Large Gas Fluxes Research Articles

Origin of Basaltic Subplinian Eruption at Shishaldin Volcano (Alaska): A Vigorously Degassing Magma Reservoir Hosting Small Bubbles

AbstractThe 1999 basaltic eruption of Shishaldin volcano (Alaska) displayed a transition between Subplinian and Strombolian activity. Strombolian bubbles indicate the presence of a periodically unstable foam at the top of magma reservoir. In contrast, a long foam, whose rupture led to the eruptive column, was also able to collect in the conduit. Laboratory experiments show that long foams can be produced in a conduit by the spreading of a stable foam accumulated at the top of the reservoir. The existence of a Taylor bubble at the onset of the Subplinian phase, also reproduced by my laboratory experiments, suggests that the foam in the reservoir was just at the transition between stable and unstable. This constrains the bubble diameter prior to the Subplinian phase to be 0.034–0.038 mm when using the foam dimensionless analysis and the underlying gas flux (0.52–0.80 m3/s). The increase in bubble diameter and potentially gas flux prior to the Strombolian activity, 0.81–1.4 m3/s, is sufficient to explain the foam in transition to be unstable. The radius of the magma reservoir is small, 200–210 m, as expected. The bubble diameter is the smallest of those estimated for classical basaltic eruptions (Etna, Kı̄lauea, Erta 'Ale), while the gas flux is among the largest. A dilute suspension of small and isolated bubbles cannot explain the large gas flux at Shishaldin. This implies numerous bubbles with a gas volume fraction ≥0.63−2%, a regime for which the bubbles form bubble clusters. The diameter of these bubble clusters, 3.0–5.4 mm, is sufficient to explain large gas fluxes.

Numerical study on the effect of reservoir heterogeneity and gas supply on hydrate accumulation in subsea shallow formations

Seepage-type gas hydrate accumulation in subsea shallow formations involves complicated thermo-hydro-solid coupling processes and matching problem between various accumulation elements. The formation physical properties control local natural gas migration pathway and thus the final reservoir characteristics of hydrates. In this paper, a novel mixed-flux model for gas hydrate accumulation is established and then used to simulate the process of methane gas migration into the shallow stratum to form a hydrate reservoir. The effects of reservoir heterogeneity and gas source conditions on the distribution of pore fluid and hydrate accumulation are examined. The simulation results show that reservoir heterogeneity is conducive to the retention and lateral migration of CH4 in a hydrate stability zone. CH4 can contact more pore water to form a large hydrate reserve, but the formed hydrate is often dispersed. Low-permeability layers enhance the trapping of CH4 and form a uniform and large hydrate saturation. Besides, gas source conditions have an important impact on the hydrate accumulation in reservoirs. Large gas flux, small pore water flux, continuous gas supply, high content of heavy components in natural gas, and numerous gas source points contribute to large amounts of hydrates generation in a certain time period. The presented work will deepen our understanding of the controls of natural gas hydrate systems in subea shallow formations.

Linear stability analysis of bubble-induced convection in a horizontal liquid layer.

We investigate with a linear analysis the stability of a horizontal liquid layer subjected to injection of gas bubbles through a bottom wall. The injection is assumed uniform in space and constant in time. Injected bubbles ascend in the liquid layer due to the Archimedean buoyancy force and are ejected from the top free surface of the liquid layer. Modeling this two-phase flow system as two interpenetrating liquid and gas continua, we show that homogeneous upward gas flows become unstable at large gas fluxes. We determine the critical conditions of this homogeneous-heterogeneous regime transition and show that the critical modes are made of stationary convection rolls, either multi- or whole-layered depending on liquid viscosity, the radius of bubbles, and the thickness of liquid layer. By examining the energy transfer from base to perturbation flows, we indicate that liquid convective motion is driven by the buoyancy on heterogeneously distributed bubbles. We also reveal that the lift forces on bubbles have significant stabilizing effects by homogenizing bubble distribution close to the bottom wall.

Initiation and Flow Conditions of Contemporary Flows in Martian Gullies.

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0–1.5 m and initial release volumes of 100–200 m3. Entrainment leads to final flow volumes that are ∼2.5–5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross‐channel flow velocities decrease from 3–4 m/s to ∼1 m/s from release area to flow terminus, while flow depths generally decrease from 0.5–1 to 0.1–0.2 m. The mean cross‐channel erosion depth and deposition thicknesses are ∼0.1–0.3 m. Back‐calculated dry‐Coulomb friction ranges from 0.1 to 0.25 and viscous‐turbulent friction between 100 and 200 m/s2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of ≪1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility.

