Fixed Temperature Difference Research Articles

Analytical and numerical study of Soret mixed convection in two sided lid-driven horizontal cavity: Optimal species separation

The coupling between shear-driven convection and thermo-diffusion is a complex phenomenon due to the interactions between the different forces inside the fluid mixture. This paper studies the species separation that may appear in a parallelepipedic cell filled by a binary mixture. Dirichlet boundary conditions are imposed with differentially heated horizontal walls. Each wall can be animated with a uniform velocity. More precisely, two velocities UPex→,andfUPex→ are applied at the upper and the lower horizontal walls respectively. An analytical solution, based on the parallel flow approximation in the core region of the cavity of large aspect ratio, is obtained as a function of the dimensionless parameters of the problem. The species separation is optimized according to the multiple control parameters of the problem. The velocity field obtained is the superposition of the flow generated by the velocity of the walls under weightless conditions and thermoconvective flow under gravity only. The combination of these two flows leads to a mixed convection flow with a single convective cell for f∈-1,0 and two superposed cells for f∈0,1. Only unicellular flow leads to a notable species separation. This study shows that the effective species separation admits a partial optimum as a function of f and Up. As in the case of thermogravitational columns, there is no optimum species separation with respect to parameters ΔT and H but, for a fixed temperature difference ΔT, there is a thickness H that optimizes the separation and vice versa. 2D and 3D direct numerical simulations are performed using a finite element method in order to corroborate the analytical results. The influence of mass Péclet number on the diffusive relaxation time is determined numerically. Taking into account the confinement of the medium in the third dimension, perpendicular to a vertical plane, we show that the aspect ratio in the y-direction has an influence on the species separation.

Controlling and Optimizing Entropy Production in Transient Heat Transfer in Graded Materials

This paper presents a numerical analysis of the transient heat transfer problem arising when a functionally graded material is subjected to a fixed temperature difference. Varying the gradation of the system, the thermal performance of the material is assessed both in time-dependent and steady-state conditions by means of temperature profiles and entropy production. One of the main contributions of this paper is the analysis of the system in the transient, from which it is found that the entropy production has a non-monotonic behaviour since maximum and minimum values of this physical quantity could be identified by varying the grading profile of the material. The latter allows to propose an optimization criterion for functionally graded materials which consists of the identification of spatial regions where temperature gradients are large and find thermal conductivity profiles that attenuate those gradients, thus reducing the thermal stresses present inside the material.

Performance comparison of thermoelectric generators using different materials

Abstract Thermoelectric generators (TEGs) are a promising device to harvest waste heat for power generation. Three material properties of TEGs, including the Seebeck coefficient, electrical resistivity, and thermal conductivity, are often considered as constants in the simulation. However, the performance of TEGs changes with different temperature. In this study, the performances, such as output power and efficiency of one constant property material and three temperature-dependent property materials are investigated numerically. The influences of the temperature difference, the temperature at the cold side surface, and current on the performance of the TEGs are analyzed. The predictions indicate that the output power of the TEGs deviate from the formula when their properties are temperature-dependent. In other words, the output power of the TEGs is not directly proportional to the temperature difference squared. The simulations also satisfy the result of self-consistency. In the case of the fixed temperature difference of 100 oC, when the temperature at cold side surface is lower, different performances of TEG are followed by different materials. The results also tell that one material whose ZT value increases with the rise in temperature has a similar trend in output power and efficiency to the constant-property material. However, another material that has the maximum value in the ZT curve has different trend in output power and efficiency from those of the constant-property material.

Investigation on the optimal cooling tower input capacity of a cooling tower assisted ground source heat pump system

Coupling auxiliary cooling devices with ground source heat pump (GSHP) systems in cooling dominated areas can effectively solve the ground thermal imbalance problem. However, the input capacity of the auxiliary heat rejecter directly affects the performance of such hybrid ground source heat pump (HGSHP) systems. This paper presents an investigation on the optimal cooling tower input capacity of a cooling tower assisted GSHP system through both experiments and simulations. The experiments were carried out based on a HGSHP system implemented in an office building and the experimental results were used to validate a numerical model. A simulation system of the cooling tower assisted GSHP system was then developed using TRNSYS and validated against the experimental data collected. The impacts of the cooling tower input capacity on the soil temperature and the system performance were simulated. The results showed that the soil heat accumulation could be effectively alleviated when the cooling tower input capacity ratio (CTICR) was greater than 50%. The optimal cooling tower input capacity was highly dependent on the operation scenario used. The optimal CTICR under two operation scenarios considered was around 54%, while that under the other operation scenarios was around 63%. The control strategy based on the fixed temperature difference between the cooling water leaving the heat pump and the ambient air dry-bulb temperature was found to be the optimal control strategy for the system studied.

Investigating spatio-temporal surface urban heat island growth over Jaipur city using geospatial techniques

Urban heat island (UHI) effect is one of the major impacts at local scale. A criterion based on UHIindex and transect methods have been proposed to estimate the spatio-temporal surface UHI (SUHI) growth. The study has been carried out to analyze the spatial and seasonal variations of SUHI growth over Jaipur city of the years from 2003 to 2015 for six years’ time interval using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Analysis of land surface temperature (LST) data shows that significant SUHI growth exists over the study area. UHIindex has been used to mark the locations of hot spots (HS) of Jaipur city. Significant increase of up to 230% in the area of HS has been observed between 2003 and 2015. Main focus of the research is to find the increase in area, over a period of time, for fixed temperature gradient from the LST of HS. This increase in area can be used as a measure to indicate UHI growth of the city. Isothermal lines representing fixed temperature difference with respect to LST of HS have been created. Increase in area enclosed by different isothermal lines varies from 20% to above 400% during different seasons between 2003 and 2015.

Enhanced thermoelectric response in the fractional quantum Hall effect

We study the linear thermoelectric response of a quantum dot embedded in a constriction of a quantum Hall bar with fractional filling factors nu=1/m within Laughlin series. We calculate the figure of merit ZT for the maximum efficiency at a fixed temperature difference. We find a significant enhancement of this quantity in the fractional filling in relation to the integer-filling case, which is a direct consequence of the fractionalization of the electron in the fractional quantum Hall state. We present simple theoretical expressions for the Onsager coefficients at low temperatures, which explicitly show that ZT and the Seebeck coefficient increase with m.

Performance optimization of thermoelectric generators designed by multi-objective genetic algorithm

The purpose of this study is to investigate the output power and efficiency of a TEG system using waste heat from heat pipes, and then optimize its performance. The TEG material is Bi0.4Sb1.6Te3 and its figure-of-merit (ZT) is 1.18 at room temperature. The predictions indicate that a longer length of the elements has greater power output and efficiency based on a fixed heat flux on the hot side surface, whereas a shorter length has greater output power based on a fixed temperature difference. The geometry of the TEG is designed through a multi-objective genetic algorithm (MOGA) to maximize its efficiency. When the temperature difference is fixed at 40 °C, the output power and efficiency of the TEG with optimization is increased by about 51.9% and 5.4%, compared to the TEG without optimization. Once the impedance matching, namely, the internal resistance is equal to the external load resistance, is used, the output power can be further enhanced by about 3.85–4.40%. When the heat flux is fixed at 20,000 Wm−2 along with the temperature difference of 40 °C, the output power and efficiency of a pair of elements can be increased to 7.99 mW and 9.52%, respectively. These results are much higher than those reported in other studies. Accordingly, it is concluded that the MOGA is a powerful tool to design the geometry of a TEG for maximizing its performance and real applications in industry.

Optimization of heat exchanger size of a 10 kW organic Rankine cycle system

Most of the heat exchangers design in a conventional Rankine cycle power generation system is based on the assumption of a fixed temperature difference between the heat source and the inlet temperature of the expander. Since the temperature difference between the evaporator and the condenser is high, the effect of this estimated temperature difference on the system performance is not important. However, for an ORC system, since the temperature of heat source is low, a minor difference on the temperature estimation will affect the system thermal efficiency significantly. This study provides an experimental measurement of a practical 10 kW organic Rankine cycle system using HFC-245fa as working fluid subject to the influence of heat exchangers with different NTU. The effect of NTU of evaporator on heat transfer rate, network output, system thermal efficiency and investment payback years were studied. The results show that with increasing NTU of evaporator leads to the increase of total heat transfer rate, work output and system thermal efficiency. However, since the system installation cost increases with increasing heat exchangers NTU, in combining with the increase of power output, an optimal heat exchanger NTU for the shortest investment payback years is found.

Revisiting the difference between traveling-wave and standing-wave thermoacoustic engines - A simple analytical model for the standing-wave one

There are two major categories in a thermoacoustic prime-mover. One is the traveling-wave type and the other is the standing-wave type. A simple analytical model of a standing-wave thermoacoustic prime-mover is proposed at relatively low heat-flux for a stack much shorter than the acoustic wavelength, which approximately describes the Brayton cycle. Numerical simulations of Rott’s equations have revealed that the work flow (acoustic power) increases by increasing of the amplitude of the particle velocity (|U|) for the traveling-wave type and by increasing cosΦ for the standing-wave type, where Φ is the phase difference between the particle velocity and the acoustic pressure. In other words, the standing-wave type is a phase-dominant type while the traveling-wave type is an amplitude-dominant one. The ratio of the absolute value of the traveling-wave component (|U|cosΦ) to that of the standing-wave component (|U|sinΦ) of any thermoacoustic engine roughly equals the ratio of the absolute value of the increasing rate of |U| to that of cosΦ. The different mechanism between the traveling-wave and the standing-wave type is discussed regarding the dependence of the energy efficiency on the acoustic impedance of a stack as well as that on ωτα, where ω is the angular frequency of an acoustic wave and τα is the thermal relaxation time. While the energy efficiency of the traveling-wave type at the optimal ωτα is much higher than that of the standing-wave type, the energy efficiency of the standing-wave type is higher than that of the traveling-wave type at much higher ωτα under a fixed temperature difference between the cold and the hot ends of the stack.

Intermittent flow regimes near the convection threshold in ferromagnetic nanofluids.

The onset and decay of convection in a spherical cavity filled with ferromagnetic nanofluid and heated from below are investigated experimentally. It is found that, unlike in a single-component Newtonian fluid where stationary convection sets in as a result of supercritical bifurcation and where convection intensity increases continuously with the degree of supercriticality, convection in a multicomponent ferromagnetic nanofluid starts abruptly and has an oscillatory nature. The hysteresis is observed in the transition between conduction and convection states. In moderately supercritical regimes, the arising fluid motion observed at a fixed temperature difference intermittently transitions from quasiharmonic to essentially irregular oscillations that are followed by periods of a quasistationary convection. The observed oscillations are shown to result from the precession of the axis of a convection vortex in the equatorial plane. When the vertical temperature difference exceeds the convection onset value by a factor of 2.5, the initially oscillatory convection settles to a steady-state regime with no intermittent behavior detected afterward. The performed wavelet and Fourier analyses of thermocouple readings indicate the presence of various oscillatory modes with characteristic periods ranging from one hour to several days.

Geometry optimization of thermoelectric coolers using simplified conjugate-gradient method

This paper develops an inverse problem approach to optimize the geometric structure of TECs (thermoelectric coolers). The approach integrates a complete multi-physics TEC model and a simplified conjugate-gradient method. The present TEC model couples the heat and electrical conductions and accounts for all physical mechanisms occurred within TECs. Three geometric parameters, the semiconductor pair number, N, leg length of semiconductor column, Hpn, and base area ratio of semiconductor columns to TEC, γ, are optimized simultaneously at fixed current and fixed temperature difference. The cooling rate on the cold end is the objective function to be maximized to obtain the optimal TEC geometry. The effects of applied current and temperature difference on the optimal geometry are discussed. The results show that at temperature difference of 20 K, the geometry optimization increases the TEC cooling rate by 1.99–10.21 times compared with the initial TEC geometry, and the optimal N decreases from 100 to 47 with invariable γ = 0.95 and Hpn = 0.2 mm, as the applied current varies from 1.0 A to 3.0 A. With the increase in temperature difference, the optimal N increases at smaller currents of I ≤ 1.0 A, however, it is almost invariable at larger currents of I ≥ 1.5 A.

Heat transport and flow structure in rotating Rayleigh–Bénard convection

Here we summarize the results from our direct numerical simulations (DNS) and experimental measurements on rotating Rayleigh–Bénard (RB) convection. Our experiments and simulations are performed in cylindrical samples with an aspect ratio Γ varying from 1/2 to 2. Here Γ=D/L, where D and L are the diameter and height of the sample, respectively. When the rotation rate is increased, while a fixed temperature difference between the hot bottom and cold top plate is maintained, a sharp increase in the heat transfer is observed before the heat transfer drops drastically at stronger rotation rates. Here we focus on the question of how the heat transfer enhancement with respect to the non-rotating case depends on the Rayleigh number Ra, the Prandtl number Pr, and the rotation rate, indicated by the Rossby number Ro. Special attention will be given to the influence of the aspect ratio on the rotation rate that is required to get heat transport enhancement. In addition, we will discuss the relation between the heat transfer and the large scale flow structures that are formed in the different regimes of rotating RB convection and how the different regimes can be identified in experiments and simulations.

