Thermal Irreversibilities Research Articles

The Development and Mechanisms of the High Pressure Turbine Vane Wake Vortex

The wake vortex is an important origin of unsteadiness and losses in turbines. In this paper, the development and underlying mechanisms of the shedding vortex of a high-pressure transonic turbine vane are studied and analyzed using the delayed detached eddy simulation (DDES) and proper orthogonal decomposition (POD). The goal is to understand the unsteadiness related to the wake vortex shedding and the wake evolution and mixing. Special attention is paid to the development of the wake vortex and the mechanisms behind the length characteristics. Interactions of the wake vortex with the shock wave and pressure waves are also discussed. First, the DDES simulation results are compared with published experimental data, Reynolds Averaged Navier-Stokes, and large eddy simulation (LES) simulations. Then, the development of the vane wake vortex, especially the different length characteristics from the cylinder vortex, is discussed. The reason of stronger pressure-side vortex shedding compared to suction-side vortex shedding is revealed. Wake-shock wave interaction and wake-pressure wave interaction are also investigated. The pressure waves are found to have a stronger effect than the shock wave on the spanwise motion and the dissipation of the wake vortex. An analysis of the losses through the turbine vane passage is carried out to evaluate the contributions of thermal and viscous irreversibilities. Losses analysis also confirms the strong interaction between the wake vortex and pressure waves. After that, POD study of the wake behavior was carried out. The results indicate that the shedding vortex is dominant in the unsteady flow. The phase relation between the pressure side wake vortex (PSVP) and the suction side wake vortex (SSVP) is confirmed.

Entropy generation analysis for nanofluid flow inside a duct equipped with porous baffles

In this research, a numerical simulation is performed to investigate thermal and viscous irreversibilities for Al2O3–water nanofluid inside a duct equipped with porous baffles. The effects of different parameters including Reynolds number, Darcy number, solid volume fraction of nanoparticles, and the number of baffle were investigated on thermal and viscous entropy generation rates and Bejan number. The results indicated that the viscous and thermal entropy generations decrease by increasing the number of baffles for N > 4. The reductions in the viscous and thermal entropy generations are 32 and 14%, respectively as the number of baffles increases in the range of 4–16. Moreover, the thermal entropy generation is dominant term in most part of the duct except along the centerline of the duct in the space between bottom and top baffles where the velocity gradients are intense.

Second law of thermodynamics analysis for nanofluid turbulent flow inside a solar heater with the ribbed absorber plate

Employing rough surfaces and nanofluids in solar heater ducts are recognized as effective techniques to improve thermal performances of these devices. In this paper, the second law of thermodynamics analysis is performed for nanofluid turbulent flow in a solar heater duct with rib roughness on the absorber plate. The effects of different parameters including rib height, rib wedge angle, rib pitch, Reynolds number, and nanofluid concentration on thermal and frictional irreversibilities are investigated. The results indicated that the thermal entropy generation decreases about 11.1% by using nanofluid with concentration of 0.04. The frictional and thermal entropy generations decrease by increasing the rib pitch. Moreover, the thermal entropy decreases about 21.05% by increasing the rib height in the range of 0.025–0.033 for Re = 3200. Finally, it is found that the thermal entropy generation decreases by increasing the wedge angle of the rib.

Thermal conduction of stable graphite suspensions considering structural characteristics of graphite aggregations

In this article, a new simple approach that can investigate thermal conduction of graphite suspensions is discussed. Using the three-level homogenization model, the fractal dimension (df) of graphite suspensions is determined as df = 1.76 and 1.70 for graphite/EG and for graphite/PAO, respectively. From these values, the highly anisotropic heat conduction is expected to be dominant through aggregations of graphite flake. Based on these observations, a simple thermal expression is proposed to describe the anisotropic heat conduction of graphite suspensions incorporating the effect of the interfacial thermal resistance. The effects of aggregated sphere (AS) distribution on thermal irreversibilities of nanofluids are also investigated by comparing the entropy creations between spatially uniform and random distributions of AS.

