Flame Suppressants Research Articles

Performance assessment of residential heat pumps operating in extreme cold climates using zeotropic mixtures

Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO2-based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO2, as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO2 (10%) yields the highest COP and CO2-rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO2-rich mixtures tend to allow more compact systems, especially in colder climates.

Regulating the Thermal Behavior of Nitrocellulose by Stable Incorporation of a Green Supramolecular Salt

Commonly used inorganic potassium salts present challenges in nitrocellulose (NC)-based propellants due to poor dispersion, destabilization, and toxic byproduct formation. Environmental concerns drive interest in green flame suppressants. Here, we report a green supramolecular salt, carboxymethyl-cyclodextrin potassium (CM-CD-K) through alkalinization and etherification reaction. Molecular dynamic simulations reveal stronger interactions between CM-CD-K and NC. The loading of CM-CD-K within NC forms a stable structure with resistance to migration and defense against crystalline precipitation. Remarkably, static combustion tests show reduced flame areas and harmful gas concentrations compared to traditional potassium salt-doped NC. Kinetic analysis reveals a shift from order to Kamal-Sourour reaction with CM-CD-K addition. CD-based hybrid organic salts hold potential as eco-friendly propellant alternatives.

Effectiveness of flame suppressants on cool flames and hot flames

While the effectiveness of various flame suppressants such as bromotrifluoromethane and trimethylphosphate on hot flames has been relatively well studied over the years, such suppressants have not been examined in the context of low-temperature cool flames. This investigation solves this issue by exploring the extinction limits of six suppressants on both hot flames and cool flames in the counterflow geometry using n-dodecane as the fuel. In contrast to hot flames, it is found both experimentally and numerically that cool flames are relatively impervious to chemically based suppressants such as bromotrifluoromethane; these suppressants are essentially diluents at low temperatures. Detailed examination of the computed flame structure reveals that the reactions composing the catalytic cycles that interfere with hydrogen radical and hydroxyl radical production in hot flames are orders of magnitude lower in cool flames. Furthermore, mildly flammable suppressants such as trimethylphosphate and 2‑bromo-3,3,3-trifluoropropene are observed to ignite under the conditions necessary to initiate cool flames, which limits measurements of the cool flame extinction limits. This premature oxidation is not predicted by kinetic models describing the suppressant chemistry.

Viability of Various Sources to Ignite A2L Refrigerants

Environmental considerations are motivating the adoption of low global warming potential refrigerants. Most of these are mildly flammable, i.e., A2L. Their susceptibility to ignition from various ignition sources is poorly understood, particularly for the stoichiometric and quiescent mixtures that are emphasized here. The viability of fifteen residential ignition sources to ignite four A2L refrigerants is considered. Tests are performed in a windowed chamber with a volume of 26 L. The refrigerants are R-32 (difluoromethane); R-452B (67% R-32, 26% R-1234yf, and 7% pentafluoroethane); R-1234yf (2,3,3,3-tetrafluoropropene); and R-1234ze (1,3,3,3-tetrafluoropropene). Two types of ignition sources are confirmed here to be viable: a resistively heated wire at 740 °C and open flames. When the refrigerant concentration was increased slowly, candle flames and butane flames extinguished before initiating any large deflagrations. Eleven other sources were not viable: a smoldering cigarette, a butane lighter, friction sparks, a plug and receptacle, a light switch, a hand mixer, a cordless drill, a bread toaster, a hair dryer, a hot plate, and a space heater. The difficulty to ignite these refrigerants in air is attributed to their long quenching distances (up to 25 mm). Under some conditions the refrigerants were observed to act as flame suppressants.

Influence of Flame Suppressants on the Level of Nonequilibrium Radiation during Ignition of Hydrogen-Oxygen Mixtures behind Shock Waves

The nonequilibrium radiation occurring during ignition of a 10% stoichiometric hydrogen-oxygen mixture with flame suppressant additives diluted with argon behind shock waves was studied. Instead of the expected reduction in the super-equilibrium radiation of active radicals in the ignition zone, the addition of halogenated flame suppressants led to increased UV radiation around wavelengths of 220 and 411 nm characteristic of the HO2 radical and H2O2 and H2O molecules. Therefore, the hypothesis about the suppression mechanism due to quenching of the excited HO*2 radical is not confirmed, and the effect of flame-suppressing additives is due to the binding of H and O atoms.

