Outflow Face Research Articles

In vitro fluid dynamic characteristics of the Medtronic-Hall pivoting disc heart valve prosthesis.

The need for better low-profile mechanical valves led to the design and development of the Medtronic-Hall (formerly known as the Hall-Kaster valve) pivoting disc heart valve prosthesis in 1976. In vitro flow studies indicate that it has improved pressure drop characteristics compared to the Lillehei-Kaster and convexoconcave Björk-Shiley (60 degrees model) tilting disc valves. It does, however, have a somewhat larger regurgitant volume compared to the Björk-Shiley valve design. Velocity measurements with a laser-Doppler anemometer in the immediate downstream vicinity of the Medtronic-Hall valve indicate no region of stagnation near the outflow face of the disc. Regions of stagnation were, however, observed adjacent to the two titanium "pivot stops" situated on either side of the disc in the major orifice and along the pivot post in the minor orifice, together with a region of flow separation adjacent to the sewing ring of the minor outflow region. The results of the present in vitro study indicate a small but significant improvement in the overall fluid dynamic performance of the Medtronic-Hall valve, compared to the convexo-concave Björk-Shiley (60 degrees model) and Lillehei-Kaster tilting disc prostheses in current clinical use.

Approximation for Salt‐Water Intrusion in Unconfined Coastal Aquifer

ABSTRACTA numerical model based on a finite difference technique has been developed to analyse and solve the problem of an unconfined coastal aquifer of infinite extent with free surface and interface along with the width of outflow face as unknown boundaries. The problem has been formulated in a complex potential plane and a five‐point operator of a finite difference scheme has been used to reduce the governing flow equation into a set of linear algebraic equations which is solved by the Gauss‐Seidel iterative method with an over‐relaxation factor. Results of the analysis provide a generalised dimensionless plot for assessing the fresh‐water flow to the sea from the known field information of aquifer characteristics and water‐table head landward from the shoreline.

In vitro velocity measurements in the near vicinity of the Bjork-Shiley aortic prosthesis using a laser-Doppler anemometer

In this study,in vitro velocity measurements in the near vicinity of a Bjork-Shiley aortic valve, one of the more commonly used aortic valve prostheses, were made using a laser-Doppler anemometer. The velocity measurements identified a zone of stagnation, about 20 mm wide, immediately downstream from the fully open disc. The measurements also showed that the flow through the valve was divided into two unequal regions, namely, the major and minor outflow regions. Because of the low flow in the minor outflow region, the shear stresses along the perimeter of the valve in that region were considerably lower than the shear stresses along the sewing ring of the major outflow region. Pathologic studies of nine recovered Bjork-Shiley aortic valves indicated varying amounts of thrombus formation on the outflow face of the disc and excess growth of endothelial tissue along the perimeter of the minor outflow region. If the large stagnation zone and the relatively low shear in the minor outflow region which were observed in thein vitro measurements also existin vivo, they could lead to the clinically observed thrombus formation and tissue overgrowth, respectively.

Impaired function of cloth-covered Starr-Edwards mitral valve prosthesis. Detection by phonocardiography.

Malfunction of a cloth-covered Starr-Edwards mitral valve prosthesis (6300 series) was suspected because of phonocardiographic abnormalities. A prolonged interval from aortic closure to opening of the mitral ball suggested an impediment to movement of the ball. This was confirmed by catheterization and cinefluorography. At autopsy the impaired ball movement was found to be due to thrombus buildup on the outflow face of the mitral valve orifice. It is suggested that frequent postoperative phonocardiographic observations may be useful in detecting malfunction of valvular prostheses.

