Penetration height of transitional round fountains in a linearly stratified fluid

The transition to asymmetry of transitional fountains is further complicated by the stratification of the ambient fluid. In this study, a series of three-dimensional direct numerical simulations were carried out for transitional round fountains in linearly-stratified fluids over the ranges of 100 ≤ Re ≤ 400, 1 ≤ Fr ≤ 8 and 0.0 ≤ s ≤ 0.3, where Fr, Re and s are the Froude, Reynolds, and dimensionless temperature stratification parameters, respectively, to examine, both qualitatively and quantitatively, the effect of these parameters on their axisymmetric transition and their maximum penetration heights. The results show that when Fr or Re are small enough, a fountain remains axisymmetric for all time; however, when Fr or Re are increased sufficiently, the fountain will be axisymmetric initially, before becoming asymmetric and unsteady, ultimately reaching a fully developed quasi-steady stage when the maximum fountain penetration height fluctuates over a constant, time-average, value. The stratification is found to play a positive role to stabilize the flow and to reduce or even to eliminate the asymmetric behavior. The numerical results were also used to develop a series of scaling relations, in conjunction with the scalings obtained with dimensional analysis, for the initial and time-averaged fountain penetration heights, and the time for the fountain to attain the initial penetration height. It is further found that in general these characteristic parameters are strongly dependent on Fr and s, while only weakly dependent on Re.     

Experimental study on buoyant flow stratification induced by a fire in a horizontal channel

Experiments were carried out in a reduced-scale horizontal channel to investigate the fire-induced buoyant flow stratification behavior, with the effect of the velocity shear between the hot buoyant flow and the cool air flow considered. This shear intensity was controlled and varied by changing the exhaust rate at the ceiling with one of the end of the channel opened. The flow pattern was visualized by the aid of a laser sheet. The horizontal traveling velocity, vertical temperature profile and stratification interface height of the buoyant flow were measured. The stratification pattern was found to fall into three regimes. Buoyancy force and inertia force, as the two factors that dominate the buoyant flow stratification, were correlated through the Froude number and the Richardson number. At Region I (Ri > 0.9 or Fr < 1.2), the buoyant flow stratification was stable, where a distinct interface existed between the upper smoke layer and the lower air layer. At Region II (0.3 < Ri < 0.9 or 1.2 < Fr < 2.4), the buoyant flow stratification was stable but with interfacial instability. At Range III (Ri < 0.3 or Fr > 2.4), the buoyant flow stratification becomes unstable, with a strong mixing between the buoyant flow and the air flow and then a thickened smoke layer.     

Transition to asymmetry of transitional round fountains in a linearly stratified fluid

In this study, a series of three-dimensional direct numerical simulations were carried out for transitional round fountains in a linearly-stratified fluid over the ranges of 100 ≤ Re ≤ 400, 1 ≤ Fr ≤ 8 and 0.0 ≤ s ≤ 0.3, where Fr, Re and s are the Froude, Reynolds, and dimensionless temperature stratification parameters, respectively, to examine, both qualitatively and quantitatively, the effect of these parameters on their transition to asymmetry and the asymmetric behavior. It is found that the transition to asymmetry are well represented and quantified by azimuthal velocity, with non-zero or noticeable azimuthal velocity indicating asymmetry. The results show that when Fr or Re are small, a fountain remains axisymmetric for all time; however, when Fr or Re are increased to be sufficiently large, the fountain will be axisymmetric initially, before becoming asymmetric and unsteady, ultimately reaching a fully developed quasi-steady stage when each quantity fluctuates over a constant, time-average, value. The stratification is found to play a positive role to stabilize the flow and to reduce or even to eliminate the asymmetric behavior. The numerical results were also used to develop the scaling for the time for transition to asymmetry, which is found to be strongly dependent on Fr and s, while only weakly dependent on Re.     

Simulation by solutal convection of a thermal plume in a confined stratified environment: application to displacement ventilation

This paper describes the experimental study, by solutal simulation, of a thermal plume in a confined stratified environment, a situation encountered in displacement ventilation systems. The criteria enabling similarity to be established between the thermal plume in air and the solutal plume in a hydraulic model are discussed. Density stratification is detected by a planar laser-induced fluorescence (PLIF) technique. Criteria for defining the interface height and thickness are determined After validation of these criteria in the fully developed region of the plume in a confined stratified environment, a formulation of the stratification height in the region close to the source has been established.     

