연속파형 Nd:YAG 레이저를 이용한 Hastelloy C-276의 용접특성에 관한 연구

나기대,유영태,신호준,오용석,Na, Gee-Dae,Yoo, Young-Tae,Shin, Ho-Jun,Oh, Yong-Seok 대한용접접합학회 2008 대한용접·접합학회지 Vol.26 No.5

Hastelloy C-276, corrosion resistant alloy at high temperature, is used in chemical plant and power generation industry. In this study, process parameter of laser welding for welding property in Hastelloy C-276 using a continuous wave Nd:YAG laser was studied. As the result of experiment, laser welding did not show segregation or crack at heat affected zone compared to conventional GTWA welding. The melting zone showed cell dendritic structure along with welding line. In addition, planer front solidification is occurred from welding structure, and it was progressed to cellular solidification. Optimal process parameter for butt welding was 1.2kW and 2.0 m/min for laser power and welding speed, respectively. While heat input, output density, tensile stress, and longitudinal strain was $441.98{\times}103$ J/cm2, $29.553{\times}103$ W/cm2, 768 MPa, and 0.689, respectively. Lap welding of the same material showed greater discrepancy in tensile property during 1 line and 2 line welding. For 1 line welding, tensile stress was about 320 MPa, and 2 line showed slightly larger tensile stress. However, strain was decreased by 20%. From this result, lap welding of the same material, Hastelloy C-276, with 2 line welding is considered to be more effective process than 1 line welding with consideration of mechanical property.

밀페공간 내에 가연성가스가 누출될 때 가연혼합기 생성에 관한 이론적 연구

나기대,홍명석,정낙규,박기남 조선대학교 환경공해연구소 1998 環境公害硏究 Vol.14 No.-

Numerical simulation have been conducted to obtain the basic information for the prevention of explosion in a rectangular enclosure with n-Butane(n-C_(4)H_(10)) gas leak. A numerical Model was established to predict the characteristics of flammable mixture formation with the variation of variables: leak location, leak time. leak velocity and aspect ratio. When the gas leaks from the bottom of the rectangular enclosure, the closer the leak location is to the side wall, the faster mass transfer towards the horizontal direction than the vertical direction is as the leak velocity increases, the fuel reaches higher and in the case of the same leak velocity and leak time, mass transfer in the leak direction is less than that in the horizontal direction because of the gravitational force. Aspect ratio of the rectangular enclosure also affects the formation of the flammable mixture proportional to the leak time and the leak velocity.

유압관로에서 비정상 압력측압공의 동특성에 관한 연구

나기대,유영태,김지환 조선대학교 에너지.자원신기술연구소 2000 에너지·자원신기술연구소 논문지 Vol.22 No.2

In this study, the analysis of the dynamics characteristics and performance investigation of the pressure measurement in the hydraulic pipe. The exact solution of the frequency response is obtained by using the complicated transfer function. The discrepancy with the exact and the approximate is small, so the approximation solution is adopted to compare the experimental result with the theoretical analysis. The pressure measurement in the hydraulic pipe is caused by length of capillary and according to the end volume size. The experimental and calculate result show that characteristic of the pressure .measurement in the hydraulic pipe varies with the length of capillary and volume size. Therefore present theoretical formulation and experimental result can be used generally in designing and performance evaluation of all the hydraulic pressure measurement in the hydraulic pipe

충돌수분류계에서 단상강제대류 열전달에 관한 연구

나기대 조선대학교 동력자원연구소 1995 動力資源硏究所誌 Vol.17 No.2

For the three-dimentional single phase convection, local Nusselt number was subjected to the influence of Re number, Pr number, height of supplementary water and position of flat plate. Nusselt number in the direction of nozzle length had maximum value on Y-axis. It was simply decreased with the increment of X/B within Y/B<2, then the secondary peak of Nu number appeared in the near of X/B=4. If nozzle exit velocity is greater than 6.2㎧, its peaks valve however was shown in the region of Y/B<2. The empirical equation which is applicable in the stagnation point is as follews: Nu_0 = F(S/B) Re^1.05 Pr^0.4(Z/B)^-0.30 The optimum height of supplementary water to augement heat transfer rate at stagnation point was S/B = 2, which that heat transfer rate was 1.2 times of that when supplementary water was not used.

충돌수분류계(衝突水噴流系)에서 와이어 메쉬를 사용(使用)한 열전달(熱傳達) 증진(增進)에 관(關)한 연구(硏究)

나기대,Na, G.D. 대한설비공학회 1994 설비공학 논문집 Vol.6 No.3

This paper presents the promotion of heat transfer through the use of wire-mesh screens. To improve heat transfer in an impingement water system, the wire-mesh screens are installed between the nozzle-to-heater surfaces. When the wire-mesh screens are not employed, this report exhibits the maximum heat transfer and the secondary maximum value at the stagnation point. But in case of using the wire-mesh screens, the transfer coefficient value of maximum heat exists at the stagnation point, and the second maximum value doesn't occur. Therefore, the heat transfer is more improved than 4~6 times that of the mean Nusselt numbers of simple water jet system, Also, within the region presented in this study, the heat transfer was promoted by using the wire-mesh screens at the stagnation point ; thus, the heat transfer was more increased than 6-7. 5 times that of simple water jet system.

