Influence Of Initial Pressure Research Articles

Effect of initial pressure on the critical characteristics and overpressure of hydrogen-air premixed gas combustion and explosion

In order to ensure the safe utilization of hydrogen, the flame characteristics and explosion overpressure at lean-burn, stoichiometric and rich-burn conditions are investigated systematically by varying initial pressures. Experiments were conducted in a constant-volume combustion chamber using the Schlieren system to investigate the critical characteristics of hydrogen-air premixed flames at different equivalence ratios (from 0.2 to 6.5) and initial pressures (from 50 kPa to 150 kPa). The cellular instabilities of hydrogen-air premixed flames were interpreted and evaluated from the viewpoint of thermal-diffusive and hydrodynamic instabilities. The influence of initial pressure on the cellular instability and combustion and explosion characteristics were analyzed. The results show that the variation of thermal-diffusive instability dominantly governs the cellular instability at lean-burn conditions (φ = 0.3–1.0). In contrast, the cellular instability at rich-burn conditions (φ > 1.5) is governed by the variation of hydrodynamic instability. The increase of initial pressure promotes the cellular instabilities noticeably caused by the rise of the hydrodynamic instability. With the increase of P0, the Lewis number Le and thermal expansion ratio σ almost remain the same while flame thickness δ noticeably decreases. Increasing initial pressure at lean-burn and stoichiometric conditions (φ < 2.0) facilitates cellular instability, making the explosion more intense and resulting in higher explosion overpressure. In contrast, the variation of initial pressure suppresses the laminar combustion at rich-burn conditions (φ > 3.0).

Experimental study on the explosion characteristics of NH3/DME/air mixtures

Co-firing ammonia (NH3) fuel with dimethyl ether (DME) is a promising approach of achieving clean combustion. However, the fundamental explosion characteristics of NH3/DME blends have not been well understood. This paper presents an experimental study of the explosion characteristics of NH3/DME blends using a spherical constant-volume combustion chamber. The effects of equivalence ratio (ϕ = 0.7–1.5), initial pressure (Pu = 0.05–0.5 MPa), and DME fraction (XDME = 0–100%) on the explosions were examined. The results show that peak explosion pressure Pmax, maximum pressure rise rate (dP/dt)max, and deflagration index KG, vary non-monotonically with equivalence ratio, and reach peak values at around ϕ = 1.1–1.2. DME addition has a positive effect on explosion intensity. Both (dP/dt)max and KG show a linear increase trend with increasing DME, while the growth rate of Pmax decreases with DME fraction. The relatively higher increase in Pmax between 0 %DME and 20 %DME is mainly due to greater heat release. Pmax increases with increasing initial pressure, and this is because of the increase in mixture mass promoting the heat release, and the enhanced possibility of effective collision between molecules of gas mixtures. The influence of initial pressure on (dP/dt)max and KG depends on the equivalence ratio. Both (dP/dt)max and KG show a linear increase with increasing initial pressure at ϕ = 1.0 and 1.5, whereas they are insensitive to initial pressure at ϕ = 0.7. This is because the decrease in laminar burning velocity leads to a significant increase in explosion time at ϕ = 0.7 compared to ϕ = 1.0 and 1.5. In addition, the actual explosion pressure is closely associated with non-negligible heat loss during flame propagation, and the heat loss decreases with the addition of DME, but increases with increasing initial pressure.

Detonation behaviors in stoichiometric CH4-H2-O2 under different initial pressures conditions

In this study, the detonation behaviors (i.e., detonation limit, detonation overpressure, mean velocity, velocity deficit, and cell sizes) of CH4-H2-O2 mixtures in a small-bore tube at different initial pressures (ranging from 6 kPa to 300 kPa) were investigated. Mixtures with different proportions of H2 in the total fuel (0, 10%, 20%, 25%, 30%, 50%, 75%, 100%) were tested. Steady one-dimensional ZND calculations are used to obtain detonation parameters such as induction zone length, stability parameters, and cell sizes. Results indicate that increasing the hydrogen fraction decreases the detonation limit. The addition of hydrogen can reduce velocity deficit, especially near the detonation limit pressure. Velocity deficits were found to be within 3.5% when the initial pressure higher than 50 kPa. Higher initial pressure may result in the mean detonation velocity surpassing the theoretical CJ velocity. The detonation overpressure increases with the initial pressure and decreases with hydrogen fraction. The influence of initial pressure on cell sizes weakens as hydrogen fraction increases. Besides, the induction zone length and stability parameter decrease as hydrogen fraction increases Notably, the stability parameter of mixtures with different hydrogen fractions shows different trends as the initial pressure increases. A linear relationship between actual measured cell sizes and induction zone length is given as λ = 24.56Δi for hydrogen-methane-oxygen mixtures with the initial pressure range of 100 to 300 kPa. If experimental measurement error is considered, the prediction of the Ng model has reasonable accuracy.