Characteristics of hydrogen leakage sound from a fuel-cell vehicle by hearing

Fuel-cell vehicle, run on hydrogen, is known that it has better energy efficiency than existing gasoline cars. The vehicles are designed so that hydrogen leaks from the tank are stopped automatically upon detection of hydrogen leakage or detection of impact in a collision. However, we investigated the characteristics of hydrogen leakage sound from a hydrogen-leaking vehicle and the threshold of discrimination of hydrogen leakage from noise at a crossing with much traffic to examine a method to rescue people safely depending on the sense of hearing in the event of a continuous hydrogen leak. Here, in the discrimination threshold test, we conducted the test by using helium, which is alternative gas of hydrogen leakage sound. We clarified that hydrogen leakage sound from vehicles has directivity, height dependence, and distance dependence. Furthermore, we confirmed the threshold flow rate for distinguishing hydrogen gas when hydrogen leakage is heard at a distance of 5–10 m from the center of the hydrogen leaking vehicle in a 74 dB traffic noise environment.     

A theoretical framework for calculating full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage

Previous experimental results on full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage can only be suitable for solving practical engineering problems, or testing the limitation of previous models. Thus, this paper presents a theoretical framework for the high-pressure hydrogen/natural gas leakage and the subsequent jet fire. The proposed framework consists of a transient leakage model, a notional nozzle model, a jet flame size model, a radiative fraction correlation and a line source radiation model. The framework is validated by comparing the model predictions and experimental measurements of mass flow rate, total flame height and thermal radiation field of hydrogen, natural gas, hydrogen/natural gas mixture jet fires with a flame height up to 100 m. The comparison shows that the theoretical framework can give considerable predictions to properties of full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage.     

GASFLOW-MPI: A new 3-D parallel all-speed CFD code for turbulent dispersion and combustion simulations Part II: First analysis of the hydrogen explosion in Fukushima Daiichi Unit 1

The core-melt in Fukushima-Daiichi Unit 1 represents a new class of severe accidents in which combustible gas from core degradation leaked from the containment into the surrounding air-filled reactor building, formed there a highly reactive gas mixture, and exploded 25 h after begin of the station black-out. Since TMI-2 hydrogen safety research and management has focussed on processes and counter-measures inside the containment but the reactor building remained unprotected against hydrogen threats. The code GASFLOW-MPI is currently under development to simulate hydrogen behaviors, including distribution and combustion, for scenarios with containment leakage.This paper describes a first analysis of the hydrogen explosion in Unit 1. It investigates gas dispersion in the reactor building, assuming a leak at the drywell head flange, shows the evolution of a stratified, inhomogeneous H₂–O₂–N₂–steam mixture in the refueling bay, simulates the combustion of the reactive gas mixture, and predicts pressure loads to walls and internal structures of the reactor building. The blast wave propagated essentially sideways, which explains why all side walls were blown out and the ceiling just collapsed onto the floor of the refueling bay. The blast wave propagation into the free environment was also simulated. The over-pressure amplitudes are sufficiently high to cause damage to adjacent buildings and to injure people. The hitherto existing presumption that the blow-out panel of Unit 2 was removed by the Unit 1 explosion can be confirmed which likely prevented a hydrogen explosion in the Unit 2.GASFLOW-MPI provides validated models for the integral simulations of leakage related core-melt scenarios. Furthermore, the code contains extensively tested submodels for catalytic recombiners, igniters and burst foils, which allow design of new hydrogen risk mitigation systems for currently unprotected spaces in reactor buildings.     

The effect of stable stratification and thermophoresis on fine particle deposition in a bounded turbulent flow

Small inertial particle transfer and deposition in a thermally stratified turbulent channel flow is studied by large eddy simulation. The Lagrangian tracking approach is used to describe the dynamics of particles. The objective of this study is to examine the influences of thermal stratification and the thermophoresis on the preferential concentration and deposition of small particles with different particle relaxation time scales. The diameters of particle are ranged from 0.6 μm to 5 μm. Numerical results show that the thermophoresis strengthens the deposition of particles near the cold wall while it weakens the deposition near the hot wall. With the Richardson number increases, the particle mean velocity increases regularly in the core of the channel, and the particle fluctuation intensities decrease due to the re-laminarization tendency of stably stratified turbulent boundary layer. The magnitude of the thermophoretic force on particles decreases in the stable stratified flow, which results in smaller deposition rate with increasing the Richardson number.     

