Low Initial Pressure Research Articles

Particle behavior close to the lower wall of a tokamak type geometry during a loss of vacuum accident

The behavior of tungsten and beryllium particles close to the wall of a tokamak type geometry (i.e. a toroidal vacuum vessel) during a loss of vacuum accident (LOVA) is explored in this paper.Values of the flow field and temperature of the air surging into the torus were calculated by Computational Fluid Dynamics (CFD) simulations from an initial low pressure of 1000 Pa by Gelain et al., Fusion Engineering and Design, 100:87–99, 2015. A new CFD calculation is performed here for an initial low pressure of 500 Pa. The aerodynamic forces on particles in rarefied flow are derived from the calculated friction velocity and temperature in the lowest wall region. The focus is on spherical particles, but these values of forces are complemented for comparison with an estimate of the force on an elongated particle following Sentman (1961). Typical particles with diameters of 1, 2, 5, 10, 20 μm are considered. The possibility for particles which are detached from the lower wall to be entrained away the wall is explored. A particle may be detached by the flow field provided it escapes the adhesion force with the wall, which occurs only for a rough wall and large particles. It is then entrained away from the lower part of the wall provided its weight is smaller than the aerodynamic force, which occurs even at low pressure for all sizes of studied beryllium particles and only for specific sizes and a pressure above 1500 Pa for tungsten particles. This key influence of particle weight on the particle dynamics in rarefied gas flow is the focus of the paper. As a result, only particles of intermediate diameters may be resuspended and transferred away from the wall during the whole pressurisation sequence of the vessel. Using the data for the initial low pressure of 500 Pa, these particle diameters are 5, 10 μm for tungsten and 5, 10, 20 μm for beryllium.

Predictive role of central venous oxygen saturation in patients with acute type i respiratory failure

Background Global oxygenation can be indirectly measured by the measurement of venous oxygen saturation.Objective To estimate the value of central venous oxygen saturation (ScvO2) to predict the outcome in type 1 acute respiratory failure (ARF) patients admitted to the respiratory ICU and to assess its potential association with mortality.Patients and methods The study included 62 type 1 ARF patients admitted to respiratory ICU with indication for a central venous catheter. ScvO2 was measured from right atrium blood samples at admission, 3 and 7 days later. Arterial blood gas was assessed simultaneously. The patients were classified into three groups according to the ScvO2 level (at admission): high more than 75%, normal 65–75%, and low less than 65% (9, 34, and 19 patients, respectively). The best ScvO2 cutoff value associated with the mortality was considered the critical value of ScvO2. Acute physiology and chronic health evaluation (APACHE) II score severity, ICU, and hospital stay, and hospital mortality were reported.Results Sixty-two type 1 ARF patients with a mean age of 60±17.4 years were included. Seventy-nine percent patients had pneumonia. Mechanical ventilation was indicated in 77.4% of the patients with a mean duration of 6±3.6 days. The mean ScvO2 at day 0, 3, and 7 were 67.7±8.1, 66±11, and 69.3±9.3, respectively. The mean hospital stay was 6.8±4 days; 72.6% of the patients died in the hospital. Overall mortality in patients with low, normal, and high ScvO2 (at admission) were 42.2, 40, and 17.8%, respectively. Abnormal low and high ScvO2 was significantly related to mortality (P<0.0001). The cutoff point of ScvO2 at admission for mortality prediction was less than or equal to 65% (P=0.0008), 100 specificity, 49 sensitivity, 100 positive predictive value, and 42.5 negative predictive value. Mechanical ventilation, APACHE II score more than 27, development of complications, low initial arterial oxygen saturation and partial arterial oxygen pressure, the ScvO2 cutoff point less than or equal to 65% at admission, less than or equal to 70% at third and less than or equal to 66% at seventh day were significantly related to the mortality (<0.0001, <0.0001, <0.0001, 0.01, 0.03, 0.0008, 0.04 and 0.04, respectively).Conclusion In type1 ARF patients, abnormal low and high ScvO2 after day 0, 3, and 7, % of ScvO2 change in third and seventh day, and high APACHE II score were significantly linked to high mortality.Clinical implications In critically ill patients, ScvO2 is useful in the estimation of tissue oxygenation and may replace the use of mixed venous oxygen saturation.