Research on Characteristics of Hydrogen Dynamic Leakage and Combustion at High Pressure

Hydrogen is promoted as an alternative energy given the global energy shortage and environmental pollution. A scientific basis can be provided for the safe use and emergency treatment of hydrogen based on hydrogen leakage and combustion behavior. This study examined the stagnation parameters of dynamic hydrogen leakage and flame propagation in turbulent jets under normal temperatures and high pressure. Based on van der Waals’ equation of state for gas, a theoretical model for completely predicting stagnation parameters, outlet gas velocity, and flow rate changes in the process of high-pressure hydrogen leakage could be proposed, and the calculation result of this model was compared with the experimental result, with an error within ±10%. The progression and propagation of the flame in turbulent jets after ignition were recorded using the background-oriented schlieren image technology, and the propagation speed of flame from the ignition position downward and upward was calculated. Moreover, the influence of initial pressure, nozzle diameter, and ignition position on the flame propagation process and propagation speed was analyzed.

Investigation of the formation behaviors of CO2 hydrate in porous media below the freezing point

AbstractThe formation process of gas hydrate is a stochastic phenomenon considered to be significant for its application in the areas such as hydrate‐based technology and natural gas hydrate exploitation. The formation behaviors of CO2 hydrate were studied in porous media below the freezing point. In addition, the formation experiments were conducted in the high‐pressure vessel under different temperature and pressure conditions. The formation process of CO2 hydrate showed different formation behaviors in the porous media under different experimental conditions. The results showed that the formation rate was higher in earlier stage and the largest formation rate attained 7.18 × 10−4 mol h−1. The influence of initial pressure on the formation rate was indistinctive in porous media below the freezing point. Particle size of the porous media played an important role in affecting the formation characteristic of CO2 hydrate below the freezing point. The results indicated that, the smaller the particle size of porous media was, the larger the formation rate and the conversion rate of CO2 hydrate. The maximum conversion rate was attained 90.61%. The results also indicated that CO2 hydrate had a good gas storage capacity in porous media below the freezing point. The gas storage capacity was close to 160 L/L in frozen quartz sand with the particle size of 250 μm in the experiments. The relative results will provide some insight for sequestration and storage process of CO2 gas in the form of hydrates into sediments.

Experimental studies on gas dissipation during the coring process of shale

A series of experiments were performed to study the influence of initial pressures and pressure drop rates on the amount of lost gas during the coring process. The change laws of cumulative gas production and gas production rate were investigated. The proportion changes of lost gas, desorbed gas, and residual gas were analyzed. The results show that the cumulative gas production can be divided into three stages. There is a step with a relatively stable gas production rate in the gas production rate-time curve. The proportional change curves of the lost gas and desorbed gas have the same turning point.

Influence of Initial Pressure and Vessel’s Geometry on Deflagration of Stoichiometric Methane–Air Mixture in Small-Scale Closed Vessels

In the present work experimental data on closed vessel explosions of stoichiometric methane-air mixture are reported for mixtures at ambient initial temperature and various initial pressures between 0.2 and 1.2 bar, for small-scale closed vessels. The data are collected as pressure-time records in a spherical vessel and two cylindrical vessels with central ignition. The influence of initial pressure and vessel size and shape on maximum rates of pressure rise and deflagration (severity) indices is examined. In each explosion vessel a linear correlation of the maximum rate of pressure rise with the initial pressure is found. At all pressures, the rates of pressure rise and the severity factors reached in the spherical vessel exceed those reached in cylindrical vessels, as a result of differences in heat losses within these enclosures. The experimental deflagration indices are compared with the adiabatic deflagration indices, calculated by means of peak explosion pressures and laminar burning velocities; the...

Parametric studies on startup transients in multiple parallel channels of a natural circulation boiling system

The nature of startup transients in a natural circulation boiling system with multiple parallel channels are governed by the interplay between numerous geometric and operating parameters. Present work focuses on numerically ascertaining the relative influence of such variables, with primary objective being to identify possible options of suppressing concerned oscillations. A scaled-down version of AHWR, comprising of three parallel channels, has therefore been simulated using RELAP5. Both geysering and type-I instabilities have been observed in all three channels on the initiation of two-phase operation, commonly separated by a span of near-stable reversed flow in one of the channels. Adoption of higher initial pressure has been found to have a strong stabilizing effect, as it can completely eliminate geysering and can substantially shorten the duration of type-I instability. Increase in inlet loss coefficient and decrease in either core outlet or riser outlet loss coefficients can also subside type-I oscillations, without having any noteworthy effect on geysering. Channels with smaller diameter can also reduce both amplitude and duration of type-I fluctuations, albeit at the expense of a deteriorated steady-state flow rate. The effects of wall roughness and thermal capacitance have also been explored. The influence of initial pressure has been found to be significantly stronger than the others, particularly in removing the geysering instabilities. An augmented initial pressure is capable of neutralizing the system even when the channels are subjected to unequal power, thereby identifying itself as a very potent option for practical implementation.

