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This paper describes an investigation of thermo-convective structures called Rayleigh-Bénard-Poiseuille (RBP) convection in a wide rectangular water channel formed above a heated bottom-wall. The phenomenon was investigated experimentally using planar Particle Image Velocimetry (PIV) for velocity vector field data and 2-color Laser-Induced Fluorescence (LIF) for temperature scalar field data. The PIV and LIF records were taken simultaneously on three cameras (one for PIV and two for LIF) in the central vertical plane of the channel in order to detect the secondary flow caused by the natural convection. The result is the combined temperature and velocity data used to evaluate the onset point of the secondary flow. The manifestation of this phenomenon on both velocity and temperature fields is described. The LIF method was confirmed to give more satisfactory results because the PIV method is generally less suitable for lower velocity measurement in boundary layers, both because of adverse reflections and because of the limitation for measuring very low velocities near walls. The measured data were compared with theoretical dependencies and with the literature data. Other limitations of the measurement were described, and steps for its improvement were suggested. In the present setup, temperature influence by the laser or wall accumulation was observed, which needs to be investigated more thoroughly and eliminated in future work.

This paper describes the preliminary measurements of the temperature field in water channel by the Planar Laser-Induced Fluorescence (pLIF) method in the one-colour mode using commercial LaVision system. The aim of the work is to find suitable parameters of the experiment (concentration of fluorescent dye, laser power, arrangement of the experiment, etc.) for measuring the temperature field in a closed glass channel of rectangular cross-section with a heated bottom-wall and to verify an applicability of this method in general. The intention is to carry out measurements in the transverse plane of the duct such that the specific thermos-convective structures of the Rayleigh-Bénard-Poiseuille convection, i.e. mixed convection, are detected. For simplicity, the initial measurements were performed in the natural convection regime, i.e. the water in the channel was not driven by the circulation pump. In this case, a larger temperature gradient was created near the wall compared to the force convection case when a cold liquid flows continuously into the experimental area. The presented work serves as a methodological guide for future measurements and provides valuable experience for the design of a new channel for investigation of the thermal flow instabilities.

Vibrating tube densimeters (VTDs) have become relatively popular instruments for measuring the density of a large variety of gases and liquids since their development about fifty years ago. The apparatuses measure the characteristic frequencies of vibrating U-shaped tube filled with a fluid sample, when the fluid fill shifts the dynamic characteristics of the U-tube. This study describes design and testing of the in-house developed supporting setup for a commercial high-pressure VTD Anton Paar DMA HP. The instrument covers density range from 0 to 3000 kg/m3 at pressures up to 700 bar and temperatures from -10 to 200 °C. Whereas the temperature of density measurement is fully controlled by the apparatus using a thermal block with Peltier unit, pressure has to be generated and monitored externally. Therefore, a high-pressure sampling system was designed and assembled. Special care is taken when measuring temperature with an external resistance thermometer connected to precise thermometry bridge. In order to avoid unfavorable condensation of air moisture during the measurement, VTD apparatus is protected by a dry box of own design. Technical details together with the calibrations of pressure and temperature probes are described in detail. A preliminary data for temperature dependence of density of the selected liquids (water, ethanol or ethylene glycol) obtained at the barometric pressure are also provided and compared to the reference density correlations.

In this article, a literature survey of the experimental channels used for investigation of a non-isothermal flow is presented. Particular interest is focused on the flow stability with a heated wall by a uniform heat flux. The aim of this work is to prepare a comprehensive overview of the technical design and experimental settings used in previous studies. Collected information shall help to design and manufacture new horizontal channel with a rectangular cross-section with a width of around 200 mm and a height of 20 mm. The channel will be equipped with a constant heat flux section on a bottom wall for investigation of the vortex structures and the flow instabilities using modern optical methods. The channel will operate with liquid (most probably water) in the range of low Reynolds numbers. Parallel laminar flow with Poiseuille velocity profile at the inlet of a heated section and a uniform heat flux predefine the essential requirements on the channel design.

