Global Entropy Generation Rate Research Articles

Assessment of thermohydraulic performance and entropy generation in an evacuated tube solar collector employing pure water and nanofluids as working fluids

This study conducts a numerical comparison of the thermal performance of three distinct working fluids (pure water, TiO2, and SiO2 water-based nanofluids) within an evacuated tube solar collector using Computational Fluid Dynamics. The study evaluates thermohydraulic performance alongside global and local entropy generation rates, while considering variations in solar radiation values and inlet mass flow rates. Results indicate that nanofluids demonstrate superior performance under low solar radiation, exhibiting higher outlet temperatures, velocities, thermal efficiency, and exergy efficiency compared to pure water. However, at the higher solar radiation level, the efficiency of SiO2 water-based nanofluid diminishes due to its impact on specific heat. Furthermore, the entropy generation analysis reveals significant reductions with TiO2 water-based nanofluid in all the phenomena considered (up to 79 %). The SiO2 nanofluid performance aligns closely with pure water under high radiation value. This investigation offers valuable insights into the utilization of nanofluids in solar collectors across diverse operating conditions, emphasizing their pivotal role in enhancing overall performance.

Fluid–structure-interactive elasto- and thermo-hydrodynamics of electrokinetic binary fluid flows in compliant micro-confinements

We explore the intricate two-way fluid structure interaction arising due to the flow of a binary system of immiscible Newtonian fluids, composed of upper electrically conducting and lower electrically insulating fluids, flowing within a compliant microchannel, whose walls behave as linear elastic solids. The transport of the fluids along the domain occurs due to the collective impact of pressure gradient and applied electric field. We solve the closed-form system of equations and obtain semi-analytical expressions for the velocity fields and channel wall deformation from the coupled elasto-hydrodynamic problem. We then delineate the effect of four pivotal parameters: (a) Debye–Hückel parameter κ¯, (b) upper wall slip length, ls¯, (c) viscosity ratio, μr, and (d) elasticity ratio, Nr, on the morphological evolution of the wall deformation characteristics and the spatial distribution of the velocity profile of the fluids. Observations establish a positive co-relationship of wall deformation with fluid pressure, showcasing an increased collapsibility with augmented pressure gradients. Consequently, the channel walls show enhanced deformation with a decrease in κ¯, ls¯, μr and with an increase in Nr. We also demonstrate from our model that by properly tuning the applied pressure gradient and electric field, it is possible to achieve counterflow of the two fluids. We also consider the effect of heat generation in the fluids due to viscous dissipation and Joule heating, which dissipates to the surrounding by the mechanism of conjugate heat transfer. Accordingly, we provide semi-analytical expressions for the temperature profile distribution within the channel, and discuss their variation with three thermo-physical parameters: (a) Biot number of the top wall (Bi1), (b) Peclet number of the top fluid (Pe1), and (c) ratio of the thermal conductivities of the upper conducting fluid to that of the upper solid wall (kr2). We establish from our investigation that with the increase in Pe1 and with the decrease in Bi1 and kr2, the overall system temperature enhances. Finally, in order to design a thermally efficient system, we compute the global entropy generation rate in the system and evaluate optimum values of, Pe1, Bi1, and kr2 for which the system exhibits highest second law efficiency. We expect our findings to contribute toward the development of optimized microfluidic devices fabricated from deformable elastic materials.

Investigation of thermal performance and entropy generation rate of evacuated tube collector solar air heater with inserted baffles and metal foam: A CFD approach

This study investigates the impact of metal foam embedding into evacuated tube collector solar air heaters with inserted baffle (IBMF-ETC-SAH) on the device thermal performance using a computational fluid dynamics (CFD) approach. A three-dimensional model is employed to analyze the system under steady-state conditions. The key finding of this study is that the utilization of metal foam leads to a substantial increase in thermal performance, with an average improvement of approximately 300 %. The investigation examines the influence of metal foam porosity and pores per inch (PPI) on the performance of the IBMF-ETC-SAH across Reynolds numbers ranging from 6000 to 18,000. The results indicate that increasing the PPI from 20 to 40 at a constant porosity of 0.93 has negligible effects on device performance. However, the pressure drop increases by 48 % for a PPI of 40 compared to a PPI of 20. Furthermore, reducing the porosity from 0.95 to 0.9 leads to a 7 % enhancement in thermal performance. The IBMF-ETC-SAH exhibits a significant decrease in the total global entropy generation rate (EGR) compared to the device without metal foam. The maximum value of total EGR for the IB-ETC-SAH case is 2.32 times higher than the corresponding values for the IBMF-ETC-SAH case.

