Terms Of Entropy Generation Research Articles

On Entropy Generation and Heat Transfer Due to Magneto-Marangoni Convective γ Al2O3-H2O/C2H6O2 Nanofluid Flow Over a Porous Surface

A numerical analysis is carried out for water/Ethylene Glycol based γ Al2O3 nanofluid movement over a horizontal permeable sheet placed within a porous medium using MATLAB package Bvp4c solver. Other flow controlling conditions like Non-linear thermal radiation and uniform Magnetic field are also considered for this study. The present study is novel in terms of entropy generation and heat transfer rate investigation for mentioned fluid system in the existence of dissipation (an irreversible process) and heat generation/absorption impact. First, a mathematical pattern is prepared in the form of partial differential equations to represent the Marangoni convection flow and temperature, considering suitable boundary conditions. Using similarity parameters, we convert our mathematical model in dimensionless form and then solved it. Accuracy of obtained data is also cross-checked with another numerical technique “Runge-Kutta fourth order” along with shooting process. Using velocity and temperature fields, entropy is measured for present system. From the plots, it is noted that entropy as well as Bejan number is qualitatively changed for parameters namely, volume fraction parameter, radiation parameter, Brinkmann number and heat generation/absorption parameter. It is noticed that heat transfer rate and entropy generation number is higher for γ Al2O3-C2H6O2 nano fluid then γ Al2O3-H2O nano fluid.

Evolution of Entropy Generation on Mixing Process Using a Novel Chaotic Micromixer

This study aims to compare numerically the laminar steady flow of shear-thinning non Newtonian fluids in novel chaotic micromixer. The process is verified for non-Newtonian flow in a complex geometry that is heated with a constant flux. The selected geometry produce secondary flow structures that greatly improve fluid dynamic performance. To measure this performance, the Poincaré map method is used for various cases of fluid power-law index in different geometries. In addition, thermal mixing behavior is studied for shear-thinning fluids in the chosen geometry with two different inlet temperatures.In different cases of fluid power-law indexes, the novel geometry resulted in an 82% increase in thermal mixing degree relative to the thermal mixing degree observed in the C-shaped and serpentine channels. The second law of thermodynamics is examined in terms of entropy generation caused by thermal and hydrodynamic processes, as a function of low rates of generalized Reynolds number and power-law index, while taking into consideration the effects of chaotic advection. As a result, the new configuration showed a significant improvement in the degree of mixing compared to other geometries considered, while also minimizing friction and thermal irreversibilities.

Thermal energy analysis of multi-walled carbon nanotubes-Fe3O4/H2O flow over non-uniformed surface with Darcy–Forchheimer model

In this study, a new cavity shape was filled with an extension multi-walled carbon nanotubes-Fe2O3/H2O nanofluid under a constant magnetic field. The Darcy–Forchheimer model is used to account for the inertial impact of advection in the porous layer while maintaining the laminar and incompressible nature of the nanofluid flow. The dimensionless version of the governing equations is used to describe the issue and the finite element approach is used to resolve it. Through this complex geometry, various thermophysical factors such as Rayleigh number [Formula: see text], Hartmann number [Formula: see text], and nanoparticle concentration are considered [Formula: see text]. The porous layer's numerous characteristics are also explored. For example, its porosity [Formula: see text] and Darcy number [Formula: see text], which indicates the permeability of the porous medium. The content of the hybrid nanofluid is considered to be Newtonian, stable, incompressible, and following a constant Prandtl number for the base fluid [Formula: see text]. Calculations are made according to the finite element method. The results of this work are presented in terms of rheology, isotherms, entropy generation, and mean Nusselt numbers. They have demonstrated that increasing the Rayleigh and Darcy numbers improve heat transfer in the enclosure.

