Yu Daren

Nonlinear coordinated control of drum boiler power unit based on feedback linearization

This work analyzes the feasibility of applying feedback linearization technique to the nonlinear control of superheater steam pressure and power output of a boiler-turbine generating unit. A nonlinear coordinated controller has been designed and evaluated through computer simulation using a well-established and validated nonlinear boiler-turbine model. The simulation results are presented to illustrate the performance of a nonlinear coordinated control system and compare the proposed strategy with the conventional direct energy balance control strategy. The simulation results show that improvement can be achieved in control with a nonlinear control system. The real benefits obtained include improved dispatch rate, tighter regulation of steam pressure, and reduced maintenance over a wide range of operating conditions, and these improvements in control can be maintained for large changes in operating conditions.

Topological Geometry Interpretation of Supersonic Inlet Start/Unstart Based on Catastrophe Theory

U NSTART phenomenon is one of the most important issues of hypersonic inlets. If an inlet is not started, the mass capture will be greatly reduced, and the spillage drag will be excessive. For hypersonic air-breathing engines, inlet unstart will cause a large drop of both engine thrust and specific impulse, and it may cause catastrophic damage during hypersonic flight. Over the past decades, many investigations have been conducted to examine the mechanisms of inlet start and unstart. Mayer and Paynter [1,2] simulated an axisymmetric inlet unstart due to the variation of freestream variables such as temperature, velocity, and pressure. Neaves and McRae [3] simulated the 3-D inlet unstart caused by a combustor perturbation. Zha et al. [4,5] investigated unstart transient mechanism of a typical axisymmetric inlet at angle of attack. Cox et al. [6] presented several mechanisms of hypersonic inlet unstart, including backpressure unstart, overcontraction unstart, and angle of attack unstart. Van Wie and Kwok [7], Yuan and Liang [8], and others have observed the nonlinear catastrophe and hysteresis phenomena of inlet start/unstart. These studies have made contributions from different aspects. As from these studies, the start/unstart process is influenced by many factors, and the physical law is complex with strong nonlinearities such as catastrophe, hysteresis, etc. Because inlet unstart may cause catastrophic damage to the aircraft, it must be avoided and controlled during hypersonic flight. With this consideration, it is important to study the physical law of inlet start/unstart, for example, to study the catastrophe boundaries of inlet start/unstart at different operation conditions (different flight Mach number, angle of attack, flight height, and back pressure). The physical law of inlet start/unstart will be important for further application (i.e., avoid or control inlet unstart). As known, the transition between inlet start and unstart is mainly characterized by catastrophe, and such systems can be studied by catastrophe theory. With this consideration, this paper will try to use the topological geometry method in catastrophe theory to study the physical law of inlet start/unstart. In general, catastrophe theory permits the establishment of qualitative and general solutions of the catastrophe behaviors, and it can deal with complex systems with catastrophe properties. In this paper,first, the catastrophemechanism of inlet start/unstart is analyzed. Second, the nonlinear catastrophe, hysteresis, and bifurcation features of inlet start/unstart are simulated based on the topological geometrymethod in catastrophe theory, and the catastrophe boundaries of inlet start/unstart are obtained. Third, the physical laws inlet start/unstart are interpreted along typical control routes around the catastrophe boundaries.

Computer simulations of Hall thrusters without wall losses designed using two permanent magnetic rings

A type of Hall thruster without wall losses is designed by adding two permanent magnet rings in the magnetic circuit. The maximum strength of the magnetic field is set outside the channel. Discharge without wall losses is achieved by pushing down the magnetic field and adjusting the channel accordingly. The feasibility of the Hall thrusters without wall losses is verified via a numerical simulation. The simulation results show that the ionization region is located in the discharge channel and the acceleration region is outside the channel, which decreases the energy and flux of ions and electrons spattering on the wall. The power deposition on the channel walls can be reduced by approximately 30 times.

Effect of the Hollow Cathode Heat Power on the Performance of an Hall-Effect Thruster

Effect of the hollow cathode heat power on the performance of a Hall-effect thruster is investigated. The variations in the Hall-effect thrusters performance (thrust, specific impulse and anode efficiency) with the hollow cathode heat power was obtained from the analysis of the experimental data. Through an analysis on the coupling relationship between the electrons emitted from the hollow cathode and the environmental plasma, it was found that the heat power would affect the electron emission of the emitter and the space potential of the coupling zone, which would lead to a change in the effective discharge voltage. The experimental data agree well with the results of calculation which can be used to explain the experimental phenomena.

