Dong In Yu

Self-assembled foam-like graphene networks formed through nucleate boiling

Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400°C) SFG film were 11.8 S·cm–1 and 91.2 Ω□−1, respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (~10 S·cm–1). The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs).

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 on spatially controlled heterogeneous surfaces: Wettability patterns on microstructures

We investigated nucleate boiling heat transfer with precisely controlled wetting patterns and micro-posts, to gain insights into the impact of surface heterogeneity. To create heterogeneous wetting patterns, self-assembled monolayers (SAMs) were spatially patterned. Even at a contact angle <90°, bubble nucleation and bubble frequency were accelerated on SAM patterns, since this contact angle is larger than that found on plain surfaces. Micro-posts were also fabricated on the surface, which interrupted the expansion of generated bubbles. This surface structuring induced smaller bubbles and higher bubble frequency than the plain surface. The resistance provided by surface structures to bubble expansion broke the interface between the vapor mushroom and the heating surface, and water could therefore be continuously supplied through these spaces at high heat flux. To induce synergistic effects with wetting patterns and surface structures on boiling, we fabricated SAM patterns onto the heads of micro-posts. On this combined surface, bubble nucleation was induced from the head of the micro-posts, and bubble growth was influenced by both the SAM pattern and the micro-post structures. In particular, separation of the vapor path on the SAM patterns and the liquid path between micro-post structures resulted in high heat transfer performance without critical heat flux deterioration.

Enhanced heat transfer is dependent on thickness of graphene films: the heat dissipation during boiling

Boiling heat transfer (BHT) is a particularly efficient heat transport method because of the latent heat associated with the process. However, the efficiency of BHT decreases significantly with increasing wall temperature when the critical heat flux (CHF) is reached. Graphene has received much recent research attention for applications in thermal engineering due to its large thermal conductivity. In this study, graphene films of various thicknesses were deposited on a heated surface, and enhancements of BHT and CHF were investigated via pool-boiling experiments. In contrast to the well-known surface effects, including improved wettability and liquid spreading due to micron- and nanometer-scale structures, nanometer-scale folded edges of graphene films provided a clue of BHT improvement and only the thermal conductivity of the graphene layer could explain the dependence of the CHF on the thickness. The large thermal conductivity of the graphene films inhibited the formation of hot spots, thereby increasing the CHF. Finally, the provided empirical model could be suitable for prediction of CHF.

Dynamics of Contact Line Depinning during Droplet Evaporation Based on Thermodynamics

For several decades, evaporation phenomena have been intensively investigated for a broad range of applications. However, the dynamics of contact line depinning during droplet evaporation has only been inductively inferred on the basis of experimental data and remains unclear. This study focuses on the dynamics of contact line depinning during droplet evaporation based on thermodynamics. Considering the decrease in the Gibbs free energy of a system with different evaporation modes, a theoretical model was developed to estimate the receding contact angle during contact line depinning as a function of surface conditions. Comparison of experimentally measured and theoretically modeled receding contact angles indicated that the dynamics of contact line depinning during droplet evaporation was caused by the most favorable thermodynamic process encountered during constant contact radius (CCR mode) and constant contact angle (CCA mode) evaporation to rapidly reach an equilibrium state during droplet evaporation.

Wetting state on hydrophilic and hydrophobic micro-textured surfaces: Thermodynamic analysis and X-ray visualization

In this study, the wetting state on hydrophobic and hydrophilic micro-textured surfaces was investigated. High spatial resolution synchrotron X-ray radiography was used to overcome the limitations in visualization in previous research and clearly visualize the wetting state for each droplet under quantified surface conditions. Based on thermodynamic characteristics, a theoretical model for wetting state depending on the chemical composition (intrinsic contact angle) and geometrical morphology (roughness ratio) of the surfaces was developed.

Apparent Contact Angle on the Hydrophilic/Hydrophobic Surfaces with Micro-pillars

In this study, the apparent contact angle on the hydrophilic/hydrophobic surfaces with micro-pillars was studied. The previous researches showed that the Wenzel equation and the Cassie-Baxter equation were thermodynamically derived for the rough hydrophilic/hydrophobic surfaces and generally referenced on the field of wetting phenomena. For the verification of both equations, the apparent contact angle on the hydrophilic/hydrophobic surfaces with micro-pillars was measured. In the comparison between the measured and estimated apparent contact angles with the equations, the differences between the apparent contact angles were analyzed. Conclusively, the available range and limitation of theoretical equations were investigated and further researches about the apparent contact angle on the rough surfaces were proposed.

Development of New Correlation and Assessment of Correlations for Two-Phase Pressure Drop in Rectangular Microchannels

There are two kinds of models in two-phase pressured drop; homogeneous flow model and separated flow model. Many previous researchers have developed correlations for two-phase pressure drop in a microchannel. Most correlations were modified Lockhart and Martinelli`s correlation, which was based on the separated flow model. In this study, experiments for adiabatic liquid water and nitrogen gas flow in rectangular microchannels were conducted to investigate two-phase pressure drop in the rectangular microchannels. Two-phase frictional pressure drop in the rectangular microchannels is highly related with flow regime. Homogeneous model with six two-phase viscosity models: `s, `s, Cicchitti et `s, Dukler et `s, Beattie and `s, Lin et `s models and six separated flow models: Lockhart and `s, `s, Zhang et `s, Lee and `s, Moriyama and `s, Qu and `s models were assessed with our experimental data. The best two-phase viscosity model is Beattie and Whalley`s model. The best separated flow model is Qu and Mudawar`s correlation. Flow regime dependency in both homogeneous and separated flow models was observed. Therefore, new flow pattern based correlations for both homogeneous and separated flow models were individually proposed.

Study of Wettability Effect on Pressure Drop and Flow Pattern of Two-Phase Flow in Rectangular Microchannel

Wettability is a critical parameter in micro-scale two-phase system. Several previous results indicate that wettability has influential affect on two-phase flow pattern in a microchannel. However, previous studies conducted using circular microtube, which was made by conventional fabrication techniques. Although most applications for micro thermal hydraulic system has used a rectangular microchannel, data for the rectangular microchannel is totally lack. In this study, a hydrophilic rectangular microchannel was fabricated using a photosensitive glass. And a hydrophobic rectangular microchannel was prepared using silanization of glass surfaces with OTS (octa-dethyl-trichloro-siliane). Experiments of two-phase flow in the hydrophilic and the hydrophobic rectangular microchannels were conducted using water and nitrogen gas. Visualization of twophase flow pattern was carried out using a high-speed camera and a long distance microscope. Visualization results show that the wettability was important for two-phase flow pattern in rectangular microchannel. In addition, two-phase frictional pressure drop was highly related with flow patterns. Finally, Two-phase frictional pressure drop was analyzed with flow patterns.

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)