Enthalpy Effects Research Articles

Significance of binary chemical reaction, enthalpy and variable thermophysical properties over a radioactive heated rotated disk

In present work, we assume the thermo-physical characteristics of viscous fluid on a heated rotating disk. We used the modified Newtonian fluid model for temperature dependent viscosity and calculated the mean flow profiles under the viscosity effect near the heated rotating disk. We determine the behavior with modified activation enthalpy and radiation effects by utilizing the heat phenomena while the mass transfer process is investigated by using thermophoretic, chemical reaction and activation energy. The rudimentary flow can be solved numerically via von Kàrmàn similarity solution that has been cracked by using a shooting method. We regulate the base flow profiles and provide a physical clarification of the problem through graphs and tables. Correlations in term of velocity, viscosity, temperature and concentration are plotted graphically.

Alterations in dynamics of counterjet injection technique in presence of real gas effects

Large wave drag is common problem encountered for flights driving at high speeds. It should be dealt with utmost care for better design of long range flights cruising at high speeds. Studies found in the literature only deals with computations or experiment that are carried at lower stagnation temperature or considering perfect gas flow. Thus, the present work incorporates the real gas effects on various flow features like Mach disk diameter, Mach disk location and recirculation pressure. Further the effect of various freestream Mach number (M∞) and freestream total enthalpy (Ho) are observed on wall pressure for blunt body hemisphere considering real gas reactions by means of in house perfect and real gas solvers at hypersonic speeds. Pressure ratio of opposing jet is fixed as 15 while temperature ratio is fixed as 1. Results reveals that the pressure on the object lowers with the increase in M∞ especially upstream of maximum surface pressure point both for the reacting as well as the perfect solver. Surface pressure is found to be much higher at elevated freestream total enthalpy in comparison of smaller freestream total enthalpy for reacting solver while surface pressure almost shows similar trend at all Freestream total enthalpy for perfect solver case. Surface pressure for perfect solver is found to be lower than surface pressure for reacting solver for all the cases studied and this reduction is significant at higher M∞. Drag on body is rising with Freestream total enthalpy but this rate of drag enhancement diminishes at higher M∞.

Heat transfer in generalized Carreau fluid flow near a radioactive heated rotating disk

A theoretical study is examined to investigate the effects of enthalpy and radioactive heat transfer in generalized Newtonian fluid flow due to an inclined rotating disk. In order to examine the shear thickening phenomenon, Carreau fluid model is utilized. Here for the first time, Carreau fluid model under the enthalpy, thermophoretic effect, binary mixture and solar radiation effects are examined near an inclined rotating disk. In this study, the fluid flow does not give an analytical solution of the Navier–Stokes equations, except for the limit case, that is for high Reynolds number. The flow inside three-dimensional boundary layer flow is determined by the von Kármán transformations. The boundary layer equations are converted into system of ordinary differential equations by using von Kármán inequalities and solved numerically by employing shooting method. The effects of shear thinning and shear thickening fluids show a increase behaviour for base flow profiles. Thermal conductivity and mass diffusivity increases the temperature and concentration distributions. The azimuthal velocities increases by increasing values of λ T and λ C parameters. We present the numerical solutions for different physical parameters and discussed them for radial and azimuthal velocities and for heat and mass transfer profiles through graphs and tables.

Temperature distribution of long-length high temperature superconducting cable cooled by slush-nitrogen

A numerical model is established to calculate the temperature and pressure distributions of the 3 km long high temperature superconducting (HTS) power cable. The model assumes that the configuration of HTS power cable is three phase co-axial shape with counter-flow cooling method. In the numerical model, the circulating nitrogen enters the inner channel of the HTS power cable for cooling at the one end, flows to the other terminal, returns through the outer channel, and exits from the HTS power cable. The numerical model considers the mass, the momentum, and the energy balances of nitrogen, and the heat diffusion inside the HTS layer to obtain the realistic temperature and pressure distributions of the cable. When the long HTS cable is cooled by subcooled liquid nitrogen, the result shows that the maximum temperature of the compact HTS layer cannot be kept under 80 K even though the subcooled liquid nitrogen is supplied at the melting temperature. In order to supply nitrogen with lower enthalpy for ensuring sufficient cooling through the whole cable region, we suggest using slush nitrogen, and analyze it in the detailed numerical model. Thermo-hydraulic equations for slush nitrogen are incorporated in the numerical model. The results show that the maximum temperature of the HTS layer decreases by 6.4 K when slush nitrogen with solid volume fraction of 30% is supplied to the HTS cable. The effect of the inlet enthalpy of nitrogen and the heat transfer characteristics to the temperature distribution is discussed and further elucidated in this paper.

