Dong Eok Kim

Enhanced critical heat flux by capillary driven liquid flow on the well-designed surface

Based on the unique design of the surface morphology, we investigated the effects of gravity and capillary pressure on Critical heat flux (CHF). The micro-structured surfaces for pool boiling tests were comprised with both the rectangular cavity and microchannel structures. The microcavity structures could intrinsically block the liquid flow by capillary pressure effect, and the capillary flow into the boiling surface was one-dimensionally induced only through the microchannel region. Thus, we could clearly establish the relationship between the CHF and capillary wicking flow. The driving potentials for the liquid inflow can be classified into the hydrostatic head by gravitational force, and the capillary pressure induced by the interactions of vapor bubbles, liquid film, and surface solid structures. Through the analysis of the experimental data and visualization of vapor bubble behaviors, we present that the liquid supplement to maintain the nucleate boiling regime in pool boiling condition is governed by the gravitational pressure head and capillary pressure effect.

Boiling crisis controlled by capillary pumping and viscous friction: Liquid penetration length and dry spot diameter

A boiling crisis, or critical heat flux (CHF), is a condition that determines the upper bound on removable thermal energy at a boiling surface. In such situations, the liquid cannot wet the surface because a vapor film completely covers it. CHF is enhanced on micro-structured surfaces when under boiling conditions. CHF values were measured for surfaces with rectangular microchannel geometries of various channel widths, (10–30 μm) and generally increased in value as channel widths decreased. However, the CHF value for the 5-μm channel-width surface was found to be lower than the wider channel-width surfaces. This observation contradicts models based on vapor recoil and classical instability mechanisms. Hence, we present a fluid-dynamics model that considers capillary pumping and viscous friction. With a focus on the spatial distribution of the liquid penetration region and the local dry spot under a large vapor bubble, this model can accurately predict the CHF variation associated with different channel wi...

Dynamic Leidenfrost temperature on micro-textured surfaces: Acoustic wave absorption into thin vapor layer

The dynamic Leidenfrost phenomenon is governed by three types of pressure potentials induced via vapor hydrodynamics, liquid dynamic pressure, and the water hammer effect resulting from the generation of acoustic waves at the liquid-vapor interface. The prediction of the Leidenfrost temperature for a dynamic droplet needs quantitative evaluation and definition for each of the pressure fields. In particular, the textures on a heated surface can significantly affect the vapor hydrodynamics and the water hammer pressure. We present a quantitative model for evaluating the water hammer pressure on micro-textured surfaces taking into account the absorption of acoustic waves into the thin vapor layer. The model demonstrates that the strength of the acoustic flow into the liquid droplet, which directly contributes to the water hammer pressure, depends on the magnitude of the acoustic resistance (impedance) in the droplet and the vapor region. In consequence, the micro-textures of the surface and the increased spacing between them reduce the water hammer coefficient ( kh) defined as the ratio of the acoustic flow into the droplet to total generated flow. Aided by numerical calculations that solve the laminar Navier-Stokes equation for the vapor flow, we also predict the dynamic Leidenfrost temperature on a micro-textured surface with reliable accuracy consistent with the experimental data.The dynamic Leidenfrost phenomenon is governed by three types of pressure potentials induced via vapor hydrodynamics, liquid dynamic pressure, and the water hammer effect resulting from the generation of acoustic waves at the liquid-vapor interface. The prediction of the Leidenfrost temperature for a dynamic droplet needs quantitative evaluation and definition for each of the pressure fields. In particular, the textures on a heated surface can significantly affect the vapor hydrodynamics and the water hammer pressure. We present a quantitative model for evaluating the water hammer pressure on micro-textured surfaces taking into account the absorption of acoustic waves into the thin vapor layer. The model demonstrates that the strength of the acoustic flow into the liquid droplet, which directly contributes to the water hammer pressure, depends on the magnitude of the acoustic resistance (impedance) in the droplet and the vapor region. In consequence, the micro-textures of the surface and the increased spa...

