Wide Range Of Pressure Conditions Research Articles

Understanding AP/HTPB composite propellant combustion from new perspectives

Combustion of the AP/HTPB composite propellant at the mesoscale is simulated by a partitioned numerical framework, in which the unsteady solid-phase conduction, gas-solid interface regression, and gaseous combustion are sequentially calculated on two sets of overlapped grids. The gas-solid interface is traced by the level-set method. The mass and species exchange conditions at the gas-solid interface are converted into source terms by the immersed-boundary method. Effectiveness of the entire framework is justified by applying the integrated solver to a Miller pack example. The predicted burning rates show good agreement with experimental data over a wide range of pressure conditions, and the gas-phase temperature field clearly depicts the final diffusion flame. The flame zone closely above the regressing interface can be identified by the fluctuation of the projected velocity components perpendicular to the main stream direction. Statistical results conditional on Lagrangian tracing of fluid parcels released from the AP and binder parts show distinction between the oxidizer-rich and fuel-rich flames in terms of the reaction intensity and the flame structure. Statement of SignificanceFrom the algorithm/simulation perspective: a partitioned numerical solution framework is built upon overlapped structured (for the gas phase) and unstructured (for the solid phase) meshes; the level-set approach is used on the unstructured mesh to identify solid-gas interface with complex geometry. From the analysis perspective: the flame front is newly defined in terms of the tangential velocity components; the fuel/oxidizer evolution pattern is analyzed from conditional statistics based on Lagrangian tracing; new insights in heterogeneous combustion include, for instance, effects of pressure on the tangential velocity fluctuation and flame properties conditional on different ingredients.

The behaviour of a low- to medium-density chalk under a wide range of pressure conditions

Experiments are described which provided the basis for advanced numerical modelling of large-scale axial and lateral pile tests undertaken chalk to assist the design of offshore wind and other projects in northern Europe. The research explored the mechanical behaviour of chalk from a UK research site under effective cell pressures up to 12.8 MPa. When sheared from low confining pressures the chalk’s interparticle bonds contribute a large proportion of the peak deviator stresses available to specimens that crack, bifurcate and dilate markedly after failing at relatively small strains. Progressively more ductile behaviour is seen as pressures are raised, with failures being delayed until increasingly large strains and stable critical states are attained. Loading invokes very stiff responses within the chalk’s (Y1) linear elastic limits and behaviour remains stiff, although non-linear, up to large-scale (Y3) yield points. Near-elliptical Y1 and Y3 yield loci can be defined in q-p′ stress space and a critical state v-p′ curve is identified. The chalk’s initially bonded, high porosity, structure is explored by normalising the shearing and compression state paths with reference to both critical state and intrinsic compression lines. The results have important implications for pile test analysis and practical design in this challenging geomaterial.

An accurate correlation for calculating natural gas compressibility factors under a wide range of pressure conditions

The natural gas compressibility factor is a vital parameter in petroleum engineering calculation. Currently, the calculation models for calculating natural gas compressibility factors in engineering are not accurate and efficient enough. To solve this problem, multivariate nonlinear regression is used to fit the 6988 data of the Standing–Katz chart, a novel empirical formula of natural gas compressibility factors is obtained, which is suitable for the pseudo-reduced pressure range of 0.2 to 30. The mean absolute error, mean relative error and root mean square error between the fitted values and the Standing–Katz chart values are 0.01140, 0.01282 and 0.01663 respectively, hence the fitted values accord well with the chart values. The proposed correlation is verified by 270 measured data of natural gas compressibility factors under low and medium pressure conditions and 219 measured data under high pressure conditions and compared with the other five representative calculation models. The verification result shows that the mean absolute error, mean relative error and root mean square error between the calculated values and the measured values are 0.01962, 0.01626 and 0.02511 respectively, and the proposed correlation is superior to the other five methods because of its higher calculation accuracy. The new correlation is practical and accurate for predicting the natural gas compressibility factor under an extensive range of pressure conditions.

Effect of pressure on the heat transfer of pool fire in a closed chamber.

