Low Pressure Gradient Research Articles

Performance Characteristics and Energy Loss Analyses of a High-Speed Centrifugal Pump with Straight Blades

Along with the rapid growth of cutting-edge petrochemical technology and the pressing demand for efficiency improvement, evaluation of the performance characteristics of high-speed pump is becoming increasingly important. In this paper, numerical simulation is presented on the flow instability of a 16 straight-blade high-speed centrifugal pump with flow rate of 3 m3/h and rotating speed of 8500 rpm. Combined with the analysis of flow stability, the entropy production method is introduced to evaluate regions of high mechanical energy loss and its distribution at different flow rates. Results show that approximately 96% of the energy loss of the pump is produced in the volute, gap, and front and back chambers. Large energy loss is observed near the trailing edge of the blade and volute tongue, which are caused by the small region including both the high and low pressure gradients and large momentum exchange by the flow separation, respectively. Moreover, the rotor–stator interaction causes much energy loss at the wall of the volute and front and back chambers. Owing to the circumferential pressure gradient and the 90° leading edge of the straight blade, the fluid tends to form counter-rotating recirculation vortices. The large number of blades narrows the passage and limits the formation of large vortices in flow channels, thus the backflow phenomenon seems not to worsen with the rise of flow rates. Hence, the entropy production in most of the flow parts are insensitive to flow rates.

Low-pressure chromatographic separation and UV/Vis spectrophotometric characterization of the native and desialylated human apo-transferrin

Low-pressure pH gradient ion exchange separation provides a fast, simple and cost-effective method for preparative purification of native and desialylated apo-transferrin. The method enables easy monitoring of the extent of the desialylation reaction and also the efficient separation and purification of protein fractions after desialylation. The N-glycan analysis shows that the modified desialylation protocol successfully reduces the content of the sialylated fractions relative to the native apo-transferrin. In the optimized protocol, the desialylation capacity is increased by 150 %, compared to the original protocol provided by the manufacturer. The molar absorption coefficients in the near-UV region for the native and desialylated apo-transferrin differ by several percent, suggesting a subtle dependence of the glycoprotein absorbance on the variable sialic acid content. The method can easily be modified for other glycoproteins and is particularly appropriate for quick testing of sialic acid content in the protein glycosylation patterns prior to further verification by mass spectrometry.

Multi-centre cross-sectional study on vascular remodelling in children following successful coarctation correction.

Coarctation of the aorta is an arteriopathy with life-long sequelae, with remarkably increased cardiovascular events in young adults even after successful repair and despite blood pressure status. There are data on arterial remodelling in adults after coarctation correction, however, these data are scarce in childhood. Thus, the aim of this cross-sectional study was to evaluate changes in arterial wall function and morphology in children following successful coarctation repair and to compare these changes among patients with different blood pressure status and coarctation correction modes. Blood pressure status, echocardiographic parameters, arterial wall structure and stiffness, endothelial function and central blood pressure measurements were evaluated in 110 children aged 6-18 years following successful coarctation repair with right arm blood pressure not exceeding leg blood pressure by ≥20 mmHg. The prevalence of arterial hypertension was 50%. The mean carotid intima-media thickness SDS was 3.1 ± 1.5 and above 1.65 SDS in 91 of 110 patients. Increased right carotid intima-media thickness was associated with left ventricular hypertrophy, office blood pressure difference between leg and right arm, recoarctation in the past and interventional coarctation correction. Increased local common carotid artery stiffness was associated with increased pulse pressure and central systolic blood pressure. Potentially decreased endothelial function was related to a slight increase of peak and mean systolic gradient in the descending aorta. After successful coarctation repair and with a low blood pressure gradient, children still have a high prevalence of arterial hypertension and significant arterial remodelling, indicating accelerated biological age and advanced arteriosclerosis.

