Flammability Limits Research Articles

Flame inhibition using nanotechnology

Flame inhibition is significant as it directly contributes to fire safety and prevention. Effective flame inhibition can reduce the spread and intensity of fires, protecting lives, property, and the environment. The research focused on the significant role of nanomaterials, specifically aluminum oxide (Al₂O₃), in improving fire retardation and enhancing fire inhibition performance. Nanoparticles can be used in flame inhibition, as they might merge two flame inhibition techniques: cooling and physical barriers. They offer unique properties due to their small size and high surface area, which can enhance their effectiveness in disrupting the combustion process. Nanoparticles can act as heat sinks, absorbing heat and reducing the temperature of the combustion zone (cooling method). They can also catalyze the formation of non-combustible gases or form a protective layer on the material's surface (physical method). Due to these properties, nanoparticles are increasingly studied and utilized for advanced flame retardant systems. Experimental work was carried out to accomplish the goal of the research. The fire inhibition test was performed within a pressure range between 3.2 and 5.3 cm of Hg, corresponding to a fuel-oxygen ratio of 9.3 % to 14.8 %, respectively. A novel nanoparticle material was involved in this analysis, including Al₂O₃. Based on the research analysis, the critical research results revealed that Al₂O₃ employment as a fire inhibitor provided a more enhanced impact of fire retardance and flame inhibition compared with conventional retardation materials, reflected in more sustainability and less adverse environmental effects.Furthermore, the experimental findings indicated that the flammability limits range between a lower flammability limit of approximately 9.3 % and an upper one of 14.8 % under specified amounts of pressure and temperature. In addition, increasing the inhibitor quantity could raise the lean flammability limit. Raising the nanomaterial mass would extend the flammability limits, altering the concentration of these nanoparticles to increase inhibitory effectiveness and change the lower and upper flammability limits.The study concluded that adjusting nanoparticle concentrations can significantly alter inhibitory effectiveness. The novelty lies in the use of a novel nanoparticle material, demonstrating improved sustainability and effectiveness in fire retardation of a highly flammable gaseous mixture.

Laminar burning velocity of Ammonia/Air mixtures at high pressures

Ammonia (NH3) is one solution for decarbonizing the economy. This study used high-speed schlieren imaging to investigate the NH3/air flame morphology and propagation characteristics (laminar burning velocity (LBV) and flame thickness) inside a constant volume combustion chamber (CVCC), at conditions relevant to different types of combustors used in power generation. Experiments were carried out for equivalence ratios (ϕ) from 0.7 to 1.3, initial pressures from 1 atm to 10 atm, and an initial temperature of 298 K using a variable gap ignition system. A novel passivation method that minimized the effect of NH3 adsorption on stainless steel surfaces was used during the CVCC filling to reduce the uncertainty in the actual ϕ before ignition. Results showed that the low speed of NH3/air flames increased the effect of buoyancy on the flame shape, especially at high initial pressures or near the flammability limits when flames adopted a bean-shaped structure. A comparison of Rayleigh and Peclet numbers (indicators of buoyancy and diffusion phenomena in flames, respectively) supported these findings. Maximum LBV was measured at ϕ = 1.1 and decreased non-linearly with the initial pressure. Similarly, flame thickness decreased with pressure, and it was thinnest at ϕ = 1.1 for all the examined conditions. Numerical simulations using Cantera produced similar LBV values as the experiment. A sensitivity analysis indicated that reactions involving NH2 and NO have an important role on LBV. Reactions leading to H, O, and OH radicals generation had the strongest effects on LBV enhancement, while LBV-inhibiting reactions had more accentuated effects at high pressures at which LBV is lower.

