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A detailed experimental and numerical investigation of the combustion characteristics of carbon-free NH 3 /H 2 blends in N 2 O was performed in the present work. The experiments with various hydrogen fractions (α = 0–1.0) and equivalence ratios (φ = 0.6–1.4) were performed at ambient pressure (1 atm) and temperature (283 K) in a standard 3.375-L cubic combustion vessel. The explosion behaviors (maximum explosion pressure P max and maximum pressure rise rate (d p / d t) max) were obtained from the pressure-time curves. Further, the combustion characteristics (e.g. laminar burning velocity (S L), thermal expansion ratio (σ), flame thickness (δ), effective Lewis number (L e e f f ) and Markstein length (L b)) were calculated numerically by means of chemical kinetic analysis using Zhang's mechanism (Zhang et al. , Combust. Flame, 2017, 182: 122–141). Besides, sensitivity analyses of the explosion pressure and laminar burning velocity were performed to identify the contribution of dominant elementary reactions. The results show that, for combustion supported by N 2 O, the flame instability and the potential of deflagration to detonation transition (DDT) should be considered for the evaluation of P max and (d p / d t) max. Moreover, the hydrodynamic instability of premixed H 2 /NH 3 /N 2 O flame reaches its peak near the stoichiometric ratio and increases monotonously with the hydrogen fraction. The diffusion-thermal instability is more significant in the leaner mixture, and it is not invariably enhanced with the increase of hydrogen fraction but is relevant to the composition of the mixture. In addition, the sensitivity analysis of explosion pressure demonstrates that R80 (N 2 O + H 2 N 2 +H 2 O) and R70 (N 2 O+(M) = N 2 +O (+M)) are the most dominant reactions contributing to the pressure rise. The sensitivity analysis of laminar burning velocity shows that R70 (N 2 O+(M) = N 2 +O (+M)), R71 (N 2 O + H N 2 +OH) and R80 (N 2 O + H 2 N 2 +H 2 O) are the most dominant reactions enhancing laminar burning velocity. • The combustion characteristics of NH 3 /H 2 blends in N 2 O were firstly investigated. • The flame instability and DDT were considered for the combustion supported by N 2 O. • The parameters of hydrodynamic and diffusion-thermal instability were analyzed. • The sensitivity analysis based on different mechanisms was performed. [ABSTRACT FROM AUTHOR]

Nitrous oxide has a wide range of applications, and the explosion in presence of nitrous oxide can cause serious disaster. In this study, we reported the explosion characteristics of hydrogen-methane-nitrous oxide mixtures with different equivalence ratios (ϕ) in a closed cylindrical explosion vessel at sub-atmosphere pressure (40 kPa). The equivalence ratio (ϕ) is defined as the ratio of the actual N2O–blended fuel ratio to the stoichiometric N2O–blended fuel ratio in this study. The key parameters were obtained experimentally which reflected the explosion characteristics such as the maximum explosion pressure, maximum pressure rise rate, and deflagration index. The results demonstrated that the experimental explosion pressure was greater than the adiabatic explosion pressure and N2O made explosion have potential of deflagration to detonation transition (DDT). The addition of methane increased the peak of both the maximum explosion pressure and the maximum pressure rise rate when the value of ϕ is constant. The effect of methane fraction on the maximum pressure rise rate was negligible when the equivalence ratio was higher than 0.6. In addition, with the increase of methane content, the equivalence ratio of the maximum explosion pressure peak would increase, but the peak of the pressure rise rate was stable at where the equivalence ratio is about 0.7. [ABSTRACT FROM AUTHOR]

The cellular instabilities of expanding hydrogen-propane-nitrous oxide spherical flames were experimentally and theoretically investigated at various equivalence ratios (ϕ =0.4–1.6), hydrogen fractions (X H =0–0.8), and initial pressures (P u =1.1–1.5 bar). The experiments were conducted in a constant volume combustion chamber (CVCC) using high-speed schlieren technology to record the flame morphology, while the theoretical analysis was based on linear stability theory. The controlling parameters, critical conditions for the onset of instability, flame front oscillation intensity, logarithmic growth rate of perturbation, etc. were studied. The results indicate that the instability of premixed hydrogen-propane-nitrous oxide flames is dominated by hydrodynamic instability at X H ≤ 0.6 , while the competitive mechanism between thermal-diffusion and hydrodynamic instabilities is considered at X H = 0.8. The oscillation intensity in the horizontal direction of the flame profile is more prominent than those in other directions, and with the increasing hydrogen fraction, the equivalence ratio at which the highest oscillation strength occurs shifts from ϕ =1.0 to ϕ =0.6. The trends of the experimental critical radius and Peclet number with each initial condition are completely identical to that of the theoretical value, but the experimental value is somewhat underpredicted. The three main reasons are proposed in the paper. Novelty and Significance Statement: This work is the first study on the cellular instabilities of hydrogen-propane-nitrous oxide flames by experiment and theory. It is under a small scale and reduced initial pressure condition that the cellular destabilization of hydrogen-propane flames is realized in the nitrous oxide atmosphere, while in the air atmosphere, the initial condition at which flame cellular destabilization occurs is harsher. Flame front oscillation strength, growth rate of disturbance, etc. were used to characterize quantitatively the degree of wrinkles and instabilities. Some novel findings were presented in the study. The present work is of great significance for the combustion community to understand deeply the cellular instabilities characteristic of expanding spherical flames supported by nitrous oxide, as well as for the engineering community to optimize the engine, propulsion, and energy supply systems, etc. [ABSTRACT FROM AUTHOR]

