고온벽면 충돌에 의한 미립화

전인곤,전흥신 경희대학교 레이저공학연구소 2004 레이저공학 Vol.15 No.-

The breakup behaviors of impinging droplet on a hot surface are studied experimentally. The droplets are produced by the dripping method and the breakup behaviors of liquid droplet are recorded by photographs. Experimental conditions are, droplet diameter di : 2.5, 3.2 [mm], Weber number : 30∼140, surface temperature : 28℃ (room temperature) 450℃. Water is used as a liquid. As Weber number of droplet increases, a liquid sheet, which is formed after the impingement on a hot surface, is disintergrated by the dynamical effect. But at low Weber number, it has effected by thermodynamical effect. The breakup behaviors of droplet are divided into three patterns with Weber number and surface temperature, non-disintegration, transition and disintegration region. Further, these boundary values are affected by the hot surface temperature and Weber number. S11M of breakup droplets are calculated in according to surface temperature and Weber number. The minimum SMD of breakup droplets are observed at Weber number 65.49, temperature 250℃ and Weber number 99.08, temperature 350℃

액적의 Weber 수에 따른 냉각특성의 변화

방창훈,양창호 한국산업안전학회 2003 한국안전학회지 Vol.18 No.1

The objective of the present work is to examine the variation of cooling characteristics due to the Weber number of droplet on a heated surface. The surface temperatures varied from 72.5~106.1℃ on steel and Teflon, when Weber number was 60, 180, 300. The results are as follows; In the case of the same droplet size, the initial temperature of solid increases the indepth temperature of solid more drop. In the case of the same surface temperature, Weber number increases with increasing the cooling effect of droplet. The time-average heat flux increases with increasing the initial temperature of solid and Weber number. The evaporation time decreases with increasing the initial temperature of solid and Weber number.

홍수파 선단의 축척효과에 대한 보정계수 산정

정석일(Seok-il Jeong),김수영(Soo young Kim),이승오(Seung Oh Lee) 한국산학기술학회 2016 한국산학기술학회논문지 Vol.17 No.6

최근의 이상기후로 인하여 수공구조물의 안전성이 재고되고 있으며, 이에 따라 수공구조물의 붕괴에 대한 연구가 증가하고 있다. 특히 제방 등 수공구조물의 월류로 인한 붕괴로 발생하는 홍수파 선단의 초기 이동속도는 제내지의 위험성을 예측하고, EAP(Emergency Action Plan) 등을 수립하는 기초자료가 된다. 이러한 선단홍수파의 정확한 예측을 위해 수리실험 이 다수 수행되고 있으나, 많은 연구에서 초기의 홍수파 선단에서 발생하는 표면장력에 의한 축척효과(Scale effect)를 고려하 지 않고 있는 실정이다. 이에 본 연구에서는 계면활성제(surfactant)와 영상 분석 장비를 이용한 표면장력 도출 수리실험 및 급 개폐가 가능한 수조를 이용한 홍수파 전파 실험을 통하여, 계면활성제 농도와 표면장력 관계, 표면장력과 홍수파 전파속도 관계, 표면장력과 Weber Number 관계를 도출하여, 초기 홍수파 선단에서 발생하는 표면장력에 의한 축척효과 보정계수 및 축척효과를 받지 않는 한계조건(critical condition)을 제시하였다. 연구결과 Weber Number가 약 12.2 이하일 경우 보정 계수가 필요한 것으로 나타났으며, 그 이상인 경우 축척효과를 무시할 수 있는 것으로 나타났다. Recently, there has been increasing concern regarding the collapse of hydraulic structures due to abnormal climate conditions. Therefore, numerous studies of the collapse of hydraulic structures have been carried out. In particular, the velocity of the propagation of a flood wave-front is important for predicting the inundation safety and establishing an EAP (Emergency Action Plan). Although many hydraulic tests have been conducted for precise predictions of a flood wave-front, the scale effect from downsizing has not considered. In this study, the relationships between surface tension and the concentration of surfactant, between surface tension and the velocity of flood wave propagation, and between surface tension and the Weber Number were derived through hydraulic tests using a surfactant and image analysis equipment. Based on these relations, the modification factor for the scale effect of the front of flood wave propagation was suggested. The results highlight the necessity of a modification factor when the Weber Number is lower than 12.2, but the scale effect can be ignored when the Weber Number over 12.2.

친수성 및 소수성 표면에 충돌하는 액적의 퍼짐 특성

문주현(Joohyun Moon),이재봉(Jae Bong Lee),이성혁(Seong Hyuk Lee) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11

This article reports on spreading characteristics of impinging droplets on various solid surfaces at low Weber number for a variety of engineering applications such as inkjet printer and spray cooling. We also studied the effect of hydrophobicity on maximum spreading of water droplet after impact. Experiment was extensively conducted for impinging droplet of 3.6μl DI-water on different solid surfaces, i.e., Aluminum (Al 6061), PDMS, Slide glass for Weber number ranging from 5 to 45. The drop impact process was visualized and analyzed by using a high-speed camera and Image-J software. From the results, we observed two different regimes for a droplet spreading behavior, and suggested some useful correlations of maximum spreading diameter as a function of Weber number. Moreover, it was found that as equilibrium contact angle increased, maximum spreading diameter decreased, indicating that spreading behavior was substantially affected by Weber number and surface hydrophobicity.