Diffusive-thermal instabilities in unstrained H2O-diluted syngas diffusion flames

A new version of the unstrained diffusion flame burner that can be operated with gaseous fuels containing high vapor content is introduced. Being a good approximation of the classical chambered diffusion flame solution, the flames generated are nominally unstrained, unlike common research burners where hydrodynamic effects are significant. This permits quantitative comparison with theoretical models that are often based on this simple configuration, and paves the way for fundamental experimental studies with vaporized fuels. In this paper, the capabilities of the new burner design are exploited to study diffusive-thermal instabilities (DTIs) in H2O-diluted H2-CO-CH4-CO2 mixtures. H2O dilution can be significant in biomass-derived syngas mixtures that are not cooled prior to combustion, and that are often burned directly to lower losses as waste heat and pollutant emission in practical combustors. Flammability limits are first presented for a broad range of fuel blends, where the destabilizing effect of H2O dilution is discussed. Instability maps in terms of the Damköhler number are then provided to illustrate the different types of superimposed cellular-pulsating instabilities that onset from the simultaneous presence of H2 with high diffusivity, and CO/CH4 with much lower mobility. The characteristics of these peculiar instabilities are highly dependent on the H2O dilution fraction, which increases both the fuel blend and oxidizer Lewis numbers. The degree of cellularity superimposed in the pulsating multi-fuel flame is reduced at higher water content, as the number of observed cells decreases. The opposite effect is observed on the pulsation frequency, which increases at higher water concentrations.

Experimental investigation of the plasma-assisted spray combustion of methanol/water mixtures

To achieve the goal of net zero by 2050, carbon dioxide emissions must be reduced using carbon–neutral fuels, and methanol is a feasible choice for gas turbines, IC engines, and furnaces. Methanol can be prepared by bio-process or thermal catalytic processes, but the crude product contains much water. Crude methanol must be dehydrated into high-grade methanol to enable its use as fuel. The dehydration process is the most energy-intensive step in methanol production. Therefore, this study investigated the feasibility of adding different proportions of water to methanol as a fuel for plasma-assisted spray combustion by using low-power gliding arc plasma. Based on the current sprayer setup, using plasma to sustain water-content methanol is more energy-efficient than refining it. The results delineate that plasma can sustain the combustion of methanol in a spray, even when the methanol contains 60 % water. The length and lift-off height of the flame are related to the water content of the fuel. To illustrate this distinguishing phenomenon, this study used CH and OH fluorescence to conduct a more in-depth analysis. The distribution of CH* radicals indicated that the gliding arc plasma enhanced the flame heat release rate, which may be why the flammability limit extended to greater proportions of water with methanol. It's fascinating to observe that blended fuel with higher water content can result in higher OH chemiluminescence signals. This suggests that the dissociation of water vapor, caused by the thermal effect of the gliding arc, plays a crucial role at the flame base locations. In addition, the flammability limits, emissions, and combustion efficiency of methanol/water spray combustion with and without plasma assistance were all examined.

Field experimental study of high expansion foam coverage after LNG leakage

With the rapid development of the world’s natural gas industry, LNG (Liquefied Natural Gas) has received widespread attention as a clean energy source. However, the leakage accidents of LNG are usually accompanied by safety hazards such as explosions, low temperatures, and phase transitions. In this study, the field experiment is carried out to cover LNG leakage pool with HEF (High-Expansion Foam). In the process of LNG leakage, when the ambient temperature at the bottom of the reservoir drops to about − 82.3 °C (the critical temperature of LNG), the ambient temperature would rise briefly due to the effect of phase transition. Meanwhile, the temperature distribution of different vertical and horizontal positions in the reservoir during the process of leakage is analyzed. Besides, it is also found that the density of the spilled vapor is much lower than that of the air, but the water in the surrounding air encounters the cryogenic vapor to condense into small droplets, the small droplets entrap part of the flammable gas during the process of settling to the ground, therefore, the flammable vapor still shows a heavy gas effect. During the initial period of HEF coverage, evaporation from the liquid pools accelerates, resulting in a 386% increase in peak concentration of methane above the reservoir. Nevertheless, when HEF application is maintained an effective thickness, it can effectively reduce the methane vapor concentration downstream of the reservoir near the surface to less than 1/2 LFL (1/2 of the Lower Flammability Limit). In general, the large-scale field experiments have far-reaching engineering significance and reference value for the occurrence and emergency treatment of LNG leakage accidents under actual working conditions.

