► 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.
Herring is the third most commercialized fish species in the EU and a common host of Anisakis spp. larvae. The aim of this study was to assess the occurrence, distribution and viability of Anisakis ...spp. larvae in different kind of ready to eat (RTE) herring products. One hundred and thirty-five products, consisting of 50 smoked whole specimens and 85 filleted products were collected in Italy from 2016 to 2018. Viscera and muscle of whole herrings were visually inspected and separately submitted to artificial digestion. Filleted products were also visually inspected and digested. Natural and UV light were used for examining the residual material. Nematodes larvae viability was assessed, then they were counted, collected and identified to genus level by optical microscopy. In addition, the cox2 gene was targeted for the identification of part of the larvae. Anisakis spp. larvae were found in 56 products (41.5%) and overall 1715 larvae were collected (range 0–172 larvae/product). Most of the larvae (1559, 91%) were found in the viscera of 49 of the 50 whole herrings (98%). A statistically significant difference (p < 0.001) was observed in the positivity rate and the larval density of the remaining 156 larvae found at muscle level, as 149 larvae were found in the muscle of 31 whole herrings (positivity rate 62%, 0.022 larval density/g), while only 7 larvae were found in the 85 filleted products (positivity rate 7%, 0.001 larval density/g). Larvae were molecularly identified as A. simplex. Although all the larvae were dead, the high level of contamination of whole herrings on the market poses some issues related to the presence of a potentially hazardous defect. In particular, the significant difference between infection levels of muscle tissue of whole and filleted herrings, likely due to differences in the production process, results in a different risk of exposure to parasitic antigens. Therefore, a better management of the herring supply chain is required to improve the whole quality and to protect consumers’ health.
•Different kinds of herring ready to eat products were analysed by digestion.•41.5% of the products were positive for at least one visible Anisakis spp. larva.•A total of 1715 dead larvae were collected.•Whole smoked products were heavily contaminated at visceral and muscle level.•The product category influenced the positivity rate and larval density.
This paper describes the commissioning of the flexible asymmetric shock tube (FAST), a novel Ludwieg tube-type facility designed and built at Delft University of Technology, together with the results ...of preliminary experiments. The FAST is conceived to measure the velocity of waves propagating in dense vapours of organic fluids, in the so-called non-ideal compressible fluid dynamics (NICFD) regime, and can operate at pressures and temperatures as high as 21 bar and 400
∘
C, respectively. The set-up is equipped with a special fast-opening valve, separating the high-pressure charge tube from the low-pressure plenum. When the valve is opened, a wave propagates into the charge tube. The wave speed is measured using a time-of-flight technique employing four pressure transducers placed at known distances from each other. The first tests led to the following results: (1) the leakage rate of
5
×
10
-
4
mbar
l
s
-
1
for subatmospheric and
5
×
10
-
2
mbar
l
s
-
1
for a superatmospheric pressure is compatible with the purpose of the conceived experiments, (2) the process start-up time of the valve has been found to be between 2.1 and 9.0 ms, (3) preliminary rarefaction wave experiments in the dense vapour of siloxane
D
6
(dodecamethylcyclohexasiloxane, an organic fluid) were successfully accomplished up to temperatures of
300
∘
C
, and (4) a method for the estimation of the speed of sound from wave propagation experiments is proposed. Results are found to be within 2.1 % of accurate model predictions for various gases. The method is then applied to estimate the speed of sound of
D
6
in the NICFD regime.
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.
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 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.
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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