Flame Inhibition Chemistry: Rate Coefficients of the Reactions of HBr with CH3 and OH Radicals at High Temperatures Determined by Quasiclassical Trajectory Calculations

Reactions of HBr with radicals are involved in atmospheric chemistry and in the mechanism of operation of bromine-containing flame retardants. The rate coefficients for two such reactions, HBr + OH and HBr + CH3, are available from earlier experiments at near or below room temperature, relevant for atmospheric chemistry, and in this domain, the activation energy for both has been found to be negative. However, no experimental data are available at combustion temperatures. In this work, to provide reliable data needed for modeling the action of brominated flame suppressants, we used the quasiclassical trajectory (QCT) method in combination with high-level ab initio potential energy surfaces to evaluate the rate coefficients of the two title reactions at combustion temperatures. The QCT calculations have been validated by reproducing the experimental rate coefficients at room temperature. At temperatures between 600 and 3200 K, the QCT rate coefficients display positive activation energies. We recommend the...

Advances in understanding combustion phenomena using non-premixed and partially premixed counterflow flames: A review

Counterflow flames provide an ideal platform for understanding the flame structure and as a model to study the effect of physical and chemical perturbations on the flame structure. This article reviews the advances made in the understanding of combustion dynamics and chemistry through experimental and numerical studies in counterflow non-premixed and partially premixed flames. Key contributions on fundamental aspects such as extinction, ignition and effect of perturbations on the stability of diffusion flames are first summarized and analysed. The review then focuses on the progress made in the understanding of the effect of inert particles and flame suppressants on the flame characteristics. A review of detailed studies on edge flames facilitates further understanding of local quenching and re-ignition phenomena in highly turbulent flames. The influence of radiation model and unsteady flow-conditions on the flame kinetics and dynamics along with work on NOx kinetics has been discussed. The review also outlines that specific experiments need to be carried out over a wide range of conditions for further understanding and validation of numerical models.

The opposite influences of flame suppressants on the ignition of combustible mixtures behind shock waves

The experiments were performed to investigate the influence of the flame suppressants CCl4, CF3H, and C2F4Br2 on the ignition of hydrogen–oxygen, acetylene–oxygen and methane–oxygen mixtures behind shock waves. The temperature dependencies of the ignition delay times and temperature profiles in the induction zone were measured. The experiments show that CCl4 and C2F4Br2 inhibit the ignition of hydrogen and acetylene and promote the ignition of methane. The influence of CF3H on the inhibition is much smaller. A qualitative kinetic analysis suggests that at low temperatures typical for hydrogen and acetylene ignition, the flame suppressants participate in only chain termination reactions inhibiting the ignition. For methane ignition at higher temperatures, the pyrolysis of admixture results in the formation of active radicals, which initiate chain reactions of combustion. The results prove that the chemical stability of the suppressants molecules is essential for successive chemical suppression of ignition under given conditions. The strong inhibiting effect of C2F4Br2 on ignition of H2/O2 mixtures behind shock waves is notable.

Promotion of methane ignition by the fire suppressants CCl4 and CF3H

The experiments were performed to investigate the influence of the flame suppressants CCl4, CF3H, and C2F4Br2 on the ignition of hydrogen–oxygen, acetylene–oxygen and methane–oxygen mixtures behind shock waves. The temperature dependencies of the ignition delay times and temperature profiles in the induction zone were measured. The experiments show that CCl4 and C2F4Br2 inhibit the ignition of hydrogen and acetylene and promote the ignition of methane. The influence of CF3H on the inhibition is much smaller. A qualitative kinetic analysis suggests that at low temperatures typical for hydrogen and acetylene ignition, the flame suppressants participate in only chain termination reactions inhibiting the ignition. For methane ignition at higher temperatures, the pyrolysis of admixture results in the formation of active radicals, which initiate chain reactions of combustion. The results prove that the chemical stability of the suppressants molecules is essential for successive chemical suppression of ignition under given conditions. The strong inhibiting effect of C2F4Br2 on ignition of H2/O2 mixtures behind shock waves is notable.