Detailed kinetic analysis of synthetic fuels containing ammonia

The imperative to adopt environmentally sustainable energy sources has propelled the exploration of zero-carbon alternatives, such as hydrogen and ammonia for energy supply. Integrating these species with methane is recognized as a viable short-term solution. However, a complete understanding of their chemical behaviour in a wide range of conditions remains elusive, especially in terms of safety parameters, limiting the applicability of this solution. To this aim, this work presents a detailed analysis of the overall reactivity, expressed either in terms of laminar burning velocity or flammability limits, of hydrogen-ammonia-containing fuels. Additional considerations were obtained by the analysis of the maximum pressure rise and maximum pressure obtained in adiabatic conditions. A spectrum of nitrogen/oxygen mixtures was scrutinized, and the influence of initial temperature within the range of 300–500 K was systematically investigated. The estimation quality of the adopted mechanism was evaluated by comparing numerical predictions with experimental measurements obtained by different systems, when available. A tendency in slightly more conservative results on the safe side was detected for the assessment of flammability limits and minimum oxygen concentration. This trend was attributed to the assumption of perfectly adiabatic conditions posed for the numerical analysis, which does not perfectly match the experimental conditions. Conversely, an excellent agreement between numerical and experimental laminar burning velocity was observed. Therefore, the collected data were used for the quantification of input parameters required by well-established correlations suitable for synthetic fuels.

Teorijska analiza niskozapaljivih i ekološki prihvatljivih smjesa ukapljenog naftnog plina i tetrafluoretana kao hladila

Liquefied petroleum gas (LPG), like other hydrocarbon gases, has good thermo-physical and ecological properties, but its high-flammability has limited its direct use in refrigeration applications. Therefore, this paper investigates the thermodynamic performance of low flammable and ecological-save mixtures of LPG and tetrafluoroethane refrigerant in a refrigeration system. The computational results of global warming potential (GWP) and low flammability limit shown that the blends with 50 – 90 % LPG contents have GWP below 750 and fall within the lower flammability safety class of which five were selected for further analysis. The results showed that the thermal conductivity and the refrigerating effects produced by the selected blends are greater than those of the reference refrigerant (R134a). Also, the selected blends exhibited appropriate lower pressure ratio, discharge temperature and specific power than R134a refrigerant. Four of the blends required low power input and exhibited higher coefficient of performance which are clear indication that they are more energy efficient than R134a in the refrigeration systems.

Quantitative prediction of the flammability limits of filter paper in microgravity conditions

Under JAXA's FLARE project, ongoing since May 2022, long-duration microgravity combustion experiments have been conducted using filter paper. These experiments utilize the Solid Combustion Experiment Module (SCEM) in the ISS/Kibo module. The dimensions of the filter paper used were 130 mm in length and either 40 mm or 20 mm in width, with the experiments conducted in an environment of 298 K and 100 kPa. This paper highlights the results of 19 flame spread experiments performed in opposed-flow environments and compares the predetermined flammability limiting curve with the flammability map obtained in orbital experiments. One of the objectives of the FLARE project is to predict the flammability limiting curve in microgravity environments based on the results of tests that can be conducted on the ground. These experiments produced intriguing findings, demonstrating the effects of both radiation loss and blow-off during flame spread, as the opposed flow velocity was incrementally increased from 10 cm/s to 18 cm/s in 2 cm/s steps, within an environment containing 15.5 % oxygen concentration. When comparing with the combustible map obtained from orbital experiments, it was discovered that the original limit curve accurately replicated the blow-off branch but overestimated the flammable region in areas with opposed-flow velocity below 10 cm/s. This discrepancy was attributed to the inadequate consideration of the effect of opposed-flow velocity on the radiation quenching branch. Therefore, by considering the velocity distribution measurement results obtained in orbit and modeling the impact of the effective flow velocity opposing the flame front on both the radiation quenching branch and the blow-off branch, we successfully quantitatively replicated the results of the orbital experiments. These findings indicate that the flammability limits of thin flat plate samples can be accurately predicted by properly assessing the effect of the boundary layer on the sample surface.