Performance Comparison of Different Flat Plate Solar Collectors by Means of the Entropy Generation Rate Using Computational Fluid Dynamics

In this work, a numerical analysis of three different flat plate solar collectors was conducted using their entropy generation rates. Specifically, the Computational Fluid Dynamics (CFD) technique was used to compare the detailed performance of conventional and zigzag tube geometries of flat plate solar collectors (FPCs) in terms of their entropy generation rates. The effects of fluid viscosity, heat transfer, and heat loss of the flat plate solar collectors were considered for the local and global entropy generation rate analyses. Variations on the inlet volumetric flow rate of the FPCs from 1.0 to 9.0 L/min were simulated under the average solar radiation for one year in the state of Guanajuato, Mexico. The results illustrate and discuss the temperatures, pressures, and global entropy generation rates for volumetric flow variations. The velocity, temperature, and pressure distributions and the maps of the local entropy generation rates inside the collectors are presented and analyzed for the case with a flow rate of 3.0 L/min. These results demonstrate that the zigzag geometries achieved higher outlet temperatures and greater entropy generation rates than the conventional geometry for all the volumetric flow rates considered.

Loss evaluation and aerodynamics investigation of an aggressive intermediate turbine duct under off-design conditions

To maximize the performance of the intermediate turbine duct (ITD) under off-design conditions, the loss generation in a one-half stage turbine was quantified using entropy generation and the global entropy generation rate. The numerical results solved by the unsteady Reynolds-averaged Navier–Strokes equations were first verified. Then, the aerodynamic losses within the high-pressure turbine stage were evaluated by efficiency loss under nine operating conditions composed of three rotor speeds and three rotor tip gaps. Finally, the disturbance modes caused by the upstream wake were captured by the dynamic mode decomposition method. Different from the influence of tip gaps, losses of the high-pressure turbine and the ITD are due to the swirl angle display an opposite trend. Under the influence of the interaction between the tip leakage flow and the shroud flow of the ITD, the viscous dissipation and turbulent dissipation increase with a larger tip gap owing to the dominant counter-rotating vortices and secondary flow occurring near the upstream of the ITD shroud. In addition, a large gap seems to enlarge the swirl component of the inflow angle, especially over 80% passage height, leading to greater dissipation losses in these areas. At the ITD inlet, two pairs of counter-rotating vortices at the shroud and the hub are, respectively, captured by the axial velocity mode. Large tip gaps enhance endwall vortices near the shroud and make the up vortex pairs merge into one pair.

Influence of conjugate heat transfer on the minimization of entropy generation for forced convective flow through parallel plate channel filled with porous material

AbstractA fluid–solid conjugate heat transfer model is developed to analyze the characteristics of entropy generation for forced convective steady hydrodynamically fully developed laminar flow of a Newtonian fluid through a parallel plate channel filled with porous material by modulating the following parameters: substrate thickness, the ratio of thermal conductivity of wall to fluid, Biot number, the axial temperature gradient in the fluid, and Peclet number. The exteriors of both the walls are subjected to the thermal boundary conditions of the third kind. The mass and Brinkman momentum conservation equations in the fluidic domain and the coupled energy conservation in both the solid and fluidic domain are solved analytically using the local thermodynamic equilibrium model, so as to derive closed‐form expressions for the velocity in the fluid and the temperature both in the fluid and solid walls in terms of relevant parameters. Suitable combinations of influencing factors, namely the geometric parameters of the system, fluid, flow, and substrate properties are identified for which global entropy generation rate is minimized. The findings may be helpful in the design of thermal systems frequently used in diverse engineering applications having heat transfer in the solid wall being a crucial parameter.

