Thermal Entropy Research Articles

Thermal enhancement and entropy investigation in dissipative ZnO-SAE50 under thermal radiation: a computational paradigm

Exploring heat transport mechanism and entropy generation in new heat transfer fluids generation are imperative and broadly applicable in many industrial and engineering domains. Therefore, the study of heat and entropy generation inside a rocket engine chamber filled with ZnO-SAE50 nanolubricants is designed, and the main version is attained by exercising similarity equations. After that, the numerical computation was performed under varying physical constraints and explored fascinating outcomes. The results revealed that the fluid motion drops inside the chamber by strengthening Re and ZnO fraction factors, respectively. Furthermore, the entropy generation for ZnO-SAE50 is very high than regular liquids, and this characteristic allows nanofluids for effective industrial and manufacturing purposes. Implementation of thermal radiations over the chamber is a prominent factor for enhancing the Nusselt number at both ends of the chamber. Finally, the reported study would be valuable for applied thermal engineering problems.

Entropy generation characteristics of phase change material in a variable wavy walled triplex tube latent heat storage unit for battery thermal management system

This study focuses on the thermal and frictional entropy generation characteristics of phase change material (PCM) in a triplex tube latent heat storage unit which is a promising technique for thermal management of lithium-ion batteries. Four different wavy structures for the triplex tube heat exchanger with variable waviness are considered, namely Case 1 to 4. Regarding the numerical analysis, the variations in mean temperature and liquid fraction of PCM are first revealed and presented where the crossflow heat transfer fluid in the system is considered water. Furthermore, thermal and frictional entropy generation rates in local and global means are separately discussed for both charging and discharging processes. The results showed that the difference in mean temperature and melting rates among the cases are more noticeable at the end of the process due to intensified natural convection, and Case 3 has 240 s (12.3%) shorter melting time compared to Case 1 and 4. Although solidification rates have similar trends in all cases, the solidification time is different, e.g., it is 110 s (5.2%) shorter in Case 4, compared to Case 1. The entropy generation trends are relatively similar during the solidification, while they are noticeably different in the melting process due to effect of liquid PCM flow.

Island, Page curve, and superradiance of rotating BTZ black holes

We study the Page curve and the information paradox for the BTZ black hole coupled to two thermal baths by applying the island paradigm. We prove that as the island locates outside the event horizon, the entanglement entropy of Hawking radiation for finite temperature black holes obeys the Page curve. However, for extremal rotating black holes, the Page time and the scrambling time become divergent. To avoid the ill-defined of the Page time, we consider the contribution of superradiance, which is a process that can extract rotational energy from black holes. The superradiance continues in a period much shorter than the Page time when the central charge $c$ is not too large. In this process, the Page time, the scrambling time and the black hole thermal entropy all decrease. Whenever the superradiance finishes, the black hole turn out to be a neutral black hole and the Hawking radiation dominates. The Page curve can then be well reproduced.

Investigating thermo-hydraulic behavior of pillow plate heat exchangers using entropy generation approach

Pillow plate heat exchanger (PPHE) is a modern type of plate heat exchanger composed of wavy shape parallel plates. Adjustment of the geometrical parameters has been broadly made to enhance the heat transfer performance of PPHEs. However, there has been limited attention to the thermo-hydraulic features and entropy generation of fluid flow through such heat exchangers. Addressing this gap could help further understanding of such heat exchangers. In the present study, Ansys CFX is used to simulate turbulent flow and heat transfer in inner pillow plate channels. The local entropy generation due to thermal effects and friction is calculated as a post processed task. The influence of the channel height, Prandtl number, diameter and arrangement of spot welds on the overall entropy generation of pillow plates are analyzed. The results indicate that by increasing the channel height, thermal entropy generation increases. Besides, increasing the spot weld diameter leads to a higher frictional loss in pillow plate channels, while the effect of spot weld size on thermal entropy generation is negligible. Also, the spot weld arrangement significantly affects the thermal and hydraulic characteristics of PPHEs. The results show that exergy analysis can be considered as a potential tool for investigating thermo-hydraulic behavior of PPHEs.

