This article presents the development of technical equations of state for four siloxanes using the 12-parameter Span–Wagner functional form. Siloxanes are used as heat transfer fluids and working ...media in energy conversion applications. The investigated fluids are two linear dimethylsiloxanes, namely MM (hexamethyldisiloxane, C
6H
18OSi
2) and MD
4M (tetradecamethylhexasiloxane, C
14H
42O
5Si
6), and two cyclic dimethylsiloxanes, namely D
4 (octamethylcyclotetrasiloxane, C
8H
24O
4Si
4) and D
5 (decamethylcyclopentasiloxane, C
10H
30O
5Si
5). Available measured properties are critically evaluated and selected for the optimization of the equation of state (EoS) parameters. Due to the insufficient number of experimental values, several other properties are estimated with the most accurate ad hoc methods. These estimates are included in the optimization of the equation of state parameters. Moreover, experimental saturated liquid density and vapor pressure data are correlated with the equations proposed by Daubert and Wagner–Ambrose, respectively, to provide short, simple, and accurate equations for the computation of these properties. The performance of the obtained equations of state is assessed by comparison with experimental data and also with estimates obtained with the Peng–Robinson cubic EoS with the modification proposed by Stryjek and Vera. This equation was adopted in previous technical studies. The improvements obtained with the newly developed EoS's are significant. Exemplary state diagrams are also reported as a demonstration of the consistency of the obtained thermodynamic models. Sound speed measurements in the vapor phase are planned for the near future and results will be incorporated in future improvements of the newly developed thermodynamic models.
► The fundamental derivative of gas dynamics, Γ, diverges near the critical point. ► The power law of divergence is universal for all 3-dimensional Ising-like systems. ► Negative values of Γ exist in ...a delimited equilibrium two-phase near-critical domain.
This document describes the results of an investigation on the variation of the so-called fundamental derivative of gas dynamics, Γ, in the vapor–liquid critical region of well-measured substances, namely methane, carbon dioxide and water, for which accurate, scaled fundamental equations are available. The results demonstrate that for a pure fluid in the single-phase thermodynamic regime, Γ diverges to +∞ independent of the direction of approach of the vapor–liquid critical point. Furthermore, in the two-phase thermodynamic regime, Γ diverges to −∞ independent of the direction of approach of the vapor–liquid critical point. These two qualitative results, as well as the value of the exponent giving the power-law dependence of Γ along the critical isochore as a function of |T−TC|/T (T is the temperature and “C” indicates its critical point value), namely ≈−0.89, are similar for all pure, non-ionized fluids belonging to the class of 3-dimensional Ising-like systems, i.e., systems governed by short-range forces.
This article presents the continuation of the work on the development of technical equations of state for linear and cyclic siloxanes already documented in this journal. The fluids considered ...herewith are octamethyltrisiloxane (MDM, C
8H
24Si
3O
2), decamethyltetrasiloxane (MD
2M, C
10H
30Si
4O
3), dodecamethylpentasiloxane (MD
3M, C
12H
36Si
5O
4), dodecamethylcyclohexasiloxane (D
6, C
12H
36Si
6O
6). The 12-parameter functional form proposed by Span and Wagner has been selected because of its positive characteristics. Siloxanes are produced in bulk quantities and are mostly utilized in the cosmetics industry and, mixed, as high-temperature heat transfer fluids. Furthermore, they are used as working fluids in high-temperature organic Rankine cycle power plants. The available property measurements are carefully evaluated and selected for the optimization of equation of state parameters. For some of the fluids, experimental values are scarce, therefore ad hoc estimation methods have been used to supply more information to the procedure for the optimization of the parameters of the equation of state. In addition, saturated liquid density and vapor pressure measurements are correlated with the equations proposed by Daubert and Wagner–Ambrose, respectively, to provide short, simple, and accurate equations for the computation of these properties. The recently developed isobaric ideal-gas heat capacity correlation for the selected siloxanes is included in the thermodynamic models. The performance of the newly developed equations of state is tested by comparison with experimental data and also with predictions calculated with the Peng–Robinson–Stryjek–Vera cubic EoS, as this model was adopted in previous technical studies. The new thermodynamic models perform significantly better than cubic equations of state.