A review of recent developments in flammability of polymer nanocomposites

AbstractPolymer nanocomposite flame retardancy has become a critical parameter in industrial material application. Recent environmental policies have prohibited the incorporation of halogenated flame-retardant compounds into polymers owing to the high level of environmental degradation caused by high levels of toxic gas and smoke emission. The demand for zero-halogen flame-retardant compounds by both researchers and manufacturers is due to the inherent advantages accruable from their incorporation like very minimal toxic emission, minimal smoke release, zero corrosive gas release, reduced corrosion activities and absence of dripping in fire condition. This has necessitated the quest for eco-compliant replacements for halogenated flame suppressants. Recent insight has shown the eco-compliancy of exfoliated graphene nanoplatelets as flame retardants when incorporated into polymer nanocomposites (PNCs). Relative to the propensity to retard flame, increasing quantities of exfoliated graphene nanoplatelets have exhibited the capability to significantly repress critical flammability parameters like heat release rate (HRR), peak HRR (PHRR), rate of carbon monoxide production, smoke production rate and total mass loss rate while simultaneously increasing limiting oxygen index, time of ignition and total PHRR, thereby retarding flammability and creating better chance to reduce loss and casualty in real-life fire situation through the formation of even layers of carbonaceous char in the condensed phase capable of efficiently suppressing the thermal decomposition caused by oxygen and heat to the polymer matrix and cutting off the flaming path. This paper gives an insight into recent developments in flame retardancy of PNCs, with special emphasis on the flame-retardancy propensity of exfoliated graphite nanoplatelets.

Numerical study of detonation transmission in mixtures containing chemical inhibitors

In this article, we report on numerical simulations of the evolution of gaseous detonation waves in mixtures that contain chemical inhibitors. In general, these are compounds that consume the radicals that are produced during combustion, thereby inhibiting the exothermic chain-terminating reaction. Also, some of them participate in endothermic reactions, such as dissociation. These properties make them very efficient flame suppressants. In this study, we employ a chemical kinetics model that consists of a three-step chain-branching mechanism for the fuel combustion and a one-step mechanism for the reaction between inhibitor and radicals. Results from both one- and two-dimensional simulations are presented and discussed. It is shown that radical consumption and heat absorption due to the inhibitor's reaction result in longer induction zones. This, in turn, leads to a detachment of the reaction zone from the precursor shock. For small and medium inhibitor concentrations, this detachment is temporary. Eventually, the radical concentration behind the induction zone becomes sufficient to initiate rapid fuel consumption, thus producing pressure waves which reach the precursor shock and re-ignite the detonation. This is followed by large over-pressures and highly irregular oscillations of the shock. Nonetheless, sufficiently high inhibitor concentrations can yield permanent detonation quenching.

Testing ogranophosphorus, organofluorine, and metal-containing compounds and solid-propellant gas-generating compositions doped with phosphorus-containing additives as effective fire suppressants

The development and investigation of new effective and environmentally clean flame suppressants is a promising direction in fire extinguishing. Organophosphorus and metal-containing compounds are the most promising candidates for the replacement of CF3Br. However, for many of these compounds, data on their minimum extinguishing concentration are not available. In the present work, the minimum extinguishing concentrations for a number of new phosphorus-and metal-containing compounds and some of their mixtures, solid-propellant gas-generating compositions containing phosphorus additives were determined using the cup-burner technique and a setup with a turbulent flame source and transient application of flame suppressants.

Flame suppression by aerosols derived from aqueous solutions containing phosphorus

Aerosols derived from aqueous solutions containing phosphorus are investigated as possible alternatives to halon-based flame suppressants. Phosphorus-containing compounds (PCCs) have been shown to be effective flame suppressants in the gas phase; a water-based solution would provide a practical means of delivering a condensed-phase PCC to the flame. Flame suppression was characterized by measuring the global extinction strain rate of a counterflow, nonpremixed methane–air flame with and without a PCC additive. An aqueous solution of the additive was introduced as an aerosol into a heated chamber upstream of the air flow tube. A high-efficiency nebulizer produced a polydisperse spray of droplets with a Sauter mean diameter of 8 μm, as measured with a phase-Doppler particle analyzer. The droplet size distribution was nearly independent of the composition and flow rate of the liquid. Externally maintaining the reactant streams at 360 K allowed the water in the aerosol to evaporate prior to reaching the flame. Evaporation of water leaves behind residual particles for solutes that are solids at 360 K, and thus solid particulates, not droplets of solution, enter the flame zone. Comparisons of different solutions with different phase solutes were used to provide an estimate of the physical effect of particles on the flame. Three neat phosphorus-containing compounds, dimethylmethylphosphonate (DMMP), diethylmethylphoshonate (DEMP) and dimethylphosphonate (DMP) and five 1.6% molar aqueous solutions, orthophosphoric acid (OPA), phosphorus acid, phosphonic acid, methylphosphonic acid, and dimethylmethylphosphonate (DMMP), were investigated. The acid solutions (with solid solutes) all displayed similar effectiveness, and were all slightly more effective, on a per mole basis, than the DMMP solution (a liquid solute). The effectiveness of a DMMP/water solution is found to be a weighted average of the effectiveness of its constituents. Per mole of water delivered, a 1.6% molar solution of a PCC is approximately twice as effective in fire suppression compared to neat water vapor. Flame modeling calculations for the extinction condition with added gas-phase phosphorus compounds using a published phosphorus reaction mechanism grossly underpredict the total effectiveness of the compounds investigated.

