The perturbation expansion of the Helmholtz free energy of a square-well fluid is analyzed. A closed-form expression is found for the first-order term in that expansion as a function of the density and of the attractive range of the well. The procedure is based on the interpolation of the extreme cases of long and short ranges, for which exact asymptotic results are known. The proposed interpolation formula is tested against Monte Carlo calculations of the same first-order term for different values of the range and the density, obtaining a root-mean-square deviation of 2.2%. Previous analytic approximations as that of van der Waals are discussed.

This paper focuses on the numerical modeling of the effect of the height of a combustion chamber on the development of a reference calorimeter whose objective is to measure the calorific value of natural gas. The impacts of temperature, velocity, and mass fraction on the exhaust gases were evaluated by varying the height of the combustion chamber. The eddy dissipation concept (EDC) approach was used to model combustion with two different chemical kinetic mechanisms: one with three steps, called the three-step mechanism defined by default in the software used, and second skeletal model, which consists of 41 steps, through the ChemKin-import file with 16 species. The main result of this study is the selection of a combustion chamber height for the reference calorimeter that produces the best performance in the combustion process, which is 70 mm, as well as the main differences in using a three-step mechanism and a skeletal model to simulate an oxy-fuel combustion reaction.

This research obtains a mathematical formulation to determine the heat transfer in a transient state, in a calorimeter cell, considering an adiabatic system. The development of the cell was established and the mathematical model was transiently solved, which approximated the physical phenomenon under the cell operation. A numerical method for complex geometries was used to validate performance. The results obtained in the transient heat transfer in a cylinder under boundary and initial conditions were compared using an analytical solution and numerical analysis employing the finite-element method with commercial software. The study from the temperature distribution can afford, selection between a cylindrical and spherical geometry, design criteria that are generated by changing parameters such as dimension, temperature, and working fluids to develop an adiabatic calorimeter to measure the heat capacity in fluids. We show the mathematical solution with its initial and boundary conditions as well as a comparison with a numerical solution for a cylindrical cell with a maximum error from 0.075% in the temperature value, along with a theoretical and numerical analysis for a temperature difference of 1 °C.

In this article, a theoretical–experimental study was conducted on a two-channel solar air collector (SAC-2C). For the experimental study, a prototype of the SAC-2C was designed, built, and instrumented with dimensions of 1.860 m in length, 0.605 m in width, and featuring two air channels of 5.5 mm and 5 mm thick each, respectively. The collector operates via forced convection and was positioned at an inclination angle of 18.88 deg at the Tecnológico Nacional de México CENIDET campus (TecNM/CENIDET) located in Cuernavaca, Morelos, Mexico. For the theoretical analysis, the method of global energy balances in two dimensions (2D) and under transient conditions was applied. Temperature differences of up to 3.0∘C are observed with respect to mathematical models that do not consider heat conduction terms in solid elements. These differences are accentuated in the glass cover. Furthermore, altitude’s impact on air density calculations could influence theoretical temperature profiles up to 3.0∘C. The theoretical results of the numerical model were validated with the information obtained from the experimental tests, which showed good similarity. It was observed that the elements of SAC-2C are sensitive to sudden changes in meteorological conditions. The system’s response time is not only associated with the characteristics of the materials but also with the thermal bridges between the absorber plate and the casing. The calculation of the appropriate heat transfer coefficients allowed the evaluation of energy gains or losses in the SAC-2C collector.

The present work focuses on the numerical modeling of two combustion chambers to be used inside an isoperibolic calorimeter to measure the Superior Calorific Value (SCV) from natural gas. This work shows performance of both chambers working under isoperibolic principle, through simulations based on Computational Fluid Dynamics (CFD). The aim of the work is expose the performance of chamber combustion published in the literature versus another one proposed in this work, and show how was improved the performance of the chamber which proposed in this work by changing the geometry. And it is checked by analyzing temperature of burned gases at exit of combustion chamber.

ResumenEl estudio de la transferencia de calor y de humedad en el secado de materiales sólidos porosos es importante, debido al amplio interés en reducir el agua presente en los materiales, ya que el exceso de humedad tiene efectos adversos en los materiales. En la actualidad, la investigación del secado se realiza de manera experimental, ya que no se cuenta con un modelo universal de secado que aplique satisfactoriamente a todos los materiales sólidos. Los modelos desarrollados se aplican a grupos restringidos de materiales. Uno de los modelos más aceptados para describir la transferencia de humedad en materiales capilares porosos es el modelo de Luikov, el cual se deriva de la termodinámica de procesos irreversibles. Además de la extensa aplicación de este modelo, hace falta identificar las propiedades termofísicas que tienen mayor influencia en los perfiles de secado, ya que existen pocas comparaciones contra datos experimentales. En este trabajo se utilizó la solución del modelo de Luikov obtenida por Liu et al. (1991) para alcanzar los perfiles de humedad en el proceso de secado de muestras de ladrillo a diferentes temperaturas, en donde se realizó una evaluación de los factores de influencia que afectan al perfil obtenido. Se encontró en el análisis, que la difusión es uno de los principales mecanismos que controla la transferencia de humedad. También el modelo se validó experimentalmente en muestras de ladrillo secados a temperaturas de 60 y 80°C; asimismo los resultados teóricos y experimentales concuerdan satisfactoriamente en la región final del secado, lo cual es útil en la determinación del contenido de humedad.AbstractThe heat and moisture transfer in the drying of porous solid materials is important, since excessive moisture has adverse effects on these materials; then, it exist a wide interest on the reduction of moisture in them. Nowadays, the research of the drying process is carried out both theoretical, as experimentally, because does not exist a universal model which satisfactorily describe the drying process of all kind of solid materials. The developed models apply to restricted groups of materials. To describe the moisture transfer in capillary porous materials, one of the more accepted theoretical models is the Luikov model, which is derived from the principles of thermodynamics of irreversible processes. This model has been applied to many materials; however, to describe the moisture transfer in capillary porous materials, it is needed a comprehensive analysis to identify the thermophysical properties which have major influence in the drying profiles. Also, it is needed to validate the model for those materials in which the model is applied. In this work, the solution of Luikov model (as obtained by Liu et al., 1991) was used to obtain moisture profiles during the drying of brick at different temperatures. Also, an evaluation of the influence factors which affects to the obtained moisture profiles was carried out. It was found that one of the most important mechanisms which control the drying process is the diffusion of moisture. Also, the experimental validation using brick was performed at temperatures of 60 and 80°C; the theoretical and experimental results agree in the falling rate period, which is useful in the moisture content determination