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• DUGKS is extended to moving boundary rarefied gas flows with heat transfer. • The complex advancing boundary condition is handled by the IB method. • Least square method is applied to the information construction at the ghost cell. • Six flow problems are performed to test the versatility of the method. Rarefied gas flows in complex geometries with heat transfer and moving boundaries have received remarkable attention due to the prevailing of micro/nano science and engineering applications. This study presents an immersed boundary (IB)-based discrete unified gas kinetic scheme (DUGKS) for the simulation of complex moving boundary thermal rarefied flows. The fluid and temperature fields are solved according to the DUGKS with Shakhov collision model (SDUGKS), which is capable of modeling non-unit Prandtl number problems in all Knudsen regimes. The IB approach, featured by its superior performance to incorporate complicated boundary constraints on Cartesian grids, is applied here to handling the interaction between the micro gas and curved boundaries. Specifically, for the IB adopted in this study, we use the least square method for the reconstruction of the distribution functions near the boundary. The accuracy of the IB-SDUGKS is explored comprehensively in six benchmark problems, including the micro lid-driven cavity flow, thermal equilibrium gas in a circular domain, thermal transpiration in an enclosure, moving shuttle in a rarefied gas, movement of a piston with pressure differences and particle free motion in a lid-driven cavity. The present results agree well with the corresponding analytical and direct simulation Monte Carlo (DSMC) solutions, illustrating the versatility of the present IB-SDUGKS complex thermal rarefied flows. [ABSTRACT FROM AUTHOR]

In an effort to reduce the emission level of carbon dioxide (CO 2) from metallurgical, thermal, cement burning and other enterprises, an innovative solution is proposed in this study, which aims to sequester CO 2 with alkali activated fly ash (FA) and ground granulated blast furnace slag (GGBFS) based foam concrete. The fluidity of alkali activated foam concrete (AAFC) fresh paste, dry density and compressive strength after hardening, micro-pore structure, carbonation products and CO 2 sequestration capacity were comprehensively explored. The experimental findings indicated that the fluidity of AAFC slurry was almost unaffected by the content of foaming agent hydrogen peroxide (H 2 O 2). When the H 2 O 2 content ranges between 1 % and 3 %, the dry density of uncarbonated AAFC ranges approximately 752.3–365.2 kg/m3, and the compressive strength of carbonated AAFC ranges about 1.01–0.21 MPa. Accelerated carbonation increased the dry density of uncarbonated AAFC samples. The porous structure of AAFC facilitated CO 2 gas penetration into the matrix, leading to rapid carbonation. When the H 2 O 2 content was 1 %, the compressive strength of fully carbonated AAFC was lower than that of non-carbonated AAFC; however, when the H 2 O 2 content exceeded 1.5 %, the trend in compressive strength reversed. The carbonation kinetics of AAFC demonstrated a linear relationship with the square root of carbonation time, and the carbonation coefficient increases proportionally with H 2 O 2 content. The porosity of AAFC increased from 61.05 % to 71.51 % as the H 2 O 2 content increased from 1 % to 2.5 %. The main pore size distribution range of AAFC was 30–400 µm and 5–50 nm. Accelerated carbonation minimally impacted the total porosity of AAFC but transformed certain large pores into capillary pores. Accelerated carbonation would generate calcite, aragonite, vaterite and additional amorphous phases. The maximum carbon sequestration capacity of AAFC, as determined in this study, reached 26.41 kg/m3, indicating significant potential for CO 2 sequestration. • An innovative solution was proposed for CO 2 sequestration using AAFC. • Accelerated carbonation generates calcite, aragonite, vaterite, and additional amorphous phases. • The maximum CO 2 sequestration capacity of AAFC reached 26.41 kg/m3. • AAFC exhibits excellent CO 2 sequestration performance and potential. [ABSTRACT FROM AUTHOR]

Nanoparticles excel in foam stability enhancement. In this experiment, xanthan gum (XG) short links were branched on the surface of TiO 2 nanoparticles, and the synthesis of novel nanomaterials was verified by analyzing the structure of the nanoparticles by FT-IR, XRD, and XPS, and investigating the dispersion of the nanoparticles by using SEM as well as capturing the cross-sectional images of XG-TiO 2 by using TEM and performing elemental analyses. The foams prepared by using cationic surfactant Cetyltrimethylammonium bromide (CTAB) and XG-TiO 2 synergistically had excellent performance, and XG-TiO 2 was able to effectively prevent the disproportionation reaction of the foams. By adding hydroxide and changing the air pressure, the half-life and foaming volume of the prepared foams were compared and analyzed, and the half-life of the foams was more than twice as much as that of the traditional materials, and the foam morphology of the foams with XG-TiO 2 was smaller and more homogeneous. When XG-TiO 2 foam was tested for core drive, the total crude oil recovery reached 81.2 %, which was 45.4 % higher than the water drive recovery. This experiment demonstrated the potential of XG-TiO 2 nanoparticles in enhancing foam stability and foam drive application. • In this study, xanthan gum (XG) was grafted onto the surface of TiO 2 nanoparticles by surface modification of TiO 2 nanoparticles to enhance the stability of the nanoparticles in solution in order to solve the problem of serious agglomeration and precipitation of nanoparticles in solution. • TiO 2 nanoparticles were used to enhance foam stability with obvious effects, but xanthan gum grafted TiO 2 nanoparticles (XG- TiO 2) improved foam half-life more than twice as much as pure TiO 2 , and foaming volume was more than 15 % higher than TiO 2 foam. • XG-TiO 2 can build up the shell layer structure more stably at the foam interface because the branched chains of XG attached to TiO 2 interact or cross-link with each other, and finally form a gel-like reticulation. This structure can enhance the adsorption of nanoparticles at the interface. • The reticular structure formed by XG-TiO 2 on the foam liquid film effectively reduces the discharge of the foam, and it plays a role similar to the skeleton support, which is a good protection for the internal foam, and greatly slows down the rupture speed of the foam. • The XG-TiO 2 foam was applied to foam drive oil, and the original total recovery rate reached 81.2 %, which was 45.4 % higher than the water drive recovery rate. This experiment proves the potential of modified nanoparticles in foam oil drive application. [ABSTRACT FROM AUTHOR]

