Pressure Recovery Phenomenon Research Articles

Flash boiling and pressure recovery phenomenon during venting from liquid ammonia tank ullage

Liquid ammonia stored at elevated pressures undergoes phase change during tank venting. The paper describes a CFD model able to reproduce the INERIS experiment performed using 12 m3 volume tank when 300 kg of ammonia were vented during 460 s from the ullage through the pipe to the atmosphere. The model is based on the volume-of-fluid method combined with Lee’s model for mass transfer between phases. Extensive calibration trials were conducted to examine the impact of surrounding temperature conditions. It allowed to establish the range of time relaxation parameter values of Lee evaporation-condensation model specific for liquid ammonia tank venting. Simulations reproduced accurately the experimentally measured final pressure in the tank and the amount of ammonia released during venting. Liquid ammonia boiling is triggered by reduction of pressure and takes place throughout the liquid causing the pressure recovery phenomenon. Numerical simulations demonstrated that pressure recovery phenomenon is driven by the characteristic time delay associated with the rise of vapour bubbles generated by flash boiling and the subsequent release of evaporated ammonia into the tank ullage space. The developed CFD model can be used as a contemporary tool for safety engineering and development of tank management strategies for liquid ammonia storage.

LES model of flash-boiling and pressure recovery phenomena during release from large-scale pressurised liquid hydrogen storage tank

The paper presents large eddy simulations (LES) model of two-phase (liquid-gas) hydrogen behaviour during pressurised tank venting. The results of simulations are compared against published experiment on flash-boiling of liquid hydrogen (LH2) in a large-scale tank of 7.622 m height, 2.3 m internal diameter, filled with LH2 to 6.248 m (80% tank filling) at pressure 321.8 kPa gauge. The volume-of-fluid model is applied for tracking phases and Lee's model is used to simulate evaporation and condensation. The effect of various Lee's model evaporation-condensation coefficients on the process dynamics is assessed. The simulations are compared against experimental pressure dynamics monitored at the venting pipe and experimental temperature measured by thermocouples in gaseous and liquid phases during the first 15 s of venting. The LES model reasonably predicts physical phenomena during depressurisation of LH2 storage tank, including the pressure recovery phenomenon, LH2 level and temperature dynamics in the tank.

Analytical and numerical investigation of flow distribution in PEMFC stacks

The flow distribution in the PEMFC stack is of significance to its performance and durability. To determine the flow uniformity criterion of the stack manifold, a “flow field analysis scheme” that combines theoretical analysis and numerical simulations has been employed. In the analysis, an analytical model is developed for the manifold flow system. Two dimensionless configuration parameters η and λ determining the flow distribution are clarified and the coefficient of variation (CV) has been quantitatively derived, which is the function of the combined variable η·λ. On this basis, a simple and explicit criterion for flow uniformity has been proposed (CV < 5% when ηλ < 0.59). Additionally, the detailed numerical simulations systematically investigate the stack configuration parameters, further validating the aforementioned criterion. The pressure recovery phenomenon in the inlet header has inspired us to conceive the basic principles of the stack manifold design.

New Evidence About Aortic Valve Stenosis and Cardiovascular Hemodynamics.

Aortic stenosis (AS) is the most common degenerative valvular disease in western word. In patients with severe AS, small changes in aortic valve area can lead to large changes in hemodynamics. The correct understanding of cardiac hemodynamics and its interaction with vascular function is of paramount importance for correct identification of severe AS and to plan effective strategies for its treatment. In the current review with highlight the importance of pressure recovery phenomenon and valvular arterial impedance as novel tools in the evaluation of patients with aortic stenosis.

Different measurements require different benchmarks: One size does not fit all

Key Points Echocardiographic Doppler measurements do not account for the pressure recovery phenomenon and therefore estimate higher transvalvular pressure gradients (TPG) than invasive catheter‐based measurements. An observed 3‐4 mmHg mean TPG difference between self‐expanding and balloon‐expandable transcatheter valves has been observed consistently, but no clinical significance has been reported. A necessary novel classification system must provide a range of normal and abnormal values for each measurement modality and each valve type.