Explosion characteristics of liquid fuels at low initial ambient pressures and temperatures

An experimental study on the explosion characteristics of diethyl ether-air, epoxypropane-air and n-pentane-air mixtures at various initial ambient pressures (57.4–101.3 kPa) and temperatures (263–293 K) was performed in a 20 L sphere explosion vessel with central ignition. The explosion pressures, maximum pressure rise rates, explosion indexes and flammability mass concentration limits of the three fuel-air mixtures were tested and analyzed. The explosion pressure of n-pentane-air mixture had the highest sensitivity to the initial ambient temperature and pressure. In terms of the maximum pressure rise rates and explosion indexes, the influences of the initial ambient pressure and temperature on the fuel-air mixtures decreased in the order of epoxypropane-air > n-pentane-air > diethyl ether-air. The epoxypropane-air mixture had the widest flammability mass concentration scale of 125–985 g·m−3.

Experimental study of detonation limits in methane-oxygen mixtures: Determining tube scale and initial pressure effects

In this paper, detonation limits in stoichiometric methane-oxygen mixtures with varying tube inner diameter and initial mixture pressure were investigated. Detonations in tubes with different inner diameter (D = 36 mm, 25 mm, 20 mm and 13 mm) and low initial pressure from 3.5 to 18 kPa were studied. Smoked foils were applied to observe the evolution of the detonation cellular structure for various initial conditions. An alternate length scale at the limits is examined, Ldcs, which is the maximum length from the beginning of the test section after which cellular patterns can no longer be observed. Simultaneous local velocity measurements were obtained by photodiodes to complement the Ldcs results. The study also aims to reveal relation between the near-limit detonation dynamics, the tube geometry, and the thermodynamic properties of the mixture. Past the failure limit, Ldcs decreases with decreasing initial mixture pressure for a given tube diameter, and Ldcs decreases faster in a smaller diameter tube. In the D = 13 mm tube, galloping detonation mode is observed, and the length of the galloping cycle is reduced with an increase in initial pressure. To further characterize the onset of detonation limits, a scaling analysis of Ldcs with tube inner diameter (D) and detonation cell size (λ) was performed. The experimental results show that the decrease of Ldcs/D and Ldcs/λ are more abrupt in smaller diameter tubes with decreasing initial pressure. At low initial pressure, the boundary layer displacement thickness growth is significant in the flow structure. Since the distribution of global curvature over the whole detonation front is faster in smaller tube, it thus leads to a more abrupt decrease sensitive to initial pressure. For increasing pressure closer to the critical failure limit, the boundary layer displacement thickness is becoming less comparable to the tube diameter. The failure mechanism appears to be more dominant by the rate of transverse waves attenuation or cell disappearance. Lastly, by comparing the detonation cell size and the tube scale at the critical limits condition in different tubes, λ = π⋅D is shown to be an appropriate limit criterion of detonation propagation in agreement previous studies.

Study on the effects of initial pressure and temperature on the explosion characteristics of DME-blended LPG mixtures in an obstructed confined pipeline

Numerical analyses were conducted to study the explosion characteristics of premixed LPG/DME/Air mixture in an obstructed confined pipeline of length 3 m and diameter 0.1 m under a wide range of initial pressures (100 to 300 kPa) and temperatures (−20 to 200 °C). The explosion overpressure, flame temperature and flame propagation along axial direction were investigated. The results indicated that the maximum explosion overpressure was raised significantly with increasing initial pressure, while decreased with the increase of initial temperature. Under lower initial pressure (P0 ≤ 150 kPa), there was an obvious increase in the peak flame temperature as the initial pressure increased. However, when the initial pressure exceeded 150 kPa, the effect of the initial pressure on the peak flame temperature was not significant. At the same time, however, the peak flame temperature increased almost linearly with the increase of initial temperature. The structure of flame front experienced a change of “spherical flame - finger flame - tulip flame” at all conditions. The change of flame velocity was also divided into three stages in the study, namely rapid growth, gradual decline, backward propagation. In the “rapid growth” stage, both the velocity and the acceleration of the flame increased with initial pressure and temperature rising, but the influence of the initial pressure on the flame velocity became weak when the initial pressure exceeded 200 kPa.