PCO2 and CO2 fluxes of the metropolitan river network in relation to the urbanization of Chongqing, China

AbstractMost rivers of the world are supersaturated with carbon dioxide (CO2), resulting in a large gas flux that has only recently been included in global carbon budgeting. However, little is known about CO2 emissions from urban river networks suffering anthropogenic disturbance in the context of global urbanization. We surveyed the partial pressure of carbon dioxide (pCO2) and CO2 flux from 84 locations in the Chongqing metropolitan river network, with an area of 5494 km2. The overall mean pCO2 and CO2 fluxes were 2152 µatm and 163.0 mol C m−2 yr−1, respectively, with 318.9 mol C m−2 yr−1 from rivers in the completely urban area and 49.6 mol C m−2 yr−1 from the least urbanized area. The riverine pCO2 level increased with the proportion of urban land, with 2–4 times higher CO2 flux in the urban areas than the remote rural ones. Sites with low flow velocity, narrow channel, and factitious bottom sediment appeared to be local hot spots of CO2 emission. The pCO2 and CO2 degassing was positively correlated with the nutrients content of surface water (i.e., nitrogen and phosphorus). Carbon was released to the atmosphere as CO2 from Chongqing metropolitan river network at a rate of 12.3 × 109 mol C yr−1. pCO2 exhibited a clear seasonality with lower values in March 2015 and June 2015 but a higher value in September 2014, which was coregulated by temperature, flood dilution, and in situ photosynthesis. The results highlight that rapid urbanization, with increasing nutrients loading and anthropogenic activities, will alter the carbon biogeochemical cycle in the terrestrial‐aqueous‐atmospheric ecosystem and then impact global CO2 budgets.

Gas hydrate saturation at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough

AbstractThe degree of gas hydrate saturation at Integrated Ocean Drilling Program (IODP) Site C0002 in the Kumano Basin, Nankai Trough, was estimated from logging‐while‐drilling logs and core samples obtained during IODP Expeditions 314 and 315. Sediment porosity data necessary for the calculation of saturation were obtained from both core samples and density logs. Two forms of the Archie equation (‘quick‐look’ and ‘standard’) were used to calculate gas hydrate saturation from two types of electrical resistivity log data (ring resistivity and bit resistivity), and a three‐phase Biot‐type equation was used to calculate gas hydrate saturation from P‐wave velocity log data. The gas hydrate saturation baseline calculated from both resistivity logs ranges from 0% to 35%, and that calculated from the P‐wave velocity log ranges from 0% to 30%. High levels of gas hydrate saturation (>60%) are present as spikes in the ring resistivity log and correspond to the presence of gas hydrate concentrations within sandy layers. At several depths, saturation values obtained from P‐wave velocity data are lower than those obtained from bit resistivity data; this discrepancy is related to the presence of free gas at these depths. Previous research has suggested that gas from deep levels in the Kumano Basin has migrated up‐dip towards the southern and seaward edge of the basin near Site C0002. The high saturation values and presence of free gas at site C0002 suggest that a large gas flux is flowing to the southern and seaward edge of the basin from a deeper and/or more landward part of the Kumano Basin, with the southern edge of the Kumano Basin (the location of site C0002) being the main area of fluid accumulation.