• The explosion behaviors of gaseous binary fuel (H 2 /C 3 H 8) at N 2 O atmosphere were firstly investigated. • The presence of N 2 O makes explosion extremely unstable due to its heat release during decomposition. • A spot of propane can dramatically alter the explosion characteristics of H 2 -N 2 O mixtures. • The maximum explosion pressure and maximum pressure rise rate both present three characteristic trends with propane fraction. • The effect of initial pressure on flame acceleration and heat loss was found and analyzed. In the present work, the explosion of hydrogen-propane-nitrous oxide mixtures with various equivalence ratios (ϕ = 0.2 - 2.4), propane fractions (ω C = 0 - 1.0) and initial pressures (P 0 = 50 - 100 kPa) at ambient temperature (288 K, room temperature) was experimentally performed in a standard 3.375-L cubic vessel. The maximum explosion pressure (P max), maximum pressure rise rate ((d p / d t) max) and explosion time (t e) were obtained in light of pressure–time curves, and the normalized maximum explosion pressure (P max / P 0) and normalized explosion time (t d / t e) were analyzed as well. Moreover, the adiabatic pressure (P ad) and laminar burning velocity (S L) were also calculated for argumentation. The results indicate that with the increase of ω C , P max and (d p / d t) max both present three characteristic variation trends for different ϕ , while the inflections of P max - ϕ and (d p / d t) max - ϕ transient gradually from lean to rich mixtures. Besides, t e and t d / t e are monotonously rising with ω C , illustrating that propane addition can remarkably decelerate the explosion process. In addition, with the increase of P 0 , P max / P 0 raises monotonously and is asymptotic to P ad / P 0 at ω C ⩽ 0.2 , demonstrating that heat loss decreases under higher P 0. Linear dependence is also found between (d p / d t) max and P 0 for any composition. Furthermore, t e monotonically decreases with P 0 , due to the flame acceleration occurring in the combustion period. In general, the presence of nitrous oxide (N 2 O) with a large content makes the explosion extremely unstable, and a spot of propane can dramatically alter the explosion characteristics of H 2 -N 2 O mixtures. These results will be meaningful for the development of explosion-mitigation devices and technology in case of underlying explosion and fire hazards. [ABSTRACT FROM AUTHOR]

The laminar burning velocity and cellular instabilities of premixed flame in H 2 -N 2 O mixtures were studied experimentally at various equivalence ratios (φ) and initial pressures (p 0). High-speed schlieren was used to record the morphology of the spherical flame. Based on the spherical flame method, the laminar burning velocity and Markstein length were derived. In addition, the Lewis number, flame thickness, thermal expansion and Zel'dovich number were evaluated numerically. The results show that both the laminar burning velocity and Markstein length increase with the equivalence ratio. The Lewis number (less than unity) increases with the equivalence ratio while the flame thickness exhibits an inverse evolution. Hence, the most unstable flames were found to be at φ = 0.4–0.6 due to the combined effects of the thermal-diffusion and hydrodynamic instabilities. At lower initial pressure, the flame front is smooth in spite of the diffusion-thermal instability. With the increase of initial pressure, both the diffusion-thermal instability and hydrodynamic instability dominate the cellular structure of the flame front. The flames undergo self-acceleration at φ = 0.4–0.6 and p 0 = 40 kPa, with the acceleration exponents close to 1.5 (the value of self-turbulization). Due to the local oscillation of velocity, the flame propagation speed decreases after the self-similar regime. [ABSTRACT FROM AUTHOR]

Explosions can occur in different oxidizer atmospheres in confined spaces, which should be given attention in regard to the fire safety in process industries. In this study, experiments were carried out in a closed cylindrical vessel (7.3 L) to compare the explosion behaviors of hydrogen (H 2) and ethylene (C 2 H 4) with air, nitrous oxide (N 2 O), and oxygen (O 2) at various equivalence ratios. The initial pressures of the hydrogen and ethylene mixtures were 50 kPa and 30 kPa, respectively. Explosion indices such as the maximum explosion pressure (P m a x ), maximum rate of pressure rise ( (d p / d t) m a x ) and deflagration index (K G) were determined experimentally. The results showed that the pressure evolutions obtained with N 2 O and O 2 exhibited strong oscillations due to flame instabilities and cellular structures. The maximum explosion pressure increased in the order of air, O 2 and N 2 O while the maximum rate of pressure rise increased in the order of air, N 2 O and O 2. Unlike those of air and O 2 , the maximum values of (d p / d t) m a x obtained with N 2 O were found to be at the fuel-lean side. This indicated that the instability feature of N 2 O supported combustions at the fuel-lean side. Compared with air, the maximum explosion pressure obtained in N 2 O and O 2 atmospheres could be very close to the adiabatic pressure when considering no heat loss during an explosion. Therefore, there was a flame acceleration and deflagration to detonation transition (DDT) mechanism in the explosions with N 2 O and O 2. Additionally, two dimensionless pressure rise coefficients were derived and discussed. The flames in very lean H 2 -N 2 O were very unstable, resulting in scattered coefficients as a function of equivalence ratio. At the fuel-rich side, the dimensionless coefficients were very close. In practice, the potential DDT mechanism in the explosions, especially in N 2 O and O 2 atmospheres, needed to be considered. [ABSTRACT FROM AUTHOR]