Pool Boiling Characteristics on the Microstructured surfaces with Both Rectangular Cavities and Channels

Based on a surface design with rectangular cavities and channels, we investigated the effects of gravity and capillary pressure on pool-boiling Critical Heat Flux (CHF). The microcavity structures could prevent liquid flow by the capillary pressure effect. In addition, the microchannel structures contributed to induce one-dimensional liquid flow on the boiling surface. The relationship between the CHF and capillary flow was clearly established. The driving potentials for the liquid supply into a boiling surface can be generated by the gravitational head and capillary pressure. Through an analysis of pool boiling and visualization data, we reveal that the liquid supplement to maintain the nucleate boiling condition on a boiling surface is closely related to the gravitational pressure head and capillary pressure effect. § 이 논문은 대한기계학회 창립 70주년 기념 학술대회 (2015.11.10-14., ICC제주) 발표논문임. †Corresponding Author, hsahn@inu.ac.kr C 2016 The Korean Society of Mechanical Engineers 1. 서 론 비등 열전달은 단상대류 열전달에 비해 월등히 높은 열전달계수로 인해 고열유속 제거를 위한 가장 효과적인 수단으로 활용되어 왔다. 그러나 비등현상은 제거 가능한 열유속에 있어서 본질적 인 한계를 지닌다. 표면 열유속이 증가함에 따라 비등 표면위에서 수많은 증기 기포가 발생, 성장 및 이탈을 반복하는 핵비등 영역(Nucleate boiling regime)으로부터 비등 표면이 증기막으로 덮여 액 김동억.박수청.유동인.김무환.안호선.명병수 384 체의 공급이 차단되는 막비등 영역(Film boiling regime)으로의 천이가 발생하고, 이는 급격한 표 면 온도 상승을 유발하여 결국 표면을 손상시키 게 된다. 이것이 임계열유속(CHF, Critical Heat Fulx)이다. 즉, CHF는 비등 열전달을 통해 제거 가능한 열에너지의 상한을 제시한다. 2000년대 초반부터 이러한 비등표면에서의 CHF를 증가시 키기 위한 많은 시도들이 이루어져 왔으며, 가장 최근에는 마이크로 및 나노미터 사이즈의 복합구 조(Hierachical structure)를 표면에 형성시켜 CHF 를 증진시킨 연구들이 보고되고 있다. 최적의 CHF 증진성능을 보유한 비등 표면의 설계를 위해서는 마이크로구조에서의 CHF 증진 현상에 대한 물리적 이해가 필수적이다. CHF 현 상을 설명하기 위한 기존의 대표적인 이론으로는 Kutateladze 및 Zuber에 의해 제안된 수력학적 불안정성 모델(Hydrodynamic instability theory), Haramura 및 Katto의 Macrolayer dryout model 등 이 있다. 그러나 이러한 이론적 모델들은 CHF 현상의 원인을 설명하기 위해 증기 및 액체의 수 력학적 거동만을 고려하고 있어, 마이크로구조 표면에서의 CHF 증진현상을 설명하는데 한계를 지니고 있다. 이러한 CHF에 대한 표면의 효과를 설명하기 위한 초기연구는 주로 표면젖음성 (surface wettability)에 주목하였다. 또한 증기반 동력(vapor recoil force) 및 고체-액체 접착력의 균 형이론, 표면열전도, 모세관 윅킹 (capillary wicking) 및 Rayleigh-Tayolor 파장변 화 등이 마이크로구조에서의 CHF 증진 메커니 즘으로 제안되었으나, 정성적인 해석수준에 머무 르고 있으며, 일반화된 이론은 아직까지 정립되 지 못했다. 본 연구에서는 CHF에 대한 중력 및 수 마이크 로미터 스케일의 구조의 영향을 명확히 설명하기 위해 사각 공동 및 채널(rectangular cavities and channels)이 복합적으로 형성된 마이크로구조 표 면에서 수조비등실험을 수행하였다. 이 표면에서 마이크로 공동구조는 모세관 압력(capillary pressure)에 의한 액체의 유입을 차단하여, 액체유 동은 마이크로 채널영역에서만 존재한다고 가정 할 수 있다. 이러한 마이크로구조 표면에서의 수 조비등 실험을 통해, CHF 증진현상을 중력수두 및 모세관압력에 의한 액체공급의 관점에서 분석 하였다. Cases Hole size (μm) Channel size(μm) # of channels f Bare 1 HS 5×5×20 2.78 MC6 5×5×20 5×20 400 2.51 MC12 5×5×20 5×20 666 2.33 Table 1 Test surface geometries and roughness factor Fig. 1 SEM images of the micro-structured test surfaces (a) and static contact angle on the flat silicon oxide surface (    ) (b)