Experiments were performed to enable the development of a new theoretical model for the enhancement in CHF commonly observed with flow boiling on concave heaters as compared to straight heaters. High-speed video imaging and photomicrography were employed to capture the trigger mechanism for CHF for each type of heater. A wavy vapor layer was observed to engulf the heater surface in each case, permitting liquid access to the surface only in regions where depressions (troughs) in the liquid–vapor interface made contact with the surface. CHF in each case occurred when the pressure force exerted upon the wavy vapor–liquid interface in the contact regions could no longer overcome the momentum of the vapor produced in these regions. Shorter interfacial wavelengths with greater curvature were measured on the curved heater than on the straight heater, promoting a greater pressure force on the wavy interface and a corresponding increase in CHF for the curved heater. A theoretical CHF model is developed from these observations, based upon a new theory for hydrodynamic instability along a curved interface. CHF data are predicted with good accuracy for both heaters.

Issue Section:
Boiling Heat Transfer
Keywords:
Augmentation and Enhancement, Boiling, Flow Instability
Topics:
Boiling, Critical heat flux, Flow (Dynamics)
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