T–
s and
P–
v
diagrams for all the substances are also reported.
An efficient method is proposed to evaluate the Vapour-Liquid Equilibrium (VLE) curve for complex multi-parameter technical and reference thermodynamic equations of state, in connection with ...Computational Fluid Dynamics (CFD) simulations of compressible flows of real gases. Differential algebra techniques are used to obtain an approximation of the VLE curve from the reference equation of state of carbon dioxide. Seven fourth-order Taylor polynomials are required to approximate the VLE curve for a reduced pressure between 0.7 and 1, with an error on density below 0.04%, except near the critical point where the error is around 0.1%. The proposed approach is proved to be a suitable alternative to standard Look-Up Table (LUT) techniques, with comparable accuracy and computational burden. Moreover, the explicit use of the model analytical expression in the determination of the polynomial expansions allows to reduce the number of expansion poles and it will possibly simplify the approximation of different fluids, including mixtures.
Blood cultures remain the gold standard for detecting bacteremia despite their limitations. The current practice of blood culture collection is still inefficient with low yields. Limited focus has ...been given to the association between timing of specimen collection at different time points during admission and their yield.
We carried out a retrospective observational study by analyzing all 3,890 sets of cultures collected from the 1,962 admitted patients over the seven-month period of this study. We compared the blood culture yield between the early group (≤24 hours after admission) and the late group (> 24 hours of admission). We also investigated the effect of prehospital oral antibiotics and pre-analytical time on the first cultures in the emergency department. Epidemiology and efficiency of blood cultures were studied for each medical specialty.
In total, 3,349(86.1%) blood cultures were negative and 541(13.9%) were positive for one or more microorganisms. After correcting for contamination, the overall yield was 290 (7.5%). The early group (n = 1,490) yielded significantly more true-positive cultures (10.1% versus 5.8%, P<0.001) than the late group (n = 2,400). The emergency department had a significantly higher yield than general wards, 11.2% versus 5.7% (p<0.001). Prehospital oral antibiotic use and pre-analytical time did not affect the yield of first cultures at the emergency department (p = 0.735 and 0.816 respectively). The number of tests needed to obtain one true-positive culture varied between departments, ranging from 7 to 45.
This study showed that blood cultures are inefficient in detecting bacteremia. Cultures collected during 24 hours after admission yielded more positive results than those collected later. Significant variations in blood culture epidemiology and efficiency per specialty suggest that guidelines should be reevaluated. Future studies should aim at improving blood culture yield, implementing educational programs to reduce contamination and cost-effective application of modern molecular diagnostic technologies.
The iPRSV equation of state van der Stelt, T.P.; Nannan, N.R.; Colonna, P.
Fluid phase equilibria,
09/2012, Letnik:
330
Journal Article
Recenzirano
► There is a non-physical discontinuity in the properties of PRSV cubic EoS. ► This discontinuity can affect process and CFD simulations. ► The issue of the discontinuity of properties of the PRSV ...CEoS is solved. ► The proposed solution does not require changes in fluid databases for the PRSV. ► Performance assessments and a CFD simulation with clear improvements are showed.
The Peng–Robinson cubic equation of state with the Stryjek–Vera modification (PRSV) is widely adopted in scientific studies and engineering. However, it is affected by a discontinuity in all the properties, which is caused by a discontinuity of the α-function. Aside of being non-physical, this discontinuity causes robustness and accuracy issues in numerical simulations. The discontinuity in thermodynamic properties is eliminated here without affecting the overall accuracy of the model. In addition, the functional form of α(T) is optimized in such a way that it is not required to change the values of the fluid-dependent parameters stored in the many available databases. The performance of the improved equation of state (iPRSV) is assessed by comparing calculated properties with those obtained with the original PRSV equation of state, the Gasem et al. equation of state (PRG), which is also continuous in temperature, a reference multiparameter equation of state, and experimental data. It is shown that the accuracy of the new model approaches the accuracy of the original equation of state and that it performs better than the PRG equation of state. The modified PRSV equation of state solves the issue of the artificial discontinuity in the calculation of properties relevant to scientific and industrial applications, at the cost of a small decrease in overall accuracy.