Analysis and improvement of flow distribution in manifold for proton exchange membrane fuel cell stacks

The flow distribution in manifold has significant effect on the output performance of the stack according to the Cannikin Law, especially for multi-cell proton exchange membrane fuel cell (PEMFC) stacks. This study presents an analytical model which simultaneously considered local pressure losses, pressure recovery phenomenon, electrochemical reactions, and liquid water to calculate the flow distribution using the flow network method. Both U-shape and Z-shape flow configuration stack are investigated. The analytical model is simultaneously and quantitatively validated with three distinct variables (the dimensionless mass flow rates, the stack pressure drop and the pressure distribution in manifold) to ensure the accurateness of modelling results. Effects of reactant consumption and two-phase flow in unit cell on the flow distribution are quantitatively investigated. Results show that the flow maldistribution is overestimated when ignoring gas consumption and two-phase flow for both 200-cell U-shape and Z-shape stack. Balance the pressure drop of inlet and outlet manifold significantly improves the flow distribution for U-shape stack. Finally, the differentiated assembly strategy is first proposed to improve the flow distribution in manifold for multi-cell PEMFC stacks. Compared to the original design, the ratios of the improvements for 200-cell U-shape and Z-shape stack are 85% and 25%, respectively.

Determining prosthesis‐patient mismatch after TAVR: Which is the best method?

Determining prosthesis‐patient mismatch after TAVR: Which is the best method?

DOPPLER VERSUS CATHETER TRANSVALVULAR PRESSURE GRADIENTS IN SELF-EXPANDING VERSUS BALLOON EXPANDABLE TRANSCATHETER AORTIC VALVES, AN IN VITRO STUDY

Because of pressure recovery (PR) phenomenon, transvalvular pressure gradients (TPGs) measured by Doppler are generally higher than those measured by catheterization. Recent studies suggest that the extent of PR and thus of the Doppler-catheter TPG difference may be more important in balloon-

Prospective assessment of the frequency of low gradient severe aortic stenosis with preserved left ventricular ejection fraction: Critical impact of aortic flow misalignment and pressure recovery phenomenon

The frequency of paradoxical low-gradient severe aortic stenosis (AS) varies widely across studies. The impact of misalignment of aortic flow and pressure recovery phenomenon on the frequency of low-gradient severe AS with preserved left ventricular ejection fraction (LVEF) has not been evaluated in prospective studies. To investigate prospectively the impact of aortic flow misalignment by Doppler and lack of pressure recovery phenomenon correction on the frequency of low-gradient (LG) severe aortic stenosis (AS) with preserved LVEF. Aortic jet velocities and mean pressure gradient (MPG) were obtained by interrogating all windows in 68 consecutive patients with normal LVEF and severe AS (aortic valve area [AVA] ≤1cm2) on the basis of the apical imaging window alone (two-dimensional [2D] apical approach). Patients were classified as having LG or high-gradient (HG) AS according to MPG <40mmHg or ≥40mmHg, and normal flow (NF) or low flow (LF) according to stroke volume index >35mL/m2 or ≤35mL/m2, on the basis of the 2D apical approach, the multiview approach (multiple windows evaluation) and AVA corrected for pressure recovery. The proportion of LG severe AS was 57% using the 2D apical approach alone. After the multiview approach and correction for pressure recovery, the proportion of LG severe AS decreased from 57% to 13% (LF-LG severe AS decreased from 23% to 3%; NF-LG severe AS decreased from 34% to 10%). As a result, 25% of patients were reclassified as having HG severe AS (AVA ≤1cm2 and MPG ≥40mmHg) and 19% as having moderate AS. Hence, 77% of patients initially diagnosed with LG severe AS did not have "true" LG severe AS when the multiview approach and the pressure recovery phenomenon correction were used. Aortic flow misevaluation, resulting from lack of use of multiple windows evaluation and pressure recovery phenomenon correction, accounts for a large proportion of incorrectly graded AS and considerable overestimation of the frequency of LG severe AS with preserved LVEF.

