Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms)

Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms)

Comparative Study on Autoignition Characteristics of Methylcyclohexane and Cyclohexane

Ignition delay times were measured behind reflected shock waves for cyclohexane and methylcyclohexane at pressures of 1.1, 5.0, and 16.0 atm, temperatures from 1075 to 1750 K, and equivalence ratios of 0.5, 1.0, and 2.0. Correlations of the ignition delay times were performed at the three equivalence ratios in Arrhenius form. Measured ignition delay times showed fairly good agreements with previous data. Several accepted mechanisms (JetSurF 2.0, Wang et al., Sirjean et al., Orme et al., and Silke et al.) were used to simulate the experimental measurements and conduct flux analyses and sensitivity analyses. Comparisons of the simulations and analyses between the mechanisms give insights into the oxidation of methylcyclohexane and cyclohexane. Methylcyclohexane has an evidently longer ignition delay time than cyclohexane at ϕ = 0.5, while its ignition delay time becomes comparative to those of cyclohexane at ϕ = 1.0. Chemical kinetic interpretation is given for this observation

Ignition Delay Characteristics and Kinetic Investigation of Dimethyl Ether/<i>n</i>‑Pentane Binary Mixtures: Interpreting the Effect of the Equivalence Ratio and Dimethyl Ether Blending

Measurements of the autoignition delays of dimethyl ether (DME)-enriched <i>n</i>-pentane mixtures were conducted at the equivalence ratios of 0.5–2.0, pressure of 20 atm, and temperatures of 1100–1600 K using a shock tube, and new data were provided for dual-fuel engine design, kinetic model development, and computational simulation. A recently published pentane isomer model was validated and used for kinetic analysis. It is found that the autoignition delay of <i>n</i>-pentane becomes longer as the equivalence ratio (fuel/air) increases. However, for DME, the dependence upon the equivalence ratio is inverse to that of <i>n</i>-pentane. Autoignition delays of DME/<i>n</i>-pentane mixtures become shorter with the increased DME proportion, except for the ϕ = 0.5 conditions, where the autoignition delays of DME and <i>n</i>-pentane are identical

Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms)

Numerical Study on the Effects of Diluents on the Laminar Burning Velocity of Methane–Air Mixtures

The effect of diluents on the laminar burning velocity of the premixed methane–air–diluent flames was numerically studied using the Chemkin package. The mechanisms of dilution, thermal-diffusion, and chemical effects of diluents on the laminar burning velocity were analyzed quantitatively at different dilution ratios for different diluents. Results show that the laminar burning velocity is decreased in the order from helium, argon, nitrogen, and carbon dioxide. In the case of N₂, the thermal-diffusion and chemical effects can be negligible and the decrease of the laminar burning velocity is largely caused by the dilution effect. The dilution, thermal-diffusion, and chemical effects of CO₂ suppress the laminar burning velocity, where the dilution effect plays a dominant effect among them. For helium and argon diluents, the chemical effect can be negligible and the thermal-diffusion effect enhances the laminar burning velocity. Therefore, the dilution effect has a much larger suppression effect on decreasing the laminar burning velocity to counteract the thermal-diffusion effect of helium and argon. An empirical formula of the laminar burning velocity that takes into account the adiabatic flame temperature and thermal diffusivity is obtained. Good correlations between the laminar burning velocity and mole fraction of H + OH at the position of the maximum mole fraction of the H radical in the flame are also demonstrated. The laminar burning velocity has the same tendency with the product of thermal-diffusion and chemical reaction terms as a function of the dilution ratio for different diluents. The adiabatic flame temperature plays a dominant influence on the laminar burning velocity, and thermal diffusivity has a secondary influence on methane–air–diluent flames

Shock-Tube Experiments and Kinetic Modeling of 2‑Methylfuran Ignition at Elevated Pressure

Ignition delays of 2-methylfuran were measured behind reflected shock waves over a wide range of experimental conditions: equivalence ratios from 0.25 to 2.0, average pressures from 1.25 to 10.65 bar, temperatures from 1120 to 1700 K, and oxygen concentrations up to 20%. Results show that the ignition delay decreases with increasing the pressure and decreasing the dilution ratio. For a given dilution ratio, there exists a crossover in the ignition delay time dependence upon the equivalence ratio and the crossing point shifts to the higher temperature at a higher pressure. The measured ignition delays of 2-methylfuran show good agreement with the previous data at atmospheric pressure. The 2-methylfuran model NUI_MF2 well predicts the ignition delays of 2-methylfuran at 1.25 bar but gives the underprediction when pressures are elevated to 4.25 and 10.65 bar. Sensitivity analysis identifies the importance of the reactions involving the <i>n</i>-butadienyl radical (C₄H₅-<i>n</i>) in the ignition process of 2-methylfuran. Better prediction on ignition delay times is achieved by perturbing the rate constants of β-scission reactions for the C₄H₅-<i>n</i> radical, and these perturbations do not affect the primary fuel consumption flux based on the reaction pathway analysis

Experimental and Kinetic Study on Ignition Delay Times of <i>iso</i>-Butanol

Ignition delay times of <i>iso</i>-butanol with fuel concentration of 0.5–1.0% were measured behind reflected shock waves. The experiments were conducted in the temperature range of 900–1700 K, at pressures of 1.2–10.0 atm and equivalence ratios of 0.5–2.0. The measured ignition delay times were compared with previous data under the same conditions and were correlated through multiple linear regression. Using the correlation, the ignition delay times of <i>iso</i>-butanol were compared to those of <i>n</i>-butanol. It was found that <i>iso</i>-butanol presented longer ignition delay time than that of <i>n</i>-butanol. Three available models were used to simulate the ignition delay times. Results showed that Sarathy model exhibited the best predictions at high temperature, but none of these models could well reproduce the measured ignition delay times at intermediate–low temperature. Reaction pathway analysis was performed to chemically interpret the observed difference in ignition delay times of <i>n</i>-butanol and <i>iso</i>-butanol. In addition, sensitivity analysis was made to ascertain the key reactions that control the intermediate–low temperature ignition in the Sarathy model

Ignition Delay Characteristics and Kinetic Investigation of Dimethyl Ether/<i>n</i>‑Pentane Binary Mixtures: Interpreting the Effect of the Equivalence Ratio and Dimethyl Ether Blending

Measurements of the autoignition delays of dimethyl ether (DME)-enriched n-pentane mixtures were conducted at the equivalence ratios of 0.5–2.0, pressure of 20 atm, and temperatures of 1100–1600 K using a shock tube, and new data were provided for dual-fuel engine design, kinetic model development, and computational simulation. A recently published pentane isomer model was validated and used for kinetic analysis. It is found that the autoignition delay of n-pentane becomes longer as the equivalence ratio (fuel/air) increases. However, for DME, the dependence upon the equivalence ratio is inverse to that of n-pentane. Autoignition delays of DME/n-pentane mixtures become shorter with the increased DME proportion, except for the ϕ = 0.5 conditions, where the autoignition delays of DME and n-pentane are identical

Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms)