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Thermoelectric generators (TEGs) demonstrate great potential for flexible and wearable electronics due to the direct electrical energy harvested from waste heat. Good wearability requires high mech...

Highly foldable and flexible energy materials are a requisite for actual applications in self-powered wearable electronics and complex environments. Unfortunately, thermoelectric (TE) materials showing excellent foldability and flexibility under external mechanical stresses have rarely been reported so far. Herein, foldable and flexible TE materials based on Chinese traditional Xuan paper (a bio-based material) and commercialized conducting polymer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) are reported. With a facile dip-coating and subsequent drying process, PEDOT:PSS is wrapped on the surfaces of cellulose fibers of Xuan paper via hydrogen-bond interactions. The resulting composite films exhibit outstanding mechanical and decent TE properties. In particular, the PEDOT:PSS/Xuan paper composite films exhibit excellent foldability and flexibility, and can be easily processed into various shapes with no distinct damage to their electrical conductivity and TE performances. This work paves the way for imparting TE performance to insulating paper-based materials, and the unique foldability may widen the TE applications in complex and extreme environments.

Organic polymer thermoelectrics (TEs) that can realize direct heat-to-electricity conversion hold great potential in flexible and wearable applications and thus are receiving tremendous attention. Constructing polymer-based nanocomposites represents an effective approach in achieving high TE performance, while current studies on the underlying mechanisms for the improvement of TE properties in aspects of interfacial nanostructures are insufficient. In this work, flexible ternary nanocomposite films with unique interfacial architectures are developed by sequential electrochemical polymerization of conducting polymers and subsequent anion treatment. The optimized interfacial architectures contribute to enhanced π electron conjugation, which facilitates interfacial charge transfer and favours large-area charge transport. The anion treatment further enables the molecular chains to arrange in a more ordered configuration, leading to improved carrier mobility. As a result, the nanocomposites exhibit high power factors of more than 500 μW m−1 K−2 that outperform most of the literature-reported peer composites. The feasible interfacial architecting and anion treatment methods proposed in this study demonstrate high potential in designing high-performance TE nanocomposites.

The design of effective energy systems is crucial for the development of flexible and wearable electronics. Regarding direct converting heat into electricity, thermoelectrochemical cells (TECs) are particularly suitable for low-grade heat harvesting to enable flexible and wearable applications, despite the fact that the electrolyte leakage and complex packaging issues of conventional liquid-based TECs await to be further addressed. Herein, a quasi-solid-state TEC is assembled from polyacrylamide/acidified-single-walled carbon nanotubes (PAAm/a-SWCNTs) composite hydrogel, developed via a facile in situ free-radical polymerization route with tin (IV) chloride/tin (II) chloride (Sn4+/Sn2+) as a redox couple. The as-fabricated TEC with 0.6 wt% a-SWCNTs content presents a large thermoelectrochemical Seebeck coefficient of 1.59 ± 0.07 mV K-1, and exhibits excellent stability in thermoelectrochemical performance against large mechanical stretching and deformation. Owing to this superior stretchability, the as-fabricated TEC is further assembled into an energy-autonomous strain sensor, which shows high sensitivity. The strategy of utilizing quasi-solid-state TEC for energy-autonomous strain sensing unveils the great potential of heat-to-electricity conversion in flexible and wearable electronics.

Heat transfer of in-tube supercritical fluid cooling accompanying out-tube pool boiling was investigated. A smooth horizontal circular tube with an inside-diameter of 20 mm was submerged in a water pool at atmospheric pressure. Test parameters of in-tube were as follows: Pressure: 23–28 MPa, mass flux: 600–1000 kg·m−2·s−1, fluid temperature: 400–725 K, and the temperature difference between bulk and wall: 300–374 K. A thermal amplification system based on out-tube pool boiling was used to improve the measurement accuracy of local heat duty near pseudocritical region. According to the experiment, the transition from nucleate boiling to film boiling in the pool occurred near the pseudo-critical fluid region. Sharp variation on thermo-physical properties led to the peak value of heat transfer coefficient in the pseudo-critical region. The pool boiling heat flux increased gradually to 1.19 MW·m−2 near the pseudocritical point. Based on the experimental data, a modified Gnielinski equation was adopted to predict the heat transfer coefficient of in-tube supercritical fluid cooling with out-tube pool boiling. The thermophysical property ratio of the wall to the bulk as well as the effect of buoyancy were taken into consideration in the new correlation. The predicted correlation has an error of less than 20% to the experimental data.

