Transport equations for reaction rate in laminar and turbulent premixed flames characterized by non-unity Lewis number

Transport equations for (i) the rate $W$ of product creation and (ii) its Favre-averaged value $\stackrel{?}{W}$ are derived from the first principles by assuming that $W$ depends solely on the temperature and mass fraction of a deficient reactant in a premixed turbulent flame characterized by the Lewis number $Le$ different from unity. The right hand side of the transport equation for $\stackrel{?}{W}$ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum $\overline{T_{\text{Σ}}}$ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar premixed flames characterized by $Le<1$ are performed and the linear relation between $T_{\text{Σ}}$ integrated along the normal to a laminar flame and a product of (i) the consumption velocity $u_c$ and (ii) the stretch rate ${\dot{s}}_w$ evaluated in the flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of $\overline{T_{\text{Σ}}}\propto \overline{\rho W\dot{s}}$ is hypothesized, where $\rho$ is the density and $\dot{s}$ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, premixed turbulent flames in the case of a single-step chemistry and $Le=0.34$, 0.6, or 0.8. In all three cases, the agreement between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ extracted from the DNS is good with exception of large ($\overline{c}>0.4$) values of the mean combustion progress variable $\overline{c}$ in the case of $Le=0.34$. The developed linear relation between $\overline{T_{\text{Σ}}}$ and $\overline{\rho W\dot{s}}$ helps to understand why the leading edge of a premixed turbulent flame brush can control its speed.     

Effects of content of hydrogen on the characteristics of co-flow laminar diffusion flame of hydrogen/nitrogen mixture in various flow conditions

Effect of content of hydrogen (H₂) in fuel stream, mole fraction of H₂$\left(X_{H_2}\right)$ in fuel composition, and velocity of fuel and co-flow air $\left(V_{avg}\right)$ on the flame characteristics of a co-flow H₂/N₂ laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with $X_{H_2}$ varying between 0.25 and 1, and $V_{avg}$ between 0.25 and 1 ms^?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either $X_{H_2}$ or $V_{avg}$. Significant effect on the flame temperature and NOx emission are obtained from a higher $X_{H_2}$ in fuel whereas the flame size and heat release are the result of increasing $V_{avg}$. Along with this finding, the role of N₂ and its higher content reducing the flame temperature and NOx emission are presented.     

Direct numerical simulation of auto-ignition of a hydrogen vortex ring reacting with hot air

Direct numerical simulation (DNS) is used to study chemically reacting, laminar vortex rings. A novel, all-Mach number algorithm developed by Doom et al. [J. Doom, Y. Hou, K. Mahesh, J. Comput. Phys. 226 (2007) 1136–1151] is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H₂/air combustion proposed by Mueller et al. [M.A. Mueller, T.J. Kim, R.A. Yetter, F.L. Dryer, Int. J. Chem. Kinet. 31 (1999) 113–125]. Diluted H₂ at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratios, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto-ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a ‘most reactive’ mixture fraction, ${\zeta }_{MR}$ (Mastorakos et al. [E. Mastorakos, T.A. Baritaud, T.J. Poinsot, Combust. Flame 109 (1997) 198–223]). Subsequent evolution of the flame is not predicted by ${\zeta }_{MR}$; a most reactive temperature $T_{MR}$ is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke ratios greater than the formation number, ignition initially occurs behind the leading vortex ring, then occurs along the length of the trailing column and propagates toward the ring. Lewis number is seen to affect both the initial ignition as well as subsequent flame evolution significantly. Non-uniform Lewis number simulations provide faster ignition and burnout time but a lower maximum temperature. The fuel rich reacting vortex ring provides the highest maximum temperature and the higher oxidizer temperature provides the fastest ignition time. The fuel lean reacting vortex ring has little effect on the flow and behaves similar to a non-reacting vortex ring.     

Numerical investigation of a high pressure hydrogen jet of 82 MPa with adaptive mesh refinement: Concentration and velocity distributions

