H2O Mass Fraction Research Articles

LES-CMC simulations of a turbulent hydrogen jet in oxy-combustion regimes

The paper presents the results of numerical simulations of a combustion process for a pure hydrogen jet in the atmosphere of oxygen and water vapor. The research was performed using Large Eddy Simulations (LES) combined with Conditional Moment Closure (CMC) model and included the analysis of auto- and forced (spark) ignition, flame development and flame propagation. In the configuration analyzed in the study, the fuel jet issues into a hot co-flowing oxidizer stream (O2/H2O). We considered two different co-flow temperatures, 1030 K and 1045 K, and various oxidizer compositions with H2O mass fraction over the range of Y H2O=0.1−0.9 and with YO2=1−YH2O. The analyses were carried out with assuming constant 30% excess of the mass flow rate of oxygen relative to the hydrogen. It was found that: (i) the auto-ignition process occurs only for a low content of H2O in the co-flow (YH2O≤0.5), (ii) and is additionally determined by its temperature; (iii) depending on O2/H2O proportion the flames stabilizes as attached or lifted. By alteration of YH2O in the oxidizer, the maximum flame temperature and the lift-off height (LH) can be changed in the range of 1500K→3000K and LH=0→11 jet diameters, respectively.

Comparative study on the cooling characteristics of high level water collecting natural draft wet cooling tower and the usual cooling tower

With the rapid development of large capacity power units, high level water collecting natural draft wet cooling towers (HNDWCTs) are now attractive. To study the cooling characteristics of HNDWCT, a three-dimensional (3D) numerical model for a HNDWCT was established and validated. Combining with the verified 3D numerical models for usual natural draft wet cooling towers (UNDWCTs), the parameters such as air pressure drop, air mass flow rate, exit water temperature, cooling load percentage of each zone, air velocity, air temperature and H2O mass fraction were elaborated for HNDWCT and UNDWCTs under the same work conditions. Comparing with UNDWCTs, the cooling characteristics of HNDWCT were analyzed and highlighted. The cooling load percentage of fill zone in HNDWCT is far larger than those in UNDWCTs for the large air mass flow rate and large heat transfer potential. Under high velocity crosswind impacts, blocking the cross ventilation through HNDWCT is favorable to its cooling performance. Suggestions concerning the orientations of water collecting gutter and cross wall are presented.

Numerical calculations of the WR-40 boiler with a furnace jet boiler system

The aim of this work was to develop a numerical model of combustion in the furnace of a WR-40 boiler equipped with a FJBS (furnace jet boiler system). Based on previous calculations [1] and measurements [2], appropriate competent sub-numerical models were developed to match a temperature value and composition of the flue gas obtained with measurements taken at the real working boiler. The FJBS System has eight in-furnace jet-pump ventilators that uses gas propulsion (compressed air). Its task is to mix the flue-gas and any substrates of combustion at vortex, fire it up, and alignment align the temperature field. Numerical modelling results for the combustion chamber of the WR-40 boiler was shown. The concentration of O2, CO, CO2 in the flue gas, the temperature of flue gas for the whole combustion chamber, the change of the mass fraction of H2O in the coal, and the change in the mass of coal particles along the grate were measured to demonstrate that the processes taking place during combustion of fuel on the grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). Numerical calculations confirm measurement results qualitatively rather than quantitatively. Generally, the CFD (Computational Fluid Dynamics) modelling confirmed the conclusions from measurements. Obtaining reliable temperatures at the furnace chamber outlet using numerical modelling is possible based on the 0-dimensional model of the boiler. In order to get reliable results from numerical modelling, it is necessary to validate the model with real measurement data. The results obtained by numerical simulation show that the FJBS system aligns the field temperature and concentration and decreases CO in flue gas.

Reprint of: Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.

Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.

