Detonation cellular structure in NO2/N2O4-fuel gaseous mixtures

Experimental and numerical studies of the detonation in NO₂-N₂O₄/fuel (H₂, CH₄, and C₂H₆) gaseous mixtures show that for equivalence ratio Φ>0.8-1, (1) the detonation has a double cellular structure, the ratio between the cell size of each net being at least one order of magnitude; (2) inside the detonation reaction zone the chemical energy is released in two successive exothermic steps. Their chemical induction lengths, defined between the leading shock front and each local maximum heat release rate associated with each step, differ by at least one order of magnitude. The chemical reaction NO₂ + H → NO + OH is mainly responsible for the first exothermic step (fast kinetics), NO being the oxidizer on the second one (slow kinetics). Existence of correlations between calculated induction lengths and corresponding cell sizes strengthen the assumption that the cellular structure originates from local strong gradients of chemical heat release inside the detonation reaction zone.     

Effect of orifice plate on the transmission mechanism of a detonation wave in hydrogen-oxygen mixtures

In this paper, a square orifice plate with 60 mm thick and the blockage ratio (BR) of 0.889 is employed to systematically explore the transmission regime of a steady detonation wave in hydrogen-oxygen mixtures. The influence of hydrogen mole fraction is also considered. The average velocity of combustion wave can be determined by evenly mounting eight high-speed pressure sensors on the tube wall, and the detonation cellular patterns can be also registered by the soot foil technique. The experimental results indicate that for the condition of smooth tube, the hydrogen concentration limits range of detonation successful propagation is 37.5%–73.68%. Two propagation modes can be obtained, i.e., the regimes of fast flame and steady detonation. The hydrogen concentration limits range is narrowed to 42.53%–69.51% in the tube with a square orifice plate. Three propagation regimes are observed: (1) near the low limit, a steady detonation wave can be produced before the obstacle, and the phenomenon of detonation decay is seen across the square orifice plate because of the influence of diffraction resulting in the mechanism of detonation failure. The failed detonation wave is not re-ignited because of the lower hydrogen concentration; (2) as the hydrogen mole fraction is increased to 42.53%, the mechanism of detonation re-ignition can be seen after the detonation decay. Well within the limits, the same detonation re-initiation phenomenon also can be observed; (3) as the hydrogen concentration is further enhanced to 69.7% beyond the upper limit, a stable detonation wave is not produced prior to the orifice plate, and the combustion wave front maintain the mode of fast flame until the end of the channel. Finally, it can be found that the detonation wave can successfully survive from the diffraction only when the effective diameter (d_eff) is at least greater than one cell size (λ).     

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.     

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.     

Detonation behaviors of syngas-oxygen in round and square tubes

The present study reports the detonation behaviors of syngas-oxygen mixtures in three 2 m long tubes, including two circular tubes ($D$ = 32 mm and 18.5 mm, $D$ is the inner diameter) and a square tube ($H$ = 32 mm, $H$is the inner side). Three stoichiometric syngas-oxygen mixtures, i.e., CO+H₂+O₂, 2CO+H₂+1.5O₂, 3CO+H₂+2O₂ were used. Evenly spaced photodiodes were used to determine the detonation velocity while the soot foil technique was adopted to record the cellular structure. The results indicate that well with the limits, the detonation propagates at a steady velocity close to the Chapman-Jouguet value. The velocity deficits are more prominent at decreased initial pressure (equivalently, larger cell size and less sensitivity). At the limiting pressure, the velocity deficits of three mixtures in various tubes are approximate 14%–17%. The experimental velocity deficits were compared with a modified model based on Fay's theory, in which the cell size as well as the corresponding cell length are measured. The experimental velocity deficit is in excellent agreement with the theoretical prediction. The effective diameter, $D_e$, rather than the hydraulic diameter, is found to be a more appropriate parameter for the characterization of the detonation velocity in both round and square tubes. Further, a linear correlation between the normalized detonation velocity and ${(D_e·p_0)}^{?1}$ is observed. The cellular structure shows that the single-headed spin occurs in all tubes when the detonation limits are approached.     

Thermal modes of bimolecular exothermic reactions: Concentration limits of ignition

In the present work, the analytical investigation of the phase trajectories structures on the plane: heating rate-temperature for bimolecular exothermic reactions was carried out. This gave a possibility to divide the parametric plane Todes parameter–Semenov parameter into five characteristic regions with various modes of self-heating. It was established that all regions converge at a single point which determines the thermal explosion degeneration conditions. The correlation between Todes parameter, Semenov parameter and concentration ratio of initial reactants at the limit of the thermal explosion degeneration was found. The critical conditions for thermal explosion at any ratio of concentrations of initial reactants were determined. The approach proposed is a general one and allows analyzing both first and second order reactions.     

