The Infona portal uses cookies, i.e. strings of text saved by a browser on the user's device. The portal can access those files and use them to remember the user's data, such as their chosen settings (screen view, interface language, etc.), or their login data. By using the Infona portal the user accepts automatic saving and using this information for portal operation purposes. More information on the subject can be found in the Privacy Policy and Terms of Service. By closing this window the user confirms that they have read the information on cookie usage, and they accept the privacy policy and the way cookies are used by the portal. You can change the cookie settings in your browser.
The current study presents a numerical computation of combined gas radiation and forced convection through two parallel plates. A laminar flow of a temperature-dependent and non-grey gas in the entrance region of the channel was investigated. Over-heated water vapor was chosen as a gas because of its large absorption bands. Some special attention was given to entropy generation and its dependence...
The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical...
This paper addresses the estimation of local entropy generation rate for diabatic saturated two-phase flow of a pure fluid. Two different approaches have been adopted for this thermodynamic characterization: the separated flow model using the classical vapor flow quality, and the mixture model, using the thermodynamic vapor quality. Based on these two models, two distinct expressions for the local...
Optimization for convective heat transfer plays a significant role in energy saving and high-efficiency utilizing. We compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability. We derived the optimization equation for each optimization principle...
In this paper a second law analysis of a cross-flow heat exchanger (HX) is studied in the presence of a balance between the entropy generation due to heat transfer and fluid friction. The entropy generation in a cross-flow HX with a new winglet-type convergent–divergent longitudinal vortex generator (CDLVG) is investigated. Optimization of HX channel geometry and effect of design parameters regarding...
The dissipative processes that arise in a microchannel flow subjected to electromagnetic interactions, as occurs in a MHD (magnetohydrodynamic) micropump, are analyzed. The entropy generation rate is used as a tool for the assessment of the intrinsic irreversibilities present in the microchannel owing to viscous friction, heat flow and electric conduction. The flow in a parallel plate microchannel...
In this paper, the local entropy generation analysis has been conducted based on a two-dimensional, two-phase, non-isothermal DMFC (direct methanol fuel cell) model, the entropy generation contributed by the chemical reactions, heat transfer, mass diffusion, and viscous dissipation is investigated. Then, the performance of fuel cells with different methanol barrier layers and electrolyte membranes...
Entropy generation during chemical reactions can cause significant irreversibility in chemical engines. These irreversibilities reduce the exergy of the fuel resource preventing potential work production. Understanding the cause of these irreversibilities and developing strategies for reducing them is critical for increasing engine efficiency. All chemical engines can be separated into two categories...
Turbulent double-diffusive natural convection is of fundamental interest and practical importance. In the present work we investigate systematically the effects of thermal Rayleigh number (Ra), ratio of buoyancy forces (N) and aspect ratio (A) on entropy generation of turbulent double-diffusive natural convection in a rectangle cavity. Several conclusions are obtained: (1) The total entropy generation...
An inevitable problem challenges heat exchanger designers is that the heat transfer augmentation in a thermal system is always achieved at the expense of an increase in pressure loss. Thus, the trade-off by choosing the most proper configuration and best flow condition has become the critical problem for design work. The brief survey on literature shows that optimal Reynolds number of laminar forced...
The present paper aims at investigating the main sources of irreversibility in a Proton Exchange Membrane Fuel Cell (PEMFC) using a Fermat spiral as flow distributor and also to direct possible improvements in its design. The numerical analysis is based on a finite volume technique with a SIMPLE algorithm as numerical procedure. In order to have a more complete and rigorous analysis a new dimensionless...
The viscous dissipation effect on the thermodynamic performance of the curved square microchannels in laminar flow is numerically investigated. The classical Navier–Stokes equations are adopted; aniline and ethylene glycol are selected as the working fluids. The results show that the heat transfer entropy generation number and frictional entropy generation number augment relatively under viscous dissipation...
Repeated ribs are considered an effective technique to enhance forced convection heat transfer in channels. In order to establish the performance of rib-roughened channels, both heat transfer and friction characteristics have to be accounted for. In the present paper, heat transfer coefficients and friction factors have been experimentally investigated for a rectangular channel having one wall roughened...
In this study, energy and exergy analyses are carried out for the charging period of an ice-on-coil thermal energy storage system. The present model is developed using a thermal resistance network technique. First, the time-dependent variations of the predicted total stored energy, mass of ice, and outlet temperature of the heat transfer fluid from a storage tank are compared with the experimental...
A comprehensive understanding of energy flow and entropy generation is needed for an optimal process design via reducing irreversibilities in terms of ‘entropy generation’. In this study, analysis on entropy generation during natural convection in a trapezoidal cavity with various inclination angles (φ = 45°, 60° and 90°) have been carried out for an efficient thermal processing of various fluids...
In the present study, the first and second law analyses associated with combined pressure-driven and electroosmotic flow of power-law liquids through a uniform microannulus are performed numerically. In the first law analysis, the Poisson–Boltzmann and incompressible Navier–Stokes–Fourier (NSF) equations in the cylindrical polar coordinate reference frame as the governing equations are numerically...
Recently, Teh et al. (2008) showed that constant internal energy–volume (UV) combustion (with equilibrium products) is the optimal strategy for minimizing entropy generation (S gen ) in idealized internal combustion (IC) engine processes. The present paper examines the effects of fuel type (CH 4 , C 2 H 5 OH, and C 8 H 18 ), reactant temperature (300–1200 K),...
In this paper, the entropy generation due to flow and heat transfer of nanofluids between Co-rotating cylinders with constant heat flux on the walls is studied, analytically. The governing equations in cylindrical coordinates are simplified and solved to find the effect of using nanofluids with different volume fractions on the entropy generation rate in the annulus. The results are presented at various...
The entransy theory has been developed for heat transfer optimization. This paper extends it to optimize thermodynamic processes. The entransy balance equation of thermodynamic processes is introduced, with which the concept of entransy loss is developed. For the Carnot cycle and the irreversible thermodynamic processes where the working fluid is heated by the streams with prescribed inlet temperatures...
This paper presents the relationships for the heat transfer performance of two-stream heat exchangers (THEs) and two-stream heat exchanger networks (THENs) with the entropy generation, the entropy generation number, the revised entropy generation number, the entransy dissipation, the entransy dissipation number and the entransy-dissipation-based (EDB) thermal resistance. The results indicate that...
Set the date range to filter the displayed results. You can set a starting date, ending date or both. You can enter the dates manually or choose them from the calendar.