| Bubble columns are commonly used as gas-liquid contactors and reactors,which are widely applied in chemical,environmental,bio-chemical and energy industries for the advantage of simple structures,easy operation,high mass transfer rates and high heat transfer rates.A great deal of studies concerning the factors determining hydrodynamic behavior of bubble column under ambient pressure and temperature have been conducted,such as operation condition(superficial velocity),geometrical parameter(distributor structure,bubble column diameter,height-diameter ratio,internals)and physical property parameter(gas property,liquid property).However,practical industries equipment is often operated under high pressure and high temperature.The previous studies concerning high pressure and temperature are mainly about macroscopic property of bubble column,such as gas volume fraction and critical velocity.Researches about microscopic properties(bubble scale)are still lack,thus microscopic flow mechanism and characteristics of bubble motion under high pressure and temperature are still uncertainly.Owing to the influence of pressure and temperature on physical properties are coupled with each other,it is of great significance to know about characteristics of bubble motion so as to understand two-phase bubbling flow mechanism under such severe conditions.In addition,bubble interaction plays an important role in bubble column,which influences the whole flow field.An important phenomenon relevant for the large-scale properties of bubble swarms is the tendency of bubbles to form larger structures,so called clusters.However,experimental study about bubble cluster is still lack.In light of this,both experiment and CFD simulations were performed to investigate bubble behavior under the effect of pressure,temperature,gas velocity and liquid viscosity,and so on.In addition,correlations are established to predict bubble behavior.The content of this thesis covers following aspects.1.Bubble formation under high temperature and pressure.The effects of temperature and pressure on bubble formation under constant flow conditions in different liquids(silicone oil with high viscosity and high boiling point features,tetradecane with low viscosity and high boiling point features,water with low viscosity and low boiling point features)were experimentally investigated.The experiments were carried out in a stainless steel bubble column of 50 mm I.D with three pairs of high strength quartz windows.The bubble flow was visualized and recorded through high speed camera.The experiment conditions covered orifice velocity from about 0 up to 1500 cm·s-1,temperature from 283 up to 473 K,pressure from 0 up to 6 MPa,orifice diameter 1.12 up to 2.5 mm.The results show that the transition velocity of bubble formation model decreases with increase of pressure for all three liquids.However,pressure plays a different role in bubble diameter for different liquids.The general trend is that the bubble diameter decreases with increasing pressure.In detail,at lower temperature,the pressure effect is not obvious.However,bubble diameter decreases significantly with pressure under higher temperature.In addition,temperature also plays a different role in bubble diameter for different liquids.Under high pressure,bubble diameter decreases with temperature.However,the phenomenon is different under ambient pressure.For silicon oil system,bubble diameter decreases with increase of temperature in which condition the ratio of saturated vapor pressure to system pressure is negligible.For tetradecane and water system,the effect of temperature is complicated.At lower temperature bubble diameter decreases with temperature.However,bubble diameter increases with temperature at higher temperature,in which condition evaporating phenomenon cannot be ignored.Bubble formation under high temperature and pressure is different from that under ambient condition.The gas density increases with pressure which causes larger gas momentum force.In addition,evaporating phenomenon may exist when the ratio of saturated vapor pressure to system pressure cannot be neglected,which enlarges bubble diameter.By revising Gaddis’s model,considering the effect of temperature and pressure,a new correlation is developed to predict the bubble detachment diameter,which is not only available for ambient condition but also for high temperature and pressure conditions.2.Bubble shape and rising velocity in viscous liquids under high temperature and pressure.The effects of high pressure(0 to 6 MPa)and high temperature(293 to 483 K)on the bubble shape and rising velocity in silicone oil and paraffin were experimentally investigated.The experiments were carried out in a stainless steel bubble column of 50 mm I.D with three pairs of high strength quartz windows.The bubble flow was visualized and recorded through high speed camera.The experiments indicate that bubble rise velocity decreases and bubble aspect ratio increases with increasing pressure and decreasing temperature,which could be attributed to the variations of liquid viscosity,gas density and bubble surface property.A modified correlation of Fan correlation is recommended for bubble rise velocity valid at high temperature,high pressure and viscous liquids.New correlations for bubble aspect ratio E are proposed by use of the experimental data.The correlations are divided into three parts in terms of Weber number and Morton number.For We>12,bubble aspect ratio is independent of Weber number,and is only related to Morton number.For We<12 and Mo>3,bubble aspect ratio is only related to Reynolds number.For We<12 and Mo<3,the aspect ratio could be expressed in terms of the Eotvos number and Reynolds number.3.Interaction of two in-line bubbles of equal size rising in viscous liquid.Two-dimensional axis-symmetry computational fluid dynamics simulations on the interactive bubbles were performed with VOF method.To validate the model,the results of simulations for trailing bubble were compared with those of the experiments.Influences of liquid viscosity,surface tension and gas density at different diameters on interactive bubbles were numerically investigated.It is testified that several different bubble interactive behaviors could be acquired at different condition.Firstly,for large bubbles(d:4,6,8,10 mm),the physical properties have little effect on leading bubble,but the trailing bubble rising velocity and aspect ratio have a negative correlation with liquid viscosity,surface tension and gas density.Secondly,for smaller bubbles(d:1,2 mm),the results are complicated.The two bubbles tend to move together due to attractive force by wake and the pressure gradients repulsive force.Especially for high viscous liquid,the bubble pairs undergo several times acceleration and deceleration.By comparing the trailing bubble rising velocity with the previous correlations it is found that bubble deformation plays an important role at bubble interaction which cannot be neglected.4.Experimental study on bubble swarm behavior.Firstly,the spatial distributions of bubbles,in different liquid system and orifice diameter,were investigated experimentally in two dimensional bubble column.And then bubble size distribution and state of aggregation under high temperature and pressure were investigated.For lower gas velocity,bubble cluster with horizontal orientation is evident in liquid with high viscosity,but not obvious with lower viscosity.With the increase of gas velocity,bubble cluster exists in the form of several compact small bubbles of equal size or a larger bubble entraining several smaller bubbles.Under high pressure and temperature,bubble cluster exists in the form of larger bubble with smaller bubbles entrainment or bubble coalescence. |
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