29 - Frequency of paradoxical low flow low gradient severe aortic stenosis: critical impact of aortic flow misalignment and pressure recovery phenomenon

29 - Frequency of paradoxical low flow low gradient severe aortic stenosis: critical impact of aortic flow misalignment and pressure recovery phenomenon

Significant intra-valvular pressure loss across EPIC SUPRA and perimount magna supra-annular designed aortic bioprostheses in patients with normal aortic size

Doppler-derived trans-prosthetic gradients are higher and the estimated effective valve area is smaller than the catheter-derived and directly measured hemodynamic values, mostly due to pressure recovery phenomenon. Pressure recovery to a varying extent is common to all prosthetic heart valves including bioprostheses. Pressure recovery-related differences are usually small except in patients with bileaflet metallic prosthesis, wherein high-pressure local jets across central orifice have been documented since long back and also in patients with narrow aortic root. We describe two patients with normally functioning stented aortic bioprostheses with supra-annular design (EPIC SUPRA and PERIMOUNT MAGNA), wherein very high trans-prosthetic gradients and critically reduced estimated effective valve orifice areas in presence of normal aortic size were consistently recorded over long periods of follow-up. The valve leaflets, however had normal excursion, were thin, opened with a triangular or oblong shape and had expected geometric valve area (1.7 and 1.6cm2 respectively) measured by 3D trans-oesophageal echocardiographic planimetry. Pressure recovery upstream the valves accounted for 20% and 12% of total pressure gradients respectively. Dominant site for pressure drop was intra-valvular (75–85%). Such a phenomenon has not been reported in vivo for these two valve designs.

IMPACT OF CORRECTION OF PRESSURE RECOVERY PHENOMENON IN CALCULATION OF EFFECTIVE ORIFICE AREA AFTER AORTIC VALVE REPLACEMENT

To test the impact of correction of pressure recovery phenomenon (PRP) on accurate diagnosis of patient-prosthesis mismatch (PPM) after aortic valve replacement (AVR) and to evaluate the effect of different valve types, post-AVR echocardiographic data of 494 consecutive patients (male 241 [48.8%])

Discrepancies between Doppler and catheter gradients in ventricular septal defect: a correction of localized gradients from pressure recovery phenomenon

Although very high gradient levels were measured during the evaluation of ventricular septal defect (VSD) in daily practice, these measurements are generally interpreted as erroneous and thus neglected. Our aim was to assess the features of VSD's having erroneous interventricular pressure gradients by echocardiography. A 46 patients were enrolled in the study. The patients with higher Doppler-derived interventricular gradient than brachial systolic blood pressure were compared with patients with lower gradient (group 1, n = 26; group 2, n = 20, respectively) in terms of echocardiographic characteristics of VSD. No significant relations were observed in systolic and diastolic blood pressure and interventricular synchronicity between two groups (117.1 ± 6.7 vs 110.2 ± 6.3 mmHg, p = 0.145; 74.7 ± 4.3 vs 73.2 ± 4.9 mmHg, p = 0.32; 31.2 ± 5.5 vs 33.2 ± 4.9 msn, p = 0.29, respectively). Left ventricular end-diastolic and end-systolic diameters were greater in group 2 (46.6 ± 3.5 vs 49.5 ± 4.5, p = 0.022; 30.3 ± 2.5 vs 32.9 ± 3.2, p = 0.004, respectively). Doppler-derived interventricular pressure gradients were significantly higher in group 1 (144.4 ± 13.6 vs 75.7 ± 5.1 mmHg, p < 0.001, respectively). Defect width was significantly lower (3.20 ± 0.40 vs 4.8 ± 1.8 mm, respectively, p < 0.05), and length was greater in group 1 patients (5.75 ± 0.90 vs 2.80 ± 0.80 mm, p < 0.05, respectively). There was a significant positive correlation between pressure gradient and defect length (r = 0.84, p < 0.001), and a negative correlation between pressure gradient and defect width (r = -0.66, p < 0.001). Defect length/width was significantly greater in group 1. With the cut-off value of 1.2, defect length/width was able to predict tunnel-type VSD with sensitivity of 88.5% and specificity of 72.7%. Continuous-wave Doppler method may overestimate interventricular pressure gradients in patients with tunnel-type ventricular septal defect.