Effect of ionization distribution on the low frequency oscillations mode in Hall thrusters

It is found that the discharge parameters have notable effects on the mode of discharge current low frequency oscillation in Hall thrusters, but different discharge parameters might form the similar low frequency oscillation mode. In order to study the mechanism of oscillation mode formation, the ionization distribution in the discharge channel was measured experimentally, and one-dimensional quasi-neutrality hydrodynamic model was used to study the relationship between ionization distribution and the oscillation mode. Researches show that the low frequency oscillation with a narrow and condensed ionization distribution has the mode of lower amplitude and scattered frequency. The low frequency oscillation amplitude would become high and have dominative frequency component with the relative wide ionization distribution. Therefore, it can be concluded that the difference of ionization distribution characteristics is the main reason of the oscillation mode variation, and discharge parameters are only the external control parameters of ionization distribution characteristics.

A Novel Feature Selection Approach Using Classification Complexity for SVM of Stock Market Trend Prediction

As a preprocessing method of data mining with SVM, feature selection can eliminate irrelevant or redundant attributes and increase the density of samples in feature spaces that can improve classification performance. In the field of financial time series pattern recognition, the study of feature selection has been receiving increasing attention. Different from other studies, this work use two new coefficients: neighborhood dependence (ND) and neighborhood decision error (NDEM) to measure the classification complexity as a method of feature selection. In the real example, we use ASH, ASNN, ND and NDEM to measure the classification complexity of technical indicators data set of Shanghai stock exchange (SSE). Then we take the reduced sets which selected by the above four coefficients as input data for SVM. Compared with the forecasting results of SVM for the other three coefficients, the SVM with NDEM reduced data set has higher classification accuracy.

The mathematical models of the periodical literature publishing process

Abstract Based on a theoretical analysis, the paper establishes the mathematical models of the periodical publishing process: a continuous model and a discrete model — a group of difference equations, which is derived from the former. The validity of the models is demonstrated by a particular solution of the continuous model at steady state. To some degree, the paper provides an effective mathematical tool for the quantitative study of the periodical literature publishing process.

Catastrophe Model for Supersonic Inlet Start/Unstart

Mathematical models may be very valuable for studying the flow laws of supersonic inlet start/unstart. From a viewpoint of structure stability, this paper makes a link between the catastrophe points of inlet start/unstart and the singular points in Whitney’s singularity theory, from which we take notice of the conception of singularity classification, which may be helpful for modeling the catastrophe points of inlet start/unstart. By further research, Thom’s catastrophe theory is found to be the right mathematical tool to classify general singularities by topological method. This paper makes an effort to model the catastrophe points of inlet start/unstart by the elementary catastrophe model in catastrophe theory and puts forward a new method to fit data to the catastrophe model. The method is based on nonlinear topological transformation, which has provided an improvement over the existing Cobb’s method. Simulation results have shown that the present model is in good agreement with the data.

Thermodynamic analysis for a chemically recuperated scramjet

Endothermic hydrocarbon fuel is regarded as an optimal fuel for a scramjet with regenerative cooling, which provides extra cooling through endothermic chemical conversion to avoid the severly limited cooling capacity when conventional fuels are adopted for cooling. Although endothermic cooling is proposed from the view point that the heat sink of a conventional fuel is insufficient, the heat-absorbing through endothermic chemical reaction is actually a chemical recuperation process because the wasted heat dissipated from the engine thermal structure is recovered through the endothermic chemical reaction. Therefore, the working process of a scramjet with endothermic hydrocarbon fuel cooling is a chemical recuperative cycle. To analyze the chemical recuperative cycle of a chemically recuperated scramjet engine, we defined physical and chemical recuperation effectivenesses and heating value increment rate, and derived engine performance parameters with chemical recuperation. The heat value benefits from both physical and chemical recuperations, and it increases with the increase in recuperation effectiveness. The scramjet performance parameters also increase with the increase in chemical recuperation effectiveness. The increase in chemical recuperation effectiveness improves both the performances of the fuel cooling system and the combustion system. The results of analysis prove that the existence of a chemical recuperation process greatly improves the performance of the whole scramjet.

Experimental study on the role of a resistor in the filter of Hall thrusters

A filter is a mainly component applied to reduce the discharge current low frequency oscillation in the range of 10–100 kHz. The only form of the filter in actual use involves RLC networks, whose design originates from the 1970s, but even now, researchers are unaware of the actual primary motivations for the resistor’s presence [S. Barral et al., AIAA Paper 2008-4632, 2008]. Therefore, the role of the resistor in the filter is experimentally studied and discussed through the analysis of control system and electric circuit theory. Experimental results and analysis indicate that the presence of a resistor makes the filter having the phase compensation function. The proper phase-angle and amplitude provided by the filter would increase or decrease the ion mobility and be helpful to balance the ion production in the discharge channel and then to decrease the fluctuation of the plasma density and lower the low frequency oscillation.