Detailed simulations of the transient hydrogen mixing,leakage and flammability in air in simple geometries

During an accidental release, hydrogen disperses very quickly in air due to a relatively high density difference. A comprehensive understanding of the transient behavior of hydrogen mixing and the associated flammability limits in air is essential to support the fire safety and prevention guidelines. In this study, a buoyancy diffusion computational model is developed to simultaneously solve for the complete set of equations governing the unsteady flow of hydrogen. A simple vertical cylinder is considered to investigate the transient behavior of hydrogen mixing, especially at relatively short times, for different release scenarios: (i) the sudden release of hydrogen at the cylinder bottom into air with open, partially open, and closed tops, and (ii) small hydrogen jet leaks at the bottom into a closed geometry. Other cases involving the hydrogen releases/leaks at the cylinder top are also explored to quantify the relative roles of buoyancy and diffusion in the mixing process. The numerical simulations display the spatial and temporal distributions of hydrogen for all the configurations studied. The complex flow patterns demonstrate the fast formation of flammable zones with implications in the safe and efficient use of hydrogen in various applications.     

Particle image velocimetry experimental and computational investigation of a blood pump

Blood pumps have been adopted to treat heart failure over the past decades.A novel blood pump adopting the ro-tor with splitter blades and tandem cascade stator was developed recently.A particle image velocimetry (PIV) experiment was carried out to verify the design of the blood pump based on computational fluid dynamics (CFD) and further analyze the flow properties in the rotor and stator.The original sized pump model with an acrylic housing and an experiment loop were constructed to perform the optical measurement.The PIV testing was car-ried out at the rotational speed of 6952±50 r/min with the flow rate of 3.1 l/min and at 8186±50 r/min with 3.5 l/min,respectively.The velocity and the Reynolds shear stress distributions were investigated by PIV and CFD,and the comparisons between them will be helpful for the future blood pump design.     

Atomistic simulations of hydrogen embrittlement

It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement. Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in α-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method. We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of α-Fe under monotonic loading.     

Advancing the hydrogen safety knowledge base

The International Energy Agency's Hydrogen Implementing Agreement (IEA HIA) was established in 1977 to pursue collaborative hydrogen research and development and information exchange among its member countries. Information and knowledge dissemination is a key aspect of the work within IEA HIA tasks, and case studies, technical reports and presentations/publications often result from the collaborative efforts. The work conducted in hydrogen safety under Task 31 and its predecessor, Task 19, can positively impact the objectives of national programs even in cases for which a specific task report is not published. The interactions within Task 31 illustrate how technology information and knowledge exchange among participating hydrogen safety experts serve the objectives intended by the IEA HIA.     

Forced ventilation for sensing-based risk mitigation of leaking hydrogen in a partially open space

This study performs the numerical simulation of hydrogen dispersion in a partially open space with a single roof vent. The effects of various roof vent positions, leak positions, leak flow rates and exhaust flow rates on the forced ventilation of leaking hydrogen, are shown and discussed. Based on the results, a proper roof vent position and the disadvantage of ventilation with constant exhaust flow rates are established. To overcome the disadvantage, a new control strategy to change exhaust flow rates with the roof vent fixed at the proper position is proposed. First a plot is constructed to show acceptable exhaust flow rates to various inflow rates and leak positions. Assuming real-time sensing of hydrogen concentration and height-direction velocity, volume flow rates of leaking hydrogen are then estimated. Based on the estimated leak flow rates and hydrogen sensor information near the roof, control is conducted considering the plot of acceptable exhaust flow rates to various inflow rates and leak positions. The proposed method is validated against various leak positions, leak flow rates and leak modes. This paper proposes an innovative approach to sensing-based risk mitigation control of hydrogen dispersion and accumulation in a partially open space by forced ventilation.     

Heat transfer and crevice flow in a hydrogen-fueled spark-ignition engine: Effect on the engine performance and NO exhaust emissions

The present work investigates the effect of heat and mass transfer on the combustion process of a hydrogen-fueled spark-ignition engine, using an in-house CFD code. The main scope is to compare the calculated local heat fluxes with the available measured ones, using three heat transfer models of increasing complexity (two existing and one developed by the authors). Moreover, the effect of mass transfer through the crevice regions is also investigated using a phenomenological crevice model. The calculated results (cylinder pressure traces, local heat fluxes and NO exhaust emissions) are compared with the corresponding measured data, at various operating conditions, maintaining constant engine speed and altering the compression ratio and the equivalence ratio. It is revealed, that the proposed heat transfer model is more accurate than the standard wall-function formulation, while with the use of the crevice model a more reliable prediction of engine performance is achieved.     

Assessment of a novel numerical model for combustion and in-flight heating of particles in an industrial furnace

The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2?MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178?K higher compared to a particle, which trajectory led through zones with lower gas temperatures.     