A combined experimental and analytical study was performed to determine the effect of pressure on the heat transfer of pool fire in a closed chamber. A series of ethanol pool fires with nominal diameters from 4 cm to 10 cm were conducted for a wide range of pressure conditions in between ~ 60 kPa to 250 kPa. Considering the effect of pressure, a theoretical formulation was proposed to estimate the radiation flux received by the pool surface from the chamber wall and hot gases. The relationship between the modified mass burning flux of pool fire and pressure was obtained. It was found that external radiation heat transfer received by pool surface depends significantly on gas temperature and its radiative properties and wall temperature. Results showed that these parameters are highly coupled in determining the effect of pressure to the external radiation. Furthermore, the average mass burning flux modified by the obtained external radiation was fitted with pressure by a power function and the fitted exponent value was consistent with theoretical analysis.

Combustion stability of inverse oxygen/hydrogen coaxial jet flames at high pressure

Turbulence-chemistry interaction of inverse gaseous oxygen/hydrogen coaxial jet flames in a test model combustor at elevated and high pressures (up to 23.6 bar) has been studied experimentally. Since it is difficult to experimentally determine the Damkohler number (Da), an important parameter representing the turbulence-chemistry interaction, for turbulent nonpremixed flames, a new method to estimate Da using the flame visualization by OH planer laser induced fluorescence and OH∗ chemiluminescence is suggested. The proposed modified Da that is based on the flame base location where the maximum OH∗ intensity is observed and the mean OH layer thickness provides the limits for the occurrence of combustion instability leading to local flame extinction. Thus, it is found to be useful for representing the regimes that involve the local flame extinction, particularly when considering a wide range of pressure conditions where thermochemical and transport properties simultaneously vary and thus complicated phenomena are observed. The combustion instability is observed only for Da ≤ 0.4, and it is found that the OH layer thickness generally decreases, the disconnected OH reaction zone is reduced, the occurrence of the local flame extinction is suppressed and the maximum value of flame surface density is enhanced with increasing combustion pressure.

Measurement of methane solubility in pure water in equilibrium with hydrate by using high–pressure optical capillary cell

The solubility of methane in aqueous solution in equilibrium with hydrate is an important control on the crystallization and dissolution of methane hydrate. Accurate calibration of Raman spectroscopic measurements of dissolved CH4 with the newly developed Unsaturated Homogenized Solution Method makes possible more accurate measurement of methane solubility across a wide range of pressure conditions. In this study, methane solubilities in pure water in equilibrium with methane hydrate in the absence and presence of a vapor phase were experimentally determined by Raman spectroscopy at temperatures from 273.15 to 293.15 K and pressures from ~2.6 to 120 MPa. The results show that the solubility of CH4 under H–Lw equilibrium increases with temperature at constant pressure but decreases slightly with increasing pressure at constant temperature. The pressure effect on CH4 solubility in water at H–Lw equilibrium becomes more significant with the increase of temperature. Both the isobaric relationship between ln(mCH4) and 1/T, and the isothermal relationship between ln(mCH4) and P are linear, indicating that the enthalpy change can be considered constant at fixed pressure over the temperature range of 273.15–293.15 K, and the volume change at a given temperature is constant for the methane hydrate dissolution reaction. The values of ΔV range from 9.3 to 12.0 cm3 per mole of hydrate at temperatures between 273.15 and 293.15 K. The Gibbs free energy change is 3.99 to 7.81 kJ∙mol−1 at temperature range from 273.15 K to 293.15 K and pressure range from 10 to 120 MPa. The enthalpy and entropy changes of the methane hydrate dissolution reaction over the studied T–P range are 41.52 to 48.12 kJ∙mol−1 and 0.1282 to 0.1476 kJ∙K−1∙mol−1, respectively. The values increase slightly with increasing pressure.

