Entropy Generation Decreases Research Articles

Analysis of Entropy Generation of a Magneto-Hydrodynamic Flow Through the Operation of an Unlooped Pulsating Heat Pipe

The aim of the study is the analysis of a uniform magnetic field effect on fluid flow, heat transfer, and entropy generation through the operation of a pulsating heat pipe (PHP). An open loop PHP with three neighboring vapor plugs and two liquid slugs has been considered. The governing equations such as momentum, energy, and mass equations are solved using an explicit method except for the energy equation of liquid slugs. For each case study, Bejan number has been derived to find the heat transfer share in entropy generation. According to the results, the performance of the oscillating heat pipe decreases by applying uniform magnetic field. Moreover, the obtained results illustrate the effects of the applied magnetic field position on the heat transfer and the entropy generation. The latent and sensible heat transfer into the PHP enhance as a result of increase in the pipe diameter, so that the liquid slugs oscillate with high amplitudes. In addition, the entropy generation value increases with an augmentation in the value of the pipe diameter. The evaluated Bejan numbers indicate that the viscous effects in entropy generation decrease as the pipe diameter increases. Furthermore, the results depict that the heat transfer performance of PHP improves by increasing temperature difference between evaporator and condenser sections. With an increase in the value of the evaporator temperature, the Bejan number will increase, as a result, this phenomenon reveals the inconsiderable role of viscous impacts in high evaporator temperatures. In order to validate the calculations, the calculated results have been compared with the previous studies which show good agreement.

Investigating heat transfer and entropy generation for mixed convection of CuO–water nanofluid in an inclined annulus

Mixed convection of the CuO–water nanofluid inside an inclined annulus is investigated based on the first and second laws of thermodynamics. The Boussinesq approximation is employed, and all properties (except the density) are considered to be constant. The same heat fluxes are employed on both walls of the annulus. The effects of Richardson number, volume fraction and inclination angle on the second law characteristics and heat transfer are evaluated. The Bejan number and entropy generation rates are calculated to investigate the irreversibility types due to friction and heat transfer. At greater Richardson numbers, the velocity profile becomes flatter owing to easy formation of the natural convection. The results show that increase of the inclination angle from 0° to 75° leads to increase of convective heat transfer and decrease of total entropy generation and consequently, a proper heat exchange together with a low irreversibility are achieved. Additionally, with increment of Richardson number, both total entropy generation and convective heat transfer increase. Meanwhile, frictional entropy generation decreases while thermal entropy generation increases with the concentration increment. In all the cases, Bejan number is close to 1, which proves dominance of the heat transfer in entropy generation.

Numerical Investigation of Nanofluid Thermocapillary Convection Based on Two-Phase Mixture Model

Numerical investigation of nanofluid thermocapillary convection in a two-dimensional rectangular cavity was carried out, in which the two-phase mixture model was used to simulate the nanoparticles-fluid mixture flow, and the influences of volume fraction of nanoparticles on the flow characteristics and heat transfer performance were discussed. The results show that, with the increase of nanoparticle volume fraction, thermocapillary convection intensity weakens gradually, and the heat conduction effect strengthens; meanwhile, the temperature gradient at free surface increases but the free surface velocity decreases gradually. The average Nusselt number of hot wall and the total entropy generation decrease with nanoparticle volume fraction increasing.

Irreversibility analysis for flow of a non-Newtonian hybrid nanofluid containing coated CNT/Fe3O4 nanoparticles in a minichannel heat exchanger

Irreversibilities caused by heat transfer and friction are evaluated for a non-Newtonian hybrid nanofluid flow in a double-tube minichannel heat exchanger by calculating entropy generation rates. The nanofluids are prepared by dispersing two different nanoparticles, namely Tetra Methyl Ammonium Hydroxide (TMAH) coated Fe3O4 nanoparticles and Gum Arabic (GA) coated Carbon Nanotubes (CNTs). Variable thermophysical properties are employed such that thermal conductivity is considered dependent on temperature and concentration, while viscosity is dependent on temperature, concentration and shear rate. The results show that heat transfer is the main cause in entropy generation at low concentrations while friction is the main factor at high concentrations. By increasing water temperature at the annulus side, the nanofluid thermal entropy generation increases while the nanofluid frictional entropy generation decreases. For low magnetite concentration, maximum thermal entropy generation occurs at the lowest CNT concentration while for high magnetite concentration, it happens at the highest CNT concentration. By increasing the Reynolds number, minimum point of thermal entropy generation moves toward smaller magnetite concentrations. In addition, at low magnetite concentration, total entropy generation rate possesses a minimum (optimal) point with respect to CNT concentration while an ascending trend is observed at high magnetite concentrations.

Slip Velocity in Flow and Heat Transfer of Non-newtonian Fluids in Microchannels

The steady-state fully-developed laminar flow of non-Newtonian power-law fluids is examined in a circular microchannel with slip boundary condition and under an imposed constant wall heat flux. Effects of slip as well as the hydrodynamic and thermal key parameters on heat transfer and entropy generation are investigated. The results reveal that increasing the Brinkman number and the flow behavior index both lead to increasing the entropy generation and decreasing the Nusselt number. In heating process, the temperature difference between the fluid and the wall decreases as the slip coefficient increases; similar trend may or may not be observed for cooling process, determined by the range of the slip coefficient as well as the Brinkman number. An increase in the slip coefficient leads to an increase in both the Nusselt number and the Bejan number, whereas it gives rise to a decrease in entropy generation. For each particular value of the slip coefficient, the Nusselt number approaches a specific value as the Brinkman number and/or the flow behavior index increases. An increase in the flow behavior index or a decrease in the slip coefficient results in incrementing the average entropy generation.

Entropy Generation Analysis and Performance Evaluation of Turbulent Forced Convective Heat Transfer to Nanofluids

The entropy generation analysis of fully turbulent convective heat transfer to nanofluids in a circular tube is investigated numerically using the Reynolds Averaged Navier–Stokes (RANS) model. The nanofluids with particle concentration of 0%, 1%, 2%, 4% and 6% are treated as single phases of effective properties. The uniform heat flux is enforced at the tube wall. To confirm the validity of the numerical approach, the results have been compared with empirical correlations and analytical formula. The self-similarity profiles of local entropy generation are also studied, in which the peak values of entropy generation by direct dissipation, turbulent dissipation, mean temperature gradients and fluctuating temperature gradients for different Reynolds number as well as different particle concentration are observed. In addition, the effects of Reynolds number, volume fraction of nanoparticles and heat flux on total entropy generation and Bejan number are discussed. In the results, the intersection points of total entropy generation for water and four nanofluids are observed, when the entropy generation decrease before the intersection and increase after the intersection as the particle concentration increases. Finally, by definition of Ep, which combines the first law and second law of thermodynamics and attributed to evaluate the real performance of heat transfer processes, the optimal Reynolds number Reop corresponding to the best performance and the advisable Reynolds number Read providing the appropriate Reynolds number range for nanofluids in convective heat transfer can be determined.

Entropy Generation and Heat Transfer Performances of Al2O3-Water Nanofluid Transitional Flow in Rectangular Channels with Dimples and Protrusions

Nanofluid has great potentials in heat transfer enhancement and entropy generation decrease as an effective cooling medium. Effects of Al2O3-water nanofluid flow on entropy generation and heat transfer performance in a rectangular conventional channel are numerically investigated in this study. Four different volume fractions are considered and the boundary condition with a constant heat flux is adopted. The flow Reynolds number covers laminar flow, transitional flow and turbulent flow. The influences of the flow regime and nanofluid volume fraction are examined. Furthermore, dimples and protrusions are employed, and the impacts on heat transfer characteristic and entropy generation are acquired. It is found that the average heat transfer entropy generation rate descends and the average friction entropy generation rate rises with an increasing nanofluid volume fraction. The effect of nanofluid on average heat transfer entropy generation rate declines when Reynolds number ascends, which is inverse for average friction entropy generation rate. The average wall temperature and temperature uniformity both drop accompanied with increasing pumping power with the growth in nanofluid volume fraction. The employment of dimples and protrusions significantly decreases the average entropy generation rate and improve the heat transfer performance. The effect of dimple-case shows great difference with that of protrusion-case.

Analytical study of parameters affecting entropy generation of nanofluid turbulent flow in channel and micro-channel

In this study, thermo-physical and geometrical parameters affecting entropy generation of nanofluid turbulent flow such as the volume fraction, Reynolds number and diameter of the channel and micro-channel with circular cross section under constant flux are examined analytically. Water is used as a base fluid of nanofluid with nanoparticles of Ag, Cu, CuO and TiO2. The study is conducted for Reynolds numbers of 20000, 40000 and 100000, volume fractions of 0, 0.01, 0.02, 0.03 and 0.04, channel diameters of 2, 4, 6 and 8 cm and micro-channel diameters of 20, 40, 60 and 80 micrometers. Based on the results, the most of the generated entropy in channel is due to heat transfer, and also, with increasing the diameter of the channel, Bejan number increases. The contribution of entropy generation due to heat transfer in the micro-channel is very poor and the major contribution of entropy generation is due to friction. The maximum amount of entropy generation in channel belongs to nanofluids with Ag, Cu, CuO and TiO2 nanoparticles, respectively, while in the micro-channel this behavior is reversed; and the minimum entropy generation happens in nanofluids with Ag, Cu, CuO and TiO2 nanoparticles, respectively. In channel and micro-channel, for all nanofluids except for the water-TiO2, with increasing volume fraction of nanoparticles, the entropy generation decreases. In channel and micro-channel, the total entropy generation increases as Reynolds number augments.

Effect of Magnetic Field on Entropy Generation in a Microchannel Heat Sink with Offset Fan Shaped

In this study, convection flow in microchannel heat sink with offset fan-shaped reentrant cavities in sidewall filled with Fe3O4-water is numerically investigated. The effects of changing some parameters such as Reynolds number and magnetic field are considered. The nanofluid flow is laminar, steady and incompressible, while the thermo-physical properties of nanoparticles were assumed constant. A finite volume method and two phase mixture models were used to simulate the flow. The obtained results show that the frictional entropy generation increases as Reynolds number increases, while a reverse trend is observed for thermal entropy generation. By applying a non-uniform magnetic field, the entropy generation due to heat transfer decreases at first and then increases. When using the uniform magnetic field, the frictional entropy generation and thermal entropy generation is negligible. For all studied cases, the total entropy generation decreases using non-uniform magnetic fields. The results indicate that by increasing the magnetic field power, the total entropy generation decreases.

Improved design of Wells turbine for wave energy conversion using entropy generation

In this paper, a comparison of total entropy generation between a suggested design and a constant chord Wells turbine is presented. A variable chord rotor Wells turbine design is suggested in order to have a more uniform axial velocity distribution at blades leading edge in comparing conventional design to improve turbine efficiency. Each model is analysed numerically with respect to the total entropy generation due to viscous dissipation around rotor blades. Simulations have been performed by numerical solving of the steady, incompressible, three-dimensional Reynolds-averaged Navier–Stokes together with RNG k–e turbulence model equations in a non-inertial reference frame rotating with the turbine rotor. A comparison of the computed results with the available experimental data exhibits agreement in terms of efficiency, torque and input coefficients at different flow rates. Finally, the detailed comparisons of the results are done between the suggested design and the conventional Wells turbine, demonstrating a 26.02 % average decrease in entropy generation throughout the full operating range, hence supporting the superiority of the new design.

Overall heat transfer coefficient and pressure drop in a typical tubular exchanger employing alumina nano-fluid as the tube side hot fluid

Nano-fluids are used to improve the heat transfer rates in heat exchangers, especially; the shell-and-tube heat exchanger that is considered one of the most important types of heat exchangers. In the present study, an experimental loop is constructed to study the thermal characteristics of the shell-and-tube heat exchanger; at different concentrations of Al2O3 nonmetallic particles (0.0, 2, 4, and 6 %). This material concentrations is by volume concentrations in pure water as a base fluid. The effects of nano-fluid concentrations on the performance of shell and tube heat exchanger have been conducted based on the overall heat transfer coefficient, the friction factor, the pressure drop in tube side, and the entropy generation rate. The experimental results show that; the highest heat transfer coefficient is obtained at a nano-fluid concentration of 4 % of the shell side. In shell side the maximum percentage increase in the overall heat transfer coefficient has reached 29.8 % for a nano-fluid concentration of 4 %, relative to the case of the base fluid (water) at the same tube side Reynolds number. However; in the tube side the maximum relative increase in pressure drop has recorded the values of 12, 28 and 48 % for a nano-material concentration of 2, 4 and 6 %, respectively, relative to the case without nano-fluid, at an approximate value of 56,000 for Reynolds number. The entropy generation reduces with increasing the nonmetallic particle volume fraction of the same flow rates. For increase the nonmetallic particle volume fraction from 0.0 to 6 % the rate of entropy generation decrease by 10 %.

Analysis of performances of a manifold microchannel heat sink with nanofluids

Hydraulic and thermal performances of a manifold microchannel heat sink (MMHS) with and without nanofluids as working fluids have been investigated by a finite volume method. Effects of volume fraction, particle diameter of nanoparticles, and Reynolds number on the Nusselt number, pumping power, performance index, and entropy generation in a 3D unit cell were evaluated. The results showed that with increasing volume fraction of nanoparticles, Nusselt number and pumping power increase, but total entropy generation decreases. Increasing particle diameter leads to decreasing Nusselt number, pumping power, and performance index, but increasing the total entropy generation. Finally increasing the Reynolds number leads to increasing the Nusselt number and pumping power, but decreasing performance index and total entropy generation.

Entropy Generation during Turbulent Flow of Zirconia-water and Other Nanofluids in a Square Cross Section Tube with a Constant Heat Flux

The entropy generation based on the second law of thermodynamics is investigated for turbulent forced convection flow of ZrO2-water nanofluid through a square pipe with constant wall heat flux. Effects of different particle concentrations, inlet conditions and particle sizes on entropy generation of ZrO2-water nanofluid are studied. Contributions from frictional and thermal entropy generations are investigated, and the optimal working condition is analyzed. The results show that the optimal volume concentration of nanoparticles to minimize the entropy generation increases when the Reynolds number decreases. It was also found that the thermal entropy generation increases with the increase of nanoparticle size whereas the frictional entropy generation decreases. Finally, the entropy generation of ZrO2-water was compared with that from other nanofluids (including Al2O3, SiO2 and CuO nanoparticles in water). The results showed that the SiO2 provided the highest entropy generation.

Entropy generation analysis of turbulent convection flow of Al2O3–water nanofluid in a circular tube subjected to constant wall heat flux

A parametric investigation of entropy generation of nanofluid turbulent forced convection inside a circular section tube subjected to constant wall heat flux is presented. The analysis is developed for different inlet conditions (i.e. constant Re, fixed mass flow rate and constant velocity) and in a concentration range from 0% up to 6%. The impact of the dispersed nanoparticles on total, thermal and frictional entropy generation is investigated and optimal working conditions are highlighted. The study shows that at the increase of Re, the optimal particles concentration to minimize entropy generation decreases. The impact of nanoparticles dimension is also taken into account, showing that its effect depends on the flow conditions. When working with constant mass flow rate, an optimal concentration is determined to minimize the frictional entropy generation, whereas thermal entropy generation results to increase. The study shows that to minimize total entropy generation when velocity is kept constant, a low concentration of nanoparticles is necessary.

Optimizing Natural Gas Fueling Station Reservoirs Pressure Based on Ideal Gas Model

At CNG fuelling station, natural gas is usually stored in a cascade storage system to utilize the station more efficient. The cascade storage system is generally divided into three reservoirs, commonly termed low, medium and high-pressure reservoirs. The pressures within these three reservoirs have huge effects on the performance of a CNG fuelling station and a fast filling process of natural gas vehicle’s (NGV) cylinder. A theoretical analysis is developed to study the effects of the reservoirs pressures and temperatures on the performance of the CNG station. The analysis is based on the first and the second law of thermodynamics, conservation of mass and ideal gas assumptions. The results show that as the reservoir temperature decreases, the fill ratio increases and the pressure within the filling station reservoirs has no effects on the fill ratio. The non-dimensional entropy generation and filling time profiles have opposite trends and as entropy generation decreases, the filling time increases. The optimized non-dimensional low and medium pressure-reservoir pressures are found to be as 0.24 and 0.58 respectively in thermodynamic point of view.

