Collective self-organization as a precursor to critical heat flux (CHF) in non-equilibrium boiling systems

Critical heat flux (CHF) is generally regarded as a hydrodynamic or thermal instability that limits boiling heat transfer. However, the collective structural evolution of boiling systems approaching CHF remains insufficiently understood. In this study, boiling structures approaching CHF were investigated for water and NOVEC 649 using high-speed visualization, spatial autocorrelation, Shannon entropy, and effective potential reconstruction. As CHF was approached, boiling structures evolved from dispersed bubbles to interconnected vapor configurations, accompanied by enhanced spatial correlation, increased correlation length, fluid dependent configurational evolution, and changes in state-space confinement. A phenomenological self-organization index was introduced to integrate these characteristics into a measure of collective boiling behavior. The normalized index exhibited fluid dependent evolution: NOVEC 649 showed a pronounced maximum at CHF followed by a post-CHF decrease, whereas water exhibited a broader, modest maximum near CHF. Despite these differences, both fluids exhibited enhanced index values near the critical condition. These results indicate that the approach to CHF is accompanied by progressive collective structural reorganization rather than solely by increased vapor generation. The proposed framework provides a phenomenological description of structural evolution preceding CHF without implying that self-organization causally governs or triggers the boiling crisis.