The Influence of Forced Convective Heat Transfer on Hybrid Nanofluid Flow in a Heat Exchanger with Elliptical Corrugated Tubes: Numerical Analyses and Optimization

The capabilities of nanofluids in boosting the heat transfer features of thermal, electrical and power electronic devices have widely been explored. The increasing need of different industries for heat exchangers with high efficiency and small dimensions has been considered by various researchers and is one of the focus topics of the present study. In the present study, forced convective heat transfer of an ethylene glycol/magnesium oxide-multiwalled carbon nanotube (EG/MgO-MWCNT) hybrid nanofluid (HNF) as single-phase flow in a heat exchanger (HE) with elliptical corrugated tubes is investigated. Three-dimensional multiphase governing equations are solved numerically using the control volume approach and a validated numerical model in good agreement with the literature. The range of Reynolds numbers (Re) 50 < Re < 1000 corresponds to laminar flow. Optimization is carried out by evaluation of various parameters to reach an optimal case with the maximum Nusselt number (Nu) and minimum pressure drop. The use of hybrid nanofluid results in a greater output temperature, a higher Nusselt number, and a bigger pressure drop, according to the findings. A similar pattern is obtained by increasing the volume fraction of nanoparticles. The results indicate that the power of the pump is increased when EG/MgO-MWCNT HNFs are employed. Furthermore, the thermal entropy generation reduces, and the frictional entropy generation increases with the volume fraction of nanoparticles and Re number. The results show that frictional and thermal entropy generations intersect by increasing the Re number, indicating that frictional entropy generation can overcome other effective parameters. This study concludes that the EG/MgO-MWCNT HNF with a volume fraction (VF) of 0.4% is proposed as the best-case scenario among all those considered.

Numerical study for entropy optimized radiative unsteady flow of Prandtl liquid

Entropy analysis for magnetohydrodynamic (MHD) unsteady flow of Prandtl fluid by a stretchable surface is addressed. Thermal transport with radiation, magnetic field and dissipation is taken. Physical behaviors of Soret and Dufour impacts are examined. Features of entropy have been deliberated. Mass transfer with first order chemical reaction is addressed. Dimensionless problems are simulated by finite difference method. Graphical illustrations show outcomes of fluid motion, thermal and mass transport and entropy. An intensification in Reynold number improves the velocity. Higher magnetic variable lead to enhance thermal field and entropy rate. Temperature enhancement is observed through radiation while reverse trend holds for Prandtl number. Higher Soret number rise the concentration while opposite holds for Schmidt number. Higher estimation of radiation lead to improve both entropy rate and thermal field.

Quantum extremal islands made easy. Part IV. Massive black holes on the brane

We study two-dimensional eternal black holes with non-zero mass, where each asymptotic boundary is in contact with a CFT on a circle, following the doubly holographic braneworld models constructed in [1–3]. We compute the Page curve of the black hole (or the bath CFTs), which amounts to finding different geodesics in the bulk BTZ geometry with a Randall-Sundrum brane falling into the black hole. We also explore the possibility of including an intrinsic JT gravity action on the brane. As expected, the generalized entropy rises linearly at early times. However, there is a transition to a late-time phase in which the entropy remains constant. The value of the late-time entropy depends on the size of the thermal baths. For a small size, it corresponds to the thermal entropy of the baths, while for large size, it corresponds to twice the horizon entropy of the black hole. The critical size and the Page time are proportional to ratio of the central charges of the conformal defect and the bath CFT.

