Nucleate boiling in thin film annular flow is a complex phenomenon influenced by a variety of heat transfer mechanisms. While the fundamentals are rooted in the comparatively well-understood fields of pool and flow boiling, thin film flow boiling introduces additional challenges that make the heat transfer behavior hard to predict. Specifically, the relative importance of the heat transfer mechanisms controlling nucleate boiling in thin annular films and their roles in flow regime transitions remains inadequately understood. In light of this, the foundational aspects of nucleate boiling and the relationship between flow parameters - such as liquid film thickness, system pressure, surface tension, channel geometry, mass flow rate, and applied heat flux - and the intensity of nucleate boiling in thin annular films are analyzed. The importance of these parameters is demonstrated through the presentation of multiple correlations, with a critical examination of the limitations inherent in correlation-based modeling. Additionally, recent research has identified disturbance waves as a significant factor in enhancing bubble nucleation, yet the underlying mechanisms driving this phenomenon remain undefined. Therefore, the insights and deficiencies of three theories seeking to explain wave-based nucleation are extensively considered. This research aims to introduce and categorize the wide breadth of literature regarding thin film annular flow nucleate boiling to identify gaps in understanding and facilitate future physics-based modeling efforts.

Carbon nanotubes (CNTs) must be ordered into densely aligned arrays to fully exploit their electronic properties in next-generation integrated circuits. Recent advances have shown that CNTs can accumulate and self-order at liquid-liquid interfaces, from which the CNTs can be transferred onto a substrate to create dense CNT arrays with remarkable electronic characteristics. Here, by leveraging in situ polarized optical microscopy, we investigate the self-assembly of CNTs at organic solvent-water interfaces and answer key questions about CNT assembly structure and formation kinetics. We find that CNTs spontaneously form liquid crystalline (LC) phases at the liquid-liquid interface with a density strongly dependent on the concentration of CNTs in the organic solvent ink. This LC behavior is robust across a range of polymer wrappers, including polyfluorenes, triblock copolymers, and polycarbazole (PCz). Polarized microscopy reveals that the resulting LC domains are polycrystalline in nature with domain size governed by the kinetics of LC formation. Additives can alter interfacial dynamics─either by promoting Marangoni flow or by enhancing CNT transport─offering an avenue to tune domain characteristics. We find that the LC domain structure formed at the interface is largely preserved upon transfer to a solid substrate, indicating that optimizing interfacial ordering is key to achieving high-quality CNT arrays for electronic applications. In cases where distortions occur during transfer, they often arise from a mismatch between the substrate translation speed and the transport velocity of the LC to the solid surface.

Flow boiling is utilized in many energy systems, including vapor compression and nuclear power cycles. Although there has been significant past work on boiling heat transfer and dryout characteristics in smooth tubes, many systems have obstructions on the heated surface that alter the flow. This study investigates the effect of obstructions under annular two-phase flow conditions flowing through a vertical, upward channel. A refrigerant with similar properties to high-pressure steam, R-245fa, is used as the working fluid. The flow around a cylindrical obstruction, tested with two different diameters, is characterized using high-speed video, temperature measurements, and optically based liquid film thickness measurements. The experiments investigate the flow phenomena over a range of operating conditions: mass fluxes of 130-170 kg/m2-s, inlet qualities of 0.63-0.95, and heat fluxes of 0-29 kW/m2. Unlike prior literature focusing on relatively high mass fluxes or obstructions separated from the heated surface - which have shown enhanced heat transfer coefficients downstream and delayed critical heat flux - this work examines an obstruction in direct contact with the heated wall to relate the movement of the liquid film to heat transfer performance. The results indicate that the obstruction induces sustained film dryout in its wake without a severe reduction in heat transfer coefficient at heat flux values less than that of unobstructed flows. This behavior is attributed to reduced wave activity and thinner liquid films compared to unobstructed flows at similar conditions.

Direct Contact Condensation (DCC) transfers heat to a process fluid by directly injecting a high-pressure vapor into a subcooled liquid stream. Direct contact condensers have a smaller overall footprint compared to traditional heat exchangers and are used in several industries, including milk pasteurization, emergency cool-down in nuclear power plants, and wastewater treatment. During DCC, vapor is accelerated through small nozzles that generate vapor plumes that penetrate the process fluid. Under certain conditions, the vapor plume becomes unstable and can generate significant noise and vibrations. The goal of this study is to understand the effect of nozzle geometry on plume stability and noise when steam is injected into a crossflow water stream. In these experiments, steam plume flow regimes are characterized and noise level through single- and two-port nozzles are measured. The results quantify the impact of water temperature (25-80 degrees C), pressure ratio (0.47 and 0.58), nozzle diameter (1.65-3.18 mm), and the nozzle length-to-diameter ratio (1-8) on these parameters. This study shows that nozzles with a smaller diameter and length-to-diameter ratio produce less noise and that two smaller holes can have the same mass flow rate (and heating) as one larger nozzle, but less noise.

This paper discusses the extension of an optical liquid film thickness measurement technique to characterize liquid film flow rate in wavy thin liquid film flow. The technique, based on laser refractometry, is used to measure wave height, shape, frequency, and velocity. A two-zone model to process the measured wave characteristics is used to estimate the liquid film flow rate. The method is validated in a falling film facility where easy optical access allows comparisons of the wave velocity measurements with high-speed videos and where the calculated liquid film mass flow rate can be compared with actual measurements. The paper provides a framework for analyzing time-resolved film thickness data using multizone models in more complex liquid film flows, such as in two-phase annular flow.

The entire liquid-film dryout process in vertical annular two-phase boiling flow is characterized experimentally, from inception to completion, in a newly defined flow regime between annular two-phase flow and mist flow, called 'intermittent dryout'. Experiments are conducted using saturated R245fa flowing vertically through a heated transparent channel. Liquid-film thickness measurements are taken during dryout events. The state (wet or dry) of the heated surface is determined using a laser reflectance measurement, and the signal is used to calculate the time-averaged dry fraction, f(dry). The local heat transfer coefficient (HTC) is characterized as a function of f(dry). For all the investigated mass fluxes, optimum boiling conditions (OBC), corresponding to the heat flux that results in maximum HTC, consistently occur when the time-averaged dry fraction is f(dry )approximate to 0.05. Subsequently, the critical boiling transition (CBT), corresponding to the heat flux that results in a significantly lower HTC, can occur for dry fractions of f(dry )>0.1. Further insight into the liquid film behavior within the intermittent dryout regime is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. In general, the dryout mechanism is dominated by disturbance waves.Mechanistic model frameworks are presented for the prediction of two-phase HTC and heat flux associated with dryout. The model inputs are local flow quantities (pressure, mass flux, vapor quality, and liquid-film thickness) and output a prediction for local heat transfer coefficient. The model framework accurately predicts the local two-phase HTC for a range of mass flux and vapor quality in the annular flow regime. The model framework captures the observed initial increase in the local HTC and subsequent decrease that is associated with liquid-film dryout. The mechanistic approach provides predictions for time-averaged HTC and OBC heat flux that both agree well with experimentally measured values.

High packing density aligned arrays of semiconducting carbon nanotubes (CNTs) are required for many electronics applications. Past work has shown that the accumulation of CNTs at a water-solvent interface can drive array self-assembly. Previously, the confining interface was a large-area, macroscopic feature. Here, we report on the CNT assembly on microscopic water features. Water microdroplets are formed on 10-100 μm wide hydrophilic stripes patterned on a substrate. Exposure to CNTs dispersed in solvent accumulates CNTs at the microdroplet-solvent interface, driving their alignment and deposition at the microdroplet-solvent-substrate contact line. Compared with macroscopic methods in which the contact line uncontrollably moves across the substrate as it is pulled out of the liquids, the hydrophilic patterns and microdroplets allow pinning of the contact line. As CNTs deposit, the contact line self-translates, allowing for dense CNT packing. We realize monolayer CNT arrays aligned within ±3.9° at density of 250 μm-1 and field effect transistors with a high current density of 1.9 mA μm-1 and transconductance of 1.2 mS μm-1 at -0.6 V drain bias and 60 nm channel length.

