Deflagration Flame Research Articles

Effects of nozzle diameter on marine fuel injection and deflagration performance

The marine fuel injection and deflagration performance with different nozzle diameters in large compartments are researched in this work. The results show that marine fuel could quickly form a stable fuel spray field. Increasing nozzle diameter dramatically enhances the fuel spray concentration and aggravates the fuel spray deflagration degree, resulting in rapid increases in flame propagation speed, deflagration overpressure, and deflagration temperature. A larger nozzle diameter causes the deflagration flame to propagate further forward. With the increase of deflagration time, the flame propagation speed shows a trend of first rising and then decreasing, with fluctuations. When the nozzle diameter is small, the overpressure declines toward the back. In the case of large nozzles (≥0.8 mm), the overpressure distribution in the compartment fluctuates greatly, with a tendency to increase first and then decrease. Furthermore, deflagration peak overpressure ascends linearly with the logarithm of fuel spray concentration. The peak deflagration overpressure is 1.875 MPa with 1.0 mm nozzle diameter. The deflagration temperature is highest at the center of the deflagration. The research results can guide the assessment and prevention of fire and deflagration accidents on ships.

Study on flame quenching characteristics inside the Tesla valve structure flame arrester unit

Flame arrester is a kind of safety device designed to prevent the spread of flames. In order to achieve a more efficient flame arrester unit structure, this paper proposes a flame arrester unit based on the Tesla valve structure. A comparative study of the quenching characteristics of deflagration flames of propane-air premixtures in traditional crimped ribbon and Tesla valve structure flame arrester units is conducted through numerical simulations and experiments. The results indicate that the flame arrester unit based on the Tesla valve structure exhibits more efficient flame quenching performance compared to the crimped ribbon flame arrester unit. The flame quenching process fusion image shows that the flame quenching length of Tesla valve flame arrester unit with the same cross-sectional area is only 61% of the crimped ribbon flame arrester unit. In addition to heat transfer and wall effect, the collision of airflow in the branched pipe and main pipe within the Tesla valve structure flame arrester unit also plays a significant role in hindering the further advancement of flames. The quenching process of the Tesla valve structure flame arrester unit can be divided into four stages: expansion, diversion, convergence, and contraction stages. Among the five structural parameters that make up the Tesla valve structure flame arrester unit, the angle is the most critical factor affecting quenching performance, with the angle and entrance structure having a significant impact on flow performance. Furthermore, experimental evidence confirms the typical morphological changes the flame front structure undergoes during flame propagation: spherical (hemispherical),finger-shaped and tulip-shaped. However, tulip-shaped flames do not always propagate continuously to the end of the pipeline.

On the pressure wave emanating from a deflagration flame front

We consider a spherical flame expanding from an ignition point through a homogeneous, flammable gaseous mixture. We analytically predict the transient pressure and velocity fields ahead of the flame as a function of the flame front position, which is assumed to evolve in time according to a power-law relation. The predictions are successfully validated by CFD simulations. We show that the model is also effective for analyzing real deflagration problems, by predicting measurements taken in hydrocarbon deflagration tests.

The Formation of a Flame Front in a Hydrogen–Air Mixture during Spark Ignition in a Semi-Open Channel with a Porous Coating

An experimental study of ignition and flame front propagation during spark initiation in a hydrogen–air mixture in a semi-open channel with a porous coating is reported. The bottom surface of the channel was covered with a porous layer made of porous polyurethane or steel wool. The measurements were carried out for a stoichiometric mixture (equivalence ratio ER = 1.0) and for a lean mixture (ER = 0.4) of hydrogen with air, where ER is the molar excess of hydrogen. The flame front was recorded with a high-speed camera using the shadow method. Depending on the pore size, the velocity of the flame front and the sizes of disturbances generated on the surface of the flame front were determined. Qualitative features of the deflagration flame front at ER = 0.4, consisting of disturbances resembling small balls of flame, were discovered. The sizes of these disturbances significantly exceed the analytical values for the Darrieus–Landau instability. The effect of coatings made of porous polyurethane or steel wool is compared with the results obtained for an empty smooth channel. Depending on the hydrogen concentration in the hydrogen–air mixture, the velocity of the flame front compared to a smooth channel was three times higher when the channel was covered with steel wool and five times higher when the channel was covered with porous polyurethane.

