Entropy Generation Mechanisms Research Articles

Turbulence model comparison for ventilated cavitation around a hydrofoil

Ventilated cavitation involves complex multiphase flow, phase change, and turbulence, posing challenges for accurate prediction and control. This paper investigates the performance of three turbulence models—Large Eddy Simulation (LES), Detached Eddy Simulation (DES), and Shear Stress Transport (SST)—in predicting the unsteady characteristics and entropy generation mechanisms in ventilated cavitation around a hydrofoil. A comparative analysis with experiments reveals each model's strengths and limitations in capturing cavitation and turbulence at various scales. The results demonstrate that the LES model excels in resolving small-scale turbulent structures, cavitation formation and shedding, as well as reverse flow and vortex dynamics, making it highly suitable for detailed analysis of complex cavitation phenomena. The DES model shows robustness in predicting large-scale flow features but exhibits delays when handling bubble motion near the hydrofoil's trailing edge. The SST model achieves a balance between computational accuracy and efficiency in predicting flow separation and vortex structures, making it ideal for trend analysis in engineering applications. By analyzing boundary vorticity flux lines, Q-criterion, and Omega methods, the study confirms the LES model's superior capability in capturing intricate vortex structures in high-turbulence regions, while also revealing the dynamic evolution of entropy generation during cavitation. Overall, the LES model proves to be the most effective for precise simulation of ventilated cavitation phenomena, while the DES and SST models are better suited for large-scale flow analysis and practical engineering applications, respectively. This research provides theoretical and technical support for the prediction and control of complex ventilated cavitation flows.

Optimization of entropy generation and thermal mechanism of MHD hybrid nanoliquid flow in a sinusoidally heated porous cylindrical chamber

The present computational investigation explores the fluid and thermal characteristics along with entropy generation due to buoyancy-driven magnetohydrodynamic (MHD) convective flow of aqueous hybrid nanoliquid filled in a nonuniformly heated porous cylindrical chamber. The fluid motion in the enclosure is modeled by Brinkman — extended Darcy model. The modeled equations are resolved by finite difference approach. The computations are conducted for Rayleigh number (103–105), Hartmann number (0–50), Darcy number (10−5–10−1), different nanoparticle shapes and nanoparticle volume fraction (Ag/MgO: 0–0.05) to understand the characteristic of flow, thermal and irreversibility distribution. With vast numerical simulations, the outcomes reveal that though the buoyancy force is greater, the fluidity cannot be enhanced unless the permeability and magnetic field strength are optimally maintained. As Ra,Ha and Da is varied respectively from 103 to 105, 50 to 0 and 10−5 to 10−1, the fluidity has been enhanced by 99.39%,83.26% and 99.79%. Among all considered parametric combinations, it has been noticed that the choice of Ha=0,Ra=105,Da=10−1 with proper ratio of nanoparticles enhance the system efficiency. However, minimal entropy generation can be achieved with greater Hartmann and lower Darcy numbers. Furthermore, it has been found that blade shaped nanoparticles lead to increase the performance of thermal system.

Modeling fatigue of pre-corroded body-centered cubic metals with unified mechanics theory

Corrosion is a common degradation problem in engineering structures. The unified mechanics theory (UMT) was used to develop a model to predict the fatigue life of pre-corroded steel samples with BCC structure. The model was also verified experimentally. For this purpose, A656-grade steel samples were immersed in a 5 wt% sodium chloride (NaCl) solution at a pH of 7. Electrochemical measurements were made with a potentiostat to monitor corrosion. Then, a series of ultrasonic vibration fatigue tests were conducted on these corroded samples at 20 kHz. UMT is ab-initio unification of the laws of Newton and the 2nd law of thermodynamics. Hence no empirical degradation/dissipation function is needed. However, the thermodynamic fundamental equation for metal corrosion and ultrasonic vibration must be analytically derived. The thermodynamic fundamental equation formulates all the entropy generation mechanisms during the corrosion and then during ultrasonic vibrations. The cumulative entropy generation was then used to calculate the Thermodynamic State Index (TSI), which starts at zero and asymptotically approaches one at failure. Hence, TSI predicts the life span. The proposed model results agree well with the experimental results, thus proving that the UMT-based model can predict the high cycle fatigue life of previously corroded samples accurately.

