Hydrogen-air Flow Research Articles

Modelling of hydrogen-air supersonic mixing and combustion in near-wall region

Abstract Turbulent mixing, ignition, and flame stabilization in the non-premixed supersonic hydrogen-air flow is numerically modelled in a near-wall region. Mixing algorithm based on the turbulence approach SARANS (Reynolds Averaged Navier–Stokes equations closed with Spalart–Allmaras turbulence model) with a diffusion model and a detailed kinetic model for hydrogen-air chemical reactions are employed. The system of governing equations that consists of basic conservation laws and the turbulence model equation is solved in a coupled manner with the LU–SGS–GMRES method. The model is applied to simulate the process of hydrogen injection into a M = 2.44 air flow with their subsequent mixing, ignition, and combustion in the Burrows– Kurkov chamber. The results are compared to available experimental and reference computational data. All calculations are carried out on the ‘MVS-10P’ JSCC RAS supercomputer cluster.

Formation of ultra-lean comet-like flame in swirling hydrogen–air flow

In this study, a hydrogen–air premixed flame in a partially tapered swirl burner in which a stable counterflow of unburned and burned gases is expected to be formed, was investigated. The experimental results indicate the formation of almost steady flames at equivalence ratios of as lean as 0.084, and the resulting ultra-lean flames in the swirling flow had a comet shape. Furthermore, the flame was numerically reproduced, and the mechanisms behind the phenomenon were identified by checking the balance among the chemical enthalpy through diffusion, heat flux by conduction, and transport of these parameters by convection. It was determined that the region around the tip of the flame head was almost dominated only by diffusion and heat conduction similar to a flame ball, but its formation mechanism was found to be essentially different from that of a flame ball because the comet-like flame can be numerically reproduced even without a radiative heat loss, in contrast to a flame ball.

Numerical investigation of flow structures with an oblique detonation wave in a hypersonic annular cylindrical chamber

A new supersonic flow-type annular detonation combustor is designed in which steady oblique detonation waves in the channel are generated using a compression body in the form of a solid single-wound spiral with a constant pitch angle. A two-dimensional unsteady mathematical model of the reacting flow in this device is formulated. The flow dynamics at the start of the chamber operation and steady supersonic flow structures for a stoichiometric hydrogen-air flow with an inlet Mach number M0=5 are numerically investigated. Two-dimensional numerical simulation is carried out for different spiral angles and geometrical dimensions of the chamber. A bifurcation of steady flow structures with respect to the initial condition of the problem is observed.

Analytical Modeling of Hydrogen–Air Mixture in a Catalytic Microchannel

In the present study, the surface catalytic reaction in a microchannel with nonreactive hydrogen–air flow is investigated. Here, the microchannel wall is coated with a catalyst. A solution to the catalytic microchannel flow is gained by applying an iterative method to solve the energy, mole fraction, and catalytic reaction equations. To validate the model, the results of fuel conversion are compared with the results of experimental studies available in the literature, and a good agreement between them is achieved. Moreover, the results show that increasing the hydraulic diameter of the microchannel decreases normalized temperature of the mixture and catalyst heat transfer. Steeper variations are observed in the case of . Furthermore, when the wall-to-inlet-temperature ratio is smaller than 2.5, the fuel conversion is very small; and for , the fuel is fully converted.

Ignition Time Measurements in Repetitive Nanosecond Pulse Hydrogen–Air Plasmas at Elevated Initial Temperatures

