•Bubble formation process in freezing droplets were experimentally investigated.•Effect of the dissolved gas on the nucleation and growth of ice crystals were studied.•A quantitative analysis of the ...tip angles of frozen droplet was proposed.•Effect of the dissolved gas on the initial frost crystal was experimentally investigated.
The solidification process of water (liquid) can cause an intricate deformation of its structures, from the complex of ice crystals to dendritic frost crystal growth. The dissolved gas in water (liquid) is rejected and accumulated ahead of the ice-water interface during solidification and the resulting bubbles are incorporated into the growing ice crystal. In this study, the bubble formation process and the effect of the dissolved gas on the nucleation and growth of ice crystals in freezing droplets at various gas concentrations were experimentally investigated. The dissolved gas acted as a heterogeneous medium, and promoted the nucleation of the liquid droplets. Compared with the pure solid ice crystal, during the freezing process, the existence of bubbles altered the solid-liquid density ratio and the shape of the droplets. Coupled with experimental observations, a quantitative analysis of the tip angles was proposed to elucidate this influence, which indicated that the tip angle of the frozen ice droplet decreased with the gas concentration, and promoted the growth of the initial frost crystal.
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Carbon dioxide is a promising natural working fluid that can be used in transcritical Rankine cycles due to environmental and safety concerns. However, the high operation pressure has to be reduced ...and the relatively low efficiency of the system has to be increased. Traditional working fluids have been widely investigated to reclaim low-grade heat energy, and most of them have high GWPs (global warming potentials) or are flammable or even toxic. Consequently, to mitigate the above disadvantages, we studied zeotropic mixtures of carbon dioxide blends with 7 low GWP working fluids for use in a TRC (transcritical Rankine cycle) for low-grade heat conversion. The results revealed that these zeotropic mixtures can help improve the thermal efficiency of the TRC and decrease the operation pressure compared to that of pure CO2. Owing to the perfect thermal match in the heat transfer process, higher exergy efficiencies were achieved for the entire system when zeotropic mixtures were used than pure CO2. Maximum exergy efficiencies exist for the TRC at the corresponding optimal pressures for each mixture. Finally, the mixture CO2/R161 is recommended for small capacity instruments for its high efficiency, in spite of its high flammability; the mixtures CO2/R1234yf and CO2/R1234ze can be used in TRCs with larger capacities due to their lower flammability.
•Frosting-defrosting process on clean and dusty surfaces were studied.•The effect of surface wettability on the dust removal was addressed.•Super-hydrophobic surface exhibits a wrapping effect during ...frost cleaning process.•A sliding surface was prepared and it is promising in frost cleaning application.
Utilizing the frosting-defrosting process for dust removal is a simple and novel method to keep heat exchangers clean. With the development of surface treatment technology, the effect of the wettability of a surface on cleaning effects has drawn great attention. As the dynamic behavior of the frost layer varies with surfaces wettability, the interaction between the dust layer and frost layer also varies. Through experiments on four kinds of copper surfaces, we studied the effect of surface wettability on dust removal characteristics during the frosting cleaning process. The four surfaces were: bare surface, hydrophilic surface, super-hydrophobic surface, and sliding surface. The results show that dust particles get removed with the movement of the melted water on the hydrophilic, bare and sliding surfaces, whereas on the super-hydrophobic surface, dust particles get wrapped into the curling frosting layer and fall with chunks of frost-water mixture. The dust removal rates during the frosting cleaning process were 24.1%, 28.1%, 86.6%, and 32.7% on the four kinds of surfaces. The super-hydrophobic surface has a shorter defrosting time and exhibits the wrapping effect of the dust layer, which gives it the optimal dust removal property. The sliding surface is also promising in the dust removal applications.
HFO1234yf has been proposed for mobile air-conditioners due to its low global warming potential (GWP) and performance comparable to that of R134a. However, its performance is inferior to that of ...R410A. This makes it difficult to be applied to residential air-conditioners. In order to apply the low-GWP refrigerant to residential air-conditioners, refrigerant mixtures of HFO1234yf and R32 are proposed, and their flow boiling heat transfer performances were investigated at two mass fractions (80/20 and 50/50 by mass%) in a smooth horizontal tube with an inner diameter of 2mm. The experiments were conducted under heat fluxes ranging from 6 to 24kW/m2 and mass fluxes ranging from 100 to 400kg/m2s at the evaporation temperature of 15°C. The measured heat transfer coefficients were compared with those of pure HFO1234yf and R32. The results showed that the heat transfer coefficients of the mixture with an R32 mass fraction of 20% were 10–30% less than those of pure HFO1234yf for various mass and heat fluxes. When the mass fraction of R32 increased to 50%, the heat transfer coefficients of the mixture were 10–20% greater than those of pure HFO1234yf under conditions of large mass and heat fluxes. Moreover, the heat transfer coefficients of the mixtures were about 20–50% less than that of pure R32. The performances of the mixtures were examined at different boiling numbers. For refrigerant mixture HFO1234yf and R32 (80/20 by mass%), the nucleate boiling heat transfer was noticeably suppressed at low vapor quality for small boiling numbers, whereas the forced convective heat transfer was significantly suppressed at high vapor quality for large boiling numbers. This indicates that the heat transfer is greatly influenced by the mass diffusion resistance and temperature glide of the mixture.
