Experimental study on the detonation propagation behaviors through a small-bore orifice plate in hydrogen-air mixtures

In this study, the regimes of detonation transmission through a single orifice plate were investigated systematically in a 6-m length and 90-mm inner diameter round tube. A series of experiments on the detonation propagation mechanisms in hydrogen-air mixtures were performed. A single obstacle with different orifice size (d) from 10 to 60 mm was adopted to study the effects of the induced perturbations on the detonation propagation. Here, the thickness of orifice plate (δ) was fixed at 10.33 mm. Detonation velocity was determined from the time-of-arrival (TOA) of the detonation wave recorded by eight high-speed piezoelectric pressure transducers (PCB102B06). Detonation cellular size was obtained by the smoked foil technique. The characteristic of detonation velocity evolution were quantitatively analyzed after it passes through a single obstacle, and particular attention was paid to the cases for which the blockage ratio (BR) is greater than 0.9, i.e., the cases of small hole diameter of d < 25 mm. The experimental results showed that, in a smooth tube, only super-critical condition and sub-critical condition can be observed. After the orifice plate is introduced into the tube, critical condition occurs. The detonation re-initiation with distinct cellular structures was experimentally observed. Of note is that when the blockage ratio (BR) values in the range of 0.802–0.96, it was easier to detonate at the fuel-lean side. Finally, the critical condition for detonation propagation through an orifice plate was quantified as d/λ > 1 where λ is the detonation cell size.     

Effect of bundle geometries on the detonation velocity behaviors in stoichiometric hydrogen-air mixture

In this study, the effects of pipe bundle geometries on the detonation velocity behaviors are examined systematically in a circular tube with 6 m long and an inner diameter of 90 mm. The tube bundle structures are created by inserting several small pipes (20 mm outer diameter, 2 mm wall thickness) into the tube. Three different bundle structures can be obtained by varying the number of small pipes n of 3, 4 and 5. The ionization probes and pressure transducers (PCB102B06) are used to determine the average velocity while the smoked foil technique is employed to register the detonation cellular structures. The experimental results indicate that detonation can propagate at about the theoretical CJ velocity with a small deficit when the initial pressure (P₀) is greater than the critical value (P_c). The average velocity gradually decreases and deviates from the CJ value as the approaches of critical condition by gradually decreasing the initial pressure. The failure of detonation can be observed below the critical pressure. In the smooth tube, three different propagation mechanisms can be observed, i.e., super-critical condition, critical condition and sub-critical condition. After the bundle structures are introduced into the tube, a sudden velocity drop is seen at the critical pressure. Moreover, the detonation re-initiation phenomenon occurs with the velocity from the flame back to over-driven state quickly. Of note is that nearly no difference is seen between n = 3 and 4. However, in the case of n = 5, the detonation velocity experiences a more violent fluctuation with a high frequency, and the critical pressure is also increased to 28 kPa sharply. Finally, the critical condition analysis of detonation successful transmission is performed. The critical condition can be quantified as D_H/λ > 1. However, the critical values of D_H/λ are not uniform among various bundle geometries, but in a small range, i.e., from 1.52 to 1.97.     

Experimental study of detonation propagation in a square tube filled with orifice plates

DDT experiments were conducted in a 6000 mm long square cross-section (112 mm × 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for $S=3D$, where $S$ is the obstacle spacing and $D$ is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion $d_{eff}/\lambda \approx 1$ is not pervasive, i.e., $d_{eff}/\lambda$ decreases with the obstacle spacing increase, in which $d_{eff}$ and $\lambda$ are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re-initiation. This also accounts for the significant velocity fluctuation for larger spacing ($S=2D$ and $S=3D$) when the limits are approached.     