Extended validation of a developing post-dryout heat transfer correlation over a wide range of pressure conditions

Abstract The present paper deals with the validation of a post-dryout heat transfer correlation, which has been proposed by Nguyen and Moon (2015), to show its extended applicability to developing post-dryout region. It is also shown that the well-known film-boiling look-up table (LUT) method currently having limited application to fully-developed flow can be improved to extend its applicability to the developing post-dryout region. In order to show that the correlation can be utilized for nuclear reactor safety analysis, eight different sets of post-dryout data covering flow conditions both in both large break loss-of-coolant accident (LB LOCA) and prolonged station blackout (SBO) scenarios have been assessed. It turns out that the correlation predicts well the measured wall temperature with a total average error of 3.81% and a root-mean-square error of 13.46% in case of given CHF (dryout) condition. The prediction accuracy of the correlation is strongly influenced by how well the local vapor temperature and the CHF locations can be predicted.

Estimation of asphaltene precipitation in light, medium and heavy oils: experimental study and neural network modeling

Asphaltene can precipitate in oil reservoirs as a result of natural depletion and/or gas injection crippling the oil production performance. Most of the conventional models for asphaltene precipitation cannot precisely capture the asphaltene precipitation at a wide pressure range and for different oil types. To have a precise model that can be used for various oil types at a wide range of pressure conditions, a comprehensive artificial neural network (ANN) model was proposed to estimate the weight percent of precipitated asphaltene in different oil types (three oil types, namely light, medium and heavy). The dilution ratio, pressure, molecular weight of solvent, API gravity and resin-to-asphaltene ratio were considered as the model input parameters. The oil samples were thus categorized based on the differences in their API gravity and resin-to-asphaltene ratio. Five hundred and fifty experimental precipitation datapoints were obtained from our experimental apparatus in a wide range of pressure, dilution ratio and injected fluid molecular weight, and used to make a comprehensive databank for model calibration and verification. At the test stage, the coefficient of correlation (R2) was higher than 0.98 and mean square error was less than 0.04 indicating the good performance of the proposed model. Furthermore, a comparison between the prediction of ANN model and two types of alternative approaches, namely the thermodynamic and the fractal/aggregation approaches, was performed. For this purpose, the prediction of two of the widely used solubility models, Flory---Huggins and Modified Flory---Huggins and also a polydisperse thermodynamic model was compared to the prediction of the proposed ANN model. In addition to those, as a fractal/aggregation model, a scaling model was also selected and employed to compare its performance against that of the proposed ANN model. The ANN model showed a better performance as compared to the other conventional models. The results demonstrated that the proposed model provides acceptable prediction for different oil types over a wide range of pressure which is a difficult task for most of the conventional techniques.

Prediction of compressibility factor for gas condensate under a wide range of pressure conditions based on a three-parameter cubic equation of state

The gas compressibility factor as an essential thermodynamic parameter is often used to analyze PVT behavior in natural gas engineering. To accurately predicate it for various gas condensates, a data base containing 916 data sets covering a wide range of experimental conditions is employed to establish and test the prediction model based on a three-parameter cubic equation of state (EoS). The presented model which based on EPT EoS combines with Elliott–Daubert binary interaction coefficients correlation, Ahmed et al. splitting and Hosseinifar–Jamshidi characterization methods is superior to conventional empirical correlations with three mixing rules and two-parameter EoSs as SRK, PR, PRSV and MPR2. Statistical error analysis shows that it outperforms empirical correlations with average absolute relative errors of 1.45% and coefficient of determination of 0.989. At pressure and temperature up to 95.04 MPa and 429.3 K, respectively, the model also outperforms two-parameter EoSs with average absolute relative errors of 1.65% and coefficient of determination of 0.992. The presented model is efficient and practical for predicting the compressibility factor of gas condensate.

Navier–Stokes Simulation of Graphite Nozzle Erosion at Different Pressure Conditions

Graphite is commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in solid rocket motors. Because of the extremely harsh environment in which these materials operate, they are chemically eroded during motor firing, with a resulting performance reduction. The objective of the present work is to study the thermochemical erosion of graphite nozzles under a wide range of pressure conditions for both metallized and nonmetallized propellants. Numerical simulations are performed to reproduce recent literature experimental tests for nominal chamber pressures up to 250 bar. The adopted approach relies on a validated full Navier–Stokes flow solver coupled with a thermochemical ablation model that takes into account finite-rate heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary layer, ablation species injection in the boundary layer, heat conduction inside the nozzle material, and variable multispecies thermophysical properties. Results show that numerical simulations are able to correctly evaluate the erosion rate, provided that modeling aspects such as entrance length and combustion efficiency are suitably accounted for.

