Enthalpies Of Mixtures Research Articles

Numerical investigation on the transient transport and heat transfer characteristics of transpiration cooling with liquid phase change during coolant adjustment

• A new transient model is proposed for phase-change transpiration cooling. • Dynamic fluid–solid coupling heat transfer during coolant adjustment is studied. • Dramatic variation of injection pressure with coolant mass flux is simulated. • Influence of coolant phase change location on transport characteristic is analyzed. Aimed at learning the variation law of injection pressure with coolant mass flux and promoting the active control of phase-change transpiration cooling, this study proposes a new transient mathematical model by simplifying the two-phase mixture model (TPMM) with the introduction of modified mixture enthalpy and enthalpy ratio. The smooth change with time of mixture enthalpy and enthalpy ratio improves the convergence and stability of TPMM. Experiment is first carried out to verify the accuracy of the new model. Then the two-phase flow and corresponding fluid–structure coupling heat transfer processes of transpiration cooling are studied with the new model under both steady and transient conditions. Besides, the relationship link from coolant adjustment, corresponding pressure and mass flux to the cooled wall temperature is discussed. The steady results show that injection pressure is closely related with phase change location, and the huge change of fluid kinematic viscosity and pumping effect of capillary force induces the non-linear variation of injection pressure with coolant mass flux. The transient results reveal that injection pressure changes with different laws when the mass flux increases and decreases. The changing speed of mass flux and scale of external heat flux also have a vital influence on the fluctuation of injection pressure, the faster the mass flow rate changes and the greater the heat flux is, more tempestuously the injection pressure varies.

μFlowCal – High‐Resolution Differential Flow Microcalorimeter for the Measurement of Heats of Mixing

AbstractThis work presents the description, calibration, test and performance evaluation of a new flow microcalorimeter. The μFlowCal is a differential heat conduction, flow microcalorimeter, designed to work in isothermal conditions, in differential mode and in the absence of gas phase, requiring small amounts of sample (200 μL). Both the microcalorimeter and its operational methodology were optimized to measure the heat of mixing of fluids with moderate viscosity and volatility. The microcalorimeter has a sensitivity of 116 μV mW−1 and a level of detection of 170 nW. A very good performance (uncertainty <5 %) was obtained for the selected test processes: enthalpy of dilution of aqueous sucrose solutions and excess enthalpy of binary mixtures of decane‐octanol. The μFlowCal prototype is a new tool in micro‐scale physical chemistry and a step forward concerning signal resolution and sample volume. Nowadays, these are major requirement for the study of novel promising solvents and their mixtures.

Performance evaluation of alternative refrigerants for R134a in automotive air conditioning system

AbstractAiming to explore environmentally friendly working fluids replace R134a for automotive air conditioning applications, seven potential pure refrigerants were analyzed first in terms of safety, environmental characteristics, and cycle performances. It was found that the pure refrigerants have some defects when applied individually. However, using the principle of complementary advantages, the mixtures employed in automotive air conditioning can make up for the shortcomings existing in the pure refrigerants. On this basis, the thermophysical models of vapor‐liquid equilibrium, enthalpy, entropy, and critical properties of binary mixtures were established. Also, the properties and cycle performances of 14 blends were investigated, and the results indicated that the predicted models of vapor‐liquid equilibrium have high accuracy, where the average absolute relative deviations of pressures and the average absolute deviations of vapor‐phase mole fractions were less than 2% and 0.005, respectively. M1 (R13I1/R152a [30/70]) and M11 (R13I1/R290 [40.3/59.7]) showed superiorities in cycle performances and environmental and safety characteristics. Moreover, the operating pressures of which were close to that of R134a, and they were expected to become the substitutes for R134a in automotive air conditioners.