Pressure propagation and flow restart in a pipeline filled with a gas pocket separated rheomalaxis elasto-viscoplastic waxy gel

A clogged waxy crude oil pipeline, entails a high pressure to de-structure the gelled oil and resume its normal operation. Our earlier work, which utilized generalized Newtonian fluid based model, demonstrated that the presence of a gas pocket in-between the gel plugs, delays pressure propagation in the downstream gel plug [L. Tikariha, L. Kumar, Pressure propagation and flow restart in the multi-plug gelled pipeline, Journal of Fluid Mechanics. 911 (2021)]. This delay in pressure propagation is effectively utilized for sequential gel degradation. In the present work, we consider gel as a rheomalaxis elasto-viscoplastic (REVP) fluid, capable in predicting flow stoppage and pressure signal stoppage. The governing equations for mass, momentum balance, and phase equation are solved together with the REVP rheology model. The propagation of pressure in a REVP gel-filled pipeline is directly related to the gel material's elastic response. For a REVP gel, the flow-restart in a gelled pipeline depends on the conditional statement γ>γs (i.e. mean gel deformation overcoming the yield strain). In the case of a single gel plug, no-pressure propagation is predicted at intermediate compressibility, whereas pressure propagation without flow restart is predicted at low compressibility. In these cases, the presence of a small gas pocket instigates the gel breakage process, resulting in a flow restart. The gas pocket delays the pressure propagation in the second gel plug, resulting in a pressure gradient higher than yield stress across the first gel plug. The first gel plug, which deforms under a high-pressure gradient, compresses the gas pocket. This results in a larger mean deformation in the first gel plug, and it overcomes the threshold yield strain limit (γs). At strain values larger than yield strain the gel structure breaks, resulting in a lower elastic strength in the first gel plug. The lower elastic gel strength ascertains a lower pressure gradient in the first gel plug, allowing a largely unattenuated pressure to propagate in the second plug. The propagation of unattenuated pressure in the second gel plug results in a pressure gradient higher than the yield stress. This causes sequential gel breakage, resulting in a successful flow restart in a pipeline longer than a critical length. The present study also demonstrates that the pressure propagation and initial transient flow depend on the gel properties (i.e. gel strength and compressibility) and the number and size of gas pockets in the gel. The insights gained from the present work provide a cost-effective solution to mitigate the flow restart problem.

Prognostic impact of echocardiographic mean transvalvular gradients in patients with aortic stenosis and low flow undergoing transcatheter aortic valve implantation.

Blunted left ventricular hemodynamics reflected by a low stroke volume index (SVI) ≤35 mL/m2 body surface area (low flow [LF]) in patients with severe aortic stenosis (AS) are associated with worse outcomes even after correction of afterload by transcatheter aortic valve implantation (TAVI). These patients can have a low or high transvalvular mean pressure gradient (MPG). We investigated the impact of the pre-interventional MPG on outcomes after TAVI. Patients with LF AS were classified into those with normal (EF ≥ 50%; LF/NEF) or reduced ejection fraction (EF < 50%; LF/REF) and were then stratified according to an MPG < or ≥ 40 mmHg. Patients with SVI >35 mL/m2 (normal flow; NF) served as controls. 597 patients with LF/NEF, 264 patients with LF/REF and 975 patients with NF were identified. Among all groups those patients with a low MPG were characterized by higher cardiovascular risk. In patients with LF/REF, functional improvement post-TAVI was less pronounced in low-MPG patients. One-year survival was significantly worse in LF AS patients with a low vs. high MPG (LF/NEF 16.5% vs. 10.5%, p= 0.022; LF/REF 25.4% vs. 8.0%, p= 0.002), whereas no differences were found in NF patients (8.7% vs. 10.0%, p= 0.550). In both LF AS groups, a low pre-procedural MPG emerged as an independent predictor of mortality. In patients with LF AS, an MPG cut-off of 40 mmHg defines two patient populations with fundamental differences in outcomes after TAVI. Patients with LF AS and a high MPG have the same favorable prognosis as patients with NF AS.

Current Status and Effective Suggestions for Efficient Exploitation of Coalbed Methane in China: A Review

The commercial production of coalbed methane (CBM) in China has made significant achievements. To further improve the production of CBM and realize its development, it is necessary to establish a unique, high-efficiency CBM production method that considers the characteristics of coal reservoirs in China. This paper presents the current status of China’s CBM exploitation over the past 15 years from three aspects: reservoir characteristics, production mode, and exploitation technology. The achievements, in terms of exploration and development of CBM resources in China, were summarized. The CBM reservoirs in China are characterized by high coal rank, low permeability, high gas content, and low reservoir pressure gradient. Strategic measures for coal and CBM co-mining were proposed creatively as “Huainan”, “Jincheng”, and “Songzao” modes. CBM production enhancement measures have achieved significant innovations and breakthroughs in drilling and completion technologies and reservoir reconstruction, effectively reducing development costs and increasing single well production. Finally, based on existing basic research, this study proposes efficient CBM exploitation technologies suitable for the characteristics of CBM reservoirs in China. In particular, the uniform permeability enhancement measures of hydraulic grid slotting and fracturing have the advantages of safety, high efficiency, economy, feasibility, and broad application prospects under complex mining conditions. It is of great strategic significance to improve the CBM recovery rate and optimize China’s energy structure.

