Volume 40, Issue 3

Starting out with an entropy identity, we establish the entropy flux, the entropy production, and the corresponding Gibbs and Gibbs–Duhem equations of general-covariant continuum thermodynamics. Non-dissipative materials and equilibria are investigated. We prove that equilibrium conditions only put on material properties cannot generate equilibria, rather the Killing property of the 4-temperature is a necessary condition for space-times in which equilibria are possible.

We extend Maxwellian velocity distributions to long observational timescales in much the same way that short timescale statistical mechanics distributions are averaged to yield normal laboratory timescale thermodynamic distributions. This long timescale view has several novel effects: Fluctuating overall velocities (i.e. “wind”) thermalizes into an additional component of temperature, while returning a Maxwellian velocity distribution. However, fluctuating temperature results in a new distribution with a Gaussian core but heavy polynomial tails. The power of the polynomial tail is either -3 or -2 depending on whether the precision of the temperature is allowed to extend to ± infinity or is required to remain strictly positive. The distribution is also interesting in the way it remains almost exactly Gaussian up to a certain velocity after which it quickly breaks off to become polynomial. The distributions are carefully analyzed mathematically, and physical consequences are drawn.

Phonon transport across silicon thin film pair with minute gap (Casimir limit) between the films is studied. Phonon transport characteristics across the gap are examined for various gap sizes, and the transient solution of the frequency-dependent Boltzmann transport equation is presented according to relevant boundary conditions incorporating the gap between the film pair. Since the gap size is minute (Casimir limit), the radiative energy transport between the edges of the film pair is incorporated. In addition, phonon transmission and reflection is introduced at the gap edges, thus satisfying energy conservation. The thermal conductivity predicted is validated through experimental data reported in the open literature. Predicted thermal conductivity data agree well with the experimental data reported in the open literature. Increasing gap size alters the phonon transport characteristics across the film pair. Increasing gap size enhances temperature difference between the edges of the gap; in which case, the effect of phonon transmittance is more significant on the temperature difference than that corresponding to the radiation heat transfer due to Casimir limit.

The performance of a pilot scale flat plate solar water heater system is investigated theoretically and experimentally. The effect of the operating conditions and characteristic factors of the collector on the system efficiency is studied. A conceptual mathematical model is developed in order to analyze the system behavior in different operating conditions by considering the physical and constructive aspects of the system. The accuracy of the model result is estimated by comparing the model results with the existing experimental data. The highest obtained system thermal efficiency is 45%, and the optimum local values for surface azimuth and tilt angles are obtained at 180 degrees from north and 33 degrees, respectively, for the constructed solar water heater in Isfahan, Iran, with the local latitude of 32.6333∘$32.6333^\circ $ N.

Two collectors with the same area of 945×420$945\times 420$ mm have been designed and fabricated: one flat plate collector with internally grooved fins inserted inside the riser tubes and the other one with plain riser tubes. Provisions were made for inserting the T-type thermocouples at six different locations to monitor temperature variations inside the riser tubes of the collector. The two identical systems of collectors (finned and unfinned tube) were initially run with base working fluid (water) and then with Al 2 O 3 nanoparticles with a size of 40 nm for different weight fractions of 0.2$0.2$% and 0.4$0.4$% in incremental order to study the effect on the efficiency of collectors. Mixing of nanoparticles in water thus improves the efficiency of the collector. Results show the efficiency of finned tube collector using the nanoparticles and water is substantially higher when compared to that of unfinned riser tubes collector.

Prediction of the available vapor pressure data in the case of C-H-O hydrocarbons led to derivation and recommendation of standard equations for this property. The accuracy of vapor pressure estimations is essential because it is used as a basis to calculate the acentric factor and the thermal and equilibrium properties. In this study, an accurately developed equation for calculation of vapor pressure is presented for 213 pure C-H-O hydrocarbons as a function of reduced temperature and critical pressure. With this equation, vapor pressures have been calculated and compared with the experimental data reported in the literatures for 213 pure hydrocarbons for more than 3800 data points, and the overall average absolute percentage deviation is only 0.286%. The accuracy of the developed equation has been compared to Antoine, Wagner, and other most used equations, and the comparison indicates that the developed method provides more accurate results than other methods used in this work.

Researchers in thermoelectricity with backgrounds in non-equilibrium thermodynamics, thermoelectric engineering or condensed-matter physics tend to use different choices of flux densities and generalized forces. These choices are seldom justified from either the dissipation function or the entropy production rate. Because thermoelectric phenomena are a primary focus in several emerging fields, particularly in recent energy-oriented developments, a review of the different formalisms employed is judged timely. A systematic classification of the transport equations is presented here. The requirements on valid transport equations imposed by the invariance of the entropy production are clearly explained. The effective Peltier and Seebeck coefficients, and the thermal conductivity, corresponding to the different choices of flux densities and generalized forces, are identified. Emphasis is made on illustrating the compatibility of apparently disparate formalisms. The advantages and drawbacks of these formalisms are discussed, especially from the point of view of the experimental determination of their thermoelectric coefficients.