Investigation of a large top wall temperature on the natural convection plume along a heated vertical wall in a square cavity

The characteristics of the laminar natural convection in an air-filled square cavity heated and cooled on the side walls was studied for cases where the temperature of the top wall was significantly larger than the heated vertical wall. Experiments were performed for a horizontal Grashof number of 1.3 × 10⁸, and non-dimensional top wall temperatures from 1.4 to 2.3. The results show that the plume formed on the heated vertical wall separated from this wall before reaching the top wall. As a result, three different regions were observed in the cavity: a stratified core region, a buoyant plume region, and a highly stratified region above the plume after it had separated from the vertical wall. The highly stratified region above the plume became larger and more stable with an increase of the top wall temperature, stabilizing the motion of the plume across the cavity. The similarity solutions developed by Kulkarni et al. [A.K. Kulkarni, H.R. Jacobs, J.J. Hwang, Similarity solution for natural convection flow over an isothermal vertical wall immersed in thermally stratified medium, Int. J. Heat Mass Transfer 30 (1987) 691–698] to characterize the natural convection heat transfer along an isothermal single vertical plate did not agree with the results for the current measurements; however, the non-similarity model of Chen and Eichhorn [C.C. Chen, R. Eichhorn, Natural convection from a vertical surface to thermally stratified fluid, J. Heat Transfer 98 (1976) 446–451] was in good agreement over most of the wall. There were some discrepancies in the temperature distributions and the heat transfer characteristics, especially at y/H ? 0.8 due to the separated flow in this region.     

Studies on Thermal Stratification Phenomenon in LH2 Storage Vessel

A study of convective heat transfer in a cryogenic storage vessel is carried out numerically and experimentally. A scaled down model study is performed using water as the model fluid in a rectangular glass tank heated from the sides. The convective flow and the resulting thermal stratification phenomenon in the rectangular tank are studied through flow visualization, temperature measurement, and corresponding numerical simulations. It is found that a vortex-like flow near the top surface leads to a well-mixed region there, below which the fluid is thermally stratified. In addition, in an attempt to simulate the actual conditions, a numerical study is performed on a cylindrical cavity filled with liquid hydrogen (LH₂) and heated from the sides. The results are compared with our model study with water, and the qualitative agreement is found to be good.     

Ecoulements induits par la force gravifique dans une cavité cylindrique en rotation

A spectral Tau-Chebyshev method is used for the prediction of the motion of a rotating Boussinesq fluid driven by buoyancy from a horizontal temperature gradient. The analysis is made for the axisymmetric regime in annular cavities when the physical parameters vary as: 0≤ Re≤ 2500; Ra = 10², ± 10⁴, 10⁵; 0 ≤ Fr ≤ 25. The transition from the thermal convection regime to the rotation dominated regime results from the competition between the various buoyancy, centrifugal and Coriolis forces. The validity of the asymptotical solutions derived from Hunter (1966) when Fr ⪡ 1 andRe ⪢ 1, is discussed when Re varies.     

Three-dimensional driven-cavity flows with a vertical temperature gradient

Numerical studies of three-dimensional flows in a cubical container with a stable vertical temperature stratification are carried out. Flows are driven by the top lid, which slides in its own plane at a constant speed. The top wall is maintained at a higher temperature than the bottom wall. The end walls and the side walls are thermally insulated. Numerical solutions are obtained over a wide range of physical parameters, i.e. 10² ≤ Re ≤ 2 × 10³, 0 ≤ Ri ≤ 10.0 and Pr = 0.71, where the mixed-convection parameter Ri Gr. Re⁻². Systematical comparison of the three-dimensional numerical solutions with the previously reported two-dimensional results illuminates the impact of thermal stratification on the three-dimensional flow characteristics. When Ri < O(1), the effect of the vertical temperature gradient is minor, and the flow structures are similar to those of the non-stratified fluid flows in a conventional lid-driven cavity flow. Fluids in the primary vortex are well mixed, and the temperature is fairly uniform in the main circulating region. When Ri ≥ O(1), the stable temperature distribution tends to suppress the vertical fluid motion. Much of the fluid motion takes place in the vicinity of the top sliding lid and the bulk of the cavity region is nearly stagnant. When Ri > O(1), the fluid motion exhibits vertically layered vortex structures. The Nusselt number is computed at the top and bottom wall, and this also illustrates the varying flow characteristics as Ri encompasses a broad range. Extensive numerical flow visualizations are conducted. Plots demonstrating the primary flows in the (x−y) plane and the secondary flows in the (y−z) plane are presented. These display the influences of Re and Ri on the basic character of the flow and the three-dimensional effects.     