Thermodynamic optimization of reverse Brayton cycles of different configurations for cryogenic applications

The thermodynamic optimization of differing Reverse Brayton Refrigeration (RBR) cycle configurations is presented in this study. These cycle configurations include: Conventional 1-stage compression cycle; Conventional 2-stage compression cycle; 1-stage compression Modified cycle with intermediate cooling of the recuperator using an auxiliary cooler; and an Integrated 2-stage expansion RBR cycle. For high pressure ratio applications, multi-stage compressors with intercooling are considered. Analytical solutions for the conventional cycles are developed including thermal and fluid flow irreversibilities of the recuperators and all heat exchangers in addition to the compression and expansion processes. Exergy analysis is performed and the exergy destruction of different components of the RBR cycles for different configurations is presented and the effects of important system parameters on performance are investigated. Thermodynamic optimization of the cycles with intermediate cooling of the recuperator is included. Effects of the 2nd law/exergy efficiency of the auxiliary cooler on the total system efficiencies are presented.

Tube pattern effect on thermalhydraulic characteristics in a two-rows finned-tube heat exchanger

The present study deals with thermalhydraulic characteristics and entropy production analysis in a two-rows Plate Fin-and-Tube Heat Exchanger (PFTHE). Using Unsteady-RANS simulations, the effect of the iso-sectional tube shape modification (from circular to elliptic) on the air-side thermalhydraulic characteristics and entropy production rate was studied in three geometrical configurations of PFTHE and two airflow velocities. Spanwise and time-averaged representative quantities as well as the flow structure were used for local characterization of heat transfer, pressure drop, thermal and viscous volumetric entropy production rates. A global analysis of the thermalhydraulic performances and entropy production was also performed. This study highlights the direct relationship between the flow structure, local heat transfer, wall shear stress and the different components of the volumetric entropy production rate. It is shown that the reduction of the tube ellipticity significantly increases the thermalhydraulic performance of the heat exchanger up to 80% when compared to circular tube shape. Similarly a reduction of the thermal and viscous irreversibilities respectively down to 15% and 50% was observed in the modified shapes when compared to circular ones.

Role of heat exchangers in helium liquefaction cycles: Simulation studies using Collins cycle

Energy efficiency of large-scale helium liquefiers generally employed in fusion reactors and accelerators is determined by the performance of their constituting components. Simulation with Aspen HYSYS ® V7.0, a commercial process simulator, helps to understand the effects of heat exchanger parameters on the performance of a helium liquefier. Effective UA (product of overall heat transfer coefficient U, heat transfer surface area A and deterioration factor F) has been taken as an independent parameter, which takes into account all thermal irreversibilities and configuration effects. Nondimensionalization of parameters makes the results applicable to plants of any capacity. Rate of liquefaction is found to increase linearly with the effectiveness of heat exchangers. Performance of those heat exchangers that determine the inlet temperatures to expanders have more influence on the liquid production. Variation of sizes of heat exchangers does not affect the optimum rate of flow through expanders. Increasing UA improves the rate of liquid production; however, the improvement saturates at limiting UA. Maximum benefit in liquefaction is obtained when the available heat transfer surface area is distributed in such a way that the effectiveness remains equal for all heat exchangers. Conclusions from this study may be utilized in analyzing and designing large helium plants.

Entropy generation of droplet motion with surface tension hysteresis in a closed microchannel

A transient heat transfer and entropy analysis is conducted to investigate the processes of thermocapillary droplet motion in closed microchannels. Both theoretical predictions and experimental data are presented for time-dependent temperature changes during the droplet acceleration. This paper develops a predictive model of the entropy production due to thermal and fluid irreversibilities in the microchannel. Thermocapillary, pressure and friction forces are modelled within the droplet, as well as surface tension hysteresis during start-up of the droplet motion. The spatial temperature change in the axial direction is measured experimentally, as well as the displacement of the droplet over time. The variation of the entropy generation number is reported for open and closed channels. Close agreement is obtained between the predicted and experimental data. The results show that water droplets have lower thermal and fluid irreversibilities than toluene and mineral oil.

