The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform
Yongqiang Yang - One of the best experts on this subject based on the ideXlab platform.
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A study on the Steady-State Flow behavior of the flashing-driven open natural circulation system
Applied Thermal Engineering, 2021Co-Authors: Xiaofan Hou, Jinrong Qiu, Zhongning Sun, Shiwei Yao, Xu Yang, Shiming Wang, Yongqiang YangAbstract:Abstract The Steady-State Flow behavior serves as an important indicator of a natural circulation system, since it directly reflects the Flow and heat transfer capacity of the system. Many studies have been conducted into the Steady-State behavior of high-pressure natural circulation, while a few have focused on the open natural circulation. The Steady-State Flow behavior of the open natural circulation is however more complex due to its low-pressure property, which increases the likelihood of flashing. The position of flashing front and the distribution of void fraction in the riser are never fixed. Furthermore, due to the nonlinear characteristic of flashing and nonzero integral result of accelerated pressure drop along the loop direction, it is difficult to obtain an accurate mathematical expression of the relationship between Flowrate and heating rate by using the integral of momentum conservation equations. Therefore, relevant theoretical formulations are inadequate and unavailable. In this paper, a theoretical formula to determine the Steady-State Flow behavior of a flashing-driven natural circulation system was derived by using a quasi-linear flashing mode and a special formula processing of accelerated pressure drop integral result. The prediction results of the formula were validated by experimental data. Furthermore, the influence of geometric and thermal-hydraulic parameters on the Steady-State Flow behavior was studied through calculation and validated by experimental data. This paper aims at providing technical support for the design of open natural circulation systems.
Zekâi Şen - One of the best experts on this subject based on the ideXlab platform.
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Steady State Flow with hydraulic conductivity change around large diameter wells
Hydrological Processes, 2010Co-Authors: Abdüsselam Altunkaynak, Zekâi ŞenAbstract:Darcian Flow law in aquifers assumes that the aquifer hydraulic conductivity is constant and the groundwater movement is due only to the piezometric level changes through hydraulic gradient. In practice, after the well development the aquifer just around the well has comparatively larger hydraulic conductivity and gradient. Patchy aquifer solutions in the literature consider sudden hydraulic conductivity changes with distance for the Steady State Flow. The change of transmissivity is demonstrated by the application of slope-matching procedure to actual field data. It is the main purpose of this paper to derive simple analytical expressions for aquifer parameter evaluations with steadily decreasing hydraulic conductivity around the well. Spatial nonlinear hydraulic conductivity changes around a large-diameter well within the depression cone of a confined aquifer are considered as exponentially decreasing functions of the radial distance. Copyright © 2010 John Wiley & Sons, Ltd.
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A graphical method for storage coefficient determination from quasi-Steady State Flow data
Hydrology Research, 1996Co-Authors: Zekâi ŞenAbstract:A simple, approximate but practical graphical method is proposed for estimating the storage coefficient independently from the transmissivity value, provided that quasi-Steady State Flow data are available from a pumping test. In the past, quasi-Steady State Flow distance-drawdown data have been used for the determination of transmissivity only. The method is applicable to confined and leaky aquifers. The application of the method has been performed for various aquifer test data available in the groundwater literature. The results are within the practical limits of approximation compared with the unSteady State Flow solutions.
Xiaofan Hou - One of the best experts on this subject based on the ideXlab platform.
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A study on the Steady-State Flow behavior of the flashing-driven open natural circulation system
Applied Thermal Engineering, 2021Co-Authors: Xiaofan Hou, Jinrong Qiu, Zhongning Sun, Shiwei Yao, Xu Yang, Shiming Wang, Yongqiang YangAbstract:Abstract The Steady-State Flow behavior serves as an important indicator of a natural circulation system, since it directly reflects the Flow and heat transfer capacity of the system. Many studies have been conducted into the Steady-State behavior of high-pressure natural circulation, while a few have focused on the open natural circulation. The Steady-State Flow behavior of the open natural circulation is however more complex due to its low-pressure property, which increases the likelihood of flashing. The position of flashing front and the distribution of void fraction in the riser are never fixed. Furthermore, due to the nonlinear characteristic of flashing and nonzero integral result of accelerated pressure drop along the loop direction, it is difficult to obtain an accurate mathematical expression of the relationship between Flowrate and heating rate by using the integral of momentum conservation equations. Therefore, relevant theoretical formulations are inadequate and unavailable. In this paper, a theoretical formula to determine the Steady-State Flow behavior of a flashing-driven natural circulation system was derived by using a quasi-linear flashing mode and a special formula processing of accelerated pressure drop integral result. The prediction results of the formula were validated by experimental data. Furthermore, the influence of geometric and thermal-hydraulic parameters on the Steady-State Flow behavior was studied through calculation and validated by experimental data. This paper aims at providing technical support for the design of open natural circulation systems.
Shlomo P. Neuman - One of the best experts on this subject based on the ideXlab platform.
