Effect of hydrogen addition on the dynamics of premixed C1–C4 alkane-air flames in a microchannel with a wall temperature gradient

The effect of hydrogen (H₂) addition on the flame dynamics of premixed C₁–C₄ alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H₂ mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH₄/air mixture shows a higher HRR than C₃–C₄ alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH₄/air mixture, whereas it decreases monotonically for C₃H₈/air and C₄H₁₀/air mixtures with H₂ addition. The amount of HRR and flame propagation velocity decreases with increase in H₂ addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH₄/air mixture, which disappears due to H₂ addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H₂ in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel.     

Semi-free-jet simulated experimental investigation on a valveless pulse detonation engine

The total pressure recovery coefficient, flow coefficient and intake resistance of a valveless pulse detonation engine (PDE) with five different induction systems were measured based on the semi-free-jet simulated ground experiments. The proof-of-principle experiments of pulse detonation engine mockup with above five different induction systems were all successfully carried out, by using liquid gasoline-air mixture with low-energy ignition system (total stored energy less than 50 mJ). The process of back-propagation of pressure and average thrust of PDE mockup with these different induction systems were compared. The results indicated that induction system 1 had higher performance in total pressure recovery coefficient, flow coefficient or flight resistance. The time interval (Δt) between the time of detonation initiation and the time of backward pressure perturbation propagating to the PDE inlet decreased gradually at the increased operating frequency. The average thrust of the PDE with induction system 4 was the highest in case of lower operating frequencies, while that of PDE with induction system 1 was the highest when operating at higher frequencies.     

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.     

Propagation characteristics of unstable detonation waves in a round tube with annular perturbation

The detonation propagation characteristics of the mixtures, 2H₂ + O₂+3Ar and CH₄ + 2O₂, were investigated. Accordingly, the mixtures were tested in round tube with inner diameter of D = 80 mm and annular tubes with widths of w = 25 mm, 15 mm, and 5 mm. The two mixtures represent stable mixture with regular cell pattern and unstable mixture with irregular cell pattern, respectively. Smoked foils were utilized to record cellular structure under various initial conditions. Subsequently, the length scale L_dsc was measured, which represents the length from the start of the test section to the position where the cellular structure changes drastically (the cell size obviously increases or the structure disappears). The results reveal that both mixtures can successfully propagate in round tube and annular tubes of 25 mm and 15 mm, but fail in 5 mm annular tube. The L_dsc value of 2H₂ + O₂ + 3Ar is higher than that of CH₄ + 2O₂ in 80 mm and 15 mm tubes, but it is opposite in 25 mm tube. Moreover, the relationship between L_dsc and hydraulic diameter D_H was analysed. For a given tube, the values of L_dsc and L_dsc/D_H increased when the initial pressure increased. And the variation trend of L_dsc and L_dsc/D_H of CH₄ + 2O₂ is steeper. Furthermore, the mixtures 2H₂ + O₂ + 3Ar and CH₄ + 2O₂ resulted in over-driven detonation in 15 mm and 25 mm annular tubes, respectively. The ratio between the total reaction length (sum of the induction length and exothermic length) and the hydraulic diameter (D_H/(Δ_i + Δ_e)) correspond to critical values of 18 for hydrogen-oxygen-argon and 6 for methane-oxygen, below which the detonation will fail.     

提高脉冲爆震发动机工作频率的实验研究

实验采用汽油为燃料,压缩空气为氧化剂,通过改善气源、点火结构以及燃油雾化效果的方法,对如何提高脉冲爆震发动机工作频率进行了实验研究．脉冲爆震发动机的最高工作频率可达35Hz,此时得到的平均推力为142．8N．测试了不同爆震频率下的爆震波压力、推力及爆震波速,所测得的爆震波压力、波速和结构接近充分发展的理想爆震波,说明已产生了稳定的、充分发展的爆震．脉冲爆震发动机的平均推力随着工作频率的提高接近线性增大．     