Although conjugated polymer/carbon nanotube (CNT) thermoelectric (TE) composites have witnessed explosive achievements in recent years, the in-depth mechanism of CNT network evolution and its influence on composite TE performance are still unclear. Herein, we systematically investigate the evolution of single-walled CNT (SWCNT) network structure with a wide SWCNT content range for poly (3,4-ethyienedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/SWCNT composites based on TE properties and atomic force microscopic (AFM) observations. The whole curves of TE performance can be divided into three regions with the increase of SWCNT loading. In Region I (

The experiment was conducted by immersing a smooth horizontal tube in a pool tank to simulate the flow condition of Passive Residual Heat Removal System (PRHRS) in a SuperCritical Water-cooled Reactor (SCWR). Hydraulic resistance and friction factor of in-tube cooling supercritical water accompanying out-tube pool boiling were investigated in this study with test pressure ranging from 23 to 28 MPa and mass flux ranging from 600 to 1000 kg·m−2·s−1. The influence of pressure and mass flux on pressure drop in adiabatic and cooling flows was analyzed. This paper also discussed the effect of deceleration in the cooling flow and assessed various friction-factor correlations by employing the experimental data. Results showed that the friction factor of the adiabatic flow exists a steep “pit” approaching to pseudocritical region. A noticeable “Λ-shaped” profile was observed in the vicinity of the pseudocritical temperature, due to deceleration-effect of frictional pressure drop in the cooling flow. The deceleration factor of supercritical cooling flow led to the axial fluid element shrinkage and radial bulk fluid velocity parabolic distribution. Taking into account the effect of deceleration-factor, a modified correlation was proposed for in-tube cooling supercritical water accompanying out-tube pool boiling, of which the average error and root mean square error are −2.51% and 15.28% respectively.

In order to investigate the water cooling technology for the divertor in the International Thermonuclear Experimental Reactor (ITER), heat transfer tests of subcooled water flow boiling in a circular channel with a twisted tape (TT) insert under high and non-uniform fluxes have been performed at Xi’an Jiaotong University. The vertically upward channel was off-center in a nickel rectangular block and the twist ratio of the twisted tape is 2. Experimental parameters covered the pressure range of 3–5 MPa, mass flux range of 3000–8000 kg m−2 s−1, the inlet fluid temperature of 40–220 °C and the average heat flux on the inner wall surface of 2–5 MW m−2. The influences of parameters on heat transfer coefficients have been discussed in detail. A wide set of empirical heat transfer correlations from forced convection to fully developed boiling (FDB) have been assessed with the experimental results, and most of the correlations could not fit the experimental data well. A new correlation for the FDB region and a modified Liu and Winterton correlation for the entire subcooled boiling region were proposed to predict the heat transfer coefficients in swirl flow under high and non-uniform heat fluxes. The average deviations (ADs) of the two correlation were 0.46% and 3.65%, respectively, and the root mean square deviations (RSMDs) of them were 4.85% and 13%, respectively.

Heat transfer experiments on subcooled water flow boiling were carried out in a vertical upward circular channel, which was set off-center in a rectangular nickel block. Particular attention was paid to the heat transfer of water under high and heat fluxes and high mass fluxes under one-side heating conditions, with respect to the heat removal technologies for the divertor in the International Thermonuclear Experimental Reactor (ITER). The test section was electrically heated by a large AC power supply with an effective heated length of 360 mm. The test parameters covered pressures of 3–5 MPa, mass fluxes of 3000–8000 kg·m−2·s−1, inlet bulk temperatures of 40–220 °C, and equivalent heat fluxes up to 8 MW·m−2. The effects of the parameters on the heat transfer coefficients have been discussed in detail, from single-phase forced convention to fully developed nucleate boiling. A series of heat transfer correlations were evaluated using the experimental data, and most of the correlations did not adequately fit the experimental results. A modified Liu-Winterton correlation and modified Jens-Lottes correlation were used to predict the heat transfer coefficients in the partially boiling region and fully developed boiling region, respectively. The average errors (AEs) of the two correlations were −0.28% and 0.6%, and the root mean square errors (RMSEs) of them were 7.93% and 2.89%.

Despite the rapid development for thermoelectrics (TEs) and aerogels, elastic aerogel TE generators (TEGs) with vertical-type architectures have not been found to harvest heat utilizing vertical temperature difference. Herein, an elastic poly(3,4-ethylenedioxytiophene)-tosylate(PEDOT-Tos)/single-walled carbon nanotube (SWCNT) TE aerogel was fabricated via convenient chemical oxidative polymerization, physical mixing and subsequent freeze-drying process. Then, we report the first elastic TEGs consisting of vertical-type aerogel legs, which not only inherit the advantages of ultralight weight and compressibility of aerogels, but also display high TE performance. The output performance can be conveniently adjusted by the number of legs, the temperature difference and compressive strain. An optimum output power of 1967 nW (output power density of 30.73 mW m−2) is achieved for the TE generator using 10 unipolar aerogel legs at 50% strain and temperature difference of 50 K. Furthermore, the ultralight and compressible TEG displays excellent capability of harvesting heat under vertical temperature difference and mechanical deformation, such as on hot oil/plate or compression by human body. The results will greatly facilitate the development of TE aerogels and TEGs, and widen the versatile application scenarios.