Thermo-fluidic behaviour of diamond and hourglass microchannels: Investigation from the second law viewpoint

The potential of diamond and hourglass microchannels from the second law perspective is explored to amplify the effectiveness of the thermal device for various thermodynamic systems. The chosen design variables cover all relevant physical traits, such as the divergence-convergence effect (divergence-convergence angle, width ratio), geometrical considerations (depth, length), input variables (Reynolds number, supplied heat flux), and conjugate effect (substrate material and thickness). The performance is quantified in terms of thermal and frictional entropy generation, total entropy generation, and augmented entropy generation number. Results reveal that the proposed geometries outperform uniform microchannels for all cases except at higher Reynolds numbers for high divergence-convergence angle, width ratio, length, and low supplied heat flux. The outcome also demonstrates that the diamond microchannel performs slightly better than the hourglass microchannel for all cases except at shorter lengths and higher aspect ratio for Reynolds numbers above 100, owing to flow recirculation zones and the conjugate effect. Additionally, the first and second law attributes of diamond/hourglass microchannels are compared and analyzed. Diamond/hourglass microchannels with superior second law performance at a given design constraint need not exhibit better first law performance simultaneously, and vice versa. Lastly, the current investigation sheds light on design possibilities that enhance the potential of proposed geometries through prudent exploitation of the physical characteristics. The outcomes of this research have the potential to design diamond/hourglass microchannels with the appropriate geometry for a plethora of heat transfer applications.

Entropy Generation and Mixed Convection of a Nanofluid in a 3D Wave Tank with Rotating Inner Cylinder

The generation of entropy and mixed convection in a nanofluid-filled 3D wavy tank containing a rotating cylinder is investigated. The top wavy surface of the tank is heated and all vertical surfaces are assumed to be adiabatic, while the bottom horizontal surface remains isothermally cold. The tank contains a solid cylinder and is saturated with an Al2O3–water nanofluid. The numerical simulations using the FEM are performed for the Richardson number (0.01≤Ri≤100), nanoparticle volume fraction (0≤ϕ≤0.04) and number of oscillations (0≤N≤4). The numerical results of the present work are given in terms of 3D streamlines, isotherms and local entropy generation, as well as average heat transfer and Bejan number. The results show that for low values of the Richardson number and oscillation, heat transfer enhancement can be achieved by increasing the nanoparticle volume fraction.

Entropy generation study due to MHD double-diffusive convection in the rectangular cavity with built-in rectangular blockage

In the present work, the influence of the aspect ratio of the rectangular cavity (length > width) and the built-in rectangular blockage (length > width) on the local and global entropy generation (EG) characteristics due to double-diffusive convection and external magnetic field have been explored. Particularly, the effect of the pertinent parameters, such as the aspect ratio of the cavity and blockage Lewis number Rayleigh number Hartmann number and buoyancy ratios on the EG characteristics have been explored. The working fluid is considered to be a liquid metal-sodium–potassium alloy (Pr = 0.054). Lattice Boltzmann method numerical scheme is employed to solve the double-diffusive natural convection phenomenon. The increase in Ra induces an enhancement in the localized EG terms, whereas the augmentation in Ha diminishes the local total EG (S Total) for a given Ra. The dominance of thermal and species transport irreversibility in the cavity was noticed for Conversely, for AR = 4, the significance of flow friction irreversibility within the cavity was observed (). The results of the work are also summarized in terms of simpler empirical correlations of and ST , which might be useful for possible engineering design purposes.

Flow and heat transfer irreversibility in partial filled metal foams

This paper presents the analysis of entropy generation, exergy and heat transfer through metal foams partially and completely filled in a vertical channel. The assembly of heater located in between two aluminum plates is located at the core of the channel and for the purpose of heat transfer augmentation aluminum metal foam of pore density 10 with porosity 0.95 are located on either side of the plates. Four different filling rates of 25%, 50%, 75% and 100%, and three metal foam thicknesses of 10 mm, 20 mm and 30 mm are examined in this research for a fluid velocity ranging from 0.1 to 3 m/s. Darcy Extended Forchheimer (DEF) model and local thermal non-equilibrium (LTNE) model is used for forecasting the flow features and heat transfer through the metal foam. The numerical methodology adopted in this research is confirmed by comparing the results with the literature and found fairly good agreement between them. The flow physiognomies in terms of friction factor, heat transfer performance in terms of Nusselt number and performance factor, exergy transfer in terms of mean exergy based Nusselt number, and entropy generation i.e., total irreversibility in terms of Bejan number and entropy generation number are presented and discussed. Results showed that the effect of partially filled metal foams are found to be better in terms of entropy generation, and exergy transfer, and the effect of higher metal foam thickness is found to be better in terms of exergy and heat transfer, but in terms of total entropy generation resulting in terms of both heat transfer and pressure drop smaller metal foam thickness exhibits better performance.

