Craig D. Gerardi

Infrared thermometry study of nanofluid pool boiling phenomena

Infrared thermometry was used to obtain first-of-a-kind, time- and space-resolved data for pool boiling phenomena in water-based nanofluids with diamond and silica nanoparticles at low concentration (<0.1 vol.%). In addition to macroscopic parameters like the average heat transfer coefficient and critical heat flux [CHF] value, more fundamental parameters such as the bubble departure diameter and frequency, growth and wait times, and nucleation site density [NSD] were directly measured for a thin, resistively heated, indium-tin-oxide surface deposited onto a sapphire substrate. Consistent with other nanofluid studies, the nanoparticles caused deterioration in the nucleate boiling heat transfer (by as much as 50%) and an increase in the CHF (by as much as 100%). The bubble departure frequency and NSD were found to be lower in nanofluids compared with water for the same wall superheat. Furthermore, it was found that a porous layer of nanoparticles built up on the heater surface during nucleate boiling, which improved surface wettability compared with the water-boiled surfaces. Using the prevalent nucleate boiling models, it was possible to correlate this improved surface wettability to the experimentally observed reductions in the bubble departure frequency, NSD, and ultimately to the deterioration in the nucleate boiling heat transfer and the CHF enhancement.

Assessment of Distributed Fiber Optic Sensors for Flow Field Temperature Mapping

Distributed fiber optic temperature sensing based on Rayleigh scattering is a relatively new technique offering data density unachievable with point sensors such as thermocouples and RTDs. Thousands of temperature measurements can be generated by a single fiber optic cable suspended within a flow field. And unlike imaging techniques such as laser induced fluorescence, fiber optic sensors are suitable for applications involving opaque fluids. But verifying measurement accuracy along a distributed temperature sensor (DTS) can be problematic. Unlike traditional sensors such as thermocouples, DTS calibration shifts can accompany sensor handling or movement because they respond to strain as well as temperature. This paper describes an assessment of a Rayleigh scattering-based sensing system used to measure air temperature within a 1 × 1 × 1.7 m tank used for thermal mixing experiments. Two 40 m-long DTSs were strung across the tank midplane at 16 levels. Stability in stagnant air was examined over seven days and found to be generally better than ± 0.5°C with local regions of drift up to 1.5°C. DTSs were also tested in isothermal flow to assess signal degradation associated with flow-induced vibration. Noise increased with flow velocity, inducing data loss that grew with distance along the fiber. Despite data losses >50% in high noise regions, mean temperatures after simple filtering agreed with low noise regions to within ∼4°C.Copyright

Development of Process for Cleanup of Sodium-CO 2 Reaction Products

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Fiber Optic Distributed Sensors for High-resolution Temperature Field Mapping

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists chiefly of velocity and temperature readings, both ideally having high spatial and temporal resolution to facilitate rigorous code validation. While high resolution velocity data is readily obtained through optical measurement techniques such as particle image velocimetry, it has proven difficult to obtain temperature data with similar resolution. Traditional sensors such as thermocouples cannot fill this role, but the recent development of distributed sensing based on Rayleigh scattering and swept-wave interferometry offers resolution suitable for CFD code validation work. Thousands of temperature measurements can be generated along a single thin optical fiber at hundreds of Hertz. Sensors function over large temperature ranges and within opaque fluids where optical techniques are unsuitable. But this type of sensor is sensitive to strain and humidity as well as temperature and so accuracy is affected by handling, vibration, and shifts in relative humidity. Such behavior is quite unlike traditional sensors and so unconventional installation and operating procedures are necessary to ensure accurate measurements. This paper demonstrates implementation of a Rayleigh scattering-type distributed temperature sensor in a thermal mixing experiment involving two air jets at 25 and 45 °C. We present criteria to guide selection of optical fiber for the sensor and describe installation setup for a jet mixing experiment. We illustrate sensor baselining, which links readings to an absolute temperature standard, and discuss practical issues such as errors due to flow-induced vibration. This material can aid those interested in temperature measurements having high data density and bandwidth for fluid dynamics experiments and similar applications. We highlight pitfalls specific to these sensors for consideration in experiment design and operation.

Description of the First Observed Sodium-C02 Reactions in the Sodium CO2 Interaction Experiment (SNAKE)

One appealing feature of the supercritical carbon dioxide Brayton cycle energy conversion system is the small footprint that the hardware requires, which is in part due to the use of Printed Circuit Heat Exchangers (PCHEs) as the heat source heat exchanger (sodium-to-CO2) as well as the recuperator and cooler modules. Although PCHEs have a high degree of structural integrity, the potential for leaks to develop between the sodium and CO2 coolant channels in the secondary heat exchanger cannot be ruled out, and this would lead to discharge of high pressure CO2 into the secondary coolant circuit. Due to the robustness of the PCHE design, catastrophic failure leading to CO2 jet blowdown into the secondary sodium loop is not deemed likely. Rather, small cracks (or micro-leaks) may develop in which CO2 will bleed into the secondary system at a relatively low rate and chemically react with the sodium. The goal of the sodium-CO2 interaction tests is to gain a fundamental understanding of sodium-CO2 interactions under prototypical conditions of compact diffusion-bonded heat exchanger failure, a fundamental understanding of self-plugging if it occurs, and the development of one-dimensional phenomenological models for the interactions between highpressure CO2 issuing into liquid sodium from a micro-leak across a stainless steel pressure boundary. These models will be validated using experiment data. Therefore, an experiment program at Argonne was initiated in Fiscal Year 2010 to investigate the reaction characteristics between sodium and CO2 under micro-leak conditions. Several reports have described the facility scaling rationale and design. Assembly of the SNAKE (SCO2, Na Kinetics Experiment) began in Fiscal Year 2011 and was completed in July 2012. Approximately 44 lbs (20 kg; ~21 L/5.5 gal) of sodium was transferred into the SNAKE dump tank from a drum of clean sodium in July 2012. The first sodium-CO2 interaction experiment was carried out at SNAKE in September 2012. This test was successful in that supercritical carbon dioxide was sparged into a pool of sodium through a 64 μm diameter nozzle. A series of sodium-CO2 interaction experiments were carried out in Fiscal Year 2013 in the SNAKE experiment. These tests successfully injected supercritical carbon dioxide into a pool of sodium through a 64 μm diameter nozzle. A reaction between the CO2 and sodium was detected. The extent of this reaction was unexpected since the initial sodium temperature was 145 ̊C, a temperature range where previous researchers have detected little or no chemical reaction between these species. The important difference between the SNAKE experiment and previous research is that the SNAKE geometry and conditions promote high-interfacial area and mixing between the CO2 and sodium. These characteristics could be very important in promoting accelerated chemical reactions and will be studied further as the SNAKE test matrix is carried out. Approximately 325 standard liters of CO2 were injected into a 45 cm (15 inch) high column of sodium at a nominal temperature of 150 ̊C over the course of 3 hours. The inlet CO2

Report on the Initial Fundamental Sodium-CO2 Interaction Experiment

...................................................................................................................................... i List of Figures ........................................................................................................................... iv List of Tables............................................................................................................................. vi