Results 1 - 10 of 248
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[en] Thermal optimization is very important for improving the performances of thermal systems. In engineering, the entropy generation minimization (EGM) has been widely used to optimize and evaluate the performances of thermal systems. However, the consistency between the EGM and the optimization objective should be specified when the EGM is used. In this paper, we discuss the view angle of irreversibility of entropy generation, and show that entropy generation directly reflects the exergy destruction or the ability loss of doing work. As the design objective in a thermal system is not often consistent with the view angle of irreversibility of entropy generation, the EGM may not lead to the optimal value of the design objective. In heat transfer and heat-work conversion, the inconsistence between the design objectives and the EGM is shown with some examples, and the applicability of the EGM is found to be conditional. The “entropy generation paradox” in heat exchanger analyses is also discussed, and it is shown that there is no direct monotonic relation between the minimum entropy generation rate and the best heat transfer performance of heat exchangers. (paper)
[en] As it is known, in material media the distribution of heat is always associated with thermal movement of structural units. If the heat transfer process is complex, methods that summarize results of various simple methods are used to study it. One of these methods is the phase plane development method. Based on this method, in this work, the stationary distribution of heat flux as a function of temperature in condensed matter has been studied. The equilibrium point of phase trajectory has been numerically determined. (author)
[en] The last two decades has seen the discovery of several nanomaterials ranging from one-dimensional carbon nanotubes to two-dimensional, homo-atomic and hetero-atomic nanosheets. These nanomaterials, in their isolated faun, exhibit exotic properties such as ballistic electron transport, high specific surface area, exceptionally high thermal conductivity and high tensile strength. Translating these properties from their isolated state to ensembles and assemblies has been a major roadblock for realizing practical applications of these materials. For instance, an isolated single wall carbon nanotube (CNT) exhibits a thermal conductivity of 4000 W/mK which degrades by more than 99% to 1 W/mK when present as ensembles
[en] Predictions of geo-neutrino fluxes and the Earth’s internal heat flux made by the Hydride Earth model are discussed. The prediction of geo-neutrino fluxes can be consistent with experimental measured fluxes. The predicted value of the Earth’s internal heat flux is significantly larger than the value experimentally obtained under the assumption that the main mode of heat transport is thermal conductivity. We consider another mode of heat transport in the Earth’s crust: heat transport by hot gases created in the Earth’s crust at great depth. We discuss also experimental data supporting this idea, particularly the temperature profiles obtained in the Kola superdeep borehole.
[en] Present article is devoted to study of heat conductivity in a limited media by artificial exaggerate method. Therefore, the problem of heat transfer in a limited medium at given initial temperature distribution and known thermal regime at the boundary of body has been studied. The temperature distribution at subsequent time has been determined. These problems have been solved on the basis of artificial exaggerate method. The proposed algorithm has the property of resistance to small perturbations of initial data. In practice, an important role is played by the conditions for matching and stabilization of regularization parameter, described in detail in the paper. (author)
[en] In this work, we designed the elliptical thermal cloak based on the transformation thermotics. The local entropy generation rate distribution and entransy dissipation rate distribution were obtained, and the total entropy generation and entransy dissipation of different types of elliptical cloaks were evaluated. We used entropy generation approach and entransy dissipation approach to evaluate the performance of the thermal cloak, and heat dissipation analysis was carried out for models with different parameters. Finally, the optimized elliptical thermal cloak with minimum entropy generation and minimum entransy dissipation is found, and some suggestions on optimizing the structure of elliptical thermal cloak were given. (paper)
[en] Here we have studied locally rotationally symmetric Bianchi-V Universe in the presence of modified theory for gravitation [f(R,T)theory] and for that, we considered a perfect fluid with heat conduction as the energy source. We used the law of variation for the deceleration parameter (DP) to solve field equations, as it gives a constant value of DP and is related to the average scale factor metric. Also, we have discussed the physical and geometrical properties of the model in detail. (author)
[en] A formal analogy between the Friedmann equation of relativistic cosmology and models of convective–radiative cooling/heating of a body (including Newton’s, Dulong-Petit’s, Newton-Stefan’s laws, and a generalization) is discussed. The analogy highlights Lagrangians, symmetries, and mathematical properties of the solutions of these cooling laws.
[en] We experimentally study the cavity formation when heated spheres impact onto water at low and high subcooling. The observations present that the formation and appearance of the cavity are affected by the boiling modes and the heat transfer intensity. In the nucleate-boiling regime, a rough cavity can be formed at a rather low impact velocity, while at the same velocity, the cavity formed in the film-boiling regime may have a very smooth interface with a stable vapor layer around the sphere. We discuss the effects of the impact speed, water and sphere temperatures on the stability of the vapor layer. For low subcooled water, the stable vapor layer will be disturbed when increasing the impact velocity, leading to a disturbed cavity. For high subcooled water, the film boiling has a particular boiling model in which the vapor layer around the sphere cannot keep its stability. In this particular film-boiling regime, no cavities can be formed at low impact velocities and only broken cavities can be formed at high impact velocities. (paper)