Dynamic responses for inhomogeneous hydrogen deflagrations in a 7 m3 box-type chamber with a vent: Effects of ignition position and equivalent volume concentration

The aim of this paper is to investigate the effects of ignition position and equivalent volume concentration C‾H2 on dynamic responses for vented inhomogeneous hydrogen deflagrations in a 7 m3 chamber. The results indicate that the narrow-band and wide-band processes of overpressure signals are formed during Helmholtz-type and thermo-acoustic oscillations, respectively. The displacement response depending on C‾H2 is characterized by the weak vibration or a sharp increase. The acceleration impulsive response is dominated by the forced vibration component with the high-frequency and high-amplitude, while the acceleration dynamic response is dominated by the free vibration component with the low frequency and low amplitude. The high-frequency component and frequency overlapping in structural responses become more significant with the increase of C‾H2. Structural responses generated by bottom ignition are the largest due to the most violent thermo-acoustic oscillation induced by a coupled flame structure of deflagration flame and jet fire.

Effect of airflow velocity on flame propagation and pressure of starch dust explosion in a pneumatic conveying environment

To investigate the flame propagation and pressure characteristics of dust explosions during the airflow transportation of dust particles, a 2 m transparent pipeline, supplied with a constant airflow from a fan, was constructed as a dust explosion testing apparatus. The distribution patterns of airflow velocity were measured using an anemometer and subjected to statistical analysis. Dust explosion experiments were conducted with airflow velocities of 5, 10 and 15 m/s, and an ignition energy of 1 kJ was applied to the transported dust particles. The results revealed that the flame propagation of starch explosions undergoes distinct stages: ignition, slow propagation, accelerated propagation. The flame propagation speed and explosion pressure initially increased and subsequently decreased as the airflow velocity was raised. As the airflow velocity increased from 5 m/s to 10 m/s, the maximum flame propagation speed rose from 53.2 m/s to 67.7 m/s, and the maximum explosion pressure increased from 25.62 kPa to 31.96 kPa. At an airflow velocity of 15 m/s, both the maximum flame propagation speed and explosion pressure decreased, reaching 32.6 m/s and 19.67 kPa, respectively. In contrast to conventional closed container powder injections, chemical igniters in the pneumatic conveying environment exhibit elongated flames, with the airflow influencing the timing of dust deflagration flame appearance and accelerating flame propagation. Due to the connectivity of the airflow transport pipeline, the explosion pressure tends to be lower. Influenced by non-uniform airflow and temperature differentials, dust deflagration flames within the airflow transport pipeline manifest a spiral propagation structure.

Experimental and numerical study on suppressing coal dust deflagration flame with NaHCO3 and MPP

In this paper, the flame propagation behavior and free radical reaction mechanism of two powder explosive suppressants, sodium bicarbonate (NaHCO3) and polyphosphate melamine (MPP), at different additions were investigated using Hartmann experimental system and CHEMKIN-PRO numerical simulation. The results showed that NaHCO3 and MPP showed different inhibitory advantages. MPP mainly suppressed the propagation of flame during the development and rapid rise stages, while NaHCO3 had a better suppression effect at the preliminary stage of explosion. The gas phase kinetic analysis also proved that the NaHCO3 reaction was rapid, and it could decompose and endothermy at the beginning of the explosion, and generate gaseous H2O and CO2 to dilute the concentration of volatile components. Meanwhile, NaOH, NaO, and NaO2 produced by decomposition consumed free radicals and reduced the initial reaction rate of the explosion. In the process of gas phase reaction, MPP continuously reduced the amount of flame free radicals through a inhibition cycle of HPO3 ⇔ PO2, and generated a large number of inert products for inerting fixed carbon, exhibiting better long-term effectiveness. This study can provide a reference for the selection and design of explosion suppressants.