An experimental channel intended for investigation of a wake behind a cooled cylinder was designed and constructed in this work. The setup is designed for measurements with air at low Reynolds numbers. The measuring techniques based on the standard visualization with a fog generator and the constant temperature anemometry (CTA) were used. The flow performance was investigated behind two cylinders placed inside a channel with square cross section. One cylinder was kept at a constant flow temperature, while the second cylinder could be cooled using a water circuit with a thermostatic bath. The dependency of the Strouhal number on the Reynolds number was evaluated and compared to the literature data and the empirical correlations. The concept of an effective temperature in a non-isothermal flow was also applied. Unfortunately, a considerable difference between the preliminary measurements and the literature data was detected. The air-flow was found to have unsatisfactory high turbulence intensity. Consequently, some modifications of the current channel design are proposed.

This article represents a part of an ongoing work on the preparation of an experimental rectangular channel for the PIV and LIF measurements of vortex structures and temperature field in a non-isothermal water flow. The main aim of the current study is to develop a sufficiently accurate simplified numerical model of the real problem. The basic requirements for thermal properties of heated bottom-wall are specified. In the computational model, there are several simplifications such as 2D case and a constant temperature of a heated surface along the longitudinal coordinate. Results of the numerical simulation of the fluid flow and heat transfer are verified on the experimental data obtained in a laboratory channel with the same geometry and similar flow conditions. The presented results helped to define additional requirements on the design of a new experimental channel intended for investigation of the flow instabilities in a non-isothermal liquid flow.

The study presents progress in the development of mixture preparation device (MPD) representing an important part of the larger experimental setup intended for investigation of homogeneous droplet nucleation in CO2-rich systems. MPD allows for accurate adjustment of flow parameters, i.e. temperature, pressure, and flow rate, of CO2 in either superheated vapor or supercritical fluid phases and of other gas components such as argon or nitrogen. Through accurate settings of flow rates of individual components, the mixture composition can continuously be defined. MPD is going to be connected to the expansion chamber, where the droplet nucleation will experimentally be observed. In this work, CO2-branch, i.e. the core part of MPD, was modified and tested. Several components, e.g., pressure transducers and safety valve, had to be calibrated and adjusted to assure well-defined and safe operation. Most attention was paid to the design and performance of throttling capillary tubes installed in thermostatic bath, which define final flow parameters of CO2 coming from the CO2 branch. The flow characteristics of two capillary tubes with lengths of 7.8 and 4.0 m and inner diameter 0.1 mm were measured and compared to the predictions of a numerical model. The 1-D model of isothermal capillary flow was found to provide quite good agreement with the measured data.

An experimental apparatus originally developed for the measurement of surface tension of supercooled water was modified such that it allows for measurement of binary aqueous mixtures. The measuring principle based on the capillary rise technique is similar to that employed in the previous measurements with pure water [J. Hrubý et al., J. Phys. Chem. Lett. 5 (2014) 425 and V. Vinš et al., J. Phys. Chem. B 119 (2015) 5567]. The temperature dependence of the surface tension is determined from the measured height of the liquid column elevated in a vertical capillary tube with an inner diameter of 0.32 mm. The aqueous liquid rises to a height of 60 mm to 95 mm depending on the solute concentration and the temperature. Preliminary tests performed with the binary mixtures of water with methanol, ethanol, and n-propanol at various concentrations and temperatures down to –28 °C approved functionality of the measuring technique. Some difficulties, such as influence of impurities on the liquid column elevation or formation of bubbles in the liquid column, were observed. The experimental apparatus is further being modified in order to obtain more accurate data for various aqueous mixtures.