Analysis of counter flow injection technique at elevated enthalpy hypersonic reacting flows

Large wave drag and high surface heating are common problems encountered at hypersonic speeds and these should be properly dealt for the effective and safe flights. Many studies have been carried out to mitigate these problems by employing various active and passive techniques but most of these computational or experimental studies account lower stagnation enthalpy flows or perfect gas assumption. Hence the current study examines the effect of higher freestream stagnation enthalpy on flow field alteration for counter - jet drag reduction technique for a hemispherical object. Results also includes the real gas effects on flow field, wave drag and wall heat flux. Further the effect of various flow parameters is observed on surface pressure distribution, surface heat flux and drag force for the hypersonic flow over the hemisphere, using the in house developed perfect gas and non - equilibrium N - S flow solvers. Results reveal that the perfect gas assumption overestimates surface properties and wave drag value. Drag coefficient reduces with freestream total enthalpy (Ho) in the presence of real gas effects. Around 30% drag reduction is observed at Ho=1 MJ/kg for Mach number 5 as compared to no - jet case and this reduction increases at higher freestream total enthalpy for same injection pressure ratio. Higher pressure ratio of the jet results in lower surface pressure and Stanton number on the object which gives lower wave drag.

Computational design and experimental substantiation of conformationally constrained peptides from the complex interfaces of transcriptional enhanced associate domains with their cofactors in gastric cancer

Transcriptional enhanced associate domains (Teads) are the downstream effectors of the hippo signaling pathway and have been recognized as attractive druggable targets of gastric cancer. The biological function of Teads is regulated by diverse cofactors. In this study, the intermolecular interactions of Teads with their cognate cofactors were systematically characterized at structural, thermodynamic and dynamic levels. The Teads possess a double-stranded helical hairpin that is surrounded by three independent structural elements β-sheet, α-helix and Ω-loop of cofactor proteins and plays a central role in recognition and association with cofactors. A number of functional peptides were split from the hairpin region at Tead–cofactor complex interfaces, which, however, cannot maintain in native conformation without the support of protein context and would therefore incur a considerable entropy penalty upon competitively rebinding to the interfaces. Here, we further used disulfide and hydrocarbon bridges to cyclize and staple the hairpin and helical peptides, respectively. The chemical modification strategies were demonstrated to effectively constrain peptide conformation into active state and to largely reduce peptide flexibility in free state, thus considerably improving their affinity. Since the cyclization and stapling only minimize the indirect entropy cost but do not influence the direct enthalpy effect upon peptide binding, the designed conformationally constrained peptides can retain in their native selectivity over different cofactors. This is particularly interesting because it means that the cyclized/stapled, affinity-improved peptides can specifically compete with their parent Teads for the cofactor arrays as they share consistent target specificity.

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

Evaluation of texture, retrogradation enthalpy, water mobility, and anti-staling effects of enzymes and hydrocolloids in potato steamed bread

The functionalities of hydrocolloids and enzymes in texture, retrogradation enthalpy, water mobility and distribution, and anti-staling effects of potato steamed bread stored for 0, 24, and 48 h at 25 °C were investigated. Four kinds of hydrocolloids, including carrageenan, xanthan gum, arabic gum, sodium alginate, and one kind of enzyme (xylanase) showed little effects on the hardness reduction and springiness retention of potato steamed bread during storage, while the presence of α-amylase and lipase could slow down its staling rate. Potato steamed bread with combination of α-amylase (20 mg/kg) and lipase (40 mg/kg) exhibited the lowest hardness, with a significant reduction of 44.20%, besides improving the specific volume, L*, and overall acceptability in sensory evaluation. The addition of α-amylase and lipase could decrease the retrogradation enthalpy and bound water, and increase the mobility of mobile water. These findings shed efficient methods to retard staling of potato steamed bread.