Effects of gas-solid nonequilibrium in filtration combustion

To determine the effects of gas-solid nonequilibrium on forced filtration combustion (FC) waves, a two-temperature model is employed, with separate temperature fields for the solid and gas particles. We consider heterogeneous (solid/gas) combustion in a porous sample with a prescribed gas flux at the inlet. If the reaction is initiated at the inlet (outlet) of the sample and the combustion wave travels in the direction of (opposite to ) gas filtration it is referred to as coflow (counterflow) FC. We determine the effects of gas-solid nonequilibrium on various aspects of forced FC waves. First, we consider coflow FC waves, in which case the gas infiltrating through the hot product region significantly enhances the propagation of the combustion wave. For a relatively small gas flux, the infiltrating gas delivers heat from hot product to the combustion layer, thus increasing the combustion temperature and, hence, the combustion rate. Propagation of such waves is controlled by conduction of the heat released in the reaction to the preheat zone. Conductively driven coflow FC waves have been studied extensively using one-temperature models, which assume a very large rate of interphase heat exchange between the solid and the gas, so that thermal equilibrium is attained almost immediately. However, if the gas flux is sufficiently large, the phases do not have sufficient time to equilibrate and, hence, the underlying assumption of one-temperature models is no longer valid. One-temperature models are only appropriate for describing slowly propagating coflow FC waves in which the time of contact between the solid particles and the gas is sufficiently large for rapid thermal equilibrium to occur. However, not all coflow FC waves are slowly propagating. There can also be rapidly propagating coflow FC waves, in which case a two-temperature model, with the solid and the gas attaining distinct temperatures, is more appropriate. For a relatively large gas flux, an alternative mechanism of enhancement occurs in the combustion temperature is increased as a result of increasing the effective initial temperature of the unurned solid ahead of the reaction zone. Propagation of such waves is controlled by convection of heat stored in the product to the preheat zone. Convectively driven coflow FC waves depend on a pronounced temperature difference between the phases and, therefore, cannot be described by a one-temperature model. We employ a two-temperature model to study coflow FC waves. The two-temperature model must also account for the fact that the oxidizer concentration at the surface of the solid particles, where the reaction occurs, may be significantly less than the average oxidizer concentration in the gas stream. Effects of gas-solid nonequilibrium are determined by the interphase heat transfer rate, as well as the rate of oxidizer diffusion to the solid particle surface. In the appropriate limits, the model can describe both conductive and convective coflow FC waves. We determine steady solutions in each of the limits and compare and contrast the structures, the combustion characteristics, and the parameter dependences of the two modes of propagation. Transition between conductive and convective steady-traveling coflow FC waves is discussed. We determine how gas-solid nonequilibrium affects such features as the wave velocity, the extinction limit, net gas production or consumption in the reaction, and the ability of the wave to accumulate energy near the reaction site. The results of our analyses are verified by numerical computations of the time-dependent problem. Next, we consider the effects of gas-solid nonequilibrium on counterflow FC waves, in which case the solid near the reaction site loses heat to the cool infiltrating gas. Counterflow FC waves are necessarly driven by conduction because the convective mechanism relies on the gas to deliver heat to, rather than carry heat away from, the unburned solid fuel. It is known from one-temperature-model analyses that extinction of steady-traveling waves will occur if the gas influx is sufficiently large. However, when the gas flux is sufficiently large to cause extinction, the two phases may not have sufficient time to attain thermal equilibrium, in which case a two-temperature model is required. Though only conductively driven counterflow FC waves are possible, we find that thermal nonequilibrium has a significant effect on the intrinsic extinction limit and the combustion characteristics.

Rapid, upward buoyant filtration combustion waves driven by convection

A two-temperature model is employed to analyze upward buoyant filtration combustion (BFC) waves. We consider heterogeneous (solid/gas) combustion in a porous sample open to gas flow only at the top and the bottom. The reaction is initiated at the bottom of the sample and the combustion wave travels in the direction of gas filtration. There is a localized region of high temperature either ahead of or behind (depending on certain parameters) the combustion layer. Hot gas contained within this region rises due to gravity induced buoyant forces, thus drawing cool fresh gas containing oxidizer in through the bottom of the sample. We focus on the reaction leading structure, in which the reaction occurs at the leading edge of the heated portion of the sample. The gas infiltrating through the hot product region significantly enhances the propagation of the combustion wave. For a relatively small gas flux, the infiltrating gas delivers heat from the hot product region to the reaction zone, thus increasing the maximum burning temperature, and hence, the combustion rate. This is referred to as the superadiabatic effect. The propagation of such waves is controlled by the diffusion of heat released in the reaction to the preheat zone. Diffusively driven BFC waves have been studied extensively using one-temperature models. One-temperature models assume a very large rate of interphase heat exchange between the solid and the gas, so that thermal equilibrium is attained almost immediately. However, if the gas flux is sufficiently large, the solid and the gas do not have sufficient time to equilibrate, and hence, the underlying assumption of one-temperature models is no longer valid. That is, one-temperature models are only appropriate for describing slowly propagating BFC waves in which the time of contact between the solid and the gas is sufficiently large for rapid thermal equilibrium to occur. However, not all BFC waves are slowly propagating. There can also be rapidly propagating BFC waves, in which case a two-temperature model, with the solid and the gas attaining distinct temperatures, is more appropriate. For a relatively large gas flux, an alternative mechanism of enhancement occurs, in that the maximum combustion temperature is increased as a result of increasing the effective initial temperature of the solid. The propagation of such waves is controlled by the convection of heat stored in the product to the preheat zone. Convectively driven BFC waves depend on a pronounced temperature difference between the solid and the gas, and therefore, cannot be described with a one-temperature model. We employ a two-temperature model to study upward propagating BFC waves. In the appropriate limits, we consider both diffusively and convectively driven BFC waves, with the main interest lying in the latter, which requires that a two-temperature model be used. We analyze both modes of propagation and compare and contrast the results. The filtration of gas may be driven by various mechanisms. A process that is simpler than, yet similar to upward BFC, is forced forward FC, in which the incoming gas flux is fixed by an external source. In BFC, gas flux is induced by the combustion process and must be determined, whereas in forced forward filtration combustion (FC), the hydrodynamic description is reduced to prescribing the gas flux at the inlet of the sample. By comparing and contrasting convective upward BFC waves and convective forced forward FC waves, we determine the effects of a buoyancy driven gas flux, as opposed to a fixed gas flux, on convective FC waves.