A unique in-house designed experimental apparatus for investigation of nucleation of droplets in CCS relevant systems is being developed by the present team. The apparatus allows simulating various processes relevant to CCS technologies. Gaseous mixtures with CO2 are prepared in a Mixture Preparation Device (MPD) based on accurate adjustment of flow rates of individual components [EPJ Web of Conferences 143, 02140 (2017)]. The mixture then flows into an expansion chamber, where it undergoes a rapid adiabatic expansion. As a consequence of adiabatic cooling, the mixture becomes supersaturated and nucleation and simultaneous growth of droplets occurs. In this study, we describe the design and testing of the expansion part of the experimental setup. The rapid expansion was realized using two valve systems, one for low pressures (up to 0.7 MPa) and the other for high pressures (up to 10 MPa). A challenge for a proper design of the expansion system is avoiding acoustic oscillations. These can occur either in the mode of Helmholtz resonator, where the compressible gas in the chamber acts as a spring and the rapidly moving gas in the valve system as a mass, or in the “flute” mode, where acoustic waves are generated in a long outlet tubing.

The temperature and pressure correlations for the volume of gas hydrates forming crystal structures sI and sII developed in previous study [Fluid Phase Equilib. 427 (2016) 268-281], focused on the modeling of pure gas hydrates relevant in CCS (carbon capture and storage), were revised and modified for the modeling of mixed hydrates in this study. A universal reference state at temperature of 273.15 K and pressure of 1 Pa is used in the new correlation. Coefficients for the thermal expansion together with the reference lattice parameter were simultaneously correlated to both the temperature data and the pressure data for the lattice parameter. A two-stage Levenberg Marquardt algorithm was employed for the parameter optimization. The pressure dependence described in terms of the bulk modulus remained unchanged compared to the original study. A constant value for the bulk modulus B0 = 10 GPa was employed for all selected hydrate formers. The new correlation is in good agreement with the experimental data over wide temperature and pressure ranges from 0 K to 293 K and from 0 to 2000 MPa, respectively. Compared to the original correlation used for the modeling of pure gas hydrates the new correlation provides significantly better agreement with the experimental data for sI hydrates. The results of the new correlation are comparable to the results of the old correlation in case of sII hydrates. In addition, the new correlation is suitable for modeling of mixed hydrates.

A unique in-house designed experimental apparatus for investigation of nucleation of droplets in CCS relevant systems is being developed by our team. The nucleation is measured with the help of a rapid pressure drop within an expansion chamber. In this study, a CO2 branch representing an important part of the mixture preparation device (MPD) was designed, assembled and tested. MPD is intended for preparation of CO2-rich mixtures in a sense of accurate setting of flow rates of individual gaseous components through the experimental section with the expansion chamber. In the CO2 branch, the saturated liquid CO2 coming from a cylinder with a dip tube is pressurized by a supercritical pump with a constant flow rate to the pressures of up to 340 bar. The required mass flow of the supercritical CO2 of up to 180 g/h at pressures between 74 bar and 150 bar is accurately tuned with a help of a set of stainless steel capillary tubes with inner diameter of 0.1 mm. Some of the tubes are submerged in a thermostatic bath set to a constant temperature of 70 °C in order to assure the supercritical flow conditions of CO2. Needed lengths of the capillary tubes were determined with a help of a one-dimensional numerical model allowing prediction of both isothermal and adiabatic flow conditions.

An experimental apparatus with a horizontal capillary tube for measurement of the surface tension of supercooled liquids, i.e. liquids in a metastable state below the equilibrium freezing point, was designed and tested in the previous study [V. Vinš et al., EPJ Web Conf. 92, 02108 (2015)]. In this work, recent modifications of both the experimental setup and the measurement analysis are described. The main aim is to improve the accuracy and the reproducibility of measured surface tension and to achieve higher degrees of supercooling. Temperature probes measuring the temperature of cooling medium near the horizontal capillary tube were calibrated in the relevant temperature range from – 31 °C to + 45 °C. An additional pressure transducer was installed in the helium distribution setup at the position close to the capillary tube. The optical setup observing the liquid meniscus at the open end of the horizontal capillary tube together with the video analysis were thoroughly revised. The red laser illuminating the liquid meniscus, used at the original apparatus, was replaced by a fiber optic light source, which significantly improved the quality of the meniscus image. The modified apparatus was used for the measurement of surface tension of supercooled water at temperatures down to – 11 °C. The new data have a lower scatter compared to the previous horizontal measurements and show a good agreement with the other data obtained with a different measurement technique based on the modified capillary rise method.