Numerical investigation of the effect of corrugation profile on the hydrothermal characteristics and entropy generation behavior of laminar forced convection of non-Newtonian water/CMC-CuO nanofluid flow inside a wavy channel

The force convection flow of non-Newtonian water/Carboxy-Methyl Cellulose (CMC)-CuO Nanofluid (NF) in three wavy channels and a straight channel was numerically investigated. The two dimensional governing equations were solved in the computational domain using the FLUENT software. The influence of Re and nanoparticle concentration (φ) on the pressure drop (ΔP), Nu, and global thermal and frictional entropy generation rates (Ṡg,th and Ṡg,f) were studied for different Re and φ values. The results showed that the influence of Re on the Nu is much higher than that of φ. The increasing of the Re and decreasing of φ, results in increasing the Nu for four channels of the study. Besides, the sinusoidal channel has the highest Nu as compared to the other channels. Also, the highest pressure drop and total global entropy generation rate were obtained for the trapezoidal channel.

Entropy Study on the Enhanced Heat Transfer Mechanism of the Coupling of Detached and Spiral Vortex Fields in Spirally Corrugated Tubes

The present work numerically studied the enhanced heat transfer mechanism of the coupled fields of detached and spiral vortices in symmetrical and asymmetrical spirally corrugated tubes. The heat transfer and viscous dissipation were analyzed by evaluating the entropy combined with the flow patterns, and the global entropy generation rate was analyzed by considering the local Nusselt number and friction factor. The results indicated that, both the leeside and windward corrugation angles had obvious effects on the strength of detached vortex and spiral flow, and also the location of the vortex. The maximum values of the heat transfer entropy were located at the boundary layers, and the heat transfer entropy of the secondary flow region was more distinguished than in other parts of the main flow region. The maximum values of friction entropy generation were located at both the boundary layers and the core of detached vortex. The performance evaluation criterion (PEC) presents nearly the same values for the four cases, when the Reynolds number (Re) is less than 6,300. In addition, the PEC of symmetrical spirally corrugated tube with corrugation angle equal to 25° showed the best performance, when Re is greater than 6,300. To keep the PEC above 1, Re should not exceed 33,000.

Heat transfer and entropy generation of water–Fe3O4 nanofluid under magnetic field by Euler–Lagrange method

The Euler–Lagrange method is considered to simulate the water–Fe3O4 nanofluid inside a circular tube. The non-uniform magnetic field is employed to a part of the tube. The effects of Reynolds number, concentration and magnetic field are investigated. The obtained results show that by increasing the magnetic field strength, global frictional entropy generation rate enhances. Meanwhile, with the application of the magnetic field, the frictional entropy generation rate in the central region of the tube and the vicinity of the wall decreases and increases, respectively. Additionally, the Bejan number is approximately 1 near the outlet. Also, there is a non-uniform distribution for nanoparticles, and the concentration of nanoparticles in the tube center is higher than the wall adjacency. Moreover, the wall temperature of the tube decreases in the part where the magnetic field is applied. The use of nanoparticle leads to an increase in the convective heat transfer coefficient. The velocity of the nanofluid in the central part of the tube decreases with the application of the magnetic field. But, the flow velocity near the wall increases with increasing magnetic field strength.

Fluid-structure interaction analysis of entropy generation and mixed convection inside a cavity with flexible right wall and heated rotating cylinder

The current work concentrates on the transient entropy generation and mixed convection due to a rotating hot inner cylinder within a square cavity having a flexible side wall by using the finite element method and arbitrary Lagrangian-Eulerian formulation. Effects of various relevant parameters like Rayleigh number (104⩽Ra⩽107), angular rotational velocity (-1⩽Ω⩽1), dimensionless elasticity modulus (1012⩽E⩽1015) on the convective heat transfer characteristics and entropy generation rates are analyzed for dimensionless time 10-8⩽τ⩽3.5. It is observed that various complex shaped wall deformations are established depending on the non-dimensional elastic modulus of the flexible right wall and cylinder rotation direction. The local and average Nusselt numbers rise with Ra and secondary peaks in the local Nusselt number are established for lower values of Ra. The local heat transfer along the hot cylinder does not change for the case of clockwise rotation of the heated cylinder even if there is a wall deformation in the positive x-direction. The highest average heat transfer and global entropy generation rates are achieved for the case of counter-clockwise rotation of the circular cylinder and for lower values of the flexible wall deformation.