To investigate the safety properties of high-pressure hydrogen discharge or leakage, an under-expanded hydrogen jet flow with a storage pressure of 82 MPa from a small jet orifice with a diameter of 0.2 mm is studied by three-dimensional (3D) numerical calculations. The full 3D compressible Navier-Stokes equations are utilized in a domain with a size of about 3 × 3 × 6 m which is discretized by employing an adaptive mesh refinement (AMR) technology to reduce the number of grid cells. By AMR, the local mesh resolutions can narrowly cover the Taylor microscale $l_T$ and direct numerical simulations (DNS) are performed. Both the instantaneous and mean hydrogen concentration distributions in the present jet are discussed. The instantaneous concentrations of hydrogen $C_{{\text{H}}_2}$ on the axis presents significant turbulent pulsating oscillations. The centerline value of the intensity of concentration fluctuation ${\stackrel{?}{\sigma }}_{{\text{H}}_2}$ asymptotically comes to 0.23, which is in a good agreement with the existing experimental results. It substantiates the conclusion that the asymptotic centerline value of ${\stackrel{?}{\sigma }}_{{\text{H}}_2}$ is independent of jet density ratio. The probability distributions function (PDF) of instantaneous axial $C_{{\text{H}}_2}$ agree approximately with the Gaussian distribution while skewing a little to the higher range. The time averaged hydrogen concentration ${\overline{C}}_{{\text{H}}_2}$ along the radial directions can also be described as a Gaussian distribution. The axial ${\overline{C}}_{{\text{H}}_2}$ of 82 MPa hydrogen jet tends to obey the distribution discipline approximated with ${\overline{C}}_{{\text{H}}_2}=4200/(z/\theta )$ where z is the axial distance from the nozzle and $\theta$ is the effective ejection diameter, which is consistent with the experimental results. In addition, the hydrogen tip penetration $Z_{\text{tip}}$ is found to be in a linear relationship with the square root of jet flow time $\sqrt{t}$. Meanwhile, the jet's velocity half-width $L_{\text{Vh}}$ approximately gains an linear relation with z which can be expressed as $L_{\text{Vh}}=0.09z$.     

Thermodynamic efficiency analysis of zinc oxide based solar driven thermochemical H2O splitting cycle: Effect of partial pressure of O2, thermal reduction and H2O splitting temperatures

In this paper, the thermodynamic efficiency analysis of ZnO-based solar-driven thermochemical H₂O splitting cycle is performed and compared with the SnO₂-based H₂O splitting cycle. The HSC Chemistry 7.1 software is used for this analysis and effects of thermal reduction ($T_H$) and water splitting temperature ($T_L$) on various thermodynamic parameters associated with the ZnO-based H₂O splitting cycle are explored. The thermodynamic equilibrium compositions allied with the ZnO reduction and re-oxidation of Zn via H₂O splitting reaction are identified by varying the $T_H$, $T_L$, and $P_{O_2}$ in the inert gas. The efficiency analysis indicates that the highest cycle and solar-to-fuel energy conversion efficiency equal to 41.1 and 49.5% can be achieved at $T_H$ = 1340 K and $T_L$ = 650 K. Both efficiencies can be increased further by more than 10% via employing heat recuperation (50%). Based on the cycle and solar-to-fuel energy conversion efficiency values, the ZnO-based H₂O splitting cycle seems to be more attractive than SnO₂-based H₂O splitting cycle.     

Hydrogen production by coal plasma gasification for fuel cell technology

Coal gasification in steam and air atmosphere under arc plasma conditions has been investigated with Podmoskovnyi brown coal, Kuuchekinski bituminous coal and Canadian petrocoke. It was found that for those coals the gasification degree to synthesis gas were 92.3%, 95.8 and 78.6% correspondingly. The amount of produced syngas was 30–40% higher in steam than in air gasification of the coal.The reduction of the carbon monoxide content in the hydrogen-rich reformate gas for low-temperature fuel cell applications normally involves high- and low-temperature water gas shift reactors followed by selective oxidation of residual carbon monoxide. It is shown that the carbon monoxide content can be reduced in one single reactor, which is based on an iron redox cycle. During the reduction phase of the cycle, the raw gas mixture of ${\mathrm{H}}_2$ and CO reduces a ${\mathrm{Fe}}_3{\mathrm{O}}_4–{\mathrm{CeO}}_2–{\mathrm{ZrO}}_2$ sample, while during the oxidation phase steam re-oxidizes the iron and simultaneously hydrogen is being produced. The integration of the redox iron process with a coal plasma gasification technology in future allows the production of ${\mathrm{CO}}_x$-free hydrogen.     