Development of microthermoelectric generators using integrated suspending bridge-type polysilicon thin-film thermopiles

We describe the development of novel suspension bridge-type microthermoelectric generators (μ -TEGs) having 64,000 to 147,000 serial-connected thermocouples in a 1-centimeter-square chip area using surface micromachining techniques. Each microthermocouple is constructed by a pair of n/p bridge-type polysilicon thin-film thermolegs and a pair of cold- and hot-side Cr/Au metal planes. Under a controlled fixed temperature difference between the cold/hot sides, the open-circuit voltage and the output power of the proposed μ -TEGs are simulated by commercial software (ANSYS). The influences of thermocouple thermo-leg dimensions and number of thermocouples on the thermoelectric characteristics of presented μ -TEGs are investigated. The implemented suspension bridge-type thermopile has a 2.5-μm-height air-gap separation from substrate and its fabrication yield is higher than 75% in the laboratory environment. The measured maximum temperature difference between the cold/hot sides of the proposed μ -TEGs is about 1.29°C, a maximum open-circuit voltage of 4.64 V/cm 2 and output power of 0.65 μW/cm 2 can be obtained.

Molecular dynamics modeling of thermal resistance at argon-graphite and argon-silver interfaces

Heat conduction between two parallel solid walls separated by liquid argon is investigated using three-dimensional molecular dynamics (MD) simulations. Liquid argon molecules confined in silver and graphite nano-channels are examined separately. Heat flux and temperature distribution within the nano-channels are calculated by maintaining a fixed temperature difference between the two solid surfaces. Temperature profiles are linear sufficiently away from the walls, and heat transfer in liquid argon obeys the Fourier law. Temperature jump due to the interface thermal resistance (i.e., Kapitza length) is characterized as a function of the wall temperature. MD results enabled development of a phenomenological model for the Kapitza length, which is utilized as the coefficient of a Navier-type temperature jump boundary condition using continuum heat conduction equation. Analytical solution of this model results in successful predictions of temperature distribution in liquid argon confined in silver and graphite nano-channels as thin as 7 nm and 3.57 nm, respectively.

Numerical Investigation of Natural Convection Heat Transfer in Porous Medium Saturated With Carbon Dioxide in Supercritical State

Natural convection in a porous medium saturated with supercritical carbon dioxide (CO2) whichis bounded by square cavity is studied numerically by solving the governing Darcy equation using finite-differences method.The side walls of cavity were assumed to be isotherm with fixed temperature difference of 0.0152K, while the upper and lower sides are assumed to be thermally insulated, the aspect ratio is chosen to be one, with cavity height of 0.01 m, the Darcy number adopted is , usually these values lead to very small modified Rayleigh number. Despite all of this, the state of supercritical takes the fluid (CO2) to a huge changes in its properties ( ) which allow to transfer a relatively large amount of heat between the hot and cold sides of the cavity with a very small temperature difference with maximum Nusselt number over 13 . The results of this study show clearly these concepts. The state of carbon dioxide in supercritical region is specified by reduced pressure and reduced temperature , the reduced temperature and pressure have been varied as . Keyword: Natural convection, porous media, supercritical carbon dioxide

Thermally driven spin transport through a transverse-biased zigzag-edge graphene nanoribbon

In the framework of the Landauer–Büttiker formalism, we investigate coherent spin transport through a transverse-biased magnetic zigzag-edge graphene nanoribbon, with a temperature difference applied between the source and the drain. It is shown that a critical source temperature is needed to generate a spin-polarized current due to the presence of a forbidden transport gap. The magnitude of the obtained spin polarization exceeds 90% in a wide range of source temperatures, and its polarization direction could be changed by reversing the transverse electric field. We also find that, at fixed temperature difference, the spin-polarized current undergoes a transition from increasing to decreasing as the source temperature rises, which is attributed to the competition between the excited energy of electrons and the relative temperature difference. Moreover, by modulating the transverse electric field, the source temperature and the width of the ribbon, we can control the device to work well for generating a highly spin-polarized current.

Lattice Boltzmann simulation of natural convection heat transfer of Al 2O 3/water nanofluids in a square enclosure

In the present study, mathematical modeling is performed to simulate natural convection of Al 2O 3/water nanofluids in a vertical square enclosure using the lattice Boltzmann method (LBM). Results indicate that the average Nusselt number increases with the increase of Rayleigh number and particle volume concentration. The average Nusselt number with the use of nanofluid is higher than the use of water under the same Rayleigh number. However, the heat transfer rate of the nanofluid takes on a lower value than water at a fixed temperature difference across the enclosure mainly due to the significant enhancement of dynamic viscosity. Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The present results are well validated with the works available in the literature and consequently LBM is robust and promising for practical applications.