Effect of pH Cycling and Zinc Ions on Calcium and Magnesium Carbonate Formation in Saline Fluids at Low Temperature

The formation of dolomite is very challenging in the laboratory under ambient conditions due to kinetic inhibition. The goal of this study was to test the impact of pH cycling and zinc ions on the formation of magnesium-rich carbonates in saline fluids at a low temperature. Batch reactor experiments were conducted in two series of pH cycling experiments, one without and one with zinc ions, at 43 °C. The results after 36 diel pH cycles indicate a reaction product assemblage of hydromagnesite, aragonite and magnesite in the experiments without zinc ions, and of magnesite and minor aragonite in the experiments with zinc ions. The presence of zinc ions leads to a decrease in the pH in the acid phase of the cycling experiments, which likely plays a role in the reaction product assemblage. Moreover, the hydration enthalpy and other specific ion effects could be additional factors in the formation of magnesium-rich carbonate. The results show a clear evolution towards increasing incorporation of magnesium in the carbonate phase with cycle number, especially in the experiments with zinc ions, reflecting a ripening process that is enhanced by pH cycling. Hence, repeated pH cycling did not lead to more ordered dolomite (from protodolomite), but rather to the formation of magnesite with 92 mol% MgCO3 after 36 cycles, even though geochemical models indicate a higher saturation index for dolomite than for magnesite.

Uncertainty of small enthalpy effects measured by isothermal calorimetric titration

Uncertainty of small enthalpy effects measured by isothermal calorimetric titration

Pressure and Temperature Dependence of the Hydrogen Oxidation and Evolution Reaction Kinetics on Pt Electrocatalysts via PEMFC-based Hydrogen-Pump Measurements

The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated due to its importance for a broad variety of applications especially in electrolysis and fuel cells. However, owing to its extremely fast kinetics, kinetic data can only be obtained with experimental setups that provide very fast mass transport, so that the effect of hydrogen partial pressure (pH2) and kinetic limitations at high overpotentials are not yet fully understood. Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC-based H2-pump approach. By using ultra-low platinum loadings, we could show that the apparent activation energy of the HOR/HER on platinum increases with increasing pH2, due to a diminishing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to pH2 also depends on the temperature. We further observed that the HOR reaches a limiting current at high HOR overpotentials that showed a direct proportionality to pH2 and a pressure-independent activation energy. We ascribe this to a limitation of the hydrogen adsorption rate either by a rate limiting Tafel-reaction or mass transport limitations.

Shear fracture in bulk metallic glass composites

Fracture behavior of bulk metallic glass matrix composites (BMGCs) with both transforming and non-transforming β-Ti dendrites under shear and opening modes was examined. Experimental results show that the fracture toughness of all three BMGCs is considerably lower in mode II than in mode I, due primarily to the shear dominant stress state in the former, which renders easy shear band initiation. However, stable crack growth in mode I is insignificant whereas it is considerable in mode II. The toughness of BMGCs reinforced with coarse but non-transforming dendrites in both the modes is higher than the respective values in BMGCs with transforming β-Ti inspite of their ability to strain harden and hence enhanced ductility. Fracture surface features and shear band patterns at notch tips of both mode I and mode II specimens indicate that despite the interactions of dendrites with the shear bands, the fracture criterion and mechanism of BMGCs is identical to that in BMGs. The differences in the fracture behaviour of these BMGCs is rationalized by considering the effect of relaxation enthalpy of the amorphous matrices in addition to the length scale and transforming tendency of the dendrites on the transformation of a shear band into a crack. Implications of these results in terms improving the fracture toughness of BMGCs with transforming dendrites is discussed.

Stopped‐Flow 19F NMR Spectroscopic Analysis of a Protodeboronation Proceeding at the Sub‐Second Time‐Scale

AbstractIn‐situ NMR spectroscopic analysis of homogeneous reactions is an essential tool for mechanistic analysis in organic and organometallic chemistry. However, rapid non‐equilibrium reactions, that are initiated by mixing, require specialized approaches. We report herein on a study of the factors that ensure quantitative results in a recently‐developed technique for stopped‐flow NMR spectroscopy. The influence of some of the key parameters on quantitation is studied by 19F NMR spectroscopic analysis of the kinetics and activation parameters for the base‐catalyzed protodeboronation of highly‐reactive polyfluorinated arylboronic acids, with half‐lives as low as 0.1 seconds. The effects of spin relaxation, pre‐magnetization, heat‐transfer versus reaction enthalpy, and mixing‐efficiency are analyzed in detail. We also compare and contrast choice of pulse angle, interscan delay, and use of pseudo real‐time by interleaving, as means to achieve an optimal balance between temporal resolution and sensitivity.

