Research On Flooding In Vertical Narrow Rectangular Channels

The phenomenon of flooding widely exists in many industrial equipments, such as thefalling film evaporator, reflux condenser, heat pipes and so on. The possible counter currentflow of gas-liquid two-phase flow occurring within reactor, especially reflux condensing limitwithin the steam generator under the accident will generate critical impact toward the reactorsecurity. With the technolgy development and the continuous improvement of reactor design,the channel used for core heat removal changes from a traditional larger diameter to narrowand even small one. In further study of the flooding problem in the narrow channel to providetheoretical and experimental basis for the development of new reactors, has importantpractical significance. In this paper, the theoretical analysis and experimental study of verticalnarrow rectangular channel flooding were investigated. Firstly, the internal relation of the gasentrance conditions, channel length and the gap of flow etc. with the flooding starting point,completely carrying point, flow reverse and liquid the vanishing point were studied by theresearch of experimental phenomenon and pressure drop of relationship. And floodingmechanism in vertical narrow rectangular channel was described. Moreover, the reason ofvertical narrow rectangular channel flooding and the influencing factors of completelycarrying point, flow reverse and flooding disappear were analyzed respectivly. Finally, theflooding of the narrow rectangular channel was compared with traditional channel. The mainresults are as follows：1. As differenting from jadging onset of flooding by pressure mutation in traditionalchannel, four characters should be had on flooding in vertical narrow rectangular channels:sudden rise in the pressure gradient in the test section; appearance of a sustained liquid flowabove the liquid inlet; reduction in the down flow rate of the liquid; appearance of a highlydisturbed film flow.2. At the same liquid flow rate, the gap size of channel is smaller, the gas flow rate islower on flooding. Flooding gas velocities decrease as the pipe length increases. Gas entrancecondition is the important factor effecting on onset of flooding. The gas flow rate is lowest onflooding at plane entrance, highest at bell-mouthed entrance and intermediate at obliqueentrance. Onset of flooding of plane entrance and oblique entrance are produced in the gasentrance but bell-mouthed entrance is produced in the liquid entrance. 3. Carryover of flooding is no related with liquid flow rate, entrance conditions andliquid flow rate. Reversal point related to channel dryness, but no correlation with gas flowrate, entrance conditions and liquid flow rate; with the same water flow rate, gas flow ishigher in a dry channel than in a dry channel at reversal point.4. In different channel length, gap size and gas entrance conditions, flooding disappearedthere are some hysteresis. At the low water flow rate,, onset of flooding and the vanishingpoint are similarly, with the liquid flow increasing, flooding disappeared hysteresisphenomenon more obvious.The hysteresis of flooding becomes more apparently with the gapsize decreasing. At the same liquid flow rate, the length of channel is smaller, the gas flow rateis lower on vanish.5. The results show that the flow characteristics and the tendency of pressure drop invertical narrow rectangular channels were similarly with the conventional channels. However,the maximum of pressure drop appeared at the completed carrying up of flooding in verticalnarrow rectangular channels, and it appeared at the onset of flooding in conventionalchannels.