An Equivalent Mechanical Model for Representing the Entropy Generation in Heat Exchangers. Application to Power Cycles

One of the most common difficulties students face in learning Thermodynamics lies in grasping the physical meaning of concepts such as lost availability and entropy generation. This explains the quest for new approaches for explaining and comprehending these quantities, as suggested by diagrams from different authors. The difficulties worsen in the case of irreversibilities associated with heat transfer processes driven by a finite temperature difference, where no work transfer takes place. An equivalent mechanical model is proposed in this paper. Heat exchangers are modelled by means of Carnot heat engines and mechanical transmissions; the use of mechanical models allows an easy visualization of thermal irreversibilities. The proposed model is further applied to a power cycle, thus obtaining an “equivalent arrangement” where irreversibilities become clearly apparent.

Analysis of welding conditions based on induced thermal irreversibilities in welded structures: Cases of welding sequences and preheating treatment

Preheating treatment and employing a proper welding sequence are introduced as two efficient approaches for reducing residual stresses in welding processes. Selecting a right welding sequence and a proper preheating temperature for a weld system are crucial tasks, since welding residual stresses are inevitably produced in a welded structure. However, obtaining residual stresses is very complex, and much time is needed in some cases, particularly for the modeling and prediction of residual stresses in welded structures. So in this work, we attempt to show that these stresses depend on generated entropy during the arc welding process. Thus by obtaining thermal irreversibility effects from temperature distribution, due to a variation of different welding parameters, selection of optimum factors to achieve minimum residual stress is not only possible, but easier to undertake. In the proposed method, it is not a necessity to obtain residual stress values, and one can predict the behavior of residual stress qualitatively. To do so, 3D numerical models are employed to study the similar behavior of created residual stresses and entropy due to a variation of different preheating temperatures and three common welding sequences.

Analysis of entropy generation for distributed heating in processing of materials by thermal convection

Analysis of entropy generation has been carried out for square cavities with distributed heated sources filled with various materials involving wide range of Pr(=0.015, 0.7, 10, 1000) during the conduction and convection regime within Ra(=10 3 − 10 5). Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on elemental basis set via Galerkin finite element method. Local entropy maps are analyzed in detail for various cases and the dominance of thermal and frictional irreversibilities is studied via average Bejan number. The heat transfer irreversibility is found to dominate during conduction regime while the fluid friction irreversibility dominates the entropy generation in the convection regime, except for the low Pr fluid based on the heating configuration of the cavity. Further, the variation of total entropy generation has been observed to be similar for different heating configurations for higher Pr fluids (=10, 1000) whereas, the configuration of cavity has been found to have little effect on total entropy generation for fluids with Pr = 0.7 during both conduction and convection regimes. Thermal mixing and degree of temperature uniformity due to distributed heating in various cases are also reported and optimum cases for processing of various fluids are presented based on minimum entropy generation.

Effects of Thermal Boundary Conditions on Entropy Generation during Natural Convection

A comprehensive numerical study on entropy generation during natural convection is studied in a square cavity subjected to a wide variety of thermal boundary conditions. Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on the elemental basis set via the Galerkin finite element method. The thermal and fluid irreversibilities during the conduction and convection dominant regimes are analyzed in detail for various fluids (Pr = 0.026,988.24) within Ra = 103–105. Further, the effect of Ra on the total entropy generation and average Bejan number is discussed. It is observed that thermal boundary conditions significantly affect the thermal mixing, temperature uniformity, and the entropy generation in the cavity. A case where the bottom wall is hot isothermal with linearly cooled side walls and adiabatic top wall is found to result in high thermal mixing and a higher degree of temperature uniformity with minimum total entropy generation.