초음파 진동이 비등열전달 과정에 미치는 영향에 관한 실험적 연구

나기대,오율권,양호동 한국에너지학회 2006 에너지공학 Vol.15 No.1

본 연구에서는 비등열전달 과정동안, 초음파 진동이 열전달 과정에 미치는 영향에 관하여 실험적으로 조사해 보았다. 실험은 등온가열조건하에서, 40 kHz의 초음파 진동을 가진한 경우와 가진하지 않은 경우로 나누어 비등과정동안의 온도분포를 측정하였고, 대류상태와 과냉상태 그리고 포화상태에서의 열전달계수를 측정하여 열전달 향상율을 비교하여 보았다. 또한, 하이드로폰을 이용하여 초음파 가진시 매질내에 발생하는 음압분포를 측정하고 열전달 향상율과 비교하여 보았다. 실험결과, 비등열전달 과정에 초음파 진동을 가진한 경우, 가진하지 않은 경우와 비교하였을 때 열전달계수가 높게 나타나는 것을 확인하였으며, 특히 대류상태에서 열전달계수가 급격하게 증가하였다. 또한, 초음파 진동의 가진으로 인해 형성되는 음압은 진동자가 부착된 지점에서 주위보다 상대적으로 높게 형성되는 것을 실험적으로 확인하였으며, 초음파 진동으로 인해 형성된 높은 음압이 열전달 향상율에 영향을 미치는 원인의 하나로 작용하고 있음을 알 수 있었다. 결국, 초음파 진동에 의해 매질내에 발생하는 음압은 열전달 향상과 밀접한 관련이 있다고 사료된다.

경계조건변화에 따른 동력전달관로의 동특성

나기대,유영태,김지환 한국공작기계학회 2002 한국생산제조학회지 Vol.11 No.6

Design for a quiet operation of fluid power system requires the understanding of noise and vibration characteristics of the system. It's not easy to analyze noise problem in hydraulic cylinder used in typical actuator Because they've got complex fluid dynamics. One of the fundamental problems associated with the hydraulic system is the pulsating flow in pipe lines, which can e tackled by the analysis under simplifying assumptions. The present study focuses on theoretic analysis and experimental study on the dynamics of laminar pulsating flow in a circular pipe. We analyze the propagation characteristics of the pressure pulse within a hydraulic pipe line taking into account the pulsating flow frequency variation. We also measure instantaneous pressure pulses within pipe line to identify the transfer functions. We conduct series of experiments to investigate the propagation characteristics of pressure pulse for various pressure of pulsating flow. The working fluid of the present study is ISO VG46 and the temperature ranges from 20 to 60℃ with normal pressure at 4000kPa. The flow rate is measured by using an ultrasonic flow meter. Pressures at fixed upstream and downstream positions are measured concurrently. The electric signals of the pressure sensor are stored and analyzed using a system analyzer(PKE 983 series). The frequency is varied in the range of 10~500Hz. The Reynolds number is kept below 2,000. In the present study, boundary condition was varied by installing a surge tank and an orifice at the end of pipe. Experimental and theoretical results were compared each other under various boundary conditions.

밀폐공간 내에 가연성가스가 누출될 때 가연혼합기 생성에 관한 이론적 연구

나기대,홍명석,정낙규,박기남 조선대학교 환경연구소 1998 環境硏究 Vol.14 No.1

Numerical simulation have been conducted to obtain the basic information for the prevention of explosion in a rectangular enclosure with n-Butane(n-C_4H_10) gas leak. A numerical Model was established to predict the characteristics of flammable mixture formation with the variation of variables: leak location, leak time, leak velocity and aspect ratio. When the gas leaks from the bottom of the rectangular enclosure, the closer the leak location is to the side wall, the faster mass transfer towards the horizontal direction than the vertical direction is as the leak velocity increases, the fuel reaches higher and in the case of the same leak velocity and leak time, mass transfer in the leak direction is less than that in the horizontal direction because of the gravitational force. Aspect ratio of the rectangular enclosure also affects the formation of the flammable mixture proportional to the leak time and the leak velocity.

고압관로내의 과도응답특성에 관한 연구

나기대,모양우,유영태,서동진 조선대학교 생산기술연구소 1997 生産技術硏究 Vol.19 No.1

Functional operator is derived for the propagation factor and characteristic impedance in hydraulic pipe lines when the effects of varying velocity profile and heat transfer are included. The results of theoretical analysis are compared with experimental results obtained from response characteristics. These results are particularly used to determine the impulse and step responses of a line. The impulse and step responses of an infinitely long uniform rigid line are computed in a viscous liquid. The waveform is measured due to propagation factor by means of oscilloscope whenever ball valve constructed in a pipe downstream was closed and opened repeatedly.

軸對稱 衝突噴流의 熱傳達機構에 關한 實驗的 硏究

羅基大 조선대학교 동력자원연구소 1991 動力資源硏究所誌 Vol.13 No.1

Axisymmetric impinging water jet is applied to various uses In the industrial fields, In the free jet from nozzle outlet to just before impingement, configuration of its liquid column was visualized. In the visualization experiment, the results are as follows : the jet number is 0.089 in the case of dripping, is 7.181 in the case of smooth jet, and is above 10,76 in the case of wavy jet. In the present experiment on impinging jet heat transfer, the liquid column of free jet is wavy jet and its jet number is above 10. The results of impinging jet heat transfer is divided into two important regions. The first is on impinging jet region including stagnation point with the highest local heat transfer effectiveness and the second is on wall jet region. In the stagnation point(X/D=O), heat transfer empirical equation about nondimensional parameters is as follows : Nu=1.9339×10^-4 Re^1.47 Pr^0.33 In the above equation, exponential value on the Reynolds number is more than that of the laminar theoretical value 0.5.