Performance analysis of a triple pressure hrsg

Operating characteristics of a triple pressure reheat HRSG are analyzed using a commercial software package (Gate Cycle by GE Enter Software). The calculation routine determines all the design parameters including configuration and area of each heat exchanger. The off-design calculation part has the capability of simulating the effect of any operating parameters such as power load, process requirements, and operating mode, etc., on the transient performance of the plant. The arrangement of high-temperature and intermediate-temperature components of the HRSG is changed, and its effect on the steam turbine performance and HRSG characteristics is examined. It is shown that there could be a significant difference in HRSG sizes even though thermal performance is not in great deviation. From the viewpoint of both economics and steam turbine performance, it should be carefully reviewed whether the optimum design point could exist. Off-design performance could be one of the main factors in arranging components of the HRSG because power plants operate at various off-design conditions such as ambient tempera-ture and gas turbine load, etc. It is shown that different heat exchanger configurations lead to different performances with ambient temperature, even though they have almost the same performances at design points.     

Octanethiol coating of nano-sized copper powders using the vapor self-assembled monolayer method

Recent research has focused on preventing copper oxidation for direct wiring applications. In this study, a novel coating method was developed for nano-sized copper powders using vapor self-assembled monolayer (VSAM). The surfaces of oxidized and non-oxidized copper powders were studied to determine whether they can be coated with a vapor form of octanethiol. It was found that non-oxidized copper powders were successfully coated with the vapor form of octanethiol according to X-ray photoelectron spectroscopy (XPS) analysis. However, due to the partial coating of self-assembled monolayer (SAM) on a copper surface, a very small amount of copper oxide was detected when the coating mechanism was verified using Atomic Force Microscopy (AFM). Overall, the vapor form of octanethiol successfully adhered to the surface of nanosized copper powders, constituting the novel method of preventing oxidation. A complete coating method for the nano-sized copper powders must be developed in future research.     

High performance hydraulic design techniques of mixed-flow pump impeller and diffuser

In this paper, we describe a numerical study about the performance improvement of a mixed-flow pump by optimizing the design of the impeller and diffuser using a commercial computational fluid dynamics (CFD) code and design-of-experiments (DOE). The design variables of impeller and diffuser in the vane plane development were defined with a fixed meridional plane. The design variables were defined by the vane plane development, which indicates the blade-angle distributions and length of the impeller and diffuser. The vane plane development was controlled using the blade-angle in a fixed meridional plane. The blade shape of the impeller and diffuser were designed using a traditional method in which the inlet and exit angles are connected smoothly. First, the impeller optimum design was performed with impeller design variables. The diffuser optimum design was performed with diffuser design variables while the optimally designed impeller shape was fixed. The importance of the impeller and diffuser design variables was analyzed using 2^k factorial designs, and the design optimization of the impeller and diffuser design variables was determined using the response surface method (RSM). The objective functions were defined as the total head (Ht) and the total efficiency (?t) at the design flow rate. The optimally designed model was verified using numerical analysis, and the numerical analysis results for both the optimum model and the reference model were compared to determine the reasons for the improved pump performance. A pump performance test was carried out for the optimum model, and its reliability was proved by a comparative analysis of the results of the numerical analysis and an experiment using the optimum model.     

Reliability-based design optimization of axial compressor using uncertainty model for stall margin

Reliability-based design optimization (RBDO) of the NASA stage 37 axial compressor is performed using an uncertainty model for stall margin in order to guarantee stable operation of the compressor. The main characteristics of RBDO for the axial compressor are summarized as follows: First, the values of mass flow rate and pressure ratio in stall margin calculation are defined as statistical models with normal distribution for consideration of the uncertainty in stall margin. Second, Monte Carlo Simulation is used in the RBDO process to calculate failure probability of stall margin accurately. Third, an approximation model that is constructed by an artificial neural network is adopted to reduce the time cost of RBDO. The present method is applied to the NASA stage 37 compressor to improve the reliability of stall margin with both maximized efficiency and minimized weight. The RBDO result is compared with the deterministic optimization (DO) result which does not include an uncertainty model. In the DO case, stall margin is slightly higher than the reference value of the required constraint, but the probability of stall is 43%. This is unacceptable risk for an aircraft engine, which requires absolutely stable operation in flight. However, stall margin obtained in RBDO is 2.7% higher than the reference value, and the probability of success increases to 95% with the improved efficiency and weight. Therefore, RBDO of the axial compressor for aircraft engine can be a reliable design optimization method through consideration of unexpected disturbance of the flow conditions.     