Many studies have been published on a variety of clinical applications of artificial intelligence (AI) for sepsis, while there is no overview of the literature. The aim of this review is to give an ...overview of the literature and thereby identify knowledge gaps and prioritize areas with high priority for further research.
A literature search was conducted in PubMed from inception to February 2019. Search terms related to AI were combined with terms regarding sepsis. Articles were included when they reported an area under the receiver operator characteristics curve (AUROC) as outcome measure.
Fifteen articles on diagnosis of sepsis with AI models were included. The best performing model reached an AUROC of 0.97. There were also seven articles on prognosis, predicting mortality over time with an AUROC of up to 0.895. Finally, there were three articles on assistance of treatment of sepsis, where the use of AI was associated with the lowest mortality rates. Of the articles, twenty-two were judged to be at high risk of bias or had major concerns regarding applicability. This was mostly because predictor variables in these models, such as blood pressure, were also part of the definition of sepsis, which led to overestimation of the performance.
We conclude that AI models have great potential for improving early identification of patients who may benefit from administration of antibiotics. Current AI prediction models to diagnose sepsis are at major risks of bias when the diagnosis criteria are part of the predictor variables in the model. Furthermore, generalizability of these models is poor due to overfitting and a lack of standardized protocols for the construction and validation of the models. Until these problems have been resolved, a large gap remains between the creation of an AI algorithm and its implementation in clinical practice.
•Artificial Intelligence has potential to improve identification of septic patients.•Generalizability of artificial intelligence (AI) algorithms is still poor.•AI models are often at high risk of bias due to predictor variables in the outcome.•Insufficient availability of data will decrease AI accuracy in clinical practice.•There is a large gap between creation and clinical implementation of algorithms.
The value of the fundamental derivative of gas dynamics,
Γ
, is a quantitative measure of the variation of the speed of sound with respect to density in isentropic transformations, such as those ...occurring, for example, in gas-dynamic nozzles. The accurate computation of its value, which is a constant for a perfect gas, is key to the understanding of real-gas flows occurring in a thermodynamic region where the polytropic ideal gas law does not hold. The fundamental derivative of gas dynamics is a secondary thermodynamic property and so far, no experiments have been conducted with the aim of measuring its value. Several studies document the estimation of
Γ
for fluids composed of complex molecules using mainly simple thermodynamic equations of state, e.g., that of Van der Waals. A review of these studies has revealed that the calculated values of
Γ
are affected by large uncertainties; these uncertainties are due to the functional form of the adopted equations and because of uncertainties in the available fluid property data on which these equations were fitted. In this work, the fundamental derivative of gas dynamics of molecularly simple fluids is computed with the aid of, among other models, modern reference equations of state. The accuracy of these computations has been assessed. Reference thermodynamic models however, are not available for molecularly complex fluids; some of these molecularly complex fluids are the substances of interest in studies on the so-called nonclassical gas dynamics. Therefore, results of the computation of
Γ
for few, molecularly simple hydrocarbons, like methane, ethane, etc., are used as a benchmark against which the performance of simpler equations of state, can be assessed. For the selected substances, the Peng–Robinson, Stryjek–Vera modified, cubic equation of state yields good results for
Γ
-predictions, while the modern multiparameter technical equations of state, e.g., the one in the Span–Wagner functional form, are preferable, provided that enough accurate thermodynamic data are available. Another notable result of this study, is that
Γ
for a fluid composed of complex molecules is less affected by the inaccuracy of
C
v
-information (
C
v
is the isochoric heat capacity), if compared to the estimation of
Γ
for simple molecules. Inspection of the results of the calculation of
Γ
in the proximity of the critical point confirms that analytical equations of state fail to predict the correct physical behavior, even if they include terms which allow for the correct estimation of thermodynamic properties.
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