Second Law Analysis of Al2O3-Water Nanofluid Turbulent Forced Convection in a Circular Cross Section Tube with Constant Wall Temperature

The present paper proposes an analysis based on the second principle of thermodynamics applied to a water-Al2O3 nanofluid. The nanofluid flows inside a circular section tube subjected to constant wall temperature. The aim of the investigation is to understand, by means of an analytical model, how entropy generation within the tube varies if inlet conditions, particles concentration, and dimensions are changed. To gather these information is of fundamental importance, in order to optimize the nanofluid flow. The results show that according to the inlet condition, there is a substantial variation of the entropy generation, particularly when Reynolds number is kept constant there is an increase of entropy generation, whereas when mass flow rate or velocity are taken constant, entropy generation decreases.

Entropy Generation Due to Natural Convection in a Partially Open Cavity with a Thin Heat Source Subjected to a Nanofluid

This article presents a numerical study of natural convection cooling of a heat source mounted inside the cavity, with special attention being paid to entropy generation. The right vertical wall is partially open and is subjected to copper–water nanofluid at a constant low temperature and pressure, while the other boundaries are assumed to be adiabatic. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for a Rayleigh number in the range 103 < Ra < 106, and for solid volume fraction 0 <ϕ <0.05. In order to investigate the effect of the heat source and open boundary location, six different configurations are considered. The effects of Rayleigh numbers, heat source and open boundary locations on the streamlines, isotherms, local entropy generation, Nusselt number, and total entropy generation are investigated. The results indicate that when open boundary is located up, the fluid flow augments and hence the heat transfer and Nusselt number increase and total entropy generation decreases.

Analysis of the Magnetic Field Effect on Entropy Generation at Thermosolutal Convection in a Square Cavity

Thermosolutal convection in a square cavity filled with air and submitted to an inclined magnetic field is investigated numerically. The cavity is heated and cooled along the active walls with a mass gradient whereas the two other walls of the cavity are adiabatic and insulated. Entropy generation due to heat and mass transfer, fluid friction and magnetic effect has been determined in transient state for laminar flow by solving numerically the continuity, momentum energy and mass balance equations, using a Control Volume Finite—Element Method. The structure of the studied flows depends on four dimensionless parameters which are the Grashof number, the buoyancy ratio, the Hartman number and the inclination angle. The results show that the magnetic field parameter has a retarding effect on the flow in the cavity and this lead to a decrease of entropy generation, Temperature and concentration decrease with increasing value of the magnetic field parameter.

Optimal paths of piston motion of irreversible diesel cycle for minimum entropy generation

A Diesel cycle heat engine with internal and external irreversibility’s of heat transfer and friction, in which the finite rate of combustion is considered and the heat transfer between the working fluid and the environment obeys Newton’s heat transfer law [q≈ Δ(T)], is studied in this paper. Optimal piston motion trajectories for minimizing entropy generation per cycle are derived for the fixed total cycle time and fuel consumed per cycle. Optimal control theory is applied to determine the optimal piston motion trajectories for the cases of with piston acceleration constraint on each stroke and the optimal distribution of the total cycle time among the strokes. The optimal piston motion with acceleration constraint for each stroke consists of three segments, including initial maximum acceleration and final maximum deceleration boundary segments, respectively. Numerical examples for optimal configurations are provided, and the results obtained are compared with those obtained when maximizing the work output with Newton’s heat transfer law. The results also show that optimizing the piston motion trajectories could reduce engine entropy generation by more than 20%. This is primarily due to the decrease in entropy generation caused by heat transfer loss on the initial portion of the power stroke.

Performance Evaluation of Some Tube Inserts as Heat Transfer Enhancement Techniques

Different heat transfer enhancement techniques can be used successfully in the retrofit design of shell-and-tube heat exchangers that can lead to significant cost savings. Tube inserts are frequently used in such applications. Twisted tapes, wire coils, helically coiled ribbons (as tube inserts in smooth pipes), and rifled pipes have been evaluated as heat transfer enhancement techniques. To assess the benefit of using these kinds of techniques, extended performance evaluation criteria (PEC) have been implemented under different constraints. The decrease in entropy generation can be combined with the relative increase in the heat transfer rate or decrease of the heat transfer area to determine the geometrical parameters of the tubes for optimal thermodynamics performance.