High speed turbulent deflagrations and transition to detonation in H2air mixtures

An experimental investigation on flame acceleration and transition to detonation in H₂air mixtures has been carried out in a tube which had a 5 cm cross-sectional diameter and was 11 m long. Obstacles in the form of a spiral coil (6 mm diameter tubing, pitch 5 cm, blockage ratio BR = 0.44) and repeated orifice plates spaced 5 cm apart with blockage ratios of BR = 0.44 and 0.6 were used. The obstacle section was 3 m long. The compositional range of H₂ in air extended from 10 to 45%, the initial pressure of the experiment was 1 atm, and the mixture was at room temperature. The results indicate that steady-state flame (or detonation) speeds are attained over a flame travel of 10–40 tube diameters. For H₂ ? 13% maximum flame speeds are subsonic, typically below 200 m/s. A sharp transition occurs at about 13% H₂ when the flame speed reaches supersonic values. A second transition to the so-called quasi-detonation regime occurs near the stoichiometric composition when the flame speed reaches a critical value of the order of 800 m/s. The maximum value of the averaged pressure is found to be between the normal C-J detonation pressure and the constant volume explosion value. Of particular interest is the observation that at a critical composition of about 17% H₂ transition to normal C-J detonation occurs when the flame exits into the smooth obstacle-free portion of the tube. For compositions below 17% H₂, the high speed turbulent deflagration is observed to decay in this portion of the tube. The detonation cell size for 17% H₂ is about 150 mm and corresponds closely to the value of πD that has been proposed to designate the onset of single-head spinning detonation, in this case for the 5 cm diameter tube used. This supports the limit criterion, namely, that for confined detonations in tubes, the onset of single-head spin gives the limiting composition for stable propagation of a detonation wave.     

Research on H2/Air rotating detonation in the hollow chamber with double injection

In order to investigate the relationship between high frequency tangential instability and continuous rotating detonation, series of H₂/Air rotating detonations are experimentally achieved in the hollow chamber with double injection sections. In the center part, gaseous H₂ and air injected by co-axial injector. Near the outer wall, the same propellants are injected in the form of slit-orifice collision. By keeping the total air mass flow rate approximately constant, varying the mixture of the inner and outer injection, series experiments are conducted in the test model with or without Laval nozzle. The results verify the possibility of rotating detonation in the hollow chamber with co-axial injector. To clarify the relationship between continuous rotating detonation and high frequency tangential combustion instability, the intrinsic frequencies of the test model are captured to be compared with propagation frequencies of detonation waves. The results show that they are close to each other when enough propellant assembled near the outer wall. In the combustor, the flame direction in constant pressure mode can change itself into rotating direction spontaneously. The results indicate that rotating detonation is an implication to high frequency tangential instability.     

Deflagration-to-detonation transition via the distributed photo ignition of carbon nanotubes suspended in fuel/oxidizer mixtures

Here the promotion of flame acceleration and deflagration-to-detonation transition (DDT) using the distributed photo ignition of photo-sensitive nanomaterials suspended in fuel/oxidizer mixtures is demonstrated for the first time. Distributed photo ignition was carried out by suspending single-walled carbon nanotubes (SWCNTs) with Fe impurity in quiescent C₂H₄/O₂/N₂ mixtures and flashing them with an ordinary Xe camera flash. Following the flash, the distributed SWCNTs photo ignite and subsequently provide a quasi-distributed ignition of the C₂H₄/O₂/N₂ mixture. In a closed detonation tube the quasi-distributed photo ignition at one end of the tube leads to the promotion of flame acceleration and DDT and, for sensitive C₂H₄/O₂ mixtures, appears to lead to direct detonation initiation or multiple combustion fronts. The DDT run-up distance, the distance required for the transition to detonation, was measured using ionization sensors and was found to be approximately a factor of 1.5× to 2× shorter for the distributed photo ignition process than for traditional single-point spark ignition. It is hypothesized that the increased volumetric energy release rate resulting from distributed photo-ignition enhances DDT due to the decreased ignition delay and greater early-time flame area and turbulence levels, which in turn result in accelerated formation and amplification of the leading shock and accelerated DDT.     

Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot

Rotating detonation engines have attracted considerable attentions in recent years. In this study, the experiments of initiating rotating detonation waves were performed on a H₂/air rotating detonation wave with the variable air-inlet slot. The results showed that the stability of detonation-wave pressure and velocity both initially increased and then decreased with the increase of slot width, and it could improve the stability of detonation-wave velocity via increasing the equivalence ratio. The intensity of reflected wave was strong for the tests of d = 0.5 mm, which leaded to the advance ignition of fresh mixture and a velocity deficit reaching up to 20%. The strong interaction between air plenum and combustor and bad mixing effect may be the reasons of forming unstable detonation wave for the tests of large-scale slots. The air-inlet slot of d = 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave.     

Formation of NO in high temperature hydrogenair and ethyleneair detonative combustion

The formation of NO in overdriven H₂air and C₂H₄air detonations was experimentally and numerically investigated under various temperature and pressure conditions. The NO concentrations were measured by (0,1)NO-γ resonance absorption with additional OH and CO + O emission measurements. It was found that the NO formation rates near the detonation front are greater by a factor of 1≤ψ≤4 than predicted by the Zeldovich three-reaction mechanism assuming equilibrium O and OH concentrations. By computer simulation of a one-dimensional detonation wave together with hydrogen oxidation and NO formation kinetics, a nearly complete fit of the NO profiles measured under different conditions was possible. In the same way the simulation of C₂H₄air detonative combustion was successful using a scheme of 52 elementary reactions to describe the C₂H₄ oxidation and of 9 reactions for NO formation. It was possible to fit all measured NO profiles by use of an adjusted rate coefficient for the reaction CH + N₂, which is part of the NO formation scheme.     

Experimental study of using biogas in Pulse Detonation Engine with hydrogen enrichment

Pulse Detonation Engine (PDE) is one of the pressure gain engines that provide prominent features over conventional engines such as higher thermodynamic efficiency, higher thrust, and less design complexity. Biogas is one of the promising alternative energy sources. The aim of this paper is to study the influence of hydrogen addition in biogas on detonation characteristics performance of PDE. The ideal detonation combustion characteristics of biogas are estimated using NASA Chemical Equilibrium with Applications (CEA). The equivalence ratio range is varied from 0.7 to 1.4. It was found that the higher the percentage of methane concentration in biogas increased the detonation characteristics of biogas such as pressure, temperature, and Mach number. However, a reduction in the flame speed, temperature, and pressure of biogas compared to pure methane was stipulated to be due to the presence of CO2 that acts as a dilutant in biogas. It was found also that, the optimal percentage of hydrogen addition into the biogas fuel mixture is 15%. At this percentage, the detonation pressure improved by 23%. The performance of PDE fuelled by biogas is presented experimentally at a frequency of 10 Hz. The results show a successful and stable detonation initiation in PDE by using biogas with 15% and 20% addition of hydrogen.     

Effects of hydrogen addition on oblique detonations in methane–air mixtures

As a low-carbon fuel, methane has been used in various engines; however, the studies on its application in hypersonic propulsion are few. Here, oblique detonation waves (ODWs) in methane–air mixtures have been simulated to facilitate methane applications in shock-induced combustion ramjets. The shortcoming of using methane in hypersonic air-breathing propulsions has been presented via examining initiation distance of ODWs. Results demonstrate that ODWs are difficult to be initiated in the methane–air mixture, and similar to normal detonations studies before, this leads to a long initiation length; therefore, methane-fueled ODW is only applicable for high flight Mach number (M₀). To broaden the M₀ regime, hydrogen has been added to methane to decrease the initiation length. An increasing in the hydrogen percentage leads to the nonlinear decrease of the initiation length, and the initiation structures also vary simultaneously. To elaborate the physical mechanism of the initiation length variation, a theoretical model of the initiation length for fuel blends has been proposed. Meanwhile, the advantages of methane fuel in ODW-based propulsion have been discussed by analyzing on the effects of hydrogen addition on the total pressure.     