A numerical model for segmented flow in a microreactor

A numerical model for segmented flow in a microreactor has been developed. The model is based on computational fluid dynamics (CFD) which means that the flow field and mass transfer are described by a set of partial differential equations. A general purpose CFD code was extended in order to predict the internal flow patterns of fluid segments and the transfer of dissolved chemical species within segments and across fluid segment interfaces. The model has been validated by comparing predicted data with experimental microreactor titrations.     

Shape effect of cavity flameholder on mixing zone of hydrogen jet at supersonic flow

Cavity flameholder is known as an efficient technique for providing the ignition zone. In this research, computational fluid dynamic is applied to study the influence of the various shapes of cavity as flameholder on the mixing efficiency inside the scramjet. To evaluate different shapes of cavity flame holder, the Reynolds-averaged Navier–Stokes equations with (SST) turbulence model are solved to reveal the effect of significant parameters. The influence of trapezoidal, circle and rectangular cavity on fuel distribution is expansively analyzed. Moreover, the influence of various Mach numbers (M = 1.2, 2 and 3) on mixing rate and flow feature inside the cavity is examined. The comprehensive parametric studies are also done. Our findings show that the trapezoidal cavity is more efficient than other shapes in the preservation of the ignition zone within the cavity. In addition, the increase of free stream Mach number intensifies the main circulations within cavity and this induces a stable ignition zone within cavity.     

CFD based simulation of hydrogen release through elliptical orifices

Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through different shapes of orifices. The effect of orifice geometry on the structure, development and dispersion of a highly under-expanded hydrogen jet close to the exit is numerically investigated. Various shapes of orifices are evaluated, including holes with constant areas such as elliptical and circular openings, as well as, enlarging circular orifices. A three-dimensional in-house parallel code is exploited to simulate the flow using an unstructured tetrahedral finite volume Euler solver. The numerical simulations indicate that, for a high pressure reservoir hydrogen release, the area of the orifice is the main geometric parameter influencing the centerline pressure at the hydrogen-air interface and the transient peak temperature, while the elliptical or expanding orifices slightly mitigate the auto-ignition risks associated with the accidental release of hydrogen. Therefore, circular openings represent the most conservative geometry for the study of auto-ignition of hydrogen.     

Canadian hydrogen safety program

The Canadian hydrogen safety program (CHSP) is a project initiative of the Codes & Standards Working Group of the Canadian transportation fuel cell alliance (CTFCA) that represents industry, academia, government, and regulators. The Program rationale, structure and contents contribute to acceptance of the products, services and systems of the Canadian Hydrogen Industry into the Canadian hydrogen stakeholder community. It facilitates trade through fair insurance policies and rates, effective and efficient regulatory approval procedures and accommodation of the interests of the general public. The Program integrates a consistent quantitative risk assessment methodology with experimental (destructive and non-destructive) failure rates and consequence-of-release data for key hydrogen components and systems into risk assessment of commercial application scenarios. Its current and past six projects include Intelligent Virtual Hydrogen Filling Station (IVHFS), Hydrogen clearance distances, comparative quantitative risk comparison of hydrogen and compressed natural gas (CNG) refuelling options; computational fluid dynamics (CFD) modeling validation, calibration and enhancement; enhancement of frequency and probability analysis, and Consequence analysis of key component failures of hydrogen systems; and fuel cell oxidant outlet hydrogen sensor project. The Program projects are tightly linked with the content of the International Energy Agency (IEA) Task 19 Hydrogen Safety.     

Numerical simulation of wall mass transfer rates in capillary-driven flow in microchannels

Microchannels are believed to open up the prospect of precise control of fluid flow and chemical reactions. The high surface to volume ratio of micro size channels allows efficient mass transfer rates. The capillary effect can be used to pump fluids in microchannels and the flow generated can dissolve chemicals previously deposited on the walls of the channel. The purpose of this work is to analyze the wall mass transfer rates generated by a capillary driven flow in a microchannel. The results have implications in the optimization and design of devices for biological assays. We performed simulations of the capillary-driven flow in two-dimensional rectangular and circular microchannels by solving numerically the governing momentum and mass transfer equations with a second order accuracy finite volume code. The effects of the Reynolds number, of the contact angle and of the channel geometry on the time evolution of the local and averaged wall mass transfer rates are reported and analyzed. The flow field behind the meniscus, viewed from a reference frame moving at the velocity of the meniscus, showed to have two recirculations that enhance the wall mass transfer rates close to the triple point. A correlation between the Sherwood number and the Reynolds number, the contact angle and the time is reported. The correlation can be a useful tool for design purposes of microfluidic devices with capillary driven flows in which a fast heterogeneous reaction occurs on the wall.