Modeling of non-homogeneous premixed hydrogen-air flame acceleration and deflagration to detonation transition in an obstructed channel

This paper uses Large Eddy Simulation (LES) technology to study the dynamic behavior of flame acceleration (FA) and deflagration-to-detonation transition (DDT) in a duct with different obstacle layouts in non-homogeneous concentration fields. A turbulent premixed combustion model with three additional source terms was established using the OpenFoam code library. It was found that the predicted results of the coupled model were in good agreement with the measured values of the experiment. Numerical results show that turbulent combustion and self-ignition effects are the main factors affecting flame acceleration. Still, self-ignition effects are a necessary and sufficient condition for inducing DDT phenomena. Detonation flames have around-flow and diffraction effects and do not evolve into jet and vortex flames, which differ from deflagration flames. Two initiation mechanisms were found under different obstacle layouts: hotspots were generated in the area swept by the Mach stem at the flame front, triggering detonation near the turbulent flame through the reactive gradient mechanism; focusing of sufficiently strong reflected waves deposited enough energy in a particular area near the flame brush and directly triggered detonation. In addition, the central obstacle layout is most conducive to FA and DDT, while the symmetrical layout is the least favorable.

Experimental study on deflagration characteristics of non-uniform oil mist in an enclosed chamber

The deflagration characteristics of oil mist in the marine enclosed chamber are studied. The results show that oil mist deflagration in a non-uniform enclosed chamber is a violent process. When the nozzle size is 1.0 mm, the maximum oil mist deflagration overpressure in the chamber can reach 0.945 MPa. Increasing the nozzle size strengthens the deflagration intensity of oil mist. As the nozzle size increases, the oil mist deflagration flame propagation velocity, peak deflagration overpressure, and gas temperature increase. In addition, the oil mist deflagration process is facilitated by the increase in injection pressure. The maximum flame propagation velocity and maximum deflagration overpressure gradually improve with the increase in injection pressure. This is mainly because the increase in injection pressure leads to better atomization of oil mist droplets, increasing surface area of the oil mist, and the increase in oil mist concentration, which makes the intensity of oil mist deflagration in the enclosed chamber more intense. Nozzle size on the intensity of oil mist deflagration is more influential than the injection pressure.

Effects of transverse concentration gradients on the vented explosion of methane-air mixtures

Experiments were performed in a 90-L vessel with a side vent to investigate the effects of transverse concentration gradients on the vented explosion of non-uniform methane-air mixture. Controlled by a mass flow meter, methane was injected uniformly into the vessel through 18 nozzles that are evenly distributed at the top to configure a methane-air mixture with an average methane volume fraction of 10.9%. Various transverse concentration gradients were obtained by altering the free diffusion time of the methane in the vessel. The deflagration flame was recorded through a high-speed camera at a frequency of 1000 Hz, and the internal overpressure at different locations was captured through three piezoelectric sensors. Experimental results reveal that larger concentration gradient resulted in lower flame propagation rate, longer time for the burst of the vent cover, and stronger pressure oscillations. Concentration gradient affects the typical flame structure, especially the formation and development of the tulip flame. Under the present experimental conditions, Helmholtz oscillations occurred in all tests. Concentration gradient has a negative correlation with the frequency of Helmholtz oscillations but a positive correlation with the duration of oscillations. In conclusion, the variation of concentration gradient obviously affects flame propagation and pressure evolution during vented explosions, which deserves more attention in the design of explosion protection.