Entropy generation mechanisms from exothermic chemical reactions in laminar, premixed flames

This paper analyzes the mechanisms of entropy production that stem from exothermic chemical reactions in laminar, premixed flames. This paper is particularly motivated by the problem of indirect combustion noise, where acceleration of entropy fluctuations through a nozzle leads to acoustic fluctuations. In general, entropy generation in reacting systems occurs through molecular transport and chemical reaction, with the reaction terms being dominant in combustion systems. However, there are multiple mechanisms for entropy production from exothermic reactions, including heat release, change in number of moles of species, and changes in species. Most past analyses of indirect combustion noise have equated the heat release term with the flames entropy production, implicitly neglecting the other reaction-induced entropy sources. This paper shows the decomposition of the chemical source term from the entropy equation into three sub-terms, and then use detailed kinetic calculations from a one-dimensional reacting flow solver to calculate their magnitudes. Illustrative results are presented in the paper for several different fuels with varying thermodynamic properties methane, propane, octane, n-dodecane and hydrogen. Calculations are also performed for both air and oxygen as an oxidizer as well as at different parameter sweeps, in order to vary the flames fractional molar production and levels of product dissociation. These results show that heat release is by far the dominant production term in air-fueled flames. However, in oxygen fired systems, heat release has comparable magnitude as the other reaction terms. An important practical implication of this result is that indirect combustion noise source terms can be equated with the heat release terms for air-breathing systems but cannot be for rockets.

Unified mechanics theory based flow stress model for the rate-dependent behavior of bcc metals

A unified mechanics theory-based micromechanical model is developed to investigate the rate-dependent post-yield behavior of body-centered cubic(bcc) metals. The entropy generation mechanisms in bcc metals at a high strain rate are investigated. Thermodynamic State Index(TSI) in the unified mechanics theory is used to model the state of material as a function of entropy generation rate and then a dynamic yield stress function is derived. This work introduces a novel approach to modeling the internal heat generation and vacancy concentration change, at high strain rates. The derived rate-dependent model is validated for a wide range of strain rates, using the test data [1] for mild steel and compared with an irreversible thermodynamics-based model, available in the literature.

Entropy Generation during Head-On Interaction of Premixed Flames with Inert Walls within Turbulent Boundary Layers.

The statistical behaviours of different entropy generation mechanisms in the head-on interaction of turbulent premixed flames with a chemically inert wall within turbulent boundary layers have been analysed using Direct Numerical Simulation data. The entropy generation characteristics in the case of head-on premixed flame interaction with an isothermal wall is compared to that for an adiabatic wall. It has been found that entropy generation due to chemical reaction, thermal diffusion and molecular mixing remain comparable when the flame is away from the wall for both wall boundary conditions. However, the wall boundary condition affects the entropy generation during flame-wall interaction. In the case of isothermal wall, the entropy generation due to chemical reaction vanishes because of flame quenching and the entropy generation due to thermal diffusion becomes the leading entropy generator at the wall. By contrast, the entropy generation due to thermal diffusion and molecular mixing decrease at the adiabatic wall because of the vanishing wall-normal components of the gradients of temperature and species mass/mole fractions. These differences have significant effects on the overall entropy generation rate during flame-wall interaction, which suggest that combustor wall cooling needs to be optimized from the point of view of structural integrity and thermodynamic irreversibility.