The dryout of liquid film and the role of disturbance wave frequency is investigated. The Multiphase Flow Visualization and Analysis Laboratory (MFVAL) at UW-Madison has built a facility capable of providing vapor pulses (or density-wave oscillations) with consistent flow rate for long periods of time. Experimental results for heat transfer coefficient (HTC), liquid-film thickness, temporal dry fraction, and dryout statistics under pulsatile conditions are presented. A normalized time-averaged HTC is compared with the fraction of time that the surface is dry. The maximum in HTC associated with local optimal boiling conditions (OBC) occurs when the surface is dry 5% of the time, independent of pulse amplitude and frequency. Liquid-film measurements, dryout statistics, and direct observation indicate that disturbance-wave frequency can be manipulated by density-wave oscillations in the flow field.

The assembly of a two-dimensional (2D) nematic liquid crystal at an interface between two liquids can be exploited to assemble densely packed and highly aligned arrays of rod-like nanoparticles. This method is especially relevant to creating arrays of semiconducting carbon nanotubes (CNTs) for high-performance electronics. When a dense solvent containing CNTs flows over a less dense water subphase in a confined channel, the locally aligned arrays of nanoparticles align globally with the flow direction and can be transferred to the substrate. For large substrates and long channels, the dense solvent tends to slow and create a pool, which then drops through the interface and disturbs the delicate deposition process. Understanding this phenomenon is critical to improving and scaling up similar manufacturing processes. Here, data are collected, and an empirical model is developed to understand and predict the pooling behavior of a suspended fluid flowing over a less dense subphase. The model is demonstrated with two different solvents and proves to be accurate within +/- 15%. With a better understanding of the physics governing the system, the model is then used to suggest methods for minimizing pooling behavior.

Direct contact condensation (DCC) is an efficient way of transferring heat between fluids’ vapor and liquid phases by mixing them directly rather than using a device like a heat exchanger. However, in the field, DCC may produce destructive and unwanted vibrations due to the instability of the gaseous plume condensing in the process fluid under certain conditions. Previous research on this topic has focused on correlating the flow conditions of the liquid and vapor phases to engineering quantities, such as the heat transfer coefficient, plume length, stability, and the dominant frequency exhibited by the DCC process. The present study investigates the impact of nozzle geometry on plume stability. It was motivated by the drastic difference observed experimentally between the stability of two identically designed nozzles with slightly different microfeatures. Tests were performed with several nozzle geometries, including converging/diverging, straight, and converging nozzles. The results from these experiments show that a nozzle with a supersonic flow at the exit plane is more likely to be stable than one with a subsonic exit flow. It is, therefore, possible to correlate the pressure ratio across the nozzle and nozzle geometry to the stability of the plume. This understanding allows a DCC nozzle geometry to be analyzed and designed for high stability with relatively simple one- and two-dimensional compressible flow modeling tools capable of resolving only the single-phase, compressible flow in the nozzle itself.

Selective shear deposition of polymer-wrapped semiconducting carbon nanotubes (s-CNTs) into densely packed, highly aligned arrays of s-CNTs using removable chemical and topographical patterns.

The stability of a steam plume during direct-contact condensation into a crossflow of subcooled water is investigated for mass fluxes that are higher (>600 kg/m(2)s) and a nozzle diameter (2.4 mm) that is smaller than typically seen in the literature. The transition from a stable steam plume to an unstable plume associated with the formation and collapse of steam bubbles is characterized by high-speed imaging and high-frequency pressure measurements. Four regimes are observed: stable, condensation oscillation, transition, and unstable. A regime map and spectral signatures of the different flow regimes are provided. Results are compared with correlations from the literature, which are typically derived for lower mass fluxes, larger nozzles, and injection into stagnant pools of water.

Semiconducting carbon nanotubes promise faster performance and lower power consumption than Si in field-effect transistors (FETs) if they can be aligned in dense arrays. Here, we demonstrate that nanotubes collected at a liquid/liquid interface self-organize to form two-dimensional (2D) nematic liquid crystals that globally align with flow. The 2D liquid crystals are transferred onto substrates in a continuous process generating dense arrays of nanotubes aligned within ±6°, ideal for electronics. Nanotube ordering improves with increasing concentration and decreasing temperature due to the underlying liquid crystal phenomena. The excellent alignment and uniformity of the transferred assemblies enable FETs with exceptional on-state current density averaging 520 μA μm −1 at only −0.6 V, and variation of only 19%. FETs with ion gel top gates demonstrate subthreshold swing as low as 60 mV decade −1 . Deposition across a 10-cm substrate is achieved, evidencing the promise of 2D nanotube liquid crystals for commercial semiconductor electronics.

Selective deposition of semiconducting carbon nanotubes (s-CNTs) into densely packed, aligned arrays of individualized s-CNTs is necessary to realize their potential in semiconductor electronics. We report the combination of chemical contrast patterns, topography, and pre-alignment of s-CNTs via shear to achieve selective-area deposition of aligned arrays of CNTs. Alternate stripes of surfaces favorable and unfavorable to s-CNT adsorption were patterned with widths varying from 2000 nm down to 100 nm. When the chemical and topographical contrast patterns are reduced to less than the width of individual nanotubes (≤ 500 nm), confinement effects become dominant enabling the selective-area deposition of much more tightly aligned CNTs (~7 degrees). At a trench width of 100 nm, we demonstrate the lowest standard deviation in alignment degree of 7.6 ± 0.3° at a deposition shear rate of 4,600 s-1, while maintaining an individualized s-CNT density > 30 CNTs µm-1. Chemical contrast alone enables selective area deposition but chemical contrast in addition to topography enables more effective selective area deposition and stronger confinement effects, with the advantage of removal of nanotubes deposited in spurious areas via selective lift-off of the topographic features. These findings provide a methodology that is inherently scalable, and a means to deposit spatially selective, aligned s-CNT arrays for next-generation semiconducting devices.

The entire liquid-film dryout process in a vertical two-phase annular flow is characterized experimentally, from inception to completion. Experiments are conducted using saturated R245fa at high vapor qualities in a heated rectangular channel with a hydraulic diameter of 18 mm and an aspect ratio of 1/3. The walls of the test section are made of glass coated with fluorine-doped tin oxide (FTO). Heat fluxes up to 50 kW/m(2) are generated at the inner surface of the window by passing an electrical current through the FTO coating. Instantaneous pressure and temperature in the test section, temperature on the outer wall of the test section, liquid-film thickness, and high-speed videos are recorded simultaneously during the dryout events. In addition, the state (wet or dry) of the heated surface is measured using a non-invasive laser reflectance technique at high sampling rate (2000 Hz) and over long periods of time (> 1000 s). The laser reflectance measurement is used to calculate the time-averaged dry fraction, f(dry), which is the fraction of time that the wall is dry during intermittent cycles of dryout and rewet. Data show that cyclic dryout starts before the critical heat flux (CHF) is reached. The dryout heat flux (DHF), which marks the onset of dryout, is typically 90% of the CHF, except at very high quality (x > 0.95), where it can be as low as 50% of CHF. For all the investigated mass fluxes, CHF, where the heat transfer coefficient peaks, occurs consistently at f(dry) approximate to 0.05. Further insight into the liquid-film behavior at the onset of dryout is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. The rewetting process is driven by disturbance waves. In the wake of disturbance waves, the liquid film is almost stationary. Calculations of the characteristic time it takes for this stationary film to evaporate predict well the characteristic time during dryout events measured with the laser reflectance method. (C) 2021 Elsevier Ltd. All rights reserved.

