Bong-Jun Cha

Effects of Fuel Injectors and Cavity Configurations on Supersonic Combustion

In this study, the effects of the diameter and quantity of fuel nozzles combined with various cavity-type flame holders for supersonic combustion were investigated empirically and numerically. Although increasing the quantity of fuel nozzles yields a greater fuel-to-air interface area, which results in a higher fuel–air mixing rate, smaller nozzle diameters yield a lower penetration depth, which results in a lower combustion performance at the top wall region. This study adopts the cavity shapes of fuel injectors from previous research studies to design new injector configurations with different sizes and quantities of fuel nozzles. The diameter and quantity of nozzles are determined to have the same equivalence ratio and momentum ratio as were used in previous research under a given fuel pressure. A comparison of results from the present tests with previous research shows that the plain cavity and zigzag cavity have distinct characteristics. Configurations using these cavity types were investigated via t...

Aerodynamic Optimization of Axial Turbine Tip Cavity With Approximation Model

Design optimization of unshrouded rotor tip cavity of a high pressure turbine stage with low aspect ratio was carried out to maximize the turbine stage efficiency. Cavity shapes were parameterized with 4 design variables including rim thickness, cavity depth, cavity front blend radius and cavity aft blend radius. Initially the CCD method was utilized for sampling experimental points and the Kriging method was chosen to construct an approximation model. The optimum points derived from the approximation model were assessed by CFD analyses to verify the approximation model. The approximation model was refined repeatedly by adding more experiment points to minimize difference of CFD result and predicted value from the approximation model at the optimum point.The optimization result showed that there is an optimum ratio of cavity depth to tip clearance height, while the optimum design suggests cavity front blend radius and cavity aft blend radius be as small as possible within the design range. As the tip clearance height increases, the optimized tip cavity depth increases. However, the rim thickness has little effect on the optimum tip cavity depth. Without the tip cavity, leakage flow at fore part of the blade suction surface develops large vortex flow from the starting point of the unguided turning region due to adverse pressure gradient. The tip cavity prevents the early leakage flow from flow to the suction surface, which suppresses the leakage flow dissipation to the loss. It results in efficiency improvement. The effect of the tip cavity on the efficiency increases at the larger tip clearance.On the other hand, the cavity rim thickness effect on the efficiency becomes noticeable when the tip cavity depth is over than the optimum value. The rim thickness effect mainly appears on the tip leakage flow after the blade throat. The leaked flow after the blade throat generates a high loss region near the blade tip, especially when the rim thickness is small. The loss from the thick tip cavity rim gradually increases as the tip clearance increases. However, the rim thickness effect is most sensitive when the tip clearance is small. The loss generation mechanism due to the rim thickness is totally different to the tip cavity depth effects on the total pressure loss.Copyright

Thermodynamic and aerodynamic meanline analysis of wet compression in a centrifugal compressor

Wet compression means the injection of water droplets into the compressor of gas turbines. This method decreases the compression work and increases the turbine output by decreasing the compressor exit temperature through the evaporation of water droplets inside the compressor. Researches on wet compression, up to now, have been focused on the thermodynamic analysis of wet compression where the decrease in exit flow temperature and compression work is demonstrated. This paper provides thermodynamic and aerodynamic analysis on wet compression in a centrifugal compressor for a microturbine. The meanline dry compression performance analysis of centrifugal compressor is coupled with the thermodynamic equation of wet compression to get the meanline performance of wet compression. The most influencing parameter in the analysis is the evaporative rate of water droplets. It is found that the impeller exit flow temperature and compression work decreases as the evaporative rate increases. And the exit flow angle decreases as the evaporative rate increases.

Experimental Research on the Altitude Performance of an Auxiliary Power Unit for Helicopters

An APU(Auxiliary Power Unit) for helicopters has been developed in Korea and tested at the AETF(altitude engine test facility) in KARI(Korea Aerospace Research Institute) for the purpose of the military qualification. A cell correlation test was performed before the official test, and the results are within the tolerance. The APU has the capability of supplying electric power as well as compressed air to the helicopters. It was tested at bleed extraction conditions, electric power extraction conditions, and maximum continuous concurrent power conditions within the entire helicopter flight envelop. Some special test equipments were implemented for the measurement of air flowrate, electric power and so on. The tests were successfully performed and their results satisfy the requirements of the helicopters.

Modeling and Experimental Evaluation on Torque Loss in Turbine Test Rig

To evaluate the performance of a turbine through turbine rig test, torque or power generated from the test turbine should be measured. This is measured from a dynamometer or torquesensor installed in the test rig. So, to evaluate the performance of turbine with high precision, the accurate measurement of power or torque is necessary. However, there is an intrinsic difficulty as not all the power generated by the turbine is measured by the dynamometer or torquesensor. A small portion of power generated from test turbine is dissipated through bearing loss and windage loss. This dissipated energy is called mechanical loss of test rig. Therefore, it is necessary to measure the mechanical loss of the test rig for the accurate evaluation of the turbine performance. This paper divides mechanical loss as bearing loss, disk windage loss and extra windage loss. Spin down tests are performed in a 1-stage axial turbine test rig to evaluate each losses. It is found that the dominant loss is bearing loss and the total mechanical loss amounts to 0.78∼1.4% of energy generated at the turbine. And the effect of bearing temperature is investigated and it is found that the mechanical loss is dependent on the bearing temperature and that it increases with decreasing bearing temperature.Copyright

Performance Test of Centrifugal Compressor for Microturbine with Running Tip Clearance

Tip clearance of centrifugal compressor affects the performance. Larger tip clearance results in lower efficiency. What really affects the performance is the running tip clearance, not the cold tip clearance. When the compressor is operating, blade strain and the pressure difference between impeller backplate and hub affects the running tip clearance. This paper describes measured running tip clearance and its effects on the performance of centrifugal compressor. Cold tip clearance before operation was 0.4 mm and running tip clearance varied from 0.86 mm to 0.25 mm with impeller exit pressure. As the pressure at impeller exit increases, the running tip clearance tends to decreases. The target running tip clearance for Compressor at speed was 0.3 mm, and it turned out to be exactly 0.30 mm from experiment.