Modeling ultrasonic vibration fatigue with unified mechanics theory

A series of ultrasonic vibration tests are performed on A656 grade steel samples, at a frequency of 20 kHz. A fatigue life model based on the unified mechanics theory is introduced to predict the very high cycle fatigue life of metals. Then, the fatigue life test data results are compared with the unified mechanics theory based model simulation results. It is shown that the physics-based unified mechanics theory can predict very high cycle fatigue life very well, without the need for the traditional empirical curve fitting a fatigue damage evolution function. The model does not require any curve fitting parameters obtained from fatigue test data. However, it does require deriving analytical thermodynamic fundamental equation of the material subjected to ultrasonic vibration fatigue. The thermodynamic fundamental equation of the material formulates the entropy generation mechanisms during the fatigue process. There are more than half a dozen entropy generation mechanisms during fatigue process. Entropy is an additive property, hence, the entropy generation due to all active mechanisms can be added.

The Determination of the Thermal Characteristics of the Microchannel Heat Exchangers for Single-Phase R600a Flow

Bu çalışmada, özellikle iklimlendirme sistemlerinde ön ısıtma/soğutma, aşırı kızdırma/soğutma gibi tek fazlı soğutkan akışının olduğu uygulamalar için mikrokanal eşanjörlerin matematiksel modellemesi deneysel doğrulamalı olarak ele alınmıştır. Çalışma akışkanının R600a olduğu panjurlu kanatlı mikrokanal eşanjörler için R600a’nın çıkış sıcaklığını, toplam ısı transfer kapasitesini ve entropi üretimini tahmin eden bir matematiksel benzetim modeli geliştirilmiştir. Modelin doğruluk hassasiyetini arttırmak için geçişlerdeki farklı kütle hızları ve uniform olmayan hava hızı modelde dikkate alınmıştır. Bu etkileri dikkate almak için literatürde yer alan diğer modellerden farklı olarak iki seviye ayrıklaştırma ile oluşturulan bir hesaplama sistemi modelde uygulanmıştır. Modelde mikrokanal ısı eşanjörünün ön yüzü hava akış bölgelerine bölünerek uniform olmayan hava hızları dikkate alınmıştır. Model sonuçlarını doğrulamak için deneysel çalışma yapılmıştır. Deneysel doğrulama sonucunda modelin, incelenen tüm test koşulları için çıkış sıcaklığını ±%10 aralığında bir ortalama mutlak sapma ile öngördüğü sonucuna varılmıştır. Mikrokanal ısı eşanjörünün, ısı transfer performansını tahmin etme kabiliyeti, deneylerle değerlendirilmiş, uniform olmayan hava hızının modele dahil etmenin, modelin doğruluk hassasiyetini arttırdığı görülmüştür. Mikrokanal ısı eşanjöründeki entropi üretim mekanizmaları incelenmiş ve akışkan akımı tersinmezliklerinin entropi oluşumuna katkısının, ısı transfer tersinmezliklerine kıyasla oldukça düşük olduğu tespit edilmiştir.

Designing of microsink to maximize the thermal performance and minimize the Entropy generation with the role of flow structures

Designing a novel microsink with geometric structures for better cooling of compact tiny-sized electronic devices at lower pressure drop is the utmost desire of recent technological development. A three-dimensional numerical study of conjugate heat transfer has been carried out to propose a microsink with the disruptive structures placed symmetrically about the vertical midplane in a rectangular microchannel. The channels are made of silicon with the structures like triangular cavity (TC), secondary branches (SB) and blockage (B) with water as a working fluid. The effects of these structures are illustrated by heat transfer, friction factor, thermal performance (TP), entropy generation (EG) due to heat transfer, entropy generation (EG) due to pressure drop and entropy generation number (EGN) over the range of Reynolds number (Re) from 146 to 618. The best channel has been identified from the rigorous analysis among the microchannels with the only triangular cavity (TC), triangular cavity with the secondary branch (TCSB) and triangular cavity along with secondary branch and blockage (TCSBB) from the highest TP and minimum EGN. These two models have been chosen to find out a best channel as criteria of highest heat transfer with the same pumping power from the highest TP and minimum temperature of electronic sink from the lowest EGN. The highest TP and lowest EGN are obtained from the channel TCSBB compared to other three channels. The parametric variation of relative angle of the secondary branch, length of cavity, width of secondary branch and cavity, pitch distance have been varied to find the maximum TP and the minimum EGN. The values have been obtained equal to 2.01 and 0.41 respectively at Re of 480. The significant roles of longitudinal vortex on the augmentation of heat transfer and minimizations of EG mechanism with the alteration of geometrical parameters are exploited and described explicitly. Finally, the correlations of heat transfer enhancement and rise in pressure drop dependent on geometric parameter have been inscribed based on Response surface methodology.

