breakup mechanism of a superheated liquid jet and modeling a flash boiling spray.

This paper develops and applies a model to characterize the vaporization of gasoline–methanol blends in applications such as homogeneous direct injection spark ignition (DISI) engines. In DISI engines, the fuel is injected during intake or early in the compression stroke. Thus, the fuel droplets must vaporize in a low-temperature environment. Because methanol’s relatively high latent heat, the gases cool significantly during vaporization. Opposed stagnation flow is a computationally efficient platform on which to base the study. The study predicts the effects of initial droplet size, temperature, fuel–air ratio, methanol fraction, and operating pressure. Results include liquid- and gas-phase profiles during the vaporization process. Parameter studies show the state of the droplets, the gas phase temperature, and composition near the end of the droplet’s lifetime. The results provide quantitative insight about optimizing ignition and combustion, including under engine cold-start conditions.

Laser-induced-fluorescence/Mie-scattering (LIF/MIE) was proven to be a useful diagnostic for Sauter Mean Diameter (SMD) measurements in non-evaporating sprays. However, the measurement is not reliable for cases of an evaporating spray due to the interference of the fluorescence signal from the vapor phase. In this work, simultaneous Laser-induced-exciplex-fluorescence/Mie-scattering (LIEF/MIE) imaging techniques were proposed to obtain the SMD distribution of evaporating sprays. A special experimental condition was used to generate the flash-evaporating spray, which includes a fuel temperature of 60 °C and an ambient pressure of 20 kPa. Different from the conventional LIF/MIE technique, the combination of LIEF and Mie techniques allows eliminating the effect of tracer fluorescence from vapor phase in an evaporating spray. In addition, carefully selected tracers and specially designed filters were used to decrease the effects of variation on tracer concentration and temperature dependency of fluorescence intensity during evaporation. The numerical analysis based on geometrical optics approximation (GOA) and experimental analysis was conducted to determine the calibration coefficient K . Finally, SMD distribution of an evaporating spray measured by both LIEF/MIE and conventional LIF/MIE techniques was compared to PDI measurement. The results show that the SMD of the flash boiling spray obtained from LIEF/MIE is very close to those measured by PDI, while the results measured by LIF/MIE and PDI show a large deviation of around 40%. It indicates that the evaporation effect cannot be ignored for evaporating sprays.

The macroscopic spray characteristics were quantified using dimensionless analysis by examining the role of the dominating forces associated with liquid-jet breakup. The Weber number, Reynolds number, and air-to-liquid density ratio dimensionless numbers were used to capture the primary forces including the inertia, viscous, surface tension, and aerodynamic drag forces. Planar Mie-scattering technique was applied to generate spray images over a broad range of conditions found in today’s spark-ignition-direct-injection (SIDI) engines, providing a relatively large range of dimensionless numbers. The effect of fuel properties were examined using gasoline, methanol and ethanol fluids. Six regions described on a Weber number versus Reynolds number domain were selected to identify the relative importance of the inertia force, surface tension force, and viscous force on macroscopic spray structure. The effect of aerodynamic drag was individually determined by characterizing the spray over a range of ambient air-to-liquid density ratios. As a result, for the non-flash-boiling multi-hole sprays in this study, the Weber number and air-to-liquid density ratio have much more profound effect on the spray penetration and spray–plume angle compared to the Reynolds number contribution. The inertia force and air drag force are more important factors compared to the viscous force and surface tension force. This analysis yielded dimensionless correlations for spray penetration and spray–plume angle that provided important insight into the spray breakup and atomization processes.

