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This paper presents an investigation of transient pool boiling heat transfer phenomena in water at atmospheric pressure under exponentially escalating heat fluxes on plate-type heaters. Exponential power escalations with periods ranging from 5 to 100 ms, and subcooling of 0, 25 and 75 K were explored. What makes this study unique is the use of synchronized state-of-the-art diagnostics such as infrared (IR) thermometry and high-speed video HSV, which enabled accurate measurements and provided new and unique insight into the transient boiling heat transfer phenomena. The onset of nucleate boiling (ONB) conditions were identified. The experimental data suggest that ONB temperature and heat flux increase monotonically with decreasing period and increasing subcooling, in accordance with the predictions of a model based on transient conduction and a nucleation site activation criterion. Various boiling regimes were observed during the transition from ONB to fully developed nucleate boiling (FDNB). Onset of the boiling driven (OBD) heat transfer regime and overshoot (OV) conditions were identified, depending on the period of the power escalation and the subcooling. Forced convection effects have also been investigated and are discussed in the companion paper (Part II). Keywords: Exponential power escalation; Heat transfer mechanisms; Infrared thermometry; High speed video; Transient pool boiling

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.

An experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular mini-gap channel was conducted using deionized water. The local heat transfer coefficient, onset of nucleate boiling, and flow pattern of subcooled boiling were investigated. The influence of heat flux and mass flux were studied with the aid of a high-speed camera. The results show that the flow pattern was mainly isolated bubbly flow when the narrow microchannel was placed vertically. The bubbles generated at lower mass fluxes were larger and did not easily depart, forming elongated bubbly flow and flowing upstream. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewet. The local heat transfer coefficient near the exit of the test section was larger.

The onset of nucleate boiling (ONB) serves as the thermal-hydraulic operating limit for many research and test reactors. However, boiling incipience under forced convection has not been well-characterized in narrow channel geometries or for oxidized surface conditions. This study presents experimental data for the ONB in vertical upflow of deionized (DI) water in a simulated materials test reactor (MTR) coolant channel. The channel gap thickness and aspect ratio were 1.96 mm and 29:1, respectively. Boiling surface conditions were carefully controlled and characterized, with both heavily oxidized and native oxide surfaces tested. Measurements were performed for mass fluxes ranging from 750 to 3000 kg/m2s and for subcoolings ranging from 10 to 45°C. ONB was identified using a combination of high-speed visual observation, surface temperature measurements, and channel pressure drop measurements. Surface temperature measurements were found to be most reliable in identifying the ONB. For the nominal (native oxide) surface, results indicate that the correlation of Bergles and Rohsenow, when paired with the appropriate single-phase heat transfer correlation, adequately predicts the ONB heat flux. Incipience on the oxidized surface occurred at a higher heat flux and superheat than on the plain surface., United States. Department of Energy. Office of Nonproliferation and National Security, United States. National Nuclear Security Administration. Global Threat Reduction Initiative (Contract No. #25-30101-0004A) (, United States. National Nuclear Security Administration (contract no. DE-AC04-94AL85000)

Critical Heat Flux (CHF) plays a key role in nuclear reactor safety both during normal operation as well as in accident scenarios. In particular,when an in-vessel retention (IVR) strategy is used as a severe accident management strategy, the reactor pressure vessel (RPV) cavity is flooded with water, to remove the decay heat from the corium relocated in the lower plenum by conduction through the RPV wall and flow boiling on the outer surface of the RPV. The CHF limit must not be exceeded to prevent RPV failure.Therefore, knowledge of the CHF under realistic conditions is necessary to assess coolability margins. Previous studies for prediction of CHF in the IVR situation were mostly based on data for as fabricated un-oxidized stainless steel. However, the RPV is made of low carbon steel and its surface has an oxide layer that results from pre-service heat treatment as well as oxidation during service. This oxide layer introduces significant differences in surface wettability, porosity, and roughness in comparison to an un-oxidized stainless steel surface. In this study, test heaters were fabricated out of RPV low carbon steel, pre-oxidized in a controlled high temperature wet air environment, which emulates the surface oxides present on the outer surface of the actual RPV; the heaters were then tested in a flow boiling loop designed specifically for the IVR conditions. Up to 70% enhancement in CHF value was observed for the oxidized in low carbon steel in comparison to the stainless steel.

This paper presents an investigation of transient pool boiling heat transfer phenomena for water at atmospheric pressure under rapidly escalating heat fluxes on plate-type heaters, which were used to simulate the interface between cladding and coolant in plate-type fuel during Reactivity Initiated Accidents (RIAs). The investigation utilized Infrared (IR) thermometry and High-Speed Video (HSV) to gain insight into the physical phenomena and generate a database that can be used for development and validation of accurate models of transient boiling heat transfer. The test matrix included exponential power escalations with periods in the range from 5 to 100 milliseconds, and subcooling of 0, 25 and 75 K. The onset of nucleate boiling (ONB), onset of significant void (OSV) and overshoot (OV) conditions were identified. The experimental data suggest that ONB, OSV and OV temperature and heat flux increase monotonically with decreasing period and increasing subcooling. The transient boiling curves were measured up to the fully developed nucleate boiling (FDNB) regime.