Coexisting cardiac diseases and pressure recovery phenomenon contribute to discrepancy between the echocardiographic severity of aortic stenosis and left ventricular hypertrophy

Assessing left ventricular (LV) hypertrophy (LVH) is an important step in the echocardiographic diagnosis of aortic stenosis (AS). We aimed to investigate the causes of discrepancies between the degrees of AS and LVH. The study subjects consisted of 149 consecutive patients with AS having aortic valve area <2.0cm(2) (mean age 72.5±11.9years, 67 men and 82 women). Coexisting cardiac diseases were determined based on echocardiographic findings and comprehensive clinical judgment. Echocardiographic measurements included LV mass index (LVMI), aortic valve area index (AVAI), transaortic mean pressure gradient (MPG), valvulo-arterial impedance (Zva), energy loss coefficient (ELCo), and energy loss index (ELI). LVMI was not significantly correlated with AVAI and Zva, and had a weak correlation with MPG (r=0.305, p=0.0001). There were 55 patients in group A (non-severe AS without significant LVH), 58 in group B (non-severe AS with significant LVH), 7 in group C (severe AS without significant LVH), and 29 in group D (severe AS with significant LVH). Coexisting cardiac diseases were more frequently observed (p=0.0003) in group B (50%) than in group A (18%). In group C, ELCo and (ELI - AVAI)/ELI were significantly greater than in group D (p=0.043 and 0.007, respectively). Significant LVH seen in less than moderate AS is often due to coexisting cardiac diseases, and there may be an overestimation of AS severity due to pressure recovery among patients with apparently severe AS who do not have significant LVH.

IMPACT OF DESIGN AND HEMODYNAMIC CONDITIONS ON MECHANICAL HEART VALVE DYSFUNCTION ASSESSMENT

Function assessment of mechanical heart valve (MHV) dysfunction relies on pressure gradient and Effective Orifice Area (EOA) through echographic measurements. On the one hand, Doppler pressure measurements are based on the simplified Bernoulli equation. However, comparisons of Doppler and catheter pressure measurements have been largely discussed and have led Food and Drug Administration (FDA) to recommend the verification of Bernoulli’s equation before any valvular prosthesis test. Moreover, pressure recovery phenomenon was described in case of MHV in echographic recommendation [Zogbhi, 2009] and is particularly significant in case of valve dysfunction. On the other hand, EOA calculations still remain unsatisfying as MHV have a Geometrical Orifice Area (GOA) divided into three orifices. This experimental study focuses on validation of Doppler measurements, velocity profiles through mitral prostheses and consequences of hemodynamic conditions and design of MHV on dysfunction assessment.

Marked discrepancy in pressure gradient between Doppler and catheter examinations on Medtronic Mosaic valve in aortic position

A 71-year-old woman underwent aortic valve replacement with 23 mm Medtronic Mosaic Ultra valve 4 years ago because of aortic stenosis. Although she had been asymptomatic since the operation, echocardiography showed 4 m/s of transprosthetic valve flow that implied early prosthetic valve failure. Catheter examination revealed that the mean transvalvular pressure gradient during systole was 15.1 mmHg on simultaneous pressure recording, and calculated valve area 1.82 cm(2). Her body surface area was 1.56 m(2). Prosthetic valve failure and prosthesis-patient mismatch were both denied. The discrepancy between Doppler study data and catheter data seemed to be due to fluid dynamical pressure recovery phenomenon. Net pressure difference between the left ventricle and the aorta may be significantly smaller than that estimated using Bernoulli's equation from transvalvular flow speed in some patients after aortic valve replacement.

