Volume 47, Issue 4

A class of two finite-heat-reservoir endoreversible heat engine with the generalized models of both the reservoir thermal capacities and heat resistances is investigated. The optimality condition for cycle maximum work output is derived by applying optimal control theory, and impacts of both thermal capacity characteristics of heat reservoirs and heat transfer laws on the optimal configurations are discussed. The results obtained in some previous researches are special cases of those obtained herein, which can provide some guidelines for optimal design of actual heat engines.

This work describes the nonlinear Thomson effect produced by a transient current source powering a thermoelectric cooler. The electric effect of the capacitive impedance in the semiconductors was considered in the equations as a novelty term that naturally appears by solving the Boltzmann equation to find the mathematical form of the current density. Thus, considering the new term and heath energy balances, a one-dimensional mathematical model for a thermoelectric cooler (TEC) powered by a time-dependent current was developed, finding a new nonlinear Thomson effect in the heath transfer equations. To evaluate the impact of the nonlinear effect on the thermodynamic behavior of the thermoelectric cooler, a continuous, sinusoidal and square-pulse current conditions were simulated. The temperature profile, temporal evolution, and the effective coefficient of performance (COP) were calculated. The results revealed a new thermoelectric heat transfer in addition to the Thomson flow created by virtual junctions throughout the semiconductors caused by the instantaneous change of current. This fact was evidenced by three results: the shifting of the temperature mean value due to the peak current change 0.45 A is 1.68 K1.68\hspace{0.1667em}\mathrm{K} and 2.56 K2.56\hspace{0.1667em}\mathrm{K} to sinusoidal and square current supplies, respectively; it was determined that a TEC powered by a square-pulse current signal had greater effective efficacy, having more pronounced cold side supercooling temperature peaks compared to those powered by a constant sinusoidal current signal.

In actual operation, the operating environment temperature of thermoelectric devices are constantly changing and rarely remain stable, and the electrical output characteristics of thermoelectric devices are largely determined by thermoelectric materials. In response to this question, the thermoelectric properties of thermoelectric materials (p and n type Bi 2 Te 3 {\mathrm{Bi}_{2}}{\mathrm{Te}_{3}}) are measured under different temperature difference environments. The Seebeck coefficient, resistivity, and thermal conductivity of the specimens at T = 300 – 600 KT=300\text{--}600\hspace{0.1667em}\text{K} were measured by CTA-4 and CLA1000 (laser flash method), respectively; the thermal and electrical output responses of the thermoelectric materials under different temperature difference conditions were collected in real time by using a self-built thermoelectric performance test platform, thermal/electrical test system with infrared thermal imager, and voltage acquisition system, respectively. The experimental results show that when the temperature difference between the two ends of the specimen increases uniformly, the electrical output signal amplitude also increases uniformly; when the temperature difference is stable, the two ends of the specimen also produce a stable electrical output signal. After stabilization, the electrical output signal amplitude also decreases uniformly when the temperature decreases at a uniform rate. In the temperature range of 298 ∼ 573 K298\sim 573\hspace{0.1667em}\text{K}, the larger the temperature difference between the two ends of the specimen was, the larger the amplitude of the electrical output signal was after stabilization; and vice versa. The greater the loading rate of the thermal load was, the greater the rate of increase of the electrical output signal amplitude at both ends of the specimen was, and the steady-state equilibrium time required was less.

The current study is made to analyze the impact of local thermal nonequilibrium ( LTNE ) on the steady, incompressible, and viscous Ostwald-de-Waele nano-liquid over a rotating disk in a porous medium with the various power law index, due to many remarkable applications, such as aeronautical systems, rotating machineries, air cleaning machineries, electrical power-generating systems, heat exchangers, gas turbines, centrifugal pumps. To describe the modeling of the nano-liquid, Brownian movement and thermophoresis are employed with the passive control boundaries. Three temperature model is adopted to distinguish the temperature among the fluid, particle, and solid. The governing transport equations have been converted to a system of nonlinear coupled ordinary differential equations by employing von Karman transformation. Numerical results of the flow and heat and transfer characteristics of the fluid, particle, and solid are obtained by applying Runge–Kutta–Fehlberg method (RKF) together with the shooting technique. The numerical results in the present work are compared with the published results for the case of thermal equilibrium and found that they are in good agreement. It is observed that the temperature profile significantly varies with the fluid-particle, fluid-solid interphase heat transfer coefficients and the modified thermal capacity ratios.

This paper explores the hyperbolic heat transfer effects in processes involving high heating rates. The behavior of the model is analyzed in detail under different boundary conditions and the circumstances under which a non-Fourier law could be used to describe thermal conduction processes established from physical mathematical analysis. Finally, the model developed here is coupled to a previous population balance framework to predict the bubbling phenomenon that occurs during the fast pyrolysis of biomass. We found that a transient overheating occurs in the central zone of the generated liquid phase due to the high heating rates that take place during that process.

In this study, the results of an investigation of the performance of a gas-steam combined cycle system (GSCCS) under exergetic and exergo-economical criteria are reported. The effective power (P ef ), destroyed exergy (X), efficiency of exergy (ε), unit electric generation cost (C elec ) and exergy-dependent economic worth of electrical energy (C ex,elec ), which is novelly determined in this study, have been analyzed. The impacts of speed ( N ), pressure ratio of the gas cycle (λ), equivalence ratio (ϕ), the flow rate of the air mass ( m ˙ a {\dot{m}_{a}}), the flow rate of the fuel mass ( m ˙ f {\dot{m}_{f}}), inlet temperature of the air into the compressor ( T 1 {\mathrm{T}_{1}}), steam temperature ( T 6 {\mathrm{T}_{6}}) and pressure (P 6 ) of the heat exchanger, outlet pressure (P 7 ) of the high pressure steam turbine and condenser pressure (P 9 ) on P ef , ε, C elec and C ex,elec have been parametrically evaluated. It was revealed that the stream and component characteristics of the system have significant influences on the performance characteristics of the GSCCS.

A finite source heat engine’s optimal configuration is studied. The model includes thermal resistance, heat leakage, a complex heat transfer law, and a heat source with variable temperature. The optimization objective is that the output work is the largest. The influences of factors such as the heat transfer law and heat leakage are analyzed. The results of this paper are universal and inclusive, and provide certain theoretical support for the performance improvement of actual heat engines.