Negative Temperature Coefficient Phenomenon Research Articles

On the Three-Stage Explosion Characteristics of Diethyl Ether-Oxygen Mixtures

ABSTRACT In this article, the three-stage explosion limits of diethyl ether (a typical alternative fuel with a high cetane number and high energy density) are taken as the research object. A comprehensive and in-depth analysis of the double negative temperature coefficient (NTC) behavior of the cool flame region and hot ignition region, through the detailed kinetic analysis, have been carried out. The results show that with the increase of the equivalence ratio, the double NTC phenomenon becomes more remarkable. In very lean conditions, such as the equivalence ratio of less than 0.5, the boundary between the first NTC region and the second NTC region becomes obscure, and a deeper single NTC phenomenon is formed as these two NTC regions merge. The whole explosion limit curve shifts to higher pressure conditions with the increase of the addition of inert gas. The NTC phenomenon in the moderate temperature region becomes more significant and the boundary between these two NTC regions is more obvious. In addition, the hot flame explosion limit also shows such two NTC response. The heat release rate (HRR) profile exhibits three-stage, two-stage, single-stage ignition, or non-ignition depending on the initial conditions. To elucidate the differences in the ignition process between the different explosion limit regimes, the sensitivity and reaction pathway analyses are performed. This study not only provides insight into the oxidation characteristics of ether fuels, but also provides theoretical support for high-performance engines.

High-Resolution Two-Dimensional Numerical Simulation of n-Heptane/Air Autoignition Under the Influence of Temperature Fluctuation

ABSTRACT High-resolution two-dimensional numerical simulations are performed to investigate n-heptane/air autoignition process under different initial temperatures with varied levels of temperature fluctuations. It is found that temperature fluctuation has an important impact on the autoignition process, and its effects depend on the initial mean temperature. When is located at the upper turning point of the negative temperature coefficient (NTC) regime, as the temperature fluctuation increases, the autoignition mode evolves from a single-stage ignition mode controlled by high-temperature chemistry (HTC) to a mode characterized by the coexisting of multi-stage and single-stage ignition. Both the first-stage heat release and ignition delay increase, but the overall heat release and the total ignition delay decrease. In the presence of a large temperature fluctuation, the low-temperature chemistry (LTC), and HTC reactions, single-stage and two-stage ignitions could coexist in the domain. Chemical explosive mode and sensitivity analyses are performed to identify the reaction mode and the key element reactions in the mutli-stage autoignition process. The temperature fluctuation has a significant promotion effect on the overall ignition except when is located at the lower turning point of the NTC regime. Overall, the NTC phenomenon is weakened under the influence of temperature fluctuation, approaching to zero temperature coefficient (ZTC) phenomenon.

Effects of non-equilibrium plasma and equilibrium discharge on low-temperature combustion in lean propane/air mixtures

A zero-dimensional (0D) non-equilibrium plasma assisted ignition/combustion (PAI/PAC) model is established by coupling plasma kinetics and combustion kinetics to investigate the effect of non-equilibrium nanosecond repetitively pulsed discharge (NRPD) plasma and equilibrium discharge on low-temperature combustion (LTC) stage of lean propane/air mixtures. The results show that the NRPD case has the fastest ignition, in comparison to equilibrium discharge (ED) with the same discharge energy. With NRPD before LTC, electrons, O(1D), as well as O and OH, are formed during discharge, promoting propane consumption dramatically. Electron collision with propane can promote propyls (NC3H7/IC3H7) and alkenes, but there is a competitive relationship in terms of discharge energy between them. The formed propyls are mainly consumed to produce propyl peroxyl (NC3H7O2/IC3H7O2) and accelerate low-temperature product formation, such as ketones and OH. The relaxations of vibrationally excited states are the dominant pathways of heat release, leading to the continuous temperature rising after NRPD. Finally, the negative temperature coefficient (NTC) performances under auto-ignition, NRPD, and ED are investigated. ED only shifts the NTC to the low-temperature zone, while NRPD eliminates the NTC phenomenon. The provided path fluxes of intermediates show that the promoted NC3H7/IC3H7 and NC3H7O2/IC3H7O2 induced by NRPD weaken the negative correlation between the oxidation rate of NC3H7/IC3H7 and temperature. The higher concentrations of NC3H7/IC3H7 and HO2 radicals during discharge can result in an advanced intermediate/high temperature reaction process.

