Bubble elongated

Slug/semi-annular flow. Here both slug and semi-annular flows were present. The vapor velocity increased with the heat flux and the rear of elongated bubbles began to break up (Fig. 2.30d). Coalescence was no longer clean and created a churn-like zone where the liquid slug had been. 

Region along the channel after venting of an elongated bubble... 

After venting of the elongated bubble, the region of liquid droplets begins. The vapor phase occupies most of the channel core. The distinctive feature of this region is the periodic dryout and wetting phenomenon. The duration of the two-phase period, i.e., the presence of a vapor phase and micro-droplet clusters on the heated wall, affects the wall temperature and heat transfer in micro-channels. As the heat flux increases, while other experimental conditions remain unchanged, the duration of the two-phase period decreases, and CHF is closer. 

The periodic phenomenon described above indicates that the entire channel acts like the area beneath a growing bubble, going through periodic drying and rewetting. The cycle was repetitive with venting of the elongated bubble. Such a behavior affects the mean flow characteristics that usually are measured at the manifolds. 

At relatively low liquid superficial velocities, increasing the mixture volumetric flux led to longer bubbles and shorter liquid slugs, eventually leading to the merging of elongated bubbles, and the development of the slug-annular flow pattern, repre-... 

Such a behavior agrees with results reported by Agostini et a. (2008). It was found that the elongated bubble velocity increased with increasing bubble length until a plateau was reached. An analytical model has been proposed that is able to predict this trend. 

The collision of elongated bubbles has been studied by Revellin et al. (2008) along adiabatic glass micro-channels of 509 and 709 pm internal diameters for refrigerant R-134a. A model for the collision of elongated bubbles in micro-channels was proposed to predict the bubble length distribution at the exit of the microevaporator. 

Agostini B, Revellin R, Thome J (2008) Elongated bubbles in micro-channels. Part I Experimental study and modeling of elongated bubble velocity. Int. J. Multiphase Flow 34 590-601 Bankoff SG, Haute T (1957) Ebullition from solid surfaces in the absence of pre-existing gaseous phase. Trans ASME 79 735-740... 

Figure 12 shows the estimated liquid flow rate versus the actual liquid flow rate. The tests for Qa — 0.6 m3/h are of the stratified flow type. The tests of 1.8 < Qa < 7.5 m3/h are of the elongated bubble and slug flow type (Brennen, 2005 Govier and Aziz, 1972). In this study, the flow conditions are compared with the flow regime charts of Govier and Omer (1962 in Govier and Aziz, 1972) and Mendhane (1974 in Brennen, 2005). 

Second, a peak-intensity ultrasound echo can be used to detect the gas-liquid interface, but in this case the aim is the development of a flow meter capable of estimating the ratio of component phases accurately and in real time. Our results are promising for the estimation of the liquid flow rate of gas-liquid two-phase flow further research will produce valuable data that will allow the estimation of flow rates for the two phases simultaneously. The results presented here show the liquid flow rate estimated by the peak echo intensity method can provide an accurate estimate of the actual liquid flow rate. This method can be applied to pure liquid as well as to a two-phase flow where the void fraction is as high as 50%. The flows tested are of the stratified, elongated bubble, and slug flow types. Other types of flow such as wave flow and dispersive flow were not tested the present experimental setup does not provide the gas and liquid flow rates needed to achieve such flows. 

Jacobi, A.M., Thome, J.R., (2002), Heat transfer model for evaporation of elongated bubble flows in microchannels, J HEAT TRANSFER, 124, 6, pp. 1131-1136. 

Jacobi, A. M. and. Thome J. R., (2002) Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels, HSA7E Jowraa/ of Heat Transfer, Vol.124, pp.1131-1136. 

Figure 6. Image of an elongated bubble (top) and a schematic diagram of the three-zone evaporation model (bottom).

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