Advanced diagnostics to resolve long-lasting controversies in boiling heat transfer

Boiling heat transfer has been investigated for several decades, but still there are many open questions and controversies about the mechanisms associated with this process and particularly its limit, known as critical heat flux (CHF). This lack of understanding often arose from the absence of experimental capabilities to perform measurements at the space and time scales characteristic of these phenomena. However, improvements have been possible in the last decade thanks to the rapid development of InfraRed (IR) thermometry diagnostics to measure temperature distributions on the boiling surface [1-13]. In this work, we discuss first-of-a-kind investigations enabled by advanced IR diagnostics developed in the Nuclear Science and Engineering (NSE) department at the Massachusetts Institute of Technology (MIT). Specifically, a new technique has been developed [14], requiring the solution of a couple conduction/radiation inverse problem, to measure temperature and heat flux distributions on the boiling surface with a space resolution of approximately 100 μm, and frame rates from 2500 to 4000 fps. This technique is being leveraged to shed light on many phenomena still debated within the boiling heat transfer community. The list of controversial issues addressed in this talk includes but is not limited to: CHF in steady-state [15] and transient conditions, such as power escalations; onset of nucleate boiling (ONB) temperature in steady-state and transient conditions [16,17]; and CHF enhancement on microand nanoengineered surfaces. Here, a specific controversy is addressed, about the partition of the wall heat flux in flow boiling heat transfer. INTRODUCTION Mechanistic models for the prediction of heat transfer coefficients in boiling heat transfer rely on the so-called wall heat partitioning approach, where the total heat flux transferred at the boiling wall is represented as the sum of several contributions associated with different heat transfer mechanisms. Traditional models have been adapted from the original heat partitioning formulation proposed by Judd and Hwang [18] and adopted by Kurul and Podowski[19] for pool boiling conditions. As such, they consist of three contributions: evaporation, quenching and single-phase convection. More recently, Basu et al. [20], Yeoh et al. [21] and Gilman and Baglietto [22] introduced a further contributions observed in flow boiling conditions, associated with the sliding of bubbles on the boiling surface. However, due to a lack of experimental capabilities to provide a direct measurement of heat flux partitioning, only indirect validation has been possible, using integral quantities such as the heat transfer coefficient and wall superheat. Surprisingly, these models can predict consistent heat transfer coefficient and superheat using different heat partitioning schemes [23]. Precisely, there is a significant difference in the evaporation term, which results in different vapor production rates. To date, no clear consensus on the weight of the different terms has been achieved. Very little work has been done to experimentally capture flow boiling phenomena at the resolutions needed for the validation of these models. To this end, we present here a firstof-a-kind experiment for capturing subcooled flow boiling data, including unique measurements of the wall heat flux partitioning. EXPERIMENTAL METHODS The Platform for Experimental ThERmal-hydraulics (PEThER) is a flow-boiling loop designed, constructed and operated at MIT. The whole loop is built with stainless steel 316L and is designed for operation with DI-water up to 10 bar at saturation conditions and mass fluxes as large as 2500 kg/ms. At the heart of the facility is a test section (see Figure1) with a 3cm × 1cm flow channel running the length of the structure. The test section connects to an entrance region providing more than 60 L/D to establish fully-developed turbulent flow. The main body of the test section consists of four sides, three are used for quartz windows to provide high-speed video (HSV) optical access, the fourth wall contains the cartridge used to hold an InfraRed (IR) heater. The cartridge is made of a ceramic material called Shapal, which is electrically insulating and mechanically strong enough to act as a pressure boundary. The IR heater (see Figure 2) is installed inside the cartridge. It consists of a sapphire substrate coated with indium-tin-oxide (ITO). The sapphire substrate is a 20×20 mm slide, 1 mm thick. The ITO coating, in contact with water, is wrapped around the substrate and serves as a Joule heater. The ITO is nano-smooth, but has localized imperfections in the range of a few microns. It has a thickness of 0.7 μm and a resistivity of 2.5 /square. Silver pads are deposited on top of the ITO and wrap around the filleted edges 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics