Subcooled pool boiling on nanotextured surfaces is explored in this study. The experiments are performed in an enclosed viewing chamber. Two silicon wafers are coated with Multiwalled Carbon Nanotubes (MWCNT), 9 microns (Type-A) and 25 microns (Type-B) in height. A third bare silicon wafer is used for control experiments. The test fluid is PF-5060, a fluoroinert with a boiling point of 56°C (Manufacturer: 3M Co.). The apparatus is of the constant heat flux type. Pool boiling experiments in nucleate and film boiling regimes are reported in this study. Experiments are carried out under low subcooling (5 °C and 10 °C) and high subcooling conditions (20°C to ~ 38°C). At approximately 38°C, a non-departing bubble configuration is obtained on a bare silicon wafer. Increase in subcooling is found to enhance the critical heat flux (CHF) and the CHF is found to shift towards higher wall superheats. Presence of MWCNT on the test surface led to an enhancement in heat flux. Potential factors responsible for boiling heat transfer enhancement on heater surfaces coated with MWCNT are identified as follows: a. Enhanced surface area or nano - fin effect b. Higher thermal conductivity of MWCNT than the substrate c. Disruption of vapor-liquid vapor interface in film boiling, and of the “microlayer” region in nucleate boiling d. Enhanced transient heat transfer caused by local quasi-periodic transient liquid-solid contacts due to presence of the “hair like” protrusion of the MWCNT e. Enhancement in the size of cold spots f. Pinning of contact line, leading to enhanced surface area underneath the bubble leading to enhanced heat transfer Presence of MWCNT is found to enhance the phase change heat transfer by approximately 400% in nucleate boiling for conditions of low subcooling. The heat transfer enhancement is found to be independent of the height of MWCNT in nucleate boiling regime in the low subcooling cases. About 75%-120% enhancement in heat transfer is observed for surfaces coated with MWCNT under conditions of high subcooling in the nucleate boiling regime. Surfaces coated with Type-B MWCNT show a 75% enhancement in heat transfer in the film boiling regime under conditions of low subcooling.

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<p> An experimental study of temperature profiles in the near vicinity of a horizontal copper surface on which water was boiled is reported in this thesis. A series of three tests is reported for heat fluxes of 20,000, 50,000 and 100,000 BTU/HRFT^2 respectively. Four levels of subcooling were achieved in each series in the range of 0°F < θsub < 105°F. Using the superheat-layer thickness. as defined by Han and Griffith, results are reported indicating an increase in superheatlayer thickness for an increase in the degree of subcooling at a constant heat flux and a reverse effect for an increase in heat flux for a constant degree of subcooling. </p> <p> In addition, five tests are reported in which incipience of boiling was achieved. Using this data, Hsu's mathematical model for predicting bubble nucleation is tested. In general, good agreement is found between the Hsu model and the experimental data. </p> / Thesis / Master of Engineering (MEngr)

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19994号 / 工博第4238号 / 新制||工||1656(附属図書館) / 33090 / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 功刀 資彰, 教授 佐々木 隆之, 准教授 横峯 健彦 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This present study seeks to contribute detailed visualization data on a pool boiling experiments using HFE-7000. Particle Image Velocimetry (PIV) was used to measure the time resolved whole field liquid velocity. Bubble dynamic parameters such as nucleation site density, bubble departure diameter, contact angles and frequency were obtained in shadowgraphy measurements. Infrared thermometry with an IR camera was used for observation of temperature fluctuations of nucleation sites. The experiments were taken for the heat flux from 0.042 kW/m^2 to 0.266 kW/m^2, six experimental conditions in total. To provide a supplementary description of heat transfer mechanism, a novel bubble characterization technique, reflection interference contrast microscopy (RICM), was used to obtain detailed information on bubble dynamic parameters on the microscopic scale. Bubble diameter was obtained from RICM pictures. Comparison between the experiments results and previous empirical correlation were made. Agreements and discrepancies were discussed.

The primary purpose of this investigation was to experimentally determine the effect of operational parameters on the onset of flow instability (OFI) in narrow, uniformly heated, vertical, rectangular channels. The geometry investigated was a 9.0 cm long rectangular channel with a 1.0mm by 1.3cm cross section. This geometry closely matches the coolant channel geometry in an accelerator target. Nitrogen-saturated subcooled water was used as the coolant, with mass fluxes ranging from 250 to 1336 kg/m^2 s, and an inlet temperature of 26ºC for the OFI experiments. The exit pressures investigated ranged from 275kPa to 620kPa, while the heat flux ranged from 0.729 to 2.236 MW/m^2. The primary data collected from these experiments were used to develop two correlations for the heat flux and mass flux at OFI. Wall temperature data were also collected in order to develop a Nusselt number correlation for the single-phase regime. This correlation is valid for the Reynolds number range of 6x103 to 1.7x104. The data obtained in this investigation will aid designers of high-power-density systems establish design limits to prevent over heating and possible damage due to the onset of flow instability. The data obtained in this investigation will aid designers of high-power-density systems establish design limits to prevent over heating and possible damage due to the onset of flow instability.

The objective of this investigation is to measure and analyze surface temperature fluctuations in pool boiling. The surface temperature fluctuations were recorded on silicon surfaces with and without multi-walled carbon nanotubes (MWCNT). Novel Thin Film Thermocouples (TFT) are micro-fabricated on test substrates to measure surface temperatures. A dielectric liquid refrigerant (PF-5060) is used as test fluid. Both nucleate and lm boiling regimes are investigated for the silicon test substrates. Dynamics of nucleate boiling is investigated on the CNT coated substrates. High frequency temperature fluctuation data is analyzed for the presence of determinism using non-linear time series analysis techniques in TISEAN(copyright) software. The impact of subcooling and micro/nano-scale surface texturing using MWCNT coatings on the dynamics of pool boiling is assessed. Dynamic invariants such as correlation dimensions and Lyapunov spectrum are evaluated for the reconstructed attractor. A non-linear noise reduction scheme is employed to reduce the level of noise in the data. Previous investigations in pool boiling chaos, reported in literature were based on temperature measurements underneath the test surface consisting of single or few active nucleation sites. Previous studies have indicated the presence of low-dimensional behavior in nucleate boiling and high-dimensional behavior in CHF and film boiling. Currently, there is no study detailing the effects of multiple nucleation sites, subcooling and surface texturing on pool boiling dynamics. The investigation comprises of four parts: i) in situ micro-machining of Chromelalumel (K-type) TFT, ii) calibration of these sensors, iii) utilizing these sensors in pool boiling experiments iv) analysis of these fluctuations using techniques of nonlinear time series analysis. Ten TFT are fabricated on a rectangular silicon surface within an area of ~ 3.00 cm x 3.00 cm. The sensing junctions of the TFT measure 50 mm in width and 250 nm in depth. Surface temperature fluctuations of the order of i) 0.65-0.93 degrees C are observed near ONB ii) 2.3-6.5 degrees C in FDNB iii) 2.60-5.00 degrees C at CHF and iv) 2.3-3.5 degrees C in film boiling. Investigations show the possible presence of chaotic dynamics near CHF and in film-boiling in saturated and subcooled pool boiling. Fully-developed nucleate boiling (FDNB) is chaotic. No clear assessment of the dynamics could be made in the onset of nucleate boiling (ONB) and partial nucleate boiling (PNB) regimes due to the effects of noise. However, the frequency spectra in these regimes appear to have two independent frequencies and their integral combinations indicating a possible quasiperiodic bifurcation route to chaos. The dimensionality in FDNB, at CHF and in film-boiling is lower in saturated pool boiling as compared to values in corresponding regimes in subcooled pool boiling. Surface temperature fluctuations can damage electronic components and need to be carefully controlled. Understanding the nature of these fluctuations will aid in deciding the modeling approach for surface temperature transients on an electronic chip. Subsequently, the TFT signals can be employed in a suitable feedback control loop to prevent the occurrence of hotspots.

An experimental investigation was carried to examine the flow/ thermal field characteristics with/without phase change in the microchannels and compared with the traditional results. There are three parts in this study. The first part investigated the 2-D flow field measured by the micro particle image velocimetry (£gPIV) in a single PMMA microchannel fabricated by an ArF excimer laser. The slip boundary condition in the microchannel wall was also discussed. The second part studied the influence of surface condition (hydrophilic vs hydrophobic) on the flow/thermal field in a micro cooling device which included twenty parallel microchannels, which was fabricated by SU-8 microfabrication technique and replicated by the PDMS replica technique. The UV/ozone device was used to change the PDMS microchannels¡¦ surface condition from hydrophobic to hydrophilic and the £gPIV/£gLIF system was also used to measure the velocity and temperature distribution. The third part investigated the two-phase subcooled flow boiling phenomena (onset of nucleate boiling, boiling curve, flow patterns, bubble departure diameter and frequency) in the seventy-five parallel microchannels fabricated by SU-8 microfabrication technique, and aimed to raise the critical heat flux (CHF) and heat transfer coefficient to enhance the cooling efficiency. Three major methods were used in this study, as follows: (1) To add the cavity angle of £c = 60¢X, 90¢X, and 120¢X on the microchannel side walls. (2) To coat 2 £gm diamond film on the Cu heated surface. (3) To add 1 vol. % Multi-walled Carbon Nanotube (MCNT) into the working medium (deionized water). The goal of this paper is to develop a high heat flux cooling technique and apply the experimental results to solve the cooling problem resulting from the exceedingly high heat flux from the electronic component.

Nucleate boiling is widely used as a means of heat transfer in thermal management systems because of its high heat transfer rates. This study explored the effects of heat flux and surface geometry on heat transfer behavior and bubble dynamics of nucleate pool boiling in microgravity. A single platinum wire, a twist of three platinum wires, and a twist of four platinum wires were used as boiling surfaces for two separate experiments performed in microgravity on board NASA’s parabolic flight aircraft. Wire temperature, thermocouple, and video measurements were taken during a total of 44 microgravity parabolas. Results show that the crevices formed by wire twisting provide regions of localized superheating and are able to reduce the heat flux necessary for boiling onset to occur. This localized heating results in a lower average heater temperature and shortened superheating periods, but this effect decreases when more wires are present in the twist. This behavior was investigated and confirmed with a finite volume, transient conduction model. This model also showed that the water temperature profile at the bubble onset indicates that water at a certain distance from the wire surface, in this experiment 50 μm, needs to be heated to above saturation temperature in order to initiate and generate a burst of bubbles. A relative bubble area analysis method was able to quantify vapor production and bubble behavior across multiple frames of video. Application of this method revealed a transition of bubble behavior from large isolated bubbles to jet flows of small bubbles, and this method allowed the heat flux contribution of jet flows to be approximated. Additionally, a new mode of jet flows was observed. Particle image velocimetry was used to provide approximate velocities of small bubble jet flows and their influence on heat transfer to the bulk fluid.

On the efforts to provide a reliable source of experimental information on turbulent subcooled boiling ow, time resolved Particle Tracking Velocimetry (PTV) experiments were carried out using HFE-301 refrigerant ow through a vertical rectangular channel with one heated wall. Measurements were performed at liquid Reynolds numbers of 3309, 9929 and 16549 over a wall heat flux range of 0.0 to 64.0 kW=m2. From the PTV measurements, liquid two dimensional turbulence statistics are available, such as: instantaneous 2-D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses. The present results agree with previous works and provide new information due to the 2-D nature of the technique, for instance, this work shows that by increasing heat ux, the boiling bubbles influence on the liquid phase is portrayed as a persistent increase of axial velocity on regions close to the heater wall. This persistent increase on the axial velocity reaches a maximum value attributed to the terminal bubble velocity. These new observed phenomena must be considered for the development and improvement of two-phase ow turbulence models. To this end, an extensive error analysis was also performed with emphasis on the applicability of the PTV measurement technique on optically inhomogeneous flows. The error quantification exhibited negligible optically induced errors for the current conditions, making the data acquired in this work a vast and reliable source.