Kook-Young Ahn

Optimal Design of a Centrifugal Compressor Impeller Using Evolutionary Algorithms

An optimization study was conducted on a centrifugal compressor. Eight design variables were chosen from the control points for the Bezier curves which widely influenced the geometric variation; four design variables were selected to optimize the flow passage between the hub and the shroud, and other four design variables were used to improve the performance of the impeller blade. As an optimization algorithm, an artificial neural network (ANN) was adopted. Initially, the design of experiments was applied to set up the initial data space of the ANN, which was improved during the optimization process using a genetic algorithm. If a result of the ANN reached a higher level, that result was re-calculated by computational fluid dynamics (CFD) and was applied to develop a new ANN. The prediction difference between the ANN and CFD was consequently less than 1% after the 6th generation. Using this optimization technique, the computational time for the optimization was greatly reduced and the accuracy of the optimization algorithm was increased. The efficiency was improved by 1.4% without losing the pressure ratio, and Pareto-optimal solutions of the efficiency versus the pressure ratio were obtained through the 21st generation.

A Dynamic Model of PEMFC System for the Simulation of Residential Power Generation

A proton exchange membrane fuel cell (PEMFC) system of residential power generator (RPG) has a different operating strategy from the PEMFC system of transportation application because of its environmental difference. In this study, a dynamic simulation model of the PEMFC system is introduced, which has a model for a turbo blower, a membrane humidifier, two cooling circuits, and a PEMFC stack. The thermal efficiency of the PEMFC system for the RPG is very high because it supplies the electricity and hot water to the house. This study is designed to study the dynamic response of individual components during the dynamic change of current density. In particular, since the operation of the turbo blower is very sensitive at low current density the parasitic power consumption of the blower is significant. Additionally, the system performance and the operating strategy are also presented.

Forces and Surface Pressure on a Blade Moving in Front of the Admission Region

The partial admission technique is widely used to control the output power of turbines. In some cases, it has more merits than full admission. However, additional losses, such as expansion, mixing, or pumping, are generated in partial admission as compared with full admission. Thus, an experiment was conducted in a linear cascade apparatus having a partial admission region in order to investigate the effect of partial admission on a blade row The admission region was formed by a spouting nozzle installed at the inlet of the linear cascade apparatus. Its cross section was rectangular and its size is 200 X200 mm 2 . The tested blade was axial-type and its chord was 200 mm. Nineteen identical blades were applied to the linear cascade for the partial admission experiment. The blades moved along the rotational direction in front of the admission region, and then operating forces and surface pressures on the blades were measured at the steady state. The experiment was conducted at a Reynolds number of 3 × 10 5 based on the chord. The nozzle flow angle was set to 65 deg with a solidity of 1.38 for performance test at the design point. In addition, another two different solidities of 1.25 and 1.67 were applied. From the experimental results, when the solidity was decreased, the maximum rotational force increased but the maximum axial force decreased.

Mass and Heat Transfer Analysis of Membrane Humidifier with a Simple Lumped Mass Model

The performance of proton exchange membrane fuel cell (PEMFC) is seriously changed by the humidification condition which is intrinsic characteristics of the PEMFC. Typically, the humidification of fuel cell is carried out with internal or external humidifier. A membrane humidifier is applied to the external humidification of residential power generation fuel cell due to its convenience and high performance. In this study, a simple static model is constructed to understand the physical phenomena of the membrane humidifier in terms of geometric parameters and operating parameters. The model utilizes the concept of shell and tube heat exchanger but the model is also able to estimate the mass transport through the membrane. Model is constructed with FORTRAN under Matlab/Simulink O environment to keep consistency with other uf0d2 components model which we already developed. Results shows that the humidity of wet gas and membrane thickness are critical parameters to improve the performance of the humidifier

A System Simulation Model of Proton Exchange Membrane Fuel Cell for Residential Power Generation for Thermal Management Study

초록:이온교환막연료전지는전세계적인에너지고갈문제와온실효과에대한대응책의하나이다.특히,이온교환막연료전지는전기화학반응에의해전기를생산함과동시에열을발생하기때문에가정용으로적용하기에적당하다.가정용연료전지의열관리목적은연료전지가최적조건에서운전할수있도록적절히온도를제어해주는것으로, 본연구에서는부하변화시가정용연료전지시스템의응답특성과열관리특성을알아보기위한해석모델을개발하였다. 열관리해석모델은연료전지의온도를조절하기위한펌프와열교환기로구성된1차측,주택에온수를공급하기위한탱크와펌프계통의2차측으로구성되었다.부하를순차적으로증가시킬때와감소시킬때를구분하여열관리계통의응답특성을확인하였다.결과적으로탱크의초기승온에많은시간이소요되기때문에부하를다단으로오랜시간동안서서히증가시키면서시스템응답특성을확인하였다.또한,본연구에서는가정용연료전지의부하변화시의열관리특성을고려한운전전략에대해서도조사하였다.Abstract:APEMFC(protonexchangemembranefuelcell)isagoodcandidateforresidentialpowergenerationto be coped with the shortage of fossil fuel and green house gas emission. The attractive benefit of thePEMFC is to produce electric power as well as hot water for home usage. The thermal management ofPEMFC for RPG is to utilize the heat of PEMFC so that the PEMFC can be operated at its optimalefficiency.Inthisstudy,thermalmanagementsystemofPEMFCstackismodeledtounderstandthedynamicresponse during load change. The thermal management system of PEMFC for RPGFC is composed of twocoolingcircuits,oneforcontrolingthefuelcelltemperatureandtheotherforheatingupthewaterforhomeusage. The different operating strategy is applied for each cooling circuit considering the duty of those twocircuits. Even though the capacity of PEMFC system (1kW) is enough to supply hot domestic water forresidence, heat-up of reservior takes some hours. Therefore, in this study, time schedule of the simulationreflects the heat-up process. Dynamic responses and operating strategies of the PEMFC system areinvestigatedduringloadchanges.