Particle creation in FLRW higher dimensional universe with gravitational and cosmological constants

We study the mechanism of particle creation in the higher dimensional FLRW-type cosmological models by using variable cosmological and gravitational constants. The solution of the corresponding field equations is obtained by assuming a linear function of the Hubble parameter (H) (i.e., q = α + βH) (Dixit et al. Pramana. J. Phys. 94, 25 (2020). doi: 10.1007/s12043-019-1884-2 ), which gives a scale factor [Formula: see text], where k and β are positive constants. Here we consider the time-dependent higher-dimensional field equations, including the general formulation of particle creation and entropy generation mechanisms. We also investigate a few quantities, such as the deceleration parameter q, particle creation rate ψ, the entropy S, the cosmological constant Λ, Newton’s gravitational constant G, and the energy density ρ, and discuss their physical significance. We have observed that all quantities, except the gravitational constant (G) and the entropy (S), decrease with time in all dimensions characteristically. However, the entropy S and the gravitational constant G increase with time. Additionally, we have also discussed the look-back time, luminosity distance, distance modulus, and age of the universe with redshift z and observed the role of particle formation in universe evolution in early and late times. For the derived model, we have calculated various physical parameters, which are in good agreement with the recent observations.

Exploring the physical aspects of nanofluid with entropy generation

Characteristics of entropy generation for magnetized non-Newtonian nanofluid in the presence of heat sink–source are elaborated in this research. Heat transfer features are accounted in view of viscous dissipation and nonlinear thermal radiation. More specifically, thermophoretic deposition and Brownian moment aspects in addition to chemical processes are considered in the mathematical modeling. Non-dimensionalization procedure is adopted by introducing apposite variables. Bvp4c algorithm is opted for nonlinear analysis. Graphical analysis is prepared to portray the features of distinct variables against dimensionless quantities. We found opposing behavior for thermophoretic deposition and Brownian moment against concentration field. One can observe from graphical analysis that magnetized nanofluid is significantly affected by entropy generation mechanisms. More specifically, it has been noticed that the Hartmann and Brinkman numbers have reverse characteristics against entropy generation and Bejan number.

Particle Creation in Friedmann–Robertson–Walker Universe

Using variable gravitational and cosmological constants, the mechanism of particle creation in the universe is studied for Friedmann–Robertson–Walker (FRW) space-times at high dimensions to explain the early deceleration and present accelerating phases. To investigate the dynamics of these phases, we consider two scale factor of the forms $$a(t)=\sqrt{t^\alpha{e^t}}$$ and $$a(t)=\sqrt[m]{{\rm{sin}}h(kt)}$$, which yield two different time-dependent deceleration parameters. Firstly, we modify the d-dimensional time-dependent field equations, including the general formulation of particle creation and entropy generation mechanisms. Then, we investigate the time dependence of a few quantities such as the particle creation rate ψ, the entropy S, the gravitational constant G, the cosmological constant Λ, the energy density ρ, the deceleration parameter q, etc. We show that all constants and quantities, except the gravitational constant G and the entropy S, characteristically decrease with time for two kinds of scale factors in all dimensions. However, the gravitational constant G and the entropy S increase with time. Additionally, we show that the cosmological constant Λ is unexpectedly independent of the particle creation mechanism, unlike G.

