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© 2020 Capturing the dynamics of the triple contact line at the base of a vapor bubble is crucial to the development and validation of boiling heat transfer and boiling crisis models. To this end, laser-based techniques are the state-of-the-art. However, they have technical limitations, e.g., due to the presence of parasite interference patterns or laser speckle, and are often complicated to implement and operate. In this paper, we propose and discuss the rationale of alternative phase detection techniques based on LED lights. Colored LED lights have a coherence length of several micrometers, which is long enough to create interference patterns when reflected by the microlayer and short enough to prevent the formation of the parasite interference patterns caused by lasers. We show that, even with a simple optical setup, these LED-based techniques can create images of superior quality and uniformity, making it possible to resolve smaller boiling features, e.g., size and shape of bubble footprint at high-pressure conditions. Moreover, LED lights are much less expensive, safer, and easier to operate than lasers, which is an obvious advantage for researchers that want to either enter the field or expand their measurement capabilities. We demonstrate the use and the potential of these LED-based techniques with experimental results obtained in subcooled flow boiling of water, at both ambient and high-pressure conditions. We also demonstrate how this technique can be combined with infrared thermometry to study the formation and evaporation of the microlayer, other heat transfer mechanisms associated with the growth and departure of a vapor bubble, and the heat flux partitioning over the entire boiling surface.

We present an experimental and theoretical investigation of single-phase heat transfer under exponential power inputs. We conduct forced flow experiments with water, covering a broad range of mass fluxes (from 0 to 19,300 kg/m2/s), bulk temperatures (from 25 to 100 °C), pressures (from 0.1 to 1.2 MPa), exponential power escalation periods (from 2.5 to 200 ms), and considering two different heater and channel geometries. We use a high-speed infrared thermometry technique to measure the space- and time-dependent heat transfer coefficient between the heated surface and the coolant, building a database covering 73 different experimental conditions. We consider turbulence as a diffusive process and develop an analytic model that can predict 80% of our database within a ±10% error, and the entire database within a ±20% error. We discuss the presence of three heat transfer regimes, i.e., transient conduction, transient turbulent diffusion, and quasi-steady turbulent heat transfer, and derive analytically the two associated transition criteria. These transitions depend on the power escalation period, fluid properties, and are connected to the profile of the turbulent diffusion properties across the boundary layer.

Using a combination of high-speed infrared thermography and electrical-conductance sensors, we observe the surprisingly large effect of surface waves on the instantaneous heat transfer response at the heated wall in nominally-steady upward annular flow of steam-water mixtures. While the heat flux applied at the wall is constant, the passage of the waves causes significant periodic oscillations of the local heat transfer coefficient and wall superheat in the forced convective evaporation regime, with amplitudes and periods much higher than the fluctuations due to turbulent eddies. We discuss the physical mechanisms potentially responsible for the experimental observations.

Simultaneous measurement of liquid film characteristics and heat transfer in annular two-phase flow is a major experimental challenge. In this study, we implement an innovative integrated infrared (IR) heater and liquid film electrical-conductance sensor in combination with high-speed IR thermography, which results in an accurate, synchronized and co-located measurement of liquid film characteristics and local heat transfer at the heated surface in diabatic annular two-phase flow. The measurements reveal that disturbance waves have a significant effect on the instantaneous heat transfer response in nominally-steady vertically upward annular flow of steam-water mixtures. Although the heat flux applied at the wall is constant in time, the passage of the waves causes significant periodic oscillations of the local heat transfer coefficient and wall superheat in the forced convective evaporation regime, which are reported in this study.

The effects of hydrophilic/hydrophobic surface patterning on critical heat flux (CHF) and heat transfer coefficient (HTC) were studied using custom-engineered testing surfaces. Patterning was created over a sapphire substrate and tested in a pool boiling facility in MITs Reactor Hydraulics Laboratory. The hydrophilic and hydrophobic matrices were created using layer by layer deposition of 50 nm thick SiO2 nanoparticles and monolayer thickness fluorosilane, respectively. Ultraviolet ozone patterning was then used with chrome-printed masks to create the desired geometric features. Hexagon, ring, star, and mixed patterns were tested to determine their abilities to affect CHF and HTC through prevention of bubble pinning at high heat fluxes. During testing, an infrared camera was used to measure the surface temperature distribution as well as locate nucleation sites for data analysis. It was found that CHF values were enhanced over the bare sapphire values by approximately 90% for hexagons, 60% for stars, 65% for rings, and 50% for mixed patterns. Contrary to expectations, patterning did not seem to affect the HTC values significantly. Although patterning did improve CHF performance over bare heaters, both CHF and HTC were found to be statistically similar to those for unpatterned, uniformly hydrophilic surfaces. Copyright © 2013 by ASME.

