Fixed Temperature Difference Research Articles

Probing the melting dynamics in a phase change Rayleigh–Bénard system under low gravity conditions

The present work aims to investigate dynamic characteristics of Rayleigh–Bénard convection during the solid–liquid phase change process under the influence of variable low-gravity conditions. Low-gravity conditions are crucial to gain physical insights into the complex convection dynamics relevant in terrestrial and space environments that feature diverse gravitational fields. The melting dynamics of a paraffin-based phase change material with Prandtl number Pr ≈ 71 in a square rigid walled enclosure is analyzed by subjecting it to a fixed temperature difference resulting in a Stefan number Ste ≈ 0.33. The enthalpy porosity method is employed to simulate the solid-liquid phase change process in Rayleigh number range 105<Ra <107 to take wide range of gravitational acceleration (0.0g, 0.1g, 0.2g, 0.4g, 0.6g, 0.8g, and g) into account. The simulation results allow detailed insights on the buoyancy-driven convective flow phenomena in a phase-change Rayleigh–Bénard system. Various characteristic flow regimes, the criteria for the onset of convective instabilities, and scaling analysis are presented under varied gravitational acceleration. The findings reveal that the critical Rayleigh number (Racr) for the onset of convection is found to vary between 1351.5 and 1810.5 under the gravity range investigated in contrast to the Racr ≈ 1707.7 for classical Rayleigh-Bénard system under the standard gravity. Furthermore, the scale analysis based on the numerical results shows that the relationship between effective Nusselt number (Nue) and effective Rayleigh number (Rae) follows a power law behavior Nue = 0.27Rae0.25 under each gravity condition, similar to the classical Rayleigh–Bénard system.

Thermoelectrohydrodynamic convection in a finite cylindrical annulus under microgravity

Numerical simulations of thermoelectrohydrodynamic convection in a dielectric liquid inside a finite-length cylindrical annulus with a fixed temperature difference have been performed with increasing high-frequency electric tension under microgravity conditions. The electric field, coupled with the permittivity gradient, generates a dielectrophoretic buoyancy force whose non-conservative part can induce thermoelectric convection in the liquid. The liquid remains in a conductive state below a critical value of the applied electric voltage. At a critical value, a supercritical bifurcation occurs from the conductive state to a convective state made of stationary helicoidal vortices. A further increase of electric voltage leads to oscillatory helicoidal vortices and then to wavy patterns before spoke patterns dominate the convective flow. The dielectrophoretic force is shown to enhance the heat transfer from the hot to cold walls due to induced convective flows. Particularly, these results demonstrate that the dielectrophoretic buoyancy force holds promise to replace the gravitational force to induce efficient heat transfer in microgravity conditions, and they contribute to a better fundamental understanding of heat transfer in microgravity.

Wetsuit Thermal Resistivity Measurements.

In recent years, attention to the realization and characterization of wetsuits for scuba diving and other sea sports or activities has increased. The research has aimed to establish reliable and standardized measurement methods to objectively assess wetsuit quality, particularly focusing on their mechanical and thermal properties. In this work, we describe and compare two different measurement methods for the characterization of neoprene wetsuit thermal resistivity. The first method follows the existing regulations in the field, while the second one, which we are originally proposing in this paper, offers an alternative yet accurate way based on a simplified experimental set-up and easier measurements. In both cases, the wetsuit sample under testing was shaped in the form of a cylindrical sleeve of proper dimensions and wrapped around a phantom containing water at a higher temperature and surrounded by water at a lower temperature. The wetsuit's cylindrical surface allows heat flow from the warmer water on the inside to the colder water on the outside through the wetsuit area. In the first case, a thermal steady state was achieved, with constant heat flow from the phantom to the exterior. This was obtained with a power balance between two homogenous quantities. Electrically supplied thermal heating within the phantom was used to balance the thermal energy naturally flowing through the wetsuit's surface. In this first case, a stable and fixed temperature difference was obtained between the inner and the outer surfaces of the wetsuit sample. In the second case, a thermal transient was analyzed during the cooling process of the phantom, and the thermal time constant was measured, providing the sample thermal resistance once the phantom thermal capacity was known. In both cases and methods, the heat flow and thermal resistance of other elements than the wetsuit must be evaluated and compensated for if they are not negligible. Finally, the thermal resistivity per unit area of the wetsuit material was obtained with the product of the wetsuit sample's thermal resistance and the wetsuit area. The measurements, conducted until now by immersing the phantom in a free surface tank, show that both methods-under stationary and under transient temperature conditions-were valid to assess the wetsuit's thermal resistivity. The stationary method somehow provided better accuracy while involving less well-known parameters but at the expense of a more complicated experimental set-up and additional energy consumption. The transitory method, on the other hand, is quite easy to implement and, after careful characterization of the phantom's parameters, it provided similar results to the stationary one. An uncertainty budget was evaluated for both methods, and they did provide highly compatible measurement results, with resistivity values of 0.104(9) m2·K/W (stationary method) and 0.095(9) K·m2/W (transient method) for the same wetsuit sample under testing, which is also consistent with the values in the literature. We finally propose that the novel method is a valid alternative for characterization of the thermal insulation properties of a scuba diving wetsuit.