Thermal analysis on a nanofluid-filled rectangular cavity with heated fins of different geometries under magnetic field effects

Rectangular cavity filled with a non-Newtonian shear-thinning nanofluid was investigated considering varied fin geometry under a vertical magnetic field. Irreversibility and entropy generation mechanisms were investigated by finite element method. The effect of geometry parameter (a), Hartmann number (Ha), nanoparticles volume fraction (φ) and nanoparticles type (Fe3O4, CuO and Al2O3) on the Nusselt numbers, entropy generations and Bejan numbers were investigated by the response surface methodology (RSM). Increasing a leads to an increase in both local and average Nusselt numbers and increment in different terms of entropy generation, while it reduces the Bejan number. Fe3O4 nanoparticle shows the highest Nusselt number compared to other nanoparticles. RSM analysis revealed that a = 0.29 and φ = 0.04 are the optimized values for the goal of maximum Nusselt number and minimum Bejan number.

Transport of radiative heat transfer in dissipative Cross nanofluid flow with entropy generation and activation energy

In this paper, features of entropy generation for mixed convective flow of a Cross nanoliquid are discussed. Properties of Hall characteristics for nonlinear mechanisms are elaborated on for greater Loren’s force aspects. Thermophoretic and Brownian diffusion effects are taken into account to investigate colloidal analysis for a non-Newtonian fluid. Moreover, a convectively heated surface is considered in the modeling of a physical problem. The modeled problem is simplified through a lubricant procedure. A numerical solution of the proposed physical problem is obtained through a bvp4c scheme. More specifically, we detect that the colloidal property of nanomaterials significantly affects the properties of the base liquid and entropy generation mechanism. Significant features of the Bejan number entropy generation rate are explained via graphical data.

Entropy generation and activation energy mechanism in nonlinear radiative flow of Sisko nanofluid: rotating disk

The theme of the present communication is to explore the novel analysis of entropy generation optimization, binary chemical reaction and activation energy for nonlinear convective flow of Sisko model on a radially stretchable rotating disk in the presence of a uniform vertical magnetic field. Nonlinear mixed convection, nonlinear thermal radiation, MHD, viscous dissipation, Joule heating and non-uniform heat generation/absorption are also considered. Nanofluid model includes significant slip mechanism of Brownian motion and thermophoresis. Apposite transformations are endorsed to get the nonlinear coupled ODEs system. The resultant system of ordinary differential equations is endeavoured for series solutions through homotopic technique. Total entropy generation is inspected through numerous emerging flow variables. Comparative study is made for temperature, velocity, heat transfer rate, Bejan number, entropy generation and mass transfer Nusselt number by considering shear thickening and thinning fluids. Finally, a comparison is specified with the previous existing results.

Energy Savings in Desalination Technologies: Reducing Entropy Generation by Transport Processes

Desalination systems can be conceptualized as power cycles in which the useful work output is the work of separation of fresh water from saline water. In this framing, thermodynamic analysis provides powerful tools for raising energy efficiency. This paper discusses the use of entropy generation minimization for a spectrum of desalination technologies, including those based on reverse osmosis (RO), humidification–dehumidification (HDH), membrane distillation (MD), electrodialysis (ED), and forward osmosis (FO). Heat and mass transfer are the primary causes of entropy production in these systems. The energy efficiency of desalination is shown to be maximized when entropy generation is minimized. Equipartitioning of entropy generation is considered and applied. The mechanisms of entropy generation are characterized, including the identification of major causes of irreversibility. Methods to limit discarded exergy are also identified. Prospects and technology development needs for further improvement are mentioned briefly.

