On Bubble Behaviors In Subcooled Flow Boiling Under Inclination And Rolling Condition

With the development of economy,the shortage of energy becomes more and more serious.The narrow rectangular channel,which has the advantage of higher heat exchange capacity and space saving,was widely used in heat exchange equipments.When the rectangular channel is used in marine condition,due to the ocean wave and wind,the characteristics of flow and heat transfer would be different from that under conventional condition.This attracts a lot of researchers to pay their attention to it.Unfortunately,most of these researches were performed on the flow resistance and heat transfer.The bubble behaviors under ocean condition still lacks of investigation,which blocks the microscopic way to understand the influence of ocean condition on flow and heat transfer.Therefore,in order to fill this gap,a visual experimental research was performed to study the variation of bubble behaviors in a single side heated rectangular channel under ocean condition.The working fluid was distilled water.Due to the time scale of bubble behaviors,a huge number bubble images and parameters need to be processed.On the basis of experimental method and image feature,the image pre-processed method is proposed.The bubble center coordinates are overlapped according to the thought of light flow method to form bubble tracks,which could distinguish the nucleation site and sliding bubble.Combined with the PIV/PTV algorithm,the temporal feature belong to a single bubble could be gathered.Based on this method,the bubble behaviors under vertical,inclination and rolling conditions are discussed.Three aspects are discussed about bubble cluster behaviors:bubble number density,averaged size and time-averaged volumetric void fraction.Both of bubble number density and averaged size increase with increasing heat flux,inlet temperature and decreasing mass flux.With increasing inclination angle,the bubble number density and bubble averaged size show a downtrend and uptrend respectively.Under rolling condition,only the rolling angle has a certain effect,which just enlarges the fluctuation amplitude of these two parameters.In present work,no evidential relationship is found between void fraction and inclination angle,and rolling parameters.A comparison is performed on the fluctuation curve of these 3 parameters and indicates that the averaged bubble size is the dominant factor on void fraction.And,3 types of fluctuation form are observed during rolling motion.Due to the different curve of bubble number density and averaged size,the 3 types are:sine,square and irregular.The bubble growth-condensation curve could be predicted by the modified correlation proposed by Akiyama.The values of empirical coefficient K and N,in this work,are 2.14 and 0.68 respectively.The key parameters,bubble maximum size and lifetime,illuminate a linear increase with the inclination angle.During rolling motion,the effect of rolling period is not considerable.The increase of rolling angle would increase both the parameters.Considering the effect of bulk flow velocity and bubble interaction,a correlation,including Re and Bo,is proposed to predicate the bubble maximum size and lifetime.The correlation would predict the both parameters with an uncertainty less than ±25%and 30%respectively.A simple correlation is proposed too in order to relate maximum size and lifetime directly.Experimental results show that the bubble departure size would increase with heat flux,inlet temperature and inclination angle.A higher mass flux would decline the size.During rolling motion,the departure size shows a sine fluctuation with same period and phase.The effect of rolling period also is very tiny,which could be ignored.And the increase of rolling angle would enlarge fluctuation amplitude.The parametric tendencies based on force analysis are discussed too.The tendencies indicates that the rolling motion has no significant influence on bubble departure size(<1%),but the fluctuation of bulk flow and bubble growth rate has.However,the higher pressure head supplied by pump restrain the flow fluctuation(<5%).Thus,the variation of bubble departure size under rolling and inclination conditions is caused by the change of bubble growth rate.The bubble departure frequency increases with the heat flux,mass flux and inlet temperature.Under inclination,it increases with the inclination angle.During rolling motion,a sine fluctuation with same period and opposite phase was found.And,the rolling period has no evident influence on it,but increasing the rolling angle would increase the frequency.In present work,the variation of bubble departure frequency was controlled by the waiting time.Every bubble parameter under rolling and inclination with same thermo-hydraulic conditions is compared.No matter what the bubble parameter or the condition parameters are,the results show a good agreement.This indicates that the rolling period has no significant influence.And,the influence of rolling angle is presented by the periodically change of surface orientation.In a word,the influence of rolling motion and inclination are equal to each other.