Analysis of conjugate Marangoni natural convection in a heating system with an open boundary flow

Marangoni natural convection involved in a heating system with an open boundary layer flow has been numerically analyzed in this article. An isoflux heater is placed below a steel pan (material: AISI 4340 steel) which is partially/fully filled with water to make this model more realistic. The surrounding of this pan has a two-dimensional air domain including a water-air interface. Hence, the air domain has three open boundaries within the computational domain. Conjugate natural convection heat transfer takes place from the solid pan to water and then air, whereas Marangoni convection is facilitated at the water-air interface. Two-dimensional, unsteady Navier-Stokes and energy equations are considered as the governing equations, and those are solved with the help of finite element method. Rayleigh number is varied in the range of laminar regime from 6 × 104 to 3 × 107 as the governing parameter under different water level conditions. Thermal and flow fields are visualized qualitatively, whereas quantitative distributions of average Nusselt numbers are taken for further analysis. Finally, the exergy of the system is inspected in terms of total entropy generation at individual flow domains. Here, overall heat transfer performance shows a strong dependency on the change of Rayleigh number.

Entropic Assessment of Sleeping Comfort

In the literature longevity and comfort are evaluated in terms of entropy generation and export rates. When people cannot export entropy, they experience discomfort. Effect of bedding, pajamas and levels of body coverage have been assessed in 270 cases by referring to body weight and height of 25 years old women. By 160 cm tall and 50 kg women, at 0 °C of room temperature, entropy export rates were 6.0x10--3 and 7.4x10--3 W/kg K, while they were sleeping on their back and on the side, respectively. The results showed that entropy export became more difficult as the body temperature approached the room temperature. Textile properties, including heat transfer and wicking rates of sweat removal were also important while exporting the entropy. The results of this study may be employed while designing beds or beddings, pajamas and comforters to achieve a more comfortable sleeping environment.

Investigation of thermal entropy generation and nanofluid flow in a new heatsink with effect of nanoparticles shape

In this paper, the entropy generation (ENT) in two novel micro heatsinks (MHSs) is investigated numerically. The HSs are two types of octagonal ones with circular and rectangular fins. Water/alumina boehmite nanofluid (NF) is used as a cooling fluid for HSs, which enters from the middle of the heatsink (HS) and exits from four side outputs. The Reynolds number (Re) varies from 300 to 1700 and the concentration of nanoparticles (NPs) changes from 0 to 6%. The shapes of NPs include platelet brick, blade, cylinder, and Oblate Spheroid (OS). A heat flux due to the operation of a microprocessor is applied to the bottom of the HSs made of copper. The finite element method is used for the simulations. The variables include pumping power (PP), ENT, ENT due to fluid friction, thermal ENT, temperature gradient contours, etc. The results of this study show that less ENT occurs in the HS with rectangular fins and less PP occurs in the HS with circular ones. Increasing the Re reduces the amount of ENT and increases the PP. The results demonstrate that the addition of NPs with high volume percentages is not suitable in terms of ENT and PP.

Understanding loss generation mechanisms in a centrifugal pump using large eddy simulation

Water pumps are amongst the most frequently used turbomachines, which consume about 9% of the global energy production. This paper provides insight on the loss generation mechanisms in inline centrifugal pumps operating under realistic conditions through a detailed analysis based on large eddy simulation (LES). The equations for the resolved, sub-grid and wall entropy generation terms are derived using the LES filtering and Smagorinsky-Lilly formalism. Specific exergy is calculated at the grid points. Entropy generation and exergy transfer within the flow domain are visualised to highlight the loss generation mechanisms within the pump. Results show that there is a strong component interaction. The double-bended inlet pipe initiates Dean-vortices at the eye of the impeller. This asymmetric inflow coupled with the disturbances at the cut-off cause a non-uniform flow through the blade passages. Other drivers of the asymmetrical loading of the blade passages are high leakage and unsteady and asymmetrical mass and exergy transfer between the dead zone. The paper also investigates the effect of the grid resolution on the first and second order statistics of the flow field and its turbulent characteristics. A thorough evaluation of the discretisation error is presented to provide a measure for the required grid density and demonstrate the extent and impact of the temporal and spatial discretisation error in centrifugal turbomachinery.