Aortic Root Replacement With Autologous Pericardium Valved Conduit

Available aortic prosthesis replacement options present a challenge to achieving low perioperative morbidity, low pressure gradients, and prolonged durability. Trileaflet aortic valve reconstruction using autologous pericardium offers an alternative treatment option with excellent postoperative gradients, large effective orifice areas, and the avoidance of long-term anticoagulation. The modified Bentall procedure with either tissue xenograft valved conduit or mechanical valved conduit is considered the gold standard for patients with aortic root pathology requiring surgical replacement. We report a novel adaptation of the modified Bentall procedure with a self-fabricated valved conduit with trileaflet aortic valve neocuspidization using autologous pericardium.

Numerical forward modelling of the overpressure build-up in the Cenozoic section of the Central Graben in the North Sea

One of the most widespread hypotheses for the origin of the present-day overpressure in the shale Post-Chalk section in the North Sea is the very rapid sedimentation from Neogene to present day. We tested this hypothesis by the means of numerical forward finite elements modelling and successfully simulated the overpressure build-up during the Cenozoic filling of the North Sea with relatively simple model set-up. Our model shows that overpressure of approximately 28% above hydrostatic developed in the Neogene, while during the Quaternary, it reached up to 36% above hydrostatic. At present day, the predicted onset of overpressure starts at about 800–1000 m below seafloor, while the maximum (magnitude about 1.36 sg, i.e. 36% above the normal hydrostatic pressure) is at approximately 2100 m. This overpressure profile fits reasonably well with data from wells drilled in the Central Graben. The exact magnitude of the overpressure depends on the used assumptions, the model set-up and the values of the input parameters. Especially the dynamic interaction between high sedimentation rates, clay permeability and low horizontal pressure gradient seems to be a key factor in the efficiency of dewatering of saturated clays during burial. The results indicate that, the assumption of horizontal stress isotropy results in nearly no horizontal fluid flow, despite the same magnitude for the vertical and the horizontal permeability. In these conditions, the vertical permeability plays much bigger role than the horizontal one in the effective de-watering of the sediments during burial. Further investigation is needed to explore the role of horizontal pressure gradient in fluid migration in passive sedimentary basins.

Implantation of a Sutureless Valve Into a Surgically Enlarged Aortic Root: A Bailout Option

Sutureless bioprostheses such as the Sorin Perceval Valve (SPV; Sorin Group, Srl, Saluggia, Italy) have been proposed for replacing stenotic native valves within small aortic roots of geriatric patients with significant comorbidity. Their use seems as safe as that of stented bioprostheses and enables significantly reduced length of surgery. Low transprosthetic pressure gradients have been measured. Because of the radial force of its self-expandable nitinol stent, aortic annulus interruption could be a relative contraindication to SPV use. Off-label implantation of the SPV into a surgically enlarged ascending aorta was first reported in this study, as a bailout option in the presence of a tiny aortic root.

No Evidence for Classic Thrombotic Microangiopathy in COVID-19.