Development of methods for accurate modeling of phase interfaces is important for understanding various natural processes and for applications in technology such as power production and carbon dioxide separation and storage. In particular, prediction of the course of the non-equilibrium phase transition processes requires knowledge of the properties of the strongly curved phase interfaces of microscopic droplets. In our work, we focus on the spherical vapor–liquid phase interfaces for binary mixtures. We developed a robust computational method to determine the density and concentration profiles. The fundamentals of our approach lie in the Cahn-Hilliard gradient theory, allowing to transcribe the functional formulation into a system of ordinary Euler-Langrange equations. This system is then split and modified into a shape suitable for iterative computation. For this task, we combine the Newton-Raphson and the shooting methods providing a good convergence speed. For the thermodynamic roperties, the PC–SAFT equation of state is used. We determine the density and concentration profiles for spherical phase interfaces at various saturation factors for the binary mixture of CO2 and C9H20. The computed concentration profiles allow to the determine the work of formation and other characteristics of the microscopic droplets.

In our previous study [Planková et al., EPJWeb. Conf. 92, 02071 (2015)], several molecular simulations of vapor-liquid phase interfaces for pure water were performed using the DL_POLY Classic software. The TIP4P/2005 molecular model was successfully used for the modeling of the density profile and the thickness of phase interfaces together with the temperature dependence of the surface tension. In the current study, the extended simple point charge (SPC/E) model for water was employed for the investigation of vapor-liquid phase interfaces over a wide temperature range from 250 K to 600 K. The TIP4P/2005 model was also used with the temperature step of 25 K to obtain more consistent data compared to our previous study. Results of the new simulations are in a good agreement with most of the literature data. TIP4P/2005 provides better results for the saturated liquid density with its maximum close to 275 K, while SPC/E predicts slightly better saturated vapor density. Both models give qualitatively correct representation for the surface tension of water. The maximum absolute deviation from the IAPWS standard for the surface tension of ordinary water is 10.4 mN · m−1 and 4.1 mN · m−1 over the temperature range from 275 K to 600 K in case of SPC/E and TIP4P/2005, respectively.

Molecular dynamics simulations for water were run using the TIP4P/2005 model for temperatures ranging from 250 K to 600 K. The density profile, the surface tension and the thickness of the phase interface were calculated as preliminary results. The surface tension values matched nicely with the IAPWS correlation over wide range of temperatures. As a partial result, DL_POLY Classis was successfully used for tests of the new computing cluster in our institute.

The paper describes molecular dynamics study of nucleation of water in NVE ensemble. The numerical simulation was performed with the DL_POLY. The metastable steam consisting of 10976 water molecules with TIP4P/2005 potential was driven on the desired energy level by a simulation at constant temperature, and then the nucleation at constant energy was studied for several tens of nanoseconds, which was sufficient for clusters to evolve at hundred molecules size. The results were compared with the previously published results and the classical nucleation theory predictions.

New experimental apparatus for measurement of the surface tension of liquids under the metastable supercooled state has been designed and assembled in the study. The measuring technique is similar to the method employed by P.T. Hacker [NACA TN 2510] in 1951. A short liquid thread of the liquid sample was sucked inside a horizontal capillary tube partly placed in a temperature-controlled glass chamber. One end of the capillary tube was connected to a setup with inert gas which allowed for precise tuning of the gas overpressure in order of hundreds of Pa. The open end of the capillary tube was precisely grinded and polished before the measurement in order to assure planarity and perpendicularity of the outer surface. The liquid meniscus at the open end was illuminated by a laser beam and observed by a digital camera. Application of an increasing overpressure of the inert gas at the inner meniscus of the liquid thread caused variation of the outer meniscus such that it gradually changed from concave to flat and subsequently convex shape. The surface tension at the temperature of the inner meniscus could be evaluated from the overpressure corresponding to exactly planar outer meniscus. Detailed description of the new setup together with results of the preliminary tests is provided in the study.