Thermal design of microchannel heat sinks using a contour extraction based on topology optimization (CEBTO) method

The Topology Optimization (TO) has been proved to be an effective method for designing Microchannel Heat Sinks (MCHS). However, due to the relatively high computational cost and poor numerical stability, it is still challenging to design the thermo-fluid TO under high Reynolds number (Re). In this work, we propose a Contour Extraction Based on Topology Optimization (CEBTO) method to optimize the geometry of the fins based on a threshold velocity (Ut, proportional to the maximum velocity Umax) and improve the hydrothermal performance of MCHS under high Re. The 3D conjugate heat transfer numerical models are established to study the fluid flow and heat transfer characteristics of the MCHS generated by the conventional TO method and the CEBTO method. The results showed that due to the elimination of the area of stagnation zones under high Re, the CEBTO-generated heat sinks have a better interaction of fluid with the walls and more uniform velocity distribution compared to the TO-generated ones. However, the elimination of the stagnation zones area also leads to the decrease of channel width, which increases the friction loss in CEBTO-generated heat sinks. Nevertheless, the CEBTO-generated heat sinks have better overall performance compared to the TO designs owing to the smaller total entropy generation rate. For example, the CEBTO-I (Ut=28%Umax) exhibits the best overall performance with 10.1% increment of flow entropy generation rate and 7.1% reduction of thermal entropy generation rate as compared to those of TO-I (i.e., a TO-generated heat sink with alveoli-like shape), leading to the augment entropy generation number of 0.92 (Re = 1715). Finally, the CEBTO-I (Ut=28%Umax) was manufactured by micro milling technology and its hydrothermal performance was investigated experimentally to validate the accuracy of numerical simulation. The pressure drop and temperature of the CEBTO-generated heat sink in the experiment agree well with the numerical simulation.

Thermo-hydraulic and entropy generation investigation of nano-encapsulated phase change material (NEPCM) slurry in hybrid wavy microchannel

PurposeThis study aims to capture the heat transfer and entropy generation characteristics of temperature-dependent nano-encapsulated phase change material (NEPCM) slurry in a hybrid wavy microchannel. In addition, the effect of substrate material combined with NEPCM slurry on conjugate heat transfer condition is captured for different microchannel heat sinks.Design/methodology/approachA novel “hybrid wavy microchannel” is proposed to enhance the overall heat transfer and reduce the pressure drop by combining wavy and raccoon geometry. NEPCM–water slurry is implied in the hybrid wavy, conventional wavy and raccoon microchannel. A user-defined function (UDF) is used to observe the effect of phase-change of paraffin material in thermophysical properties of NEPCM–water nanofluid. All three (hybrid, wavy, raccoon) microchannels are engraved on a rectangular substrate of 1.8 mm width (ωs) and 30 mm length (L), respectively. For hybrid, wavy and raccoon microchannel, waviness (γ) of 0.067 is selected for the investigation.FindingsThe result shows that NEPCM particle presence reduces the fluid domain temperature. The thermal performance of proposed Heat sink 2 is found better than the Heat sink 1. The effect of the geometrical modification, wall thermal conductivity, different volumetric concentrations of nanoparticles (ϕ ∼ 1 – 5%) and Reynolds number (Re ∼ 100 – 500) on thermodynamic irreversibility is also observed. Additionally, the effect of thermal and frictional entropy generation is reduced with a combination of NEPCM slurry and higher conductive material for all heat sinks.Practical implicationsA combination of NEPCM slurry with laminar flow microchannel cooling system emerged as a better alternative over other cooling techniques for higher power density devices such as microprocessors, electronic radar systems, aerospace applications, semiconductors, power electronics in modern electronic vehicles, high power lasers, etc.Originality/valueThe phase-change process of the NEPCM slurry is tracked under conjugate heat transfer in a hybrid wavy microchannel. Furthermore, the phase-change process of NEPCM slurry is captured with different heat sink materials (SS316, silicon and copper) under conjugate heat transfer situation for different heat sinks and concentrations (ϕ ∼ 1–5) of NEPCM.

Employing a novel crimped-spiral rib inside a triple-tube heat exchanger working with a nanofluid for solar thermal applications: Irreversibility characteristics