Background: Coronavirus disease-2019 (COVID-19) triggers systemic infection with involvement of the respiratory tract. There are some patients developing haemostatic abnormalities during their infection with a considerably increased risk of death. Materials and Methods: Patients (n = 85) with SARS-CoV-2 infection attending the University Medical Center, Mainz, from 3 March to 15 May 2020 were retrospectively included in this study. Data regarding demography, clinical features, treatment and laboratory parameters were analyzed. Twenty patients were excluded for assessment of disseminated intravascular coagulation (DIC) and thrombotic microangiopathy (TMA) due to lack of laboratory data. Results: COVID-19 patients (n = 65) were investigated, 19 with uncomplicated, 29 with complicated, and 17 with critical course; nine (13.8%) died. Seven patients showed overt DIC according to the ISTH criteria. The fibrinogen levels dropped significantly in these patients, although not below 100 mg/dl. Hallmarks of TMA, such as thrombocytopenia and microangiopathic haemolytic anaemia, were not detected in any of our COVID-19 patients. ADAMTS13 activity was mildly to moderately reduced in 4/22 patients, all having strongly elevated procalcitonin levels. Conclusion: DIC occurred in 7/65 COVID-19 patients but fibrinogen and platelet consumption were compensated in almost all. ADAMTS13 assays excluded TTP and hallmarks of classic TMA were absent in all investigated patients. We hypothesize that the lacking erythrocyte fragmentation and only mild platelet consumption in severe COVID-19 are due to a microangiopathy predominantly localized to the alveolar microcirculation with a low blood pressure gradient.

Even a low transmitral pressure gradient after transcatheter mitral valve repair leads to impaired dynamic performance and increased left atrial endsystolic volumes

Abstract Funding Acknowledgements Type of funding sources: None. INTRODUCTION Transcatheter mitral valve repair (TMVR) was shown to be safe and improves outcome in patients with severe functional mitral regurgitation (MR). Mean transmitral pressure gradients (MVG) &amp;lt; 5.0 mmHg are generally tolerated after TMVR. However, the prognostic relevance of this arbitrary threshold remains unclear and dynamic changes during physical exercise are not well examined. METHODS From 2017 to 2019 48 patients with an indication for TMVR were prospectively enrolled in a single center study. The total physical capacity was measured by dynamic stress echocardiography on bicycle at baseline, before discharge and follow-up (FUP, mean (± SD) of 8.2 ± 1.4 months). Patients were classified into two groups: "MVG present" (MVG ≥ 2.5mmHg, measured by transesophageal echocardiography intraoperatively after TMVR) or "MVG absent". Between group differences were compared with a two-sided t-test. For association analysis the Pearson Product-Moment correlation coefficient was used. RESULTS 48 with a mean (± SD) age of 72.8 ± 12.1 years were eligible for the analysis. There was no significant difference in baseline characteristics concerning the level of the EuroScore II or prevalence of atrial fibrillation, heart failure, cardiovascular risk factors, renal failure or cardiomyopathy and demographic characteristics as age and sex, respectively. Also, most of the standard morphological and functional echocardiographic parameters as well as modern speckle-tracking derived parameters were similar between the 2 groups. There was no difference in remaining mitral regurgitation regarding severity between the two groups (p = 1.00). While the left atrial end systolic volumes (LAESV) at baseline did not differ significantly between groups, patients with an increased MVG after clipping showed an increase in LAESV (plus 56.2 ± 33.6 ml), while the other group showed a decrease in mean value (minus 15.9 ± 42.1 ml; baseline vs FUP: p &amp;lt; 0.001). Further performance in dynamic stress echocardiography was significantly better in patients without development of a gradient (increase in metabolic equivalent of task (METS) changes between FUP and baseline 1.05 vs 0.06, p = 0.014). Most importantly, those patients who had a higher MVG after TMVR showed a significant increase in MVG already during stress-echo at baseline, compared to patients who didn´t develop a significant MVG after intervention (1.45 ± 1,08 mmHg vs. 0.67 ± 1,00 mmHg; p = 0.026). CONCLUSION After TMVR even a low MVG increase of &amp;gt; = 2.5mmHg is associated with impaired performance in dynamic stress echo and with enlargement of LAESV. Increase of MVG in stress echo at baseline may be suitable to identify patients at risk to develop pathological MVG. Therefore, periprocedural caution to hemodynamic changes in MVG should always be given. Maybe a certain residual MR could be accepted before implanting an additional clip, knowing that already a MVG &amp;gt; 2,5 mmHg might have adverse effects.

