International Journal of Thermodynamics

A theoretical model for the joint formation was developed for the diffusion brazing. The phenomena of dissolution and solidification were included into the model. A thermodynamic justification for the isothermal soldering occurrence in meta-stable conditions was developed. It involved the application of the criterion of higher temperature of the solid / liquid (s/l) interface. The dissolution of the filler metal in the substrate was described by the solute concentration within the dissolution zone (liquid film) situated at the substrate surface. The selection of the  parameter was justified by the Thermocalc calculation of the Ni-Al phase diagram for meta-stable equilibrium. According to the model assumptions, the solidification was accompanied by undercooled peritectic reactions resulting in formation of the intermetallic phases. The average Al – solute concentration measured across a given Al₃Ni₂/Al₃Ni/Al₃Ni₂ joint confirmed that the solute concentration was conserved within the joint sub-layers. The Ni-Al phase diagram for meta-stable equilibrium referred to the solidification was also calculated by means of the Thermocalc Software. It allowed to locate the solidification path, s/l interface path and redistribution path onto the mentioned diagram. Superposition of both calculated phase diagrams was also given to show that the joint formation occurred cyclically under the meta-stable conditions. The measured  parameter was introduced into the proposed model as an initial condition in order to solve the formulated differential equation describing solute redistribution after solidification. Isothermal formation of the Ni/Al/Ni joints has been performed at different temperatures. The following  temperatures have been applied: 700, 750, 800, 850, 900, 950, 1000 ⁰C. The solidification was arrested and the actual morphologies frozen. It allowed to make a measurement of the Al-solute concentration across each joint by means of the  EDAX micro-analyzer to estimate its average concentration, .  Regardless the  - temperature, the solidification path was always the same.

A new normalized model is developed to quantify and explore trends in coincidence of supply and demand in generic intermittent energy systems as key design and operating parameters are varied. This novel model is applied to seasonal-transient simulations for a solar-thermal powered adsorption system with and without heat recovery to investigate the coincidence between the solar-supplied cooling power and cooling load in terms of seasonal solar and loss fractions. Additionally, the system's basic performance trends are investigated as a number of parameters are varied. Results for the conditions explored include the following. The solar fraction increases and the loss fraction decreases with increases in storage capacity, and both fractions decrease with increases in maximum bed temperature. The required evacuated tube collector area is smaller than the flat plate collector area while the required mass of adsorbent is independent of collector and adsorption cycle types. Simulation results also show the effects of operating conditions and several design parameters on the system's COP.

Density (ρ ± 10^{-3 kgm-3}), surface tension (γ ± 10^-2 mNm^-1) for 8.4 to 83.6 mmolkg^-1 at 8 mmolkg^-1 interval aqueous polyoxyethylene sorbitan monolaurate (C=12, Tw20), monopalmitate (C=16, Tw40), monostearate (C=18, Tw60) and monooleate (C=18, 1 double bond, Tw80) nonionic surfactants at 293.15 K are reported. Apparent molar volumes (V₂) are derived from densities. The γ is used for surface excess tension (γ^excess), concentration (τ) and area per molecule. The ρ and γ were regressed for ρ⁰ and γ⁰ limiting and slopes for shift from hydrophilic to hydrophobic interactions (philicphobic). The ρ⁰_Tw20>ρ⁰_Tw40>ρ⁰_Tw80 >ρ⁰_Tw60 and V₂⁰_Tw60> V₂⁰_Tw40> V₂⁰_Tw80> V₂⁰_Tw20 as limiting densities and molal volume respectively in opposite order with stronger structural interaction with Tw20 and weaker with Tw60. The γ_water >γ⁰_Tw80 >γ⁰_Tw40 >γ⁰_Tw20>γ⁰_Tw60 inferred 18% weaker cohesive force (CF) and 22.68% with T60.

One of the most common difficulties students face in learning Thermodynamics lies in grasping the physical meaning of concepts such as lost availability and entropy generation. This explains the quest for new approaches for explaining and comprehending these quantities, as suggested by diagrams from different authors. The difficulties worsen in the case of irreversibilities associated with heat transfer processes driven by a finite temperature difference, where no work transfer takes place. An equivalent mechanical model is proposed in this paper. Heat exchangers are modelled by means of Carnot heat engines and mechanical transmissions; the use of mechanical models allows an easy visualization of thermal irreversibilities. The proposed model is further applied to a power cycle, thus obtaining an “equivalent arrangement” where irreversibilities become clearly apparent.

The organic Rankine cycle is a promising way for the conversion of low temperature heat to electricity. Different fluids can be used in Rankine cycles for the utilization of waste heat.  The suitability of a certain fluid will depend on its thermodynamic properties as well as on the conditions at which the heat is available, thus it is often unclear if an organic fluid has any advantage compared to inorganic fluids like water. Various substances starting from the refrigerants to high boiling organic liquid have been investigated as possible working fluid for the different temperature ranges at which the waste heat is available. The present communication reports exemplary the results for three different classes of substances, a hydrocarbon (n-heptane), two refrigerants 1.1.1.3.3-pentafluoropropane (R245fa) and pentafluoro butane mixture (Solkatherm, SES36), and water in the intermediate temperature range (473 to 773 K) where the exhaust gases of combustion engines may be used as energy source for cogeneration. In this range it turns out that for many conditions, water and heptane are well suited working fluids for cogeneration systems. In the present investigation, the attention was not laid on the cycle efficiency alone, but also on the total exergy usage from an enthalpy stream (e.g. exhausts gas). This is used for defining the total efficiency for the process. The results for the thermodynamic parameter total heat recovery efficiency and the surface area of the heat exchanger have been discussed. diagrams were also used for judging the suitability of a fluid. It turns out that water is well suited for many cases in the intermediate temperature regime.