Local entropy generation and exergy analysis of the condenser in a direct methanol fuel cell system

This paper aims to identify the irreversibilities in the condenser of a direct methanol fuel cell (DMFC) system and present possible enhancements in its design through local entropy generation analysis (L-EGA). For this purpose, the local entropy generation terms originating from heat and mass calculated from results of a pseudo two-phase computational fluid dynamic (CFD) model of the condenser. Through this analysis, the total irreversibilities due to heat and mass transfer are calculated locally (e.g., film boundary layer, vapour-gas boundary layer) under the variable operating conditions of a DMFC (undersaturated, saturated, and supersaturated conditions of the cathode exhaust gas). Moreover, the exergy destruction ratio of condenser is found to estimate the exergy performance of the condenser. The results show that in the case of supersaturated cathode exhaust gas (CEG) flow, the entropy generation rate due to mass transfer in the film region is found as 0.032 W/(m·K) which is 18 times higher than that for the undersaturated CEG flow. However, entropy generation rate due to mass transfer decreases significantly when the hot flow is just over the film region. In the film region, the entropy generation rates originating from heat transfer are found as 0.0055 W/(m·K) (for the undersaturated case), 0.0032 W/(m·K) (for the saturated case), and 0.0015 W/(m·K) (for the supersaturated case). Moreover, the maximum exergy destruction ratio is found as 0.72 when the CEG is undersaturated and the CEG velocity is 0.18 m/s, while the lowest exergy destruction ratio is calculated as 0.28 when the CEG is saturated.

Shockley: Queisser detailed balance limit after 60 years

AbstractThe year 2021 marks the 60th anniversary of the Shockley–Queisser paper which laid the theoretical foundations for the fundamentals of solar cell operation. This article reviews the principal results of the Shockley‐Queisser paper and the developments that followed. Starting with links to the earlier radiative balance laws of Kirchhoff and Planck, we discuss the nature of the detailed balance between the incident and emitted photon beams. The resulting efficiency limit is juxtaposed with another popular limit due to Trivich and Flinn. The dependence of the Shockley–Queisser limit on the intensity and sizes (étendues) of the two beams—in other words, on the concentration of sunlight—is discussed in some detail. This article then takes a broader view of the detailed balance with the help of the generalized Planck law and the refinements that this implies for the efficiency and the current–voltage characteristic of the cell. A natural extension of the Shockley–Queisser detailed balance into the realms of thermodynamics is outlined by considering the photon entropy in the two beams. This produces the detailed balance limit in a typical thermodynamic form, similar to the result for a heat engine, where the efficiency losses are expressed in terms of entropy generation. The paper concludes with a brief discussion of how the results can be extended to the operation of practical solar cells with a more realistic description of light absorption and nonradiative recombination. A brief overview is also given of mechanisms for how the Shockley–Queisser limit can be exceeded.This article is categorized under: Sustainable Energy > Solar Energy Emerging Technologies > Materials

Conjugate Free Convection Heat Transfer and Thermodynamic Analysis of Infrared Suppression Device With Cylindrical Funnels

AbstractA convection system can be designed as an energy-efficient one by making a considerable reduction in exergy losses. In this context, entropy generation analysis is performed on the infrared suppression system numerically. In addition, results due to heat transfer are also shown. The numerical solution of the Navier–stokes equation, energy equation, and turbulence equation is executed using ansysfluent 15.0. To perform the numerical analysis, different parameters such as the number of funnels, Rayleigh number (Ra), inner surface temperature, and geometric ratio are varied in the practical range. Results are shown in terms of heat transfer, entropy generation, irreversibility (due to heat transfer and fluid friction), and Bejan number with some relevant parameters. Streamlines and temperature contours are also provided for better visualization of temperature and flow field around the device. Results show that heat transfer and mass flow rate increase with the increase in the number of funnels. Entropy generation and the irreversibility rise with an increase in the number of funnels and geometric ratio. Also, the Bejan number decreases with an increase in Ra and the number of funnels. A cooling time is also obtained using the lumped capacitance method.

