Investigation of heat transfer, critical heat flux and dry spots dynamics during boiling of dielectric fluids HFE-7100 and Novec 649

Dielectric liquids possess unique physical and chemical properties, making them promising candidates for application in electronic cooling systems. However, available heat transfer and critical heat flux (CHF) data in the literature exhibit significant variability, complicating the design of such systems. This study presents experimental results on dry spot dynamics, heat transfer and crisis phenomena during boiling of dielectric liquids HFE-7100 and Novec 649, obtained using IR thermography and high-speed video recording with multiple illumination configurations, including LED reflection from the bottom side of a transparent sapphire heated substrate. Analysis of the boiling curves revealed that the maximum heat transfer coefficients and CHF for HFE-7100 are higher by factors of 1.47 and 1.56, respectively, compared to those for Novec 649. High-speed visualization with reflected LED light and a CNN-based model enabled quantitative analysis of the dependence of dry spot density, contact line length per unit area, void fraction, and dry spot sizes on the heat flux up to the onset of boiling crisis. In particular, it was shown that at CHF the structure of the two-phase flows near the heated wall changes drastically, which leads to a change in the dynamic picture of dry spots evolution and a bimodal distribution of their areas, prior to the appearance of irreversible dry region. The growth rate of the irreversible dry spot for both liquids was measured and compared against existing predictive models, demonstrating good agreement with the analytical traveling thermal wave model. The experimental findings support the concept of the boiling crisis as a conjugate problem, where the interplay between the two-phase flow dynamics near the heated wall and the thermal stability of dry spots must be considered comprehensively. The obtained results are important not only for verification of theoretical model and numerical simulations, but also for the development of effective prototypes of two-phase cooling systems using the HFE-7100 and Novec 649 as a coolants.