Forced ignition of cool, warm and hot flames in a laminar non-premixed counterflow of DME versus air

Recently, there has been great interest in the ignition of cool and warm flames (controlled by low-temperature chemistry, LTC, and intermediate-temperature chemistry, ITC, respectively) as they may have significant influence on the ignition and subsequent propagation of hot flame. However, it is still unclear how the flow affects the ignition of the cool/warm flame and their transition to the hot flame in a non-premixed fuel/oxidizer system. In this study, we conduct 2D simulations on the forced ignition of cool, warm and hot flames in a laminar, axisymmetric, non-premixed counterflow of dimethyl ether (DME) versus air. The objective is to assess the effects of flow on the ignition of cool, warm, and/or hot flames and the transition among them. Different ignition energies and strain rates are considered. It is found that the transient ignition process is mainly controlled by the extinction/transition limits and the minimum ignition energies of cool, warm and hot flames. When the strain rate is lower than the transition limits of cool and warm flames, all three flame types can be directly initiated but they eventually evolve into a hot flame. At intermediate strain rate between the transient limit and the extinction limit, quasi-steady cool and warm flames can be obtained without further transition to a hot flame. When the strain rate exceeds the extinction limit of cool and warm flames, only the hot flame can be ignited. An ignition regime diagram in terms of ignition energy and strain rate is proposed, in which different regimes for cool, warm and hot flames as well as transition among them are identified. Furthermore, the ignition of cool and warm flames in a non-premixed counterflow is shown to be very sensitive to ignition position as well as strain rate and ignition energy.

Исследование зависимости температуры самовоспламенения от концентрации паров горючей жидкости

Purpose. The currently used methodology for determining flammable liquid self-ignition temperature, which is well-known as the “drop method”, makes it impossible to obtain the dependence of self-ignition temperature on flammable liquid concentration that corresponds to theoretical concepts. It is related to the fact that while conducting experiments a flammable liquid is injected into the neck of an open heated vessel. The preliminary calculation of the required quantity of flammable liquid for creating its given concentration does not take into account the substance loss when it is injected into the reaction vessel. The main aim of the research is to improve the methodology for determining flammable liquid self-ignition temperature depending on the concentration of its vapors. Methods. To study the relationship between the volume of flammable liquid injected into the neck of the reaction vessel heated to a certain temperature and self-ignition temperature of this liquid the method of experimental data analysis was used. The experimental results were obtained with the help of the experimental method that involved injection of different volumes of flammable liquid and then recording flame presence or absence. Findings. A relationship was established between the fuel concentration in the vapor-air mixture and the volume of the injected sample. The methodology for determining the flammable liquid self-ignition temperature was corrected. Research application field. The results can be used in determining flammable liquids self-ignition temperature of various chemical composition, as well as in the educational process when students carry out a laboratory work on determining a flammable liquid self-ignition temperature depending on its concentration. Conclusions. The findings of the research allowed the authors to draw the following conclusions: 1) in the methodology for determining a flammable liquid self-ignition temperature when calculating the volume of the injected sample to create the required concentration of fuel in the vessel, the flammable liquid losses occurring on its injection into a heated reaction vessel were not taken into account. It is recommended to use a correction factor of 2 to take into consideration losses in calculation formulas. 2) to equally distribute the concentrations of the used fuel on the graph of the dependence under study within the lower and upper flammability limits, to calculate the volume of a flammable liquid, fuel concentration must be preset as follows: ϕ1 = 1,2ϕн, ϕ2 = ϕстех, ϕ3 = 1,8ϕстех, ϕ4 = 0,8ϕв. 3) the methodology for determining the self-ignition temperature of a flammable liquid is also valid in case of using high molecular weight aliphatic (non-cyclic) alcohols, for example, butanol, pentanol, hexanol.

Stabilization regimes and flame structure at the flame base of a swirled lean premixed hydrogen–air injector with a pure hydrogen pilot injection

Hydrogen (H2) has become a key in the decarbonation process of the aeronautical field and new gaseous injection systems must be designed to suit hydrogen’s specific properties (high laminar flame speed, high adiabatic temperature, large flammability limits and fast diffusivity (Le<1)). In this study, Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) are performed for a co-axial injector including an annular premixed hydrogen–air, swirled injection surrounding a central axial pure hydrogen lance. LES of the injector installed in a full chamber are performed at 12bar and the LES resolution issues associated with these high pressures are presented. LES reveals that the flame is stabilized on the rim separating the premixed and the pure hydrogen streams. The stabilization is produced by a structure called RSE for Rim Stabilized Edge Flame. To avoid resolution issues of the LES, the RSE flame structure is analyzed using CANTERA 1D flames and a 2D DNS of the attachment region. Extinction limits are described and implications for flame stability are discussed.