On Iso-octane Combustion with Ozone Addition under HCCI Engine-Like Conditions

Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time (IDT) of iso-octane/air mixtures under typical operating conditions of HCCI engines. 0-D and 2-D CFD simulations were carried out to compute IDT and to characterize compression, combustion, and expansion in an HCCI engine, respectively. A kinetic model was developed by merging a mechanism for iso-octane, a sub-mechanism for nitrogen oxides, and a sub-mechanism for ozone. The model was used to investigate iso-octane/air/ozone mixtures under typical operating conditions of HCCI engines running with very lean mixtures (equivalence ratio equal to 0.3). Parametric analyses were carried out considering different values of temperature (500 to 1200 K), pressure (15 to 40 bar) and ozone concentration (0 to 50 ppm). The results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures, and the NTC (Negative Temperature Coefficient) effect decreases. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. An increased pressure, on the other hand, generally promotes faster ozone decomposition and enhances ozone effect on IDT reduction for all temperatures except in the range 760-840 K, where the opposite effect occurs due to NTC phenomenon. Finally, when temperature is very high, i.e. 1200 K, both ozone and pressure have little influence on the percentage reduction of IDT.

Methanol oxidation up to 100 atm in a supercritical pressure jet-stirred reactor

Methanol (CH3OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. The experimental results show that the onset temperature of CH3OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH3OH oxidation from the literature and added some new low-temperature reaction pathways such as CH2O + HO2 = HOCH2O2 (RO2), RO2 + RO2 = HOCH2O (RO) + HOCH2O (RO) + O2, and CH3OH + RO2 = CH2OH + HOCH2O2H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO2 chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO2 chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO2 formation.

Influence of noble metals on thermoacoustic oscillations and boundaries of the region of negative temperature coefficient during combustion of n-pentane – air mixtures

Using the combustion of n-pentane – air mixtures as an example, it was shown that thermoacoustic regimes of ignition disappear in the presence of a platinum surface, which generates catalytic centers propagating into the volume. Thus, the platinum catalyst eliminates a certain stage of kinetic mechanism, and the negative temperature coefficient phenomenon vanishes. In the presence of a palladium catalytic surface that does not generate catalytic centers propagating into the volume, the phenomenon of a negative temperature coefficient occurs.

Autoignition characteristics of diesel spray under different injection pressures and cold start strategy for compression ignition engines

Poor cold start performance is one of the main factors restricting the application range of compression ignition engines. Stable flame generation in the cylinder is the prerequisite for the successful start of the engine. In order to explore ways to improve the cold start ability of the compression ignition engine, the ignition characteristics of diesel sprays at different injection pressures were studied in a constant volume combustion chamber. A high-speed photography technique was used to obtain the color images of the initial flames of diesel free sprays. And the yellow flame is introduced as the criterion for a successful flame generation. It was found that under moderate injection pressures, the reactant concentration and energy dissipation rate reached equilibrium. Under this condition, the duration of the blue flame is relatively stable, and it is most conducive to the generation of the yellow flame in the cylinder. The negative temperature coefficient phenomenon is also observed, and the temperature range in which this phenomenon occurs is identical with the change of injection pressure. A cold start strategy for the compression ignition engine is proposed by controlling the compression-end temperature in the cylinder to be higher than the threshold ignition temperature of diesel spray.

Experimental and Kinetic Study of the Promoting Effect of Nitrogen Dioxide on Ethane Autoignition in a Rapid Compression Machine

Nitrogen oxides (NOx: NO2 and NO) are the main pollutants produced in combustion. Previous studies have focused on the NOx forming mechanism and control strategies but have paid less attention to the interaction between NOx and hydrocarbon fuels; even a small amount of NOx can significantly affect the ignitions of hydrocarbons. The ignition characteristic is of great importance for the optimization design and operation control of engines such as diesel engines, which are affected by the self-ignition characteristics of fuels. In this study, a rapid compression machine was used to measure the ignition delay times of seven fuel-lean C2H6/O2/Ar mixtures with [NO2]/[C2H6] ranging from 0.0 to 50% at Tc = 760–970 K and Pc = 30 bar. The results exhibit no negative temperature coefficient phenomenon in the ignition of ethane with or without NO2. A great reduction in the ignition delay time and the global activation energy is observed in the presence of NO2, which becomes more noticeable with increasing the NO2 concentration. Subsequently, four literature models involving the C2H6/NO2 chemistry were validated against the new measurements. Moreover, the key elementary reactions and important reaction channels controlling the ignition of C2H6/NO2 mixtures were then identified by the detailed chemical kinetic analysis.