Simulation of a MILD combustion burner using ILDM chemistry

A numerical study was performed to accommodate mathematical simplification of kinetics and non-premixed reactants in three-dimensional computations. Specifically, the ILDM technique was combined with the presumed probability density function approach to simulate turbulent combustion in a burner operating at MILD combustion, which was characterised by relatively uniform temperatures with no visible flame and sound. An Eulerian solution strategy was implemented in a CFD code on a structured mesh. Predictions of the mean flow field, turbulence kinetic energy, mixture fraction and its variance, temperature, and mass fraction of CO2 and H2O were presented. It is found that the flow near the burner exhibited strong anisotropy and, thus, k – ε turbulence model has over predicted the spread of the jets. Nevertheless, the calculated mean velocities and temperatures reproduce experimental data reasonably well. The mixing mechanism in the near field of the burner was fully described with remarks on the entrainment of flue gas. Low levels of turbulence in the lifted reaction zone were particularly emphasised with relevance to the homogeneity of the temperature field.

P, ρ, T-properties and phase equilibria in the water-n-hexane system with a low content of water

Using a piezometer of constant volume, we determined experimentally the P, ρ, and T properties and the phase equilibria for the binary water-n-hexane mixtures with 0.04, 0.05, 0.06, and 0.0673 mass fraction of H2O over the density range of 0.067–0.607 g/cm3, temperature range of 380–680 K, at pressures up to 60 MPa. The equilibrium lines of the liquid-liquid and liquid-gas transition have been determined. The three-phase line, the line of the azeotrope, and the lower branch of the critical line (all lines are joined at the upper finite critical point) have been plotted in the work.

Influences of H2O mass fraction and chemical kinetics mechanism on the turbulent diffusion combustion of H2–O2 in supersonic flows

Hydrogen is one of the most promising fuels for the airbreathing hypersonic propulsion system, and it attracts an increasing attention of the researchers worldwide. In this study, a typical hydrogen-fueled supersonic combustor was investigated numerically, and the predicted results were compared with the available experimental data in the open literature. Two different chemical reaction mechanisms were employed to evaluate their effects on the combustion of H2–O2, namely the two-step and the seven-step mechanisms, and the vitiation effect was analyzed by varying the H2O mass fraction. The obtained results show that the predicted mole fraction profiles for different components show very good agreement with the available experimental data under the supersonic mixing and combustion conditions, and the chemical reaction mechanism has only a slight impact on the overall performance of the turbulent diffusion combustion. The simple mechanism of H2–O2 can be employed to evaluate the performance of the combustor in order to reduce the computational cost. The H2O flow vitiation makes a great difference to the combustion of H2–O2, and there is an optimal H2O mass fraction existing to enhance the intensity of the turbulent combustion. In the range considered in this paper, its optimal value is 0.15. The initiated location of the reaction appears far away from the bottom wall with the increase of the H2O mass fraction, and the H2O flow vitiation quickens the transition from subsonic to supersonic mode at the exit of the combustor.

216201 パッシブ形PEFCのマルチフィジックスシミュレーション(一般05 流体力学・流体工学3)

Because the operation of a passive type polymer electrolyte fuel cell (passive type PEFC) is accompanied with complex phenomena such as an electrochemical reaction, heat and flow, it is hard to obtain an appropriate orientation for the improved design. Numerical analysis is conducted for the purpose of obtaining the parameters in the improved design, which leads to reduction of time and expense in a trial production and an experiment. In this paper, using the COMSOL Multiphysics, a general commercial software, was used to compute the multiphysics of the passive type PEFC. The model uses current balances, mass transport equations (Maxwell-Stefan diffusion for reactant, water and nitrogen gas), and momentum transport (gas flow) to simulate a passive type PEFC's behavior. The results showed that the computed diffusion velocity vectors of 02 and H2O, the mass fraction of H2O, and the current density on the MEA surface were qualitatively reasonable well.

CFD Analysis of Cantilevered Expansion and Compression Type Ramp Injector

In this study the combustion process in a SHCRAMJET using cantilevered compression and expansion type ramp injector having 80 of both compression and expansion angle is analyzed using FLUENT software. After designing the model in GAMBIT, it is exported to FLEUNT software for analysis of combustion process with air inlet at Mach number 7 and hydrogen as the fuel with inlet Mach number 3.5. The results obtained explain the significance of geometries of the injector in initiating combustion process and achieving enhanced air-fuel mixing. The contours of temperature, pressure, density, turbulent intensity and mass fraction of H2O formed explain the extent of combustion taking place in this case.