Numerical assessment into the hydrothermal and entropy generation characteristics of biological water-silver nano-fluid in a wavy walled microchannel heat sink

The objective of this numerical assessment is to examine the hydrothermal and irreversibility behaviour of a biologically synthesized water-silver nano-fluid in a wavy microchannel heat sink (MCHS). The green tea leaf extract is utilized to prepare silver nanoadditives. The impacts of nanoadditives volume fraction, Reynolds number, amplitude and wavelength of the channel on the convective heat transfer coefficient, CPU surface temperature, pumping power, as well as the thermal, frictional, and total irreversibilities are investigated. The results show that enhancing the Reynolds number and nanoadditives fraction intensifies the performance of heat sink by boosting the convective heat transfer coefficient of the working fluid which favorably reduces the CPU surface temperature and the rate of thermal and total irreversibilities and importantly leads to the temperature uniformity of the CPU surface. However, increase in Reynolds number adversely affects both the pumping power and the frictional irreversibility in the system. In addition, it is found that the nano-fluid always has a better cooling performance in comparison with the pure water. Moreover, it is reported that augmenting the wavelength results in an increase in the hydrothermal performance of nano-fluid and decrease in the global total entropy generation rate.

Second law analysis of a hybrid nanofluid in tubes equipped with double twisted tape inserts

Entropy generation rates for flow of a novel hybrid nanofluid containing graphene–platinum nanoparticles in tubes equipped with double twisted tapes, which can develop diverse swirling flows, are analyzed based on the second law of thermodynamics. The twisted tapes under study include double co-twisted tapes and double counter twisted tapes with various twisted ratios. Based on the obtained results, using the double counter twisted tapes is recommended since they demonstrate the lower entropy generation rate (i.e. smaller overall irreversibility) compared with the double co-twisted tapes. The double counter twisted tapes are able to create more intense swirl flows, disturb boundary layer and reduce total entropy generation rate more than double co-twisted tapes. Smaller irreversibility for the double counter twisted tapes is a pleasant outcome based on the second law of thermodynamics. Furthermore, using lower twisted ratios is suggested since although with the twisted ratio reduction, the frictional entropy generation augments, the thermal entropy generation reduces and due to dominance of thermal entropy generation, total entropy generation decreases. In the case of double counter twisted tape, the global thermal entropy generation rate reduces around 10% with decreasing the twisted ratio from 3.5 to 2.5 at concentration of 0.06%. Moreover, the results reveal that using the nanofluid instead of the base fluid decreases total entropy generation and consequently overall irreversibility. Indeed, adding the graphene–platinum nanoparticles, even in low concentrations, profoundly reduces the thermal entropy generation while the frictional entropy generation increases slightly. Thereby, in the case of double co-twisted tapes with twisted ratio of 3, increasing the concentration from 0% to 0.1% causes 14% decrement in the global thermal entropy generation.

Effect of line dipole magnetic field on entropy generation of Mn-Zn ferrite ferrofluid flowing through a minichannel using two-phase mixture model

A numerical investigation is carried out to assess the effect of magnetic field on the entropy generation characteristics of a magnetic nanofluid flowing between parallel plates using the two-phase mixture model. The magnetic field is generated by one or multiple line dipole(s). The influences of Reynolds number, magnetic field strength, number of dipoles and also their arrangement are examined on the thermal, frictional and total entropy generation rates both locally and globally. The application of a magnetic field leads to rotation of magnetic nanofluid and consequently, mixing in the flow and more uniform temperature distribution. The results show that augmenting the magnetic field strength from 0.01 Am to 0.1 Am, at Reynolds number of 500, results in a 2.93% increase and a 5.54% decrease in the global frictional and thermal entropy generation rates, respectively. Moreover, it is reported that increasing the Reynolds number from 500 to 2000, for a magnetic field with strength of 0.1 Am, causes an increase of 63.02% and a decrease of 50.45% in the global frictional and thermal entropy generation rates, respectively. In the case of one dipole, the results reveal that the position of the dipole has no effect on the global total entropy generation rate, but this is not true for two and three dipoles cases.