Clean and effective catalytic hydrolysis of bagasse waste to small-molecular compounds over a hydrothermally stable Ru/La(OH)3

Ruthenium/lanthanum hydroxide (Ru/La(OH)3), a hydrothermally stable catalyst, was prepared and used for catalyzing hydrolysis of bagasse, a sugar industry waste, to small-molecular compounds under low initial H2 pressure (IHP 0.2 MPa, 5% H2 in 4 MPa H2 and N2 mixture) at 240 °C. Neither oligomers nor char was formed and bagasse was almost completely converted into soluble portion (SPCH, 59.1 wt%) and gaseous products by the catalytic hydrolysis (CH). Obviously different from the methanol-soluble portion and ethanol-soluble portion from the bagasse methanolysis and ethanolysis, in total 104 organic compounds were detected in SPCH, among which alkyl-substituted phenols & benzenepolyols (38.8%), alkoxy-substituted phenols (22.2%), ketones (15.1%), and carboxylic acids (12.2%) are dominant according to the analysis with a gas chromatograph/mass spectrometer (GC/MS). Based on the analysis with a quadrupole exactive orbitrap mass spectrometer, heteroatom-containing compounds, especially oxygenates, are predominant in the SPs, while more species including smaller molecular fragments, can be identified in SPCH, which is consistent with the analysis with GC/MS. In addition, the catalytic hydroconversion of phenethoxybenzene over Ru/La(OH)3 further confirms that appropriately raising the temperature and reducing the IHP facilitate the formation and transfer of active H∙. In one-pot, clean and effective conversion of bagasse with low H2 consumption and in a green solvent can be achieved through this strategy.

Hydrate dissociation conditioned to depressurization below the quadruple point and salinity addition

Techniques have been developed to evaluate the hydrate dissociation behaviour in a brine system by reducing pressure below the quadruple point. Experimentally, hydrate with low initial pressure and temperature is dissociated by a deep depressurization method (DDM) in a one-dimensional (1D) model to evaluate the gas and water production. Theoretically, simulation techniques have been developed to determine the two-phase relative permeability, permeability variation, intrinsic hydrate reaction constant by piecewisely fitting the experimental measurements with consideration of the temperature depression induced by NaCl. Subsequently, the existing semi-empirical permeability reduction models are compared with the simulated average permeabilites to identify the occurrence of hydrate in pores. It is shown from experiments that salinity enhances the potential of gas recovery by enhancing the dissociation driving force (i.e., increasing equilibrium pressure and decreasing equilibrium temperature). Temperature decreases almost in a constant rate at sensible heat dominant stages, while, after reaching the freezing point, it is decreased lowly and even recovered slightly due to heat conduction from boundary and ice fusion. By approximating the energy contributions, it is found that sensible heat is supplied for hydrate dissociation at the initial stage and that heat conduction from boundary and ice fusion accelerates hydrate dissociation at the final stage. It is concluded that the salinity lowers the freezing point from −1.7 to −1.3 °C in the experiment. For numerical simulation, the newly developed dissociation model is found to be able to not only identify fluid flow mechanisms and kinetic hydrate dissociation, but also reproduce ice fusion. With hydrate dissociation and water production, salinity continuously decreases and finally becomes less than its initial value. The Kozeny grain center-filling model is found to be able to capture the permeability variation, implying that hydrate tends to be pore filling dominant in the experiments. The fluid flow and hydrate dissociation kinetics are found to play a dominant role above the freezing point, while, below the freezing point, the significance of heat conduction from boundary and ice fusion is enhanced during the experiments.

Investigation on the performance of closed-loop pulsating heat pipe with surfactant

A three-dimensional closed-loop pulsating heat pipe (CLPHP) charged with deionized water and surfactant is numerically investigated in present work. The surfactant used in present work is hexadecyl trimethyl ammonium chloride (CTAC). The start-up performance and heat transfer characteristics of CLPHP with deionized water and surfactant are compared by changing the initial pressure (0.1 MPa, 0.07 MPa, 0.05 MPa) and input heat load (10 W, 20 W, 30 W, 40 W). It’s found that the performance of CLPHP is affected by the initial pressure, surfactant concentration and input heat load jointly. Under the condition of high initial pressure, the performance of CLPHP with surfactant is not as good as that with deionized water. However, under the condition of low initial pressure, especially at higher input heat load, the superiority of the surfactant begins to appear. Compared with the CLPHP with deionized water, the thermal resistance could be decreased by 4.78% when the initial pressure, the input heat load and the surfactant concentration is 0.05 MPa, 40 W and 50 ppm, respectively. It is also found that the CLPHP with 2000 ppm CTAC has a much lower heat transfer performance than that with deionized water under any conditions. Finally, surfactant can improve the wettability of the CLPHP wall, reduce the contact angle, and absolutely prevent the occurrence of “dry-out”.

