Phase Distribution Characteristics And Influencing Mechanisms In Rod Bundles

Two phase flow systems are widely encountered in various industrial systems,such as gas-oil storage and transportation,chemical reactor,nuclear reactor and refrigeration et al.The intrinsic complexities of gas liquid two phase flow,including the deformation of interface,uneven phase distribution and relative velocity between two phases,make the mechanisms of mass,momentum and energy transfer complicated.As a common geometry in nuclear reactor and heat exchanger,it is crucial to make clear the flow resistance,heat transfer characteristics and critical heat flux in rod bundles,which is significant for the safety and efficiency of nuclear reactor.For the two phase flow in rod bundles,the flow resistance,heat transfer coefficient and critical heat flux are always related to the phase distribution characteristics.Therefore,it is necessary to conduct experimental research on the phase distribution characteristics and analyze its influencing factors.However,the measuring instruments for gas liquid two phase flow are scarce,especially for that in rod bundles,which limits the related experimental research.Thus,based on the electrical differences between gas and liquid,the sub-channel impedance void meter has been designed to measure the sub-channel void fraction for the first time.Meanwhile,the four-sensor conductivity probe has been developed to measure the detailed local two phase flow parameters.The global phase distribution characteristics,local two phase flow parameters and their influencing factors have been analyzed.Furthermore,the transition model from the void wall peak to core peak distribution and the sub-channel distribution parameter model have been proposed,which could guide the operation of related devices,design of the industrial systems et al.The sampling frequency of the developed sub-channel impedance void meter is set to 10 kHz,which has good accuracy and has captured the dynamic void fraction characteristics of bubbly,cap bubbly,cap turbulent,churn turbulent and annular flow in the sub-channel flow for the first time.Based on the 13 feature values of the sub-channel void fraction(mean value,standard deviation,sample entropy and 10 proportion values for different void fraction regions),the sub-channel flow regime maps have been objectively acquired for the first time using random forest algorithm in different subchannels with different spacer girds.The comparison with Paranjape's global flow regime map indicates that the effect of casing tube makes the global flow regime map not applicable for the inner sub-channel and makes the flow regime transitions occur in the inner sub-channel at first,then the side and corner sub-channel at 19.5 L/D downstream of simplified spacer grid(SSG).At low liquid velocity,the transitions from bubbly to cap bubbly flow at 6.8 and 19.5 L/D downstream of SSG almost happen at the same flow condition while the transitions from cap bubbly to cap turbulent,then to churn turbulent flow appear at 6.8 L/D then at 19.5 L/D.However,at high liquid velocity,the transitions from bubbly to cap bubbly,then to cap turbulent flow occur at 6.8 L/D,then at 19.5 L/D downstream of SSG.Compared with SSG,the spacer grid with mixing vanes(MVSG)plays an important role in breaking up bubbles due to the shear force,which delays the flow regime transitions.According to the local two phase flow parameters measured by four-sensor conductivity probe,the parameters fluctuate at the radial direction because of the restriction of rod gap,whose fluctuating amplitudes are associated with void fraction,superficial gas and liquid velocity et al.With the gas velocity increasing,the wall peak distribution of Group I void fraction profiles turns to core peak distribution,and Group II bubbles might appear,which happens at void fraction ranging from 0.2 to 0.3.Moreover,the Group II void fraction profiles have core peaks both for sub-channels and for the whole flow channel.And the influence of MVSG on two phase flow has been discussed for the first time.For example,the peaks of two phase flow parameters might shift due to the lateral velocity induced by the mixing vanes of MVSG;the wall peak distribution of void fraction profiles has been enhanced by MVSG;moreover,MVSG promotes the Group II void fraction and interfacial area concentration profiles have an inclination towards one direction,and would be “reflected” towards the opposite direction due to the restriction of the casing tube.It has been demonstrated that the transition from void fraction wall peak to core peak distribution is determined by bubble size and rod gap size et al.According to the analysis in the lift force,wall lubrication force and turbulent dispersion force,the total bubble force acting in the lateral direction is related to the density difference,relative velocity,bubble size,surface tension,bubble Reynolds number,bubble E?tv?s number,the distance between bubble and wall et al.In general,the increments of superficial gas velocity and void fraction promote the transition from void fraction wall to core peaks,while the increments of superficial liquid velocity and rod gap size might delay the transition.Therefore,these influencing factors have been divided into “promoting factors” and “inhibiting factors”.Then based on the present,Yang's and Hosokawa's experimental data,the transition criteria from void fraction wall peak to core peak distribution has been proposed by taking into gas and liquid Weber numbers,Reynolds numbers,physical properties separately and rod gap size,which could be applicable for the rod bundles with different sizes.Furthermore,the distribution parameter models applicable for rod bundles have been reviewed.On the basis of the measured sub-channel void fraction,these existing models have been evaluated.However,the predicting errors of sub-channel void fraction turn out to be larger than 25% for all models.Thus,based on the drift velocity correlation proposed by Julia in the direct integral method,the sub-channel distribution parameter model has been proposed,which could predict the present and others' data with small errors.Moreover,the distribution parameter in the inner sub-channel is demonstrated to be smaller than that in the side and corner sub-channel due to the influence of the casing tube.The effects of spacer grid have been analyzed in the sub-channel flow regime transitions,local two phase flow parameter distributions and the sub-channel distribution parameter,which could be summarized as follows: bubbles' breaking up due to the shear force;stronger turbulence;the lateral velocity induced by mixing vanes;the flow recirculation behind the mixing vanes;the low pressure region behind the spacer grid.The influencing region and intensity of spacer grid are determined by the gas velocity,liquid velocity,void fraction,its geometrical structure and so on.According to the analysis for the present flow conditions,the influencing region of MVSG is larger than 19.4 L/D but smaller than 32.1 L/D for one-group two phase flow,while larger than 32.1 L/D for two-group two phase flow.