Discrepancy between Gradients Derived by Cardiac Catheterization and by Doppler Echocardiography in Aortic Stenosis: How Often Does Pressure Recovery Play a Role?

Studies have shown very good correlation between Doppler-derived gradients and gradients obtained by cardiac catheterization (cath) in aortic stenosis (AS). However, the phenomenon of pressure recovery may lead to significant overestimation of aortic valve (AV) gradients by Doppler echocardiography (echo). We hypothesized that echo-derived gradients will be higher in mild-moderate AS because of pressure recovery. We studied 94 patients who had echo and cardiac caths in a span of 1 week. The mean age was 72 +/- 13 years, 54% males, 79% had coronary artery disease, and the mean left ventricular ejection fraction was 45 +/- 22%. The mean cardiac output and cardiac indices were 5.1 +/- 1.4/2.7 +/- 0.6 (l/mt), (l/m(2)), respectively. For those with mild AS, echo overestimated gradients in 9.5% of patients (4/42) by an average of 19 mmHg, thus misclassifying the degree of stenosis. In those with moderate AS, 14% (3/21) were misclassified as severe AS (gradient overestimation by an average of 13.6 mmHg). In those with severe AS, echo underestimated gradients in 13% (4/31) by an average of 22.7 mmHg. The aorta at the sinotubular junction was 2.8 cm in those patients with mild AS in whom gradients were overestimated by more than 20 mmHg compared to a sinotubular junction diameter of 3.12 cm in those with mild AS and no overestimation of gradients. The AV area/aortic root ratio was 0.4 in those with mild AS and 0.2 in those with severe AS (P < 0.05).

Clinical Implication of Energy Loss Coefficient in Patients With Severe Aortic Stenosis Diagnosed by Doppler Echocardiography

The Doppler-derived energy loss coefficient (ELCo), which can take into account the pressure recovery phenomenon and reconcile discrepancies between the aortic valve effective orifice area (EOA) obtained by the Gorlin formula using a catheter (EOAcath) and the EOA obtained by the Doppler continuity equation (EOADop), is proposed as an equivalent index to represent EOAcath. Therefore, the purpose of this study was to evaluate the clinical impact of ELCo in patients with severe aortic stenosis (AS). Thirty-three patients with severe AS were assessed by Doppler examination [EOA obtained by the continuity equation (EOADop)<or=1.0 cm2], and referred to the cardiac catheterization laboratory for evaluation of EOA obtained by the Gorlin formula (EOAcath). Patients with ELCo<or=1.0 cm2 (n=26) had significantly lower incidence of symptoms related to AS compared with those having ELCo>1.0 cm2 (n=7) (p=0.002). Superior concordance in severity of AS was demonstrated between EOAcath and ELCo compared with EOAcath and EOADop (kappa=0.52, and kappa=0.32, respectively). In 21% of patients with "severe" AS diagnosed by Doppler echocardiography, the ELCo value indicated moderate rather than severe AS. These patients had significantly lower incidence of symptoms compared with patients who had ELCo<or=1.0 cm2.

Routine Adjustment of Doppler Echocardiographically Derived Aortic Valve Area Using a Previously Derived Equation to Account for the Effect of Pressure Recovery