Multi-objective optimization for mixed-flow pump with blade angle of impeller exit and diffuser inlet

During design optimization, the impeller and diffuser of a mixed-flow pump are generally optimized separately. In such cases, the total head can be overdesigned. In this study, the designs of the impeller and diffuser were optimized simultaneously by using computational fluid dynamics and the Response surface method (RSM). Design variables were defined according to the vane plane development of the impeller and diffuser. Three-dimensional Reynolds-averaged Navier–Stokes equations for the shear stress transport turbulence model were discretized by finite volume approximations and solved on hexahedral grids to analyze the flow in the pump. The total head and total efficiency were selected as objective functions, with four design variables related to the impeller outlet angles and diffuser inlet angles used for optimization. The RSM was constructed based on the objective functions with design points generated from the central composite method. The hydraulic performance of the optimum model was analyzed.     

Numerical study on the range enhancement of a centrifugal compressor with a ring groove system

A numerical study of casing treatments on a centrifugal compressor to improve stability and stall margin is presented. High efficiency, high pressure ratio, and a wide operating range are required for a high-performance centrifugal compressor. A ring groove casing treatment is effective for flow range enhancement in centrifugal compressors. Compressor performance was analyzed according to the ring groove location, and the results were compared with the case without a ring groove. The effect of guide vanes in the ring groove was also investigated. Four variants of grooves were modeled and simulated using computational fluid dynamics to optimize the groove location. Numerical analysis was performed using a commercial code ANSYS-CFX program. The simulation results showed that the ring groove increased the operating range of the compressor. The ring groove with guide vanes improved both performance of the compressor at low flow rates and the stall margin of the compressor.     

A dead-time minimization algorithm for improving the output characteristics of the PWM inverter

The dead time in an inverter is necessary to prevent the short circuit of the DC source. However, the dead time may cause serious problems such as waveform distortion, voltage drop, increased torque ripple and heating of the motors. In this paper, a dead-time minimization algorithm is proposed for improving the inverter output performance. The adverse effects of the dead time are investigated, focusing on the voltage drop and the distortion factor of inverter output current. The proposed algorithm consists of forbidding unnecessary triggers for the inverter switches that are not turned on although the gate drive signal is impressed. The proposed algorithm is explained in terms of the conduction modes of the output currents. The validity of the proposed method is verified by comparing the simulated and experimental results with those of the conventional methods. It is concluded from the results that the proposed algorithm can reduce the output current harmonics. Further, the output voltage can be equal to the reference value and the number of inverter switchings can also be reduced to 50% compared with those of conventional methods.     

An improved two-switch flyback converter with regenerative clamping

A novel soft-switching two-switch flyback converter is proposed in this article. This converter is composed of two active power switches, a flyback transformer and two passive regenerative clamping circuits. The proposed converter has the advantages of a low-cost circuit configuration, a simple control scheme, high efficiency, and a wide operating range. The circuit topology, analysis, design considerations and experimental results of the new flyback converter are presented.     

The relationships between interfacial friction and the conformational order of organic thin films

This report describes studies of the relationships between the structures of organic monolayers and their molecular-scale frictional properties. Several distinct self-assembled monolayers (SAMs) were formed by the adsorption of a series of spiroalkanedithiols and a single structurally related normal alkanethiol. Measurements of hexadecane wettability, infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy revealed that the films possessed a wide range of interfacial structures and conformational orders. Atomic force microscopy was used to measure the frictional properties of the well-characterized SAMs on the molecular scale. Comparison of the frictional data with structural information derived from complementary analytical techniques revealed a high correlation between the conformational order of the films and the observed frictional response.     

Performance of a shell-and-tube heat exchanger with spiral baffle plates

School of Mechanical and Industrial System Engineering, Dong-Eui University, Busan 614-714, Korea In a conventional shell-and-tube heat exchanger, fluid contacts with tubes flowing up and down in a shell, therefore there is a defect in the heat transfer with tubes due to the stagnation portions. Fins are attached to the tubes in order to increase heat transfer efficiency, but there exists a limit. Therefore, it is necessary to improve heat exchanger performance by changing the fluid flow in the shell. In this study, a highly efficient shell-and-tube heat exchanger with spiral baffle plates is simulated three-dimensionally using a commercial thermal-fluid analysis code, CFX4.2. In this type of heat exchanger, fluid contacts with tubes flowing rotationally in the shell. It could improve heat exchanger performance considerably because stagnation portions in the shell could be removed. It is proved that the shell-and-tube heat exchanger with spiral baffle plates is superior to the conventional heat exchanger in terms of heat transfer.