Comparison of base gas replacement using nitrogen, flue gas and air during underground natural gas storage in a depleted gas reservoir

ABSTRACT Base gas is considered as an important factor in the storage operation as it remains permanently in the reservoir and maintains the reservoir pressure along the production cycle. Depending on the reservoir under consideration, base gas may occupy as little as 15% or as much as 75% of the total underground gas storage (UGS) reservoirs. Providing and injecting the cushion gas has the most contribution to the cost of the storage operations. Therefore, part of the base gas can be replaced by a cost-effective gas such as nitrogen, flue gas or air to reduce the costs of the investment. Some degree of mixing takes place when two miscible gases come into contact with one another that affects the quality of the produced natural gas. Therefore, the process needs to be studied and controlled. In this study, the feasibility of underground gas storage and the substitution of the base gas by a cheaper gas, i.e., nitrogen, flue gas, and air, are investigated in a partially depleted dry gas reservoir with very low initial pressure. To do so, a comparative study is performed among nitrogen, flue gas and air as the alternative gases to the base gas. In addition, the effect of flue gas composition on the performance of base gas replacement and ultimate gas recovery is investigated. Pure CO2 is considered as flue gas with zero mole% N2. In the end, the effect of the reservoir properties on mixing between the gases is studied. The results indicated that it is possible to substitute 24.8% of the base gas by nitrogen to obtain a 16.2% increase in the gas recovery of the reservoir. In this case, the ultimate recovery reaches 50.90%. Using flue gas as the alternative gas, the results showed a 15.6% increase in the gas recovery of the reservoir, obtained by substituting 23.9% of the base gas. The ultimate recovery using flue gas is 50.31%. According to the results, flue gas can be used as an appropriate option to replace the base gas of the UGS reservoir under consideration, and hence, there would be no more need for separation and purification of N2 and CO2. Increasing the CO2 composition in the flue gas up to 46.6 mole% leads to a decrease in the base gas replacement amount. When the composition increases above 46.6 mole%, the amount of the replaced gas does not change. However, in this composition range, more flue gas is injected into the reservoir, which has environmental advantages. The highest injection rate of the flue gas is obtained when the flue gas contains 100 mole% of CO2. The main problem in using air as the base gas is the high viscosity of air which requires a high injection pressure. According to the results, using air as the replacement gas, 21.3% of the base gas is substituted by air. In this case, gas recovery increases by 13.9% with respect to the reservoir depletion scenario and the ultimate recovery reaches 48.62%.

Experimental diffusion coefficients of CO2 and H2S in some ionic liquids using semi-infinite volume method

The diffusivity of carbon dioxide and hydrogen sulfide was studied in five ionic liquids containing two different cations and four anions, namely, 1-ethyl-3-methylimidazolium (emim) based ionic liquids with bis-(trifluoromethyl)sulfonylimide (NTf2) anion and 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, (eFPT) as well as 1-(2-hydroxyethyl)-3-methylimidazolium (hemim) based ionic liquids (ILs) with different anions, viz. hexafluorophosphate (PF6), trifluoromethanesulfonate (OTf) and bis-(trifluoromethyl)sulfonylimide (NTf2). Experimental trials were carried out at temperatures of 303.15, 313.15 and 323.15 K and low initial pressures by means of semi-infinite volume method. The results show that the diffusion coefficients of CO2 and H2S in RTILs increase with increasing temperature, but with different orders, and the diffusivity of CO2 and H2S in (emim) based ILs is greater than that in (hemim) based ILs. In addition, the measured diffusion coefficients of CO2 and H2S have been fitted through Wilke-Chang empirical equation.

Abstract TP512: White Matter Lesion Volume Change and Antiplatelet Treatment in Small Vessel Disease; Quantitative Analysis From Picasso Study

