Session: K13-02 Pool Boiling Fundamentals

Paper Number: 116655

116655 - Persistent and Anti-Persistent Nature of Temperature Fluctuations in Pool Boiling 

In pool boiling, Critical Heat Flux (CHF) exists as an upper limit to the heat flux that a surface can dissipate beyond which the bubbles nucleating at the surface coalesce to form a stable vapor film enveloping the surface, denying liquid resupply, and resulting in a sharp jump in the average temperature of the surface. It is known that CHF depends on an interplay of factors like liquid-surface combination, surface roughness, and surface chemistry, all of which affect the ability of the surface to resupply with liquid a dry-spot under a bubble. Yet, a comprehensive model that captures the physics of a boiling surface to explain and predict the onset of CHF remains elusive. Recent work has demonstrated that fluctuations in pool boiling are non-stationary and exhibit long-term temporal correlations which might signify non-linear behaviour like intermittent or continuous nucleation site interactions. The Hurst Exponent, one such measure of temporal correlations, was shown to collapse data from multiple experiments of a single surface-fluid combination at different pressures. It is of interest to see whether such features extend to other pool boiling systems, such as those in which the contact angle of solid-fluid combination is varied. 

In this work, we studied temperature fluctuations on two different boiling surfaces, a platinum wire and a sapphire substrate with surface-deposited Resistance Temperature Detectors (RTDs), as they proceeded towards CHF. Wire experiments were conducted with different liquids including 3M Novec 7000, 3M Novec 7100, DI Water, and nanofluids with 0.01 and 0.1 % silica nanoparticles in DI water by volume such that CHF spanned a wide range (18 – 200 W/cm²). The sapphire substrates were coated with thin films of SiO₂, Amorphous Silicon, and Parylene-C to get a range of CHFs in 3M Novec 7000. The average and local surface temperature fluctuations, sampled at 1 kHz, were obtained from the wire and surface experiments, respectively. First, Fast Fourier Transforms (FFTs) revealed that there is a lack of characteristic frequency in the fluctuations at all levels of applied heat fluxes, indicating that boiling is a scale-free phenomena, and should be modeled as such. We also computed the Hurst Exponent of the fluctuations to study the persistent/anti-persistent nature of the fluctuations. It was observed that the fluctuations at low applied heat fluxes were persistent, meaning that the Hurst Exponent values were less than but close to 1, and that positive fluctuations about the mean were followed by positive fluctuations, and the same for negative fluctuations. Fluctuations at higher applied heat fluxes were anti-persistent, marked by the Hurst Exponent converging to values between 0.65 – 0.7, meaning that the two consecutive fluctuations were more likely to be uncorrelated. Investigation of the dependence of the Hurst Exponent on the non-dimensional heat flux, q/q_CHF, showed that for all the experiments, the Hurst Exponent followed characteristic curves with high values at low q/q_CHF which trended to values between 0.65 – 0.7 as the surface proceeded towards CHF. These results suggest that the Hurst Exponent, calculated on average or local surface temperature fluctuations, may be used as a universal indicator to presage an impending CHF event. Additionally, these results also encourage a non-linear, stochastic approach to modeling pool boiling heat transfer.

Presenting Author: Vinod Srinivasan University of Minnesota Twin Cities