Nonlinear Seebeck and Peltier effects in a Majorana nanowire coupled to leads

Abstract We theoretically study nonlinear thermoelectric transport through a topological superconductor nanowire hosting Majorana bound states (MBSs) at its two ends, a system named as Majorana nanowire (MNW). We consider that the MNW is coupled to the left and right normal metallic leads subjected to either bias voltage or temperature gradient. We focus our attention on the sign change of nonlinear Seebeck and Peltier coefficients induced by mechanisms related to the MBSs, by which the possible existence of MBSs might be proved. Our results show that for a fixed temperature difference between the two leads, the sign of the nonlinear Seebeck coefficient (thermopower) can be reversed by changing the overlap amplitude between the MBSs or the system equilibrium temperature, which are similar to the cases in linear response regime. By optimizing the MBS–MBS interaction amplitude and system equilibrium temperature, we find that the temperature difference may also induce sign change of the nonlinear thermopower. For zero temperature difference and finite bias voltage, both the sign and magnitude of nonlinear Peltier coefficient can be adjusted by changing the bias voltage or overlap amplitude between the MBSs. In the presence of both bias voltage and temperature difference, we show that the electrical current at zero Fermi level and the states induced by overlap between the MBSs keep unchanged, regardless of the amplitude of temperature difference. We also find that the direction of the heat current driven by bias voltage may be changed by weak temperature difference.

Lorenz model of instability in porous media for van der Waals gas

The article is devoted to the study of the nonlinear instability of the van der Waals gas in a gap with a porous medium of finite thickness heated from below. A fixed temperature difference is set between the upper cold surface and the lower hot one. The Lorenz approach was used to solve the problem. During a numerical solution, criteria for monotonic instability Racr1 and oscillating instability Racr2 were obtained. The results of calculations of the criterion Racr3 for the appearance of a strange attractor in the phase space are presented, which can be interpreted as a criterion for the generation of undamped turbulent fluctuations. The nature of the dependences of the critical Rayleigh numbers on the physical properties of the gas, the porosity of the medium, and the parameters of the van der Waals equation of state Waa and Wab is analyzed.The phenomenon of instability (formation of vortices in liquids or gases) in the space between two horizontal boundaries, when the lower one is heated, is widespread in technical applications. Lorenz developed a non-linear approach to the study of this phenomenon. This approach makes it possible to determine the criteria for the emergence of vortices and their transformation. This article uses the Lorenz approach to obtain instability criteria for a real gas whose properties are described by the van der Waals equation of state. This gas is in a porous medium. This combination of gas and medium has a wide technological application in the food, chemical and other industries. That is why the present study focused on this problem.