Practice guidelines of the American College of Cardiology (ACC) and the American Heart Association (AHA) define aortic stenosis (AS) as being severe when the aortic valve area (AVA) is less than or equal to 1.0 cm and as moderate when the AVA is greater than 1.0 cm and less than or equal to 1.5 cm. Routine application of these guidelines to AVA calculated by echocardiography overestimates the prevalence of severe AS compared with AVA values obtained by cardiac catheterization (AVAcath). 2 The phenomenon of pressure recovery has been shown to account for some of the discrepancy in AVA and in the observed transvalvular pressure gradient when measured separately by catheterization and echocardiography. Ejected blood achieves a maximal velocity immediately distal to the stenotic valve (an area termed the “vena contracta”). This is also the site of maximal pressure gradient between the ventricle and the aorta. Pressure recovery occurs when this ejected blood loses velocity in the proximal ascending aorta. As the jet slows, pressure increases or recovers, thus lowering the effective transvalvular pressure gradient as a result of AS. Catheters used in clinical practice measure pressure in the proximal aorta, downstream from the aortic valve after the pressure has recovered. Calculation of AVA by echocardiography using the continuity equation measures the peak velocity of the aortic jet in the vena contracta, upstream fromwhere pressure recovery occurs. Thus, AVA derived by echocardiography records a higher effective transvalvular gradient compared with AVAcath. Pressure recovery is less pronounced when the proximal aorta is dilated because energy is lost from nonlaminar flow and turbulence. Measurement of AVA using echocardiography does not account for aortic size and the degree of pressure recovery and, therefore, may overestimate the severity of AS compared with AVAcath for patients with normal or small aortic roots. 2-9 AVAcath is thought to more accurately reflect the energy loss as a result of AS because it takes into account the degree of pressure recovery. An equation has been proposed to adjust AVA calculated by Doppler echocardiography (AVADop) to account for pressure recov-

Use of intra-cardiac ultrasound in the diagnosis of prosthetic valve malfunction

A 46-year old lady was under regular follow up for aortic valve replacement done in 2002 for aortic stenosis. The valve was a Carbomedics 19 mm bi-leaflet aortic valve prosthesis for which she had adequate anticoagulation since implantation. She had a past history of end stage renal failure, type 2 diabetes, hypertension, cerebrovascular disease and systemic lupus erythematosus. She was asymptomatic and had a routine transthoracic echocardiogram performed which revealed that her aortic valve prosthesis was well seated with an elevated velocity across the valve; the aortic valve leaflets were poorly visualised but appeared to be mobile. A transoesophageal echocardiogram (TOE) confirmed the markedly increased forward velocity across the aortic valve (maximum velocity 4.6 m/s) with small aortic root raising the possibility of pressure recovery phenomenon. Once again the leaflets were not clearly seen but appeared mobile despite the use of deep trans-gastric view. Fluoroscopy was performed and revealed that one of the leaflets was not moving. The patient had an intra-cardiac ultrasound scan (ACUSON AcuNav; Siemens) with the probe of the scanner within the right atrium. A long-axis view demonstrated the prosthetic aortic valve leaflets clearly (Fig. 1). A short-axis view of the prosthetic valve revealed an echogenic area at the six o'clock position; this may be due to pannus formation (Fig. 2); colour flow across the valve during systole revealed absence of colour flow though one of the leaflets due to the leaflet being stuck (Fig. 3). The sewing cuff of the Carbomedics valve is coated with biolite carbon, which is an anti-thrombotic agent that prevents adhesion of thrombus or pannus on the sewing cuff. There are few reports of Carbomedics valve dysfunction by pannus formation in the mitral position but none in the aortic position. Fluoroscopy can be used to visualize mobility of valve leaflets but is unable to identify thrombus/pannus formation that may be causing the valve to stick. Intra-cardiac echocardiography (ICE) can provide additional information regarding potential causes of valve dysfunction such as pannus/thrombus formation. However, ICE does suffer from the same limitations as TOE in obtaining sufficiently adequate images of the aortic valve to allow appropriate assessment of leaflet motion and insight into flow channels; it is also more invasive and expensive compared with other imaging modalities. In our patient ICE provided better images than TOE, possibly due to the closer proximity of the probe to the aortic root compared with TOE. ICE is a novel tool in the assessment of prosthetic valve function, which can complement data obtained from transthoracic/transoesophageal echocardiography and fluoroscopy.