Volume 50, Issue 3

In this paper we deepen into the influence of the boundary conditions on the heat-flux behavior in the case of thin nanowires. We compare the evolution of an initially inhomogeneous heat flux between two systems at the same (constant) temperature according to two theoretical models, one only emphasizing the role played by the non-local effects, and the other only accounting for the influence of the non-linear effects. It is found that non-local effects yield a monotonic trend toward the equilibrium, whereas non-linear effects yield an oscillating trend. The behavior of the entropy in both cases is analyzed as well.

Based on the definition of exergy-based efficient ecological-function (EEF) proposed in the existing literature, which is the product of energy conversion coefficient-of-performance ( ɛ ) and exergy-based ecological-function ( E ), this paper will introduce the exergy-based EEF into performance optimization for Carnot refrigerator cycle. Via endoreversible Carnot refrigerator model established in previous literature, expression of the exergy-based EEF of refrigerator is derived based on finite-time thermodynamic theory, relationships of dimensionless exergy-based EEF versus ɛ and cooling load ( R ) are studied, and performance differences of refrigerator cycles at the maximum exergy-based EEF, at the maximum E , and at the maximum efficient cooling-load conditions are compared. The results demonstrate that relationships of dimensionless exergy-based EEF versus R and ɛ are parabolic-like ones; in actual design, the refrigerator should be designed at the larger R and ɛ points. When exergy-based EEF is taken as optimization-objective, although R decreases slightly, ɛ is increased, and entropy-generation-rate ( σ ) is greatly decreased, so exergy-based EEF does not only reflect the compromise between the R and σ , but also reflect the compromise between the R and ɛ .

Here, we investigate the effect of depositional nanoparticles (DNPs) on boiling heat transfer using molecular dynamics simulations. We discuss the type and density of DNPs and reveal their physical mechanism on boiling heat transfer. In the case of nanoparticle material changes, the onset time of explosive boiling, the heat flux, and the enhancement factor of the DNPs are calculated, and the results show that the heat transfer at the solid–liquid interface is enhanced due to the DNPs. The enhancement of Cu-DNP is the largest, followed by Ag- and Au-DNP. Compared with a smooth surface, the interfacial interaction of the DNP surface is increased, resulting in the improvement of the surface wettability, which is beneficial to heat transfer. Furthermore, the interfacial thermal resistance affects the heat transfer when the DNP material changes. The DNPs enhance the vibrational thermal matching of atoms at the solid–liquid interface, leading to heat traveling more easily across the interface, and thus the heat transfer between surface and fluid is enhanced. In the case of nanoparticle density changes, the results demonstrate that the boiling heat transfer is enhanced by an increase in DNP density, which confirms that the interfacial interaction and the thermal resistance have significant effects on the heat transfer at the solid–liquid interface.

The increase of fossil fuel powered industrial processes and vehicles has resulted in the exhaustion of petroleum reserves and pollution of the environment. Because of its clean-burning, renewable, and biodegradable qualities, biodiesel is becoming more and more recognized as a potential diesel fuel alternative. The present study investigates engine performance and emission characteristics of cottonseed oil (CSBD20) and diesel blends tested on single-cylinder compression ignition engine by several injection timings, injection pressures, and ethanol shares. Performance parameters such as brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), exhaust emissions such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO x ), carbon dioxide (CO 2 ), and smoke were considered as output factors, considering injection timing (IT), ethanol share (ES), injection pressure (IP) as input factors utilizing artificial neural network (ANN) and taguchi grey relation analysis (GRA). The ANN model accurately predicts the input-output relationships of ethanol and cottonseed biodiesel blends, as validated by experimental comparisons. The predicted values for BTE, BSFC, HC, CO, NO x , and smoke show close alignment with experimental results, with marginal errors of 6.2 %, 2.8 %, 7.1 %, 4.7 %, 6.8 %, and 5.6 %, respectively, confirming its reliability. In addition, this study utilized Taguchi grey relational analysis (GRA) to find optimum engine operating conditions. The analysis revealed that the optimal engine operating conditions were IT at 27° CA bTDC, ES at 15 %, and IP at 200 bar. Furthermore, confirmation tests are also conducted at optimum operating conditions, and the revealed values are closer to taguchi GRA experiments and ANN predicted values.