Effect of passive flow control on the aerodynamic performance, entropy generation and aeroacoustic noise of axial turbines for wave energy extractor

Wells turbine is the simplest type of an axial flow self-rectifying air turbine that can be used in conjunction with Oscillating Water Column (OWC) system in the extraction of ocean wave energy. It has been noticed that this turbine is subjected to early stall. As a consequence, several attempts for improving the energy extraction performance of Wells turbine within the stall regime have been investigated. One of these attempts was using an inclined slot as a passive flow control to obtain a delayed stall. In the following study, the impact of varying the angle for the slot on the performance of Wells turbine in the stall regime was investigated. Furthermore, the first law of thermodynamics and the entropy analysis has been used to examine the effect of the slot angle on the entropy generation features around the turbine blade. Moreover, Investigation of slot angle effect on the aerodynamics noise emission from Wells turbine airfoil during the normal operation and the stall regime is covered in this study. The blade of turbine with optimum angle of slot was investigated using the OWC based on actual data from the Egyptian Northern Coast. It was found that the optimum slot angle is 10° clockwise which results in 3% improvement in the torque coefficient before the stall and 15% after the stall as compared to the 0° slot. Otherwise, it gives a lower global entropy generation rate than the 0° slot by 4% before the stall and 3% after the stall. Furthermore, using airfoil of blade turbine with a slot resulted in a reduction of aeroacoustic noise by −21.2% at the stall regime under oscillating flow conditions.

Effect of variable thermal conductivity on entropy generation in a plate with internal energy generation

The current numerical investigation aims at analyzing the effect of variable thermal conductivity on local and global entropy generation rates in an energy generating plate dissipating heat by conjugate conduction-forced convection heat transfer. In order to fulfill this objective, the physical model of the plate dissipating heat into surrounding coolant is transformed into a mathematical model governing the temperature field in the plate as well as flow and thermal fields in the fluid. The resulting mathematical model, being a set of coupled and non linear partial differential equations, is solved by adopting stream function-vorticity formulation and by employing Alternating direction implicit scheme. Keeping Prandtl number of the fluid, temperature of the free stream coolant and maximum permissible plate temperature as fixed, numerical predictions are obtained for wide range of values of aspect ratio, conduction-convection parameter, energy generation parameter and flow Reynolds number. It is concluded that unrealistic constant thermal conductivity assumption leads to underestimation of entropy generation rates. It is also found that an increase in energy generation parameter results in significant increase in underestimation of global entropy generation rate.

Entropy generation analysis of a Solid Oxide Fuel Cell by Computational Fluid Dynamics: Influence of electrochemical model and its parameters

The aim of this paper is to evaluate numerically the effect of varying the electrochemical model and its parameters on the performance and entropy generation of a mono-block-layer build (MOLB) type geometry of a solid oxide fuel cell. Particularly, the influence of the exchange of current density, the electrical conductivity of the electrodes and the electrolyte has been studied and the prediction of the thermodynamic irreversibility by means of an entropy generation analysis is considered. The numerical analysis consider a 3-D CFD model that takes into account the mass transfer, heat transfer, species transport, and electrochemical reactions. Several numerical simulations were performed and each contribution to the local entropy generation rate was computed. The results show different trends of the current density, temperature, species, activation loss, ohmic loss, and concentration loss along the fuel cell. Also, the results show strong variations of the local and global entropy generation rates between the cases analyzed. It is possible to conclude that the fuel cell performance and the prediction of thermodynamic irreversibility can be significantly affected by the choice of the electrochemical models and its parameters, which must be carefully selected.