Dynamic hazard evaluation of explosion severity for premixed hydrogen–air mixtures in a spherical pressure vessel

To evaluate dynamic explosion severity levels of premixed hydrogen–air mixtures, pressure sensors were used to test explosion pressure in a spherical pressure vessel (inner diameter: 0.34 m). ANSYS Fluent 19.0 three-dimensional software was used to simulate the explosion process. The results revealed that when the hydrogen volume fraction was 30 vol%, the explosion pressure and pressure rise rate reached maximum values at 1.0 atm. As the initial pressure increased, the explosion pressure, and pressure rise rate increased gradually. At initial pressures of 1.0, 1.2, 1.5, and 2.0 atm, the peak pressure levels were 0.85, 0.87, 0.92, and 0.99 MPa and the pressure rise rates were approximately 198, 241, 302, and 558 MPa/s, respectively. The initial pressure exerted greater effects on the pressure rise rate than did the explosion pressure. The simulation results were consistent with the experimental findings. Moreover, the simulation yielded multi-dimensional transient explosion parameters—such as turbulent kinetic energy distribution—that are difficult to obtain in experiments. The findings pertaining to the physical experiments, numerous simulations, and influence of initial pressure on the explosion reaction mechanism of premixed hydrogen–air mixtures were discussed.

Mass flow rate prediction of shale gas considering gas diffusion and water film evaporation

The difference between organic and inorganic matters in shale matrix has significantly influence on the transport capacity of shale gas and the total production of shale gas further. Previous studies have demonstrated that the organic pores are hydrophobic and the inorganic pores are hydrophilic. Therefore, the water film can be adsorbed on the walls of the inorganic pores. However, the evaporation of water film in inorganic pore is often overlooked due to the analytical challenge and the effect of the water film evaporation on gas transport capacity is rarely discussed. Thus, it is important to accurately predict mass flow rate of the shale gas through a single nanopore whether it is organic or inorganic. In this article, the mass flow rate prediction model of the organic pore is developed firstly, which considers the interactions of slip flow, gas adsorbed at pore walls, surface diffusion for adsorbed gas and gas diffusing from the kerogen which contains dissolved shale gas in organic pores at nanoscale. This model is then used to investigate the influences of initial pressure, pressure gradient, and thickness of kerogen on the gas mass flow rate. For inorganic pores, the mass flow rate prediction model is developed secondly and the interactions of water film and the evaporation of water film have been considered. The influence of evaporation, initial humidity, water–methane diffusivity is investigated on the gas mass flow rate. Results show that diffusion from kerogen plays an important role in the transport capacity of shale gas. Both higher pressure gradient and thicker kerogen can contribute to higher gas mass flow rate and higher diffusion from kerogen. Higher initial pressure and thicker kerogen can contribute to the lower shale gas desorption. The thicker kerogen can contribute to the higher surface diffusion. The evaporation in inorganic pore should not be ignored, because it impacts gas mass flow rate and can contribute to the higher flow rate. Experimental data was compared with our prediction model, which proves the correctness and validity of our model.

Parameter extraction from spherically expanding flames propagated in hydrogen/air mixtures

The characteristics of hydrogen/air flame were studied by using the spherical expanding flame propagated in a constant volume chamber. The influence of ignition induced blast wave and the flame instability on flame propagation was investigated. The nonlinear evaluation method for laminar flame parameter evaluation was established. By using the nonlinear evaluation method and the experimental results of flame propagation, the laminar flame speed and Markstein length were extracted and the difference between the nonlinearly evaluated laminar flame speed and the linearly evaluated one was analyzed. The influence of initial pressure and equivalence ratio on laminar flame speed and flame thickness was investigated. The laminar flame speed varies with equivalence ratio and initial pressure. There exists an equivalence ratio at which the laminar flame speed gets its maximum value. And there also exists an initial pressure at which the laminar flame speed gets its maximum value. The critical radius, Markstein length and flame instability of hydrogen/air flame with different equivalence ratio at different initial pressure had been studied. In hydrogen/air flame the flame stability decreases with the increase of initial pressure, while it increases with the increase of equivalence ratio. The global stability of flame is determined by the combination of the stabilizing effect of stretch effect, thermodiffusive instability mechanism and hydrodynamic instability mechanism.