In this study, we investigate the linear transport of neutral system within the framework of relativistic kinetic theory. Under the relaxation time approximation, we obtain an iterative solution to the relativistic Boltzmann equation in generic stationary spacetime. This solution provides a scheme to study non-equilibrium system order by order. Our calculations are performed in generic hydrodynamic frame, and the results can be reduced to a specific hydrodynamic frame by imposing constraints. As a specific example, we analytically calculated the covariant expressions of the particle flow and the energy momentum tensor up to the first order in relaxation time. Finally and most importantly, we present all 14 kinetic coefficients for a neutral system, which are verified to satisfy the Onsager reciprocal relation in a generic hydrodynamic frame and guarantee a non-negative entropy production in the frame where the first order conservation laws are restored.

We study a quantum harmonic Otto engine under a hot squeezed thermal reservoir with asymmetry between the two adiabatic branches introduced by considering different speeds of the driving protocols. In the first configuration, the driving protocol for the expansion stroke is sudden-switch in nature and compression stroke is driven adiabatically, while the second configuration deals with the converse situation. In both cases, we obtain analytic expressions for the upper bound on efficiency and efficiency at optimal work output, which reveals a significant difference between the two configurations. Additionally, we find that the maximum achievable efficiency in sudden expansion case is 1/2 only while it approaches unity for the sudden compression stroke. Further, we study the effect of increasing degree of squeezing on the efficiency and work output of the engine and indicate the optimal operational regime for both configurations under consideration. Finally, by studying the full phase-diagram of the Otto cycle we observe that the operational region of the engine mode grows with increasing squeezing at the expense of refrigeration regime.

Authors give a review of the methods of finite-time thermodynamics in problems of obtaining the limiting capabilities of two- and multi-flow heat exchange systems. The boundaries of their physical reachability are constructed. The case of flows with changing heat capacity and, in particular, flows with a change in phase state is considered. The conditions for minimum heat exchange dissipation are obtained for different kinetics at fixed heat transfer coefficient and heat load. The concept of thermodynamically equivalent heat exchange systems is introduced and a method for transition from a multi-flow system to an equivalent two-flow heat exchanger in which heat capacities of the flows depend on temperature is proposed. Based on the obtained results, an algorithm for synthesis of multi-flow heat exchange systems is proposed. The synthesis takes into account the limitations on the temperatures of all or part of the flows. The synthesis of the system involves the calculation and minimization of the number of two-flow heat exchangers and the selection of the contact structure, the values of flow parameters, the distribution of contact surfaces and heat loads between two-flow heat exchange cells. In the design procedure the limitations on the temperatures of all or part of the flows are taken into account. Using thermodynamic criteria instead of technical and economic ones allows us to radically simplify the problem of designin the system.

Non-equilibrium thermal energy transfer in small scale films pairs, composing of different film materials, is important for designing semiconductor devices or thermoelectric energy generators. The present study examines thermal energy transfer in low size silicon-diamond film pairs with the quantum dots in placed. Equation for Phonon Radiative Transport (EPRT) is used to predict the distribution of phonon intensities via adopting the discrete ordinate method. Thermal energy transport is quantified in the form phonon energies via using integral form of equilibrium phonon intensities. Because of the mismatch of properties between silicon and diamond films, interface conditions are formulated after considering energy balance across both films. Findings reveal that equivalent equilibrium temperature decays gradually in the film for small size quantum dots. As the quantum dot size increases, equivalent equilibrium temperature decays sharply because films edges behave like heat sink reducing equilibrium phonon intensities in the region of film edges. Temperature jump, due to mismatch properties of the films, signifies at the mid-section of the interface and it increases slightly with increasing quantum dot size. The magnitude of heat flux vector remains higher in diamond than silicon film. The effective thermal conductivity predicted is in agreement with the previous data for silicon film.

This paper addresses the theoretical analysis of a piezothermoelastic problem involving an unbounded thermopiezoelectric medium with a spherical cavity subjected to pulse heating flux. The generalized piezo-thermo-elastic formulations of Lord and Shulman with thermal relaxation effects are used in this work. Unlike previous studies, which often consider simplified boundary conditions or steady-state thermal loading, our work incorporates generalized piezo-thermo-elastic formulations based on the Lord and Shulman model, accounting for thermal relaxation effects under dynamic thermal loading conditions. The numerical solution of the governing equations is done using the finite element approach, and temporal evolution is solved using the implicit scheme. New numerical results provide insight into the dynamic behavior of the piezoelectric medium subjected to thermal conditions. Thermal relaxation time and pulse heating flux are analyzed in their influence on the coupled thermal, mechanical and electrical fields and, thus, on the response of the system.