Role of chemical kinetics on the detonation properties of hydrogen /natural gas/air mixtures

The first part of the present work is to validate a detailed kinetic mechanism for the oxidation of hydrogen–methane–air mixtures in detonation waves. A series of experiments on auto-ignition delay times have been performed by shock tube technique coupled with emission spectrometry for H₂/CH₄/O₂ mixtures highly diluted in argon. The CH₄/H₂ ratio was varied from 0 to 4 and the equivalence ratio from 0.4 to 1. The temperature range was from 1250 to 2000 K and the pressure behind reflected shock waves was between 0.15 and 1.6 MPa. A correlation was proposed between temperature (K), concentration of chemical species (mol m^-3)${\mathrm{m}}^{-3})$ and ignition delay times. The experimental auto-ignition delay times were compared to the modelled ones using four different mechanisms from the literature: GRI [Smith PG, Golden DM, Frenklach M, Moriarty NW, Goldenberg M, et al. 〈$〈$http://www.me.berkeley.edu/gri_mech/〉$〉$], Marinov et al. [Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n  -butane flame. Combust Flame 1998; 114:192–213], Hughes et al. [〈$〈$http://www.chem.leeds.ac.uk/Combustion/Combustion.html〉$〉$], Konnov [Detailed reaction mechanism for small hydrocarbons combustion. Release 0.5 〈$〈$http://homepages.vub.ac.be/∼$\sim$akonnov/〉$〉$, 2000]. A large discrepancy was generally found between the different models. Konnov's model, which auto-ignition delay times predictions were the closest to the measured ones, has been selected to calculate ignition delay times in the detonation waves. The second part of the study concerned the experimental determination of the detonation properties, namely detonation velocity and cell size. Effect of the initial composition, hydrogen to methane ratio and the amount of oxygen in the mixture, as well as the initial pressure on the detonation velocity and on the cell size were investigated. The ratio of methane/(methane +$+$ hydrogen) varied between 0 and 0.6 for two different equivalence ratios (0.75 and 1) while the initial pressure was fixed to 10 kPa. A correlation was established between the characteristic cell size and the ignition delay time behind the leading shock of the detonation. It was clearly shown that methane has an important inhibitor effect on the detonation of these combustible mixtures.     

Detonation propagation characteristics for CH4-2H2-3O2 mixtures in a tube filled with orifice plates

In this study, the detonation propagation characteristics of stoichiometric CH₄-2H₂-3O₂ mixture are investigated comprehensively in a round tube with an inner diameter of 90-mm and 6-m in length. Three different orifice plates with the blockage ratios (BR) of 0.7 and 0.8 including circular, triangular and square orifice, are considered for the first time to investigate the effect of obstacle geometries on the detonation evolution. Eight high-speed piezoelectric pressure transducers are mounted on the outer wall to obtain the detonation velocity while the smoked foil technique is adopted to record the detonation cellular patterns. The results indicate that well within the limit, the detonation can propagate at about the theoretical CJ velocity (V_CJ). Near the limit, the velocity deficit is sharply enhanced but the detonation still can propagate at about 0.6V_CJ, which seems to be a universal phenomenon before the failure of the detonation. In the smooth tube, a sudden velocity drop and the single-headed spin can be seen near the critical condition, and the critical pressure (P_c) is 3 kPa. In the tube filled with obstacles, the effect of obstacle geometries on the detonation transmission can be ignored approximately for the BR = 0.7 case, and the critical pressures are increased to 7, 7 and 10 kPa, respectively. In the case of BR = 0.8, the effect of the orifice plates structures on the detonation propagation becomes more significant. The square orifice has the most serious impact on the detonation transmission, followed by triangular ones and the round hole has the least impact. The critical pressures are sharply enhanced to 10, 12 and 18 kPa, respectively. Finally, the effective diameter (d_eff) and the characteristic parameter (L) are introduced to analyze the critical condition of the detonation propagation. The critical condition can quantified as d_eff/λ > 1 and L/λ > 7 where λ is the detonation cell size.     

Flame acceleration and DDT of hydrogen-oxygen gaseous mixtures in channels with no-slip walls

Hydrogen-oxygen flame acceleration and transition from deflagration to detonation (DDT) in channels with no-slip walls were studied theoretically and using high resolution simulations of 2D reactive Navier-Stokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, real equation of state and a detailed chemical reaction mechanism. It is shown that in “wide” channels (D > 1 mm) there are three distinctive stages of the combustion wave propagation: the initial short stage of exponential acceleration; the second stage of slower flame acceleration; the third stage of the actual transition to detonation. In a thin channel (D < 1 mm) the flame exponential acceleration is not bounded till the transition to detonation. While velocity of the steady detonation waves formed in wider channels (10, 5, 3, 2 mm) is close to the Chapman-Jouguet velocity, the oscillating detonation waves with velocities slightly below the CJ velocity are formed in thinner channels (D < 1.0 mm). We analyse applicability of the gradient mechanism of detonation ignition for a detailed chemical reaction model to be a mechanism of the deflagration-to-detonation transition. The results of high resolution simulations are fully consistent with experimental observations of flame acceleration and DDT in hydrogen-oxygen gaseous mixtures.     