High-Performance Wearable Bi2Te3-Based Thermoelectric Generator

Wearable thermoelectric generators (w-TEGs) convert thermal energy into electrical energy to realize self-powering of intelligent electronic devices, thus reducing the burden of battery replacement and charging, and improving the usage time and efficiency of electronic devices. Through finite element simulation, this study successfully designed high-performance thermoelectric generator and made it into wearable thermoelectric module by adopting “rigid device—flexible connection” method. It was found that higher convective heat transfer coefficient (h) on cold-end leads to larger effective temperature difference (ΔTeff) and better power generation performance of device in typical wearable scenario. Meanwhile, at same h on the cold-end, longer TE leg length leads to larger ΔTeff established at both ends of device, larger device output power (Pout) and open-circuit voltage (Uoc). However, when the h increases to a certain level, optimization effect of increasing TE leg length on device power generation performance will gradually diminish. For devices with fixed temperature difference between two ends, longer TE leg length leads to higher resistance of TEGs, resulting in lower device Pout but slight increase in Uoc. Finally, sixteen 16 × 4 × 2 mm2 TEGs (L = 1.38 mm, W = 0.6 mm) and two modules were fabricated and tested. At hot end temperature Th = 33 °C and cold end temperature Tc = 30 °C, the actual maximum Pout of the TEG was about 0.2 mW, and the actual maximum Pout of the TEG module was about 1.602 mW, which is highly consistent with the simulated value. This work brings great convenience to research and development of wearable thermoelectric modules and provides new, environmentally friendly and efficient power solution for wearable devices.

Majorana tunneling in a non-Hermitian double-quantum-dot structure

In this paper, we investigate a non-Hermitian quantum transport system, which is formed by two quantum dots possessing PT-symmetric complex potentials and side coupled to Majorana fermions. We study the transmission coefficient first. It is found that the conductance is reduced by a factor of 1/4, i.e., G=e2/4h, which can be seen as a signature of Majorana fermion in this double quantum dots structure. It is also found that the joint effects of Majorana tunneling, PT-symmetric complex potentials and dot energy level are mainly as follows: merging process, asymmetry and exceptional point. Then we study the current and conductance driven by a voltage or temperature gradient. Due to the merging process and asymmetry, the thermal current and conductance change sign drastically near the exceptional point. With large fixed temperature difference, the relationship between thermal current and dot level becomes linear, which is a manifestation of Majorana fermion’s robustness. These results can be useful in understanding the quantum transport behaviors modified by the interplay between PT symmetry and Majorana fermions.

Design and Research of Thermoelectric Generator Simulation System for Boiler Flue Gas Waste Heat

One of the significant factors contributing to high energy consumption is the unutilized waste heat from flue gas in industrial boilers. Thermoelectric generator (TEG) technology can directly convert thermal energy into electrical energy, and has been gradually applied in the field of waste heat recovery due to its simple and reliable structure, environmental protection, and other advantages. In this paper, a thermoelectric generator simulation system of boiler flue gas waste heat is proposed. The experimental platform is designed by simulating the flue gas waste heat temperature condition of boiler, and the structure of cold end module and hot end module is optimized. During the experiment, the fixed temperature difference was set at 120 °C (hot end:150 °C~cold end: 30 °C). An analysis is conducted on the volt-ampere characteristics and output power of the TEG module. The output characteristics of the TEG system are analyzed under the conditions of variable load, constant load, different pump speed, different heat dissipation modes, and series and parallel connection method. The results show that the experimental platform can instantaneously and accurately test the output parameters of the TEG system, and ensure the intended design requirements. When the ratio of the load resistance to the internal resistance of the TEG module is approximately 1–1.15, the output power of the system reaches its maximum. In order to optimize the output power of the TEG system, a power prediction-based adaptive variable step size maximum power point tracking (MPPT) algorithm is introduced. Additionally, a corresponding mathematical model is formulated. Simulations demonstrate that the time of the improved algorithm to reach the stable maximum power point is 1.54 s faster than that of the traditional algorithm. The improved MPPT algorithm satisfies the criteria for speed and accuracy, diminishes superfluous energy waste, and enhances the overall system efficiency. The research results have certain guiding significance for the design and application of subsequent TEG system.

Thermal currents obtained and mutually switched by a modified Haldane model in graphene

By using the transfer matrix method, we discover three types of current, such as the 100% spin-valley polarized current, pure spin-valley current and pure charge current, in a two-terminal graphene system. These types of current can be obtained and mutually switched by modulating the parameters of the modified Haldane model (MHM). In our work, these types of current are driven by the thermal bias. Compared with this method of increasing the one-lead temperature (with a fixed temperature difference), the thermal currents can be more effectively strengthened by increasing the temperature difference (with a fixed one-lead temperature). In order to rapidly turn off these currents, we choose to enhance the intensity of the off-resonant circularly polarized light instead of canceling the temperature difference. These results indicate that the graphene system with the MHM has promising applications in the spin and valley caloritronics.

