Thermal Irreversibilities Research Articles

Redesigning heat exchanger channels: A local entropy generation approach

This paper proposes a redesign technique for heat exchanger channels based on a Local Entropy Generation (LEG) analysis and numerical simulations. The approach first divides the computational domain into zones and then identifies the zones responsible for increasing irreversibilities due to viscous and heat transfer effects. By evaluating different geometric features and flow conditions, the wall shape is modified (zone by zone) with the configurations that offer the lowest local entropy generation rates. The redesigned channels demonstrate that sudden changes in geometry along the streamwise direction reduce the size of regions with high thermal entropy generation and promote the creation of vortices, thereby reducing the magnitude of thermal irreversibilities. Smooth wall shape transitions might not be desirable when the goal is to improve heat transfer. The redesign process enhances the thermal performance of the system without significantly increasing power demand. The proposed methodology is evaluated using three wavy channels with a non-uniform amplitude-to-wavelength ratio (β1,β2,andβ3) since they allow for better management of wall geometry. As demonstrated for case β1, the LEG analysis enables the construction of new channels with a thermal performance 3.9 % superior to those with a uniform amplitude-to-wavelength ratio of α = 0.16 (best-case scenario), but with approximately 20 % less pumping power. While this study employs wavy channels as a case study, the methodology is broadly applicable to various heat exchanger configurations, including trapezoidal, triangular, and zigzag channels, making it a versatile tool for improving thermodynamic performance across different designs.

Entropy generation analysis and thermal synergy efficiency in the T-shaped micro-kenics

In this work, three different twist angles of a micro helical insert in a T-shaped are studied numerically in order to evaluate the laminar steady flow behavior of Newtonian fluid in chaotic geometry. In the geometries under consideration, thermal mixing behavior is carried out using fluids having two distinct input temperatures. Under the influence of chaotic advection and low rates of Reynolds number, the second law of thermodynamics is controlled in terms of the entropy generation caused by hydrodynamic and thermal processes. The governing equations are numerically solved using the CFD Fluent code. Thus, the micromixer's configuration demonstrated a very significant improvement in mixing degree while minimizing friction and thermal irreversibilities. The synergy coefficient, which depicts the link between velocity and heat transfer in angle form, is analyzed and the results are provided.

Thermal and Viscous Irreversibilities Analysis in a Liquid Metal Phase Change Counter Flow Heat Exchanger

Sodium, a liquid metal, is utilized in a heat exchanger system coupled to a nuclear reactor to produce hydrogen. The subcooled liquid metal enters a counterflow heat exchanger, undergoes a phase change, and exits as superheated vapor. The sodium heating process occurs through heat exchange with superheated helium vapor in three phases: subcooled liquid, saturated vapor, and superheated vapor. This article analyzes the thermal and hydraulic performance of the three stages of the heat exchanger through thermal and viscous irreversibilities using analytical simulation. The solution obtained is based on applying the thermal efficiency method and the second law of thermodynamics. The thermodynamic Bejan number, the relationship between thermal irreversibility and total irreversibility, allows a cost-benefit analysis when determining thermal performance and viscous dissipation. The essential physical quantities used in the analysis are the lengths and internal diameters of the three segments of the heat exchanger. The results are analyzed and compared with previous analytical work published in the literature. Numerical and graphical results are obtained for temperature profiles, thermal effectiveness, heat transfer rate, thermal irreversibilities, pressure drops, viscous irreversibilities, entropy generation rate, and Bejan numbers. It is demonstrated that the cost-benefit is highly advantageous when the diameter used for the internal tubes of the three segments is equal to the diameter of the saturated sodium vapor section, corresponding to ¼ of the speed of sound in the tube.

Predication of entropy generation rate in a concentrating photovoltaic thermal system with twisted tube turbulator using Boosted regression tree algorithm

Efficient energy conversion and utilization remain paramount in addressing the growing energy demand and environmental concerns. Concentrating photovoltaic thermal (CPVT) systems have emerged as promising solutions by integrating photovoltaic (PV) cells with thermal components for simultaneous electricity and heat generation. In this paper, we propose the application of the Boosted Regression Tree (BRT) algorithm to predict the entropy generation rate in a CPVT system equipped with a perforated twisted tube turbulator. Brief introduction of numerical analysis of local and global rates of frictional (S˙fr) and thermal (S˙th) irreversibilities in a CPVT system equipped with a perforated twisted tube turbulator. The results approve the efficacy of the BRT algorithm in predicting the entropy generation rate. Through comprehensive simulations and data analysis, we establish a predictive model that considers factors such as solar irradiance, fluid flow rate, tube geometry, and turbulator characteristics. The BRT model exhibits remarkable accuracy in capturing the nuanced interplay of these factors, enabling reliable estimations of entropy generation rate.

Thermal and viscous irreversibilities in the heat exchanger of individually finned heat pipes using freon R404A as the working fluid

This work aims to apply a theoretical procedure to determine the performance of the heat exchanger of individually finned heat pipes used in an air conditioning system. The relevant physical quantities are defined and specified locally in the evaporator and condenser sections. The results obtained in the sections are associated with the theoretical determination of the global performance of the heat exchanger. Global theoretical results are compared with global experimental results. Thermal effectiveness, heat transfer rate, pressure drop, thermal and viscous irreversibilities, and thermodynamic Bejan number are determined at the evaporator, condenser, and heat exchanger. The relevant variables used to determine the results are the number of fins per heat pipe and rows of heat pipes. The theoretical-experimental comparison demonstrates that the localized model applied in the analysis is consistent and can be used as a design and comprehensive analysis tool for finessed heat exchangers. The performance of the heat exchanger demonstrated exceptionalness when comparing irreversibilities through the Bejan number, indicating a favorable cost-benefit ratio for the fins less than 30 and the number of heat pipes equal to 49. Bejan’s thermodynamic number, which uses results related to thermal and viscous irreversibilities, demonstrated that one should look for the relationship between thermal irreversibility versus total irreversibility and that fin numbers between 10 and 20 for heat pipes equal to 49 provide a better cost-benefit ratio. The absolute percentage errors obtained between theoretical and experimental values, for an experimental number of fins equal to 30, for the overall heat transfer rate and overall thermal effectiveness range from 2.0% to 42.1%.

Analysis of spiral plate heat exchanger used to cool vegetable oil with nanofluid consisting of water and non-spherical boehmite alumina nanoparticles

The objective is to use dimensionless analysis through the thermal efficiency method to determine the thermohydraulic performance of a spiral plate heat exchanger (SPHE) used to cool sunflower oil. The coolant consists of water as a base fluid and non-spherical Boehmite Alumina nanoparticles with a defined volume fraction. The concept of thermal efficiency for heat exchangers is used to determine the main quantities used in the analysis. Graphical results are presented for the number of thermal units (NTU), thermal efficiency, thermal effectiveness, hot fluid outlet temperature, thermal and viscous irreversibilities, and Bejan number. The analyzed heat exchanger provides excellent thermal performance for refrigerants consisting of water and non-spherical nanoparticles in platelets or cylindrical, with a volume fraction equal to 12%. Viscous dissipation significantly increases concerning the dissipation associated with pure water, but the cost-benefit is within reason for the proposed objective, within the flow rate under analysis.

Theoretical analysis using thermal efficiency concept in straight micro channel printed circuit heat exchanger

The objective is to analyze the thermal and hydraulic performance in a Micro Channel Straight Printed Circuit Heat Exchanger. Counterflow and parallel flow configurations were analyzed for water cooling using ethylene glycol-based fluid and platelet-shaped non-spherical Boehmite alumina nanoparticles. The work presents results from applying a dimensionless theory that uses the concepts of thermal efficiency of heat exchangers and quantities associated with the second law of thermodynamics. Thermal efficiency, thermal effectiveness, thermal and viscous irreversibilities, thermodynamic Bejan number, and outlet water temperatures are presented in graph form. The data obtained allow us to conclude that the heat exchanger can work in a range of water and refrigerant flow rates below the design parameters. With the inclusion of nanoparticles with a volume fraction equal to 5.0%, the flow rates of the refrigerant fluid can be significantly reduced. The analysis performed shows that the use of nanoparticles improves the operational cost-benefit of the heat exchanger with a significant reduction in the hot water outlet temperature.

The entropy generation analysis of a PVT solar collector with internally needle finned serpentine absorber tube

The entropy generation in two different PVT absorbers with and without needle fins (WNF and WONF) was calculated by performing a 3 D numerical analysis. The effect of Re (500–2000) and nanoparticle concentration, φ (0–1%) on thermal and viscous irreversibilities (S˙thand S˙fr) were examined and two configurations of the study were compared. The outputs showed that, Re escalation within the studied range of φ decreases S˙th (or increases S˙fr) almost 8.74% (or 93.88%) and 10.53% (or 94.80%) for WONF and WNF configurations, respectively. In addition, the increase in φ from 0% to 1% reduces S˙th and S˙fr by 26% and 14%, respectively, for both configurations at different Re numbers. Moreover, the use of needle fins escalates S˙fr by 49.14%-54.08% and decreases S˙th by 63.03%-65.40% over the WONF at Re range of 500–2000. The more intensification of irreversibilities are observed near the channel turns and near the channel inlet.

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.

Numerical Analysis of Entropy Generation in a Double Stage Triangular Solar Still Using CNT-Nanofluid under Double-Diffusive Natural Convection

The analysis of entropy generation provides valuable information for the design and optimization of thermal systems. Solar stills are used for water desalination and purification. Using renewable energies, they provide a sustainable solution for drinking water supply in remote areas and off-grid situations. This work focuses on the 3D numerical study of entropy generation in a two-stage solar still subjected to the natural double diffusion convection phenomenon in the presence of CNT nanoparticles. The effects of Rayleigh number, buoyancy ratio, and nanofluid concentration on thermal, solutal, and viscous irreversibilities and flow structure were studied. The results show that increasing the buoyancy ratio leads to an increase in thermal and solutal entropy generation. The results of this study also show that total entropy is minimal for positive volume force ratios, N, at a nanoparticle volume fraction of around 3%, and for negative N ratios, at a volume fraction of around 4%.

The entropy generation analysis of a pin–fin heatsink with Fe3O4 ferrofluid coolant and considering four different pin–fin shapes (circular, square, rhumbas, and triangular) in the presence of the magnetic field

The entropy generation analysis was performed to determine the viscose and thermal irreversibilities (Ṡfr and Ṡth) inside a pin–fin heatsink under the absence (WO) and presence (W) of the magnetic field considering Res of 200, 300, 400, and 500, four different pin–fin shapes, and Fe3O4 ferrofluid coolant. The results demonstrated that Ṡfr and Ṡth escalates and diminishes, respectively, by increasing Re or by applying the magnetic field effects. For instance, the increase in Re from 200 to 500 escalates Ṡfr by 86% and 80% for 4 configurations and under WO and W scenarios of the magnetic field. Besides, the highest Ṡfr (and lowest Ṡth) was observed for the triangular configuration, which is nearly 12.79% (and 8.36%) and 13.89% (and 11.18%) higher than that for the circular configurations with the lowest Ṡfr (and highest Ṡth) among four configurations at Re = 200. Furthermore, the magnetic field escalates Ṡfr by 41.88%, 37.19%, 37.49%, and 34.06% over the base case without the magnetic field for circular, square, rhumbas, and triangular heatsinks, respectively, at Re = 200. These values diminish to 10.50%, 11.46%, 10.51%, and 7.77% as Re increases from 200 to 500. In addition, Ṡth decreases by 21.98% (or 7.39%), 19.18% (or 6.17%), 20.95% (or 7.15%), and 14.00% (or 3.46%) under the W scenario against the WO scenario for circular, square, rhumbas, and triangular configurations, respectively, at Re of 200 (or 500).

Field synergy and thermodynamic evaluation of a mini-duct with several protrusions utilizing the cross-flow method: A numerical exercise

The field synergy and entropy generation analysis of turbulent flow in a mini rectangular duct featuring surface protrusions has been numerically explored in a finite volume approach. The governing differential is solved iteratively using adequate numerical boundary conditions and a second-order upwind technique. In the post-processing, entropy generation, and irreversibilities are measured. The duct Reynolds number (ReDh, duct) has been varied from 17,831 to 53,492 at a fixed nozzle Reynolds number (ReDh, nz) of 5104. The effects of the duct Reynolds number on the thermal and frictional irreversibility have been quantified. Thermal, frictional, and total irreversibilities have been reported to rise with ReDh, duct. Furthermore, it is discovered that thermal irreversibility dominates over frictional irreversibility. It has been found that conducting triangular protrusions creates more entropy than conducting rectangular protrusions. Filed synergy analysis has also been carried out by integrating the energy equation. Heat transfer rate has been seen to increase with synergy angle (α). Triangular protrusions have a greater field synergy angle (α) than rectangular protrusions.