Interfacial area density in bubbly flow

The interfacial area per unit volume is one of the key parameters in bubbly flow. Momentum, mass and energy transfer occur through the interface between the phases. The functionality of two phase reactors with bubbly flow depends mainly on these three transfer processes. Thus, the design process of a reactor requires the prediction of interfacial area density. In the present work a simple equation for the interfacial area density is derived from the population balance, taking into account the events of coalescence and bubble break-up for each bubble fraction. The system of partial integro-differential equations is simplified. Since the integrals in these equations complicate a numerical treatment. This reduces the balance to one single partial differential equation. An approximate analytical solution is given. If the resulting equation is applied to large gas fluxes, the instability of the coalescence process causes large bubbles and gas plugs to develop. From the instability the volume fraction of the large bubbles and gas plugs may be predicted. Additives may hinder the coalescence process. Experiments show that coalescence hindrance changes the coalescence kernel only by a factor. Calculations are done for bubble columns and vertical pipe flow.

Rapid Filtration Combustion Waves Driven by Convection*

The propagation of filtration combustion (FC) waves in a porous solid which reacts with a gaseous oxidizer flowing through its pores may be significantly enhanced by increasing the infiltrating gas flux through the hot porous product region. For relatively small flux, enhancement occurs by the superadiabatic effect, i.e., by increasing the maximum temperature (Tb ), and thus, the reaction rate W(Wb ) in the combustion front. Though convection of heat from the product region increases Tb , the mechanism of FC wave propagation is controlled by diffusion of the heat released in the reaction. An alternative mechanism of enhancement, which occurs for relatively large gas fluxes, and corresponds lo pronounced temperature nonequilibrium between the gas and solid phases, leads to an increase of the FC wave velocity without increasing the combustion temperature. The propagation of such waves is controlled by the convection of heat stored in the products, rather than by diffusion of the heat released in the reaction. Such waves may propagate if diffusion is minimal or even absent altogether, due, e.g., to poor contact between the particles comprising the porous matrix. Unlike conventional (diffusion driven) combustion waves, the combustion velocity of convection driven FC waves is not controlled by the maximum temperature Tb but rather by an intermediate temperature Ti , which launches the reaction. In this sense the situation is similar to that in detonation waves. Here. Ti , the temperature at which the rate of heat release in the reaction equals the rate of heat exchange between the gas and solid phases, is the temperature at which the reaction begins to self-accelerate.

Phase fractionation at South Eugene Island Block 330

Persistent gas flux can dissolve, remobilize and alter reservoired or migrating oil through a process of phase fractionation. Moving gas, when flowing through an oil, can dissolve large fractions of that oil. The composition of the oil dissolved in the gas is dependent on the pressure-temperature conditions of the oil and the fluid flow history of the basin. The composition of the residual oil can be interpreted to yield both the depth at which the oil fractionated and the volume of gas required to fractionate the oil. South Eugene Island Block 330 in the U.S. Gulf Coast is a hydrocarbon province that has recently experienced large gas fluxes. Some of the oils in the region show signs of progressive fractionation and remobilization by gas transport. For example, the oils are more aromatic and less paraffinitic than unfractionated oils of similar maturity from the same area. The altered oils are also depleted of light n-alkanes. We have developed a computer-based model of oil alteration based on a fluid phase equilibria algorithm to simulate progressive fractionation of oil by gas. Application of the model to the South Eugene Island Block 330 area shows that several of the oils in the area have compositions that are compatible with alteration caused by equilibrating with approximately 12 to 14 mol of gas per mol of oil (2 to 2.7 g of gas per g of EI oil). The oils appear to have fractionated at approximately the depths of their present reservoirs. The model has great potential to examine hydrocarbon fluids for evidence of past migration and mixing.

Numerical solution of a hyperbolic system of conservation laws with source term arising in a fluidized bed model

A model a gas fluidized bed is considered which leads to a hyperbolic system of conservation laws with a source term. The system is solved numerically by a second-order operator splitting technique based on a Roe approximate Riemann solver. Numerical experiments demonstrate the ability of the model to reproduce qualitatively the slugging phenomenon in the case when the bed is subject to a relatively large gas flux.