Influence of Operating Conditions on the Performance of a Oxy-fuel Combustion Reference Cycle

Recently, there has been growing interest in the oxyfuel combustion cycle since it enables high-purity efficiency. However, the oxyfuel combustion cycle has some important issues regarding to its performance such as the requirement of water recirculation to decrease a turbine inlet temperature and proper combustion to enhance cycle efficiency. Also, Some of water vapour remain not condensed at condenser outlet because cycle working fluid contains non-condensable gas, i.e., . The purpose of the present study is to analyze performance characteristics of the oxyfuel combustion cycle with different turbine inlet temperatures, combustion pressures and condenser pressure. It is expected that increasing the turbine inlet temperature improves cycle efficiency, on the other hand, the combustion pressure has specific value to display highest cycle efficiency. And increasing condensing pressure improves water vapour condensing rate.

Design Parametric Analysis of PEM Fuel Cell and Hybrid Systems

Performance of PEM fuel cell systems and hybrid systems combining a PEMFC with a gas turbine have been evaluated. Two different reforming methods(steam reforming and autothermal reforming) were considered. Performances of fuel cell systems with two reforming methods were compared and effects of various design parameters on the system performance were investigated. Configurations of PEM fuel cell systems with two reforming methods have been revised to accommodate a gas turbine, resulting in PEMFC/GT hybrid systems. Performance of the hybrid systems were analyzed and compared with those of PEM systems. Influences of major design parameters on the hybrid system performance were also investigated.

Numerical Analysis of Steam-methane Reforming Reaction for Hydrogen Generation using Catalytic Combustion

A steam reformer is a chemical reactor to produce high purity hydrogen from fossil fuel. In the steam reformer, since endothermic steam reforming is heated by exothermic combustion of fossil fuel, the heat transfer between two reaction zones dominates conversion of fossil fuel to hydrogen. Steam Reforming is complex chemical reaction, mass and heat transfer due to the exothermic methane/air combustion reaction and the endothermic steam reforming reaction. Typically, a steam reformer employs burner to supply appropriate heat for endothermic steam reforming reaction which reduces system efficiency. In this study, the heat of steam reforming reaction is provided by anode-off gas combustion of stationary fuel cell. This paper presents a optimization of heat transfer effect and average temperature of cross-section using two-dimensional models of a coaxial cylindrical reactor, and analysis three-dimensional models of a coaxial cylindrical steam reformer with chemical reaction. Numerical analysis needs to dominant chemical reaction that are assumed as a Steam Reforming (SR) reaction, a Water-Gas Shift (WGS) reaction, and a Direct Steam Reforming(DSR) reaction. The major parameters of analysis are temperature, fuel conversion and heat flux in the coaxial reactor.

Performance Characteristics of a Partially Admitted Small-Scale Mixed-Type Turbine

Abstract In this study, a mixed-type turbine was designed and tested with the double or single stage to improve the specific torque when it operates at a low partial admission rate. The turbine consists of double stages and the outer diameter of its rotor is 108 mm. The turbine rotor blades were designed as an axial-type blade along the mixed flow direction because the partial admission rate was 1.7–2.0% depending on the flow direction. Performance characteristics were measured at the double and single stage rotors to investigate the effect of the second stage on the low partial admission. In addition, when the flow direction was radially inward or outward at the nozzle, turbine performances were studied. In this experiment, the specific power, torque, and total-to-static efficiency were measured at various rotational speeds to compare with the turbine performance according to different operating condition. The tested results showed that the second stage should be adopted to increase the operating torque when the operating rotational speed was less than the critical rotational speed. The specific torque was improved by 7.8% using the second stage at a radially inward flow direction turbine

A Study of the Design Technology for Developing a 100kW Class Steam Turbine

ABSTRACT Small scale steam turbines are used as mechanical drivers in chemical process plant or power generators. In this study, a design technology was developed for a 100kW class steam turbine which will be used for removing CO 2 from the emission gas on a reheated cycle system. This turbine is operated at a low inlet total pressure of 5 kgf/cm 2 . It consists of two stages and operates at the partial admission. For the meanline analysis, a performance prediction method was developed and it was validated through the performances on the operating small steam turbines which are using at plants. Their results showed that the output power was predicted within 10% deviation although the steam turbines adopted in this analysis were operated at different flow conditions and rotor size. The turbine blades was initially designed based on the computed results obtained from the meanline analysis. A supersonic nozzle was designed on the basis of the operating conditions of the turbine, and the first stage rotor was designed using a supersonic blade design method. The stator and second stage rotor was designed using design parameters for the blade profile. Finally, Those blades were iteratively modified from the flow structures obtained from the three-dimensional flow analysis to increase the turbine performance. The turbine rotor system was designed so that it could stably operate by 76% separation margin with tilting pad bearings.