Prediction of evaporation heat transfer coefficient based on gas–liquid two-phase annular flow regime in horizontal microfin tubes

A physical model of gas–liquid two-phase annular flow regime is presented for predicting the enhanced evaporation heat transfer characteristics in horizontal microfin tubes. The model is based on the equivalence of a periodical distortion of the disturbance wave in the substrate layer. Corresponding to the stratified flow model proposed previously by authors, the dimensionless quantity Fr₀ = G/[gd_eρ_v(ρ_l ? ρ_v)]^0.5 may be used as a measure for determining the applicability of the present theoretical model, which was used to restrict the transition boundary between the stratified-wavy flow and the annular/intermittent flows. Comparison of the prediction of the circumferential average heat transfer coefficient with available experimental data for four tubes and three refrigerants reveals that a good agreement is obtained or the trend is better than that of the previously developed stratified flow model for Fr₀ > 4.0 as long as the partial dry out of tube does not occur. Obviously, the developed annular model is applicable and reliable for evaporation in horizontal microfin tubes under the case of high heat flux and high mass flux.     

Effect of storage thermal behavior in seasonal storage solar heating systems

An analytical methodology has been developed to investigate the effects of storage operational strategies (or, equivalently, stratification) on the performance of a seasonal storage solar heating system with a water storage. The method is based on a relative comparison between a thermally stratified and well-mixed storage system representing probable extreme outcomes of the subsystem-to-storage loop control strategies. The effects are incorporated into a set of performance reduction factors that describe maximum changes in the solar collector yield, storage losses and solar fraction due to storage operational mode. The study indicates that the storage thermal behavior could in the worst case affect the yearly solar fraction by a factor of 2, but most likely a maximum value from 1.35 to 1.6 could be expected.     

Modeling the diffusion of flammable hydrogen cloud under different liquid hydrogen leakage conditions in a hydrogen refueling station

Constructing hydrogen refueling stations will be popular for hydrogen energy use in the future, and investigating the diffusion characteristics of hydrogen in a leakage incident is quite significant. The instantaneous evolution of flammable hydrogen clouds arising from liquid hydrogen leakage in a hydrogen refueling station is predicted using Ansys Fluent, and parametric analyses are conducted to reveal the effects of storage pressure, source height, and leakage direction on the distributions of the flammable regions. In addition, the feasibilities of heating the ceiling or the ground of the station after the leakage of liquid hydrogen to accelerate the hydrogen dilution are examined. The results show that the flammable region is stabilized at 90 s, the corresponding flammable hydrogen cloud volume is about 333 m³, and the extensions of downwind and vertical directions reach 10 m and 9.3 m. Storage pressure has a finite effect on the downwind diffusion distance of the flammable cloud. A lower source height tends to format the high-concentration hydrogen cloud near the ground while a higher source height helps separate the flammable clouds from the ground. The upward leakage direction leads to the maximum downwind diffusion distance of about 10.2 m while the downward leakage direction makes the high hydrogen concentration region confined below the ceiling. Just maintaining the ceiling at the initial temperature of 300 K is effective for accelerating the hydrogen dilution in the upward leakage. The maximum hydrogen concentration and the flammable volume can be reduced at rates of 0.35 vol % and 8% for every 50 K increase in heating temperature. For the downward leakage, keeping the ground at the initial temperature just works for the first 40 s in reducing the maximum hydrogen concentration, while increasing the heating temperature receives a gradually declined effect on reducing the flammable volume.     

Estimating leaked hydrogen gas flow in confined space through coupling zone model and point source buoyancy plume theory

A semi-analytical model combining zone model and virtual point source buoyancy plume theory is proposed to predict the gas flow and dispersion behaviors of leaked hydrogen in confined space with an opening. The height of interface between the upper and lower layers, outflow velocity and hydrogen molar fraction in outflow at steady stage are quantitively analyzed to study the effects of leakage mass flux and opening geometry. A computational fluid dynamics (CFD) tool, FLACS, is employed to simulate the interested scenarios and validate the reliability of the developed model. The results show that the interface height declines as the leakage mass flux increases, but the outflow velocity and hydrogen molar fraction exhibit inverse tendencies. The interface height is positively proportional to both opening width and height, but the opening height plays a more important role. At steady stage, the simulated interface height fluctuates within 0.1 m uncertainty range, which is identical to the grid size, and it well fits the analytical estimations. The opening width affects the outflow velocity more greatly than that of opening height. The fluctuation of the simulated interface height enhances the inaccuracy of the predicted outflow velocity in the acceptable range. Meanwhile, it is found that with fixed opening geometry the ratio of outflow velocity with higher leakage mass flux to that with lower leakage mass flux remains at 1.28 when the leakage mass flux is doubled. The variations of opening width and height have little and significant effects on outflow hydrogen molar fraction, respectively. Similarly, for given opening geometry the ratio of hydrogen molar fraction with higher leakage mass flux to that with lower leakage mass flux is about 1.58 when the leakage mass flux is doubled.     