As an inherent physical phenomenon of the contact-line motion, the contact angle hysteresis (CAH) has an important effect on dynamic behaviors of bubble/bubbles from a solid surface. In this study, boiling heat transfer considering the CAH is investigated by employing a phase change lattice Boltzmann (LB) model. Effects of the CAH on the bubble/bubbles dynamics during the boiling process and the boiling curves for both the hydrophilic and hydrophobic hysteresis windows are studied in detail. It is found that the bubble base, bubble height and phase change rate increase as the increase of the width of hysteresis window. On the other hand, the pinning of three-phase contact line is observed during the bubble growth processes. And the pinning times increase with the contact angle hysteresis. For the hydrophilic hysteresis window, the bubble completely departs from the heater if a small range of the hysteresis window is considered, while a residual bubble is left when the bubble departs from the heater in the cases of the hysteresis window larger than an approximate critical value of 45°. For the hydrophobic hysteresis window, a residual bubble can be observed regardless of the level of the contact angle hysteresis. The numerical results also indicate that the CAH affects the position of bubbles coalescence. The bubbles merge above the heater for the hydrophilic hysteresis window with a small level of hysteresis. While the bubbles merge on the heater for the hydrophilic hysteresis window with a large level of hysteresis as well as for the hydrophobic hysteresis window. Moreover, as the increase of the width of the hysteresis window, the critical heat flux (CHF) as well as the Leidenfrost temperature (the minimum temperature for film boiling) decrease, and the transition boiling regimes become shorter. Finally, we present the fitting equations between the width of the hysteresis window and the CHF/Leidenfrost temperature. The results show that the CHF and the Leidenfrost temperature decrease linearly as the width of the hysteresis window increases. [ABSTRACT FROM AUTHOR]

A honeycomb-like flow channel was proposed and investigated for the performance of proton exchange membrane fuel cells (PEMFCs). The effects of various thicknesses and porosities of the gas diffusion layer (GDL) on the honeycomb-like flow channel were studied. Compared with parallel and serpentine flow channels, the honeycomb-like flow channel exhibited the lowest oxygen non-uniformity value of 0.59 at 0.4 V, and the pressure drop was 6.9 times lower than that of the serpentine flow channel. The current density was 8034.9 A/m2, which was 14.0% and 10.4% higher than that of the parallel and serpentine flow channels. For a porosity of 0.4, the decrease in GDL thickness from 0.58 to 0.38 mm for the honeycomb-like flow channel facilitated oxygen diffusion, and the current density increased from 7717.2 to 8034.9 A/m2; the oxygen mass fraction gradually increased at the cathode channel but decreased at the center of the honeycomb-like structure. At a thickness of 0.38 mm, the porosity increased from 0.2 to 0.6, leading to a decrease in the oxygen non-uniformity value from 0.89 to 0.42. For a porosity of 0.6, the current density was 8787 A/m2, which was 60% and 9.4% greater when compared with the porosities of 0.2 and 0.4. • The novel honeycomb-like flow channel design was proposed. • The performance improvement of PEMFCs with different flow channels was characterized. • The effect of GDL thickness and porosity for the novel flow channel were studied. [ABSTRACT FROM AUTHOR]

• Two kinds of LBE models for the momentum equations derived from the van der Waals theory have been proposed. • A new thermal LBE method with a modified the equilibrium distribution function is proposed. • The thermal boundary conditions, including the Dirichlet and adiabatic boundaries, have been analyzed by Maxwell iteration. A lattice Boltzmann model (LBM) for two-phase flows with liquid-vapor phase transition based on a dynamic van der Waals theory [Phys. Rev. Lett. 94, 054501 (2005)] is proposed. The proposed model consists of two lattice Boltzmann equations (LBE): one for the Navier-Stokes-Korteweg (NSK) equations and the other for the temperature equation. In the thermal LBE, the equilibrium distribution function is redesigned by introducing a reference temperature, which is used to reduce the numerical errors of velocity divergence in the thermal LBE. A free-energy-based LBE is developed for the hydrodynamic equations and a novel force term is used to correctly recover the NSK equations. Several numerical simulations, including the liquid-vapor coexistence curve, phase separation, stationary droplet, droplet on partially wetting surface, droplet evaporation and bubble nucleate and departure, are conducted to validate the capability and performance of the present model. The numerical results of the proposed model are found to be in excellent agreement with the results of theoretical and/or the hybrid method. It is also shown that numerical stability and accuracy of the present models can be greatly improved by adjusting the reference temperature. The present models provide an effective predictive tool for two-phase flows involving phase change. [ABSTRACT FROM AUTHOR]