Flow organization in two-dimensional non-Oberbeck–Boussinesq Rayleigh–Bénard convection in water

Non-Oberbeck–Boussinesq (NOB) effects on the flow organization in two-dimensional Rayleigh–Bénard turbulence are numerically analysed. The working fluid is water. We focus on the temperature profiles, the centre temperature, the Nusselt number and on the analysis of the velocity field. Several velocity amplitudes (or Reynolds numbers) and several kinetic profiles are introduced and studied; these together describe the various features of the rather complex flow organization. The results are presented both as functions of the Rayleigh numberRa(withRaup to 108) for fixed temperature difference Δ between top and bottom plates and as functions of Δ (‘non-Oberbeck–Boussinesqness’) for fixedRawith Δ up to 60K. All results are consistent with the available experimental NOB data for the centre temperatureTcand the Nusselt number ratioNuNOB/NuOB(the label OB meaning that the Oberbeck–Boussinesq conditions are valid). For the temperature profiles we find – due to plume emission from the boundary layers – increasing deviations from the extended Prandtl–Blasius boundary layer theory presented in Ahlerset al. (J. Fluid Mech., vol. 569, 2006, p. 409) with increasingRa, while the centre temperature itself is surprisingly well predicted by that theory. For given non-Oberbeck–Boussinesqness Δ, both the centre temperatureTcand the Nusselt number ratioNuNOB/NuOBonly weakly depend onRain theRarange considered here.BeyondRa≈ 106the flow consists of a large diagonal centre convection roll and two smaller rolls in the upper and lower corners, respectively (‘corner flows’). Also in the NOB case the centre convection roll is still characterized by only one velocity scale. In contrast, the top and bottom corner flows are then of different strengths, the bottom one being a factor 1.3 faster (for Δ = 40K) than the top one, due to the lower viscosity in the hotter bottom boundary layer. Under NOB conditions the enhanced lower corner flow as well as the enhanced centre roll lead to an enhancement of the volume averaged energy based Reynolds number$Re^E \,{=}\, \langle\frac{1}{2} \bu^2 \rangle^{1/2} L/\nu$of about 4% to 5% for Δ = 60K. Moreover, we find$Re^E_{\it NOB}/Re^E_{\it OB} \,{\approx}\, (\beta (T_c)/\beta (T_m))^{1/2}$, with β the thermal expansion coefficient andTmthe arithmetic mean temperature between top and bottom plate temperatures. This corresponds to the ratio of the free fall velocities at the respective temperatures. By artificially switching off the temperature dependence of β in the numerics, the NOB modifications ofReEis less than 1% even at Δ = 60K, revealing the temperature dependence of the thermal expansion coefficient as the main origin of the NOB effects on the global Reynolds number in water.

Development of an apparatus for measuring one‐dimensional steady‐state heat flux of soil under reduced air pressure

SummaryOne of the best ways to evaluate the coupled heat and mass transfer in soil is to measure the heat flux and water distribution simultaneously. For this purpose, we developed an apparatus for measuring the one‐dimensional steady‐state heat flux and water distribution in unsaturated soil under reduced air pressure. The system was tested using four samples with known thermal conductivity (0.6–8.0 W m−1 K−1). We confirmed that the system could measure the one‐dimensional steady‐state heat flux under a fixed temperature difference between ends of the samples over a wide range of thermal conductivity values. Time domain reflectometry was used to measure the water distribution with a repeatability of less than ± 1.0%. We used the apparatus to measure the soil heat flux and distribution of water content and temperature under steady‐state conditions with reduced air pressure. The initial volumetric water content, θini, of the soil samples was set at 0.20 and 0.40 m3m−3. For a θini of 0.20, the heat flux was not significantly affected by air pressure, and the water content on the hot side decreased whilst that on the cold side increased, i.e. a pronounced water content gradient was formed. For a θini of 0.40, the heat flux increased sharply with reduced air pressure, and the water content did not change, i.e. a homogeneous water distribution was observed. The increase in the heat flux with air pressure reduction is caused by the vapour transfer in soil pores. We found that a large vapour transfer took place in the soil with the homogeneous water distribution, and that the vapour transfer was less in the soil with the pronounced water content gradient. These experimental facts were entirely different from the traditional knowledge of vapour transfer in soil under temperature gradients. A lack of data on heat flux must have resulted in the previously incorrect conclusions. The new apparatus will serve to clarify the intricate phenomena of thermally induced vapour transfer in unsaturated soil in further experiments.

TruCCOMS: real-time continuous cardiac output?

Omega Critical Care has introduced the truCCOMS system to address the need for a rapid and accurate cardiac output monitor. This monitor relates blood flow to the power required to maintain a fixed temperature difference between a coil on a pulmonary artery catheter and that of the surrounding blood. The system was tested to assess their claims of accuracy and speed.