Thermodynamics of hydrate systems using a uniform reference state

AbstractFormation of natural gas hydrates during processing and transport of natural gas has historically been a significant motivator for hydrate research. The last three decades have also seen the focus increasingly shifting towards CH4 hydrates as a potential energy source. And in the context of climate changes, the impact of hydrate‐related processes is coming more to the forefront as well. This interest is not only limited to leakage fluxes of CH4 from natural gas hydrates but also flux from conventional hydrocarbon systems entering the seafloor at temperature and pressure allowing for hydrate formation. Alternative ways to treat formally overdetermined hydrate systems is an important focus in this work. The most common method used for assessment of hydrate phase transitions involves several fitted parameters to calculate the free energy difference between liquid water and empty hydrate. This technique calls for an empirical fitting of fundamental thermodynamic properties. Numerical codes based on this method limit the models to hydrate formation only from free gas and liquid water. This is at least true for all commercial and academic codes that were examined prior to this work. This work addresses the advantages in using residual thermodynamics for all phases, including hydrates. In addition to making it possible to handle many alternative hydrate routes leading to hydrate formation or dissociation, the presented method also opens a way to calculate a variety of needed thermodynamic properties (e.g., enthalpies of pure components and mixtures) in a simple and consistent way. This approach will be illustrated through calculations of various hydrate phase transitions, examples of free energy calculations for comparison of phase stability, and calculation of enthalpies of hydrate formation. Calculated enthalpies are compared with experimental data as well as results derived from applying the Clapeyron equation. Mechanisms for conversions of in situ CH4 hydrate to facilitate safe CO2 storage are also discussed. A very simple Clapeyron‐based scheme for calculation of enthalpies for hydrate phase transitions is also proposed.

A new preferential diffusion model applied to FGM simulations of hydrogen flames

Preferential diffusion is very important in simulations of hydrogen flames. Flame stretch and curvature induce strong preferential diffusion effects in laminar premixed hydrogen flames, causing strong local deviations from the unburnt mixture fraction in the reaction zone. In tabulated chemistry methods, this necessitates the use of a partially premixed model even if the inlet mixture is purely premixed. Furthermore, in realistic combustion problems heat losses often play a dominant role. In this paper we derive a preferential diffusion model for constant but non-unity species Lewis numbers using three controlling variables, namely mixture fraction, progress variable and enthalpy. The model has been implemented in the Flamelet Generated Manifold (FGM) approach and validated by comparing with detailed chemistry simulations. As a test case we investigate a 2D laminar premixed hydrogen flame stabilised on an isothermal slit burner. Additionally, the model was compared with the standard treatment of preferential diffusion in FGM to show the increase in accuracy of the new model presented in this paper. The new model shows a significant improvement compared to the previous model, which can be attributed to the inclusion of cross-diffusion. The importance of the additional diffusion terms and its variation in mixture fraction for initially purely premixed hydrogen flames is highlighted.

A lattice Boltzmann model for reactive mixtures.

A new lattice Boltzmann model for reactive ideal gas mixtures is presented. The model is an extension to reactive flows of the recently proposed multi-component lattice Boltzmann model for compressible ideal gas mixtures with Stefan–Maxwell diffusion for species interaction. First, the kinetic model for the Stefan–Maxwell diffusion is enhanced to accommodate a source term accounting for the change in the mixture composition due to chemical reaction. Second, by including the heat of formation in the energy equation, the thermodynamic consistency of the underlying compressible lattice Boltzmann model for momentum and energy allows a realization of the energy and temperature change due to chemical reactions. This obviates the need for ad-hoc modelling with source terms for temperature or heat. Both parts remain consistently coupled through mixture composition, momentum, pressure, energy and enthalpy. The proposed model uses the standard three-dimensional lattices and is validated with a set of benchmarks including laminar burning speed in the hydrogen–air mixture and circular expanding premixed flame.This article is part of the theme issue ‘Progress in mesoscale methods for fluid dynamics simulation’.

Two-Zone Simulation of an Axial Vane Rotary Engine Cycle

An axial vane rotary engine (AVRE) is a novel type of rotary engines. The engine is a positive displacement mechanism that permits the four “stroke” action to occur in one revolution of the shaft with a minimum number of moving components in comparison to reciprocating engines. In this paper, a two-zone combustion model is developed for a spark ignition AVRE. The combustion chamber is divided into burned and unburned zones and differential equations are developed for the change in pressure and change in temperature in each zone. The modelling is based on equations for energy and mass conservation, equation of state, and burned mass fraction. The assumption is made that both zones are at the same pressure <i>P</i>, and the ignition temperature is the adiabatic flame temperature based on the mixture enthalpy at the onset of combustion. The developed code for engine simulation in MATLAB is applied to another engine and there is a good agreement between results of this code and results related to the engine chosen for validation, so the modelling is independent of configuration.

