Study Of Bubble Behavior In Subcooled Boiling Narrow Channel Under Rolling Condition

The flow boiling characteristics was different in static condition and periodic force fieldcondition. Bubble behavior in a narrow rectangular channel under subcooled flow boiling inrolling condition was experimentally studied by adopting the high speed digital cameratechnology, in order to study the effect of the rolling motion on flow boiling and to furtheridentify the effect mechanism of the rolling motion.The pre-process method of bubble image sequence was proposed based on thecharacteristics of the high speed camera. Some of the conundrum in bubble identifying wasalso figured out. Most of the fundamental information about bubblein flow boiling, includingdiameter, projection area and the location, can be obtained through the digital imageprocessing technology. The algorithm to calculate the bubble siding velocity, the bubbleefficient diameter, the void fraction and the interfacial area concentration was proposed. Thiswas helpful for the analysis of the bubble behavior in static and rolling condition.The system pressure has a great influence on the bubble nucleation in subcooled flowboiling. The distribution of nucleation site was uniformly in lower system pressure andnumerous nucleation sites could be observed in the heating surface. Most of the nucleationsites were concentrated in the onset of nucleation boiling after the system pressure wasincreased. The discrepancy of the bubble nucleation under different pressure was affected bythe bubble sliding characteristics. Bubble growth in lower system pressure was mainlyprocessed near the nucleation site. Most of the bubbles slid for a short distance anddisappeared quickly in the subcooled fluid. While in higher system pressure, bubble growthwas mainly processed during the sliding process. Most of the bubbles slid for a long distanceand the condensation was rarely happened. Bubbles in narrow channel would not lift off theheated surface due to the effect of the bubble growth force and the wall lubrication force.Bubble sliding velocity distribution in narrow channel was influenced by the workingconditions. The variation of the mean bubble velocities and the mean relative velocities hasthe same trend.The affections from the rolling motion were mainly behaved as the bubble buoyancyforce based on the bubble force models, because the introduction of the additionalacceleration filed just like to change the gravity acceleration. While some other effects which will affect the bubble behavior would also be introduced due to periodic force filed of therolling motion, such as the variation of the mass flow rate and the surface temperature.Bubble sliding velocity was periodically varied with the rolling motion. Neither of the changeof bubble buoyancy force due to the additional acceleration nor the variation of fluid flow ratearound the bubbles was the main effects which change bubble sliding velocity under rollingcondition. The fluctuation of bubble sliding velocity was mainly affected by the variation ofthe bubble diameter. Both of the void fraction and the interfacial area concentrationperiodically fluctuated under rolling condition. And the above parameters followed sametrends with the variation of rolling angles. The main affection was the variation of bubblenumber, and the effect of the fluctuation of bubble diameter was much smaller.