Flame inhibition by phosphorus-containing compounds over a range of equivalence ratios

There is much interest in the combustion mechanism of organophosphorus compounds (OPCs) due to their role as potential halon replacements in fire suppression. A continuing investigation of the inhibition activity of organophosphorus compounds under a range of equivalence ratios was performed experimentally and computationally, as measured by the burning velocity. Updates to a previous mechanism were made by the addition and modification of reactions in the mechanism for a more complete description of the inhibition reactions. Reaction pathways for HOPO 2 + H and HOPO + H are analyzed using the BAC-G2 approach. A new reaction pathway for HOPO 2 + H = PO 2 + H 2O has been identified which results in a higher rate constant than that reported in the literature. In this work, the laminar flame speed is measured experimentally and calculated numerically for a premixed propane/air flame at 1 atm, under a range of equivalence ratios, undoped and doped with dimethyl methylphosphonate (DMMP). A detailed investigation of the catalytic cycles involved in the recombination of key flame radicals is made for two equivalence ratios, fuel lean and fuel rich. From this, the importance of different catalytic cycles involved in the lean versus rich case is discussed. The chemical kinetic model indicates that the HOPO 2 ⇔ PO 2 inhibition cycle is more important in the lean flame than the rich. The OPCs are similarly effective across the range, demonstrating the robustness of OPCs as flame suppressants. In addition, it is shown that the phosphorus compounds are most active in the high-temperature region of the flame. This may, in part, explain their high level of inhibition effectiveness.

Effects of Halons and Halon Replacements on Hydrogen-Fueled Laminar Premixed Flames

Fundamental unstretched laminar burning velocities and flame response to stretch, as characterized by Markstein numbers, were considered experimentally and computationally. The investigation was limited to laminar premixed flames involving hydrogen burning in air with Halon 1301 (CF 3 Br), nitrogen, and carbon dioxide considered as flame suppressing diluents. Outwardly propagating spherical laminar premixed flames were observed for fuel equivalence ratios of 0.6-1.8, concentrations of oxygen in oxygen/nitrogen mixtures of 21% by volume before dilution with a suppressant, and normal temperature and pressure. Suppressants involved concentrations of the chemically active suppressant, Halon 1301, of 0-2% by volume and 'concentrations of the chemically passive (chemically inert) suppressants, nitrogen and carbon dioxide, of 0-50% by volume. The present flames were sensitive to flame stretch, yielding values of unstretched-to-stretched laminar burning velocities in the range of 0.6-1.2 for levels of flame stretch well below quenching conditions, for example, for Karlovitz numbers less than 0.15. The agreement between measured and predicted unstretched laminar burning velocities was good based on the mechanisms of Mueller et al. for H 2 /O 2 reactions and Babushok et al. for halogen reactions. The resulting flame structure predictions suggest that H and OH radical production and transport are important for flame suppression and preferential-diffusion/stretch interactions; this reflects the strong correlation between laminar burning velocities and the maximum concentrations of H and OH in the reaction zone of the present flames. It also was found that effects of flame suppression that reduced concentrations of H and OH in the reaction zone made the present flames more unstable to preferential-diffusion/stretch interactions, an effect that tends to counteract the beneficial effects of flame suppressants to some extent.

Binary CF 3Br- and CHF 3–inert flame suppressants: effect of temperature on the flame inhibition effectiveness of CF 3Br and CHF 3

A numerical investigation with detailed chemistry and transport was conducted on the inhibition effectiveness of binary halogenated suppressant and inert gas mixtures. Computational results demonstrate that while positive synergism persists between CF 3Br and inert gases, little or negative synergism exists between CHF 3 and inert gases. These synergistic effects are attributed to the sensitivity of flame inhibition effectiveness of the chemical suppressant, in that while the inhibition effectiveness of CF 3Br is enhanced at lower flame temperatures, it remains relatively unchanged with CHF 3 over the examined temperature range. The temperature sensitivity of CF 3Br inhibition effectiveness is a result of enhanced catalytic inhibition cycles as the flame temperature decreases. It is also demonstrated that water vapor tends to diminish the flame inhibition effectiveness of CHF 3.

