Synergistic Effect of Strontium and Melt Quenching on the Solidification Microstructure of Hypereutectic Al-Si Alloys.

The synergistic effect between strontium (Sr) and melt quenching on the solidified microstructure of hypereutectic Al-Si alloys was investigated by optical and scanning electron microscopy. The results indicate that melt quenching can suppress the growth of primary Si particles in the solidified structure of the hypereutectic Al-Si alloy, resulting in a significant decrease of in the average size of primary Si particles in Al-(18~22)Si alloys from 30.35~66.31 µm to 15.13~34.63 µm. The synergistic effect between Sr and melt quenching can further inhibit the precipitation of primary Si particles in the Al-18Si alloy. After the addition of Sr to Al-18Si alloy and undergoing melt quenching, the area fraction of primary Si clearly decreases. When the added amount of Sr increases from 0.1 wt.% to 0.5 wt.%, the area fraction of primary Si decreases from 1.13% to 0.16%. With 0.5 wt.% Sr in the tested alloy, the inhibiting effect on primary Si precipitation was significantly improved. Research has shown that the cooling rate has a significant impact on the solidified structure of the melt-quenched Al-18Si-0.5Sr alloy. There exists no primary Si in solidified structures on the area of 1/8R and 1/4R from the surface of the round bar sample, but the area fraction of primary Si increases, respectively, to 1.97% and 12.48% on the area of 1/2R and R from the surface. The higher the cooling rate, the higher the inhibitory effect on the primary Si precipitation in the Al-18Si-0.5Sr alloy.

Study on Restart Safety of Waxy Crude Pipelines Based on Reliability Principle under Constant Flow.

The restart process of waxy crude pipelines is an unsteady thermo-hydraulic coupling process, which mainly includes two modes of the constant flow and constant pressure in industry. However, some parameters involved in the restart process have obvious uncertainties, such as the operating parameters, physical parameters of crude oil, environmental parameters, and pipeline parameters, resulting in the traditional deterministic method that cannot scientifically describe the safety of the pipeline restart process. To do this, this study introduces the reliability-based limit state method and interference principle into the safety evaluation of waxy crude pipelines during the restart process. Considering the random fluctuation characteristics of the mentioned parameters, the restart physical process, the flow and heat transfer mathematical model, and the restart failure limit state function were established. On this basis, the failure probability during the restart process for one waxy crude pipeline under constant flow was determined. This research has realized the quantitative evaluation of restart safety of waxy crude pipelines.

Effect on the Surface Anticorrosion and Corrosion Protection Mechanism of Integrated Rust Conversion Coating for Enhanced Corrosion Protection.

Here, a series of integrated rust conversion agents/coatings were synthesized by esterification reaction of 3,4,5-trihydroxybenzoic acid (GA) and triethanolamine (TE). The structural features, rust conversion ability, and corrosion resistance of the synthesized rust conversion agents/coatings were analyzed using the Fourier transform infrared tests, scanning electron microscopy tests, X-ray diffraction tests, and electrochemical measurements. It was found that when the mass ratio of TE and GA was 2:1, the synthesized rust conversion agent/coating has best rust conversion ability and anti-corrosion performance (i.e., corrosion current density 7.480 × 10-7 A/cm2). In addition, different from the traditional coatings, the integrated rust conversion coating developed in this study combines the primer and topcoat of traditional coatings into one, which can significantly increase the on-site construction efficiency. Furthermore, a new rust conversion mechanism for the optimized rust conversion agent/coating was proposed. The phenolic hydroxyl functional groups in the rust conversion agent can well chelate with Fe2+/Fe3+ in the original rust layer and then form macromolecular compounds and dense chelating films inside the coating, which tightly wraps rust and also prevents the penetration and diffusion of corrosive medium, making them lose the opportunity to interact with each other.

A Novel Dehumidification Strategy to Reduce Liquid Fraction and Condensation Loss in Steam Turbines.

Massive droplets can be generated to form two-phase flow in steam turbines, leading to erosion issues to the blades and reduces the reliability of the components. A condensing two-phase flow model was developed to assess the flow structure and loss considering the nonequilibrium condensation phenomenon due to the high expansion behaviour in the transonic flow in linear blade cascades. A novel dehumidification strategy was proposed by introducing turbulent disturbances on the suction side. The results show that the Wilson point of the nonequilibrium condensation process was delayed by increasing the inlet superheated level at the entrance of the blade cascade. With an increase in the inlet superheated level of 25 K, the liquid fraction and condensation loss significantly reduced by 79% and 73%, respectively. The newly designed turbine blades not only remarkably kept the liquid phase region away from the blade walls but also significantly reduced 28.1% averaged liquid fraction and 47.5% condensation loss compared to the original geometry. The results provide an insight to understand the formation and evaporation of the condensed droplets inside steam turbines.

Study on the Kinetic Process of Asphaltene Precipitation during Crude Oil Mixing and Its Effect on the Wax Behavior of Crude Oil.