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inverse stochastic moment analysis of Steady State Flow in randomly heterogeneous media
Water Resources Research, 2006Co-Authors: A. F. Hernandez, Alberto Guadagnini, Shlomo P. Neuman, Jesús CarreraAbstract:[1] Nonlocal stochastic moment equations have been used successfully to analyze Steady State and transient Flow in randomly heterogeneous media conditional on measured values of medium properties. We present a nonlinear geostatistical inverse algorithm for Steady State Flow that makes it possible to further condition such analyses on measured values of State variables. Our approach accounts for all scales of spatial variability resolvable by the computational grid. It is based on recursive finite element approximations of exact nonlocal first and second conditional moment equations. Hydraulic conductivity is parameterized geostatistically on the basis of measured values at discrete locations and unknown values at discrete “pilot points.” Prior estimates of pilot point values are obtained (optionally) by generalized kriging. Posterior estimates at pilot points and (optionally) at measurement points are obtained by calibrating mean Flow against measured values of head. The parameters are projected onto a computational grid via kriging. Maximum likelihood calibration allows estimating not only hydraulic but also (optionally) unknown variogram parameters with or without prior information about the former. The approach yields covariance matrices for parameter estimation as well as head and flux prediction errors, the latter being obtained a posteriori. We implement our inverse approach on highly and mildly heterogeneous media under superimposed mean uniform and convergent Flows in a bounded two-dimensional domain. Our examples illustrate how conductivity and head data act separately and jointly to reduce parameter estimation errors and to model predictive uncertainty. We also evaluate the functional form of the log conductivity variogram and its parameters using likelihood-based model discrimination criteria.
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Conditioning mean Steady State Flow on hydraulic head and conductivity through geostatistical inversion
Stochastic Environmental Research and Risk Assessment (SERRA), 2003Co-Authors: A. F. Hernandez, Alberto Guadagnini, Shlomo P. Neuman, Jesús CarreraAbstract:Nonlocal moment equations allow one to render deterministically optimum predictions of Flow in randomly heterogeneous media and to assess predictive uncertainty conditional on measured values of medium properties. We present a geostatistical inverse algorithm for Steady-State Flow that makes it possible to further condition such predictions and assessments on measured values of hydraulic head (and/or flux). Our algorithm is based on recursive finite-element approximations of exact first and second conditional moment equations. Hydraulic conductivity is parameterized via universal kriging based on unknown values at pilot points and (optionally) measured values at other discrete locations. Optimum unbiased inverse estimates of natural log hydraulic conductivity, head and flux are obtained by minimizing a residual criterion using the Levenberg-Marquardt algorithm. We illustrate the method for superimposed mean uniform and convergent Flows in a bounded two-dimensional domain. Our examples illustrate how conductivity and head data act separately or jointly to reduce parameter estimation errors and model predictive uncertainty.
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Three-dimensional Steady State Flow to a well in a randomly heterogeneous bounded aquifer
Water Resources Research, 2003Co-Authors: Alberto Guadagnini, Monica Riva, Shlomo P. NeumanAbstract:[1] We consider Flow in a confined aquifer of uniform thickness due to a well of zero radius that fully penetrates the aquifer and discharges at a constant rate. If the lateral extent of the aquifer is infinite, a Steady State Flow regime never develops. It is, however, well known that if the aquifer is additionally uniform, a quasi-Steady State region extends from the well out to a cylindrical surface whose radius expands as the square root of time. On the expanding surface, head is uniform and time invariant. Inside this surface, head at any time is described by a Steady State solution. A rigorous analysis of the analogous situation in a randomly heterogeneous aquifer would require the solution of a three-dimensional transient stochastic Flow problem in an aquifer of infinite lateral extent. Here we take a different approach by developing a three-dimensional Steady solution for mean Flow to a well in a randomly heterogeneous aquifer with a cylindrical prescribed head boundary. In analogy to the uniform case we expect our solution to approximate a quasi-Steady State region whose radius is initially small in comparison to the horizontal correlation scale of log conductivity but grows with time to become eventually much larger. We treat log conductivity as a statistically homogeneous random field characterized by a Gaussian spatial covariance function that may have different horizontal and vertical correlation scales. Our solution consists of analytical expressions for the ensemble mean and variance of head in the aquifer to second order in the standard deviation of log conductivity. It is based on recursive approximations of exact nonlocal moment equations that are free of distributional assumptions and so apply to both Gaussian and non-Gaussian log conductivity fields. The analytical solution is supported by numerical Monte Carlo simulations. It clarifies the manner in which relationships between the horizontal and vertical scales of the quasi-Steady State region and those of statistical anisotropy impact the statistical moments of drawdown and the equivalent and apparent hydraulic conductivities of the aquifer. Both conductivities are shown to exhibit a scale effect by growing with distance from the well within a radius of one to two horizontal integral scales from it.