Ignition time is measured in premixed preheated hydrogen-air flows excited by a repetitive nanosecond pulse discharge in a plane-to-plane geometry. ICCD images of the plasma and the flame demonstrate that mild preheating of the fuel-air flow greatly improves plasma stability and precludes filament formation. At the initial temperatures of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 100-200°C, hydrogen-air plasmas remain stable and uniform up to at least P = 150 torr, and ignition occurs in a large volume. In contrast, ignition in less uniform preheated ethylene-air plasmas occurs locally, near the electrode edges, with flame propagating toward the center of the plasma. Ignition time in hydrogen-air mixtures is measured at initial temperatures of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 100-200°C, pressures of P = 40-150 torr, equivalence ratios of φ = 0.5-1.2, and pulse repetition rates of ν = 10-40 kHz. The results of ignition time measurements are compared with the predictions of the hydrogen-air plasma chemistry model, showing good agreement. Nitrogen emission spectra are used to measure time-resolved temperature in air and hydrogen-air plasmas. The results show that ignition begins at the plasma temperature of T ≈ 700 K and results in a rapid temperature rise. By turning off dominant plasma chemical radical generation processes in kinetic modeling calculations, while keeping discharge energy loading the same, it is demonstrated that ignition is driven by additional energy release in reactions of plasma-generated radicals with hydrogen. To determine if plasma-generated radicals may reduce ignition temperature, discharge pulse burst was terminated before the onset of ignition, and ignition delay time was measured versus plasma temperature at the end of the burst. Experimental ignition delay time is in reasonably good agreement with kinetic modeling calculations. The kinetic model predicts significant plasma-assisted ignition threshold temperature reduction at the present conditions compared to thermal ignition, up to ΔT = 180 K.

Cavity ignition and flameholding of ethylene–air and hydrogen–air flows by a repetitively pulsed nanosecond discharge

Repetitive nanosecond pulse plasma assisted ignition and flameholding of premixed and non-premixed ethylene–air and hydrogen–air flows are studied in a cavity flow at a pressure of 0.2 atm and flow velocities of up to 100 m/s. Ignition occurs via formation of multiple filaments in the fuel–air plasma, although air plasma remains diffuse until the fuel is added. After ignition occurs in the cavity, with ignition delay time of a few milliseconds, the plasma becomes diffuse and the flame couples out to the main flow. The use of a short cavity (length-to-depth ratio L/ D = 1) results in repetitive ignition and flame blow-off, caused by slow mixing between the main flow and the cavity. Increasing the length-to-depth ratio to L/ D = 3, as well as choking inlet air and fuel flows resulted in stable flameholding and nearly complete combustion in both premixed and non-premixed ethylene–air and hydrogen–air flows at u = 35–100 m/s. Air plasma temperature before fuel is added ranges from 70 °C to 200 °C. When the nanosecond pulse discharge is operated in repetitive burst mode, continuous ethylene–air flame is maintained only at a high duty cycle, which increases with the flow velocity. In hydrogen–air, the flame remains stable after the plasma is turned off. Nanosecond pulse discharge ignition of ethylene–air is compared with ignition by DC arc discharge of approximately the same power. DC arc discharge results in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower flow velocity at which ignition can be achieved. Kinetic modeling is used to identify the reduced mechanism of plasma chemical oxidation and ignition of hydrogen, and to demonstrate the mechanism of energy release low-temperature reactions of radicals generated in the plasma (primarily O and H atoms).

Study of detonation initiation in hydrogen/air flow

While extensive studies have been conducted concerning the formation of detonation waves in various combustible gaseous mixtures under static conditions since the 1950s, there is very little experimental work on simple flowing systems. In this study, experiments on the deflagration to detonation transition (DDT) of a hydrogen–air flow system were carried out to see the effects of tube diameter, equivalence ratio, and flow types in a premixed and non-premixed flow. Tube diameters used were 25, 50, and 100 mm. The premixed experiments show that the larger tube diameter provides a wider range in run-up distance, reduction of L DDT/D (ratio of the run-up distance, L DDT to tube diameter), and expansion of the detonable concentration limit by spreading the cell width. The result of the non-premixed experiments show that similar values of the run-up distance to the premixed experiments are obtained at an equivalence ratio of about 1.0, however, fluctuations of DDT occur near the DDT concentration limit. Under laminar flow conditions at a Reynolds number of less than 2,300, the difference between the two systems could not be observed. However, when the Reynolds number increases towards turbulent conditions, the DDT run-up distance decreases compared to that of static flow conditions.

Detonation waves in a reactive supersonic flow

A supersonic hydrogen—air flow is studied in detail, in particular, the fields of gas-dynamic parameters and chemical homogeneity of the mixture in various cross sections of the duct. The processes of excitation and propagation of a detonation wave in the downstream and upstream directions are considered. The detonation-wave velocity with respect to the mixture flow is found to differ from the nominal Chapman—Jouguet velocity for a quiescent mixture: the detonation-wave velocity is higher if the wave moves upstream and lower if the wave moves downstream. Some hypotheses on the reasons for these deviations of the experimentally measured velocity from the nominal value are given.