Background: White matter lesion (WML) is associated with future stroke risk and cognitive decline. Though antiplatelet treatment strategy is important in the prevention of secondary stroke, the effect on WML volume change is not clarified yet. Methods: This study was performed by a post-hoc analysis of PICASSO study which compared the efficacy and safety of cilostazol versus aspirin in patients with small vessel disease. Among those who have a follow-up imaging data, change of WML volume was measured quantitatively and automatically. Segmented lesion maps of different time points were compared using the longitudinal pipeline. The absolute difference was compared between aspirin and cilostazol. Factors associated with the change of WML volume were analyzed. Result: Among those enrolled in PICASSO trial, 505 patients (254 in aspirin group and 251 in cilostazol group) was measured. WML volume decreased 0.5±11.6cc at follow-up. There was no difference in WML change between cilostazol and aspirin (-0.7±12.5cc vs. -0.2±10.6; p=0.60). Old age (r=0.12, p=0.006), diabetes (p=0.03) and longer follow-up period (r=0.11; p=0.012) was associated with increase of WML. Interestingly, the presence of diabetes was associated with increased WML in aspirin group (2.4±10.3cc vs. -1.5±10.6cc; p=0.006), but not in cilostazol group (-0.3±15.0cc vs, -0.9±11.4cc; p=0.7; p for interaction=0.15). Furthermore, WML increased in aspirin group significantly with low initial systolic blood pressure (r=0.20, p=0.02), but not in cilostazol group (p=0.75; p for interaction=0.01). Conclusion: Though there is no significant difference in WML change between cilostazol and aspirin treatment, factors associated with WML change may differ according to antiplatelet treatment.

Numerical simulation of the effects of evaporation on the n-heptane/air auto-ignition process under different initial air temperatures

Numerical simulations are carried out to study the effects of fuel evaporation and mixing on autoignition process of n-heptane/air with complex chemistry. By assuming that n-heptane spray evaporation and mixing are infinitely fast under initial air temperature Tair of 740–1200 K and pressure of 1–24 atm, homogeneous gaseous mixture of n-heptane/air with different equivalence ratio can be obtained. Three types of the most reactive mixture fraction ηMR are identified in mixture fraction space: ηMR,LTand ηMR,ig are associated with the shortest first-stage ignition by low temperature chemistry (LTC) and the total two-stage ignition delay time respectively, while ηMR,HT corresponds to the shortest single-stage ignition delay time by the high temperature chemistry (HTC). With the increasing initial air temperature Tair, ηMR,ig and ηMR,LT will increase, but the corresponding ignition delay times decrease. When Tair is high enough to make the temperature of gaseous mixture higher than the upper turnover temperature of NTC region in very lean mixture, ηMR,HT will appear with shortest ignition delay time. Moreover, the ignition delay time at ηMR,HT could be less than that at ηMR,LT in the spray autoignition process under high initial air temperature Tair=1200K and low initial pressure, 1 atm and 6 atm. This finding suggests that in spray autoignition process, due to the cooling effect of evaporation, LTC reaction and HTC reaction may coexist if the interval between the ignition delay times of ηMR,HT and ηMR,LT is quite small, and also that the high temperature ignition may precede the low temperature ignition. In addition, sensitivity and reaction pathway analysis are performed to disclose the controlling chemical kinetics at the three types of the most reactive mixture fraction ηMR of the case with high initial air temperature 1200 K and pressure 6 atm.

Diffusivity and solubility of carbonyl sulfide and sulfur dioxide in 1-ethyl-3-methylimidazolium bis (trifluoromethyl) sulfonylimide ([emim][Tf2N]): Experimental measurement and modelling

The diffusivity and solubility of carbonyl sulfide at temperatures from (313.15 to 353.15) K and pressures up to about 1.0 MPa and also the solubility of sulfur dioxide at the same temperatures and low pressures ranges in ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl) sulfonylimide ([emim][Tf2N]) were experimentally investigated. The solubility was measured using a volumetric-base static apparatus and acid gas diffusivity was obtained using the Semi-infinite volume method with low initial pressure. Using the solubility data, Henry’s law constants at different temperatures, and the partial molar thermodynamic properties of solution, that is, Gibbs energy, enthalpy, and entropy were calculated. The solubility data were correlated using 1) the generic Redlich–Kwong (GRK) cubic equation of state approaches and 2) the extended Henry’s law combined with the Pitzer’s virial expansion for the excess Gibbs energy. Finally using solubility data regression has been done with the generic Redlich–Kwong (GRK) cubic equation of state, the precision of experimental solubility data has been inspected via thermodynamic consistency test.