Experimental research on self-initiation process of rotating detonation wave by high-temperature ethylene-rich gas

This study investigates an experimental process of high-temperature ethylene-rich gas and air for rotating detonation. After abandoning the pre-detonator method, the high-temperature ethylene-rich gas was injected and collided with the outer wall to create a localized high temperature and pressure region that ignited the gas mixture to form the deflagration state. Whether it is a single deflagration flame or double deflagration flame, it is eventually transformed through the deflagration-to-detonation transition (DDT) process into a double detonation wave. With propagation and collision of the double waves, the injection pressure increases, which results in shortening the self-initiation delay time. The equivalence ratio, gas temperature, and mass flow rate affect the success ratio of self-initiation. Rotating detonation engine (RDE) can be successfully initiated in the equivalence ratio range of 0.82 to 1.69. Equivalence ratio and gas temperature increase the propagation velocity of rotating detonation wave (RDW), for maximum RDW velocity of 1324.61 m/s, the gas temperature is 1021.3 °C. At the double wave collision point, a new weak RDW appears that decreases the RDE operating stability and a periodic quenching and re-initiation of RDW phenomenon occurs at that point.

Laminar flame properties correlations for H2/C3H8 mixtures at high temperature and pressure conditions

Laminar burning velocity (SL) and flame thickness correlations for H2, C3H8 and their mixtures at autoignition conditions for the study of ignition regime were developed. The thermodynamic conditions covered by the correlations are 750–1100 K for the unburned temperature and 5–60 atm for the pressure, typical of weak/mixed ignition regime occurrence. The data for fitting the correlation were generated using a kinetic model with a detailed reaction mechanism. A new definition of energy mixture fraction as a weighting factor in Le Chatelier's mixing rule for H2/C3H8 was proposed to SL calculations. The expression for the flame thickness calculation was developed considering the correction by the radical diffusion from the reactive zone to the preheating zone. Experimental tests and Schlieren image recording inside the combustion chamber from a Rapid Compression Machine were carried out to evaluate the Sankaran Prediction criteria (Sap) capability to predict the ignition regime. Average absolute relative residual values of 4.36% and 0.91% and the total points with an accuracy of 10% were 92.57% and 99.43% evaluating the test data group in the proposed correlations were obtained for H2 and C3H8, respectively. The new version of energy mixture fraction allows an increase in the amount of data with an accuracy of 10% from 36.4%, 34.3% and 19.2% to 96.4%, 96.6% and 75.6 % for a content of 75%, 80% and 95 % of H2 in volume, respectively. The Sap values are in agreement with observations in the RCM, where deflagrative flame fronts were identified prior to volumetric autoignition, confirming the weak/mixed ignition regime. The proposed formulations are suitable to be used in the Sap calculations and ignition regime prediction as well as to determinate the SL and flame thickness at autoignition conditions.

Study on Dust Explosion of Wheat Starch Inerted by NaHCO3

With the aim of researching the inerting effect of NaHCO3 upon the wheat starch dust explosion, the test of explosion pressure was implemented by 20L spherical explosion test system. The results showed that as NaHCO3 mass fraction increased, the maximal explosion pressure and the maximal explosion pressure rise ratio of wheat starch dust decreased gradually. When 70wt% NaHCO3 was added, the maximal explosion pressure and the maximal explosion pressure rise ratio of wheat starch dust lessened by 73.82% as well as 93.88%, separately. The deflagration flame propagation experiment of NaHCO3 inerting wheat starch was implemented inside a device of the improved Hartmann tube. These consequences showed that velocity and length of the deflagration flame propagation of wheat starch decreased gradually as NaHCO3 mass fraction increased. When 15wt% NaHCO3 was added, peak velocity and average velocity of deflagration flame of wheat starch decreased by 95.30% and 89.69%, respectively. When 20wt% NaHCO3 was added, the deflagration of wheat starch could be completely inerted.

Experimental study on the auto-initiation of rotating detonation with high-temperature hydrogen-rich gas