Effects of radiative heat loss on extinction limits of counterflow premixed ammonia-air flames

The effect of radiative heat loss on counterflow premixed ammonia-air flames has been investigated as a function of equivalence ratio and system pressure. Non-adiabatic counterflow premixed flame simulations incorporating detailed chemistry, transport, and radiative heat transfer in the optically thin limit have been employed to elucidate important underlying physics and extinction characteristics. Similar to methane-air flame systems, non-adiabatic counterflow premixed computations show that the combined effects of positive flame stretch, sub-unity Lewis number, and radiative heat loss lead to the extension of the lean flammability limit for ammonia-air flames, revealing a C-shaped curve near the lean flammability limit that exhibits both radiation-induced and stretch-induced extinction states. The computations compare favorably with the experimentally determined extinction stretch rate values for the stretch-induced extinction states over a wide range of equivalence ratios at different pressures up to 5 atm. A novel feature sensitivity analysis has also been developed to highlight important sensitive reactions for the stretch- and radiation-induced extinction states. Furthermore, the controlling chemistry at the dual-extinction states and at varying pressures are compared and discussed.

Flammability limits of theoretical azeotropic mixtures consist of R1243zf, RE170 and flame retardants (R13I1, R134a, or R227ea)

3,3,3-trifluoropropene (R1243zf) is a low climate change effect refrigerant that has great development potential. However, R1243zf is flammable and its safety classification is A2 as per ISO standard 817. To reduce its risk of flammability, the method of using mixed refrigerants in practical applications is an effective approach. In this paper, an experimental system was established according to the ASHRAE standard 34 to measure the flammability limits of four azeotropic mixtures containing R1243zf. First, the critical suppression ratio (CSR) of R13I1/R1243zf was tested out that is 0.353. Further measurements were made to determine the lower flammability limits (LFLs) and critical suppression concentrations (CSCs) of three ternary mixtures consisted of R1243zf, thermodynamic performance enhancer (RE170) and flame retardants (R13I1, R134a or R227ea), at various compositions. The experimental non-flammable proportions of R1243zf/RE170/R13I1, R1243zf/RE170/R134a, and R1243zf/RE170/R227ea were 31.81 %, 5.49 %, and 22.42 %, respectively. Besides, a comparison was made between the experimental data and the predicted results by means of three different theoretical methods (the equal effect method, thermal balance method, and group contribution method). The fresh experimental results will provide practical guidance for the applications that utilize the R1243zf blends in low flammability risk.

Numerical Simulation Study on the Dynamics of Bluff-Body Flames under Oxygen-Lean Conditions

As modern aeroengine combustors advance towards high temperatures, afterburners are inevitably affected by diminished oxygen content in incoming flows, thus affecting combustion efficiency, instability, and flammability limits. In this study, the dynamic combustion characteristics of V-shaped bluff body-stabilised diffusion flames were investigated using a large eddy simulation method with an oxygen mass fraction range of 14–23% and temperatures ranging from 900 to 1100 K. The results show the significant effects of oxygen content and inflow temperature on the flame/flow behaviours downstream of the bluff-body flame holder. In a separated shear layer, two distinct modes of flow/flame shedding are observed when varying the oxygen content and inflow temperature. The results show that BVK instability governs the far-field wake flow/flame features, whereas the oxygen concentration and temperature significantly affect their oscillation amplitudes. In addition, variations in the incoming oxygen content and temperature shift the axial position of the transition from KH instability to BVK instability. Finally, a spectral analysis is conducted to investigate the characteristics of pressure and heat release pulsations under different scenarios. This study highlights the importance of oxygen content on the combustion dynamics of bluff body-stabilised diffusion flames at various temperatures, which is essential for optimising combustion efficiency and stability in practical applications.