Near-Wall Thermal Processes in an Inclined Impinging Jet: Analysis of Heat Transport and Entropy Generation Mechanisms

In this work, near-wall thermal transport processes and entropy generation mechanisms in a turbulent jet impinging on a 45 ∘ -inclined heated surface are investigated using a direct numerical simulation (DNS). The objectives are to analyze the subtle mechanisms of heat transport in the vicinity of an inclined impinged wall, to determine the causes of irreversibilities that are responsible for the reduction of performance of impingement cooling applications and to provide a comprehensive dataset for model development and validation. Results for near-wall thermal characteristics including heat fluxes are analyzed. An entropy production map is provided from the second law analysis. The following main outcomes can be drawn from this study: (1) the location of peak heat transfer occurs not directly at the stagnation point; instead, it is slightly shifted towards the compression side of the jet, while at this region, the heat is transported counter to the temperature gradient; (2) turbulent thermal and fluid flow transport processes around the stagnation point are considerably different from those found in other near-wall-dominated flows and are strongly non-equilibrium in nature; (3) heat fluxes appear highly anisotropic especially in the vicinity of the impinged wall; (4) in particular, the heated wall acts as a strong source of irreversibility for both entropy production related to viscous dissipation and to heat conduction. All these findings imply that a careful design of the impinged plate is particularly important in order to use energy in such a thermal arrangement effectively. Finally, this study confirms that the estimation of the turbulent part of the entropy production based on turbulence dissipation rates in non-reacting, non-isothermal fluid flows represents a reliable approximate approach within the second law analysis, likewise in the context of computationally less expensive simulation techniques like RANS and/or LES.

Second-Law Analysis: A Powerful Tool for Analyzing Computational Fluid Dynamics (CFD) Results

Second-law analysis (SLA) is an important concept in thermodynamics, which basically assesses energy by its value in terms of its convertibility from one form to another.[...]

Numerical Investigation of Entropy Generation in Stratified Thermal Stores

This paper investigates the nature of entropy generation in stratified sensible thermal energy stores (SSTES) during charging using a dimensionless axisymmetric numerical model of an SSTES. Time-varying dimensionless entropy generation rates and the cumulative entropy generation in SSTES were determined from finite volume computations. The aspect ratios (AR), Peclet numbers (PeD), and Richardson numbers (Ri), for the stores considered were within the ranges 1≤AR≤4, 5×103≤PeD≤100×103, and 10≤Ri≤104, respectively. Using the Bejan number (Be), the total entropy generation was shown to be almost entirely due to thermal effects in the SSTES. The Be is practically unity for most of the SSTES' charging duration. The contributions of radial thermal gradients to the thermal entropy generation were further shown to be largely negligible in comparison to the contributions of axial thermal gradients, except at low Ri. Entropy generation numbers, Ns, in the SSTES were also computed and found to increase with decreasing AR and PeD and with increasing Ri. PeD was found to have the most significant influence on Ns. Based on this axisymmetric analyses of time-varying entropy generation in SSTES, estimates have been obtained of (1) the relative significance of radial effects on entropy generation within SSTES and (2) the relative significance of viscous shear entropy generation mechanisms within SSTES.

Analysis of entropy generation behavior of supercritical water flow in a hexagon rod bundle

The numerical simulation was carried out to investigate the local entropy generation in turbulent mixed convection heat transfer of supercritical water within the inner sub-channel of SCWR. The SSG Reynolds stress model with enhanced wall treatment was used as a turbulence model. The circumferential heterogeneity of heat transfer was introduced into the entropy generation analysis. The mechanisms of entropy generation in the near wall region within the boundary layer and the region away from the wall were discussed in detail. The results show that the buffer layer has great influence on heat transfer enhancement, and both the viscous sub-layer and buffer layer act significantly in the entropy generation with respect to circumferential heterogeneity. Usually, the entropy generation in the narrow gap is relatively larger than that in the sub-channel center when comparing the contribution of entropy production caused by heat transfer to that caused by energy dissipation. The simulation findings may provide a theoretical direction for the conceptual design of SCWR.