Myung-whan Bae

A Study on Natural Convection from Two Cylinders in a Cavity

Steady-state natural convection heat transfer characteristics from cylinders in a multiply-connected bounded region are clarified. A spectral finite difference scheme (spectral decomposition of the system of partial differential equations, semi-implicit time integration) is applied in numerical analysis, with a boundary-fitted conformai coordinate system through a Jacobian elliptic function with a successive transformation to formulate a system of governing equations in terms of a stream function, vorticity and temperature. Multiplicity of the domain is expressed explicitly.

A Study on the Effect of Recirculated Exhaust Gas with Scrubber EGR System upon Exhaust Emissions in Diesel Engines

The effects of recirculated exhaust gas on the characteristics of and soot emissions under a wide range of engine load have been experimentally investigated by a water-cooled, four-cylinder, indirect injection, four cycle and marine diesel engine operating at two kinds of engine speeds. The simultaneous control of and soot emissions in diesel engines is targeted in this study. The EGR system is used to reduce emissions, and a novel diesel soot removal device with a cylinder-type scrubber for the experiment system which has 6 water injectors(A water injector has 144 nozzles in 1.0 mm diameter) is specially designed and manufactured to reduce the soot contents in the recirculated exhaust gas to intake system of the engines. The intake oxygen concentration and the mean equivalence ratio calculated by the intake air flow and fuel consumption rate, and the exhaust oxygen concentration measured are used to analyse and discuss the influences of EGR rate on and soot emissions. The experiments are performed at the fixed fuel injection timing of BTDC regardless of experimental conditions. It is found that emissions are decreased and soot emissions are increased owing to the drop of intake oxygen concentration and exhaust oxygen concentration, and the rise of equivalence ratio as the EGR rate rises.

Reduction characteristics of NO X emissions by optimal urea injection rate in a LPG steam boiler with urea-SCR reactor

The aim of this study is to develop the urea-SCR reactor aftertreatment system for reducing NOX emissions in a LPG steam boiler. The urea-SCR system utilizes ammonia converted from 17% aqueous urea solution injected in front of the SCR catalyst as a reducing agent for reducing NOX emissions. In this study, NOX emissions and exhaust gas temperatures according to boiler load were measured and compared with before and after urea injection in a LPG steam boiler with urea-SCR reactor. Also, the equivalence ratio, urea injection rate, ammonia slip and NOX conversion rate relative to boiler load were investigated. The equivalence ratio is different by changing the fuel consumption rate or intake air rate. In this experiment, the average equivalence ratio is calculated by changing only fuel consumption rate while the intake air rate is constantly fixed at 25,957.11 cm3/sec. The average equivalence ratios are 1.38, 1.11, 0.81 and 0.57 when boiler loads are 100, 80, 60 and 40%. In this experiment, NOX conversion rates are calculated by 89, 85, 77 and 79% in cases of the boiler loads of 100, 80, 60 and 40%. NOX conversion rate is raised with increasing urea injection rate, and NH3 slip is boosted at the same time. Consequently, the optimal urea injection rate for the respective boiler load should be determined by considering NOX conversion rate and NH3 slip simultaneously. In this study, the urea flow rates of 230, 235, 233 and 231 mg/min are injected at the boiler loads of 100, 80, 60 and 40%, and the final NH3 slips are measured by 8.48, 5.58, 11.97 and 11.34 ppm at the same conditions.

Effect of concave rectangular winglet vortex generator on convection coefficient of heat transfer

This study shows an experimental result of effects of concave rectangular winglet vortex generator on convection heat transfer coefficient. There are two types of vortex generator those are rectangular winglet and concave rectangular winglet. The aim of this study is to observe effects of geometry and numbers of vortex generator to the increase of heat transfer coefficient. This experiment was done in a glass rectangular channel. Overall, this experiment was conducted with (rectangular and concave rectangular winglets) and without (baseline) vortex generator. Both types of vortex generator were placed at a 30° angle of attack, in an inline position, and variation numbers of vortex generator row of 1, 2 and 3 rows. Vortex generators were mounted on an aluminum plate which was mounted on a heater with 35 watts of heat, inside the rectangular channel. Airflow passed through vortex generators was varied by its inlet velocity to be 0.4 m/s to 2.0 m/s with an interval of 0.2 m/s. The experimental result shows t...