An experimental study on the auto-initiation process of rotating detonation waves (RDWs) was conducted with high-temperature hydrogen-rich gas as the fuel and air as the oxidant. Spontaneous combustion of high-temperature hydrogen-rich gas and air occurred after they were injected into a rotating detonation chamber (RDC), which resulted in hot spots in the RDC and induced the formation of a rotating deflagration flame. Then, an RDW formed through the deflagration-to-detonation transition process in the RDC. The auto-initiation process of high-temperature hydrogen-rich gas and the formation mechanism of RDWs were studied in detail through experiments. The influences of the equivalence ratio on the RDW propagation characteristics of high-temperature hydrogen-rich gas were analyzed. The results showed that with the increase in the equivalence ratio from 0.61 to 1.93, five RDW propagation modes appeared in the RDC: Failure, two counter rotating detonation wave (TCRDW), Mixed, intermittent single rotating demodulation wave, and single rotating detonation wave (SRDW) modes. The Mixed mode was the transition mode from the TCRDW mode to the SRDW mode. The highest RDW velocity was 1485.9 m/s when the equivalence ratio was 1.32, in which the propagation mode was the stable SRDW mode.

Investigation on quenching characteristics of parallel narrow channels for deflagration flames

AbstractA set of experimental equipment for quenching the deflagration flame in linked vessels with parallel narrow channels was proposed. The quenching effects of the deflagration flame in the linked vessels were investigated, the influence law and mechanism of parallel narrow channels on the deflagration flame was analyzed. The results showed that the Pmax and the (dP/dt)max at each position were both lower than without parallel narrow channels, which indicated that the propagation of flame was inhibited by parallel narrow channels effectively, and the explosion intensity was also reduced. The pressure oscillation phenomenon inside the linked vessels disappeared due to the inclusion of parallel narrow channels. The deflagration flame was successfully quenched when the channel gaps were 0.5, 1.5, and 3 mm, but failed to be quenched when the channel gap was 6 mm. The quenching distance and the average propagation velocity of the deflagration flame in parallel narrow channels increased with the increase of the channel gap. When the channel gap was 6 mm, the deflagration flame completely passed through parallel narrow channels, and the average propagation velocity of deflagration flame was significantly greater than that when the deflagration flame was successfully quenched.

Experimental and numerical study of the fuel effect on flame propagation in long open tubes

Previous works (Daubech et al., 2019) were dedicated to gaseous flame acceleration along long pipes with a set of cases studied both experimentally and numerically. In these cases, the flammable mixture was initially quiescent and homogenously distributed. The impact of the tube diameter and material were studied trough both approaches for rather slow flames, the fuel being methane. While main features of the real flame were recovered by the chosen CFD method, some limits remained.A new experimental dataset is detailed and analyzed with a quicker flame, the fuel being hydrogen and the same experimental set-up as the one used for measuring slow flames. Thus, the fuel effect on the flame dynamics can be directly highlighted.A simple CFD approach is tested for recovering two distinct flame behaviors: a deflagration flame and another undergoing deflagration-to-detonation transition. Furthermore, the modelling results are used to propose elements of interpretation for flame acceleration.

Study on the preparation of novel FR-245/MCM-41 suppressant and its inhibition mechanism on oil shale deflagration flame

Oil shale development is of great significance because oil and gas resources are scarce. Research on the prevention of oil shale dust explosion is particularly important for guaranteeing the safe development and utilization of oil shale resources. In this work, the flame morphology and velocity of oil shale dust with and without MCM-41 or FR-245 were compared. Furthermore, the novel green FR-245/MCM-41 inhibitor was prepared by jet mill method and used in oil shale dust explosion for the first time. The best ratio of FR-245/MCM-41 for flame inhibition was obtained, which was 9: 1. The pyrolysis oxidation behavior of oil shale before and after adding FR-245/MCM-41 was analyzed and compared by FWO and KAS methods, respectively. The results showed that the activation energy calculated by FWO and KAS methods greatly increased after adding FR-245/MCM-41, which increased by 95.36% and 115.15% than that before adding inhibitor, respectively. Significantly, the activation energy is particularly high for two methods when α between 0.2 and 0.6, due to that MCM-41 and FR-245 coexisted to limit the oxidation of oil shale. For α between 0.7 and 0.9, the activation energy is still high because of the existence of MCM-41. Combining the oil dust flame propagation behavior with the characterization results before and after explosion, the physical-chemical synergy mechanism of oil dust flame propagation inhibition was revealed.