Estimating of Surface Tension and Viscosity of Liquid Ag-Cu Alloys

The present paper compares the results obtained from the experiments for the binary liquid Cu-Ag system at a temperature of 1373K. All viscosity models available in the existing literature on the viscosity dependence of the viscosity of binary liquid alloys were used. The same process was carried out for surface tension at a temperature of 1423 K for a dual liquid Cu-Ag system. In the literature survey, the Miedema model showed little use in determining the thermophysical properties of this alloy. Using Miedema model, the mixture enthalpy and excess Gibbs free energy in the aforementioned models were calculated. In the present study, a correspondence was found between surface tension and viscosity values. Surface tension and viscosity values were inversely proportional to the temperature, at a fixed silver fraction xAg, and directly proportional to copper content at a constant temperature. The obtained results were compared to the data in the existing literature.

A pressure based solver for simulation of non-equilibrium wet steam flows

This paper describes a pressure-based solver based on the finite volume approach for the simulation of high-speed wet steam flows with non-equilibrium condensation. The governing mathematical model consists of the system of Navier–Stokes equations for common mixture density, velocity, and enthalpy equipped with an appropriate nonlinear equation of state based on IAPWS formulation and with additional equations for liquid mass fraction and droplet radius distribution. The proposed method has been implemented into open-source package OpenFOAM and validated using available experimental data for nozzle flows. Finally, it was used for simulations of flows in turbine cascades.

Enthalpic and Liquid-Phase Adsorption Study of Toluene-Cyclohexane and Toluene-Hexane Binary Systems on Modified Activated Carbons.

The liquid-phase adsorption of toluene in cyclohexane and hexane solutions on modified activated carbons was evaluated; the energy involved in the interaction between these solutions and the solids was determined by immersion enthalpies of pure solvents and their mixtures, and the contribution of the system constituents was calculated by differential enthalpies. The thermal treatment generated modifications that favored adsorption and interaction with the evaluated solutions, since it increased the textural parameters and the basic character of the samples. Cyclohexane could create greater competition with the adsorption sites compared to hexane, but it favored the increase in adsorption capacities (0.416 to 1.026 mmol g−1) and the interactions with the solid evaluated through the immersion enthalpies. The immersion enthalpies of pure solvents (−16.36 to −112.7 J g−1) and mixtures (−25.65 to −104.34 J g−1) had exothermic behaviors that were decreasing due to the possible displacement of solvent molecules when increasing the solute concentration in the mixtures. The differential enthalpies for toluene were negative (−18.63 to −2.14 J), mainly due to the π–π interaction with the solid, while those of the solvent–solid component tended to be positive values (−4.25 to 55.97 J) due to the displacement of the solvent molecules by those of toluene.

Toward In Silico Prediction of CO2 Diffusion in Champagne Wines.

Carbon dioxide diffusion is the main physical process behind the formation and growth of bubbles in sparkling wines, especially champagne wines. By approximating brut-labeled champagnes as carbonated hydroalcoholic solutions, molecular dynamics (MD) simulations are carried out with six rigid water models and three CO2 models to evaluate CO2 diffusion coefficients. MD simulations are little sensitive to the CO2 model but proper water modeling is essential to reproduce experimental measurements. A satisfactory agreement with nuclear magnetic resonance (NMR) data is only reached at all temperatures for simulations based on the OPC and TIP4P/2005 water models; the similar efficiency of these two models is attributed to their common properties such as low mixture enthalpy, same number of hydrogen bonds, alike water tetrahedrality, and multipole values. Correcting CO2 diffusion coefficients to take into account their system-size dependence does not significantly alter the quality of the results. Estimates of viscosities deduced from the Stokes–Einstein formula are found in excellent agreement with viscometry on brut-labeled champagnes, while theoretical densities tend to underestimate experimental values. OPC and TIP4P/2005 water models appear to be choice water models to investigate CO2 solvation and transport properties in carbonated hydroalcoholic mixtures and should be the best candidates for any MD simulations concerning wines, spirits, or multicomponent mixtures with alike chemical composition.

