| Two-phase flow systems are widely used in industrial productions,such as nuclear reator,heat exchanger,and bioreactor.It is of great significance to measure the thermal hydraulic parameters,which clarify the mechanisms of the two-phase flow and heat transfer characteristics.However,the interface of gas liquid two-phase flow is easily deformed,which makes the flow structure complicated and its flow parameters hard to be measured.Meanwhile,the two-phase flow phenomena have been demonstrated to be unstable,multi-scale and nonlinear.The two-phase flow resistance and heat transfer characteristics are related to the liquid film thickness,especially in the slug and annular flow.In mini-µ-channels,the small difference of liquid film thickness might have huge influence on the two-phase flow system for the small channel size.Thus,it is crucial to develop the liquid film measurements with high precision and fast response for different applications.The main content is as follows:Two kinds of liquid film thickness measurements with different measuring ranges have been developed.The first one is a sensor matrix on a printed circuit board?SMoPCB?.Combined with the visualization video taken by high speed camera,the liquid film dynamics and its influencing factors have been analyzed in narrow rectangular channel,which covers the main flow regimes,such as slug flow,churn flow,annular and wispy-annular flow.It has been demonstrated that the liquid film fluctuates with the low frequency in slug flow,fluctuates randomly in churn flow,fluctuates with high frequency and low amplitude,while fluctuates with the frequency lower than 1 Hz in wispy-annular flow.Taking the inertia force,surface tension and visious force into consideration,the liquid film thickness is considered to be determined by Weber number and capillary number by dimensional analysis.Moreover,the liquid thickness model for slug flow has been developed under low capillary number?Ca?0.03?.Different with that in slug flow,the averaged liquid film thickness increases with the increment of superficial liquid velocity and decrease of superficial gas velocity.However,the high superficial gas velocity would weaken the effect of superficial liquid velocity on the liquid film thickness.However,due to the effect of channel size,the existing models predicting the liquid film thickness in annular flow has been show large predicting errors on the present measured data in the narrow rectangular channel.Therefore,on the basis of the mechanism of the droplets entrainment in gas core,the liquid film thickness model has been developed for the rectangular narrow channel.The second liquid film thickness measurement is the sensor matrix on a chip?SMoC?,which is manufactured on the silicon chip with the help of photolithography technology.Its maximum measuring range is 83?m and uncertainty ranges from 0.5?m to 1.1?m.It has been observed that the liquid film with the thickness about several micrometers exist under the bubbles for the small capillary number(Ca<6×10-4).In order to explore the flow and heat transfer characteristics in a flow boiling system,the sample entropy has been introduced to define the flow instability for flow boiling in rectangular microchannels.The sample entropy would change with flow boiling condition.Meanwhile,the oscillation period of the flow instability system is associated by the boiling number.By taking the newly developed liquid film thickness model into consideration,the modified Thome's three-region heat transfer model has the better performance in predicting the heat transfer coefficient in micro-channels.The two kinds of electrical sesosrs developed for liquid film thickness measurement are able to detect the liquid film thickness fast and accurate under different flow conditions and different scales.The relevant experimental data and results could provide basic parameters for modeling and calculation in flow boiling system. |
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