Applied Mechanics and Materials Vols. 536-537

Abstract: Rod-shaped β-FeOOH was synthesized through a well controlled hydrothermal reaction of FeCl₃ and NH₂CONH₂. By varying the preparatory conditions such as NH₂CONH₂ concentration and reaction parameter we could establish precise control on the morphology of the sample. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were used to characterize the as-prepared products. The results showed the aspect ratio and mean size of theβ-FeOOH nanorods was achieved systematic control by regulating the concentration of urea. When the experimental time increased to 96h, the morphology of the resulting β-FeOOH changed from rod to nanocubes. Furthermore, the resulting rod-shaped iron hydroxides were transformed into pseudocube α-Fe₂O₃ by increasing the synthesis temperature at 160°C.

Abstract: Aluminum is supposed to be admirable phase change materials (PCMs) for thermal energy storage due to its excellent heat storage density and thermal conductivity. However, its application is limited because of the flowability and corrosivity when it melt. The Al/AlN-Al₂O₃ composite PCMs with stable thermal energy storage performance were prepared by direct nitridation method. The research showed that nitridation temperature and nitridation time are key parameters of heat storage performance of the composite materials. The morphology and thermal energy storage performance and components of the Al/AlN-Al₂O₃ composite PCMs were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD). The results showed that the tiny particles of aluminum powders were closely coated by AlN-Al₂O₃ encapsulating materials. This structure can prevent the flowability of the liquid aluminum and also can avoid environmental impact when the it melt, meanwhile , the composite PCMs have the performance such as stability, high thermal storage density.

Abstract: A band structure composed of a square array of parallel steel tube with narrow slits is presented.The propagation characteristic of acoustic waves in this structure is investigated theoretically by the finite element method. In particular the acoustic-solid coupling is taken into account for accurate results. The transmission coefficients of the band system with different angles of the square tubes and slits width are calculated. A large continuous band gap at a low frequency range in a compound structure is obtained due to the interaction of peaks and gaps fordifferent Structural parameters.

Abstract: Aiming at the problems of the choice of system parameters has great influence on stability of system in the push-pull DC/DC converter system. The relationship between system parameters and system stability was studied based on the push-pull DC/DC converter simulation model of MATLAB. Due to the push-pull DC/DC converter system is nonlinear, the system was analyzed by poincare mapping graph. This paper mainly analyzed the influence of the ratio of converter transformer on system stability. It is shown by simulation that the system operation state transferred from stable period to chaos, when the value of transformer ratio changed from 7 to 4. The study results are valuable for the design of switching power supply system parameters, and would have a wide application foreground.

Abstract: Aiming at the application requirements of miniaturization and energy saving in the field of high voltage power supply, a high voltage switching power supply with soft switching technology was designed. The soft switch technology which makes the loss of switch power supply decrease obviously,and the dual voltage rectifier technology can effectively reduce the volume of switching power supply. Circuit parameters of switching power supply are optimized in order to further improve the efficiency of power supply. From the experiment results, the high voltage switching power supply meets the design requirements, and suitable for the strict requirements of environment on the volume and efficiency.

Abstract: This paper analyzes the effect of stable behaviors when amplitude, phase of sine voltage compensation signal are added in the system, reveals that the dynamical behaviors mechanism that sine voltage compensation signal changes feedback voltage-mode controlled buck converter lies in changing the duty cycle without impacting the system stable error via analyzing the change of period multiplier in Monodromy matrix and conditions of period bifurcation, and finally achieves stabilization control for bifurcation and chaotic behaviors. The simulation and experimental results prove the correctness of the theoretical analysis.

Abstract: In order to stabilize the uncertainty of large-scale new energy power’s random fluctuation in the grid, there is an increasing number of large thermal power generating unit needing to do deep variable load operation. However, the pattern of steam inlet on turbine’s part load has a very significant impact on the unit operation condition of safety, stability and economy. In this paper the HP-valve’s body vibration fault of large steam turbines caused by unsteady steam flow under partial arc admission operating at part of their full load is researched, and an economic solution based on analysis and diagnosis of fault mechanism is provided by designing of complex HP-valve opening sequence rules. This solution solves the safety problem of valve vibration and avoids the economic loss by using full arc admission or replacing the valve body equipment directly, which is of great effective and practical verified by units’s actual operating experiment. As a result, it demonstrates that the optimization of HP-valve iadmission mode can not only change the stress state of high pressure rotor and prevent its vibration caused by the force of unbalanced flow to improve the shafting stability of the unit when running with variable load, but also can improve the steam instability and solve the resulting vibration problem in HP-valve. And it is of great engineering practical value to improve high-power thermal power unit depth secure efficient load operation.

Abstract: In the high-voltage direct current transmission system, the difference value between the landing phase voltage and DC transmission system commutation failure of the critical voltage drop value, as system occurred in the critical value of commutation failure. When commutation voltage lower than the critical value would reduce arc Angle, caused by commutation failure。Therefore, by using the method of reactive power compensation to keep converter bus voltage stability, can avoid commutation failure.

Abstract: In order to detect switch contact temperature of high voltage switch cabinet in the operation of the cabinet, this paper puts forward a wireless measurement way to solve the measuring temperature problem of high voltage switch of the switch contact using magnetic field coupling idea. The temperature danger can be achieved in time to reduce switch burn injury. The device is mainly composed of temperature acquisition part and temperature display part. The two parts are connected by wireless data transmission. By test determination, the method is accurate and convenient to measure the temperature of the switch contact and then completely eliminate major accidents caused by switch temperature rise.

Abstract: According to the unreasonable phenomenon of coordination of V/Hz limit and over-excitation protection, summarize the coordination principles of V/Hz limit and over-excitation protection with definite time and the principles of V/Hz limit and over excitation protection with inverse time. Analysis the operation characteristics of V/Hz limit and over-excitation protection, and understand two sets of schemes of over-excitation. By learning the relevant theoretical knowledge and their function, it can be derived that V/Hz restriction should take precedence over excitation protection action. On this basis, combined with engineering example to analysis, put forward to consider a reasonable difference to guarantee the safety of generations better when both have a reasonable coordination.