Pool boiling experiments were conducted for sandblasted stainless steel (grade 316) plate heaters submerged in deionized (DI) water and water-based zinc–oxide nanofluid, for transient heat flux conditions with power through the heaters increasing quadratically with time. Heat flux in the experiments was increased from zero to CHF in short time frames of 1, 10 and 100 s. Consistent with previous studies, transient CHF for DI water was higher than steady state CHF, and CHF increased with decreasing duration of the transient. Additionally, it was observed that for nanofluid tests, a porous and hydrophilic nanoparticle layer started to deposit on the heater surface in short time frames of 10 and 100 s, and this layer was responsible for the enhanced CHF compared to DI water. However, for the 1 s tests, nanoparticle deposition did not occur and consequently the CHF was not enhanced. Finally, experiments with heaters pre-coated with nanoparticles were performed and it was found that CHF was enhanced for all transient durations down to 1 s, establishing firmly that the CHF enhancement occurs due to surface modifications by the deposited nanoparticles, and not by nanoparticles suspended in solution., AREVA Inc. Nuclear Parts Center

A concentrating solar power system is presented which uses hillside mounted heliostats to direct sunlight into a volumetric absorption molten salt receiver with integral storage. The concentrated sunlight penetrates and is absorbed by molten salt in the receiver through a depth of 4–5 m, making the system insensitive to the passage of clouds. The receiver volume also acts as the thermal storage volume eliminating the need for secondary hot and cold salt storage tanks. A small aperture and refractory-lined domed roof reduce losses to the environment and reflect thermal radiation back into the pond. Hot salt is pumped from the top of the tank through a steam generator and then returned to the bottom of the tank. An insulated barrier plate is positioned within the tank to provide a physical and thermal barrier between the thermally stratified layers, maintaining hot and cold salt volumes required for continuous operation. As a result, high temperature thermal energy can be provided 24/7 or at any desired time. The amount of storage required depends on local needs and economic conditions. About 2500 m[superscript 3] of nitrate salt is needed to operate a 4 MW[subscript e] steam turbine 24/7 (7 h sunshine, 17 h storage), and with modest heliostat field oversizing to accumulate energy, the system could operate for an additional 24 h (1 cloudy day). Alternatively, this same storage volume can supply a 50 MWe turbine for 3.25 h without additional solar input. Cosine effect losses associated with hillside heliostats beaming light downwards to the receiver are offset by the elimination of a tower and separate hot and cold storage tanks and their associated pumping systems. Reduced system complexity also reduces variable costs. Using the NREL Solar Advisor program, the system is estimated to realize cost-competitive levelized production costs of electricity., Bill & Melinda Gates Foundation (Fellowship), Chesonis Family Foundation (Fellowship)

Experimental results are presented for single-phase heat transfer in a narrow rectangular minichannel heated on one side. The aspect ratio and gap thickness of the test channel were 29:1 and 1.96 mm, respectively. Friction pressure drop and Nusselt numbers are reported for the transition and fully turbulent flow regimes, with Prandtl numbers ranging from 2.2 to 5.4. Turbulent friction pressure drop for the high aspect ratio channel is well-correlated by the Blasius solution when a modified Reynolds number, based upon a laminar equivalent diameter, is utilized. The critical Reynolds number for the channel falls between 3500 and 4000, with Nusselt numbers in the transition regime being reasonably predicted by Gnielinski's correlation. The dependence of the heat transfer coefficient on the Prandtl number is larger than that predicted by circular tube correlations, and is likely a result of the asymmetric heating. The problem of asymmetric heating condition is approached theoretically using a boundary layer analysis with a two-region wall layer model, similar to that originally proposed by Prandtl. The analysis clarifies the influence of asymmetric heating on the Nusselt number and correctly predicts the experimentally observed trend with Prandtl number. A semi-analytic correlation is derived from the analysis that accounts for the effect of aspect ratio and asymmetric heating, and is shown to predict the experimental results of this study with a mean absolute error (MAE) of less than 5% for 4000 < Re < 70,000., United States. National Nuclear Security Administration. Global Threat Reduction Initiative (Argonne National Laboratory. Contract 25-30101-0004 A), United States. National Nuclear Security Administration. Office of Nonproliferation and International Security (Nuclear Nonproliferation Safeguards Graduate Fellowship), United States. National Nuclear Security Administration. (Sandia National Laboratories. Contract DE-AC04-94AL85000)

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.

In-Vessel Retention (IVR) through External Reactor Vessel Cooling is a severe accident management measure that is power limiting to the Westinghouse AP1000 due to critical heat flux (CHF) at the outer surface of the vessel. Increasing CHF by altering the coolant would increase safety margin at current design power or allow for higher power. Modification to current design would not require significant changes to the containment and systems.