This research attempts to study the performance of employing an innovative crimped-spiral rib within a triple-pipe heat exchanger (TPHE) operated with an alumina–water nanofluid regarding the second law viewpoint. The numerical analyses are carried out using the two-phase mixture model, and turbulence modeling is done via the Reynolds Stress Model (RSM). The parameters consist of the rib height ratio (0.23, 0.38, and 0.53), crimped intensity (2, 4, 6), rib pitch ratio (0.1, 0.2, and 0.3), and the nanofluid volume fraction (1, 2, and 3%). The nanofluid is the hot fluid flowing inside the intermediate tube, while the two other fluids are water. This paper underlines the lowest energy degradation to design a more efficient TPHE. The outcomes indicate that the idea of utilizing crimped-spiral rib is effective such that the nanofluid’s thermal entropy generation reduces about 22.92% using rib with higher crimped intensity because of the stronger swirl stream. The total entropy generation and total exergy destruction decline by the volume fraction increment, rib pitch ratio decrement, and by an increment in either crimped intensity or rib height ratio. The second law efficiency is found to be high for all the cases such that it is greater than 0.6.

Effect of a unique winglet twisted tape insert on thermal and hydraulic properties of tubular heat exchanger

ABSTRACT The present experimental investigation incorporates the winglet twisted tapes as an insert to determine the thermal and hydraulic performance of a heat exchanger tube. The purpose of connecting the winglet to the twisted tape is to improve the heat transmission while minimizing the pressure loss by not disrupting the flow at the core portion. The thermal performance of a heat exchanger with turbulent promoters is assessed by varying geometric factors, such as the size of the promoters, Twist ratio (), Winglet pitch ratio (), and Winglet height ratio (). Experiments were carried out for Reynolds numbers (Re) ranging from 3,000 to 21,000. For a set of geometric parameters, the greatest thermohydraulic performance achieved was 1.58. Regression analysis is used to build a statistical association for Nusselt number and friction factor and the anticipated data obtained is of acceptable accuracy. The entropy generation is expressed in the form of entropy generation number and the Bejan number to fetch the dominance of thermal or friction entropy using winglet twisted tape.

Study on Heat Transfer Process and Fresh Water Output Performance of Phase Change Heat Storage Dehumidifier

In the humidification-dehumidification solar desalination process, using phase change materials to recover water vapor condensation latent heat in the dehumidification can improve heat utilization and water production performances. When sodium thiosulfate pentahydrate and paraffin were used as phase change materials respectively in the phase change heat storage dehumidifier, by means of numerical simulation and experiment, the heat transfer process, entropy generation, and water production performances of heat pipes and copper wire meshes coupled phase change materials in the dehumidifier were studied. The results showed that sodium thiosulfate pentahydrate has stronger heat transfer ability, higher thermal entropy generation, and heat storage capacity than paraffin; adding copper wire meshes into the phase change material can accelerate heat transfer and shorten the time required for monitoring points to reach phase change temperature; increasing the wet air temperature at inlet of phase change heat storage dehumidifier, using copper wire meshes in the phase change material, increasing the diameter of copper wire mesh, and using a passive basin desalinator for secondary water production can improve water production performances. In brief, the use of sodium thiosulfate pentahydrate and copper wire meshes in the phase change heat storage dehumidifier leads to better heat storage and heat transfer effects.

Entropy generation in Sutterby nanomaterials flow due to rotating disk with radiation and magnetic effects

Entropy generation is a novel potential in various thermodynamic processes and presents dynamic applications in thermal polymer processing optimization. The significance of entropy generation is observed in heat exchangers, combustion, turbine systems, thermal systems, porous media, nuclear reactions etc. In view of such thermal applications, the prime objective of present analysis is to scrutinize the entropy optimized hydromagnetic flow of Sutterby nanofluid due to a stretchable rotating disk. Thermal radiation and heat source effects are considered. Effects of Brownian movement and thermophoresis diffusion are considered. Physical description of entropy generation is also addressed. Adopting procedure of transformations the non-linear PDEs are converted to ODEs. The obtained system is solved by ND-solve code in Mathematica. Influence of different parameters involved in velocity, temperature, concentration and entropy generation is discussed via graphs. Skin friction coefficient, Nusselt number and Sherwood number are discussed through Tables. Larger magnetic variable has decaying effect on velocity. An amplification in temperature distribution and entropy rate is observed for radiation variable. Larger approximation of radiation variable leads to improved thermal field and entropy rate. An opposite effect is noticed for temperature against Prandtl and heat generation variable. Variation in thermal ratio variable improves the entropy rate and temperature. Higher stretching parameter leads to produce more drag force at surface of disk. The main results are listed at the end.

