Browsing by Subject "Hurst Exponent"

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    Universality of long-term correlations in thermal fluctuations in pool boiling
    (2024-12) Saini, Ankit
    Boiling systems are characterized by Critical Heat Flux (CHF), the maximum level of applied heat flux beyond which the near-surface liquid is displaced by a thin vapor film leading to a sharp rise in the surface temperature and heater failure. Operation close to CHF is avoided because its behavior gets altered unpredictably as the surface undergoes oxidation and/or particle deposition over time. Reliable mechanistic models that can accurately predict dynamic changes to CHF do not exist. Furthermore, all surface/fluid combinations exhibit different CHFs and are represented on boiling curves with dimensional axes. The lack of a universal dimensionless quantity makes it difficult to compare across boiling systems and is indicative of how poorly the physics governing CHF is understood.In this context, non-linear dynamical analysis of boiling systems may yield information about conditions just prior to CHF. This idea is based on the fact that boiling is a non-linear, complex dynamical system that is governed by mass, momentum and energy transport in three subsystems, namely the bulk fluid, the heater surface, and the heater-fluid interface. Previous studies have demonstrated that as a result of interactions between these subsystems, local surface temperature, contact line length and surface void fraction exhibit fluctuations on a wide range of time scales and are characterized by scaling exponents. This is promising because such scaling laws can serve as characteristic fingerprints of the state in which a system is operating and act as a precursor to transition to a different state or an impending catastrophic event (like CHF). Building on the above-mentioned framework, this study investigated the statistics of local temperature fluctuations on surfaces undergoing boiling using surface-deposited, micro-scale Resistance Temperature Detectors (RTDs). Scale-free behavior of temperature fluctuations is established, and analysis of the Auto-Correlation Functions (ACF) reveals the presence of long-term correlations of temperature fluctuations when operating close to dry-out. The time series are then characterized using the Hurst Exponent, a measure of long-term memory of a time series, along the boiling curve. It is demonstrated that the trend in evolution of Hurst Exponent along the boiling curve is common across all experiments performed. Specifically, temperature fluctuations have a high degree of persistence and exhibit short-term correlations at low heat flux and have a lower degree of persistence and exhibit long-term correlations at high heat flux. Next, surface-averaged temperature fluctuations of a platinum wire undergoing boiling were investigated along similar lines. The observed values of CHF spanned one order of magnitude. Scale-free behavior and long-term correlations through analysis of ACF were observed again. It is shown that the evolution of the Hurst Exponent for surface-averaged temperature fluctuations also follows the same trend as that for local temperature fluctuations on plane surface experiments. Finally, the above analysis was extended to publicly available data of heat flux fluctuations on temperature-controlled surfaces undergoing boiling in microgravity. While the specific values of the Hurst Exponent along the boiling curve were lower for the microgravity data, the same trend as before is observed. From the spatial uniformity of this behavior as well as its universality, it may be possible to determine the proximity of a system to CHF using the Hurst Exponent, even as the surface undergoes changes in surface condition due to fouling.