Accident-tolerant fuels (ATFs) are currently of high interest to researchers in the nuclear industry and in governmental and international organizations. One widely studied accident-tolerant fuel concept is multilayer cladding (also known as coated cladding). This concept is based on a traditional Zr-based alloy (Zircaloy-4, M5, E110, ZIRLO etc.) serving as a substrate. Different protective materials are applied to the substrate surface by various techniques, thus enhancing the accident tolerance of the fuel. This study focuses on the results of testing of Zircaloy-4 coated with pure chromium metal using the cold spray (CS) technique. In comparison with other deposition methods, e.g., Physical vapor deposition (PVD), laser coating, or Chemical vapor deposition techniques (CVD), the CS technique is more cost efficient due to lower energy consumption and high deposition rates, making it more suitable for industry-scale production. The Cr-coated samples were tested at different conditions (500°C steam, 1200°C steam, and Pressurized water reactor (PWR) pressurization test) and were precharacterized and postcharacterized by various techniques, such as scanning electron microscopy, Energy-dispersive X-ray spectroscopy (EDX), or nanoindentation; results are discussed. Results of the steady-state fuel performance simulations using the Bison code predicted the concept's feasibility. It is concluded that CS Cr coating has high potential benefits but requires further optimization and out-of-pile and in-pile testing., United States. Department of Energy (Grant DE-NE0008416)

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student-submitted PDF version of thesis., Includes bibliographical references (pages [133]-138)., Subcooled flow boiling of water was experimentally investigated using high-speed video (HSV), infrared (IR) thermography, and particle image velocimetry (PIV) to generate a unique database of synchronized data. HSV allowed measurement of the bubble departure diameter. IR thermography allowed measurement of wall superheat (local distribution and surface-averaged values), heat transfer coefficient, nucleation site density, and bubble frequency. Particle image velocimetry allowed for the measurement of velocity profiles in the liquid phase for single bubble nucleation events. The tests were performed at pressures of 1.05, 1.5, and 2.0 bar and at subcoolings of 5, 10, and 15 °C. The mass flux values explored were 150-1250 kg/m2/s. The heat flux values explored were 100-1600 kW/m2. As expected, the heat transfer coefficients increased with increasing mass flux in the single-phase convection and partial boiling regions, and converged to a fully-developed boiling curve for high heat fluxes. The bubble departure diameter decreased with increasing mass flux and decreasing heat flux; in accordance with Sugrue's model. The nucleation site density increased with increasing superheat and decreasing mass flux, as predicted by Kocamustafaogullari and Ishii's model. The nucleation site density models under-predicted the nucleation site density for a given wall superheat. Wait time and frequency models did not reproduce the data accurately, and underestimated wait time by an order of magnitude. A new mechanistic model for calculating the wait time was developed that split the wall heat flux into the component that is transferred to the fluid, and the component that is transferred as sensible heat into the heater wall. Significant localized cooling was observed underneath bubbles sliding along the wall after departure from a nucleation site, an effect which should be considered in advanced models of subcooled flow boiling. The sliding bubble thermal effects were found to be insensiti, by Bren Andrew Phillips., Ph. D.

Nucleate boiling is important in many energy systems including light water reactors. Currently significant efforts are underway to develop mechanisticmodels for nucleate boiling based on computational fluid dynamics (CFD). Development and validation of these models require new high fidelity experimental data with high spatial and temporal resolutions. Previous work such as has measured temperature distributions and diameter of a growing bubble. It is important to obtain further information, such as bubble shape during growth and after departure, and velocity profiles in the liquid.This paper presents an improved experimental approach for synchronized measurement of these data.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011., Cataloged from PDF version of thesis., Includes bibliographical references (p. 130-133)., The effects on pool boiling characteristics such as critical heat flux and the heat transfer coefficient of different surface characteristics such as surface wettability, roughness, morphology, and porosity are not well understood. Layer-by-layer nanoparticle coatings were used to modify the surface of a sapphire heater to control the surface roughness, the layer thickness, and the surface chemistry. The surface was then tested in a water boiling test at atmospheric pressure while imaging the surface with high speed infrared thermography yielding a 2D time dependent temperature profile. The critical heat flux and heat transfer coefficient were enhanced by over 100% by optimizing the surface parameters. It was found that particle size of the nanoparticles in coating, the coating thickness, and the wettability of the surface have a large impact on CHF and the heat transfer coefficient. Surfaces were also patterned with hydrophobic "islands" within a hydrophilic "sea" by coupling the Layer-by-layer nanoparticle coatings with an ultraviolet ozone technique that patterned the wettability of the surface. The patterning was an attempt to increase the nucleation site density with hydrophobic dots while still maintaining a large hydrophilic region to allow for rewetting of the surface during the ebullition cycle and thus maintaining a high critical heat flux. The patterned surfaces exhibited similar critical heat fluxes and heat transfer coefficients to the surfaces that were only modified with layer-by-layer nanoparticle coatings. However, the patterned surfaces also exhibited highly preferential nucleation from the hydrophobic regions demonstrating an ability to control the nucleation site layout of a surface and opening an avenue for further study., by Bren Andrew Phillips., S.M.