Comparative analysis of means to control the thermal regime of a cooling thermoelement while minimizing the set of basic parameters

This paper reports a comparative analysis of the thermal regime control means while minimizing a set of basic parameters in various combinations with the indicators of reliability and dynamics of the functioning of a single-stage thermoelectric cooler. The connection has been established between the optimal relative operating current corresponding to the minimum of the set on the relative temperature difference and heat sink capacity of the radiator. The results of calculating the main parameters, reliability indicators, time of entering the stationary mode of operation for various current modes of operation at a fixed temperature difference, thermal load at different geometry of the branches of thermoelements are given. A comparative analysis of the main parameters, indicators of the reliability and operational dynamics of a single-stage cooler under various characteristic current modes of operation has been carried out. Minimizing the set of basic parameters in conjunction with the reliability indicators and operational dynamics of the cooling thermoelement provides a decrease in the refrigeration coefficient up to 40 % compared to the maximum cooling capacity mode, as well as the optimal heat sink capacity of the radiator, the amount of energy expended, the time of entering the stationary mode, the relative intensity of failures. The analysis of the influence of the temperature difference at a predefined thermal load on the relative operating current, the time it takes for the cooler to enter the stationary thermal regime, the heat sink capacity of the radiator, the relative intensity of failures has been performed. The devised method of optimal control over the thermal regime of a single-stage thermoelectric cooler based on minimizing the set of basic parameters makes it possible to search for and select compromise solutions, taking into consideration the weight of each of the limiting factors

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

A robust type‐2 fuzzy logic‐based maximum power point tracking approach for thermoelectric generation systems

A robust type-2 fuzzy logic (FL)-based approach for maximum power point tracking (MPPT) is proposed in this work. The method is used to increase the energy efficiency of thermoelectric generators (TEGs). Type-2 FL was applied to adapt the variable step size of incremental resistance (INR) MPPT to track the maximum available power of a TEG. The output power from a TEG relies mostly on the difference in temperature of its two sides added to the load value. Consequently, MPPT must be robust to extract the optimum operation point continually under varying operational conditions. The aim of the suggested approach is to enhance the dynamic response and eradicate fluctuations around the maximum power point (MPP). The results employing the type-2 FL are compared with conventional methods including INR, perturb and observe (P&O), and type-1 FL. With a variable load (15, 20, and 25 Ω), the proposed approach takes around 7 ms to reach a steady state with 2.6, 3.9, and 5.2 W overshoot, respectively, and almost zero oscillation. With a fixed load and a fixed temperature difference, our proposed tracker decreases the response time by 35.84%, 45.27%, and 96.50% compared to INR, P&O, and conventional FL, respectively. With a fixed load and a varying temperature difference, the proposed tracker decreases the response time by 53.33%, 94.07%, and 96.53% compared to INR, P&O, and conventional FL, respectively. The results confirmed the ability of the proposed method to keep the conversion efficiency of TEGs high and stable, reducing energy loss.

A Novel Method for Designing Fifth-Generation District Heating and Cooling Systems

District heating and cooling systems have been undergoing continuous development and have now reached the fifth-generation. In this innovative technology, connected buildings share local excess energy that otherwise would be wasted, which consequently reduces primary energy demands and carbon emissions. To date, the issue of implementing fifth-generation district systems on existing buildings has received scant attention, and our research addresses this challenging gap by proposing a novel method for designing these systems. We first explain the possible thermal interactions between connected buildings, and then present an analytical solution for the network energy balance, pipe design, and the prediction of fluid temperature under a fixed temperature difference control strategy. The analytical solution was validated against numerical simulations performed on 11 existing buildings located in Lund, Sweden using Modelica models. A diversity index metric between heating and cooling demands was also included in these models to assess the efficiency of the district system in the building cluster. The results from the analytical and numerical solutions were in complete agreement since Modelica is an equation-based modelling language. The developed models pave the way towards future investigations of different temperature control strategies and new business models that arise from the shift to the fifth-generation.