Effects of boundary layer on wedge-induced oblique detonation structures in hydrogen-air mixtures

Oblique detonation wave (ODW) structures are studied widely in recent years, but most of them are solved by the Euler equations without considering viscosity and then effects of boundary layer. In this study, the Navier-Stokes Equations are used to simulate the wedge-induced ODWs in hydrogen-air mixtures, and the two types of ODW transition structures at different incident Mach number M_i are analyzed to clarify the effects of viscosity and hence the boundary layer. Results show that the effect of boundary layer on ODW structures should be classified by the types of ODW transition patterns. As for the smooth transition pattern of ODW at high Mach numbers, the effect of boundary layer can be neglected, but for the abrupt transition pattern of ODW at low Mach numbers, the effect of boundary layer is large and it changes the ODW structure greatly. Resulting from the interaction of shock and boundary layer, a recirculation zone is formed within the viscous ODW layer at M_i = 7, which leads to the phenomenon that the straight oblique shock wave evolves into two sections, with the downstream one having a larger shock angle. Additionally, the corresponding transition position moves upstream, and the initiation length becomes only one third of that in inviscid ODW. The great importance of considering viscosity in ODW simulations and future designs of combustor of oblique detonation engine has been addressed.     

Asymptotic analysis on autoignition and explosion limits of hydrogen–oxygen mixtures in homogeneous systems

The deducted equations about chemical species and temperature are presented to calculate hydrogen–oxygen ignition delay time. Steady-state assumptions for many of the intermediate species are introduced to derive a new simplified 3-step mechanism. The simplified 3-step mechanism for hydrogen–oxygen leads the steady-state assumptions to linear differential equations. The competition among the full mechanism containing 17-, 8- and the simplified 3-step mechanisms is carried out. The resulting closed form solutions describe the low-, the intermediate- and the high-temperature ignition regimes and obtain an “S-shaped curve”. Finally, the two parameters on ignition and extinction of the continuously stirred flow reactor are discussed in detail and the temperature error analysis is given. It reduces the computational costs and supplies theory and methods for understanding autoignition and explosion limits of hydrogen–oxygen mixtures in homogeneous systems.     

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.     

The flammability limits of H2–CO–CH4 mixtures in air at elevated temperatures

The present contribution reports experimentally obtained values of the flammability limits of some fuel mixtures made up of H₂, CO, and CH₄ in air at different initial mixture temperatures of up to 300 °C. The potential catalytic effects of the surface of the test apparatus when the fuel–air mixtures were allowed to reside within the test apparatus at elevated temperatures for different time periods prior to ignition were also considered. Both stainless steel and quartz flame tubes of identical design and size were employed in the investigation.     

Intensification of turbulent free heat transport in various configurations with adequate binary gas mixtures

The present paper investigates a promising avenue for the intensification of turbulent free convection in various configurations using adequate binary gas mixtures in which helium (He) is the primary gas component and carbon dioxide (CO₂), methane (CH₄), nitrogen (N₂), oxygen (O₂) and xenon (Xe) are the secondary gas components. In the context of binary gas mixtures, the thermo-physical properties: viscosity, thermal conductivity, density and isobaric heat capacity depend on three quantities: temperature, pressure and molar gas composition. Within the platform of turbulent free convection using the five binary gas mixtures for Ra > 10⁹, results are presented for the allied convective coefficient h_mix/B varying with the molar gas composition w in the w-domain [0, 1]. Values of the maximum allied convective coefficients h_mix,max/B attained at the correlative optimal molar gas compositions w_opt are easily extracted from suitable design charts.     

Investigating the effects of injection and induction modes of hydrogen addition in a CRDI pilot diesel-fuel engine with exhaust gas recirculation

This work compares the outcomes of different flow rates of hydrogen added by induction and injection methods in three different flow rates (3, 9, and 15 LPM) through the intake manifold of a constant speed CRDI diesel engine operated at 1500 rpm. The premixed air and hydrogen mixture was ignited by injecting diesel fuel at 23? bTDC. Hydrogen addition reduced CO, HC, and smoke in both the techniques, but efficiency was decreased at a higher percentage of hydrogen induction, whereas it increased with the injection technique. The higher calorific value and flame velocity helped proper combustion and improved brake thermal efficiency by 7%, and the brake-specific energy consumption was reduced by 10.7%. In addition, CO, UHC, and Smoke were decreased by 15.8, 29.7, and 15% compared with neat diesel at full BMEP. Nitrogen oxides decreased by 5.6% for 15 LPM of hydrogen injection compared to the induction method with the same flow rate but higher than diesel fuel by 35.9%. Three different EGR percentages (5, 7.5, and 10%) were used to reduce the higher NOx emission. Though the injection process was complex compared to the induction method, the injection process can provide promising results even at higher hydrogen flow rates.     

Experimental investigation of the effect of channel length on performance and water accumulation in a PEMFC parallel flow field

Longer channels within serpentine flow fields are highly effective at removing liquid water slugs and have little water accumulation; however, the long flow path causes a large pressure drop across the cell. This results in both a significant concentration gradient between the inlet and outlet, and high pumping losses. Parallel flow fields have a shorter flow path and smaller pressure drop between the inlet and outlet. This low pressure drop and multiple routes for reactants in parallel channels allows for significant formation of liquid water slugs and water accumulation. To investigate these differences, a polymer electrolyte membrane fuel cell parallel flow field with the ability to modify the length of the channels was designed, fabricated, and tested. Polarization curves and the performance, water accumulation, and pressure drop were measured during 15 min of 0.5 A cm⁻² steady-state operation. An analysis of variance was performed to determine if the channel length had a significant effect on performance. It was found that the longer 25 cm channels had significantly higher and more stable performance than the shorter 5 cm channels with an 18% and an 87% higher maximum power density and maximum current density, respectively. Channel lengths which result in a pressure drop, across the flow field, slightly larger than that required to expel liquid water slugs were found to have minimal water accumulation and high performance, while requiring minimal parasitic pumping power.     