Influence of compositional stratification on autoignition in n-heptane/air mixtures

Autoignition of an initially quiescent stratified layer of n-heptane and air is numerically studied using multistep kinetic mechanisms. The influence of the level of stratification on the ignition characteristics of the mixture is investigated. The pressure and temperature conditions selected are relevant to compression-ignited internal combustion engines. As previously reported, for uniform temperature conditions, ignition always occurs in fuel-rich regions whereas the stable flame is located at the stoichiometric mixture condition, i.e. there is a spatial offset between the initial location and the final stabilization location of the flame. The time in which the final stabilization location is reached, once ignition initiates, is dependent on the level of mixture stratification. Consider the Damköhler number (Da) defined as the product of the inverse of scalar dissipation rate and the inverse of the chemical reaction time scale. In the presence of small gradients in the flow field, i.e. below a critical level of mixture stratification, the Da exceeds a critical value and the response of the ignition to changes in mixture stratification is controlled by the ratio of the offset distance to a diffusion length scale. In this high Da limit, an increase in gradients reduces the time required to reach the stable flame condition. Above the critical level of mixture stratification, when the Da is below the critical value, losses of species and heat from the ignition location become significant, and ignition is retarded with increasing gradients in the flow field. In a two-stage ignition event, the influence of the stratification is primarily reflected in the second stage of ignition.     

Formation of flammable hydrogen–air clouds from hydrogen leakage

The possibilities of the formation of a flammable cloud over the ground in an open atmosphere from the leakage of hydrogen stored at different temperatures are studied. The dispersion of hydrogen in the stable and unstable atmospheric conditions is determined using the Gaussian dispersion model. The efflux of hydrogen from the storage vessel is considered at velocities between 1 m/s and 1500 m/s, the latter corresponding to the upper limit of velocities arising from the choked flow. The dispersion analysis shows that flammable hydrogen–air clouds would not be formed over the ground under unstable atmospheric conditions for all efflux velocities and leakage areas and for the different temperatures of the hydrogen leak. However, under strongly stable atmospheric conditions, such as those associated with clear sky winter nights with low winds and temperature inversion in the planetary boundary layer, a flammable cloud is seen to be formed. This is particularly true for low temperature hydrogen efflux and very low velocities of the efflux.     

Safety analysis of leakage in a nuclear hydrogen production system

Due to its unique advantages, such as clean and pollution-free, hydrogen energy has gradually improved its energy transition position. Constructing nuclear hydrogen production systems is a necessary means to achieve large-scale hydrogen production, and the study of hydrogen leakage and diffusion behavior is critical to commercializing hydrogen production systems. In engineering practice, the distance between the hydrogen storage device and the nuclear power plant is an important indicator to measure the safety of nuclear hydrogen production. To study the influence of gas storage tank's own conditions and external environmental conditions on leakage diffusion, influencing factors such as wind speed, leakage direction, leakage diameter, leakage height, and leakage angle are discussed in the present study. By calculating severe working conditions combined with the above multiple factors, the longest distance of hydrogen diffusion is determined. Finally, peak overpressure impact generated by hydrogen explosion was evaluated, and the minimum separation distance required to avoid safety risks was predicted. The results demonstrate that when the wind direction is consistent with the leakage direction, and the leakage angle is 0°, the higher the wind speed, the larger the leakage diameter and the lower the leakage height, resulting in a longer diffusion distance. Under more extreme and severe working conditions, the diffusion distance of combustible hydrogen cloud can reach as far as 237 m. Once hydrogen diffusion explodes, the minimum separation distance required is about 338 m. This research provides an effective method for safety risk assessment of a nuclear hydrogen production system.     

An experimental study of the generation and characteristics of a two-layer thermally stable environment

An experimental system has been developed to generate a thermally stratified, two-layer, environment in an enclosure, with air as the fluid. Such a stably stratified circumstance, in which an essentially isothermal heated layer overlies a relatively cooler isothermal layer, frequently arises in several practical problems such as those related to heat rejection, heat transfer from isolated thermal sources in confined regions, and enclosure fires. A detailed study of the thermal field is carried out to determine the basic characteristics of the stratified environment and the dependence of the physical parameters such as interface location and temperature level on the governing variables. The location of the interface, between the two layers can be controlled by varying the temperature and the velocity of the heated fluid discharged at the top of the enclosure for stratifying the enclosed region. It is found that the interface moves down as the Richardson number Ri, based on the conditions of the heated air entering at the top of enclosure, is increased. The stratification can be maintained essentially constant with time. Also, the temperature levels in the two zones can be varied over fairly wide ranges to simulate practical circumstances. The nature of such a stably stratified region is studied, particularly the effect of the inflow and outlet conditions.