Effect of Cross-Sectional Geometry on Hydrothermal Behavior of Microchannel Heat Sink

Abstract The aim of this paper is to numerically analyze the hydrothermal behavior of different cross-sectional geometries of microchannel heat sinks (MCHSs) and conduct a comparative analysis of traditional and non-traditional designs using ANSYS Fluent. It is expected that the proposed design discussed in this paper will improve the performance of MCHSs by maximizing the cooling capability and minimizing the thermal resistance and entropy generation rate, thus leading to better energy efficiency. The channel designs include a rectangular microchannel (RMC), a circular microchannel (CMC), an elliptical microchannel (EMC), a trapezoidal microchannel (TMC), a hexagonal microchannel (HMC), and a new microchannel (NMC) which has a plus-like shape. The discussed geometry of the NMC is designed in such a way that it maximizes the cross-sectional area and the wetted perimeter of the channel, keeping the hydraulic diameter constant ( D h = 412 {D_{h}}=412 µm). The performance of various channels is compared on the basis of pressure drop, wall temperature, thermal enhancement factor, thermal resistance, thermal transport efficiency, and entropy generation rates. It has been observed that the NMC is capable of cooling effectively and it can achieve a minimum wall temperature of 305 K, thus offering the lowest thermal resistance ( R th {R_{\mathrm{th}}} ), irreversible heat loss, and entropy generation rate. Moreover, the NMC has achieved the highest value of the thermal enhancement factor, i. e., 1.13, at Re = 1 , 000 \mathrm{Re}=1,000 . Similarly, it has the highest thermal transport efficiency of almost 97 % at Re = 1 , 000 \mathrm{Re}=1,000 , followed by the TMC and the RMC. Overall, the NMC has achieved the best performance in all aspects, followed by the RMC and TMC. The performance of the EMC, the CMC, and the HMC was found to be the worst in this study.

Entropy generation of nanofluid flow in hexagonal microchannel

A new model for entropy generation number in hexagonal microchannel is developed going from the equation proposed by Bejan. The effect of Cu, CuO, Al2O3 and TiO2 mixed with pure water inside the microchannel is evaluated via entropy generation analysis. To show the contribution of entropy heat-transfer generation, Bejan number is also investigated. The effects of heat flux in the range of [2000–12000] W/m2, and volume fraction and nanoparticles size, respectively, on entropy generation numbers and Bejan number are all evaluated. The results demonstrate that increasing heat flux increases both thermal entropy generation and Bejan number but decreases the frictional entropy production. The results also show that thermal and friction entropy generation numbers exhibit an inverse variation trend when varying volume percentage and nanoparticle diameter.

Analysis of Entropy Generation on Magnetohydrodynamic Flow with Mixed Convection through Porous Media

Various industrial operations involve frequent heating and cooling of electrical systems. In such circumstances, the development of relevant thermal devices is of extreme importance. During the development of thermal devices, the second law of thermodynamics plays an important role by means of entropy generation. Entropy generation should be reduced significantly for the efficient performance of the devices. The current paper reports an analytical study on micropolar fluid with entropy generation over a stretching surface. The influence of various physical parameters on velocity profile, microrotation profile, and temperature profile is investigated graphically. The impact of thermal radiation, porous medium, magnetic field, and viscous dissipation are also analyzed. Moreover, entropy generation and Bejan number are also illustrated graphically. Furthermore, the governing equations are solved by using HAM and code in MATHEMATICA software. It is concluded from this study that velocity and micro-rotation profile are reduced for higher values of magnetic and vortex viscosity parameter, respectively. For larger values of Eckert number and thermal radiation parameters, Bejan number and entropy generation are increased, respectively. These findings are useful in petroleum industries and engineering designs.