Fluid Friction Effects on the Thermodynamic Performance of Spiral Plate Heat Exchangers

This paper presents a detailed analysis of the thermodynamic performance of spiral plate heat exchangers (SPHEs) and their channels by considering the fluid friction effect and no heat leakage to the environment. An optimal design algorithm for SPHEs is developed to find higher compactness and the overall heat transfer coefficient by increasing channels pressure drops, maintaining the geometric aspect ratio, and minimizing total costs. To determine the internal temperature distributions and the rates of heat transfer, a mathematical model is proposed. Fluid friction effects are examined as viscous heating; for this purpose, new parameters of the viscous heating entransy number and viscous heating entransy resistance are defined. Finally, various single-phase countercurrent SPHEs assuming a constant heat transfer rate, are modeled and compared in terms of energy, entropy generation, and entransy criteria. The results show the fluid friction effects as pressure drop effects in entropy generation are insignificant when compared to those caused by heat transfer. However, fluid effects as viscous heating in entransy analysis show that these effects can be rather significant in SPHEs with large spiral turn numbers or small values of heat capacity rate ratios. The obtained data suggest the significant role of entransy methods and their reliability to entropy generation methods in thermodynamic performance evaluation of SPHEs.

Influence of Pulse Characteristics on Turbocharger Radial Turbine

Abstract Turbocharging has been demonstrated as a key technology to enhance fuel efficiency in the automotive field that faces increasingly stringent emission regulations. Due to the reciprocating engine, pulsating flow feds the turbocharger turbine, which experiences conditions far from a continuous flow scenario. In this work, the effects of the characteristics of the mass flow pulse, parameterized through amplitude, frequency, and temporal gradient, are decoupled and studied via unsteady computational fluid dynamics calculations under on-engine operating conditions. First, the model is validated based on comparisons with experimental data in gas-stand conditions. Then, the effect of each parameter on the exergy budget is assessed by considering a ±10% variation with respect to a baseline pulse. The other factors defining the operating conditions (e.g., mass flow, shaft speed, and inflow exergy) are kept the same as the baseline. The adopted approach enables to completely isolate the effects of each parameter in contrast with previous literature studies. Based on the results observed, pulse amplitude is identified as the primary parameter affecting the hot-side system response in terms of turbine performance, heat transfer, and entropy generation, while frequency and temporal gradient show a smaller influence compared to it. As the pulse amplitude increases, the turbine work is reported to improve up to 9.4%. Smaller variations are otherwise observed for the frequency and temporal gradient analysis. With a 10% increase of the pulse frequency, the turbine work is registered to improve by 5.0%, while the same percentage reduction of the temporal gradient leads to an increase of the turbine work equal to 3.6%.

Improving the performance of a nanofluid-based photovoltaic thermal module utilizing dual-axis solar tracker system: Experimental examination and thermodynamic analysis

• A PVT module integrated with a novel solar tracker system (STS) was built. • This STS has two intermittent on/off switching motors for azimuth and tilt angles. • Various coolants and tracking modes were compared using thermodynamic equations. • Results were compared from energy, exergy, and entropy generation viewpoints. • PVT module with STS had 268.24/20.8 W energy/exergy rate higher than fixed PV unit. The purpose of this experimental research is to improve the electrical and thermal output power of a photovoltaic thermal (PVT) module equipped with an automatic dual-axis solar tracker system. The focus of this study is on characterizing the performance of a PV unit and a PVT module both with three different modes of fixed, one-axis tracking, and dual-axis tracking. These modules were installed at the rooftop of the engineering faculty building of Ferdowsi University of Mashhad, Iran. The test fluids are deionized water and silicon carbide nanofluid (SiC/water) with 0.5% and 1% mass fractions. The experiments are executed on non-cloudy days with stable weather conditions during September. The thermodynamic analysis is carried out in terms of energy, exergy, and entropy generation to identify and compare the performance of various modules considered in the experiments. It is found that the proposed nanofluid-based PVT module integrated with the solar tracker system has no significant inverse effect on electrical efficiency thanks to using intermittent on/off switching electro-motors. The results showed that considering the power consumption of the pump and tracking electro-motors when nanofluid-based PVT module (SiC/water 1%) with dual-axis, one-axis, and fixed modes are utilized, the increases in the obtained overall energy (exergy) rates compared to that of the fixed PV unit are 268.24 (20.8 W), 245.91 (16.51 W), and 210.32 (9.64 W), respectively.

