HEATER WIDTH EFFECT ON CRITICAL HEAT FLUX AND INSTABILITY

摘要

The sudden enhancement of the critical heat flux (CHF) of the thin flat plate heater in the saturated pool boiling was studied. Experimental data showed a large deviation from the CHF model of Lienhard and Dhir for the flat plate heater with the characteristic width less than 0.25 scaled by the Laplace length. In order to investigate the cause of CHF enhancement, the measurement of the Taylor wave length and the Helmholtz instability have been performed. For the Taylor wave length, the largest distance of the bubble departure spots were measured by changing the heater width. Also, the air-water experiments were made in the narrow slit geometry. It was found that the Taylor wave length decreases as the width of the heater and slit decrease. Furthermore, we measured the bubble departure frequency near the CHF condition to measure the dynamic information of the Kelvin-Helmholtz instability. The departure frequencies were almost constant for the heater with the characteristic width less than 0.25. A mechanical model of CHF to explain the larger CHF than the conventional Lienhard and Dhir model was developed based on the Kelvin-Helmholtz instability. The present model predicted CHF as the function of the heater width with the exponent of-1/3 rather than that of-1/4. The present study partially explains the reason of the CHF enhancement for the heater with the specific micro or nano structure.