Ignition Delay Characteristics of Kerosene with Decomposed Hydrogen Peroxide

The ignition chemical delay of kerosene with decomposed hydrogen peroxide was simulated by using -decane as a surrogate fuel for kerosene. The influences of concentration, the decomposition rate, and pressure on the chemical delay were investigated. The ignition delay time had the negative temperature coefficient phenomenon for a low decomposition rate, and the increasing pressure weakened the negative temperature coefficient phenomenon, which nearly disappeared when the pressure was greater than 4 MPa. An -dependent coefficient of the ignition delay time was almost not influenced by concentration and pressure when the pressure was less than 4 MPa, and had a noticeable influence on the ignition delay time at . Increasing the concentration effectively reduced the ignition delay time at , but it had a slight effect on the ignition delay time at .

On The Features of the Negative Temperature Coefficient Phenomenon in Combustion of N-Pentane-Air Mixtures

The ignition of n-pentane-air mixture at low temperatures is experimentally studied in a rapid mixture injection static reactor. The ignition process was monitored using a high speed color video camera. It is shown that introduction of platinum wire into the reactor eliminates the phenomenon of negative temperature coefficient; however Pt wire has no effect on the ignition delay time of thermal ignition of stoichiometric n-pentane-air mixture at lower temperatures.

Negative Temperature Coefficient of PVC Filled with Carbon Black

PVC filled with carbon black (CB) composition were prepared by a conventional melt-mixing method. The effects of CB on the negative-temperature-coefficient (NTC) effect and the resistivity-temperature characteristic of PVC/CB composites were studied. Our present research suggests that percolation threshold can be obtained for the content of CB 10%. The reasons of NTC phenomenon were also investigated from point of view of polymer structure. We also discussed relationship between the current and voltage, that is, the I-U characteristic of the composites can be divided into two regions, ohmic and nonohmic according to the different content of CB and voltage.

An experimental study of the autoignition characteristics of conventional jet fuel/oxidizer mixtures: Jet-A and JP-8

Ignition delay times of Jet-A/oxidizer and JP-8/oxidizer mixtures are measured using a heated rapid compression machine at compressed charge pressures corresponding to 7, 15, and 30 bar, compressed temperatures ranging from 650 to 1100 K, and equivalence ratios varying from 0.42 to 2.26. When using air as the oxidant, two oxidizer-to-fuel mass ratios of 13 and 19 are investigated. To achieve higher compressed temperatures for fuel lean mixtures (equivalence ratio of ∼0.42), argon dilution is also used and the corresponding oxidizer-to-fuel mass ratio is 84.9. For the conditions studied, experimental results show two-stage ignition characteristics for both Jet-A and JP-8. Variations of both the first-stage and overall ignition delays with compressed temperature, compressed pressure, and equivalence ratio are reported and correlated. It is noted that the negative temperature coefficient phenomenon becomes more prominent at relatively lower pressures. Furthermore, the first-stage-ignition delay is found to be less sensitive to changes in equivalence ratio and primarily dependent on temperature.

PTC/NTC Behavior of PVDF Composites Filled with GF and CF

Abstract Conductive polyvinylidene fluoride(PVDF) matrix composites filled with graphited fiber(GF) or carbon fiber(CF) were prepared by the melt-mixing method. The breakage and length distribution of the fibers in the polymer matrix were studied by scanning electron microscope(SEM) and optical microscope(OM) observations, respectively. The differences in the positive temperature coefficient(PTC) effects of the composites were mainly attributed to inter-fiber contact ability. The elimination of the negative temperature coefficient(NTC) effect for CF/PVDF composite was because of an increase in the viscosity of the polymer matrix. With the same filler content, CF could be more effective, to eliminate the NTC effect when compared with GF. Addition of 2% CF(mass fraction) in the PVDF composite with 7% GF(mass fraction) could effectively eliminate the NTC phenomenon of the composite.