Entropy formation analysis for magnetized UCM fluid over an exponentially stretching surface with PST and PSHF wall conditions

<abstract> <p>This article aims to demonstrate the formation of entropy due to variable thermal conductivity, radiation, and fluid friction irreversibilities for a three-dimensional upper-convected Maxwell (UCM) fluid. The fluid motion occurs as a result of exponential stretching sheets. Separate discussions are held regarding the entropy generation related to the prescribed surface temperature and prescribed surface heat flux. Additionally, the heat transport mechanism is examined in the presence of thermal radiation. The governing physical situation is first modeled and then solved by using the homotopy analysis method to acquire the solution. The physical importance of relevant flow parameters is shown graphically and in tabular form. It is noted that the entropy generated is reduced with an increase in the thermal radiation parameter. Streamline patterns are also drawn for two- and three-dimensional UCM fluid models. Finally, the current analytical solution is found to be in agreement with the solutions in the literature.</p> </abstract>

Thermo-Hydraulic Optimization of Shell and Externally Finned Tubes Heat Exchanger by the Thermal Efficiency Method and Second Law of Thermodynamics

The application aims to determine the thermal and hydraulic performance of externally finned, counter-flow, Shell, and Tube Heat Exchangers (STHE). The application refers to the cooling of machine oil flowing in the annular region, including non-spherical cylindrical nanoparticles of Boehmite Alumina. The oil inlet temperature is equal to 80 °C. Sea water is a coolant with an inlet temperature of 20°C. The main parameter in the optimization process is the number of finned tubes used for oil cooling. Another optimization factor is the number of heat exchanger units connected by hairpin. The number of fins per tube is fixed, equal to 34. The oil flow is set and equal to 4.0 kg/s. The inlet water flow varies, with a maximum flow of 5.0 kg/s. The quantities determined for analysis are: the thermal effectiveness, the actual and maximum heat transfer rates, the pressure drops caused in the tubes and by the flow in the annular region and fin system, the thermal and viscous irreversibilities, and the fluid outlet temperatures, and the thermodynamic Bejan number. It was determined that increasing the number of finned tubes from two to six tubes leads to better thermal and hydrodynamic performance. That is, it produces a favorable cost-benefit, to the detriment of the high viscous dissipation caused by the oil in the annular region. As an object of analysis, the inclusion of nanoparticles showed a significant improvement in thermal performance and an increase in viscous dissipation, with a slight decline in the Bejan number.

Thermodynamic analysis in laminar falling film evaporator

• 2D analysis of heat transfer in a laminar falling film evaporator. • The sources of irreversibilities were identified, located and quantified. • The local entropy generation varies with the operating and design conditions. • The overall entropy generation is impacted by the local one. • The thermal and evaporation efficiencies also impact the overall entropy generation. An evaporative laminar falling film flowing by gravity on a vertical plate heated by a heat transfer fluid is studied to determine the local temperature, entropy generation and evaporated mass flow, assuming constant saturation pressure at the free interface, in co-current and counter-current configurations. The heat transfer fluid and film energy equations are solved simultaneously through a 2D model using an implicit finite difference scheme. Local and global analysis were performed in order to study the impact of parameters such as the heat transfer fluid and film Reynolds numbers and inlet film temperature on entropy generation (thermal and viscous irreversibilities) and on thermal and evaporation efficiencies. Results show that the increase of the overall thermal entropy generation leads to the increase of the evaporated mass flow rate. Both increase with the increase of the heat transfer fluid Reynolds number and decreases with the increase of the film Reynolds number for both configurations. The inlet film temperature – overheated or subcooled inlet fluid flow - also impacts the evaporation process. The influence of the Reynolds number and temperature difference on the heat transfer coefficients of both fluids and for both configurations is also discussed.

The entropy generation analysis of the influence of using fins with tip clearance on the thermal management of the batteries with phase change material: Application a new gradient-based ensemble machine learning approach

The present paper deals with 3D numerical analysis of a battery thermal management system (TMS) including the Phase Change Material (PCM). The TSM comprises three annular fins located around the battery considering the tip clearance (TC) space between the fin tips and the alumina enclosure. The calculations were performed for four cases with different TCs (1.5 mm, 1 mm, 0.5 mm, and 0 mm). The entropy generation analysis was performed to determine the locations of the geometry with highest frictional and thermal irreversibilities. The results showed that the application of TC leads to improve the PCM free convection and thereby enhance the heat transfer rate. So that there is an optimum TC (0.5 mm) in which the highest heat transfer rate and lowest PCM melting time is obtained. Moreover, the magnitude of frictional entropy generation rate is much lower than that of the thermal term. For accurate estimation of the liquid fraction, fractional and thermal entropy generation rates, a new ensemble machine learning (ML), namely Gradient Boosting Decision Tree (GBDT), was developed based on the fin tip and flow time parameters as input features. The outcomes of ML-based simulation exhibited promising performance for the precision prediction of three understudy targets.

