Constant Heat Flow Conditions Research Articles

High-performance stretchable thermoelectric generator using serpentine interconnects encapsulated in an ultrasoft silicone sponge

Stretchable thermoelectric generators (S-TEGs) have the potential to utilize waste heat from sources with complex and dynamic surfaces. However, their thermoelectric performances are still lower than those of conventional hard and rigid TEGs and are easily degraded by large or cyclic deformations due to electrical failure. An approach that improves both stretchability and thermoelectric performance is required. This study presents and explores the improvements enabled by an ultrasoft silicone sponge encapsulation for S-TEGs using silicone-encapsulated serpentine interconnects for the internal electrical wiring of the bismuth-telluride-based thermoelectric elements. The ultrasoft silicone sponge is characterized by a low Young’s modulus (0.01 MPa) and low thermal conductivity (0.08 W m−1 K−1) owing to its open-cell structure. We consider that the low Young’s modulus decreases the internal stress in the interconnects under deformation and that the low thermal conductivity increases the temperature differences in the thermoelectric elements under constant heat flow conditions. We fabricated S-TEGs with three different silicone encapsulations: hard and soft silicones, as used in previous studies, and an ultrasoft silicone sponge. We experimentally measured the elongation and cycle number to failure for stretchability evaluation as well as the open-circuit voltage and maximum power for thermoelectric performance evaluation. Thus, the S-TEG with the ultrasoft silicone sponge encapsulation showed both the highest stretchability (125% elongation to failure) and thermoelectric performance (1.80 μW cm−2 maximum power per unit area on a heater at 100 °C under natural air convection). Additionally, the S-TEG showed 153 μW cm−2 maximum power per unit area on a heater at 100 °C under water cooling, and comparisons with existing S-TEGs confirm that the proposed S-TEG achieves improved stretchability and relatively high output power.

Numerical investigation of a boundary layer water-based nanofluid flow with induced magnetic field

The investigation concerns the induced magnetic field influence on the hydrodynamics and thermal build-up of water-based nanofluids containing nanoparticles in the non-isothermal and constant heat flux limits. Partial differential equations for the Boundary Layer Flow (BLF) are formulated and non-dimensionalized. Appropriate similarity variables are further introduced to arrive at a system of coupled nonlinear differential problems. Symbolic solutions are presented graphically and explained for various governing parameters. From the result, the thermal boundary layer is seen to be on the increase with increasing values of the fluid Prandtl number for both non-isothermal and constant heat flow conditions.

Investigation of effects on heat transfer and flow characteristics of Cr-Ni alloy and aluminum pins placed in AISI 304 tube

In this study, the effects of cylindrical aluminum and Cr-Ni alloy pins placed in different arrangements on the inner wall of the pipe in the turbulent flow, the effects of heat transfer and flow characteristics on different Reynolds numbers have been experimentally investigated. The experiments were carried out under forced flow and constant heat flow conditions. Air is preferred as the fluid and the fluid velocity is adjusted between Reynolds number of 10000 and 50000. It has been observed that the Nusselt values obtained over the number of Reynolds number for the 5 different test tubes are arranged in a line from large to small, sequential row aluminum pin, sequential row Cr-Ni alloy pin, diagonal row aluminum pin, diagonal row Cr-Ni alloy pin, plain tube. There are also CFD analysis for each material, arrangements and pins geometry sets. On the other hand, it was determined that friction coefficient is directly proportional to the increase of heat transfer coefficient. As a result, it is observed that experimental results are compatible with both literature and numerical study.

Romelt Process Production Indices with Partial Replacement of Coal by Natural Gas

Results are provided for calculation of the material and heat balances of cast iron smelting in a Romelt furnace using coal with a different fixed carbon content with addition of natural gas to the blast of the lower tuyeres. It is shown that under constant heat flow conditions due to gas combustion in the slag bath an increase in natural gas consumption is accompanied by a reduction in coal consumption. In this case saving coal does not depend on its form and is proportional to natural gas consumption. An increase in natural gas consumption is accompanied by a reduction in oxygen consumption in the lower tuyeres and an increase in the upper tuyeres. Saving oxygen in the Romelt process is at a maximum for coal with a low fixed carbon content. It decreases as there is an increase in fixed carbon content and with ≈ 75% Cfix saving oxygen is replaced by a small over-consumption.

