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The transition metal silicide coatings have excellent oxidation resistance. In the high-temperature oxidization environment, the "anomalous" diffusion phenomenon of the reverse concentration gradient occurs in the multilayer silicide coating structure, which has a significant impact on the coating degradation. This study is to explore the physical mechanism of "anomalous" diffusion phase transformation in the MoSi2–NbSi2 bilayer silicide coating on the Nb alloy substrate. Through vacuum annealing experiments, combined with micro-Raman spectroscopy and electron probe microanalysis measurements, the diffuisonal phase transformation of multilayer silicide coating in the oxygen-free environment at high temperatures was studied. By decoupling the oxidation and diffusion, the experiments indicate that high-temperature oxidation is not the dominant driving factor for the "anomalous" diffusion of atoms. The thermodynamic analysis reveals that the reduction in the nucleation barrier of the silicide-poor layer due to multicomponent solid solution and the non-uniform distribution of component chemical potential provide the driving force for the "anomalous" diffusion growth. Based on the diffusion kinetic modeling, the simulation of diffusion-controlled phase transformation in multilayer silicide coating was carried out, and the effect of tracer diffusion coefficients on the growth of the silicide-poor phase was analyzed. The research will have guiding significance for the recognition of failure mechanisms of silicide coating systems and performance improvement. [ABSTRACT FROM AUTHOR]

A kettle reboiler uses the latent heat from the condensation of high-temperature and high-pressure steam in the tube to produce low-pressure saturated steam in the outer shell. The deposition of particles on the tube may change the boiling heat transfer mode from nucleate boiling to natural convection, thereby deteriorating the heat transfer performance of the kettle reboiler. Therefore, it is very important to explore the deposition characteristics of particles in the kettle reboiler. In this study, the RPI boiling model based on the Euler–Euler method is used to analyze the water boiling process on the surface of the tube bundle. The DRW model and critical adhesion velocity model based on the Euler–Lagrangian method are used to calculate the motion of particles during the boiling process and the deposition (rebound) behavior. The results show that the boiling of liquid water enhances the local flow velocity of the fluid, so that the maximum flow velocity appears around the near-wall region. The local high-speed flow disperses the particles in the wake flow of the tube bundle, which inhibits the impact of particles on the wall. As the particle size increases, the wall adhesion and fluid drag on the particles are weakened, and the gravity effect gradually becomes dominant, increasing the residence time of the particles in the tube bundle and the frequency of particle impact on the wall. The particle deposition ratio first decreases and then increases. Ultimately, most particles will be deposited in the low-speed area at the end of the tube bundle. [ABSTRACT FROM AUTHOR]

[Objective] Hemispherical total emissivity is a fundamental thermophysical parameter that describes the surface thermal radiation capability of materials and has important applications in various fields, such as energy power, aerospace, and new materials. In thermal engineering series courses, the introduction to the measurement of hemispherical total emissivity parameters is insufficient, and arrangements for teaching experiments on hemispherical total emissivity measurement are lacking. Devising approaches to help students better understand the concept of hemispherical total emissivity and its application in radiation heat transfer has always been difficult in the implementation of course knowledge. [Methods] To solve the problems of low measurement accuracy and limited temperature range in traditional teaching experimental devices for measuring hemispherical total emissivity, this study designs an experimental teaching system for measuring the hemispherical total emissivity at high temperatures based on the steady-state calorimetry method. The proposed system includes a vacuum chamber, vacuum pump, vacuum gauge, power supply, sample holder, voltmeter, ammeter, and temperature sensor. A hemispherical total emissivity calculation method based on steady-state calorimetry was established, and a calibration analysis for heat loss in the sample testing area was conducted. Based on theoretical formulas, the relation between hemispherical total emissivity and spectral emissivity, as well as the correlation between hemispherical total emissivity and material thermal radiation capability, were discussed. To verify the measurement performance of the hemispherical total emissivity teaching experimental device and calculation method, oxidized and unoxidized Ni--Cr high-temperature alloy samples were selected for testing, and the hemispherical total emissivity of samples at different temperatures was measured. [Results] The experimental results showed that the experimental measurement device has good measurement accuracy under high-temperature conditions, and uncertainty in the hemispherical total emissivity measurement decreases with increasing temperature and hemispherical total emissivity values. When the temperature exceeds 600, the measu °C rement uncertainty is <1.5%, which is better than the accuracy exhibited by existing experimental teaching devices and can meet the teaching requirements. The maximum proportion of thermal conductivity and wire thermal losses caused by the temperature gradient in the testing area to heating power is approximately 3%, which decreases overall with increasing temperature. The sample with higher emissivity has a relatively small proportion of wire heat loss because of its higher radiation heat dissipation power. The heating power of the sample increases with increasing temperature and hemispherical total emissivity and is positively correlated with the fourth power of temperature and first power of hemispherical total emissivity, which is consistent with the theoretical formula. The heating power of the oxidized sample was significantly higher than that of the unoxidized sample, indicating that the sample with high hemispherical total emissivity exhibits high thermal radiation capability. The experimental results show that the corresponding relation between hemispherical total emissivity and radiation heat transfer capability can be intuitively understood. [Conclusions] The system has the advantages of simple principles and high measurement accuracy, enabling students to understand the correlation between hemispherical total emissivity and material thermal radiation capability, as well as the correlation between spectral emissivity and hemispherical total emissivity at different temperatures. The experimental results are reliable and meet the needs of research-oriented experimental teaching in thermal engineering series courses. [ABSTRACT FROM AUTHOR]

In order to improve the teaching level of engineering thermodynamics experiments and deepen students' understanding of the temperature and pressure characteristics of saturated gases and the concept of supercritical fluids, this article develops an experimental teaching equipment for saturated steam pressure temperature characteristics and supercritical phenomena. This equipment consists of a steam generation system and a data acquisition system, equipped with a large window with a diameter of 60 mm and an automatic control program, which has good visibility, safety, and operability. During the teaching process, four different refrigerants with different properties are used as research objects and teaching projects such as "Research on pressure temperature characteristics of saturated steam" and "Demonstration of critical ophthalmic phenomenon" are established. Through this experimental course, students can enhance their understanding of the micro meanings of pressure and temperature, as well as the concepts of gas-liquid equilibrium in saturated states. They can master common methods and characteristics of pressure and temperature measurement and understand commonly used data processing and error analysis methods. At the same time, students can deepen their understanding of material phases by observing the mutual transformation process between subcritical and supercritical fluids. [ABSTRACT FROM AUTHOR]