Spray formation by a swirl spray jet in low speed cross-flow

Lee, Sang-Seung,Kim, Won-Ho,Yoon, Woong-Sup The Korean Society of Mechanical Engineers 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.2

Breakup and spray formation of swirl liquid jets introduced into a low-speed cross-flow are experimentally investigated. Effects of the cross-flows on the macro and microscopic spray parameters are optically measured in terms of jet Weber number and liquid-to-gas momentum ratio. At lower jet Weber numbers, the liquid stream undergoes Rayleigh jet breakup. At higher momentum ratios, bag breakup occurs and tends to distort the liquid column into a loop-like structure. As the jet Weber number rises, stronger aerodynamic interaction and secondary flows cause multi-mode breakup. Regardless of the momentum ratio, the spray profile is hardly altered at higher jet Weber numbers. The cross-flow promotes the jet breakup and renders a finer spray in an entire range of injection velocities.

Modeling of Diesel Spray Impinging Behavior on Lubricating Oil Film-Measurement of Critical Weber Number-

( Naoto Mizobuchi ),( Hiroki Kambe ),( Eriko Matsumura ),( Takaaki Kitamura ) 한국액체미립화학회 2017 한국액체미립화학회 학술강연회 논문집 Vol.2017 No.-

Depletion of fossil fuel has become a serious problem by accelerating motorization in Asian countries. Furthermore, regulations on exhaust gas such as Particulate Matter (PM) and Nitrogen oxides (NOx) emitted from internal combustion engine are being strengthened year by year. Then, recent diesel engines are equipped with Diesel Particulate Filter (DPF) as after treatment device for exhaust gas, and PM is collected. However, with the long-term use of DPF, PM accumulates in DPF. As a result, PM collection efficiency deteriorates and pressure loss increases. Therefore, post injection has attracted attention as DPF regeneration method for burning and removing PM in DPF. In post injection, fuel is injected into the cylinder at the middle to late stage of expansion stroke, and unburned hydrocarbon is supplied to the Diesel Oxidation Catalyst (DOC). Then, oxidation heat is generated by oxidation reaction of unburned hydrocarbon and DOC. And the oxidation heat flows into the DPF, thereby burning and removing PM. However, oil dilution to the lubricating oil film on cylinder liner caused by post injection is a serious problem. Because oil dilution is leading to deterioration of sliding properties of piston and thermal efficiency, it is necessary to clarify spray impingement behavior on lubricating oil film. So in this research, we focus on the oil dilution phenomenon caused by post injection and aim for modeling oil dilution phenomenon. In this report, we measured the critical weber number which makes it possible to judge whether or not fuel droplets impinging on lubricating oil film deposits or splash. As a result, it was found that fuel droplets impingement behavior on lubricating oil film is classified into three behaviors by two critical weber numbers. First, We_crP =(272+11600δ_non^1.29)Lα^0.01,second, We_crS =(235+2255δ_non^1.05)Lα^0.05, La=ρ_ο·σ_ο·d_in/μ_ο², Here, ρ_ο is density of engine oil, σ_ο is surface tension of engine oil, μ_ο is viscosity coefficient of engine oil, and Lα is Laplace number.

하이브리드 로켓 연소에서 연료액적의 발생과 저주파수 연소불안정

김진아(Jina Kim),이창진(Changjin Lee) 한국항공우주학회 2021 韓國航空宇宙學會誌 Vol.49 No.7

파라핀 왁스는 높은 후퇴율 때문에 하이브리드 로켓의 연료로 많은 각광을 받고 있다. 하지만 파라핀 연료의 연소에서도 비정상적인 저주파수 연소압력 진동이 관찰되고 있는데, 이는 연료 표면에 형성된 액체층과 액적의 유입과 관련이 있는 것으로 추론된다. 본 연구는 액적에 의한 추가적 연소와 저주파수 연소불안정 발생과의 관계를 분석하였다. 한편, 액적의 발생은 관성력과 액체의 표면장력의 비로 정의되는 We수(Weber Number)와 액체층의 Re수(Reynolds Number)에 따라 변화하는 것으로 알려져 있다. 따라서 일차적으로 실험실 규모의 로켓을 사용하여 We수에 따른 연소불안정의 발생여부를 관찰하였다. We수의 조절은 산화제 유량 변화를 통한 관성력의 변화와 LDPE(Low Density Polyethylene) 첨가에 의한 표면장력의 변화를 통해 시도하였다. 저주파수의 연소불안정의 발생은 특정한 We수 이상에서만 관찰되었고 임계 We수가 존재하는 것을 확인하였다. Paraffin wax is attracting many attentions for promising solid fuel of hybrid rocket because of its higher regression than other fuels. However, even with paraffin fuel combustion, unsteady low-frequency oscillation of combustion pressure is still observed. And, this is related to the formation of liquid layer and the entrainment of fuel droplets entering the axial combustion gas flow. This study investigates the effect of additional combustion of fuel droplets on the occurrence of low-frequency combustion instability. On the other hand, the formation of fuel droplets depends on Weber Number (the ratio of the inertial force to the surface tension of the liquid) and Reynolds Number of the oxidizer flow. Therefore, a laboratory-scale hybrid rocket was used to monitor the occurrence of combustion instability while changing We number. A series of combustion tests were conducted to control We number by changing the oxidizer flow rate or adding LDPE (low density polyethylene) to base fuel. In the results, it was confirmed that there is a critical We number above which the low-frequency combustion instability occurs.