A novel ATH/SBA-15 suppressant prepared by in-situ synthesis and its inhibition mechanism on PE dust deflagration flame

Recently, polyethylene (PE) dust explosions have become a serious threat to the petrochemical industry. In order to prevent the occurrence of PE dust explosion, a novel aluminum hydroxide (ATH)/Santa Barbara Amorphous type 15 (SBA-15) powder explosion suppressant with the uniform dispersion of ATH was prepared by in-situ synthesis method. For ATH/SBA-15 inhibitor, SBA-15 molecular sieve and ATH was used as the carrier and active component of explosion inhibition, respectively. The inhibition effect on PE dust deflagration was studied by flame propagation experiment. The results showed that when the addition amount of ATH/SBA-15 exceeded 40 wt%, PE dust deflagration flame was almost completely inhibited. Furthermore, the Coats-Redfern method was used to study the thermal decomposition kinetics model. It was found that the thermal decomposition kinetics model of PE before and after the addition of explosion suppressants followed the random nucleation and growth reaction mechanism (A3 model). Further fitting analysis showed the apparent activation energy of PE increased significantly after adding ATH/SBA-15 inhibitor, confirming that ATH/SBA-15 had a significant inhibitory effect on PE deflagration in thermodynamics. Finally, combining with a series of characterization results, it was founded that the ATH/SBA-15 suppressant played the efficient synergy of physics and chemistry, which was efficient in inhibiting PE deflagration. The physical inhibition mainly includes: the free radicals adsorption by SBA-15 molecular sieve, the coating effect of ATH, the endothermic effect and the reduction in O2 concentration by H2O decomposed from ATH. Meanwhile, the chemical inhibition mainly lies in the elimination of the free radicals O· and H· in the explosion process by ATH.

Study on the dynamic process of premixed hydrogen-air deflagration flame propagating in a closed space with obstacles

This paper investigates the kinetic behavior of premixed hydrogen-air deflagration flame propagation in a closed tube with different obstacle arrangements with the Large Eddy Simulation (LES) technique. Comparing the computational accuracy of four different types of turbulent flame wrinkling models, it finds that the prediction results of the dynamic wrinkling model have a high match with the experimental and theoretical values in quantitative and qualitative terms. Numerical results show that the deflagration flame propagation process in the closed barrier tube has undergone a spherical flame, finger flame, jet flame, vortex flame, and tulip-like flame, five forms of the evolution process. The obstacle center arrangement and all-around arrangement induced the maximum and minimum values of flame propagation speed and deflagration overpressure, respectively; the ratio of the first speed peak, the second speed peak, and the growth rate of overpressure were 1.46, 1.51, and 1.9 times for the two arrangements, respectively. The flame propagation process is mainly influenced by the coupling of Rayleigh-Taylor (R-T) and Kelvin-Helmholtz (K-H) instability mechanisms. Furthermore, a simplified analytical model of hydrodynamic instability is developed, and it is found that instability plays a crucial role in the deformation and acceleration of the flame front.

Flame acceleration and transition to detonation in a pre-/main-chamber combustion system

Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre-/main-chamber combustion system with a stoichiometric ethylene–oxygen mixture. A Godunov algorithm, fifth-order in space, and third-order in time, is used to solve the fully compressible Navier–Stokes equations on a dynamically adapting mesh. A single-step, calibrated chemical diffusive model described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The two-dimensional simulation shows that a laminar flame grows in the pre-chamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls and interacting with the flame front. The shock–flame interactions are crucial for the development of flame instabilities, which trigger the subsequent flame development. The DDT arises due to a shock-focusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criterion is developed to study the DDT ignition mode. Preliminary one-dimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman–Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemical-diffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The two-dimensional analysis with CEMA indicates that the DDT initiated by the shock-focusing mechanism can form a strong thermal expansion region at the flame front that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignition-supported DDT, of which the emergence of excessive local autoignition mode is found to be a precursor. The effect of grid size, initial temperature, and orifice size are then evaluated, and results show that although the close-chamber DDT is highly stochastic, the detonation initiation mechanism remains robust.