Experimental determination of critical conditions for hydrogen-air detonation propagation in partially confined geometry

An experimental investigation was performed to determine critical semi-open channel height (h*) and two-sided open channel width (w*) in which hydrogen-air detonation may propagate. Three types of gaseous mixture composition were used: 25%, 29.6% and 40% of hydrogen in air. Experimental setup was based on rectangular (0.11 × 0.11 × 2 m) test channel equipped with acceleration section (0.11 × 0.11 × 1 m). Different channel heights h in range of 15–40 mm and widths w in range of 30–50 mm were used in the test channel. The critical height h* and width w* were defined for each investigated configuration. To determine representative detonation cell sizes λ and to calculate their relationship to h* and w*, the sooted plate technique was used. The results showed that detonation in stoichiometric H₂-air mixture may propagate in semi-open channel only when the channel height is very close to or higher than approximately 3λ. For less reactive mixtures critical relation h*/λ reaches 3.1 or 3.6 for mixtures with 25% and 40% of hydrogen in air, respectively. For two-sided open channel similar relations w*/λ were close to 4.9 and 5.5 for 29.6%H₂ and 40%H₂ in air, respectively.     

Detonation in the hydrogen-oxygen microfoam on the aqueous base

In the paper, the propagation of the detonation wave in the hydrogen-oxygen microfoam on the aqueous base is considered. Microfoam represents a two-phase system containing micron-sized gas bubbles filled with the hydrogen-oxygen mixture. These bubbles are dispersed in the water solution of surfactant (sodium dodecyl sulfate). The dependencies of detonation speed on the equivalence ratio and on the water content in the foam are obtained with the use of high-speed filming. It is found that the detonation speed slightly increases with the decrease in water content in the foam. Based on the pressure measurements, it is established that the detonation propagation in the foam is driven by relatively weak shock waves, which by themselves are not able to induce ignition of the hydrogen-oxygen mixture. To substantiate the fact of detonation existence in the microfoam a hypothesis of the cumulative collapse of gas bubbles under the shock wave action is proposed. The estimation for detonation speed in microfoam is analytically derived on the basis of simple phenomenological representations.     

A numerical investigation of oblique detonation waves in hydrogen-air mixtures at low mach numbers

Oblique detonation waves (ODWs) have potential applications in hypersonic propulsion, but the boundary of steady ODWs has not yet been examined comprehensively. In this study, Euler equations coupled with detailed chemical reaction models are used to simulate ODWs in hydrogen-air mixtures with relatively low flight Mach numbers (from 7 to 8) and different flight altitudes (30 km and 25 km). Decreases in the flight Mach number and altitude are shown to result in unsteady ODWs in a stoichiometric hydrogen-air mixture. These ODWs can be re-stabilized by decreasing the fuel?air equivalence ratio. Regardless of different parameters of Mach number and altitude, unsteady ODWs appear only when the velocity in the induction zone exceeds that of the corresponding Chapman-Jouguet detonation. A low equivalence ratio also induces a long initiation length, limiting the availability of decreases in the equivalence ratio to maintain a steady ODW in practical applications. A flow?combustion criterion is proposed for the application of ODWs, based on the steadiness of ODWs and fast initiation.     

Study on the effects of geometry on the initiation characteristics of the oblique detonation wave for hydrogen-air mixture

The Oblique Detonation Wave Engine (ODWE) may act as a hypersonic propulsion system operating at high Mach numbers, which is an important member in the family of Scramjet. Hydrogen is a promising fuel for Scramjet, which provides wider Mach number range and is environmentally friendly. The geometry of the engine greatly affects the performance of the ODWE using hydrogen fuel. This investigation focuses on a novel wedge proposed recently, which may be utilized in scramjet engines. The wedge consists of two sub-wedges and a step. This research focuses on how the geometry of the wedge affects the initiation characteristics of the oblique detonation. Simulations are conducted on basis of Euler equations and a 9-species and 19-reactions mechanism. It is found that a larger leading wedge angle leads to a shorter initiation length. A larger step angle induces a longer initiation length. Few effects are observed on the initiation characteristics for the current range of depth. The streamline surface at the rear of the step weakens the rear shock wave and induces a longer initiation length. The streamline surface at the tip of the step begins to take effect when the initiation position is away from the step. This research provides basis for understanding the performance of the oblique detonation wave under different geometries and provides theoretical basis for scramjet engine design.     