Experimental study on spray cooling under reduced pressures

Spay cooling is a complicated flow and heat transfer process affected by multi-factors among which the environmental pressure is extremely important. However the influence of pressure is not investigated sufficiently, especially the reduced pressure. In the present study, spray cooling under low initial environmental partial pressures and vapor partial pressures with R21 are investigated with a closed spray and condensation system. To study the influence of initial environmental partial pressure, different amounts of nitrogen are inflated into the vacuum flash chamber, while the vapor partial pressure is kept constant. To study the influence of vapor partial pressure, a cascade refrigerator is used to condense the vapor with different condensation temperatures so that the vapor partial pressure can be maintained or adjusted, while the initial environmental partial pressure is kept constant. The experimental results show that the spray cooling power increases monotonically with the increasing spray flow rate in the experimental range, while the cooling efficiency decreases with the increasing spray flow rate. The spray cooling power and cooling efficiency vary with the initial environmental partial pressure or the vapor partial pressure non-monotonously, which indicates there is an optimal pressure for the heat transfer performance. Besides, the mechanism of the non-monotonous variation trend is discussed based on the key aspects including flash evaporation, convection and boiling. Especially, the boiling heat transfer curve is applied to explain the trend.

Coupled Experimental and Computational Approach for CABRI Power Transients Analysis

CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute and operated by Commissariat a l’Energie Atomique et aux Energies Alternatives at the Cadarache Research Center. It is designed to study fuel behavior under reactivity-initiated accident conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber (3He) situated in the so-called transient rods inside the reactor core. The CABRI reactivity injection system allows us to generate structured transients based on specific sequences of depressurization. For such transients, the time difference between the openings of two valves of the reactivity injection system has an important impact on the power pulse shape. A kinetic point code, SPARTE, was developed in order to replace the older DULCINEE code dedicated to the modeling and prediction of CABRI power transients. The SPARTE code includes new models of 3He depressurization based on CFD calculations, variable Doppler coefficient based on Monte Carlo calculations, and variable axial neutron flux profile. The density and Doppler models have a large impact on power transient prediction. For low initial pressure transients, the major uncertainty comes from the reactivity injected by the 3He depressurization. For high initial pressure transients, the 3He heating during the power pulse (“TOP effect”) is responsible of an additional injection of reactivity that needs to be modeled precisely.

Explosion characteristics of a methane/air mixture at low initial temperatures

In this paper, the explosion characteristics of a methane-air mixture at low initial temperatures (up to 113 K) and elevated pressures were measured. Various parameters, such as explosion pressure, rate of the explosion pressure rise, combustion duration and flame development duration were obtained for the initial temperature range of 123–273 K. The results indicated that the maximum explosion pressure and the maximum rate of explosion pressure rise reached peak values at the equivalence ratio of 1.1. With the increase in initial pressure, the maximum explosion pressure increased significantly, and the maximum rate of explosion pressure rise increased linearly because of a larger density of the flammable mixture within the explosion vessel. With the decrease in the initial temperature the maximum explosion pressure increased monotonically, whereas the effect of initial temperature on the maximum rate of explosion pressure rise was negligible because of two opposing aspects: an increase in the amount of flammable mixture and a decrease in the flame propagation speed. Both the combustion duration and the flame development duration increased with the increase in initial pressure and decreased with the increase in initial temperature. Finally, at an equivalence ratio of 1.0, two empirical correlations were obtained to calculate the combustion duration and the flame development duration.

On the determination of laminar flame speed from low-pressure and super-adiabatic propagating spherical flames

Abstract The outwardly propagating spherical flame (OPF) method is popularly used to measure the laminar flame speed (LFS). Recently, great efforts have been devoted to improving the accuracy of the LFS measurement from OPF. In the OPF method, several assumptions are made. For examples, the burned gas is assumed to be static and in chemical equilibrium. However, these assumptions may not be satisfied under certain conditions. Here we consider low-pressure and super-adiabatic propagating spherical flames, for which chemical non-equilibrium exists and the burned gas may not be static. The objective is to assess the chemical non-equilibrium effects on the accuracy of LFS measurement from the OPF method. Numerical simulations considering detailed chemistry and transport are conducted. Stoichiometric methane/air flames at sub-atmospheric pressures and methane/oxygen flames at different equivalence ratios are considered. At low pressures, broad heat release zone is observed and the burned gas cannot quickly reach the adiabatic flame temperature, indicating the existence of chemical non-equilibrium of burned gas. Positive flow in the burned gas is identified and it is shown to become stronger at lower initial pressure. Consequently, the LFS measurement from OPF at low pressures is not accurate if the burned gas is assumed to be static and at chemical equilibrium. For super-adiabatic spherical flames, the burned gas speed is found to be negative due to the local temperature overshoot at the flame front. Such negative speed of burned gas can also reduce the accuracy of LFS measurement. It is recommended that the direct method measuring both flame propagation speed and flow speed of unburned gas should be used to determine the LFS at low pressures or for mixtures with super-adiabatic flame temperature.