Critical energy for direct initiation of spherical detonations in H2/N2O/Ar mixtures

Although the detonation phenomenon in hydrogen-nitrous oxide mixtures is a significant issue for nuclear waste storage facilities and development of propulsion materials, very limited amount of critical energy data for direct initiation - which provides a direct measure of detonability or sensitivity of an explosive mixture − is available in literature. In this study, the critical energies for direct blast initiation of spherical detonations in hydrogen-nitrous oxide-Ar mixtures obtained from laboratory experiments and theoretical predictions at different initial conditions (i.e., different initial pressure, equivalence ratio and amount of argon dilution) are reported. In the experiments, direct initiation is achieved via a spark discharge from a high voltage and low inductance capacitor and the initiation energy is estimated accordingly from the current output. Characteristic detonation cell sizes of hydrogen-nitrous oxide-Ar mixtures are estimated from chemical kinetics using a recently updated reaction mechanism. A correlation expression is developed as a function of initial pressure, argon dilution and equivalence ratio, which is fitted to provide good estimation of the experimental measured data. The direct link between cell size and critical energy for direct blast initiation is then analyzed. Good agreement is found between experimental results and theoretical predictions, which make use of the cell size estimation correlation and the semi-empirical surface energy model. The effects of the initial pressure, equivalence ratio and the amount of Ar dilution on the critical initiation energy H₂-N₂O-Ar mixtures are investigated. By comparing the critical energies with those of H₂-O₂-Ar mixtures, it is shown that H₂-N₂O mixtures are more detonation sensitive with smaller initiation energies than H₂-O₂ mixtures at the same initial pressure, equivalence ratio and amount of argon dilution, except for higher diluted condition with amount of argon in the mixture above 20%. Attempt is made to explain the critical energy variation and comparison between the two H₂-N₂O-Ar and H₂-O₂-Ar mixtures from the induction length analysis and detonation instability consideration.     

脉冲爆震燃烧强化混合过程的实验研究

对脉冲爆震发动机的混合室进行了实验研究．实验针对爆震频率为6Hz、当量比为1的汽油和压缩空气混合气,实验研究了脉冲爆震发动机模型混合室内5种不同扰流螺纹螺距内径比对其性能的影响．实验结果表明,在混合段内加入扰动装置可以大大加强燃料和氧化剂的混合程度．不同扰动螺纹螺距内径比对爆震波的形成有一定的影响,当螺距内径比约为0．77时可以产生稳定的、充分发展的爆震,而且对应的DDT距离较短．     

Deflagration-to-detonation transition in nitromethane mist/aluminum dust/air mixtures

Deflagration-to-detonation transitions (DDTs) in nitromethane mist/air mixture clouds, flaked aluminum dust/air mixture clouds, and nitromethane mist/flaked aluminum dust/air mixture clouds have been studied in an experimental tube of inner diameter 199 mm and length 29.6 m. The mist/dust/air multiphase mixtures were generated by injecting liquid/dust samples into the experimental tube. The droplet dimensions of the nitromethane mist cloud were analyzed, calculated, and measured. The multiphase fuel/air mixture clouds were ignited by means of an electric spark. The characteristics of different stages during the DDT process in the clouds of the various mixtures in the experimental tube have been studied and analyzed. A self-sustained quasi-detonation wave formed in the nitromethane mist/air mixture clouds, while a self-sustained detonation wave formed in the flaked aluminum dust/air mixture clouds and in the nitromethane mist/flaked aluminum dust/air mixture clouds. The detonation parameters of the clouds of the different mixtures have been studied and compared.     