Glass-Forming Ability and Thermal Properties of Al70Fe12.5V12.5X5(X = Zr, Nb, Ni) Amorphous Alloys via Minor Alloying Additions.

The Al70Fe12.5V12.5Ni5, Al70Fe12.5V12.5Zr5 and Al70Fe12.5V12.5Nb5 alloys were prepared via mechanical alloying. The influence of Zr, Nb or Ni addition on the glass-forming ability of Al-Fe-V amorphous alloys have been investigated. The structure of Al70Fe12.5V12.5Ni5 was amorphous and Al70Fe12.5V12.5Zr5 was not completely amorphous by transmission electron microscopy, selected area electron diffraction and differential scanning calorimetry. Different criteria were used to evaluate the influence of the addition of alloy elements on the Glass-forming ability. The Al70Fe12.5V12.5Ni5 amorphous alloys exhibits higher glass-forming ability and activation energies of crystallization. Comparison of the effective atomic size ratio and mixture enthalpy on the glass-forming ability of these amorphous alloys demonstrates that the effective atomic size ratio value becomes more significant than the values of mixture enthalpy.

Assessment on effectiveness of a shower type cooling tower

In the conventional cooling tower, filling is used to split water into small droplets in order to maximise heat transfer owing to the fungus deposited over it, the filling should be replaced regularly. The fungus accumulated over the filling will limit the free flow of water leading to poor heat transfer. A small change of the conventional pact type cooling tower to the shower type cooling tower by removing the filling material and by rotating the feed nozzle 180° upwards may increase the heat transfer zone by increasing the water flow height. The present study investigates by means of a counter-flow mathematical model the difference in height, temperature, relative humidity, change in water droplet diameter and change in air-water mixture enthalpy. The performance of the cooling tower is studied with the conventional cooling tower type.

Comprehensive performance analysis and optimization of 1,3-dimethylimidazolylium dimethylphosphate-water binary mixture for a single effect absorption refrigeration system

The energy and exergy analyses of the absorption refrigeration system (ARS) using H2O-[mmim][DMP] mixture were investigated for a wide range of temperature. The equilibrium Duhring (P-T-XIL) and enthalpy (h-T-XIL) of mixture were assessed using the excess Gibbs free non-random two liquid (NRTL) model for a temperature range ??? of 20°C to 140°C and XIL from 0.1 to 0.9. The performance validation of the ARS cycle showed a better coefficient of performance (COP) of 0.834 for H2O-[mmim][DMP] in comparison to NH3-H2O, H2O-LiBr, H2O-[emim][DMP], and H2O-[emim][BF4]. Further, ARS performances with various operating temperatures of the absorber (Ta), condenser (Tc), generator (Tg), and evaporator (Te) were simulated and optimized for a maximum COP and exergetic COP (ECOP). The effects of Tg from 50°C to 150°C and Ta and Tc from 30°C to 50°C on COP and ECOP, the Xa, Xg, and circulation ratio (CR) of the ARS were evaluated and optimized for Te from 5°C to 15°C. The optimization revealed that Tg needed to achieve a maximum COP which was more than that for a maximum ECOP. Therefore, this investigation provides criteria to select low grade heat source temperature. Most of the series flow of the cases of cooling water from the condenser to the absorber was found to be better than the absorber to the condenser.

The calculation models of vapor-liquid equilibrium, enthalpy and entropy for R152a/R1234ze(E)

To mitigate the impact of human activities on the environment, R22 with ODP not equal to 0 and high GWP must be weeded out. R1234ze has excellent environmental performance but insufficient refrigeration capacity per unit. R152a has excellent refrigeration performance but flammability. Their mixture will make up for each other’s defects and can be used as a new alternative refrigerant. Thermodynamic properties of the mixture are studied in this paper. The vapor–liquid equilibrium model, based on Peng–Robinson equation of state and VDWS mixing law, is established for R152a/R1234ze with temperature from 258.15 to 288.15 K. When the results are compared to the values of experiments, the maximal and average errors for pressures are 0.617% and 0.135%, respectively. Based on vapor–liquid equilibrium model combining residual property, enthalpy and entropy models are also established to calculate enthalpy and entropy of mixture (R152a and R1234ze with the mass ratio 60:40), which enable provide theoretical basis for the application of R152a/R1234ze.