In order to investigate the heat transfer and resistance performance of high-pressure steam in-tube condensation with pool boiling outside the tube, experiments were conducted on a short horizontal tube with a 25 mm outer diameter and 3 mm wall thickness. The test parameters were in the range of 4–10 MPa pressure, 400–1000 kg/m2 s mass velocity, and 0.1–0.9 steam quality. New measurement methods of wall temperature and heat flux were introduced. It is found that the outer wall temperature distribution is not uniform in the circumferential direction, and the wall temperature and heat flux increase with pressure, mass velocity, and steam quality. The condensation heat transfer coefficient and frictional pressure drop increase with the mass velocity and steam quality, but decrease with pressure. In addition, the pool boiling regimes were captured by a high-speed camera. The experimental condensation heat transfer coefficients were compared with available existing correlations, and the comparison results were found to be unsatisfactory. Finally, a modified condensation correlation from Akers et al. was proposed for high-pressure steam in-tube condensation with pool boiling outside in smooth horizontal tubes.

Processing protocols definitely impact polymer microstructure evolution and thermoelectric (TE) performance; however, in-depth understanding of molecular mechanism is still a challenge. Here, the mechanism of the effects of solution- and melt-processing protocols on polymer microstructure evolution and TE performance has been unraveled, where a semicrystalline poly(3-hexylthiophene) (P3HT) is chosen as a model system. The phase separations between crystallites and amorphous regions are more pronounced for melt-processed films, and solution-processed films display enlarged conjugation length and higher degree of solid-state ordering. In crystalline domains, solution-processed films reveal a completely edge-on structure, while melt-processed exhibit a co-existing of edge-on and face-on crystallites. Additionally, solution-aging strengthens aggregation and phase separation, but has little effect on conjugation length and crystalline texture. Finally, a multiphase semicrystalline model, based on crystalline texture and phase separation, is applied.

The void fraction is one of the most important parameters used to characterize gas–liquid two-phase flow, and a myriad of researchers have investigated it under the adiabatic flow conditions. The gamma ray attenuation is a frequently used non-intrusive method for measuring component volume fraction in gas–liquid two-phase flow system. In this paper, firstly, the influence of the various parameters and test conditions on the gamma ray attenuation have been completely examined, such as the calibration of Count Rate for pure gas and liquid phases, the influences of fluid temperature, phase changing point and fluid mass velocity, distance between gamma ray attenuation measuring instrument and experimental section etc. Secondly, the measurement of void fraction was taken in the vertically upward pipes under high pressure and high temperature evaporating conditions. The experimental results of void fraction were compared with the data in reference literature for measurement, the results from the gamma ray attenuation show good agreement with the literature for air–water two-phase flows, but for the evaporating conditions, a small number of compared data beyond the statistical approach for 90% of confidence interval due to some reasons, such as heat flux, the diameter of Taylor-bubbles, longitude of slugs etc. Finally, six predicted correlations from four groups were selected for comparing with the experimental data. The most of compared data were within the statistical approach for 85% of confidence interval. In general, the void fraction was rarely investigated and the available data was limited under high temperature and high pressure evaporating conditions. The investigations of present study are helpful to resolve the difficulties of measuring for gas–liquid two-phase flows concerning to the heated evaporating condition.

This study focuses on the critical heat flux (CHF) of highly subcooled water flow boiling under high mass fluxes, particularly with respect to the thermal–hydraulics analysis in fusion reactor components. The experiments were performed in vertical circular tubes those were uniformly heated by a large AC power supply. Twisted tape was employed to induce swirl flow in order to further enhance the subcooled CHF. This study obtained several high CHF data points in both smooth tubes and twisted-tape tubes, which can enhance relevant high-CHF databases. The influences of thermodynamic quality, mass flux, pressure, and twisted tape on subcooled CHF were discussed. The results show that twisted tapes can allow significant increase in subcooled CHF under high mass fluxes. The CHF enhancement effect is more obvious when using twisted tapes with smaller twist ratios. Available existing subcooled CHF correlations were compared with the experimental data from the present and others’ studies for both smooth tubes and twisted-tape tubes. For smooth tubes, the CHF look-up table has satisfactory prediction accuracy. For twisted-tape tubes, a modified correlation of Arment et al. was proposed to predict the swirl-flow subcooled CHF.