Impact of K and Ba promoters on CO2hydrogenation over Cu/Al2O3catalysts at high pressure

CO2 hydrogenation over K and Ba promoted Cu/Al2O3 catalyst was systematically investigated to study the promoter effects in a wide range of pressure conditions. The catalysts prepared by the impregnation method were characterized by XRD, physisorption, N2O-pulse chemisorption, H2-TPR, and CO2-TPD techniques. The catalytic performance was evaluated using a fixed-bed microreactor for a pressure and temperature range of 0.40–36 MPa and 443–553 K. The influence of promoters on the formation of surface species present during the reaction was examined by in situ DRIFTS. As expected from thermodynamics, high pressure and low temperature are the favourable conditions to achieve high selectivity to methanol over the Cu/Al2O3 catalyst. Improved reaction performance towards methanol synthesis and reverse water-gas shift (RWGS) reaction was observed for the Ba and K promoted Cu/Al2O3 catalysts, respectively. Notably, with the Ba promotion the selectivity to methanol was enhanced to 62.2% compared to 46.6% of the unpromoted Cu/Al2O3 catalyst at 10 MPa and 473 K at the expense of a lowered CO2 conversion. In contrast, the K promoted catalyst exhibited high selectivity to CO (95.8%) under the same reaction conditions. Formation of dimethyl ether, significant over the unpromoted Cu/Al2O3 catalyst at 0.4–10 MPa, was strongly suppressed at 36 MPa. Ba and K promoters effectively suppressed the formation of dimethyl ether under all examined pressure conditions by weakening the acidity of the alumina support. The strong promotional effects of K was explained by the predominant coverage of both Cu and alumina surface sites, creating specific active sites stabilizing surface intermediate species and preferring the RWGS pathway. On the contrary, the Ba promoter covers the alumina surface exclusively and renders Cu accessible and more easily reducible, promoting methanol synthesis. The effects of promoters on the catalytic performance were found to be valid at low and at elevated pressures.

Thermodynamic properties of methane/water interface predicted by molecular dynamics simulations

Molecular dynamics simulations have been performed to examine the thermodynamic properties of methane/water interface using two different water models, the TIP4P/2005 and SPC/E, and two sets of combining rules. The density profiles, interfacial tensions, surface excesses, surface pressures, and coexisting densities are calculated over a wide range of pressure conditions. The TIP4P/2005 water model was used, with an optimized combining rule between water and methane fit to the solubility, to provide good predictions of interfacial properties. The use of the infinite dilution approximation to calculate the surface excesses from the interfacial tensions is examined comparing the surface pressures obtained by different approaches. It is shown that both the change of methane solubilities in pressure and position of maximum methane density profile at the interface are independent of pressure up to about 2 MPa. We have also calculated the adsorption enthalpies and entropies to describe the temperature dependency of the adsorption.

SOFT COMPUTING TECHNIQUES FOR PREDICTION OF BOILING HEAT TRANSFER COEFFICIENT OF LIQUIDS ON COPPER-COATED TUBES

In this article pool boiling heat transfer coefficient (HTC) of liquids (isopropanol, methanol, and distilled water) on copper-coated heating tubes over a wide range of pressure conditions is computed experimentally. The objective is to find the applicability of soft computing techniques, swarm-intelligence based neural network, and adaptive fuzzy models in the prediction of boiling HTC. The results are compared with those computed experimentally. The performance of models for prediction of HTC is analyzed in terms of root mean square of prediction error. The minimum/maximum value obtained by zero-order fuzzy model with six membership function is 0.0023/3.4383 among all the liquids considered. The model is found to predict HTC with a maximum error of ±0.5% for boiling of liquids over all the coated tubes with pressure varying from atmospheric to subatmospheric levels. The study shows an excellent agreement between the experimental and predicted data.