The initiation and propagation of detonation in supersonic combustible flow with boundary layer

Adaptive simulations solving the Navier-Stokes equations have been conducted in order to get a better understanding on the detonation initiation and propagation in a stoichiometric H₂/O₂/Ar supersonic mixture with boundary layer. The detonation is initiated by a continuous hot jet. When reflecting on the wall, the jet induced bow shock interacts with the boundary layer and forms the shock boundary layer interaction phenomena, while in Euler result the bow shock forms Mach reflection. The investigation shows that the Navier-Stokes simulation result is structurally in better agreement with the experiment compared with that of the inviscid Euler simulation result. The bow shock interacts with the separation shock, forming the shock induced combustion behind the interaction zone. Then the combustion front couples with shock and forms Mach stem induced detonation. The Mach stem induced detonation continues to getting higher and propagating upstream, initiating the main flow. The initiated partial detonation exists with the separation shock induced combustion front, forming an “oblique shock induced combustion-partial detonation” structure in the main flow. The investigation on the influence of free stream Mach number further confirms that the boundary layer has an important influence on detonation initiation. The parametric studies also show that there exists a free stream Mach number range to initiate the partial detonation in supersonic combustible flow successfully.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.     

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.     

Transmission of near-limit detonation wave through a planar sudden expansion in a narrow channel

Transmission of single-cell and spinning detonation waves in C₂H₄ + 3(O₂ + βN₂) mixtures through a 2-D sudden expansion experimentally studied using high-speed cinematography and soot film visualization. Nitrogen dilution ratio, β, is utilized to control cell size and detonation mode. Detonation wave of ethylene/oxygen/nitrogen mixture was initiated via DDT in the 1 mm × 1 mm cross-section and 250 mm long initiator channel before propagating into the 3 mm × 1 mm receptor channel. Visualizations show that detonation waves were extinct and accompanied with abrupt decrease in visible reaction front propagating velocities right after passing through the sudden expansion. However, re-acceleration of the reaction front and re-initiation of the detonation wave were observed downstream in the expanded receptor section. Two re-initiation modes with large disparity in the re-initiation distance were experimentally characterized. For mixtures with nitrogen dilution ratio equals 0.3 or less, the cellular detonation front propagated with single cell in the initiator section before entering the sudden expansion. The re-initiation distance was less than 50 mm and was likely achieved via shock reflection. Velocity characterization shows that steady propagating speed of the detonation wave is ～100 m/s higher in receptor section than in the initiator section. Since the cell size became larger than 1 mm for mixtures with β ? 0.3, the detonation wave propagated in spinning detonation mode before transmitting into the expanded section. The reaction front would have to go through another DDT process to reach detonation state in the receptor section, and the re-initiation distance was increased to more than 150 mm. Moreover, step height of the sudden expansion was proposed as the characteristic length scale to obtain a unified non-dimensional correlation between re-initiation distance and detonation cell size.     

Experiments on combustion regimes for hydrogen/air mixtures in a thin layer geometry

A series of experiments in a thin layer geometry performed at the HYKA test site of the KIT. Experiments on different combustion regimes for lean and stoichiometric H₂/air mixtures were performed in a rectangular chamber with dimensions of 200 × 900 x h mm³, where h is the thickness of the layer (h = 1, 2, 4, 6, 8, 10 mm). To model a gap between a fuel cell assembly and a metal housing, three different layer geometries were investigated: (1) a smooth channel without obstructions; (2) a channel with a metal grid filled 25% of chamber length and (3) a metal grid filled 100% of chamber length. The blockage ratio of metal grid has changed from 10 to 60% of cross-section. Detail measurements of H₂/air combustion behavior including flame acceleration (FA) and DDT in closed rectangular channel have been done. Five categories of flame propagation regimes were classified. Special attention was paid to analysis of critical condition for different regimes of flame propagation as function of layer thickness and roughness of the channel. It was found that thinner layer suppresses the detonation onset and even with a roughness, the flame may quench or, in thicker layer, is available to accelerate to speed of sound. The detonation may occur only in a channel thicker than 4 mm.