Time periodic flow boiling heat transfer and bubble characteristics of FC-72 over a small heated circular plate due to simultaneous refrigerant flow rate and heat flux oscillations
|關鍵字:||流量以及熱量震盪;mass flux and heat flux oscillation|
This study intends to explore how simultaneously imposed time periodic coolant flow rate and heat flux oscillations affect the temporal flow boiling heat transfer and associated bubble characteristics of FC-72 over a small circular heated copper plate flush mounted on the bottom of a horizontal rectangular channel. The oscillations of the coolant flow rate and heat flux are respectively in the forms of nearly triangular and sinusoidal waves. Both the in-phase and out-of-phase mass flux and heat flux oscillations are investigated. In the experiment the time-average coolant mass flux is varied from 300 to 400 kg/m2s and the amplitude of the coolant mass flux oscillation is mainly fixed at 5, 10 and 15% of . The mean heat flux ranges from 0.1 to 10W/cm2 with the amplitude of the heat flux oscillation set at 10, 30 and 50% of for the same period of the heat flux and mass flux oscillations varied from 10 to 30 seconds Besides, the time-average liquid subcooling at the inlet of the test section ranges from 0 to 15K and the system is at slightly subatmospheric pressure. The transient oscillatory flow boiling heat transfer characteristics are illustrated by presenting the measured time variations of the heated plate temperature and boiling heat transfer coefficient. The experimental results show that the heated surface temperature also oscillates periodically in time at the same frequency as the mass and heat flux oscillations. Besides, the amplitude of the Tw oscillation is generally smaller in the flow boiling when the out-of-phase G and q oscillations are imposed. But in the single-phase flow the Tw oscillation is normally weaker by imposing in-phase G and q oscillations. These results indicate that the oscillation in Tw caused by the heat flux oscillation can be suppressed by the in-phase G oscillation in the single-phase flow and by the out-of-phase G oscillation in the boiling flow. It is further noted that by imposing the G oscillation at the time instant equal to the difference between the time lags in Tw resulting respectively from the q oscillation only behind the q oscillation, and G oscillation only behind the q oscillation, the Tw oscillation can be significantly suppressed and even completely wiped out.
|Appears in Collections:||Thesis|
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