The Critical Heat Flux (CHF) of water with dispersed alumina nanoparticles was measured for the geometry and flow conditions relevant to the In-Vessel Retention (IVR) situation which can occur during core melting sequences in certain advanced Light Water Reactors (LWRs). CHF measurements were conducted in a flow boiling loop featuring a test section designed to be thermal-hydraulically similar to the vessel/insulation gap in the Westinghouse AP1000 plant. The effects of orientation angle, pressure, mass flux, fluid type, boiling time, surface material, and surface state were investigated. Results for water-based nanofluids with alumina nanoparticles (0.001% by volume) on stainless steel surface indicate an average 70% CHF enhancement with a range of 17% to 108% depending on the specific flow conditions expected for IVR. Experiments also indicate that only about thirty minutes of boiling time (which drives nanoparticle deposition) are needed to obtain substantial CHF enhancement with nanofluids.

Nanofluids are being considered for heat transfer applications. However, their thermo-physical properties are poorly known. Here we focus on nanofluid specific heat capacity. Currently, there exist two models to predict a nanofluid’s specific heat capacity as a function of nanoparticle concentration and material. Model I is a straight volume-weighted average; Model II is based on the assumption of thermal equilibrium between the particles and the surrounding fluid. These two models give significantly different predictions for a given system. Using differential scanning calorimetry, the specific heat capacities of water based silica, alumina, and copper oxide nanofluids were measured. Nanoparticle concentrations were varied between 5wt% and 50wt%. Test results were found to be in excellent agreement with Model II, while the predictions of Model I deviate very significantly from the data.

High-speed video and infrared thermometry were used to obtain time- and space-resolved information on bubble nucleation and heat transfer in pool boiling of water. The bubble departure diameter and frequency, growth and wait times, and nucleation site density were directly measured for a thin, electrically-heated, indium–tin-oxide surface, laid on a sapphire substrate. These data are very valuable for validation of two-phase flow and heat transfer models, including computational fluid dynamics with interface tracking methods. Here, detailed experimental bubble-growth data from individual nucleation sites were used to evaluate simple, commonly-used, but poorly-validated, bubble-growth and nucleate-boiling heat-transfer models. The agreement between the data and the models was found to be reasonably good. Also, the heat flux partitioning model, to which our data on nucleation site density, bubble departure diameter and frequency were directly fed, suggests that transient conduction following bubble departure is the dominant contribution to nucleate-boiling heat transfer.

Nanofluids are being considered for heat transfer applications; therefore it is important to know their thermophysical properties accurately. In this paper we focused on nanofluid specific heat capacity. Currently, there exist two models to predict a nanofluid specific heat capacity as a function of nanoparticle concentration and material. Model I is a straight volume-weighted average; Model II is based on the assumption of thermal equilibrium between the particles and the surrounding fluid. These two models give significantly different predictions for a given system. Using differential scanning calorimetry (DSC), a robust experimental methodology for measuring the heat capacity of fluids, the specific heat capacities of water-based silica, alumina, and copper oxide nanofluids were measured. Nanoparticle concentrations were varied between 5 wt% and 50 wt%. Test results were found to be in excellent agreement with Model II, while the predictions of Model I deviated very significantly from the data. Therefore, Model II is recommended for nanofluids.

Quenching of small stainless steel rods in pure water and nanofluids with alumina and diamond nanoparticles at low concentrations (0.1 vol%) was investigated experimentally. The rods were heated to an initial temperature of ~1000 °C and then plunged into the test fluid. The results show that the quenching behavior of the nanofluids is nearly identical to that of pure water. However, due to nanofluids boiling during the quenching process, some nanoparticles deposit on the surface of the rod, which results in much higher quenching rate in subsequent tests with the same rod. It is likely that particle deposition destabilizes the film-boiling vapor film at high temperature, thus causing the quenching process to accelerate, as evident from the values of the quench front speed measured by means of a high-speed camera. The acceleration strongly depends on the nanoparticle material used, i.e., the alumina nanoparticles on the surface significantly improve the quenching, while the diamond nanoparticles do not. The possible mechanisms responsible for the quench front acceleration are discussed. It is found that the traditional concept of conduction-controlled quenching cannot explain the acceleration provided by the nanoparticle layer on the surface., Korean Research Foundation (Korean Government (MOEHRD) (KRF- 2007-357-D00026)), AREVA, Douglas C. Spreng

This paper presents the results of an experimental study on nucleate pool boiling. Experiments were performed using vapor-deposited thin films which were electrically heated. High-speed infrared and visible cameras simultaneously observed bubble growth from the heater surface. Possible experimental confirmation of microlayer dynamics is presented., King Abdulaziz City of Science and Technology (KACST, Saudia Arabia)

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)] , was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise., National Science Foundation (U.S.) (Grant no. CBET-0812804)