Effect of variable thermal conductivity on entropy generation in a plate with internal energy generation

The current numerical investigation aims at analyzing the effect of variable thermal conductivity on local and global entropy generation rates in an energy generating plate dissipating heat by conjugate conduction-forced convection heat transfer. In order to fulfill this objective, the physical model of the plate dissipating heat into surrounding coolant is transformed into a mathematical model governing the temperature field in the plate as well as flow and thermal fields in the fluid. The resulting mathematical model, being a set of coupled and non linear partial differential equations, is solved by adopting stream function-vorticity formulation and by employing Alternating direction implicit scheme. Keeping Prandtl number of the fluid, temperature of the free stream coolant and maximum permissible plate temperature as fixed, numerical predictions are obtained for wide range of values of aspect ratio, conduction-convection parameter, energy generation parameter and flow Reynolds number. It is concluded that unrealistic constant thermal conductivity assumption leads to underestimation of entropy generation rates. It is also found that an increase in energy generation parameter results in significant increase in underestimation of global entropy generation rate.

Towards the minimization of thermodynamic irreversibility in an electrically actuated microflow of a viscoelastic fluid under electrical double layer phenomenon

We discuss the entropy generation minimization for electro-osmotic flow of a viscoelastic fluid through a parallel plate microchannel under the combined influences of interfacial slip and conjugate transport of heat. We use in this study the simplified Phan-Thien–Tanner model to describe the rheological behavior of the viscoelastic fluid. Using Navier’s slip law and thermal boundary conditions of the third kind, we solve the transport equations analytically and evaluate the global entropy generation rate of the system. We examine the influential role of the following parameters on the entropy generation rate of the system, viz., the viscoelastic parameter (εDe2), Debye–Hückel parameter κ¯, channel wall thickness (δ), thermal conductivity of the wall (γ), Biot number (Bi), Peclet number (Pe), and axial temperature gradient (B). This investigation finally establishes the optimum values of the abovementioned parameters, leading to the minimum entropy generation of the system. We believe that results of this analysis could be helpful in optimizing the second-law performance of microscale thermal management devices, including the micro-heat exchangers, micro-reactors, and micro-heat pipes.

Numerical Heat Transfer and Entropy Analysis on Liquid Slip Flows Through Parallel-Plate Microchannels

In this study, two-dimensional (2D) numerical simulations of liquid slip flows in parallel-plate microchannels have been performed to obtain heat transfer characteristics and entropy generation rate under asymmetric heating conditions. Heat transfer analysis has been conducted along with second-law analysis through utilizing temperature-dependent thermophysical properties. The results indicate that temperature-dependent thermophysical properties have a positive effect on convective heat transfer and entropy generation. Nusselt numbers of the upper and lower plates and global entropy generation rates are significantly affected by slip parameter and heat flux ratio. It is shown that Nusselt number of the lower plate may have very large but finite values at a specific heat flux ratio. This finding resembles to analytical solutions, where singularities leading to an infinite Nusselt number exist.

Comparative analysis of different wave turbine designs based on conditions relevant to northern coast of Egypt

Wave energy has a great potential to solve the unrelenting energy deficiency in Egypt. The present work recommends Wells turbine as a suitable choice for the Egyptian coasts due to its simple and efficient operation under low input air flow. In addition, the possibility of extracting the wave energy from the Egyptian coasts was investigated using the oscillating water system based on real data from the site. To achieve this purpose, two-dimensional numerical models for Wells turbine airfoils, functioning under sinusoidal wave flow conditions, were built. Moreover, the running and starting characteristics under sinusoidal-flow conditions were investigated using a mathematical code. The results were discussed using the first law analysis, in addition to the second law analysis by using the entropy generation minimization method. It was found that the NACA0015 airfoil always gives a global entropy generation rate that is less than other airfoils by approximately −14%, −10.3% and −14.7% for the sinusoidal wave with time periods equal to 4, 6 and 8 s respectively. Moreover, the effects of blade profile, time period and solidity on the output power (kW) value were discussed.