Analysis of process efficiency: Role of flow and thermal characteristics on entropy production and heat transfer rates for thermal convection in porous beds confined within triangular configurations with hot slanted walls

Flow and heating characteristics associated with heat transfer rates vis-a-vis entropy generation based on flow and thermal irreversibilities are numerically studied for thermal convection in the porous bed confined within various configurations of triangular enclosures. The triangular configurations involve various vertex angles ( and ) with three tilt positions (configurations 1–3) subjected to a linear heating pattern of the slanted walls. Numerical studies are carried out for various dimensionless parameters (Prm and Dam ) at fixed Ram . Simulation results infer that the triangular configuration 3 [configuration 3: triangle is tilted perpendicularly from horizontal position (configuration 1 or 2)] involving is the optimal configuration based on the larger rate of heat transfer and moderate entropy production at larger porosities () for buoyancy dominant scenarios () irrespective of Prm .

Numerical evaluation of the effect of nano-additive type on the second-law performance of γ-AlOOH nano-fluid flow in a wavy microchannel

The present investigation addresses the impacts of nanoadditive (NA) type on the irreversibility aspects of γ-AlOOH nanofluid (NF) flow in a sinusoidal channel. To this aim, the two-phase mixture technique is employed to solve the governing formulas. The considered NA types are platelet, cylindrical, blade, brick, and spherical. The effect of Reynolds number and NA volume fraction on the local and global irreversibilities due to friction and heat transfer are numerically examined for various NA types. The results showed that the application of the platelet and spherical NA types respectively leads to the highest and lowest increment in the viscosity of the NF. Besides, it was found the highest and lowest thermal conductivity respectively belongs to the NFs composed of cylindrical and platelet NA types. In addition, it was depicted that the NF with platelet type NA presents the maximum frictional, thermal, and total irreversibilities, while the minimum values belongs to the NF with spherical NA type. Moreover, the findings revealed that boosting the and causes an increment in the frictional and thermal irreversibilities. Furthermore, it was reported that the thermal irreversibility has a more contribution in the total irreversibility compared to the frictional irreversibility.

CFD simulation of the impact of tip clearance on the hydrothermal performance and entropy generation of a water-cooled pin-fin heat sink

In this paper, the 3D numerical calculations of the water flow inside a pin-fin heat sink (PFHS) to determine the influence of tip clearance on the hydrothermal behavior and frictional & thermal irreversibilities of the fluid flow. The numerical simulations were conducted for five different fin heights (ξ = 1.5 mm to 2.5 mm) considering four different Reynolds numbers (Re=500, 1000, 1500, and 2000). The results demonstrated that the highest heat transfer coefficient is associated with ξ = 2.25 mm due to more uniform vorticity distributions, while the minimum central processing unit (CPU) average temperature is obtained for ξ = 2.5 mm due to the higher heat transfer surface area. The minimum thermal and frictional entropy generation rates are associated with PFHS with ξ = 1.5 mm. However, the PFHS with ξ = 2.25 mm has the highest Performance Evaluation Criterion (PEC) of nearly equal to 1.28 as compared to the other ξs. Besides, the fluid flow from the pin-fin base to the top and vice versa leads to create the vortices especially at the entrance of the PFHSs with tip clearance.

Numerical Investigation of Natural Convection and Irreversibilities between Two Inclined Concentric Cylinders in Presence of Uniform Magnetic Field and Radiation

In this study, the free convection with volumetric radiation in a water–alumina nanofluid flowing between two inclined concentric cylinders was simulated. The enclosure, or the cavity between the two circular quadrants in which the inner and outer pipes were respectively at high and low temperatures, was exposed to a magnetic field, and the remaining walls were insulated. Simulations were performed using control volume and the SIMPLE algorithm. The effects of the Rayleigh and Hartmann numbers, cavity or enclosure angle, magnetic field angle, nanopowder volume fraction, radiation intensity, and aspect ratio were investigated on the Nusselt number, irreversibilities and the Bejan number. According to the results, the Rayleigh number, hot pipe diameter, and adding more nanopowder have increasing and the Hartmann number has decreasing effect on the thermal performance and irreversibilities. Adding more nanopowder and increasing the enclosure angle also improved heat transfer. Increasing the magnetic field angle first increased and then decreased irreversibilities. In fact, increasing the magnetic field angle increased up to 15 ̊, the heat transfer rate increased, and further increase of the magnetic field angle from 15 to 90 ̊ decreased the heat transfer rate.