Semi-analytical Approach to Modeling Forchheimer Flow in Porous Media at Meso- and Macroscales

Darcy’s law (which states that a fluid flow rate is directly proportional to the pressure gradient) is shown to be accurate in a rather narrow range of flow velocities. Numerous studies show that at low pressure gradients gas slippage effect occurs, which gives overestimated flow rates compared to Darcy’s law. At higher flow rates, Darcy’s law is usually replaced by the Forchheimer equation which accounts for inertial forces including a quadratic term in the flow rate. Darcy’s and Forchheimer’s laws and the problem of detecting transitions between their ranges of applicability are discussed in this study. Analysis of experimental data shows that deviation from Darcy’s law is governed by the Forchheimer number, which is defined by the authors as a product of tortuosity and Reynolds number. The use of the Forchheimer number and semi-analytical approaches enables us to describe non-Darcy flow as a simple universal equation valid for any flow geometry. Comparison of the experimental data with predictions based on a semi-analytical model shows excellent agreement for a wide range of reservoir properties.

Evaluation of the influence of a dimensionless threshold pressure gradient on deviation of filtration from classical filtration law

Depletion of hydrocarbon reserves in reservoirs with high and medium permeability increases the proportion of low permeability reservoirs in which nonlinear filtration effects are observed: at low pressure gradients, filtration does not obey the classical filtration law (Darcy’s law). One of the methods to describe the nonlinear filtration effects is to apply the filtration law with a threshold pressure gradient (TPG). The influence of this parameter on the deviation of filtration from Darcy's law has not been previously analyzed. The aim of this work is to evaluate this effect of the TPG. Piezoconductivity equations are nondimensionalized and solved according to the classical filtration law and the filtration law with TPG for plane radial flow and constant well pressure. In the framework of the classical filtration law, the piezoconductivity equation is solved by applying a self-simulated variable. Using the filtration law with TPG, the conductivity equation solution is solved by the method of integral relations. Relations between dimensionless production rate and dimensionless time for both filtration laws are found from the solved piezoconductivity equations. To prove the identity of the obtained solutions, a comparative analysis is performed for a threshold pressure gradient equal to zero. Comparison of production decline curves for classical filtration law and filtration law with TPG at various values TPG is carried out. The relation between the shapes of the production decline curves shape and various dimensionless TPG values is analyzed. With increasing dimensionless TPG, the influence of nonlinear effects on filtration increases: the nonlinear filtration effects become apparent earlier, and the production decline curves for the filtration law with TPG deviate greater from the production decline curve for the classical filtration law.

Facile validated HPLC method using photodiode array detector for the combined analysis of etodolac and 5-FU in bulk and tablet dosage form

The aim of the investigation was to develop and validate a reversed-phase high-pressure liquid chromatography (RP-HPLC) method for the simultaneous estimation of etodolac and 5-FU in bulk and coated tablet dosage form. The method was validated in terms of linearity, precision, accuracy, and specificity, LOD, and LOQ according to ICH guidelines. The standard instrument parameters were optimized using C18 reverse phase Inertsil® ODS (250 × 4.6 mm, 5µ particle size) column with system equipped with photodiode array (PDA) detector. An optimized ratio of mobile phase in RP-HPLC under low pressure gradient mode comprising of 60:40 % v/v of acetonitrile and di-potassium hydrogen phosphate buffer (pH 6.4; 25 mM), provided sharp peaks (having adequate molar absorptivity at λmax of 280 nm) with short retention time of 4.3+/-0.2 and 11.5+/-0.3 min for 5-FU and etodolac, respectively. Results indicated the method was linear, precise, accurate, rugged and robust with RSD values <2%. The percentage recoveries of both the drugs were within the limits of 90-110% at various spiked concentrations. The proposed method was found to be highly effective and could be successfully employed for the quantification of 5-FU and ETD in bulk drug and tablet formulations for routine analysis.

Effect of hydrodynamic tilting at fluid contacts to reservoir production performance

Down-flush hydrodynamic movement of fluids at at transitional zones towards areas with low pressure gradients have been reported to directly influence the accurate prediction of petroleum reservoir’s production performances over time. Fluctuations in the fluid-fluid interactions and pressure changes around the reference line was the major cause for the discrepancy. In this research, a sensitivity study on the influence of hydrodynamic tilt factor h to oil, water and gas production was carried-out by means of the commercial software Eclipse Black oil simulator using the openSource Norne field data. The constant h is a function of the corrected shut-in pressure, fluid specific gravity at contact base-lines, pressure at the reference elevation and at the oil-water contact. The error in the contact depth was captured using the difference between the initial and calculated true vertical depths of the fluid’s contact zones. The obtained information was fed into the simulator to model the process dynamics. Findings revealed that the error margin for the first case hA (gas-oil contact) was insignificant. However, a mean deviation in the reservoir’s production predictions of atleast 8% for approximately a 6.5% change in the interacting fluids density was attained in-contrast to the base case.