The density gradient theory (GT) combined with a SAFT-type (Statistical Associating Fluid Theory) equation of state has been used for modeling the surface tension of associating fluids represented by a series of six alkanols ranging from methanol to 1-pentanol. The effect of nonzero dipole moment of the selected alkanols on the predicted surface tension was investigated in this study. Results of the GT + non-polar Perturbed Chain (PC) SAFT equation of state were compared to predictions of GT combined with the PC-polar-SAFT, i.e. PCP-SAFT, equation. Both GT + PC-SAFT and GT + PCP-SAFT give reasonable prediction of the surface tension for pure alkanols. Results of both models are comparable as no significant difference in the modeled saturation properties and in the predicted surface tension using GT was found. Consideration of dipolar molecules of selected alkanols using PCP-SAFT had only minor effect on the predicted properties compared to the non-polar PC-SAFT model.

In this work, we used the density gradient theory (DGT) combined with the cubic equation of state (EoS) by Peng and Robinson (PR) and the perturbed chain (PC) modification of the SAFT EoS developed by Gross and Sadowski [1]. The PR EoS is based on very simplified physical foundations, it has significant limitations in the accuracy of the predicted thermodynamic properties. On the other hand, the PC-SAFT EoS combines different intermolecular forces, e.g., hydrogen bonding, covalent bonding, Coulombic forces which makes it more accurate in predicting of the physical variables. We continued in our previous works [2,3] by solving the boundary value problem which arose by mathematical solution of the DGT formulation and including the boundary conditions. Achieving the numerical solution was rather tricky; this study describes some of the crucial developments that helped us to overcome the partial problems. The most troublesome were computations for low temperatures where we achieved great improvements compared to [3]. We applied the GT for the n-alkanes: nheptane, n-octane, n-nonane, and n-decane because of the availability of the experimental data. Comparing them with our numerical results, we observed great differences between the theories; the best results gave the combination of the GT and the PC-SAFT. However, a certain temperature drift was observed that is not satisfactorily explained by the present theories.

Gas-hydrates (clathrates) are non-stoichiometric crystallized solutions of gas molecules in the metastable water lattice. Two or more components are associated without ordinary chemical union but through complete enclosure of gas molecules in a framework of water molecules linked together by hydrogen bonds. The clathrates are important in the following applications: the pipeline blockage in natural gas industry, potential energy source in the form of natural hydrates present in ocean bottom, and the CO2 separation and storage. In this study, we have modified an analytical solid-liquid-vapor equation of state (EoS) [A. Yokozeki, Fluid Phase Equil. 222–223 (2004)] to improve its ability for modeling the phase equilibria of clathrates. The EoS can predict the formation conditions for CO2- and CH4-hydrates. It will be used as an initial estimate for a more complicated hydrate model based on the fundamental EoSs for fluid phases.

The study presents some preliminary results of the density gradient theory (GT) combined with two different equations of state (EoS): the classical cubic equation by van der Waals and a recent approach based on the statistical associating fluid theory (SAFT), namely its perturbed-chain (PC) modification. The results showed that the cubic EoS predicted for a given surface tension the density profile with a noticeable defect. Bulk densities predicted by the cubic EoS differed as much as by 100 % from the reference data. On the other hand, the PC-SAFT EoS provided accurate results for density profile and both bulk densities in the large range of temperatures. It has been shown that PC-SAFT is a promising tool for accurate modeling of nucleation using the GT. Besides the basic case of a planar phase interface, the spherical interface was analyzed to model a critical cluster occurring either for nucleation of droplets (condensation) or bubbles (boiling, cavitation). However, the general solution for the spherical interface will require some more attention due to its numerical difficulty.