Predictive model of onset of pipe failure due to a detonation of hydrogen–air and hydrocarbon–air mixtures

A fuel specific detonation wave in a pipe propagates with a predictable wave velocity. This internal detonation wave speed determines the level of flexural wave excitation of pipes and the possibility of resonance response leading to a serious structural damage. In this paper, we study the elastic response of metallic tubes and establish the resonance conditions of pipe breakage for internally loaded pipe structures. The analytical results are compared to the high strain rate calculation of a multi-material blast wave analysis using a hydrocode. Various power industries using hydrogen and hydrocarbon fuels exposed to such potential hazards may benefit from the findings of this paper.     

Experimental investigation on the near detonation limits of propane/hydrogen/oxygen mixtures in a rectangular tube

In this paper, an experimental study on the near detonation limits for propane-hydrogen-oxygen is performed. Three mixtures (i.e., 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂) are tested in a rectangular tube (52 mm × 32 mm). Photodiodes with regular intervals are mounted on the tube wall to measure the time of arrival of detonation waves, from which the detonation velocity is determined. Smoked foils are inserted into the tube to obtain the detonation cell pattern. The results indicate that well within the detonation limits, the detonation can propagate at a steady velocity. By reducing the initial pressure, the detonation velocity decreases gradually. Subsequently, the detonation fails as the initial pressure is below a critical pressure. The critical pressures for 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂ mixtures are 4 kPa, 5 kPa and 6 kPa, and the corresponding detonation velocity deficits are 10%, 9%, 10%, respectively. The cellular detonation structures show that the cell size decreases with the decrease of the hydrogen concentration, and the cell structures are very irregular near the detonation limits.     

Effect of geometry on the transmission of detonation through an orifice

An experimental study has been carried out to investigate the transmission of a planar detonation wave through an orifice into an unconfined medium. Mixtures of 2H₂ + O₂ + βN₂ and C₂H₄ + 3(O₂ + βN₂) for a range of nitrogen concentrations corresponding to 1 ≤ β ≤ 3.76 and at an initial presure of 1 atm were used in the experiments. It is found that the critical diameter for the transmission through an orifice is identical to that for a straight tube and both follow the empirical correlation of d_c ≅ 13λ. The transmission through square, triangular, elliptical, and rectangular orifices has led to the development of a correlation based on the effective diameter similar to the case of circular geometry (i.e., d_eff ≅ 13λ). The effective diameter is defined as the mean value of the longest and shortest dimensions of the orifice shape. The effective diameter correlation suggests that the criterion for transmission may be based on the mean curvature of the wave front, the implication being that it is not to exceed a certain critical value. The results suggest that local properties in the immediate vicinity of the wave front are the controlling parameters for reinitiation rather than the properties of the gas dynamic flow structure in the wake. Expressions are developed to provide estimates for the critical transmission dimensions for arbitrarily shaped openings. For the two-dimensional limit when one of the characteristic linear dimensions becomes very large compared to the other, it is found that the transmission is based on a critical value for the shorter dimension of the order of 3 times the cell diameter. This result is in accordance with the recent large scale experiments of Benedick in two-dimensional channels of an aspect ratio L.W as large as 35. These observed results can also be successfully explained in terms of the critical wave curvature criterion.     

Parametric study of hydrogen concentration on detonation initiation by shock focusing

Previous studies indicated that shock focusing is an effective way leading to detonation initiation. Hydrogen concentration as an operating condition is crucial to detonation initiation when the combustible gas is a mixture of hydrogen-air. To discuss the effect of hydrogen concentration, simulations of initiation by shock focusing with different hydrogen concentrations are performed. This paper provides an analysis of initiation by fuel in different amounts (in rich, stoichiometric, and lean). The initiation behaviors of shock focusing with different hydrogen concentrations (range from 20 vol% to 60 vol%) are discussed. The results show that high hydrogen concentration on initiation is significantly effective in accelerating the attenuation of overdriven detonation, which is conducive to initiating a quick and stable detonation initiation. An examination also shows that detonation failure is more likely caused by lean fuel mixtures. As to rich and stoichiometric fuel, there is the same physical mechanism for detonation initiation.     