Entropy generation of turbulent double-diffusive natural convection in a rectangle cavity

Abstract Turbulent double-diffusive natural convection is of fundamental interest and practical importance. In the present work we investigate systematically the effects of thermal Rayleigh number (Ra), ratio of buoyancy forces (N) and aspect ratio (A) on entropy generation of turbulent double-diffusive natural convection in a rectangle cavity. Several conclusions are obtained: (1) The total entropy generation number (Stotal) increases with Ra, and the relative total entropy generation rates are nearly insensitive to Ra when Ra ≤ 109; (2) Since N > 1, Stotal increases quickly and linearly with N and the relative total entropy generation rate due to diffusive irreversibility becomes the dominant irreversibility; and (3) Stotal increases nearly linearly with A. The relative total entropy generation rate due to diffusive and thermal irreversibilities both are monotonic decreasing functions against A while that due to viscous irreversibility is a monotonic increasing function with A. More important, through the present work we observe a new phenomenon named as “spatial self-copy” in such convectional flow. The “spatial self-copy” phenomenon implies that large-scale regular patterns may emerge through small-scale irregular and stochastic distributions. But it is still an open question required further investigation to reveal the physical meanings hidden behind it.

Finite-Time Thermoeconomic Optimization of a Solar-Driven Heat Engine Model

In the present paper, the thermoeconomic optimization of an irreversible solar-driven heat engine model has been carried out by using finite-time/finite-size thermodynamic theory. In our study we take into account losses due to heat transfer across finite time temperature differences, heat leakage between thermal reservoirs and internal irreversibilities in terms of a parameter which comes from the Clausius inequality. In the considered heat engine model, the heat transfer from the hot reservoir to the working fluid is assumed to be Dulong-Petit type and the heat transfer to the cold reservoir is assumed of the Newtonian type. In this work, the optimum performance and two design parameters have been investigated under two objective functions: the power output per unit total cost and the ecological function per unit total cost. The effects of the technical and economical parameters on the thermoeconomic performance have been also discussed under the aforementioned two criteria of performance.

Imaging Velocimetry Measurements for Entropy Production in a Rotational Magnetic Stirring Tank and Parallel Channel Flow

An experimental design is presented for an optical method of measuring spatial variations of flow irreversibilities in laminar viscous fluid motion. Pulsed laser measurements of fluid velocity with PIV (Particle Image Velocimetry) are post-processed to determine the local flow irreversibilities. The experimental technique yields whole-field measurements of instantaneous entropy production with a non-intrusive, optical method. Unlike point-wise methods that give measured velocities at single points in space, the PIV method is used to measure spatial velocity gradients over the entire problem domain. When combined with local temperatures and thermal irreversibilities, these velocity gradients can be used to find local losses of energy availability and exergy destruction. This article focuses on the frictional portion of entropy production, which leads to irreversible dissipation of mechanical energy to internal energy through friction. Such effects are significant in various technological applications, ranging from power turbines to internal duct flows and turbomachinery. Specific problems of a rotational stirring tank and channel flow are examined in this paper. By tracking the local flow irreversibilities, designers can focus on problem areas of highest entropy production to make local component modifications, thereby improving the overall energy efficiency of the system.

Power, Efficiency and Irreversibility of Latent Energy Systems

The object of this paper is to present a simple model for the latent storage systems by using a phase-change material encapsulated in the spherical capsules. When designing phase-change-material systems, it is important to minimize thermal irreversibilities, because it works best when irreversibilities are done. In this paper, second law analysis is used to evaluate the irreversibility of the system. The effects of different parameters are also discussed. The proposed model can be applied to the charge and the discharge mode. Exergy efficiency is also evaluated.

Oscillatory flow in microporous media applied in pulse – tube and Stirling – cycle cryocooler regenerators

Pulse tube and Stirling cryocoolers are widely used in aerospace and other high-demand application. A key component in these systems is the regenerator, which is typically a microporous metallic structure that is subject to periodic flow of a cryogenic fluid. The thermal and hydrodynamic irreversibilities in the regenerator, which play crucial roles with respect to the efficiency of the aforementioned cycles, are poorly understood, however. In this investigation experiments were performed where pressure drop associated with steady-periodic (axial and lateral (radial)) flows of helium in test sections packed with several widely used pulse tube and Stirling cryocooler regenerator fillers were measured under ambient temperature conditions. A computational fluid dynamic (CFD) – assisted method was developed for the analysis and interpretation of the experimental data, whereby the permeability and inertial coefficients that lead to agreement between the data and the predictions of CFD simulations were iteratively obtained. The directional permeability and Forchheimer inertial coefficients were thus obtained for the tested regenerator fillers, and were found to be independent of the frequency of flow oscillations for the frequency range 5–60 Hz. The results also show that the oscillatory flow hydrodynamic parameters are different than steady-flow parameters representing similar flow conditions.