Determination of Heat Transfer Coefficient in Pool Boiling of Organic Liquids Using Fuzzy Modeling Approach

An adaptive fuzzy model to determine pool boiling heat transfer coefficients of saturated organic liquids on plain heating tube surface is presented in this article. For this, an experimental investigation for saturated pool boiling of alcohols (methanol and isopropanol) is carried out over a wide range of pressures on a copper heating tube surface. With the availability of measured/computed input–output data of alcohols corresponding to the variables of heat flux and heat transfer coefficient, respectively, fuzzy concepts are used to develop an adaptive fuzzy model. This model is able to identify the heat transfer coefficient of alcohols under wide range of pressure conditions with a considerable error variation of ± 0.5%. The competitive identification performances of the adaptive zero-order fuzzy model having two and six membership functions are compared with the MATLAB-based ANFIS toolbox. Use of the fuzzy modeling approach has resulted in closer approximation to the experimental data as compared to empirical correlation data. The modeling generalization performance is verified on known data samples corresponding to different surface–fluid combinations.

An Integrated Catalytic and Transient Study of Sulfated Zirconias: Investigation of the Reaction Mechanism and the Role of Acidic Sites in n‐Butane Isomerization

AbstractThe transient temporal analysis of products (TAP) pulse method has been applied to investigate the isomerization of n‐butane on sulfated zirconias at very low pressure. By combining these results with findings from XRD, XPS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature‐programmed desorption (TPD), and catalytic studies at atmospheric pressure, a wide range of pressure conditions can be accessed to study the influence of surface sulfate groups on the isomerization activity and the mechanism of this reaction. The catalytic activity of powder catalysts has been correlated to the surface site density of sulfates, with pyrosulfate structures playing a major role in the initiation of the reaction. The complex interplay between sulfate‐free and sulfate‐covered zirconia surfaces for the adsorption of alkanes has been investigated in detail, with TAP pulse experiments showing a monomolecular reaction pathway at low n‐butane partial pressures. Moreover, TAP pulse experiments have allowed detection of the reaction products of this initiation process, including butene and water, and have shown their influence on the deactivation of sulfated zirconias. Heats of adsorption have been calculated from the TAP pulse experiments.

Effect of the Axial Heat Flux on the Critical Heat Flux in Low Flow Conditions with Vertical Annuli

An experimental study on the critical heat flux (CHF) has been performed for a water flow in vertical annuli with uniform and chopped cosine axial heat flux distributions under low flow and a wide range of pressure conditions. The effect of an axial heat flux distribution on the critical power is large at low-pressure conditions, but the effect decreases rapidly as the pressure increases. The ‘overall power’ hypothesis becomes more accurate as the pressure and critical quality increase, and the ‘local conditions’ hypothesis is not valid at high quality conditions. For a fixed mass flux and pressure, the relationship between the critical quality and the boiling length is represented by a single curve for the whole range of the present experimental conditions, regardless of the axial heat flux distributions. Smolin et al.'s F-factor and Bowring's Y parameter show good prediction results for the present CHF data.

An experimental study on the critical heat flux for low flow of water in a non-uniformly heated vertical rod bundle over a wide range of pressure conditions

An experimental study of the critical heat flux (CHF) has been performed for a water flow in a non-uniformly heated vertical 3 × 3 rod bundle under low flow and a wide range of pressure conditions. The experiment was especially focused on the parametric trends of the CHF and the applicability of the conventional CHF correlations to a return-to-power conditions of a main steam line break accident whose conditions might be a low mass flux, intermediate pressure, and a high inlet subcooling. The effects of the mass flux and pressure on the CHF are relatively large and complicated in the low pressure conditions. At a high mass flux or a low critical quality, the local heat flux at the CHF location sharply decreases with an increasing local critical quality. However, at a low mass flux or a high critical quality, the local heat flux at the CHF location shows a nearly constant value regardless of the increase of the critical quality. The CHF data at the very low mass flux conditions are correlated well by the churn-to-annular flow transition criterion or the flow reversal phenomena. Several conventional CHF correlations predict the present return-to-power CHF data with reasonable accuracies. However, the prediction capabilities become worse in a very low mass flux of below about 100 kg/(m 2 s).