Thermal rectification enhancement based on porous structure in bulk materials

Thermal rectification effect refers to an asymmetric heat transfer phenomenon (namely, the amount of heat flux depends on the direction of temperature gradient). A two-segment bar made of two materials that have thermal conductivities with different temperature-dependence, can realize the thermal rectification effect. In the present paper, we propose to use porous structure on the bulk material to modify the thermal conductivity of bulk material. It is found that the thermal rectification effect can be enhanced by the porous structure. The finite element method and effective medium approximation are used to analyze the influence of porosity on the thermal rectification ratio of the two-segment system. The calculation results are consistent with each other. Under low temperature bias, the effect of the porosity is weak, while its influence becomes very significant when the temperature difference is high. Usually, thermal rectification ratio decreases if the porous structure is made on the segment whose thermal conductivity increases with temperature increasing. If the porous structure is made on the segment with negative temperature-dependent thermal conductivity, an optimal porosity can be found. For low porosity, the forward heat flux keeps almost unchanged while the reverse heat flux decreases by more than half, and the thermal rectification ratio can be increased to twice or more than thrice that in the case of no porous structure. For a fixed temperature difference, the influence of porosity on the thermal rectification ratio increases with the augment of the power exponent value.

Columnar vortices induced by dielectrophoretic force in a stationary cylindrical annulus filled with a dielectric liquid

Abstract

Enhancing generation of green power from the cold of vaporizing LNG at 30 bar by optimising heat exchanger surface area in a multi-staged organic Rankine cycle

Liquefied natural gas (LNG) is warmed by seawater in a regasification terminal and the cold energy is usually wasted. Organic Rankine cycle (ORC) has emerged as the preferred thermodynamic cycle to extract green power from LNG using the temperature difference existing between LNG and the environment. The design of ORC based on a fixed temperature difference for all heat exchangers is proposed to be replaced with optimum temperature differences. Simulation with Aspen HYSYS 8.6® shows that the total surface area as obtained with a fixed temperature difference can be optimally redistributed among the heat exchangers to increase extraction of power. A three-stage cascaded ORC with ethane, ethane and propane as the working fluids generated an additional power of 123 kW with LNG vaporising at 30 kg per second without any additional surface area of heat exchangers. The paper further shows that for a delivery pressure at 6 bar(a), additional power of 630 kW can be generated from LNG vaporising at 30 bar(a) and thereafter expanded to 6 bar(a) in a turbine, instead of extracting power from 6 bar(a) LNG. The analysis shows that the third stage generates only 10% and the other two stages contribute equally to power generation.

Convective Turbulence in a Cubic Cavity under Nonuniform Heating of a Lower Boundary

The problem of heat transfer intensification using spatially nonuniform boundary conditions is of great interest. In this paper, the influence of a nonuniform, nonperiodic distribution of heating on the flow structure and convective heat flux for developed turbulent regimes (Ra = 1.1 × 109) is studied. The numerical simulation of convective turbulence in a cubic cavity is performed for a nonuniform heating distribution at the lower boundary using open-source software the OpenFoam 4.1. Calculations were carried out for three types of the temperature distribution: localized heating at the center of the lower boundary; nine heaters of the same size, equidistant from each other; and fractal heating. The heating area is the same in all of these distributions. It is shown that in all cases of a nonuniform heating distribution in the cavity a large-scale circulation is formed. The dynamics and structure of large-scale circulation depend on the temperature distribution at the lower boundary of the cavity. Spontaneous reorientations of the large-scale circulation plane by angles of ±45° or ±90° are revealed. A comparison of the intensity of the heat flux through the layer (cavity) at a fixed temperature difference at horizontal boundaries is made. It is shown that the heat transfer intensity weakly depends on the temperature distribution on the lower boundary. The maximum difference in the Nusselt numbers for three types of the nonuniform temperature distribution did not exceed 5%. Comparison of numerical simulation results for the uniform and nonuniform heating distributions at Ra = 1.1 × 109 shows that a decrease in the heating area by 70% leads to a decrease in the Nusselt number by 10%. It is shown that the heat flux also decreases with decreasing heating area, at fixed temperatures in the heating and cooling regions, not proportionally to the change in the area. For practical applications, an important role is played by the stability of the heat flux, which is characterized by the absence of pulsations. It is shown that the use of fractal heating can significantly reduce the level of heat flux pulsations without losses in the efficiency of heat transfer.