Numerical study of wedge supported oblique shock wave-oblique detonation wave transitions

The results of a numerical study of premixed Hydrogen-air flows ignition by an oblique shock wave (OSW) stabilized by a wedge are presented, in situations when initial and boundary conditions are such that transition between the initial OSW and an oblique detonation wave (ODW) is observed. More precisely, the objectives of the paper are: (i) to identify the different possible structures of the transition region that exist between the initial OSW and the resulting ODW and (ii) to evidence the effect on the ODW of an abrupt decrease of the wedge angle in such a way that the final part of the wedge surface becomes parallel to the initial flow. For such a geometrical configuration and for the initial and boundary conditions considered, the overdriven detonation supported by the initial wedge angle is found to relax towards a Chapman-Jouguet detonation in the region where the wedge surface is parallel to the initial flow. Computations are performed using an adaptive, unstructured grid, finite volume computer code previously developed for the sake of the computations of high speed, compressible flows of reactive gas mixtures. Physico-chemical properties are functions of the local mixture composition, temperature and pressure, and they are computed using the CHEMKIN-II subroutines.

Speciation Analysis of Butyltin Species in Water by Gas Chromatography with Flame Photometric Detection Using Quartz Surface-induced Tin Emission

A flame photometric detector using quartz surface-induced tin emission was designed and evaluated for quantification analysis of butyltin species. It has been demonstrated that this quartz surface-induced tin emission, centred at 390 mm, is more sensitive than the commonly used gas-phase emission at 610 mn. The dependence of detector response on quartz enclosure was studied. The operational variables such as hydrogen-air flow rate, carrier-gas flow rate and purge-gas flow rate were optimized. An analytical procedure for speciation analysis of butyltin species in water using simultaneous hydride generation with sodium borohydride and extraction into dichloromethane was established. The detection limits (defined as the signals that equal three times the deviations of the noise) were 0.3 pg of Sn for tetrabutyltin (TeBT), 5 pg of Sn for monobutyltin (MET), 18 pg of Sn for dibutyltin (DBT) and 2 pg of Sn for tributyltin (TBT), which are approximately 10- to 30-fold better than those reported for using more commonly used gas-phase emission centred at 610 mn.

Supersonic combustion in premixed hydrogen-air flows

Ignition and propagation of turbulent flame front in jet flow investigated for combustible hydrogen- air mixture

Kinetics in hydrogen-air flow systems. I. Calculation of ignition delays for hypersonic ramjets

An analytical method of estimating auto-ignition delay time based on dissociation and recombination rates of gases has been developed. In this analytical method, the one-dimensional gas-dynamics equations are combined with reaction-kinetics equations to formulate a, set of ( n +5) simultaneous, nonlinear-first-order differential equations, where n is the number of species considered. These equations describe the behavior of a mixture of reacting gases flowing supersonically down a duct. A computer program has been evolved and used to solve these equations for the particular case of hydrogen-air mixtures. The ignition delay periods obtained through this program are in good agreement with the limited amount of experimental data available for hydrogen and air.

Kinetics of hydrogen-air flow systems. II. Calculations of nozzle flows for ramjets

The performance of exhaust nozzles through which highly dissociated gases are accelerated depends greatly on the degree to which these gases are chemically recombined during the expansion process. An analytical method for calculating the extent of recombination for a specified nozzle contour has been developed from a combination of the chemical kinetic, gas dynamic, and state equations. A computer program has been evolved and used to solve these equations for hydrogen-air combustion products. The solution of these equations, leads to the determination of concentrations, temperatures, pressures, and velocities as a function of time or distance for the combustion process and nozzle expansion. To illustrate the use of the computer program, the extent of recombination of hydrogen-air combustion products in a convergent-divergent exhaust nozzle, having a throat diameter of 6.18 inches was calculated for combustion chamber entrance conditions of pressure 3000 psf, temperature 2000 o R, velocity 200 ft/sec, and an equivalence ratio equal to one. The results indicate that the flow freezes at an area ratio of approximately 1.1 in the nozzle convergent section.