A Study on the Optimal Cutting Depth upon Surface Roughness of Al Alloy 7075 in High-speed Machining

The high-speed machining in the manufacturing industry field has been widely applied for parts of vehicles, aircraft, ships, electronics, etc., recently, because the effect of cost savings for shortening processing time and improving productivity is great. The purpose in this study is to investigate the effect of cutting depth on the surface roughness of workpiece with the spindle rotational speed and feed rate of high-speed machines as a parameter to find the optimal depth in the finishing for ball end mill of the aluminum alloy 7075 which is used much in aircraft parts. When the cutting depth for the respective feed rate and spindle rotational speed is varied from 0.1 mm to 0.7 mm at intervals of 0.2 mm in the wet finishing of the aluminum alloy 7075 by the insoluble cutting oils and high-speed machining used in the rough machining of previous study, the surface roughness values and the cutting temperature are measured. In addition, the cutting surface shapes of test specimens are observed by optical microscope and compared with respectively. It is found that the surface roughness values and the temperature generated during machining are increased as the feed rate and cutting depth are raised, but those are decreased as the spindle rotational speed is increased.

A Study on Performance Characteristics of Super-mirror Face Grinding Machine Using Variable Air Pressure

The comparisons of performance characteristics between the super-mirror face grinding machine using variable air pressure developed in this laboratory to grind precisely the sliding face of a surface hardened workpiece with thermal spray and the conventional one are investigated by measuring the surface roughness and hardness for a SCM440. To process variously workpiece according to shape, size and materials, the rotating and contacting forces of the developed grinding machine can be changed by air pressure. The surface roughness of processed workpiece can be also attained to state of mirror face by grinding precisely the sliding face with changing the rotating speed of diamond wheel. It is possible to be attached to the various machine tools because the super-mirror face grinding machine using variable air pressure is a small size. The grinding efficiency is elevated because it can be worked by two or more grinding machines attached to concurrently a machine tool for the large workpiece. In this study, results show that the cusp height of the super-mirror face grinding machine for the particle size of 100 and 1500 No./mm2 is lower than that of the conventional one because the vibration is reduced by rotating very fast the diamond wheel with a pressed air and it can be processed by rotating the diamond wheel with a constantly varied air pressure perpendicular to workpiece surface, and that the workpiece in the super-mirror face grinding machine for the particle size of 3000 No./mm 2 can be processed to state of mirror face that is rarely seen by the cusp height. It is also found that the surface hardness of both the conventional and the super-mirror face grinding machines are increased as the particle size of diamond wheel is reduced, and the surface hardness of the super-mirror face grinding machine is HRC 1.1 ~ 1.8 higher than that of the conventional one.

A study on soot formation in premixed constant-volume propane combustion: Effects of pressure, temperature and equivalence ratio

The effects of pressure, temperature and equivalence ratio on soot formation in premixed propane-oxygen-inert gas combustion have been investigated over wide ranges of pressure (0.1 to 6 MPa), temperature (1200 to 2100 K) and equivalence ratio (1.5 to 2.7) in a specially designed disk-type constant-volume combustion chamber. To observe the soot formation under high pressure, premixtures are simultaneously ignited by eight spark plugs located on the circumference of chamber at 45 degree intervals. The eight converging flames compress the end gases to a high pressure. The soot volume fraction in the chamber center during the final stage of combustion at the highest pressure is measured by the in situ laser extinction technique and the burnt gas temperature during the same period by the two-color pyrometry method. The pressure and temperature during soot formation are varied by changing the initial charge pressure and by changing the volume fraction of inert gas in the premixture, respectively. It is found that the soot yield is dependent on the pressure, temperature and equivalence ratio, and the soot yield increases under the following conditions; (1) decreasing temperature and increasing equivalence ratio at constant pressure, (2) increasing pressure and decreasing temperature at constant equivalence ratio, (3) increasing equivalence ratio at constant temperature and pressure.

A Study on Durability Characteristics for Plungers of Conventional Ceramic and Surface Modification by Powder Coating Using High Velocity Oxygen Fuel Thermal Spray