Novel operation strategy for a gas turbine and high-temperature KCS combined cycle

This paper designs a high-temperature Kalina cycle system (KCS) to recover a gas turbine (GT) waste heat. A novel methodology of predicting off-design performance of GT-KCS combined cycle is presented by introducing geometric parameters of heat exchangers. The inlet guide vane of GT compressor operation strategy is applied in GT. Novel operation strategies named modified sliding pressure operation (MSPO) and novel modified sliding pressure operation (NMSPO) are proposed for KCS. The MSPO uses a fixed KCS turbine inlet temperature and an optimized ammonia concentration at KCS separator inlet under GT part-load conditions, while the NMSPO adopts an optimized ammonia concentration at KCS separator inlet and a fixed temperature difference between KCS turbine inlet and GT exhaust. The results show that the optimal ammonia concentrations at KCS separator inlet under the MSPO and NMSPO decrease with declining GT load. The MSPO has the highest bottoming KCS thermal efficiency among the sliding pressure operation (SPO), MSPO and NMSPO. The NMSPO recovers the most heat from GT exhaust, resulting in the highest KCS net power and GT-KCS thermal efficiency. The NMSPO recommended as the optimal operation strategy could produce 20.72 kW and 25.75 kW more net power than the MSPO and SPO, respectively.

A new process for the determination of the Soret coefficient of a binary mixture under microgravity

In the presence of a gravity field or under microgravity, pure thermo-diffusion leads to very weak species separation in binary mixtures. To increase the species separation in the presence of gravity, many authors use thermo-gravitational diffusion in vertical columns (TGC). For a given binary mixture, the species separation between the top and the bottom of these columns depends on the temperature difference, ΔT, imposed between the two vertical walls facing each other, and the thickness, H, between these two walls (annular or parallelepipedic column). These studies show that, for a fixed temperature difference, the species separation is optimal for a thickness, Hopt, much smaller than one millimetre. The species separation decreases sharply when the thickness H decreases with respect to this optimum value. It decreases progressively as H increases with respect to Hopt. In addition, for mixtures with a negative thermo-diffusion coefficient, the heaviest component migrates towards the upper part of the column and the lightest one towards the lower part. The loss of stability of the configuration thus obtained leads to a brutal homogeneity of the binary solution.The objective of this study in microgravity was to increase the optimum of species separation. For this purpose, the binary fluid motion was provided by uniform velocities imposed on the two walls of the cavity facing each other. This forced flow led to species separation between the two motionless walls of the cavity. In this case, the fluid motion generated in the cavity was not dependent on the imposed temperature difference, ΔT contrarily to the case of thermogravitational column. Under these conditions and for a given column of thickness H, there are three independent control parameters: ΔT and the two velocities of the walls facing each other. Using the parallel flow approximation for a cell of large aspect ratio, the velocity, temperature and mass fraction fields within the cavity were determined analytically. Thus the parameters leading to optimal species separation were calculated. The analytical results were corroborated by direct numerical simulations. The present paper thus proposes a new process for the determination of the Soret coefficient, the thermodiffusion coefficient and mass-diffusion coefficient.

MHD and heat transfer analyses of a fluid flow through scraped surface heat exchanger by analytical solver

Scraped surface heat exchangers (SSHEs) diversely used in various industries for different settings when continuous processing of fluid and related material is required. In this work, heat transfer for steady incompressible flow of a Newtonian fluid in the presence of high magnetic field with temperature variant viscosity in a small gap of SSHE is studied for a fixed temperature difference narrow gaps between rotor and stator. The fluidic system is modeled mathematically in terms of PDEs for non-isothermal megnetohydrodynamic (MHD) flow with temperature dependent viscosity for linear blade shape. The solution of modeled PDEs are obtained analytically by exploiting the strength of Adomian decomposition method (ADM). The effects of different flow parameters of the systems are studied and their results are presented with analytical expressions and graphical illustrations.