The Injection of a Hot Liquid into a Porous Medium

Abstract The injection of a hot liquid into initially cool porous media, saturated with the same liquid and porous media, saturated with the same liquid and surrounded by two impermeable but beat conducting media (cap and base rock), bas been studied both experimentally and theoretically. The temperature dependence of the viscosity was included in the theoretical model, but it was assumed that the specific heats and densities of the various materials were independent of the temperature. Solutions to the theoretical model were approximated by numerical methods. Both theoretical and experimental results indicate that centerline temperatures are significantly higher than boundary temperatures. Comparison of experimental and theoretical results with a cold/hot viscosity ratio of 19:1 were in reasonable agreement. Theoretical calculations show, that the effect of the temperature dependence of viscosity was very significant at ratios of 100:1 to 1000:1, which are typical of those that occur when injecting hot water to flood heavy oil reservoirs. Introduction We consider the problem of prediction of fluid flow and temperature distribution in an initially cold-fluid-filled reservoir on the injection of the same hot liquid by the use of mathematical and physical models. The results reported are for a physical models. The results reported are for a two-dimensional rectangular section of the reservoir, as shown in Fig. 1. The injection and withdrawal faces are assumed to be equipotentials for fluid flow. The ultimate purpose of such models would be to predict hot-water injection performance. However, we note that in the work presented here, one of the most significant aspects of the problem - the instabilities resulting from two-phase, water and oil flow - is not included. We will not give a historical review, but refer instead to the paper of Spillette and Nielsen, which contains a rather complete bibliography and critical discussion. The physical problem is exactly the same as Spillette and Nielsen, except for certain simplifications in our assumptions. The mathematical details are somewhat different, and we will present the details of our method here. The mass flow equation, which is elliptic in character, is handled by successive overrelaxation. The heat flow difficulty arose in the over-all heat balance due to by a straightforward explicit approximation. Some difficulty arose in the over-all heat balance due to small errors in the solution of the mass flow equation, and we feel that a different formulation of the heat flow equation would be desirable for future work. In addition to the mathematical solutions for the temperature distributions in a porous medium due to the injection of hot liquids, experimental data are presented to check the validity of these solutions. presented to check the validity of these solutions. A schematic diagram of the model is shown in Fig. 2. Certain qualitative physical conclusions were obtained from our numerical and experimental results These are:Assuming high (infinite) conductivity normal to the bedding plane in the reservoir is a poor approximation, and may lead to overestimates of the total heat losses (to cap and base rock) of as much as 50 percent.More heat is retained in the reservoir (per unit of heat injected) for higher viscosity changes.Any particular temperature isotherm moves more rapidly along the centerline for higher viscosity changes. Consequently, the approximation of temperature independent viscosity is not suitable for obtaining quantitatively correct results. MATHEMATICAL MODEL Our mathematical model is that of a reservoir of thickness 2h. The problem is idealized from that of a linear hot-water drive, and we imagine that the input face is sufficiently far away from the injection wells that the stream lines enter it normally. Similarly the production wells are far enough from the outflow face that the flow lines leave it normally. SPEJ P. 100

Three-Phase Relative Permeability Measurements by Unsteady-State Method

Abstract For the performance prediction of multiphase oil recovery processes such as steam stimulation, there is an acute need for three-phase relative permeability data. No fast and simple experimental technique, such as the unsteady-state method proposed by Welge for two-phase flow, is available for the three-phase flow. In this paper, an unsteady-state method is presented for obtaining three-phase relative permeability data; this method is as fast and easy as Welge's method for two-phase flow. Analytical expressions are derived by extension of the Buckley-Leverett theory to three-phase flow to express the saturation at the outflow face for all three phases in terms of the known parameters. It is assumed that the fractional flow and relative permeability of each phase are a function of the saturation of that phase. Other simplifying assumptions made include the neglect of capillary and gravity effects. The effect of saturation history upon relative permeability is acknowledged and attainment of similar saturation history in laboratory and field is recommended. The required experimental work and computations are simple to perform. The test core is presaturated with oil and water, then subjected to gas drive. During the test, required data are the rates of oil, water, and gas production, together with pressure drop and temperature. The ordinary gas-oil unsteady-state relative permeability apparatus can be readily modified to measure the required data. The proposed technique was applied to samples of a Berea and a reservoir core. The effect of saturation history upon relative permeability was studied on one Berea core. It was found that increase in initial water saturation has a similar effect upon three-phase relative permeability as it does in two-phase flow. Introduction In the light of increasing demand for three-phase, relative permeability data for predicting the performance of thermal and other multiphase-flow recovery processes, a simple and accurate method of experimental determination of such data is extremely desirable. Leverett and Lewis1 described the simultaneous flow method of obtaining three-phase relative permeability data. However, Caudle et al.2 reported that this method is very time consuming and cumbersome. Corey3 proposed calculating the three-phase relative permeability from measured krg data. Corey's theory is based on simplified capillary pressure curves,4 assuming a straight line relationship between 1/Pc2 and saturation. Also, Corey's method assumes a preferentially water-wet system. The simplest and quickest method of obtaining three-phase relative permeability data is the unsteady-state method where, for instance, oil and water are displaced by gas. However, in such a test the correlation of average saturation with relative permeability does not give a valid relationship because the rates of oil, water and gas flow in the sample change continuously from the upstream to downstream end. This difficulty in calculating valid relationships was solved by Welge for two-phase flow by deriving an expression from Buckley and Leverett frontal advance equations.5,6 In this paper, relations are established to determine the outflow face saturation and relative permeability to all phases in a three-phase flow displacement experiment. Proposed Method The fundamentals established by Buckley and Leverett5 for two-phase flow were extended to three-phase flow and used as a basis for the derivation of saturation equations. This approach is comparable to Welge's6 use of Buckley and Leverett theory in arriving at expressions to determine the outflow face saturation of the displacing fluid in a two-phase flow system.