As an important component of crude oil, asphaltene precipitation and deposition are harmful to petroleum production and processing. In previous research, the impacts of asphaltene precipitation on crude oil characteristics were preliminarily explored. In this paper, by mixing different types of crude oil, the dynamic process of asphaltene precipitation and its effect on the crystallization and gelation behaviors of mixed crude oil were in-depth analyzed and discussed using the high-speed centrifugation technique, microscopic observation, differential scanning calorimetry (DSC) thermal analysis, and rheological test. The results showed that the asphaltene precipitation mainly occurred in the early stage of crude oil mixing and was influenced by crude oil composition. As the precipitation time increased, the driving force for asphaltene precipitation was gradually weakened until a dynamic equilibrium between asphaltene precipitation and dissolution was reached. Meanwhile, once the asphaltene precipitation occurred, the crystallization and gelation processes of crude oil were significantly affected. It was discovered that the change in the existing state of asphaltenes due to their precipitation is an important factor affecting the interaction of asphaltenes and waxes, which is critical for the technological development of oil and gas flow assurance.

Study on the Operation Safety and Reliability of a Waxy Hot Oil Pipeline with Low Throughput Using the Probabilistic Method.

When the hot oil pipeline is running at a low throughput, it easily enters into an unstable condition, which seriously threatens the safety of the hot oil pipeline operation. In this study, the unsteady heat transfer and flow mathematical models for the hot oil pipeline system were established first by comprehensively considering the uncertainty of parameters during pipeline operation, such as the operating parameters (throughput and oil temperature), physical properties of crude oil (freezing point, viscosity, and thixotropic parameters), and environmental parameters (buried deep soil temperature and soil thermal conductivity). Then, the efficient Latin hypercube sampling (LHS) stochastic numerical algorithm was applied and further developed to quantitatively describe the operation safety of hot oil pipelines with low throughput in the form of probability. On the basis of the abovementioned research, the qualitative relationship between pipeline flowrate and friction loss is obtained. Finally, taking an actual crude oil pipeline as an example, the failure probabilities of the pipeline under different operating conditions were analyzed in detail. Combined with the target safety level of pipeline operation, the minimum allowable throughput of pipelines was determined. This study revealed the flow and heat transfer law of hot oil pipelines with low throughput and determined its operation safety and reliability under different operating conditions.

Graphene Modified TiO2 Composite Photocatalysts: Mechanism, Progress and Perspective.

Graphene modified TiO2 composite photocatalysts have drawn increasing attention because of their high performance. Some significant advancements have been achieved with the continuous research, such as the corresponding photocatalytic mechanism that has been revealed. Specific influencing factors have been discovered and potential optimizing methods are proposed. The latest developments in graphene assisted TiO2 composite photocatalysts are abstracted and discussed. Based on the primary reasons behind the observed phenomena of these composite photocatalysts, probable development directions and further optimizing strategies are presented. Moreover, several novel detective technologies-beyond the decomposition test-which can be used to judge the photocatalytic performances of the resulting photocatalysts are listed and analyzed. Although some objectives have been achieved, new challenges still exist and hinder the widespread application of graphene-TiO2 composite photocatalysts, which deserves further study.

Graphene based photoanode for DSSCs with high performances.

Graphene assisted photoanodes are promising because of the high performance of the resulting dye sensitized solar cells (DSSCs). A photoanode with a three-layer structure is prepared in this study and the synergy between each layer was found to play a vital role in its photovoltaic properties. The influence of interface contact between the transport layer and work layer is revealed. After ameliorating the interface contact level (enhancing the electron transport ability), the functions of the adopted reduced graphene oxide (RGO) and three-dimensional graphene networks (3DGNs) in the transport layer and work layer, respectively, can be made full use of. In order to further enhance the scattering ability for the incident light and improve the adsorption ability for dye molecules, a scattering layer based on the RGO-TiO2 is added in the photoanode. After a comprehensive optimization (including the types of functional groups and mass fractions of the RGO in the work layer and scattering layer), the resulting power conversion efficiency reaches 11.8%, which is much higher than that of previous reported graphene modified DSSCs.

Study on the growth rate of natural gas hydrate in water-in-oil emulsion system using a high-pressure flow loop.

Hydrate slurry transport technology in deep-water pipelines has become a focal point among worldwide researches, due to its high economic efficiency. However, as the key part of the hydrate slurry transport technology research, the mechanism and laws of natural gas hydrate growth dynamics are still unclear in the flow emulsion system. On this basis, we have conducted a series of growth kinetic experiments in a high-pressure loop, investigated systematically several influencing factors (i.e. the flow rate, water-cut, AA concentration and so on) of growth kinetics, obtained the quantitative relations between these factors and the gas consumption as well as the hydrate growth rate (gas consumption rate). It could be gained from analysis of these influencing factors, that the hydrate growth rate has an extreme value (maximum) during the formation process in a slurry system. The controlling factor of hydrate formation differed at the stages before and after this maximum value. The intrinsic kinetics controlled before the value while heat/mass transfer influenced after it. The time needed for the hydrate growth rate to reach the maximum point was generally within 0.5 h after the hydrate mass formation.