ABSTRACT Kinetic theory is a popular approach to model liquid-vapor phase change but accurate determination of evaporation and condensation coefficients remains a challenge. Reported values of coefficients vary by several orders of magnitude. For simplicity and convenience evaporation and condensation coefficients are assumed to be equal though there is little physical evidence to support this. This study presents a novel methodology to test this assumption using data from Constrained Vapor Bubble (CVB) experiments conducted on the International Space Station (ISS). The experiments consist of a quartz cuvette that is partially filled with n-pentane; heated and cooled at opposite ends to induce simultaneous evaporation and condensation around a central bubble. Data obtained from the NASA Physical Sciences Informatics (PSI) database enabled a three-dimensional reconstruction of the liquid–vapor interface. The net mass flux over the vapor bubble surface is zero at steady operation, providing a closure relationship for simultaneous and independent calculation of both evaporation and condensation coefficients. The resulting coefficient values are within 1% of each other but are not equal. The two coefficients are also within 2% of those predicted using transition state theory. When the evaporation and condensation coefficients are forced to be equal, the deviation from transition state theory is approximately 60%. This deviation monotonically increases with increasing rates of evaporation/condensation due to a systemic under-prediction of the bubble surface area. The agreement between derived coefficients and those predicted by transition state theory is maintained when the bubble surface area is corrected to account for Marangoni-induced interfacial instabilities.

ABSTRACT Aerogel with nano-aerogel structure is regarded as advanced thermal insulation material. Therefore, the impacts of their microstructure and physical features on thermal conductivity are vital for optimizing their fabrication. An improved random growth approach was proposed in this paper to theoretically derive the effective thermal conductivity (ETC) of nano-aerogel. This model can accurately depict the random distribution versus the real microstructure. The Lattice-Boltzmann Method (LBM) was introduced into the model to investigate the nano-scale heat transmission in nano-aerogel with porous structure. The developed model provided significant advantages beyond previous approaches for nano-aerogel by altering the particle’s parameters (e.g. particle’s diameter, specific surface area, and morphological structure) to investigate their effects on thermal performance. Comparison with existing literature was made to validate the accuracy. It indicated good agreement among the modeling results, experimental data, and reference values, implying the accuracy of the modified model. It was demonstrated that the optimal model with 150 kg·m−3 in density gained the lowest effective thermal conductivity and the ideal core distribution probability (while C d = 0.01). An inner vacuum of less than 1.0 Pa was recommended for hunting for optimal adiabatic performance. Moreover, it proved that ETC decreased by increasing the specific surface area of aerogel grain at an optimal density of about 150 kg·m−3.

ABSTRACT We put forward a prescription of getting a higher value of the spin-dependent figure of merit in comparison with its charge counterpart in the case of a 1D magnetic heterostructure (a ferromagnetic (FM) chain sandwiched between two non-magnetic 1D lattices) where all the site energies of the heterostructure follow cosine form of modulation defined in Aubry-Andre-Harper (AAH) model. It is shown that due to having a greater number of non-uniform peaks in its transmission spectra, the thermoelectric efficiency of this heterostructure becomes superior compared to that is obtained in a fully ferromagnetic 1D chain. It is also reported that the maximum value of the thermoelectric figure of merit (for both spin and charge-dependent cases) of the heterostructure in a specified energy range gets enhanced in a continuous manner as the disorder strength of the AAH modulation is increased. The Hamiltonian of the proposed quantum system is described within a tight-binding (TB) framework and after the evaluation of transmission probability by utilizing the non-equilibrium Green’s function (NEGF) technique, all the important thermoelectric quantities are determined on the basis of Landauer formalism. We believe that our study on this particular topic of spin caloritronics, a subject associated with temperature-dependent spin transmission through a quantum channel, may help to explore several fascinating ideas about spin-related thermoelectric efficiency in many other ferromagnetic quasicrystals.

ABSTRACT Rational design and adjustment of flexible thermoelectric devices are key points for sustainable and effective thermoelectric conversion, which remains a fundamental challenge due to inherent high thermal conductivity and uncontrolled carrier concentration induced by non-uniform dispersion. Under ingenious combination of weakly-coupled hollow interface and nanotube structure, thermoelectric performance of a dumbbell-like molecular junction comprised of a phenyl-terminated polyyne as central molecule and two semi-infinite 1D single-walled carbon nanotube (SWCNT) as electrodes has been investigated at certain twisted angles (θ). The results indicate that molecule twisting can be reviewed as an effective thermoelectric switch to coordinatingly control electronic and phononic transmission properties simultaneously. Resonance of molecular discrete state and electrode continuous state leads to low thermal conductance, which is sensitively affected by twist angle. Meanwhile, cyclic transformation between p-type and n-type flexible thermoelectrics can be realized by manipulating twist angle in a certain period of rotation. Thermoelectric performance of such a molecular junction can be further improved by boron atom doping at head-to-tail positions, and an excellent figure of merit (ZT = 1.75) is observed near Fermi level under 25° twisted angle. This result inspires an effective strategy to modulate and control thermoelectric conversion, which will greatly broaden applications in thermoelectric twistronics.

ABSTRACT The energy conversion performance of thermophotovoltaic (TPV) systems can be improved when the vacuum gap between the emitter and the TPV cell is at the near-field owing to the photon tunneling of evanescent waves. Among them, the back-gapped-reflector TPV systems have gained interest as a method of improving their conversion efficiency by optimizing the spectral absorption of TPV cells. In this work, we introduce an alternative concept for the back-gapped-reflector TPV systems, namely the medium-bridge near-field TPV system, by building a medium bridge between the metal reflector and the TPV cell using SU8 nanofilm. The SU8 medium-bridge achieves a noticeable improvement in output performance by increasing the spectral absorption of the InAs cell and reducing parasitic absorption losses of the Au substrate. The results indicate that, as a consequence of the improved effect of the medium-bridge, the output power density and efficiency of this system exceed those of the conventional TPV system (which lacks a medium-bridge) by 26.4% and 36.5%, respectively. Moreover, we systematically analyze the modulation of medium-bridge thicknesses and cell thickness on output performance and clarify how both affect energy losses of this near-field TPV system. Our work offers a strategy to improve the energy conversion performance of the near-field TPV system, opening new opportunities for developing near-field energy conversion.

ABSTRACT A spark plasma sintering technology has already become rather common for the fabrication of GexSi1-x nanostructured thermoelectric solid solutions. Such trend is related with a number of opportunities and technological tools that enable precise properties manipulation. The present paper is devoted to discussing the modulation of GexSi1-x spark plasma sintering technique that consists in the use of silicon phosphide as a source of n-type doping within the process of sintering. The composition of the sintered powder is investigated. The synthesis of a solid solution was carried out in the process of sintering. The SiP is a chemically stable non-toxic compound that can replace toxic phosphorus in thermoelectric technology thus reducing the safety requirements of the corresponding technology process. The paper investigates the effect of SiP concentration on thermoelectric characteristics. The impurity distribution is analyzed, and the association of phosphorus atoms into clusters at a very high doping level is shown. The distribution of impurity elements was controlled by EMF analysis in a scanning electron microscope. It was shown that sintering of Ge-Si-SiP powder mixture allowed obtaining the phosphorus doped GexSi1-x material with high electron concentration that demonstrate high level of thermoelectric properties. The obtained thermoelectric characteristics are compared with the world's best nanostructured materials

ABSTRACT The thermal conductivity of tortuous silicon nanowires with constant cross section at room temperature was investigated in use of full-spectrum Monte-Carlo simulations. Various geometric features that can be possibly used to describe the tortuosity of the nanowires were studied and their relationships with the thermal conductivity were explored. Comparison of simulation results with experimental data shows similar magnitudes and variation trend of the thermal conductivity against the nanowire hydraulic diameter. The more tortuous, the smaller the thermal conductivity is. Among all, data collapse is best when shown against the surface-to-volume ratio and the correlation length of the surface roughness does not affect the thermal conductivity at all. By taking the surface-to-volume ratio into account for the boundary scattering rate, which also depends on the phonon frequency indirectly through the phonon group velocity, we are able to obtain satisfactory predictions based on a linear spectral model, not only about the thermal conductivity but also about the spectral heat flux density distribution. The model also shows that the relative reduction caused by tortuosity decreases with increasing frequency. For highly tortuous nanowires of diameter 22 nm, simply increasing the tortuosity is sufficient to obtain simulated thermal conductivities that are smaller than the experimentally measured value.