Thermal hazard analysis and fire risk assessment of rocket kerosene in oxygen-enriched environment

Fire hazard of liquid fuel exposed in oxygen-rich environment is significantly greater than that in air, which should be considered in the safety risk assessment of scenarios where oxygen and fuel coexist. To reveal the effect of oxygen concentration on fuel properties and combustion characteristics of liquid fuel, a series of experiments were conducted to measure the liquid-phase component, flash point, lower flammability limit, heat release rate and flame height of rocket kerosene at different oxygen concentrations. The results show that the flash point of rocket kerosene (with an average chemical formula of C11.98H22.64) decreases nonlinearly with the increased oxygen concentrations, while the oxygen concentration has a great enhancement effect on the heat release rate and visible flame height of rocket kerosene pool fire. Prediction models of the flash point and lower flammability limit based on the Clausius-Clapeyron equation, Le Chatelier rule and decreasing temperature assumption, as well as empirical equations of the heat release rate and visible flame height, were derived for different oxygen concentrations in this work, providing a new comprehensive understanding of the effect of oxygen concentration on the rocket kerosene, and could be further applied to fire risk assessment in liquid fuel and oxygen coexisting scenarios.

Study of ethanol vapor explosion and prediction based on chemical kinetics under high temperature and pressure

AbstractThe study of the explosion parameters of ethanol–air mixture at high pressure and temperature is essential for the safe production of ethanol. However, the explosion characteristics of ethanol vapor at various pressures and temperatures are limited. The mechanism at the flammability limits of ethanol has not been clarified, and the corresponding prediction model is also lacking. In this study, chemical kinetics and machine learning are used to study the mechanism of ethanol explosion and build predictive models, respectively. Our findings show that an increase in the initial pressure has a more pronounced influence on the explosion pressure (Pex) and pressure rise rate (dp/dt) than an increase of temperature. The variation trend of the upper flammability limit (UFL) of ethanol is related to the different effects of temperature and pressure on OH radicals. H + O2&lt;&gt;OH + O and HO2 + CH3&lt;&gt;OH + CH3O had the greatest effect on the generation of OH radicals. The quantitative relationship between the H, O, and OH radicals and UFL was constructed by machine learning, providing a new research perspective for the prediction of the UFL of an inflammable fuel under different pressures and temperatures. The results of the study will provide theoretical and practical guidance for the prevention and control of explosions in the ethanol production process.

Analysis of influencing factors of aircraft fuel tank flammability

PurposeIt is an indispensable part of airworthiness certification to evaluate the fuel tank flammability exposure time for transport aircraft. There are many factors and complex coupling relationships affecting the fuel tank flammability exposure time. The current work not only lacks a comprehensive analysis of these factors but also lacks the significance of each factor, the interaction relationship and the prediction method of flammability exposure time. The lack of research in these aspects seriously restricts the smooth development of the airworthiness forensics work of domestic large aircraft. This paper aims to clarify the internal relationship between user input parameters and predict the flammability exposure time of fuel tanks for transport aircraft.Design/methodology/approachBased on the requirements of airworthiness certification for large aircraft, an in-depth analysis of the Monte Carlo flammability evaluation source procedures specified in China Civil Aviation Regulation/FAR25 airworthiness regulations was made, the internal relationship between factors affecting the fuel tank flammability exposure time was clarified and the significant effects and interactions of input parameters in the Monte Carlo evaluation model were studied using the response surface method. And the BP artificial neural network training samples with high significance factors were used to establish the prediction model of flammability exposure time.FindingsThe input parameters in the Monte Carlo program directly or indirectly affect the fuel tank flammability exposure time by means of the influence on the flammability limit or fuel temperature. Among the factors affecting flammability exposure time, the cruising Mach number, balance temperature difference and maximum range are the most significant, and they are all positively correlated with flammability exposure time. Although there are interactions among all factors, the degree of influence on flammability exposure time is not the same. The interaction between maximum range and equilibrium temperature difference is more significant than other factors. The prediction model of flammability exposure time based on multifactor interaction and BP neural network has good accuracy and can be applied to the prediction of fuel tank flammability exposure time.Originality/valueThe flammability exposure time prediction model was established based on multifactor interaction and BP neural network. The limited test results were combined with intelligent algorithm to achieve rapid prediction, which saved the test cost and time.