Heat and mass transfer analysis of polyacrylamide nanofluid with specific enthalpy effect

In order to further describe heat and mass transfer process of viscoelastic nanofluid, the specific enthalpy is involved in Cattaneo-Christov heat flux model. The heat and mass transfer behaviors of the polyacrylamide nanofluid in a porous medium are analysed in this study. It can be concluded that when the temperature changes from 298 K to 358 K, the viscosity decreases from 3.7204 Pa∙s to 1.5315 Pa∙s according to rheological experiment, and the thermal conductivity reduces from 0.5659 W∙m−1∙K−1 to 0.1121 W∙m−1∙K−1 according to thermal conductivity experiment. The non-linear relationship between temperature and viscosity, and the linear relationship between temperature and thermal conductivity are verified respectively. Using the inverse similarity framework, along with the analytical method, the analytical approximate solutions of the partial differential governing equations are obtained. Some new phenomena are revealed. The space stretching rate has an opposite effect on the velocities of the sheet and the free surface. Furthermore, due to the growth of specific enthalpy at infinity, the heat transfer efficiency and the concentration diffusion rate are enhanced.

Examining Some of the Challenges Students Face in Learning about Solubility and the Dissolution Process

Developing conceptual understanding of solubility and dissolution and the relationship to molecular structure, ionic salts’ charges, and enthalpy and entropy, play a significant role in the learning of chemistry. Dissolution and solubility are topics covered in general chemistry, quantitative analysis, biology, and organic chemistry. Alternative conceptions about some of the key chemistry principles can lead to students’ failure in understanding and accepting some of the future concepts. This study aims to examine some of the challenges and alternate conceptions that students face when learning about solubility and its relationship to Lewis structures and the driving force behind dissolution. A survey that consisted of ionic and molecular solubility related problems, short answers, and Likert-Type questions was given to 200 students in order to analyze their understanding of solubility, dissolution, alternate conceptions they possess, challenges they face in learning about the topics, and its relationship to Lewis dot structures and ionic charges. Our data indicates that students struggle with solving solubility and dissolution process related problems and rely on memorization to approach these problems. For ionic compounds, there seems to be a dependency on memorization and rote-learning of solubility rules instead of development of conceptual understanding of charges of ions and their relationship to entropy and enthalpy in the dissolution process. For molecular compounds, it should be noted that students lack mastery of determining molecular shape and its relationship to function but do emphasize the roles of Lewis structure and polarity in solubility. Finally, students do not attribute the combined effects of enthalpy and entropy as the driving forces behind dissolution and show very fragmented and naïve understanding of the concept.

Pressure Dependency of the Hydrogen Oxidation and Evolution Reaction Kinetics on Carbon Supported Pt Catalysts Using a PEMFC Based Hydrogen Pump Approach

The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated in electrocatalysis due to its importance for a broad variety of applications especially in electrolysis and fuel cells (FC). While the HOR/HER reaction rates on platinum and platinum alloy catalysts in alkaline environment can be determined precisely with rotating disk electrode (RDE) measurements,1-3 this is not possible at low pH due to the two orders of magnitude higher reaction rates that far exceed the slow mass transport limited rates afforded by the RDE configuration.4 As a result, meaningful measurements are limited to setups with much faster mass transport rates, such as floating electrodes or proton exchange membrane (PEM) fuel cell based electrochemical H2-pumps,4, 5 but the effect of hydrogen partial pressure and kinetic limitations at higher overpotentials are not yet fully understood.Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC based H2-pump approach. The measurements were performed on assymmetric membrane electrode assemblies (MEAs), where the working electrode has an ultra-low loading of only 1-2 µgPt cmMEA -2 (resulting in roughness factors of ≈1.5 cmPt 2 cmMEA -2), while the counter electrode has a much higher loading of 0.4 mgPt cmMEA -2. Using a fuel cell test station that allows for the feed of humidified hydrogen on both the anode and the cathode, we were able to show that the apparent activation energy of the HOR/HER on platinum increases with increasing partial pressure of hydrogen, which we ascribe to a decreasing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to H2 partial pressure is also depending on the temperature. We further observed that the HOR quickly reaches a limiting current with increasing HOR overpotential, ηHOR (orange area in Fig. 1), which we ascribe to a change in the rate-determining step (rds) from the Volmer reaction (i.e., the electrochemical oxidation of adsorbed H) at low ηHOR to the Tafel reaction (i.e., the dissociative adsorption of H2 on the Pt surface) at higher ηHOR. The Tafel-limited current shows a pressure-independent activation energy and a direct proportionality to the partial pressure of hydrogen, which is exactly what one would expect for the dissociative Langmuir adsorption as the rds. Finally, the HOR/HER reaction rates on a Pt3Co alloy and a Ru@Pt core-shell catalyst will be compared to that on a Pt/C catalyst.