Assessment of H2-CH4-air mixtures oxidation kinetic models used in combustion

Because of a wide number of applications, the potential hazards of H₂-CH₄-air mixtures have to be characterised. For hazard evaluation, an important element is a reliable detailed kinetic scheme. In the present study, three modern kinetic models, those of Konnov, of Dagaut and the GRI-mech 03, have been evaluated with respect to a large set of experimental data, including species profiles obtained in jet-stirred reactor, laminar flame speed, ignition delay time and detonation cell size, for hydrogen-methane-air mixtures. For jet-stirred reactor data, the model of Dagaut provides significantly better results. For flame speed data modeling, the three models are as reliable. For ignition delay times, the model of Dagaut seems the most reliable. For detonation cell size predictions, the model of Konnov is the best. Important chemical reactions are underlined through sensitivity and reaction pathway analysis and are discussed in the frame of rate constant values recommended by Baulch et al.     

Combustion and emissions characteristics of toluene/n-heptane and 1-octene/n-octane binary mixtures in a direct injection compression ignition engine

Successfully designing and making effective of use of the next generation of liquid fuels, which will be derived from a range of biomass and fossil sources, requires an understanding of the interactions between structurally similar and dissimilar fuel components when utilised in current engine technology. Interactions between fuel components can influence the release of energy and production of harmful emissions in compression ignition combustion through determination of the autoignition behavior of the fuel. This paper presents experimental studies carried out in a single-cylinder engine supplied with a range of binary mixture fuels to investigate the effect of fuel component interactions on autoignition in direct injection compression ignition. A range of binary mixtures consisting of toluene and n-heptane and also 1-octene and n-octane were tested so as to observe respectively the effect of an aromatic compound and an alkene on n-alkane combustion and emissions. The engine tests were carried out at constant injection timing and they were repeated at constant ignition timing and at constant ignition delay, the latter being achieved through the addition to the various fuels of small quantities of ignition improver (2-ethylhexyl nitrate). Increasing the presence of toluene in the toluene/n-heptane binary mixtures resulted in an increased ignition delay time and generated a distinct two stage ignition process. An increased level of 1-octene in the binary mixtures of 1-octene/n-octane was also found to increase ignition delay, though to a much lesser extent than toluene in the case of the toluene/n-heptane mixtures. Interactions between the fuel components during the ignition delay period appear important in the case of the toluene/n-heptane mixtures but not those of 1-octene/n-octane. At constant injection and constant ignition timings, the combustion phasing and the level of emissions produced by each binary mixture were primarily driven by the ignition delay time. With ignition delay equalised, an effect of adiabatic flame temperature on NOx production was visible.     

Comparative Analysis of Some Representative Models of Viscosity Versus Pressure in the Case of Various Hydrocarbons and Their Mixtures

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Core designing of a new type of TVS-2M FAs: neutronics and thermal-hydraulics design basis limits

One of the most important aims of this study is to improve the core of the current VVER reactors to achieve more burn-up (or more cycle length) and more intrinsic safety. It is an independent study on the Russian new proposed FAs, called TVS-2M, which would be applied for the future advanced VVERs. Some important aspects of neutronics as well as thermal hydraulics investigations (and analysis) of the new type of Fas are conducted, and results are compared with the standards PWR CDBL. The TVS-2M FA contains gadolinium-oxide which is mixed with UO₂ (for different Gd densities and U-235 enrichments which are given herein), but the core does not contain BARs. The new type TVS-2M Fas are modeled by the SARCS software package to find the PMAXS format for three states of CZP and HZP as well as HFP, and then the whole core is simulated by the PARCS code to investigate transient conditions. In addition, the WIMS-D5 code is suggested for steady core modeling including TVS-2M FAs and/or TVS FAs. Many neutronics aspects such as the first cycle length (first cycle burn up in terms of MW_thd/kgU), the critical concentration of boric acid at the BOC as well as the cycle length, the axial, and radial power peaking factors, differential and integral worthy of the most reactive CPS-CRs, reactivity coefficients of the fuel, moderator, boric acid, and the under-moderation estimation of the core are conducted and benchmarked with the PWR CDBL. Specifically, the burn-up calculations indicate that the 45.6 d increase of the first cycle length (which corresponds to 1.18 MW_thd/kgU increase of burn-up) is the best improving aim of the new FA type called TVS-2M. Moreover, thermal-hydraulics core design criteria such as MDNBR (based on W3 correlation) and the maximum of fuel and clad temperatures (radially and axially), are investigated, and discussed based on the CDBL.