Extinguishment of Liquid Heptane and Gaseous Propane Diffusion Flames

This paper presents a comparative study of the extinguishment of liquid heptane and gaseous propane diffusion flames caused by the addition of flame-suppressing agents (N2 and CO2) into a coflowing stream of air. Measurements included mass burning rate, concentrations of suppressants at flame extinction, inflame temperature profiles, and exhaust composition. Adiabatic equilibrium temperatures of the stoichiometric mixtures of the fuels in pure airstream, and in the airstream containing flame suppressants with the concentrations corresponding to flame extinction, were calculated. Although the extinction processes of liquid and gaseous fuel flames are markedly different, even when both are buoyancy dominated, the extinction mechanism of CO2 and N2 in both flames is primarily thermal. The reaction zone temperature in the flame-anchoring region of the heptane flame at extinction agrees well with the adiabatic flame temperature corresponding to the lean flammability limit. In propane flame, where the flame base is lifted off the burner before extinction, the reaction zone temperature at extinction is 100-200 K higher than the adiabatic flame temperature at the lean flammability limit.

Gas-Phase Thermochemistry of Iron Oxides and Hydroxides: Portrait of a Super-Efficient Flame Suppressant

In the search for non-ozone-depleting Halon (CF3Br) replacements, several metals, including iron, have been identified as super-efficient flame suppressants. Although some thermochemical data exist for the species that are thought to be most important in iron's flame chemistry, a more complete and accurate characterization of the thermochemistry of iron oxides, hydrides, and hydroxides is required to improve kinetic flame models. In this investigation predicted enthalpies (Δrxn ) and free energies (Δrxn ) of the reactions of several FeOxHy species in methane flames are reported. Heats of formation (Δf ) for the FeOxHy species of interest are also recommended. The hybrid B3LYP density-functional method and the CCSD(T) coupled-cluster method are employed in conjunction with a relativistic effective core potential on the iron center and a valence triple-ζ basis on all atoms in order to characterize the relative energetics of the important species.

BAC-MP4 Predictions of Thermochemical Data for C1 and C2 Stable and Radical Hydrofluorocarbons and Oxidized Hydrofluorocarbons

An ab initio bond additivity corrected quantum chemistry procedure has been applied to the development of a data base for thermochemistry of C/H/F/O species. This information has been used to construct a chemical kinetic mechanism for the prediction of the behavior of fluorocarbons as flame suppressants, with clear applications to plasma and atmospheric chemistry as well. Bond additivity corrected (BAC) Moller−Plesset fourth-order perturbation theory (MP4) calculations have been performed to obtain a large body of thermochemical data on about 100 closed and open shelled fluorocarbon species. For about 70 of these species, literature values for enthalpies of formation were available for comparison to the calculated values. The average difference between the calculated and literature values was about 9 kJ/mol. The results indicate that the BAC-MP4 procedure can provide energies that are comparable in accuracy to most experimentally derived values, at lower computational costs relative to other more computationally expensive ab initio molecular orbital methods. This work provides a substantial data base of thermochemical data for fluorinated hydrocarbons constructed in a self-consistent manner.

Effect of halogenated flame inhibitors on C 1−C 2 organic flames

The influence of CF 3 Br, CF 3 I, CF 4 , CHF 3 , C 2 F 6 , and C 2 HF 5 on laminar flame propagation in mixtures of CH 4 , CH 3 OH, C 2 H 6 , and C 2 H 4 with air has been determined by numerical simulations. Comparisons are made with experimental results and earlier calculations. Flame velocities as a function of inhibitor concentration and equivalence ratio have been determined. For all the organic fuels, the ranking of inhibitive efficiency is CF 3 Br, CF 3 I>C 2 F 6 >C 2 HF 5 >CHF 3 >CF 4 . The bromine and iodine compounds are clearly the most efficient flame suppressants. They are of equal effectiveness except in methanol, where CF 3 I is superior. The retardants are more effective for rich mixtures than for lean. With 1% inhibitor concentration, the decrease in burning velocity is most pronounced for unsaturated fuels. The general ordering is consistent with calculational results on flame thickness and radical concentrations. Reaction pathway analysis demonstrates the importance of the regeneration of reactive scavengers. Relative contributions from physical and chemical effects are estimated.