Haemodynamic Patterns of Severe Aortic Stenosis

The authors present up-to-date review of clinical pathophysiology of aortic stenosis (AS) based on differentiation of its haemodynamic patterns, and some actual issues of instrumental diagnostics and classification of AS. The variety of clinical presentations of AS is based on diverse combination of pathological changes of haemodynamics. In Russian cardiology, there is no clear pathophysiological classification of AS despite of its relevance under the progress of surgical and transcatheter treatment of AS. The authors suggest the pilot haemodynamic classification of AS which includes 6 types (0-5) based on different combination of the following variables: left ventricle ejection fraction, stroke volume, mean aortic systolic pressure gradient. Severe AS with low transaortic pressure gradient in patients with depressed systolic function of the left ventricle (so called «low flow-low» gradient phenomenon) is referred to as the most frequent, classical haemodynamic pattern of low-gradient AS. The prevalence of this variant is about 10% among European population of patients with severe AS. The inconsistence between aortic valve area and mean pressure gradient is as common as in 35-40% of patients with AS, however, in 30-50% of these cases, AS is not severe. Severe AS is a surgical disease that should be treated in a surgical way in all patients but those in whom predicted risk overbalances potential benefits of the procedure. The use of integrated clinical and instrumental approach for identification of a true sever AS is the matter of great concern, as both overestimation and underestimation can misguide the clinical decision-making process. Verification of severe AS in patients with classical and paradoxical low flow-low gradient AS with specific indications for surgical treatment regarded is further emphasized in the paper. Since transcatheter aortic valve implantation has become a commonly recognized alternative to surgical aortic valve replacement, its role in the treatment of severe AS with different haemodynamic patterns is also discussed. The authors stress on the necessity of using tailored approach for treatment of AS regarding different clinical and pathophysiological scenarios: high gradient AS with preserved ejection fraction, classical and paradoxical low flow-low gradient AS.

A Hybrid Method for the Computation of a Rarefied Gas Jet Efflux through a Very Long Channel into Vacuum

On the basis of the kinetic model, the steady efflux of monatomic gas from a high pressure camera (the Knudsen number $${\text{Kn}} \ll 1$$ ) through a long channel between two parallel plates into a vacuum camera under a constant temperature on the bounding surfaces is studied. Using asymptotic estimates for relatively long channels, the flow domain is divided into three subdomains: (1) a neighborhood of the channel entry, (2) the main part of the flow in the channel that occupies almost all channel length, and (3) a neighborhood of the channel exit. The flow in subdomain (1) is not considered due to its low speed. In the main subdomain (2), the flow is slow and is driven by a low pressure gradient (the diffusion area). In subdomain (3), the flow gets faster, and the gas expands in the channel and in the vacuum camera. In subdomain (2), we have the continuum flow regime, and the well-known results of the linear one-dimensional theory of viscous gas flows in long channels (Poiseuille flow) are used. In the subdomain of fast flow, the full nonlinear kinetic equation (S-model) is used. The condition of asymptotic matching of solutions in two subdomains is replaced by the boundary condition of solution coupling in a certain section the position of which is chosen from the smoothness condition of the full solution of the problem. The kinetic equation is solved by the method of time marching to steady state using the conservative second-order scheme with respect to all variables implemented in Nesvetay software package. The proposed solution method can be considered as a hybrid one because the Navier–Stokes and kinetic equations are solved simultaneously.

Mid-term results of Biological Bentall using a larger valve implantation technique.