The spray structural changes and vaporization processes for flash-boiling multi-hole sprays over a broad range of superheated conditions were investigated using Mie-scattering and Laser-induced-exciplex-fluorescence (LIEF) optical techniques. The fuel property effects were examined by characterizing n-hexane, methanol and ethanol fluids over a wide range of conditions consistent with that found in today’s spark-ignition-direct-injection (SIDI) engines. The macroscopic spray structure was quantified using spray penetration, spray-plume width and normalized distance between spray plumes. These structural parameters were correlated to the ratio of the ambient pressure to saturation pressure (Pa/Ps) that represents the superheated degree. Three continuous regions were identified by quantifying the spray transformation with increasing superheated degree; namely the non-flash-boiling, transitional flash-boiling and flare flash-boiling regions. Two critical values of Pa/Ps were identified, where the flash-boiling and spray collapsing transitions occurred at Pa/Ps values of 1.0 and 0.3, respectively. The evaporation process was examined using the LIEF optical technique for n-hexane, providing the relative vapor quantity throughout the spray transformation process. The correlations of the spray structural change and extent of vaporization with increasing superheated degree provided good insight into the mechanisms responsible for the observed behaviors during flash-boiling conditions.

This paper describes an investigation of the flame structure of wall-impinging diesel sprays injected by group-hole nozzles in a constant-volume combustion vessel at experimental conditions typical of a diesel engine. The particular emphasis was on the effect of the included angle between two orifices (0–15 deg. in current study) on the flame structure and combustion characteristics under various simulated engine load conditions. The laser absorption scattering (LAS) technique was applied to analyze the spray and mixture properties. Direct flame imaging and OH chemiluminescence imaging were utilized to quantify the ignition delay, flame geometrical parameters, and OH chemiluminescence intensity. The images show that the asymmetric flame structure emerges in wall-impinging group-hole nozzle sprays as larger included angle and higher engine load conditions are applied, which is consistent with the spray shape observed by LAS. Compared to the base nozzle, group-hole nozzles with large included angles yield higher overall OH chemiluminescence intensity, wider flame area, and greater proportion of high OH intensity, implying the better fuel/air mixing and improved combustion characteristics. The advantages of group-hole nozzle are more pronounced under high load conditions. Based on the results, the feasibility of group-hole nozzle for practical direct injection diesel engines is also discussed. It is concluded that the asymmetric flame structure of a group-hole nozzle spray is favorable to reduce soot formation over wide engine loads. However, the hole configuration of the group-hole nozzle should be carefully considered so as to achieve proper air utilization in the combustion chamber. Stoichiometric diesel combustion is another promising application of group-hole nozzle.

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s−1, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.

This paper uses tubular flame similarity and computational models to investigate the characteristics of premixed methane-air flames with high swirl rates. As the swirl rate increases, thus increasing centrifugal forces within the flow field, pressure variations can be large. Results show that the radial pressure field significantly affects flame structure and overall burning characteristics. Molecular species transport is affected by pressure-diffusion and absolute pressure in the reaction zone, which can be significantly reduced. To assist isolating and interpreting swirl rate effects, results are compared with comparable flames in planar unstrained and opposed-flow twin flame settings. Results show that swirl rate can influence methane-air flames differently from comparable propane-air flames. These differences are explained in terms of the pressure fields and the first Damkohler number.

This paper provides the theoretical basis for modeling tangential injection into ideal tubular flames. For most practical tubular combustors, the present formulation provides significant advantages compared to the traditional model formulation. The mass, momentum, and energy associated with tangential injectors are represented as Gaussian-shaped source terms in the appropriate conservation equations. Incorporating the source terms alters the governing equations, but does not affect the similarity properties associated with ideal tubular flow. The system of equations form a nonlinear boundary-value problem, which can be solved computationally.

This paper uses a computational model to characterize the effects of high rotation rates on the structure of swirl-type premixed tubular flames. The model incorporates an elementary propane–air reaction mechanism and multicomponent molecular transport. Because high rotation rates lead to large radial pressure variations, pressure diffusion is included in the molecular-transport model and the radial momentum equation is retained in the system of equations. A similarity formulation is used, leading to a two-point boundary-value problem. Both lean and rich mixtures are considered. Results show how rotation rate affects pressure variation, flame position, flame temperature, and flame velocity. Pressure diffusion is most significant when the flame is positioned close to the maximum circumferential velocity.