•CFD numerical calculation method of horizontal-tube falling film evaporation is summarized.•Microscopic mechanisms of thermodynamic, flow and structural parameters on liquid film flow ...characteristics and heat transfer are analyzed by summarized CFD simulation results.•Improvement and research directions of CFD simulation are suggested.
The horizontal-tube falling film evaporation is an efficient heat transfer technology for energy-saving, with good economic and environmental benefits. In the falling film evaporation technology, the thickness and uniformity of liquid film are considered as important hydrodynamic characteristics that influence the heat transfer of fluid greatly. However, through experimental methods, it is difficult to obtain the microscopic characteristics and subtle changes of the velocity and temperature distribution inside the liquid film, and hard to describe the gas-liquid interface during the evaporation process of the liquid on the tube surface. Therefore, CFD simulation is used in the researches on the falling film flow and heat transfer characteristics outside the horizontal tube. This paper mainly reviewed the CFD simulation work of falling film evaporation outside the horizontal tube in the existing literatures. The models used by researchers were made a summary. The effects and microscopic mechanisms of flow parameters such as spray density, thermodynamic parameters such as the surface tension of liquid, structural parameters such as the liquid distributor and evaporation characteristics of heat transfer tubes and tube bundle were summarized and analyzed. Based on the researches’ CFD simulation results of hydrodynamics and heat and mass transfer of falling film evaporation, it is found that these parameters have their own reasonable ranges to ensure the efficient operation of falling film evaporators under specific operation conditions. Beyond the reasonable range, the formation of liquid film on the tube surface is to be destroyed, it will result in the decrease of heat transfer performance and the dryout. In addition, by adjusting the magnitude of the surface tension, wall adhesion, inertia force and other forces, the change of these parameters leads to the generation of vortex and recirculation phenomenon, the decrease of liquid film thickness and the enhancement of liquid film fluctuation, so as to reduce the thermal boundary layer thickness and enhance the heat and mass transfer performance of the film evaporation process. It is considered that under the comprehensive action of multi-parameters, the three-dimensional distribution of flow characteristics and heat transfer coefficient of multicomponent working fluid, as well as the change of flow pattern between tube bundles, are the improvement directions of current CFD simulation. And the research directions of future CFD simulation are the microscopic hydrodynamic and heat transfer characteristics caused by wall structure reconstruction and the influence of auxiliary components on the falling film evaporation process.
Frosting cleaning process for dust removal is a simple and direct way to keep heat exchangers clean, and the dust removal effectiveness is strongly influenced by surface property. An object of this ...paper is to investigate the frosting cleaning process on super-hydrophobic surface and to determine the interaction between the frost layer and dust particles. Frosting-defrosting process on four kinds of test samples were studied by experiments. The results showed that, compared with the clean surfaces, the frost layer structure was denser and more uniform on dusty surfaces. On the super-hydrophobic surface, as a kind of heterogeneous nucleation point, the dust particle promoted frost layer growth by 57%. During the defrosting process on dusty test samples, the melted water and dust particles were reverse osmosed the frost layer. The reverse osmosed rate increased with frosting time, and it was over 0.5 under all experimental conditions. Compared with the bare surface, super-hydrophobic surface displayed a different defrosting behavior and had a shorter defrosting time. On the super-hydrophobic surface, deposited dust particles were encased in the frost-water mixture block and flaked off the surface under gravity, and the dust removal rate was up to 94.5% in this work. Because of the reverse osmosed dust movement, the wrap effect, and the weak adhesion of super-hydrophobic surface, the dust-frost mixture is easily removed. Thus, utilizing the frosting-defrosting characteristics of super-hydrophobic surface for promoting the dust removal has a good application prospect.