Equivalent Electrical Circuits of Thermoelectric Generators under Different Operating Conditions

Energy harvesting has become a promising and alternative solution to conventional energy generation patterns to overcome the problem of supplying autonomous electrical systems. More particularly, thermal energy harvesting technologies have drawn a major interest in both research and industry. Thermoelectric Generators (TEGs) can be used in two different operating conditions, under constant temperature gradient or constant heat flow. The commonly used TEG electrical model, based on a voltage source in series with an electrical resistance, shows its limitations especially under constant heat flow conditions. Here, the analytical electrical modeling, taking into consideration the internal and contact thermal resistances of a TEG under constant temperature gradient and constant heat flow conditions, is first given. To give further insight into the electrical behavior of a TEG module in different operating conditions, we propose a new and original way of emulating the above analytical expressions with usual electronics components (voltage source, resistors, diode), whose values are determined with the TEG’s parameters. Note that such a TEG emulation is particularly suited when designing the electronic circuitry commonly associated to the TEG, to realize both Maximum Power Point Tracking and output voltage regulation. First, the proposed equivalent electrical circuits are validated through simulation with a SPICE environment in static operating conditions using only one value of either temperature gradient or heat flow. Then, they are also analyzed in dynamic operating conditions where both temperature gradient and heat flow are considered as time-varying functions.

A Novel Identification Method of Thermal Resistances of Thermoelectric Modules Combining Electrical Characterization Under Constant Temperature and Heat Flow Conditions

The efficiency of a Thermoelectric Module (TEM) is not only influenced by the material properties, but also by the heat losses due to the internal and contact thermal resistances. In the literature, the material properties are mostly discussed, mainly to increase the well-known thermoelectric figure of merit ZT. Nevertheless, when a TEM is considered, the separate characterization of the materials of the p and n elements is not enough to have a suitable TEM electrical model and evaluate more precisely its efficiency. Only a few recent papers deal with thermal resistances and their influence on the TEM efficiency; mostly, the minimization of these resistances is recommended, without giving a way to determine their values. The aim of the present paper is to identify the internal and contact thermal resistances of a TEM by electrical characterization. Depending on the applications, the TEM can be used either under constant temperature gradient or constant heat flow conditions. The proposed identification approach is based on the theoretical electrical modeling of the TEM, in both conditions. It is simple to implement, because it is based only on open circuit test conditions. A single electrical measurement under both conditions (constant-temperature and constant-heat) is needed. Based on the theoretical electrical models, one can identify the internal and thermal resistances.

Fully Electrical Modeling of Thermoelectric Generators with Contact Thermal Resistance Under Different Operating Conditions

The Seebeck effect is used in thermoelectric generators (TEGs) to supply electronic circuits by converting the waste thermal into electrical energy. This generated electrical power is directly proportional to the temperature difference between the TEG module’s hot and cold sides. Depending on the applications, TEGs can be used either under constant temperature gradient between heat reservoirs or constant heat flow conditions. Moreover, the generated electrical power of a TEG depends not only on these operating conditions, but also on the contact thermal resistance. The influence of the contact thermal resistance on the generated electrical power have already been extensively reported in the literature. However, as reported in Park et al. (Energy Convers Manag 86:233, 2014) and Montecucco and Knox (IEEE Trans Power Electron 30:828, 2015), while designing TEG-powered circuit and systems, a TEG module is mostly modeled with a Thevenin equivalent circuit whose resistance is constant and voltage proportional to the temperature gradient applied to the TEG’s terminals. This widely used simplified electrical TEG model is inaccurate and not suitable under constant heat flow conditions or when the contact thermal resistance is considered. Moreover, it does not provide realistic behaviour corresponding to the physical phenomena taking place in a TEG. Therefore, from the circuit designer’s point of view, faithful and fully electrical TEG models under different operating conditions are needed. Such models are mainly necessary to design and evaluate the power conditioning electronic stages and the maximum power point tracking algorithms of a TEG power supply. In this study, these fully electrical models with the contact thermal resistance taken into account are presented and the analytical expressions of the Thevenin equivalent circuit parameters are provided.

Measurements of void fraction spatial distribution in pool boiling

A series of measurements of local vapour void fraction distribution in saturated water above the surface of a horizontal heated plate is presented. The amount of vapour was measured using a saphire optical probe which was allowed to move above the heated section. The experiments were performed at constant heat flow conditions and during slow power ramps. For high heat fluxes the void fraction as a function of the distance from the heater presents a maximum near the wall. The results of the present work confirm the general observations recently reported for similar systems.