포스터 전시회 : 웨버수에 따른 동적 접촉각 고려 시 액적 충돌 및 퍼짐 현상에 대한 수치해석적 연구

손소연 ( So Youn Son ),김소윤 ( So Yoon Kim ),고권현 ( Gwon Hyun Ko ),유홍선 ( Hong Sun Ryou ) 한국액체미립화학회 2012 한국액체미립화학회 학술강연회 논문집 Vol.2012 No.-

The contact angle is one of the significant parameters to simulate droplet spreading phenomena after impingement on the solid surface. It is usually assumed that contact angle is a material property and adapted in boundary condition to analyze the droplet spreading motion. However, in wetting phenomena, contact angle has various values according to contact line movement. In the present study, we analyze the droplet spreading and impingement motion considering dynamic contact angle function that is obtained from single droplet impingement and spreading experiment on the bare and flat surface for different Weber numbers. Fluent 6.3 is used for droplet simulation and Volume of Fluid (VOF), the most famous interface tracking method, is adapted for analysis of interface between liquid and gas phases. The numerical result considering dynamic contact angle is compared to the experimental data, numerical result using the Cubic Interpolated Pseudo-particle (CIP) method by Yabe, and numerical result considering static contact angle in terms of droplet deformation, spreading radius, spreading height at Weber numbers, 4.43 and 11.33, respectively. The study shows that numerical results agree well with the experimental data in maximum spreading radius and spreading height. Indeed, overall, the study shows increasing spreading radius and height as the Weber numbers increase.

경사진 고온평판에 충돌하는 액적의 동적거동 특성

이충현(Choong Hyun Lee),김경천(Kyung Chun Kim) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12

Dynamic behavior of impinging droplets onto inclined hot plate were studied experimentally. De-ionized water droplet was made through a 31G injection needle and the droplet size was 2 mm. A sapphire plate was heated up to 350ºC by a hotplate. Two high speed cameras were used for side and bottom visualization, and the frame rate was 20000 fps. Weber number was changed from 22.5 to 123.7 by changing initial height of droplet from 40 mm to 210 mm. Dynamic behaviour of droplet after impingement was strongly depended on surface temperature, inclination angle and Weber number. Micro-explosion was observed at the surface temperature of Leidenfrost condition in case of lower inclination angle. In case of higher inclination angle, the impinging droplet tended to recover its original spherical shape due to the vapour cushion at Lidenfrost condition. As increasing Weber number, both maximum spreading diameter and slip length were increased.

아음속 횡단 기체 유동장에서 노즐 형상 변화와 분사각 변화가 액적크기와 침투거리에 미치는 영향

이봉수(Bong-Soo Lee),고정빈(Jung-Bin Ko),조우진(Woo-Jin Cho),구자예(Ja-Ye Koo) 한국항공우주학회 2006 韓國航空宇宙學會誌 Vol.34 No.6

횡단 유동장내에서 일어나는 분무 현상에 대하여 분사각 변화에 대한 특성과 단일 노즐 형상에서 노즐의 크기와 길이에 대한 분무 특성을 연구하였다. 노즐은 단일 구멍으로 직경이 0.5 ㎜이고 노즐 대 노즐길이의 비(L/D)는 1.0에서 6.0이며 이미지는 고해상 줌 렌즈를 이용한 CCD 카메라를 통해 얻었으며, SMD와 액적의 속도는 PDPA와 상용 프로그램인 Image Express를 사용하였다. 액체 제트의 궤적은 웨버수와 모멘텀비, 노즐형상 변화(L/D)에 영향을 받아 액주가 후방으로 휘어지는 현상이 나타났다. 분사각이 낮을 때 (θ<90°)에 노즐 형상보다는 웨버수 증가가 액체 제트의 궤적에 더 크게 영향을 미쳤으며, 분사각이 높을 때(θ>90°)에 노즐 형상 변화에 의한 분열점 변화가 액체 제트의 궤적에 더 크게 영향을 미쳤다. The spray characteristics of liquid jet injected into subsonic cross-flow were investigated experimentally. Spray trajectories were captured using CCD camera. Droplet sizes were measured using PDPA and Image Express. The nozzle diameter was 0.5 ㎜, and its length-to-diameter ratios (L/D) ranged from 1.0 to 6.0. Experimental results indicate that the breakup point is delayed by increasing gas momentum ratio and the penetration length is decreased by increasing Weber number. At low injection angle(θ<90°), Weber number is dominant parameter for trajectories, but at high injection angle(θ>90°), L/D is dominant parameter for trajectories rather than Weber number.