Effect of vent size on vented hydrogen-air explosion

In this paper, the effect of vent size on vented hydrogen-air explosion in the room was studied by numerical simulation. Analysis of the explosion temperature, overpressure, dynamic pressure and wind velocity under different vent sizes indicate that these explosion parameters have different change rules inside and outside the room. Inside the room, the vent size has little effect on the explosion temperature, dynamic pressure and wind velocity, but it has a significant impact on the explosion overpressure. As the scaled vent size $K_v$ ($A_v/V^{2/3}$) increases from 0.1 to 0.3, the difference between the maximum internal peak overpressure is 87.8%. Outside the room, as the vent size increases, the high-temperature range (above 800 K) first decreases and then increases, while the explosion dynamic pressure and hurricane zone caused by explosion wind gradually decrease. The maximum high-temperature range (32.5 m for $K_v$ = 0.1) and hurricane zone (41.1 m for $K_v$ = 0.1) can reach 7.0 times and 8.9 times the length of the room, respectively. The explosion dynamic pressure can reach the same order of magnitude as the explosion overpressure under the same vent size. Therefore, these damage effects outside the room cannot be ignored. During the change of vent sizes, for $K_v$ ≤ 0.3, the explosion parameters change drastically and the disaster effect is significant. For example, external explosion that affect the discharge of internal explosion overpressure occur; explosion that occurs in masonry structures can destroy the structural integrity of the brick walls. Therefore, $K_v$ = 0.3 can be used as a reference for hydrogen-air venting safety design.     

The effects of hydrogen addition on silica aggregate growth in atmospheric-pressure, 1-D methane/air flames with hexamethyldisiloxane admixture

The effect of hydrogen addition on silica growth in burner-stabilized methane/air flames with trace amounts of hexamethyldisiloxane are reported. Profiles of the aggregates' radius of gyration $R_g$ and monomer radius $a$ versus residence time were measured by laser light scattering. Experiments were performed at equivalence ratios of 0.8, 1.0 and 1.3, with mole fractions of 0–0.4 of hydrogen in the fuel. At equal mass flux, the addition of hydrogen was found to result in decreasing $R_g$ and $a$. However, keeping the flame temperature rather than the mass flux constant upon hydrogen addition resulted in the same measured profiles.     

Hydrogen production from woody biomass by steam gasification using a CO2 sorbent

In ${\mathrm{H}}_2$ production from woody biomass by steam gasification using CaO as a ${\mathrm{CO}}_2$ sorbent, the effect of reaction parameters such as the molar ratio of CaO to carbon in the woody biomass $([\mathrm{Ca}]/[\mathrm{C}])$, reaction pressure, and reaction temperature was investigated on ${\mathrm{H}}_2$ yield and conversion to gas. In the absence of CaO, the product gas contained ${\mathrm{CO}}_2$. On the other hand, in the presence of CaO ($[\mathrm{Ca}]/[\mathrm{C}]=1,2$, and 4), no ${\mathrm{CO}}_2$ was detected in the product gas. At a $[\mathrm{Ca}]/[\mathrm{C}]$ of 2, the maximum yield of ${\mathrm{H}}_2$ was obtained. The ${\mathrm{H}}_2$ yield and conversion to gas were largely dependent on the reaction pressure, and exhibited the maximum value at $0.6\mathrm{MPa}$. It is noteworthy that ${\mathrm{H}}_2$ was obtained from woody biomass at a much lower pressure compared to other carbonaceous materials such as coal $(>12\mathrm{MPa})$ and heavy oil $(>4.2\mathrm{MPa})$ in steam gasification using a ${\mathrm{CO}}_2$ sorbent. ${\mathrm{H}}_2$ yield increased with increasing reaction temperature. Woody biomass is the one of the most appropriate carbonaceous materials in ${\mathrm{H}}_2$ production by steam gasification using CaO as a ${\mathrm{CO}}_2$ sorbent, taking the reaction pressure into account.     

Effects of inlet parameters on combustion characteristics of premixed CH4/Air in micro channels

In order to improve the stability of combustion and the working performance of the micro-combustor, this paper focuses on the effects of inlet parameters such as inlet temperature, mass flow rate and equivalence ratio. Three micro-combustors with different channel-heights are fabricated, and the three-dimensional calculation model is built to research on the combustion characteristics of premixed CH₄/Air mixture. It was found that with inlet gas mixture temperature increasing, the flammability limits of the combustion under micro-scale conditions were expanded, and the channel height under which the flame can exist in the combustor reduced from 3.0 mm to 2.0 mm after preheating. On the preheating basis, increasing the equivalence ratio of the gas mixture Ф improved the intensity of gas-phase reaction due to the simulation of the important free radical like OH. Furthermore, when the inlet mass flow rate of methane $\stackrel{?}{m_{c{h}_4}}$was increased and between $\stackrel{?}{m_{1c{h}_4}}$and $\stackrel{?}{m_{5c{h}_4}}$, it shows that the external wall temperature was higher in the micro-combustor of H = 2.5 mm compared with that of H = 3.0 mm. When in the micro-combustor of H = 3.0 mm, more fuel could be burned and $\stackrel{?}{m_{\max c{h}_4}}$ = 2.85 × 10^?6 kg/s.