Effect of NP shapes on Fe3O4 – Ag/kerosene and Fe3O4 – Ag/water hybrid nanofluid flow in suction/injection process with nonlinear-thermal-radiation and slip condition; Hamilton and Crosser's model

This study examines the flow and thermal properties of water and kerosene hybrid nanofluids in a horizontal microchannel. The effects of nonlinear thermal radiation, heat source coefficient, convective and slip boundary conditions on nanofluids in a microchannel have been studied. The impact of different nanoparticle shape factors on the temperature of nanofluids has also been analysed. The governing partial differential equations have been transformed in to ordinary differential equations using dimensionless terms. In the next step, the corresponding ordinary differential equations have been cracked using the RKF45 numerical approach, and the particles shape factor on dimensionless velocity and energy fields for hybrid nanofluid then reported. The results revealed that the spherical and lamina shape factors for both water and kerosene hybrid nanofluid differ the most, the water-based hybrid nanofluid has a higher velocity than the kerosene-based hybrid nanofluid, the thermal energy of both water and kerosene-based hybrid nanofluid was higher in the lower channel plate than the upper channel plate, flow rate falls near the lower channel wall and increases towards the upper channel wall as Reynolds number increases. Thermal energy and entropy generation decrease as the temperature parameter rises.

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study, forced flow and heat transfer of water/NDG (Nitrogen-doped graphene) nanofluid in nanoparticles mass fractions ( φ ) of 0, 2% and 4% at Reynolds numbers ( Re ) of 10, 50, 100 and 150 are simulated in steady states. Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection. Temperature of inlet fluid equals with T c = 293 K and hot source of microchannel is under the influence of oscillating heat flux. Also, in this research, the effect of the variations of attack angle of triangular rib (15°, 30°, 45° and 60°) on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L / H = 28 and nanofluid jet injection is investigated. Obtained results indicate that the increase of Reynolds number, nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop. By increasing fluid viscosity, momentum depreciation of fluid in collusion with microchannel surfaces enhances. Also, the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient. At low Reynolds numbers, due to slow motion of fluid, variations of attack angle of rib, especially in angles of 30°, 45° and 60° are almost similar. By increasing fluid velocity, the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently. In general, by using heat transfer enhancement methods in studied geometry, heat transfer increases almost 25 %.

Micropolar nanofluid thermal free convection and entropy generation through an inclined I-shaped enclosure with two hot cylinders

A numerical study is conducted to evaluate the flow, heat exchange and irreversibilities of micro-polar Al2O3-water nanofluid during the thermal buoyancy convection engendered by two heated cylinders inside an inclined I-shaped enclosure having cold top and bottom walls and adiabatic sidewalls. A numerical approach based on the finite element method was used to solve the equations that govern the phenomenon. For various scenarios for the position of the active cylinders and inclination angle of the system, the macro-flow and micro-rotations structures, temperature fields, the origin of entropy production, and heat exchange rates were determined for varied values of the control parameters, namely: Rayleigh (Ra), vortex viscosity parameter (K), geometric aspect ratio (AR), enclosure inclination angle (χ), and nanoparticles' concentration (φ). All of these variables were discovered to have significant influences on the strength of macro-and micro-nanofluid flows, as well as the entropy production and the rate of heat transfer. The rate of heat exchange escalates as the Ra and AR parameters rise but decreases when the K parameter rises.

Holographic observables at large d

We study holographically non-local observables in field theories at finite temperature and in the large $d$ limit. These include the Wilson loop, the entanglement entropy, as well as an extension to various dual extremal surfaces of arbitrary codimension. The large $d$ limit creates a localized potential in the near horizon regime resulting in a simplification of the analysis for the non-local observables, while at the same time retaining their qualitative physical properties. Moreover, we study the monotonicity of the coefficient $\alpha$ of the entanglement's area term, the so called area theorem. We find that the difference between the UV and IR of the $\alpha$-values, normalized with the thermal entropy, converges at large $d$ to a constant value which is obtained analytically. Therefore, the large $d$ limit may be used as tool for the study and (in)validation of the renormalization group monotonicity theorems. All the expectation values of the observables under study show rapid convergence to certain values as $d$ increases. The extrapolation of the large $d$ limit to low and intermediate dimensions shows good quantitative agreement with the numerical analysis of the observables.