Experimental Research On Bubble Characteristics And Lift Characteristics In Shear Flow Field

The bubble transverse migration is a basic phenomenon in two-phase flow,and the change of the transverse migration direction of bubbles will affect the two-phase distribution and heat transfer form,so it is of great significance to understand the bubble transverse migration for the prediction of two-phase flow and other key two-phase phenomena such as CCFL.However,the research on transverse migration of bubbles is still insufficient,and the effects of bubble size,fluid properties and shear flow field strength on transverse migration of bubbles are not clear.In this paper,experiments on bubble dynamics and transverse migration characteristics in low viscous water were carried out to study the bubble characteristics and transverse migration characteristics in shear flow field based on the shortcomings of previous studies.In this paper,the mechanism of the change of bubble size on transverse migration direction and the law of the effect of shear flow field intensity are revealed by analyzing the change law of bubble characteristics and transverse migration characteristics in shear flow field,so as to establish a formula for predicting the lift force.This research is helpful to establish an accurate model of two-phase flow and understand the basic process of two-phase flow.and provide theoretical support for the important mechanism of two-phase phenomenon and the safety design of key equipment.In this paper,the study of bubble characteristics in shear flow field mainly includes bubble shape,bubble drag force characteristics,bubble velocity,bubble amplitude and frequency.Bubble shape is a basic parameter for describing bubble morphology.which has an effect on bubble velocity,drag force and bubble amplitude.In this paper,the variation of aspect ratio of different bubble sizes in different shear flow fields is studied.The aspect ratio of bubbles in low viscous water decreases rapidly to a smaller value when the size of bubbles is smaller,and then decreases slowly with the increase of the size of bubbles.In view of the fact that the prediction of the aspect ratio of bubbles in droplets and high viscous fluids is not applicable to low viscous fluids,and that the bubbles in the inertial force range are quite different from those in the surface tension range,the formulas for predicting smaller bubbles and larger bubbles are proposed based on the experimental results.Bubble drag force is due to the interfacial force produced by the relative motion of bubbles and liquid phases.The increase of shear flow field does not make the bubble drag force increase significantly.This is because the existence of shear flow field does not change the relative velocity of two phases significantly.The models of Ishii and Chawla and Tomiyama can predict the drag coefficients of smaller bubbles,but the prediction is worse for larger bubbles(de>10mm).Tomiyama's prediction results for large bubbles are relatively low,while Ishii and Chawla's predictions for large bubbles are constant,which is not consistent with the experimental results.In this paper,based on Ishii and Chawla's model,the inertia force control is revised to obtain a more accurate drag force calculation model.Bubble velocity is affected by many factors,the existing bubble terminal velocity models are evaluated based on the experimental results.The results show that Tomiyama's model is the best predictor of bubble terminal velocity in water,which considers the influence of bubble shape change on bubble velocity.Bubble amplitude and frequency mainly describe bubble vibration,but there are relatively few studies on bubble amplitude and bubble frequency in existing studies.In this paper,the prediction formulas of dimensionless amplitude and dimensionless frequency are proposed based on the characteristics of bubble motion in the inertia force and surface tension control zone.There are many factors affecting the lift coefficient of bubbles.It has been found that when the size of bubbles is small,the lift of bubbles is mainly controlled by Reynolds number,but the specific law of the effect of Reynolds number is not uniform in current research.In low viscous fluids,the bubbles lift coefficient first increases to its maximum value and then decreases with the increase of Reynolds number,which is similar to the results of Adua(2009).The net migration of bubbles is mainly caused by shear flow field,and the effect of shear flow field intensity on bubbles is not clear at present.It is found that when the lift coefficient is negative,the lift coefficient decreases with the increase of shear flow field strength,and the effect of shear flow field strength decreases with the increase of bubble size.When the lift coefficient is positive,previous studies have shown that the lift coefficient is a constant independent of the shear flow field strength.Most scholars believe that the change of lift coefficient is mainly due to the effect of bubble wake.This paper finds that the effect of wake on bubbles increases with the increase of bubble size at first,and when the bubble size is large,the wake effect no longer dominates the bubble motion.Based on the results of experiments and previous analysis,the lift coefficients of surface tension control zone(1.5-7 mm)and inertia force control zone(7-20 mm)are established respectively,and the bubble lift in low viscous water can be predicted well.Through experiments and models,it is found that the critical size of bubble lift coefficient in low viscous water is 3.3-3.6 mm,and the critical size increases with the increase of shear flow field strength.