Experimental and mechanism study on the flammability of the mixtures of propylene and ethane

In the background of the global promotion of green and low-carbon transition, more attention should be paid to the combustion characteristics of environmentally friendly but flammable working fluids. This study delves into the flammability of mixtures composed of propylene (R1270) and ethane (R170) with 1,1,2,3,3,3-hexafluoro-1-propene (R1216) and 1,1,1,2-tetrafluoroethane (R134a). The flammability limit ranges of these four mixtures were measured at different concentrations. The flammability inhibition effects of R1216 and R134a on hydrocarbon (HC) working fluid R1270 were then compared and analysed under different temperature conditions. Additionally, the density functional theory (DFT) method was employed to analyse the interaction mechanisms of HCs/R1216 and HCs/R134a. The results show that, firstly, low proportional concentrations of R1216 and R134a widen the flammability limit ranges of mixtures HCs/R1216 and HCs/R134a. Moreover, elevated temperatures not only promote the combustion reactions of HCs but also facilitate the thermal decomposition reactions of R1216 and R134a. However, HCs are more sensitive to temperature changes. Therefore, the flammability limit range of mixtures expands as the temperature rises. Secondly, a combination of theoretical calculations and experiments demonstrates that R134a exhibits better physical flame retardancy than R1216, while the chemical flame retardancy of R1216 is superior to that of R134a. These results provide a valuable reference for the safe application of hydrocarbon working fluids.

Effect of two halogenated olefins on combustion characteristics of Isobutane

To reduce the greenhouse effect, environmentally friendly working fluids such as unsaturated halogenated olefins and hydrocarbons have attracted much attention. This study focuses on the combustion characteristics of two binary mixtures of isobutane (R600a) and R1234ze(E) and R1233zd(E). The results indicate that the addition of R1234ze(E) and R1233zd(E) has different effects on the combustibility of R600a under rich and lean fuel conditions. From the perspective of flammability limit range, as the ratio of R1234ze(E) and R1233zd(E) increases, the flammability limit range of both mixtures first increases and then decreases. For R1233zd(E)/R600a containing chlorine (Cl), the maximum flammable limit range corresponds to (nF + nCl)/nH equal to 1/3, which is significantly smaller than the nF/nH ratio of 1 in R1234ze(E)/R600a. Secondly, through calculation and simulation, it was found that the flame retardancy of R1233zd(E) is stronger than that of R1234ze(E), as the Cl in the former has a stronger ability to capture active radicals. Finally, based on Chemical kinetics analysis, it is found that adding R1234ze(E) to R600a during oxygen enrichment will increase the concentration of active radicals H, O and OH during combustion. This leads to higher combustion temperature and smaller flame thickness, thereby promoting the combustion process. The results will have significant implications for the safe application of new mixed working fluids and the analysis of internal reaction mechanisms of the mixture flammability.

Modeling Contact Resistance and Water Transport within a Cathode Liquid-Fed Proton Exchange Membrane Electrolyzer

Conventional proton-exchange membrane (PEM) water electrolyzers use thicker membranes (>175 μm) than their PEM fuel cell counterparts (<25 μm), which reduces hydrogen crossover but also reduces electrolyzer efficiency due to the increased resistance. Reduction of hydrogen crossover is critical in conventional systems to avoid buildup of hydrogen in the anode above the lower flammability limit. New concepts for operating PEM water electrolyzers are emerging, such as the patented concept involving liquid water supply at the cathode while operating the anode with air, which reduces the safety concern related to hydrogen crossover using thin membranes. Experimental work has demonstrated the viability of this approach, but open questions remain regarding the interplay between water transport, water consumption, and cell performance, as well as identifying the components and material properties that enable high performance. In this work, a physics-based computational model of a cathode-fed PEM water electrolyzer was developed. The model highlights the importance of limiting contact resistance and explores the effect of cell compression on non-uniformity of current distributions. Sensitivity studies found that membranes up to 50 μm thick can be used without significant water transport limitations.