Evaluation of energy flow, dissipation and performances for advanced adsorption assisted heat transformation systems: Temperature-entropy frameworks

This article presents the entropy flow and generation of an adsorption assisted heat transformation (A-HT) system. Hence the Gibbs analogy and the thermodynamic property fields of adsorbents + water systems are employed to formulate the entropy balance of adsorption beds. Later, the experimentally confirmed isotherms, kinetics and bed heat exchanger data are applied for computing the energetic performances of adsorption system in terms of entropy flow and generation, from which one can calculate (i) the system-performances with respect to heating and cooling capacities, specific water production and the overall efficiency and (ii) dissipations from each component of A-HT systems. The temperature entropy-flow (T-Sf) maps show close loops for the evaporator, the condenser and the beds indicating the stored energy under batch operating conditions of A-HT cycle. It is observed that the entropy generation (Sg) are mainly contributed by various processes such as mass and heat transfer, de-superheating and flushing of heat transfer fluids. The highest entropy generation occurs in the adsorption bed during desorption period, and the least Si is found in the evaporator. The proposed temperature-entropy maps can be applied to design each component of bed heat exchanger with minimum entropy generation. It is found that the entropy flow and generation depend on the periods of batch operation, the heating source temperature, and the quality of porous adsorbents.

Radiative thermal criticality and entropy generation of hydromagnetic reactive Powell-Eyring fluid in saturated porous media with variable conductivity

Theoretical study of inherent irreversibility and thermal runaway of an exothermic reactive Eyring-Powell fluid flow through a saturated porous fixed horizontal channel with thermal radiation and variable electrical conductivity is investigated. The reactive conducting fluid under bimolecular chemical rate law is propelled by pressure gradient. Ignoring the material assumptions, the governing dimensionless equations of the flow model are solved using semi discretization finite difference techniques coupled with weighted residual method. The results in terms of flow rate, temperature, thermal runaway, Bejan number and entropy generation are presented in graphical form. From the results, it is observed that parameter which argument the entropy production rate also enhances the Bejan number. Also, minimization of entropy generation is achieved at low values of thermal radiation, dissipation rate, electric field loading and viscosity variables.

Transport Phenomenon of Simultaneously Developing Flow and Heat Transfer in Twisted Sinusoidal Wavy Microchannel under Pulsating Inlet Flow Condition

Heat transfer operations in microchannel are of special relevance in designing micro-devices involving accumulation of high heat flux. The current study explores simultaneously developing, unsteady laminar flow inside a twisted sinusoidal wavy microchannel and heat transfer involved. A square cross-sectioned channel is taken for the study. The channel is wavy in nature as well as twisted about the flow axis. The modified geometry helps in enhancing out of plane mixing of the fluid and formation of recirculation inside the flow. The evaluation is performed for a Reynolds number range of 1–100. A sinusoidal (varying with time) normal velocity component is employed at the inlet. The study was conducted over a range of pulsation amplitude and frequency. To solve the governing equations, a finite volume based method was employed. The Nusselt number data shows significant enhancement for the sinusoidal inlet velocity as compared to that of a steady case. In terms of pressure drop, the proposed design shows that at different frequencies and amplitudes, it can keep the pressure drop within admissible limits. Entropy generation is used as the measure of dissipated energy. Performance in terms of entropy generation is also reported.