Structural, electronic, and magnetic properties of tetragonalMn3−xGa: Experiments and first-principles calculations

This work reports on the electronic, magnetic, and structural properties of the binary intermetallic compounds ${\mathrm{Mn}}_{3\ensuremath{-}x}\mathrm{Ga}$. The tetragonal ${\mathrm{DO}}_{22}$ phase of the ${\mathrm{Mn}}_{3\ensuremath{-}x}\mathrm{Ga}$ series, with $x$ varying from 0 to 1.0 in steps of $x=0.1$, was successfully synthesized and investigated. It was found that all these materials are hard magnetic, with energy products ranging from $10.1\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$ for low Mn content $(x\ensuremath{\rightarrow}1)$ to $61.6\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$ for high Mn content $(x\ensuremath{\rightarrow}0)$. With decreasing Mn content, the average saturation magnetization per atom increases from $0.26{\ensuremath{\mu}}_{B}$ for ${\mathrm{Mn}}_{3}\mathrm{Ga}$ to $0.47{\ensuremath{\mu}}_{B}$ for ${\mathrm{Mn}}_{2}\mathrm{Ga}$. The increase in the saturation magnetization as the Mn content is reduced indicates a ferrimagnetic order with partially compensating moments of the two different Mn atoms on the two crystallographically different sites of the ${\mathrm{DO}}_{22}$ structure. This type of magnetic order is supported by ab initio calculations of the electronic structure that predict a nearly half-metallic ferrimagnet with the highest spin polarization of 88% at the Fermi energy for ${\mathrm{Mn}}_{3}\mathrm{Ga}$. The Curie temperature of the compounds is restricted to approximately $770\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ because of a structural phase transition to the hexagonal ${\mathrm{DO}}_{19}$ phase. Thermal irreversibilities between zero-field-cooled and field-cooled measurements suggest that the ${\mathrm{Mn}}_{3\ensuremath{-}x}\mathrm{Ga}$ series belongs to the class of magnetically frustrated ferrimagnets. The most pronounced magnetic anomaly is found for ${\mathrm{Mn}}_{3}\mathrm{Ga}$.

Entropy-based surface microprofiling for passive near-wall flow control

In this paper, embedded surface microchannels with varying geometrical profiles are developed and applied to drag reduction and near-wall flow control along a surface. Local slip-flow conditions and a temperature discontinuity are encountered within the microchannels along the flat surface. Diverging sections of each microchannel affect both the near-wall pressure and velocity distributions, thereby influencing the boundary layer separation and entropy production. This impact of surface microchannels is characterized with respect to entropy production of friction and thermal irreversibilities. The total entropy production is expressed in terms of the microchannel base and exit angles, depth and other geometrical parameters that characterize the microprofiled surface. Numerical results are presented for the optimized geometrical configuration of the microchannel profiles. Applications of the microprofiling technique are presented for near-wall flow control along a helicopter engine cooling bay surface.

Measured turbulent entropy production with large eddy particle image velocimetry

In this paper, statistical post-processing of measured velocity, dissipation rate and turbulence data is performed to establish whole-field distributions of entropy production within a channel. Thermal irreversibilities arising from temperature variations were not included in the study, as the experiments were conducted between unheated plexiglass plates in an essentially isothermal water tunnel. Unlike velocity or temperature, the measurement of entropy cannot be performed directly, so entropy production is measured indirectly through spatial differencing of measured velocities in large eddy PIV. In contrast to single-point methods of anemometry, large eddy PIV enables whole-field, time-varying measurements of the velocity field, which can be post-processed to yield entire spatial variations of the entropy production rate. An uncertainty analysis is performed to estimate measurement uncertainties with the new experimental technique. The uncertainties are decomposed into systematic and random components, including a propagated uncertainty, due to spatial differencing of the velocity field. Close comparisons between measured results of turbulence dissipation and direct numerical simulations provide useful verification of the formulation, before post-processed results of dissipation rates are used to determine entropy production within a channel.