Experimental investigations were conducted into the heat transfer characteristics of supercritical water in a downward flowing cooling tube surrounded by pool boiling for the passive cooling system of the Generation-IV Supercritical-Water-Cooled Reactor (SCWR). The experimental parameters cover the extreme high ranges of pressure (23–27 MPa), mass flux (600–1000 kg·m−2·s−1), bulk fluid temperature (150–500 °C), and inner heat flux (98–1143 kW·m−2). A new thermal amplification system was applied to measure the heat transfer coefficient near the pseudocritical region under various mass fluxes and pressures. A sharp jump in the wall temperature and maximum heat transfer coefficient was observed at the pseudocritical point, and the mass flux and pressure exhibited different influences below and above the pseudocritical point. Based on the experimental data, a new heat transfer correlation for predicting the Nu is proposed for the subject cooling scenario. The proposed heat transfer correlation includes the density ratio and requires only the fluid properties at the bulk temperature. The mean absolute and root-mean-square error of the proposed correlation were about 7.94% and 9.60%, respectively. The proposed correlation can serve as a reference in the design of the passive cooling system of the SCWR.

Heat transfer experiments with supercritical pressure water flowing vertically upward through a 2 × 2 rod bundle have been performed at Xi’an Jiaotong University. A fuel-assembly simulator with four heated rods was installed inside a square channel with rounded corner. The outer diameter of each heated rod is 8 mm with an effective heated length of 600 mm. The experiments covered the pressure range of 23–28 MPa, mass-flux range of 350–1000 kg/(m 2 s) and heat-flux range on the rod surface of 200–1000 kW/m 2 . Heat transfer characteristics of supercritical pressure water through the bundle were examined with respect to variations of heat flux, system pressure, and mass flux. These characteristics were shown to be similar to those previously observed in tubes or annuli. The experimental data indicate a non-uniform circumferential wall-temperature distribution around the heated rod. A maximum wall temperature was observed at the surface facing the corner gap between the heated rod and the ceramic tube, while the minimum wall temperature was observed at the surface facing the center subchannel. The difference between maximum and minimum wall temperatures varies with heat flux and/or mass flux. Eight heat transfer correlations developed for supercritical water were assessed against the current set of test data. Prediction of the Jackson correlation agrees closely with the experimental Nusselt number. A new correlation has been derived based on Jackson correlation to improve the prediction accuracy of supercritical heat transfer coefficient in a 2 × 2 rod bundle.

Experiments of heat transfer characteristics of subcooled water flowing in vertical circular channels, which were off-center in rectangular blocks, were carried out under high heat fluxes up to the ITER requirements. The heating flux distributions of the channels were non-uniform in the circumferential direction, which were obtained by electrically heating the blocks directly. Two types of channels were used: smooth channel and twisted tapes channel. The surface temperature of the rectangular blocks was measured by infrared camera and thermocouples. Effect of the system pressure, mass flow rate, inlet subcooling, and equivalent heat fluxes on heat transfer were all investigated. The main attention was paid to the subcooled water heat transfer under non-uniform heating flux, and the effect of twisted tapes. Results show that subcooled boiling is more likely to become the dominant factor under the conditions of lower mass flow rates, higher heat fluxes and lower system pressures. Twisted tapes can enhance the heat transfer, which is more evident in high heat fluxes. The temperature fields in the block were calculated with a Computational Fluid Dynamics (CFD) method to obtain, which were consistent with the experimental results.

An experiment has recently been performed at Xi’an Jiaotong University to study the wall temperature and pressure drop at supercritical pressures with upward flow of water inside a 2×2 rod bundle. A fuel-assembly simulator with four heated rods was installed inside a square channel with rounded corner. The outer diameter of each heated rod is 8 mm with an effective heated length of 600 mm. Experimental parameters covered the pressure of 23–28 MPa, mass flux of 350–1000 kg/m2s and heat flux on the rod surface of 200–1000 kW/m2. According to the experimental data, it was found that the circumferential wall temperature distribution of a heated rod is not uniform. The temperature difference between the maximum and the minimum varies with heat flux and/or mass flux. Heat transfer characteristics of supercritical water in bundle were discussed with respect to various heat fluxes. The effect of heat flux on heat transfer in rod bundles is similar with that in tubes or annuli. In addition, flow resistance reflected in the form of pressure loss has also been studied. Experimental results showed that the total pressure drop increases with bulk enthalpy and mass flux. Four heat transfer correlations developed for supercritical pressures water were compared with the present test data. Predictions of Jackson correlation agrees closely with the experimental data.Copyright © 2014 by ASME