Soo-Yong Cho

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.

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.

An experimental study of the performance characteristics with four different rotor blade shapes on a small mixed-type turbine

A small mixed-type turbine with a diameter of 19.9 mm has been substituted for a rotational part of pencil-type air tool. Usually, a vane-type rotor is applied to the rotational part of the air tool. However, the vane-type rotor has some problems, such as friction, abrasion, and necessity of accurate assembly etc.,. These problems make the life time of the vane-type air tool short, but air tools operated by mixed-type turbines are free of friction and abrasion because the turbine rotor dose not contact with the casing. Moreover, it is assembled easily because of no axis offset. These characteristics are merits for using air tools, but loss of power is inevitable on a non-contacting type rotor due to flow loss, tip clearance loss, and profile loss etc.,. In this study, four different rotors are tested, and their characteristics are investigated by measuring the specific output power. Additionally, optimum nozzle location against the rotor is studied. Output powers are obtained through measured pressure, temperature, torque, rotational speed, and flow rate. The experimental results obtained with four different rotors show that the rotor blade shape greatly influences to the performance, and the optimum nozzlc location exists near the mid span of the rotor.

a study on the propagation of measurement uncertainties into the result on a turbine performance test

Uncertainties generated from the individual measured variables have an influence on the uncertainty of the experimental result through a data reduction equation. In this study, a performance test of a single stage axial type turbine is conducted, and total-to-total efficiencies are measured at the various off-design points in the low pressure and cold state. Based on an experimental apparatus, a data reduction equation for turbine efficiency is formulated and six measured variables are selected. Codes are written to calculate the efficiency, the uncertainty of the efficiency, and the sensitivity of the efficiency uncertainty by each of the measured quantities. The influence of each measured variable on the experimental result is figured out. Results show that the largest uncertainty magnification factor (UMF) value is obtained by the inlet total pressure among the six measured variables, and its value is always greater than one. The UMF values of the inlet total temperature, the torque, and the RPM are always one. The uncertainty percentage contribution (UPC) of the RPM shows the lowest influence on the uncertainty of the turbine efficiency, but the UPC of the torque has the largest influence to the result among the measured variables. These results are applied to find the correct direction for meeting an uncertainty requirement of the experimental result in the planning or development phase of experiment, and also to offer ideas for preparing a measurement system in the planning phase.

A Study on an Axial-Type 2-D Turbine Blade Shape for Reducing the Blade Profile Loss

Losses on the turbine consist of the mechanical loss, tip clearance loss, secondary flow loss and blade profile loss etc.,. More than 60 % of total losses on the turbine is generated by the two latter loss mechanisms. These losses are directly related with the reduction of turbine efficiency. In order to provide a new design methodology for reducing losses and increasing turbine efficiency, a two-dimensional axial-type turbine blade shape is modified by the optimization process with two-dimensional compressible flow analysis codes, which are validated by the experimental results on the VKI turbine blade. A turbine blade profile is selected at the mean radius of turbine rotor using on a heavy duty gas turbine, and optimized at the operating condition. Shape parameters, which are employed to change the blade shape, are applied as design variables in the optimization process. Aerodynamic, mechanical and geometric constraints are imposed to ensure that the optimized profile meets all engineering restrict conditions. The objective function is the pitchwise area averaged total pressure at the 30 % axial chord downstream from the trailing edge. 13 design variables are chosen for blade shape modification. A 10.8 % reduction of total pressure loss on the turbine rotor is achieved by this process, which is same as a more than 1 % total-to-total efficiency increase. The computed results are compared with those using 11 design variables, and show that optimized results depend heavily on the accuracy of blade design.

Performance Characteristics of a Small-Scale Axial-Type Turbine Operated in a Low Partial Admission Rate

The performance characteristics of a partially admitted small-scale axial-type turbine, which could be applied to a driver of micro air grinders are experimentally studied with various nozzles, stators, and rotors. When air tools adopt axial-type turbines as drivers, they can operate without friction and abrasion because the turbine rotor does not make contact with the casing. In order to maintain these merits on a small-scale axial-type turbine without reducing power, performance characteristics are examined through measuring the specific output power and the net specific output torque with five different stators and with three different rotors and nozzles. The tested turbine consists of two stages and its mean radius of flow passage is 8.4 mm. The experimental results show that the improvement on the first stage is important to obtain the high specific output power because partially admitted flow is fully diffused in the second stage. Blade angles greatly influence the performance of a small-scale turbine operating in partial admission, and the optimal incidence angle is about 10.3°. At the fixed nozzle flow angle, the net specific output torque is varied by 15% by changing the rotor blade angle. Key W o r d s : Partial Admission, Micro-Turbine, Axial-Type Turbine, Turbine Performance, Air Tool.

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.

An Experimental Study of the Performance Characteristics on a Multi-Stage Micro Turbine with Various Stages

An experimental study on an axial-type micro turbine which consists of maximum 6 stages is conducted to measure aerodynamic characteristics on each stage. This turbine has a 2.0 flow coefficient, 3.25 loading coefficient and 25.8mm mean diameter. The solidity of stators and rotors is within a 0.67~0.75, and the off-design performance is measured by changing the load after adjusting the mass flowrate and the total pressure to constant at inlet. A maximum specific output power of 2kW/kg/sec is obtained in one stage, but the increment of the specific output power with increasing stages is alleviated. In case of torque, the increment of the torque maintains to constant at low RPM region, but its increment become dull at high RPM region. The efficiency of the micro turbine becomes low because the tip gap effect is great due to the small blade, but it could be improved by increasing the stages.

Experimental study of the incidence effect on rotating turbine blades

Abstract Incidence is an important design parameter in the design of turbine blades, and many experiments have been conducted to ascertain an optimum incidence. However, most experiments were conducted in linear or annual cascade rows. From those experiments, one can recognize an optimum incidence that can minimize pressure losses in the passage. However, the optimum incidence suggested by the experimental results in the cascade rows could be unsuitable as a design parameter in the design of rotating parts. In this study, a turbine rotor instead of the cascade rows is used to find an optimum incidence in a rotating state. An experiment is conducted in a low-pressure and cold state with a single-stage axial-type turbine. The rotational speed and turbine output power are controlled by a dynamometer. Total-to-total efficiencies are obtained at several off-design points. Mass flow rate, torque, rotational speed, pressures and temperatures are measured at inlet or exit. Five different rotor discs are used to get some variation of blade inlet angle so that the rotor operates with different incidence even though it rotates at the same speed; that is, five cases are tested and compared with the total-to-total efficiencies obtained at various incidence angles. Experimental results show that the incidence on the rotor has a great influence on the turbine efficiency, and the optimum incidence depends on the input power as well as the rotor blade profile. The range of applicable incidence becomes narrow when the turbine operates at high input power, and turbine efficiency quickly drops down as the incidence grows to negative over the range of applicable incidence.