• A comparative study of H 2 /N 2 O/N 2 and CH 4 /N 2 O/N 2 explosions was conducted. • The extrema of explosion parameters are reached at the fuel-lean side or stoichiometric condition. • Flame instability has more dominant effect on the explosion behavior. In this study, we performed experiments in a closed cylinder vessel to investigate the explosion behaviors of H 2 /N 2 O and CH 4 /N 2 O mixtures with various equivalence ratios (φ) diluted with 40%, 50% and 60% N 2. Explosion parameters, including the maximum explosion pressure, the explosion time, the maximum pressure rise rate, the deflagration index as well as the dimensionless pressure rise coefficients were derived and evaluated. Meanwhile, the adiabatic flame temperature and the laminar burning velocity were calculated. Lastly, the heat loss to the wall during the combustion process was computed. The results show that the explosion characteristics are influenced by the flame instability more significantly than the laminar burning velocity and the adiabatic flame temperature. The maximum explosion pressures of hydrogen- and methane- mixtures are found to be at φ = 0.8 and φ = 1.0, respectively. The minimum explosion time was attained at φ = 0.6–0.8 and φ = 1.0, and the inflection of the maximum pressure rise rate and the deflagration index occur at φ = 0.6–0.8 and φ = 0.8–1.0. The normalized pressure rise coefficients are relatively scattered at the fuel-lean side due to the highly unstable feature of N 2 O supporting explosions while at the fuel-rich side, the variations of the dimensionless coefficients are very small. Furthermore, the heat loss during the explosion also reaches its minimum value at the fuel-lean side. [ABSTRACT FROM AUTHOR]

In this study, we report the explosion characteristics of H 2 /N 2 O mixtures at fuel-lean side and stoichiometric condition in a 113 L cylindrical vessel. Experiments were performed at different sub-atmospherical pressures (P 0 =10 kPa–40 kPa). Explosion pressure traces were obtained by a pressure transducer mounted on the wall. Meanwhile, the flame morphology was recorded using schlieren visualization. The maximum explosion pressure (P max) and maximum pressure rise rate ( (d p / d t) max) were derived and discussed. Results show that two competitive effects (i.e., the adiabatic flame temperature and the molecular number) make the maximum P max occurring at φ =0.5—0.6. Linear correlations were found between P max and P 0. Except the adiabatic flame temperature and flame propagation speed, the flame instability was found to participate in dominating the maximum pressure rise rate. For increased initial pressure, the equivalence ratio of the maximum (d p / d t) max switches to the lean side due to the enhanced flame instability. Depending on different equivalence ratios, both linear and quadratic correlations were found between (d p / d t) max and P 0. [ABSTRACT FROM AUTHOR]

Knowledge of detonation propagation characteristics of H 2 –N 2 O mixtures is crucial for industrial safety and development. In this study, the detonation propagation characteristics of H 2 –N 2 O mixtures with different equivalence ratios (φ = 0.4, 0.6, and 1.0) in three tubes (d = 14.5, 19.5, and 28.6 mm) were investigated experimentally. Some dominant chemical lengths were evaluated together with cell size and normalized average velocity. The results show that the periodic oscillation of local velocity is more intense as approaching the limits. Below the limits, the detonation fails and decays rapidly to a "low velocity" regime. The tube scale and initial pressure determine the velocity deficit and the minimum detonation velocity shows no obvious trend on the mixture composition and tube scale. Scaling analysis illustrates that the dimensionless parameter d / Δ I and normalized average velocity show a single exponential growth trend. Notably, the detonation tends to decay earlier at φ = 1.0 than φ = 0.4 or φ = 0.6. Besides, fuel-lean mixtures (φ = 0.4 and 0.6) and stoichiometric mixtures (φ = 1.0) share similar critical pressure. It can be obtained that a linear relationship exists between the cell size and the induction length. The cell size data shows that the cell size of φ = 0.6 is the smallest, which agrees with the results of the induction length calculated. It is also demonstrated that the maximum value of heat release ratio peak is at φ = 0.6. • Detonation propagation characteristics of H 2 –N 2 O mixtures are performed. • Detonation failure is the earliest at φ = 1.0 below the critical pressure. • Three equivalence ratios (of 0.4, 0.6, and 1.0) share similar critical pressure. • The smallest cell size is found at φ = 0.6. [ABSTRACT FROM AUTHOR]