Instantaneous temperature measurements at the interface between a solid wall and a thin, unsteady liquid film are performed using thermoreflectance, a nonintrusive optical technique with high temporal resolution. A laser beam is directed at a wall-liquid interface, and the intensity of the light reflected at that interface is measured by a photodiode. The intensity of the reflected light varies with the index of refraction of the liquid at the wall. The index of refraction is a function of temperature, which enables the instantaneous measurement of the wall temperature. In the presence of thin liquid films, reflections from the liquid-vapor interface at the free surface of the film generate noise in the measurements. We demonstrate that orienting the laser beam at a large incident angle, close to total internal reflection, minimizes noise from the liquid-vapor interface while increasing the sensitivity of the measurement. The thermoreflectance technique is validated in an unsteady two-phase annular flow. Measurements of temperature fluctuations less than 1K in amplitude are achieved, with an uncertainty of 0.1K.

The liquid-film flow in a vertical, upward, two-phase annular flow of saturated R245fa is characterized experimentally under adiabatic conditions. The experiments are conducted inside a rectangular channel with a cross section of 11.6 x 36 mm(2) for mass velocities ranging from 95 to 130 kg/m(2)s, vapor qualities from 0.63 to 0.9 and saturation temperature of 23 degrees C. Liquid-film thickness and disturbance-wave velocity and frequency are measured optically. Liquid-film thickness is recorded at a sampling frequency of 2000 Hz, while disturbance-wave velocity is recorded at a sampling frequency of 20 Hz. A parametric study as a function of quality and mass velocity is performed. Results show that the liquid-film thickness decreases linearly with increasing vapor quality. The liquid films investigated are very thin, and thinner than the critical liquid-film thickness below which momentum and mass transport are no longer driven by disturbance waves. Indeed, at high vapor quality, when the liquid film is very thin, the liquid-vapor interface becomes smoother and disturbance waves slow down and vanish. The frictional pressure gradient increases with quality until it reaches a peak. Results suggest that the subsequent decrease in pressure gradient is closely linked to the disappearance of disturbance waves. For the range of studied operating conditions, the frequency of disturbance waves remains constant within the uncertainty of the measurements. Experimental data is compared with prediction methods from the literature. Although reasonable predictions were obtained for the liquid film thickness, none of the tested methods could predict disturbance-wave velocity and frequency accurately. (C) 2020 Elsevier Ltd. All rights reserved.

An investigation into the conditional stability of direct contact steam condensation is presented. Direct contact steam condensation systems heat process fluids more effectively than indirect contact heat exchangers because the total energy of the steam is introduced directly into the process fluid, bypassing any resistances associated with having to cross the heat exchanger boundary. In the present work, superheated steam is injected perpendicular to a flow of subcooled liquid water. Conditional stability of direct contact steam condensation stems from the type of fluctuations exhibited by the steam plume structure. Under certain flow conditions the steam plume condenses in the form of a stable conical jet with little to no acoustic noise or resultant pressure fluctuations. However, other conditions cause the steam plume to increase in oscillation intensity, eventually transitioning to an intermittent bubbling sfructure that results in high-amplitude acoustic noise and large pressure fluctuations in the flow of liquid water. A regime map is presented to establish the process criteria for the onset of instability of condensation. The transitional profile to instability is also presented as an analysis of plume oscillation frequencies and associated pressure oscillations propagating from these fluctuations. It is found that, as the mode of condensation goes unstable, steam plume oscillations increase in intensity and decrease in frequency until the sfructure collapses into bubbles that provoke strong pressure oscillations in the surrounding water.