The characteristics of flame propagation in ammonia/oxygen mixtures.

As a carbon free fuel and a hydrogen-energy carrier, ammonia is expected to be one of the promising energy carriers and to be widely used in industries. In this study, the parameters of ammonia/oxygen flame, such as laminar flame speed, the Markstein length, laminar flame thickness, and critical radius of flame instability onset have been investigated experimentally and numerically. A spherical ammonia /oxygen flame propagated in a constant volume chamber and a high speed digital Schilieren photograph system were used in the study. The influences of initial pressure and equivalence ratio on flame propagation have been investigated. It was found that the maximum laminar flame speed in ammonia/oxygen mixture is 1.09 m/s. The flame thickness decreases with the initial pressure increase. Generally, Markstein length increases with the increasing of equivalence ratio while it decreases with the increasing of initial pressure. The minimum critical radius in ammonia/oxygen is 1.8 cm. And the critical radius decreases with the increase of initial pressure. The mechanisms of flame instability in ammonia/oxygen mixture were analyzed, and the stabilize effect of flame stretch in ammonia/oxygen have been observed.

Influence of initial pressure on hydrogen/air flame acceleration during severe accident in NPP

Flame acceleration (FA) and explosion of hydrogen/air mixtures remain key issues for severe accident management in nuclear power plants. Empirical criteria were developed in the early 2000s by Dorofeev and colleagues, providing effective tools to discern possible FA or DDT (Deflagration-to-Detonation Transition) scenarios. A large experimental database, composed mainly of middle-scale experiments in obstacle-laden ducts at atmospheric pressure condition, has been used to validate these criteria. However, during a severe accident, the high release rate of steam and non-condensable gases into the containment can result in pressure increase up to 5 bar abs. In the present work, the influence of the unburnt gas initial pressure on flame propagation mechanisms was experimentally investigated. Premixed hydrogen/air mixtures with hydrogen concentration close to 11% and 15% were considered. From the literature, we know that these flames are supposed to accelerate up to Chapman-Jouguet deflagration velocity in long obstacle-laden tubes at initial atmospheric conditions. Varying the pressure in the fresh gas in the range 0.6–4 bar, no effects on the flame acceleration phase were observed. However, as the initial pressure was increased, we observed a decrease in the flame velocity close to the end of the tube. The pressure increase due to the combustion reaction was found to be proportional to the initial pressure according to adiabatic isochoric complete combustion.

Investigation on natural gas hydrate dissociation from a slurry to a water-in-oil emulsion in a high-pressure flow loop

Investigations of natural gas hydrate dissociation from a slurry to a water-in-oil emulsion are conducted in a high-pressure hydrate experimental flow loop equipped with on-line microcosmic instruments including a focused beam reflectance measurement (FBRM) probe and a particle video microscope (PVM) probe. Three stages of the dissociation processes are observed and sketched in a conceptual mechanistic diagram, including initial dissociation with agglomeration, dissociation with breakage and re-aggregation, and thorough dissociation. The influences of initial pressure, flow rate, and water cut on hydrate dissociation are discussed. The pressure drop is increased by hydrate particle agglomeration, which increases the risk of hydrate plugging; thus, hydrate systems require close monitoring during the hydrate slurry dissociation process. Based on the classical kinetics theory, an improved hydrate dissociation model is developed by introducing a coupled heat- and mass-transfer influence coefficient and by considering the changes in the hydrate dissociated area based on the chord length data recorded by the FBRM probe. The intrinsic rate constant for natural gas hydrate dissociation is within the range of 1.41 × 103–7.18 × 104mol·Pa−1·s−1·m−2, and the activation energy of dissociation ranges from 68.2 to 72.3 kJ·mol−1. The reported investigation is significant for the application of hydrate slurries in risk-management methods to address issues of hydrate flow assurance in deep-water systems.

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.

High-pressure anaerobic digestion up to 100 bar: influence of initial pressure on production kinetics and specific methane yields

ABSTRACTTo ensure an efficient use of biogas produced by anaerobic digestion, in some cases it would be advisable to upgrade the biogenic gases and inject them into the transnational gas grids. To investigate biogas production under high-pressure conditions up to 100 bar, new pressure batch methane reactors were developed for preliminary lab-scale experiments with a mixture of grass and maize silage hydrolysate. During this investigation, the effects of different initial pressures (1, 50 and 100 bar) on pressure increase, gas production and the specific methane yield using nitrogen as inert gas were determined. Based on the experimental findings increasing initial pressures alter neither significantly, further pressure increases nor pressure increase rates. All supplied organic acids were degraded and no measurable inhibition of the microorganisms was observed. The results show that methane reactors can be operated at operating pressures up to 100 bar without any negative effects on methane production.