Direct numerical simulation of hydrogen turbulent lifted jet flame in a vitiated coflow

The direct numerical simulation (DNS) method with 16 steps detailed chemical kinetics was applied to a lifted turbulent jet flame with H2/N2 fuel issuing into a wide hot coflow of lean combustion products, at temperature of 1045 K and low oxygen concentrations. The chemical reactions were handled by the library function of CHEMKIN which was called by the main program in every time step. Parallel computational technology based on message passing interface method (MPI) was used in the simulation. All the cases were run by 12 CPUs on a high performance computer system. Faver-averaged DNS results were obtained by long time averaging the transient profile and compared with the experimental data. The roll-up and evolution of the vortices in jet flame were well captured. The vortices in the same rotating direction attracted each other and those in different rotating directions repulsed each other. Through complex interactions between vortices, the original symmetrical vortex structure could be converted into nonsymmetrical and more complex structures by combination, distortion and splitting of the vortices. The transient profiles of H, OH and H2O mass fraction at 5.76 ms showed the flame structure in jet flame, especially the autoignition regions clearly. The lift-off height was about 9 d–11 d, in agreement with the experimental observation. At the corner point of the flame sheet indicated by OH and H profiles, the combustion was always enhanced by the flame curvature and extended resident time. The profiles of turbulence intensities show that the flames were diffused from the original two outside flame sheets into the core. The DNS results can be considered in developing more accurate and more universal turbulence models.

Bubble Point Pressure of the Solutions of H2SiF6 + H2O and H2SiF6 + CO(NH2)2 + H2O from 323 K to 353 K

The isobaric method was used to determine the bubble point pressure of the solutions of H2SiF6 (1) + H2O (mass fraction, w1 = 0.003 to 0.006) and H2SiF6 (1) + CO(NH2)2 (2) + H2O (w1 = 0.003 to 0.006, w2 = 0.004, 0.012, and 0.020) from 323 K to 353 K. The experimental results indicated that the relationship between bubble point pressure Pv and the temperature T can be fitted as the following equation: log(Pv/kPa) = A + B/(T/K).

Liquid water and active resurfacing on Europa

Voyager images of the jovian moon Europa show a bright icy surface transected by a complex pattern of long, linear fracture-like markings with a slightly lower albedo1. Very few impact craters are observed, with sizes generally hi the range 10–20 km. Indicators of surface composition and texture include deep IR H2O absorption features2, UV absorption features characteristic of sulphur in a water matrix on the trailing hemisphere3, and a photometric function indicating much more homogeneous scattering than areas of equal albedo on Ganymede and Callisto4. Europe's mean density is 3.03 g cm−3, indicating a primarily silicate composition. However, if the density of the silicates is the same as for Io, the H2O mass fraction would be ∼6%, enough to form an outer layer of H2O > 100 km thick if completely differentiated. Several models have been suggested for the evolution and present state of Europa5–8. We present here arguments for recent resurfacing by H2O from a liquid layer, based on new interpretations of recent spacecraft and Earth-based observations and revised theoretical calculations.

On the internal structure of the major satellites of the outer planets

The structures and thermal evolutions of the large icy satellites of the outer solar system are considered. It is shown (for bodies comparable in size and mass to the Galilean satellites, having sizeable mass fraction of H2O, and with meteoritic abundances of radioactive materials contained within their silicate fractions) that the crust of solid ice over a liquid mantle predicted by conductive heat‐transfer calculations is unstable to large‐scale solid‐state convection. For appropriate material parameters, convective heat‐transfer rates are sufficient to freeze a large liquid mantle on a time scale that is short compared to the lifetime of the body. It is also concluded that the ice layer is convecting at the present time. A reevaluation of previous work, using improved values for material parameters and boundary conditions, reverses earlier conclusions and implies a rigid outer crust with resulting long‐term stability of surface features to creep deformation. The combination of a rigid crust with active internal convection presents the additional possibility of surface features that are produced and maintained by dynamic internal processes.