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Three-dimensional Steady State Flow to a well in a randomly heterogeneous aquifer
IAHS-AISH publication, 2000Co-Authors: Monica Riva, Alberto Guadagnini, Shlomo P. Neuman, Daniel M. TartakovskyAbstract:We consider three-dimensional Steady State Flow towards a well that fully penetrates a randomly heterogeneous aquifer confined between horizontal no-Flow boundaries, and bounded laterally by a cylindrical, deterministically prescribed constant head boundary. The well is represented by a line sink that produces water at a deterministically prescribed constant rate Q for unit aquifer thickness. The log hydraulic conductivity, Y= lnK, of the aquifer is multivariate Gaussian, statistically homogeneous with a Gaussian spatial autocorrelation function. We develop an analytical solution for the variance of hydraulic head as a function of dimensionless vertical and horizontal locations within the aquifer, variance σ Y 2 of Y and dimensionless ratios between the principal spatial correlation scales. Our analysis is based on the non-local theory first proposed for Steady State and transient Flows in bounded, randomly heterogeneous media by Neuman & Orr (1993), Neuman et al. (1996), Guadagnini & Neuman (1999a,b) and Tartakovsky & Neuman (1998, 1999). In particular, we develop and solve analytically recursive closure approximations of the governing non-local moment equations to second order in σ Y by means of an appropriate Green's function. We evaluate our analytical solutions by means of Gaussian quadratures for the special case of the isotropic Y field.
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nonlocal and localized analyses of conditional mean Steady State Flow in bounded randomly nonuniform domains 1 theory and computational approach
Water Resources Research, 1999Co-Authors: Alberto Guadagnini, Shlomo P. NeumanAbstract:We consider the effect of measuring randomly varying hydraulic conductivitiesK(x) on one's ability to predict numerically, without resorting to either Monte Carlo simulation or upscaling, Steady State Flow in bounded domains driven by random source and boundary terms. Our aim is to allow optimum unbiased prediction of hydraulic heads h(x) and fluxes q(x) by means of their ensemble moments, 〈h(x)〉c and 〈q(x)〉c, respectively, conditioned on measurements of K(x). These predictors have been shown by Neuman and Orr [1993a] to satisfy exactly an integrodifferential conditional mean Flow equation in which 〈q(x)〉c is nonlocal and non-Darcian. Here we develop complementary integrodifferential equations for second conditional moments of head and flux which serve as measures of predictive uncertainty; obtain recursive closure approximations for both the first and second conditional moment equations through expansion in powers of a small parameter σY which represents the standard estimation error of ln K(x); and show how to solve these equations to first order in σY2 by finite elements on a rectangular grid in two dimensions. In the special case where one treats K(x) as if it was locally homogeneous and mean Flow as if it was locally uniform, one obtains a localized Darcian approximation 〈q(x)〉c ≈ −Kc(x)∇〈h(x)〉c in which Kc(x) is a space-dependent conditional hydraulic conductivity tensor. This leads to the traditional deterministic, Darcian Steady State Flow equation which, however, acquires a nontraditional meaning in that its parameters and State variables are data dependent and therefore inherently nonunique. It further explains why parameter estimates obtained by traditional inverse methods tend to vary as one modifies the database. Localized equations yield no information about predictive uncertainty. Our stochastic derivation of these otherwise standard deterministic Flow equations makes clear that uncertainty measures associated with estimates of head and flux, obtained by traditional inverse methods, are generally smaller (often considerably so) than measures of corresponding predictive uncertainty, which can be assessed only by means of stochastic models such as ours. We present a detailed comparison between finite element solutions of nonlocal and localized moment equations and Monte Carlo simulations under superimposed mean-uniform and convergent Flow regimes in two dimensions. Paper 1 presents the theory and computational approach, and paper 2 [Guadagnini and Neuman, this issue] describes unconditional and conditional computational results.
Sidney G. Green - One of the best experts on this subject based on the ideXlab platform.
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Simultaneous determination of two-phase relative permeability and capillary pressure of porous rocks from Steady-State Flow experiments: Accounting for gravitational forces and fluid compressibility
Journal of colloid and interface science, 1998Co-Authors: Vladimir S. Mitlin, John Mclennan, Sidney G. GreenAbstract:Abstract A methodology for simultaneously measuring two-phase relative permeability and capillary pressure of porous rock samples from Steady-State Flow experiments, reported by Virnovsky et al. in 1995, is considered. The method requires measuring pressure drops in individual fluid phases across a rock sample. This paper studies the role of gravitational forces, neglected in the original derivation of the method. Direct numerical simulations of Steady-State vertical Flow experiments, performed over a typical range of rocks and Flow parameters, show that the error caused by neglecting gravitational forces in the original method, is acceptable even for extremely high (∼10 −10 m 2 ) permeability samples. Thus, if multiphase Flow measurements are performed on vertical samples, the gravitational force can be neglected for estimating relative permeability and capillary pressure. An extension of this method to the case of compressible fluids is presented.