For A-site substituted AA'BO3 perovskite, the doped A' element is easy to migrate to the surface and form segregation of A'Ox, which could partially block active B sites and thus reduce catalytic ...activity. In the present work a La0.8Sr0.2CoO3-δ perovskite-type solid was first prepared and then treated with acetic acid to eliminate SrO segregation on its surface. The obtained catalyst (LCSO-a) possessed a significantly enhanced catalytic activity for toluene oxidation in the 220–260 °C range. This promotion effect might be attributed to the increase in the concentration of active surface oxygen species and the reduction in their binding strength. In addition, the LCSO-a catalyst showed very good stability and water resistance.
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•Surface SrO segregation was formed with Sr doping into the LaCoO3.•A selective removal method was used to remove SrO on the surface of catalyst.•The catalyst showed excellent catalytic activity, stability and H2O resistance.
Current methods of manually checking whether materials meet the required standards are inefficient and harmful to workers' health. To address the complexity and variability of real-world systems, it ...is imperative to establish highly consistent digital models for subsequent operations. However, there is a challenge in unifying various modules and components into a single system and optimizing them for functionality. A mechatronic solution is proposed for material sorting within the production manufacturing process, leveraging Siemens TIA Portal and NX 2007. The solution involves constructing a physical platform and realizing a digital twin system to enable real-virtual interaction via communication protocols. Initially, a virtual material sorting simulation and debugging platform is established using NX software, while operations are conducted in the TIA Portal. Subsequently, the control program is designed within the TIA Portal, and Virtual PLCs are created using PLCSIM Advanced for virtual debugging and analysis, confirming the solution's feasibility. Following this, the physical material sorting platform is developed, utilizing Siemens S7-1500 PLC to manage inverter and servo motor operations. Ultimately, the OPC UA communication protocol facilitates information exchange between the physical PLC and the virtual platform, establishing effective linkage between the virtual system and the physical model to ensure coordination and consistency. Results demonstrate real-time synchronization and virtual sorting operations during joint real-virtual debugging. This digital twin design solution is poised to significantly enhance manufacturing efficiency, reduce initial hardware costs in factory design, and augment automation and intelligence in production.
A general threshold was proposed to subdivide a flow in a smooth or a rough micro- and mini-channel. A database was setup based on the experimental results of liquid flow in rough micro- and ...mini-channels from the literature, which includes 5569 data and covers wide ranges of the relative roughness and the Reynolds number. The effect of roughness on a flow in micro- and mini-scale channels was investigated by in-depth analyzing the friction factor and the flowing transition characteristics. Existing prediction models were used to predict the friction factors of fully developed flow and critical Reynolds number. Both experimental data and the predictions showed that roughness had little effect on flow characteristics when the relative roughness (ε/Dh) is less than 1%, while for the relative roughness larger than 1%, the friction factor and critical Reynolds number gradually deviate from the prediction values of smooth tubes. It was recommended that ε/Dh = 1% could be set as a threshold to distinguish smooth and rough micro- and mini-channels. According to this criterion, new correlations to predict friction factor and critical Reynolds number were also fitted. Moreover, it was also concluded that geometries of the channels had little effects on this threshold.
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•A database of liquid flow in rough micro- and mini-channels is generated.•A general threshold is proposed to subdivide smooth and rough micro- and mini-channels.•Existing models of friction factor and critical Reynolds are verified and evaluated.•New correlations are fitted based on the new proposed criterion.•Shape of the channels has little effect on flow characteristics.
•Combined modification is applied to the copper surface.•Condensation, freezing and the frosting processes are experimentally studied.•The increased proportion of carbon content contribute to surface ...hydrophobicity.•The combined surface shows a great advantage of frost inhibition applicability.
Heat transfer deterioration due to frost accumulation on metallic surfaces provided motivation for development of protective barriers that could effectively repel water in many forms. Currently, extensive attention has focused on superhydrophobic surfaces which have been found to be effective in reducing droplets condensation and frost accumulation. The combined modification using nanosecond laser and metal vapor vacuum arc (MEVVA) ion implantation of carbon is applied to the copper surface. The results revealed that the increased proportion of carbon content contribute to the increase of surface hydrophobicity, the contact angle of the combined surface was 152.1° ± 1° and the rolling angle was 6.7° ± 1°, which exhibited a superhydrophobic character, without thermal conductivity deterioration. The processes of droplet condensation, freezing and the frosting of three kinds of surfaces (rough #1, combined surface modification #2, silanization coating #3) have been examined. The combined surface can delay the freezing process to some extent. Compared with rough copper surface, combined surface had sparse frost structure and the growth of frost could be delayed by 43% when the frost thickness was 1.5 mm. Thus, the method of combined surface shows a great advantage of frost inhibition applicability.
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