The high velocity oxygen fuel(HVOF) thermal spray is a kind of surface modification techniques to produce the sprayed coating layer. This process is to form the coating layer after spraying the powder to molten or semi-molten state by the ultra-high speed at the high-temperature heat source and conflicting with a substrate. The efficiency of thermal spraying is dropped, however, because the semi-molten powder in a spray process become a factor that degrades the mechanical property by the formed pore within the coating layer. Therefore, it is necessary to melt completely the thermal spray powder in order to produce the coating layer with an optimal adhesive force. In this study, to improve the wear resistance, corrosion resistance and heat resistance, the plungers of high-speed and ultra-high pressure reciprocating hydraulic pumps used in ironworks are manufactured with STS 420J2 and are coated by the powders of WC-Co-Cr and WC-Cr-Ni including the WC of high hardness using a HVOF thermal sprayer developed in this laboratory. These are called by the surface-modified plungers. The surface roughness, hardness, and surface and cross-sectional microstructure of these two surface-modified and conventional ceramic plungers are measured and compared before operation with after operation for 100 days. It is found that the values of centerline average surface roughness and maximum height for conventional ceramic plunger are 9.5 to 10.8 and 5.2 to 5.7 times higher than those of surface-modified ones coated by WC-Co-Cr and WC-Cr-Ni because the fine tops and bottoms on surface roughness curve of conventional ceramic plunger are approximately 100 times higher than those of surface-modified ones. In addition, the pores and scratches in the surface microstructure are considerably formed in the order of conventional ceramic, WC-Cr-Ni and WC-Co-Cr surface-modified plungers. The greater the WC content of high hardness powder is less the change in the plunger surface.

A Study on Wear Characteristics of Cutting Tools in a Titanium Roughing Cut Machining

The application of titanium has been gradually rising because the utilizing ranges for low weight and high strength are rapidly increased by the need for improving the fuel economy in production industries such as the aviation and automotive in recent. The purpose in this study is to investigate the appropriate cutting conditions on the life of flat and round end mills by measuring the maximum cutting temperature relative to the machining time, and calculating the wear rates of cutting tool with the spindle speed and feed rate of vertical machining center as a parameter in the titanium roughing cut machining which is widely used in critical parts of aircraft, cars, etc. When the wetted roughing cut machining of titanium with a soluble cutting oil is conducted by the flat and round end mills, the maximum cutting temperatures for a variety of spindle speed and feed rate are measured at ten-minute intervals during 60 minutes by an infrared thermometer, and the wear rates of cutting tool are calculated by the weight ratios based on tool wear before and after the experiment. It is found that the maximum cutting temperature and the wear rates of cutting tool are raised as the cutting amount per tool edge is increased with the rise of feed rate, in this experimental range, and as the frictional area due to the rise of contacting friction numbers between tool and specimen is increased with the rises of cutting time and spindle speed. In addition, the increasing rate of maximum cutting temperature in the flat and round end mills are the highest for the cutting time from 50 to 60 minutes, and the wear rate of cutting tool in the flat end mill is 1.14 to 1.55 times higher than that in the round end mill for all experimental conditions.

A Study on Effect of Urea-SCR Aftertreatment System upon Exhaust Emissions in a LPG Steam Boiler

The aim of this study is to investigate the effect of SCR reactor on the exhaust emissions characteristics in order to develop a urea-SCR aftertreatment system for reducing NOx emissions. The experiments are conducted by using a flue tube LPG steam boiler with the urea-SCR aftertreatment system. The urea-SCR aftertreatment system utilizes the ammonia converted from 17% aqueous urea solution injected in front of SCR catalyst as a reducing agent for reducing NOx emissions. The equivalence ratio, urea injection amount, ammonia slip and NOx conversion efficiency relative to boiler load are applied to discuss the experimental results. In this experiment, the average equivalence ratio is calculated by changing only the fuel consumption rate while the intake air amount is constantly fixed at 25,957.11 cm/sec. The average equivalence ratios are 1.38, 1.11, 0.81 and 0.57 when boiler loads are 100, 80, 60 and 40%. The NOx conversion efficiency is raised with increasing urea injection amount, and NH3 slip is also boosted at the same time. Consequently, the NOx conversion efficiency relative to boiler load should be examined in combination with urea injection amount and NH3 slip. The results are calculated by 89, 85, 77 and 79% for the boiler loads of 100, 80, 60 and 40%. The appropriate amount of urea injection for the respective boiler load can be not discussed by only NOx emissions, and should be determined by considering the NOx conversion efficiency, NH3 slip and reactive activation temperature simultaneously. In this study, the urea amounts of 230, 235, 233 and 231 mg/min are injected at the boiler loads of 100, 80, 60 and 40%, and the final NH3 slips are measured by 8.48, 5.58, 11.97 and 11.34 ppm at the same conditions. THC emission is affected by the SCR reactor under other experimental conditions except 100% engine load, and CO emission at only 40% engine load. The rest of exhaust emissions are not affected by the SCR reactor under all experimental conditions.