This study reviews available van der Waals- and Platteeuw-based hydrate models considering multiple occupancy of cavities. Small guest molecules, such as hydrogen and nitrogen, are known to occupy lattice cavities multiple times. This phenomenon has a significant impact on hydrate stability and thermodynamic properties of the hydrate phase. The objective of this work is to provide a comprehensive overview and required correlations for the implementation of a computationally sufficient cluster model that considers up to five guest molecules per cavity. Two methodologies for cluster size estimation are evaluated by existing nitrogen hydrate models showing accurate results for phase equilibria calculations. Furthermore, a preliminary hydrogen hydrate model is introduced and compared with the results of other theoretical studies, indicating that double occupancy of small sII cavities is improbable and four-molecule clusters are predominant in large sII cavities for pressures above 300 MPa. This work lays the foundation for further exploration and optimization of hydrate-based technologies for small guest molecules, e.g., storage and transportation, emphasizing their role in the future landscape of sustainable energy solutions. [ABSTRACT FROM AUTHOR]

Homogeneous droplet nucleation has been studied for almost a century but has not yet been fully understood. In this work, we used the density gradient theory (DGT) and considered the influence of capillary waves (CWs) on the predicted size-dependent surface tensions and nucleation rates for selected n-alkanes. The DGT model was completed by an equation of state (EoS) based on the perturbed-chain statistical associating fluid theory and compared to the classical nucleation theory and the Peng-Robinson EoS. Itwas found that the critical clusters are practically free of CWs because they are so small that even the smallestwavelengths of CWs do not fit into their finite dimensions. The CWs contribute to the entropy of the system and thus decrease the surface tension. A correction for the effect of CWs on the surface tension is presented. The effect of the different EoSs is relatively small because by a fortuitous coincidence their predictions are similar in the relevant range of critical cluster sizes. The difference of the DGT predictions to the classical nucleation theory computations is important but not decisive. Of the effects investigated, the most pronounced is the suppression of CWs which causes a sizable decrease of the predicted nucleation rates. The major difference between experimental nucleation rate data and theoretical predictions remains in the temperature dependence. For normal alkanes, this discrepancy is much stronger than observed, e.g., for water. Theoretical corrections developed here have a minor influence on the temperature dependency. We provide empirical equations correcting the predicted nucleation rates to values comparable with experiments. [ABSTRACT FROM AUTHOR]

Hydrofluoroethers (HFEs) represent a new generation of promising engineering fluids for heat transfer or cleaning applications. In this work, quantum chemistry calculations (qcc) were employed to obtain ideal gas heat capacities, Cpig, for the selected HFEs and comparisons were made against the group contribution (GC) methods by Rihani and Doraiswamy, Yoneda, and Joback. Comparison between B3LYP/6-31++ G(d,p) density functional theory (DFT) and Hartree–Fock (HF) methods showed that HF method provides better representation of the available experimental gas-phase speed of sound data for HFE-7000. Critical properties and acentric factors of the selected HFEs were optimized and compared to the other reported values. The Peng–Robinson equation of state (PR EoS) combined with the Cpig correlation, allowing calculation of the ideal gas Helmholtz free energy, was used to model a complete set of thermodynamic properties of the five selected HFEs; namely HFE-7000, HFE-7100, HFE-7200, HFE-7300, and HFE-7500. The volume-translated (VT) PR EoS was also tested as an alternative. The accuracy of PR EoS for representing the phase behavior and caloric properties of the selected HFEs was analyzed based on the comparison with nearly 3500 experimental data points and a preliminary multiparameter EoS available for HFE-7000. Although relatively simple, but still widely used, PR EoS was found to provide reasonable vapor–liquid predictions for HFEs and, as such, can be used effectively in the design of their various applications. In addition, a vapor pressure correlation and the critical compressibility factor were analyzed from the view of application on various alternative refrigerants such as HFEs and hydrofluoroolefines. [ABSTRACT FROM AUTHOR]