Ion distribution models for defect fluorite ZrO2 - AO1.5 (A = Ln, Y) solid solutions: II. Thermodynamics of mixing and ordering

Thermodynamic mixing properties of AxB1-xO2-0.5xV0.5x, fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) are modelled as functions of four parameters, ΔH1, ΔH2, ΔH3 and ΔH4, which correspond to the enthalpy effects of the reactions 6A + 8B = 7A + 7B (1), 6A + 8B = 8A + 6B (2), 6B + 8B = 7B + 7B (3) and 6A + 8A = 7A + 7A (4), involving six cation species, 6A, 7A, 8A, 6B, 7B and 8B. The model predicts that the disordered configuration containing all cation species evolves with the decreasing temperature such that 6-fold coordinated cations tend to vanish within 0 ≤ x ≤ 1/2 domain, while 8-fold coordinated cations become extinct within 1/2 ≤ x ≤ 1 domain. The further evolution within the intervals of 0 ≤ x ≤ 1/3, 1/3 ≤ x ≤ 1/2, 1/2 ≤ x ≤ 2/3 and 2/3 ≤ x ≤ 1 favours the extinction of 7A, 8B, 7B and 6A cation species, respectively. With the further decrease in the temperature 6-fold B and 8-fold A cations reappear within the domains of 1/3 ≤ x ≤ 1/2 and 1/2 ≤ x ≤ 2/3 via the reaction 7A + 7B = 8A + 6B. The configurational entropy reduces along with these transformations. The model fits structural and calorimetric data on Zr-based AxB1-xO2-0.5xV0.5x systems and provides hints to understanding of ionic conductivity and radiation susceptibility data.

Process and properties of dense and porous vertically-cracked yttria stabilized zirconia thermal barrier coatings

Highly durable thermal barrier coatings (TBCs) are desired for various applications in hot sections of gas turbine engines. In this work, dense vertically-cracked (DVC) and porous vertically-cracked (PVC) yttria stabilized zirconia (YSZ) TBCs were produced using the air plasma spray process with a specially engineered powder. The effects of plasma enthalpy on the in-flight particle behavior and coating microstructures were investigated. Thermal conductivity, thermal cycling behavior, as well as, failure mechanisms of both DVC TBCs and PVC TBCs were evaluated. The thermal conductivity of PVC TBCs was found to be approximately 40%–50% lower than that of DVC TBCs due to their higher porosity. In addition, the thermal cycling life of PVC TBCs was demonstrated to be more than 1.8 times that of DVC TBCs with the same substrates and bond coats. PVC TBCs also showed excellent sintering resistance after 743 one-hour thermal cycles at 1125 °C. This work demonstrates that highly durable 8YSZ PVC TBCs with simultaneously high strain tolerance, low thermal conductivity, and high sintering resistance can be produced by the application of an engineered powder feedstock.

Simulation of Full-bore Tube Ruptures in Shell&tube Heat-exchanger

Shell & Tube heat exchanger is the most often used equipment for heat transfer between fluids in chemical plants, refineries, power generation stations and many other industries. The fluid cooled and the fluid heated are often operated at very different pressures: in this case design standards require an analysis to verify the maximum pressure surge that could be generated by an accidental fluid leak from the high-pressure side of the heat-exchanger into the other. The analysis of such accident is very complex due to a number of interacting phenomena such as vapor-liquid equilibrium, large enthalpy effects, large changes of fluid density and the formation two-phases (vapor and liquid) fluid flows. With reference to a real project case, this article presents the simulation of the pressure increase due to the full-bore rupture of a tube. This evaluation is required to assess if a rupture disk must be installed on the shell side to protect both the equipment and the operating personnel. The analysis is performed using a state-of-the-art dynamic simulation tool.

Happy Birthday to Bernd Giese

Happy Birthday to Bernd Giese