ABSTRACT In micro-scale sensitive medicinal and biochemical systems, improving mixing efficiency with small velocity limitations is critical. This work examines the influencing key parameters and their implications on mixing efficiency in a new two-dimensional electroosmotic micromixer (EM) with nonaligned input and outlet microchannels. The micromixer uses electroosmosis force generated by microelectrodes mounted on the walls of a square split and recombine (SSAR) mixing chamber to blend fluids of various concentrations, which enter into an intake microchannel from different inlets. The governing equations along with the specified boundary conditions are solved by the finite element-based solver. Thorough investigations are executed to explore how the mixing performance of the new microchannel mixer is affected by both flow (inlet velocity) and electric field (electrode potential arrangement, voltage magnitude, AC frequency, and phase difference) parameters. The results revealed that only adding electrode pairs always doesn’t increase the mixing efficiency of SSAR-EM, rather electrode polarity configuration along with an increase in electrode pair optimizes fluid mixing. Also, according to the present observations, the mixing performance of SSAR-EM is strongly sensitive to the input fluid velocity, the phase difference applied to the micro-electrodes, the AC frequency, and the amplitude of the alternating voltage. Corresponding to optimal parameters (i.e. velocity of 50 µm/s, AC-frequency of 8 Hz, voltage of 100 mV, and phase difference of 7π/36-radian), the mixing efficiency of SSAR-EM becomes 98.26%.

ABSTRACT This study investigates the impact of core-shell based nanofluids on the thermal performance of a solar water heating system by studying the changes in the useful heat gain and collector efficiency. This work would be the first to report the use of core-shell nanoparticles in solar water heating systems. The core-shell structure allows for dual improvements in thermal conductivity and better nanofluid stability, even without a surfactant. Therefore, three novel nanofluids were prepared by adding 2 wt% TiO2@SiO2, Fe3O4@SiO2, and ZnO@SiO2 core-shell nanoparticles to pure water to be used in the experiments. The experimental thermal performances of the nanofluids were individually compared with pure water by the simultaneous operation of two identical systems. The results showed that the nanofluids with Fe3O4@SiO2 and ZnO@SiO2 particles had better performance than the base fluid. In particular, 16.65% and 5.40% increase in the useful energy gain and a 17.12% and 7.39% increase in the collector efficiency were observed with Fe3O4@SiO2 and ZnO@SiO2 core-shell based nanofluids, respectively. It is possible to conclude that, with their improved performance, the Fe3O4@SiO2-based nanofluids have great potential to be used in solar hot water systems instead of water.

ABSTRACT In this study, the flow behavior of deionized water through the staggered circular micro pin fin arrays with three longitudinal spacings (S L = 2D, 3D and 4D) is investigated using flow visualization technology of micro particle image velocimetry (micro-PIV) in the range of Re = 100–800. The streamline distribution and velocity field in the three micro pin fin arrays are obtained. Experimental results indicate that the longitudinal spacing has considerable effect on both the extension of the wake region and velocity field around pin fins. The small longitudinal spacing hinders the extension of the wake region behind the pin fin and delays the vortex shedding. The micro pin fin array with S L = 2D provides maximum velocity span and transverse velocity, indicating intense local fluid mixing. The flow and heat transfer characteristics in the microchannel with a single circular micro pin fin are also studied. By comparison, the feature of the wake region in the micro pin fin array with a large longitudinal spacing is similar to that in the flow past a single micro pin fin. Moreover, vortex shedding occurs in the micro pin fin array at higher Reynolds number. The correlation between velocity field and temperature field around the pin fin is investigated. The belt zone with enhanced heat transfer around the pin fin is consistent with the distribution of fluid with high velocities. Vortex shedding can obviously enhance the heat transfer downstream of the micro pin fin.