Relevance of analytical Buckley\u2013Leverett solution for immiscible oil displacement by various gases

To generate the valid numerical simulation model, the sufficient amount of gathered data from the oil field is required. However, it is not always possible to acquire such data at the initial stage of project development. Buckley and Leverett (Pet Trans AIME 146(01):107–116, 1942) developed the analytical solution allowing to easily assess the oil displacement efficiency. One of the main assumptions of this model is incompressibility of oil and injected fluid. For slightly compressible water and oil such assumption is rational. However, that is not always the case when the gas is injected. This research aims to identify the conditions at which the usage of the incompressible gas model is appropriate. Likewise, the cases when the model of compressible gas is required are also evaluated. To accomplish the goals of this research, the comparative analysis between the injection of compressible and incompressible gases was undertaken using the numerical solution of the correspondent reservoir engineering problems. The validation of the numerical model was performed showing that it matches the analytical Buckley–Leverett solution. The findings of this research indicate that the relative and absolute density change with the pressure of the injected gas has the profound impact on the convergence between two models under consideration. With the increase in the injection pressure, the discrepancy between the models of compressible and incompressible gas raises for all the considered injection fluids (CO_2, CH_4 and N_2). Due to a steep slope of density–pressure curve for CO_2 at low initial reservoir pressure, the incompressible model cannot accurately predict the oil displacement efficiency by this gas at any reasonable injection pressure. All one-dimensional results are also representative for two-dimensional simulations (2D). However, the mismatch between two models increases considerably for 2D simulation scenarios. This study might be beneficial for those considering or researching the possibility of immiscible gas flooding by various gases at the particular oil field. Knowing some basic reservoir and technological parameters, the presented results might be used as simple screening criteria allowing to estimate the relevance of analytical Buckley–Leverett solution.

Ignition Delay Time for Hydrogen–Silane–Air Mixtures at Low Temperatures

A modified physicomathematical model of ignition of hydrogen–silane–air mixtures is applied to calculate the ignition delay time for these mixtures at low initial temperatures (300–900 K) and pressures (0.4–1 atm) of the mixture. It is shown that the diagram of the ignition delay time as a function of temperature contains a region of the so-called negative temperature coefficient. The influence of the pressure in the mixture and of the silane fraction on the length of this region is studied. It is found that an increase in both factors (silane concentration and pressure in the mixture) leads to an increase in the length of the negative temperature coefficient region.

Study on reservoir characteristic and sedimentary facies of Chang 8 Oil-bearing Formation in Wuqi-Dingbian area, Ordos Basin

Recent few years great breakthroughs have been made in oil exporation activities at Chang 8 Oil-bearing of Yanchang Formation in southwest and south-mid of Ordos basin, several hundred million geological oil reserves have been confirmed. Therefore, it is of great value to study reservoir characteristic and sedimentary facies at the Chang 8 Oil-bearing of Yanchang formation in Ordos basin. Taking Wuqi-Dingbian area in the middle of the west Ordos basin as an example, reservoir petrological characteristics, pore structure characteristics and physical property distribution characteristics are studied, based on routine thin section, casting thin section, particle size analysis, scanning electron microscope, mercury injection and other laboratory analysis data. On basis of petrofacies characteristics of drill cores and analysis of well logging electrofacies, it is studied by means of comparison profile of joint well sedimentary facies and distribution regulations of sandy land ratio. The results of the study show that reservoir characteristics studies have shown that the rock of Chang 8 reservoir of the study area is mainly composed of lithic feldspathic sandstone. Cementing materials is mainly composed of the authigenic clay mineral mainly consisting of kaolinite, chlorite and illite and carbonate cements consisting of ferrocalcite, calcite and dolomite. Pore types of the reservoir develops mainly intergranular dissolved pore. The result of the mercury penetration experiment suggest that the reservoir has a characteristic of low initial expulsion pressure and high median expulsion pressure. Chang 8 oil-bearing formation in Wuqi-Dingbian area develops frontal deposition of shallow water terrace type deltas, which were sourced from northwest, northeast and southwest, of which deltas frontal depositions of northwest and northeast sources are dominated. Distributions of subaqueous distributary channels of deltas front in different directions have the characteristics of becoming width and sheet like overiding along river mouth.