ABSTRACTThe present study evaluated our modified technique for placing a valve one size larger for Biological Bentall (Bio-Bentall) with a stented valve using a double sawing ring technique with a comparison to classical Bio-Bentall with a stent-less valve. Between December 2001 and July 2017, 44 patients (10 stent-less and 34 stented) with Bio-Bentall were included in this study. The early and mid-term surgical outcomes and re-operation free rates, including the structural valve deterioration (SVD) rate, were investigated. The mean follow-up duration was 69 months. There were two in-hospital mortalities in the stent-less group and one in the stented group, showing no significant difference. The survival at 1, 5 and 10 years was 80%, 50% and 30% in the stent-less group and 85%, 77% and 71% in the stented group, respectively, with a significant difference. There were no cases of SVD occurrence in either group. Freedom from re-operation at 1, 5 and 10 years was 100% at all points in the stent-less group and 100%, 96% and 96% in the stented group, respectively, without significance. The mean pressure gradient through the aortic valve according to the most recent echocardiogram was 8.1 mmHg in the stent-less group and 15.8 mmHg in the stented group, without significance. Our modified technique for Bio-Bentall showed a feasible short- and mid-term survival compared to classical stent-less Bio-Bentall. Both techniques had a good outcome concerning SVD occurrence and the re-operation-free rate with a low-pressure gradient in the aortic position.

Thermodynamic interpretation of the drift and noise of gradient baselines in reversed-phase liquid chromatography using mobile phase additives

The drift and noise of acetonitrile-water gradient baselines (5-95%, v/v, 5 min linear gradient) in reversed phase liquid chromatography (RPLC) are recorded at a wavelength of 215 nm using 0.1% trifluoroacetic acid (TFA) as the mobile phase additive, a 4.6 mm × 150 mm 5 μm Symmetry-C18 RPLC column, and an Arc system (low-pressure gradient proportioning valve or GPV, pump with a stroke volume of either 66 or 132 μL, no mixer) as the LC instrument. These observations are predicted from solid-liquid adsorption thermodynamics which requires the measurement of the excess adsorption isotherm of acetonitrile from water onto the RPLC column and of the variation of the Henry’s constant of TFA as a function of the volume fraction of acetonitrile in the bulk mobile phase. The incomplete mixing of the acetonitrile and water packets delivered by the low-pressure GPV is represented by a sinusoidal perturbation of the programmed volume fraction of acetonitrile during the entire gradient. The variation of the TFA absorbance at 215 nm with increasing acetonitrile concentration is measured in order to transform TFA concentration into the observable absorbance unit. The drift and noise of the gradient baseline are calculated by solving numerically (Rouchon method) the equilibrium-dispersive (ED) mass balance equations of acetonitrile and TFA.The agreement between the calculated and observed gradient baselines is very good as the proposed model of chromatography accurately accounts for the displacement of TFA between stationary and mobile phases (early excess and late deficit of TFA concentration relative to 0.1%) and for the frequency (equal to the ratio of the applied flow rate to the stroke volume) and the amplitude of the periodic noise recorded during the gradient. From a practical viewpoint, the drift of the gradient baseline can be minimized by maximizing the ratio of the gradient volume to the hold-up volume ( > 10) and/or by minimizing the retention factor of the mobile phase additive in the water-rich eluent (k < 0.2). The reduction of the noise amplitude below 0.1 mAU as requested by the pharmaceutical industry imposes the ratio of the flow rate to the stroke volume of the pump to be larger than 1 Hz. This opens avenues towards the development of new GPV, pump, and mixers in order to mix efficiently the solvent packets delivered by conventional LC instrument.

Analytical solution for one-dimensional non-Darcy flow with bilinear relation in porous medium caused by line source

One-dimensional non-Darcy flow in a porous medium caused by a line source is investigated. Owing to several reasons, a bilinear relation between the fluid velocity and the pressure gradient is considered, and the problem can be seen as an extension of a special one-phase free boundary problem reported in literature to a two-phase situation. An analytical solution for the problem is established based on the similarity type general solution of the governing equation. The analytical solution contains an unknown coefficient, which describes the free boundary movement and needs to be determined by a nonlinear equation, and the existence and uniqueness of this coefficient is proven. A numerical solution is also developed using the finite volume method, and special attention is spent on the two control volumes near the free boundary location so as to track the movement of the free boundary accurately. Computational examples are presented. The application of the analytical solution as a benchmark is introduced, and the accuracy of the numerical solution is verified. The non-Darcy flow for boreholes with different radii is studied, and the error caused by neglecting the size of borehole is discussed. The non-Darcy flow with a bilinear relation is compared with that with a threshold pressure gradient, and the error caused by neglecting the permeability under low pressure gradients is analyzed.