Consistent lattice Boltzmann model for multicomponent mixtures

Abstract

Composition-Dependent Effects of Oxygen on Atomic Structure and Mechanical Property of Metallic Glasses

Although minor alloying in metallic glasses (MGs) has been extensively investigated, the effect of O doping is still a debatable topic. In the present study, the atomic-level structures and mechanical properties of Cu-Zr MGs doped with different O contents have been analyzed using ab initio molecular dynamics simulations. It is found that O atoms prefer to bond with Zr atoms due to their low mixture enthalpy. The key finding is that we reveal the compositional dependence of O doping. For O-doped Cu64Zr36 MGs, the fraction of full icosahedra, size of the medium-range-order clusters, Young’s modulus, and shear modulus decrease sharply with O content, accompanied by a sharp increase of the non-Frank-Kasper polyhedral, ratio of bulk modulus to shear modulus, and Poisson's ratio, indicating decreased strength and improved plasticity. For O-doped Cu47Zr53 MGs, however, the above-mentioned structural and mechanical features experience little change or only change slowly after O doping, showing low oxygen sensitivity. It is reasoned that the high Zr content in Cu47Zr53 MGs weakens the effect of Zr-O bonding to some extent. The present study not only sheds light on the atomic-level structures of O-doped MGs that may provide guidelines for designing MGs with low-grade materials and low processing cost, but also help to explain the previous conflicting results based on the composition-dependence effect.

Solid-liquid equilibria and excess enthalpies in binary mixtures of propiophenone with some aliphatic amides

The solid-liquid equilibria and excess enthalpies of the binary systems of propiophenone and {N-methylformamide, or N,N-dimethylformamide, or N,N-dimethylacetamide, or N-methyl-2-pyrrolidone} were measured. The SLE data were determined by the cloud-point method, partly supplemented by the DSC technique. The excess enthalpies were measured by the titration calorimetry at 298.15 K and 308.15 K for all the systems and additionally at 293.15 K and 318.15 K for the system with N-methylformamide, and at 293.15 K for the system with N,N-dimethylformamide. The activity coefficients of both components as a function of concentration and temperature were calculated by the correlation which included both types of data. The results of the prediction by the modified UNIFAC model were compared with the experimental data. The observed trends and differences between the systems were discussed.

Laminar burning velocities of LCV syngas-air mixtures at high temperature and pressure conditions

Laminar burning velocities of low-calorific value syngas-air mixtures are measured at elevated temperature and pressure conditions using externally heated diverging channel method. The technique facilitates direct measurement of laminar burning velocities simultaneously at elevated pressures and temperatures. The mixture burning velocities are observed to increase with mixture enthalpy at elevated temperatures and decrease with an increase in the mixture pressure. The measurements are carried out for a mixture temperature range of 300–650 K and up to 5 bar pressure. Based on these measurements a new relation for the variation of laminar burning velocity with pressure and temperature is proposed. The power-exponents of temperature and pressure were expressed as linear functions of pressure and temperature ratios, respectively. The comparison of the measured burning velocities with the best performing kinetic schemes (FFCM-1 and Aramco 3) revealed that a significant deviation in the predictions is observed, when a simultaneous increase in pressure and temperature are considered.

A comparative survey on the boiling flow in different geometries using the Drift-Flux model

In this study, thermal-hydraulic parameters of a two phase flow in different channels of a same hydraulic diameter is studied. The Drift-Flux model is adopted for two phase flow investigations where four principal conservation equations are discretize phased using the mesh staggered finite volume technique. The boiling flow parameters at the sub-cooled condition are compared in three different channels with square, triangular and circular cross sections. Also, a square array sub-channel frequent in boiling structures (LWR fuel assemblies included) is added to our consideration. To perform analyses, a code is developed based on the Drift-Flux model and the resulting system of equations is solved through the Newton method. At first, results of the developed code for a circular channel is validated against the experimental data. We then focus on the changes of thermal-hydraulic parameters such as void fraction, pressure, mixture velocity and mixture enthalpy in the mentioned geometries as a result of the variation in problem parameters e.g. mass flux, inlet temperature, outlet pressure and heat flux. Solutions also demonstrate that the geometry has as a noticeable impact on the boiling flow parameters especially on the void fraction.