Detonation limits in methane-hydrogen-oxygen mixtures: Dominant effect of induction length

New-concept detonation combustors, for example, rotating detonation engines (RDEs), motivate investigations of the phenomena and theory relevant to detonation limits. Because CH₄–H₂ binary fuel mixtures have excellent combustion performance in engines, it is interesting to investigate the application possibilities of such mixtures in advanced detonation engines. Therefore, the detonation characteristics (e.g., initiation, propagation and failure) and their mechanisms for methane-hydrogen mixtures under different thermodynamic conditions need further study. In this work, the physical connection between detonation limits and induction length (Δ_I) of detonation structures for CH₄–H₂–O₂ mixtures is investigated; the dominant effect of induction length on the detonation limits is examined. The results show that a single-headed spinning structure is a unique feature of detonation limits; hence, the detonation limits can be qualitatively estimated by this phenomenon. The relation between λ and Δ_I is proportional, and the proportionality factor is 34.62; this relation is applicable in methane/hydrogen and oxygen mixtures with various fuel contents. By scaling the critical pressure p_c with Δ_I near the detonation limits, the relation between them is shown to be an exponential function: Δ_I = 201.2·(1+p_c)^−2.1.     

Effect of hydrogen addition on the detonation performances of methane/oxygen at different equivalence ratios

Detonation performances of methane/hydrogen/oxygen (CH₄/H₂/O₂) mixtures were investigated experimentally in a 3000 mm long tube with an inner diameter of 30 mm at different initial pressures $p_0$ (ranging from 10 kPa to 50.5 kPa). Mixtures with different proportions of H₂ in the total fuel $\alpha$ (0%, 14.29% and 25%) and different equivalence ratios $\Phi$ (0.8, 1.0 and 1.2) were tested. Signals of flame front and pressure were obtained by ion probes and high frequency pressure transducers, respectively. Results showed that with the increase of $p_0$, $\alpha$and $\Phi$, the average velocity of steady detonation $V_{ave}$ increased. For mixtures with the given $\alpha$, when $\Phi$ increased by 0.2, $V_{ave}$ increased by 100 m/s. In the present study, velocity deficits were found to be within 5%, and when $p_0$ was higher than 20 kPa, the velocity deficits were within 2%. The average peak pressure of steady detonation $p_{ave}$ was close to the von Neumann pressure $p_{vN}$. Both the increase of $p_0$ and $\Phi$ led to the increase of the $p_{ave}$. But the addition of H₂ led to the decrease of $p_{ave}$, and $p_{ave}$ decreased with the increased of $\alpha$.     

Improving the performance of a gasoline engine with the addition of hydrogen-oxygen mixtures

The limited fossil fuel reserves and severe environmental pollution have pushed studies on improving the engine performance. This paper investigated the effect of hydrogen-oxygen blends (hydroxygen) addition on the performance of a spark-ignited (SI) gasoline engine. The test was performed on a modified SI engine equipped with a hydrogen and oxygen injection system. A hybrid electronic control unit was adopted to govern the opening and closing of hydrogen, oxygen and gasoline injectors. The standard hydroxygen with a fixed hydrogen-to-oxygen mole fraction of 2:1 was applied in the experiments. Three standard hydroxygen volume fractions in the total intake gas of 0%, 2% and 4% were adopted. For a given hydroxygen blending level, the gasoline injection duration was adjusted to enable the excess air ratio of the fuel-air mixtures to increase from 1.00 to the engine lean burn limit. Besides, to compare the effects of hydroxygen and hydrogen additions on the performance of a gasoline engine, a hydrogen-enriched gasoline engine was also run at the same testing conditions. The test results showed that the hydroxygen-blended gasoline engine produced higher thermal efficiency and brake mean effective pressure than both of the original and hydrogen-blended gasoline engines at lean conditions. The engine cyclic variation was eased and the engine lean burn limit was extended after the standard hydroxygen addition. The standard hydroxygen enrichment contributed to the decreased HC and CO emissions. CO from the standard hydroxygen-enriched gasoline engine is also lower than that from the hydrogen-enriched gasoline engine. But NOx emissions were increased after the hydroxygen addition.