Auto ignition of a nitrogen-based monofuel as a function of pressure and concentration

Conversion of hydrogen into a transportable and environmentally friendly chemical fuels can facilitate the implementation of renewable energy sources in our energy portfolio. In this paper we analyze a liquid mono-fuel composed of an aqueous solution of ammonium nitrate and urea (UAN), a commercial fertilizer commodity and a hydrogen carrier. In order to implement this fuel, additional information regarding its safety and combustion behavior is needed. We report on an important safety parameter of UAN, namely the auto ignition temperature (AIT). The AIT was measured in an original experimental system based on an ASTM standard under applied pressure at different water contents. The water content impacts the crystallization temperature which allows utilization of the fuel at different weather conditions. Specifically, the role of water in stabilizing and inhibiting the ignition is demonstrated, where ignition occurs in a gaseous phase after nearly complete water evaporation. Additionally, the influence of initial pressure on the ignition process was investigated. The AIT increases with pressure due to the increased water vaporization temperature. This study outlines the boundaries that support auto ignition and useful combustion of this fuel.

Experiment on the continuous recovery of H2 from hydrogenation plant off-gas via hydrate formation in tetra-n-butyl ammonium bromide solution

Here, an approach toward for continuous H2 recovery from hydrogenation plant off-gas based on hydrate formation in the presence of tetra-n-butyl ammonium bromide (TBAB) is proposed. Firstly, we explored the conditions to form the CH4/H2/TBAB hydrate through pressure search method in the single-stage equilibrium separation experiments toward CH4/H2 mixture. The influence of initial pressure, temperature, and initial gas–liquid volume ratio were systematically optimized in the context of 10 wt% TBAB solution. Then, we conducted the continuous separation experiments of CH4/H2 binary mixture and hydrogenation plant off-gas using a set of laboratory-scale continuous stirred reactor according to the optimized conditions, in which the effects of pressure and liquid flow rate upon continuous separation were further studied. The experimental results that the H2 concentration in the unhydrated gas (the recovered gas) is up to 89 mol% and the yield of H2 recovery is more than 58.68% indicate the efficiency to recover H2 using continuous hydrate separation, which lays the foundation for further industrial application.

The influence of initial pressure on the characteristics of conical bubble sonoluminescence

Based on a conical bubble U-tube, conical bubble sonoluminescence was investigated by using pure water as the working medium. Intense cavitation luminescence can be obtained. With the decrease in initial pressure inside the bubble, the intensity and duration of light emission increased. The spectrum is mainly composed of the spectral bands of H2O at the initial pressure of 1000 Pa. With the decrease in initial pressure, a broad continuum background spectrum that is well fitted by blackbody radiation can be detected, on which several spectral bands emitted by water molecules are superimposed. A higher temperature inside the bubble can be obtained with the decrease in initial pressure. Moreover, the intensity of the continuum background spectrum becomes more dominant compared with that of H2O emission bands. Finally, we conclude that blackbody radiation and molecular emission contribute to luminescence of conical bubble cavitation. Moreover, the initial pressure inside the conical bubble significantly affects the emission mechanism of conical bubble sonoluminescence.

Theoretical estimation of the lower flammability limit of fuel-air mixtures at elevated temperatures and pressures

We present an approach for predicting the lower flammability limits of combustible gas in air. The influence of initial pressure and temperature on lower flammability limit has been examined in this study. The lower flammability limits of methane, ethylene and propane in air are estimated numerically at the pressure from one to 100 bar and the temperature from ambient to 1200 K. It was found that the predicted LFLs of methane, ethylene and propane decrease slightly with the elevated pressure at the high temperature. The LFLs variation for methane-air mixture is 0.17, 0.18, 0.18 volume% with the initial pressure from one to 100 bar at the initial temperature of 800 K, 1000 K and 1200 K respectively, which is significantly higher than that at lower temperature. And the LFL of methane-air mixture at 1200 K and 100 bar reaches 1.03 volume% which is much lower than that at 1 bar and ambient temperature. On the other hand, the LFLs variation is 0.11–0.12 volume% for ethylene-air mixture and 0.06–0.07 volume% for propane-air mixture with the initial temperature from 800 K to 1200 K at the same range of pressure. The LFL values at high temperatures and pressures represent higher risk of explosion.