Gas-Oil Relative Permeability Ratio Correlation From Laboratory Data

Abstract Gas-oil relative permeability ratio is an important relationship in oil reservoir predictive calculations. A correlation has been developed from 107 gas-flood krg/kro tests on Venezuelan core samples. The correlating parameter is based on restored-state water saturation tests and is applicable to both consolidated and poorly consolidated sandstone reservoirs. Data of the correlation show that there are no distinguishable differences between the mass-data groupings for the two classifications. A procedure is recommended for running sufficient relative permeability analyses to compute a geometric mean of the sample group. The geometric mean is more representative of the total core, and probably the entire reservoir. For example, while only one in four of the krg/Kro test curves agreed closely with the resultant correlation of this report, the geometric mean curves of the 16 suites (three samples or more) showed good agreement in three cases out of four. Introduction The gas-oil relative permeability, ratio is an important, fundamental relationship in most oil reservoir predictive calculations. Predictive calculations are made to estimate future reservoir production characteristics and ultimate oil recovery. The Krg/Kro relationship is specifically needed to relate the surface gas-oil ratio to the reservoir oil and gas saturation, and to calculate the relative movement of these phases within the reservoir whenever some of the more complex driving mechanisms are present. Laboratory Krg/Kro tests are not generally run as a routine analysis. Consequently, Krg/Kro data often are not available when needed because the cost of laboratory work could not be justified or the need for such data had not been properly anticipated. When laboratory Krg/Kro data are available, they are often very difficult to interpret. For example, wide divergence is sometimes shown in a family of Krg/Kro tests representative of the producing horizon in a single well. With these considerations in mind, a study was made to determine if a relationship might exist between the Krg/Kro curve and some other simple laboratory test criteria. The most probable Krg/Kro curve correlation for Venezuela described in this paper is the result of the investigation. The presented correlation defines the most probable gas-flood Krg/Kro curve through the medium of air-water capillary displacement and centrifuge water saturation tests. The laboratory procedures of these tests are relatively simple and inexpensive: test data should be widely available from routine analysis. DATA AVAILABLE, LABORATORY METHODS The report correlation utilized 107 gas-flood Krg/Kro tests run on sandstone cores of Venezuelan reservoirs. Table 1 is a general tabulation of data pertinent to the tests, while Table 2 summarizes the data. The tests include 96 from Western Venezuela and 11 from Eastern Venezuela. Eighty-two of the 107 test samples were sandstones that varied from poorly consolidated to unconsolidated; 25 were consolidated. The average sample porosity was 26.7 per cent and the average permeability was 1,121 md; these values typify the better sandstone reservoirs of Venezuela. The Welge gas-flood technique. based on fundamental Buckley-Leverett frontal displacement theory, was introducted in about 1952 and is widely accepted in the industry. The laboratory procedure is relatively simple, rapid, and can be performed on small core samples. While there have been some minor variations in sample preparation and laboratory procedure in the tests used for the correlation, these tests can be generally summarized as follows. The core sample was first solvent-extracted and dried. Connate-water saturation was restored by the oil-flushing or evaporation-blowdown methods. At the beginning of flood the hydrocarbon pore volume was completely saturated with the test oil phase. Unsteady-state gas-ail displacement then began with the injection of nitrogen or helium, while the displaced oil and gas phases were incrementally metered at the out-flow face. From the test data. the Krg/Kro curve was calculated by the Welge method. The individual oil and gas relative permeabilities were also calculated. CORRELATING PROCEDURES In attempting to establish a basis of correlation, we found that broad mid-range sections of 105 of the 107 Krg/Kro test curves could be closely duplicated by a straight line. Only two curves did not show a degree of linearity in this region. Correlation-curve definition parameters were subsequently developed from this observation of consistent mid-range linearity. Possible correlating variables were limited to the physical properties measured on core samples that were widely available as common test data and could be easily and cheaply obtained through future laboratory work. JPT P. 1111ˆ

A solution of the pattern of fresh-water flow in an unconfined coastal aquifer

Fresh-water steady flow in an unconfined coastal aquifer satisfies Laplace's equations in a domain, two of whose boundaries are free surfaces, the upper boundary being the interface between air and fresh water and the lower boundary being the interface between fresh water and salt water. A method of solution based on observations of the governing equation and boundary conditions in the complex potential plane is used. Solution of the two-free-surface problem is presented in a dimensionless form. The validity of the method of solution is tested by applying it to the problem of fresh-water pattern in a confined coastal aquifer which is a problem with a single free surface. The solution of the single-free-surface problem obtained by the present method agrees exactly with that published by others. Comparisons of the two problems confirm that the solution of the problem in a confined aquifer can be used satisfactorily for practical purposes as an approximate solution to the problem in an unconfined aquifer. If the ratio of specific weight of salt water to that of fresh water is assumed to be 1.025, the upper free surface is in error by an amount less than 1.3%; the lower free surface and the outflow face are in error by less than 2.6%.