Study on synthesis of poly(N‐isopropylacrylamide) grafted carbon black and temperature‐dependence conductivity of grafted carbon black/epoxy composites

AbstractCarbon black nanoparticle grafted with poly(N‐isopropylacrylamide) (CB‐g‐PNIPAAm) was synthesized by surface‐initiated atom transfer radical polymerization (SI‐ATRP). The temperature‐responsive behavior of CB‐g‐PNIPAAm was proved by temperature‐variable 1H NMR. A temperature‐dependent conductive composite was prepared by blending CB‐g‐PNIPAAm with epoxy resin. The relationship between temperature and resistivity of the composite was studied: the composite exhibited a negative temperature coefficient (NTC) phenomenon. Possible mechanism for the NTC phenomenon was suggested. The results showed that resultant composites can be used in intelligent temperature‐switching. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1529–1535, 2008

Positive‐temperature‐coefficient/negative‐temperature‐coefficient effect of low‐density polyethylene filled with a mixture of carbon black and carbon fiber

AbstractLow‐density polyethylene (LDPE) filled with carbon black (CB) and carbon fiber (CF) composites were prepared by a conventional melt‐mixing method. The effects of a mixture of CB and CF on the positive‐temperature‐coefficient (PTC) effect and the negative‐temperature‐coefficient (NTC) effect, as well as the percolation threshold, were examined in detail. A synergy effect between CB and CF occurred, in that continuous conductive pathways formed within the CB/CF‐filled composite. The percolation threshold was moved to a reduced filler content with the addition of CF to an LDPE/CB composite. A model was proposed to explain the difference in the PTC behavior of composites containing CB and CF and composites containing only CB or CF. In addition, the NTC effect was weakened with a mixture of CB and CF, and a relatively small radiation dose was required to eliminate the NTC phenomenon in LDPE/CB/CF composites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3094–3101, 2003

Temperature effects of electrical resistivity of conductive silicone rubber filled with carbon blacks

AbstractThe temperature effects of electrical resistivity of conductive silicone rubber filled with widely varying concentrations of carbon blacks (CBs: GPF 660 carbon black, HAF N‐330 carbon black, VXC‐72 conductive carbon black, and BP2000 superconductive carbon black) were studied from 30 to 200°C. Both low positive temperature coefficient (L‐PTC) and negative temperature coefficient (NTC) phenomena were observed, whose intensity and temperature range depend on the type and the concentration of CBs. A stronger temperature dependency of resistivity was observed when the CB concentration was in the percolation region. Unlike crystalline polymers/CBs composites, the high positive temperature coefficient (H‐PTC) phenomenon was not observed in CB‐filled silicone rubber when the CB concentration was near the percolation threshold. Generally, pronounced the NTC phenomenon appears in composites with small particles of CBs at concentrations near the percolation threshold, whereas a pronounced PTC phenomenon appears in composites with large CB particles of at concentrations exceeding the percolation region. If both PTC and NTC phenomena were in the same sample of conductive silicone rubber, the PTC phenomenon always appeared at relatively low temperatures and NTC at relatively high temperatures. The results were discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3889–3895, 2003

Experimental investigation and kinetic modeling of the negative temperature coefficient of the reaction rate in rich propane—oxygen mixtures

The phenomenon of the negative temperature coefficient (NTC) of the reaction rate of the oxidation of rich propane—oxygen mixtures was experimentally studied. The NTC phenomenon is qualitatively described by a simple kinetic model containing a minimum set of reactions related to the oxidation of the starting hydrocarbon, propane, and the propyl C3H7 . radical formed.

Negative temperature coefficient phenomenon in exchange reactions between alkene oxides and alkyl-?-hydroxyalkyl sulfides

The temperature dependence of the initial rate of the exchange reaction in the propylene oxide-octyl-β-hydroxyethyl sulfide system shows a maximum and a minimum between 60 and 110‡C, the maximum at 90‡C. This behavior may be explained by the participation in the reaction of an associate that is formed from a molecule of propylene oxide and two molecules of hydroxyethyl sulfide. Computer calculations have been made for this scheme of the reaction over a wide temperature range.