Behavior Of Weak Axisymmetric Round Fountains In Homogeneous Fluid Under The Combined Temperature And Concentration Effect

Fountains injected into homogeneous fluids with combined temperature and concentration effect are occur widely in nature and environmental and industrial applications.Such as brine,living sewage disposal,waste hot gas which is let to the atmosphere from chimne,solar desalination discharge desalination process,agriculture sewage waste water irrigation and so on.In built thermal environment,fountains form as fresh air is injected upward through floor level vents to provide task cooling.It is essential to fully understand the transient behavior and intrinsic laws of buoyant jets in ambient fluid which combined the effect of temperature and concentration if we want to effectively and optimally use their transport mechanisms and entrainment characteristics in environmental improvement and industrial applications.Two dimensional transient model established physical and mathematical model of weak axisymmetric fountain in homogeneous fluid under the combined temperature and concentration effect,and introduced user-defined function(UDS)of fluid density.By means of user-defined functions(UDF)introduced concentration transport equation is established at the same time.The numerical results are compared with the results in the literature and shows a good agreement.In this study,the transient behavior of weak axisymmetric fountains under the combined temperature and concentration effect is studied with a series of two-dimensional direct numerical simulation over 0.25?Fr_T?2.0,20?Re?300,-0.75?N?5.0 in terms of the initial and time-averaged maximum fountain height and the time for fountain to attain the initial maximum height,the upflow width,fountain width where Fr _T is the Froude number based on temperature only and N is the buoyancy ratio which represents the relative contribution to density from concentration compared to that from temperature.The results show that when Fr or Re is small,fountains usually keep its axisymmetric behavior all the time.However,when Fr is increased or Re is large(Re200,FrT1.0),the axisymmetric behavior is present only at the early development staged and the fountain will become asymmetric behavior is at the quasi-steady state,with the maximum height fluctuates around an essentially constant time-average value.For each Fr _T value,both the initial and time-average maximum fountain heights decreases when N increases,as the combined negative buoyancy due to both temperature and concentration is strengthened,whereas as Fr _T increases,they increase as the overall negative buoyancy reduces.It is found that if the overall Froude number,Fr,which is based on density due to the combined temperature and concentration contributions,is used instead of Fr _T,the previously obtained scaling relations for weak axisymmetric fountains with density coming from temperature only,are also applicable for the weak axisymmetric fountains under the combined temperature and concentration effect,when Fr(?)2.0,although there are slight differences in the quantified correlations of these scaling relations,due to the extra effect from the co-existing concentration.It is further shown that the effect from the co-existing con-centration in the present case shifts the scaling relation for intermediate fountain with 2.0(?)Fr(?)4.0 with density from only one contributor,to the scaling relation for weak fountains with Fr(?)2.0 with density coming from both temperature and concentration.The behavior of weak round fountains combined the effect of temperature and concentration with Fr _T,Re and N over the range of 0.5?Fr_T?6.0,47?Re?824,0?N?5.0 is studied experimentally by using Particle Image Velocimetry(PIV)and Planar Laser-Induced Fluorescence(PLIF).The round fountain is formed by ejecting saline water of various temperature and densities upward into a rectangular water tank filled with quiescent fixed with constant temperature and densities water at specific constant flow rates.The three group typical weak round fountain experimental results compared against the results of the corresponding DNS results,which are in good agreement both qualitatively and quantitatively,indicating the DNS runs predict the flow behavior with sufficient accuracy.The experiment results show that round fountains remain symmetric for all times at a lower Fr or Re value or at a higher N values.On the contray,when Fr or Re is large or a small N,round fountains will remains symmetric only in the early developing stage and demonstrate that the extent of both the bobbing motions increases with Fr and Re but decrease with N.Notably,it is found that the scaling relation developed by Lin for weak fountains with density come from only temperature,at z_m,i^Fr,in general also work well for the round fountains with density coming from both temperature and concentration.It also found that these characteristic parameters are under significant influences of Fr and Re,although the effect of N is in general negligible and much weaker than those of Fr and N.Direct numerical simulation are carried out to study the underfloor air distribution systems,to analyzes the effect of Re,Fr and N on indoor air quality(IAQ)and ventilation effectiveness both qualitatively and quantitatively.The results show that ventilation effectiveness increases with increase of Re and N,but decreases with increase of Fr.Ventilation effectiveness mainly influenced by Re.There are stratification of temperature and concentration in all direct numerical simulation(DNS)conditions.The DNS results can be put into practice to give references on underfloor air distribution systems preliminary design.In this thesis,the Particle Image Velocimetry(PIV)and Planar Laser-Induced Fluorescence(PLIF)experimental technique,the direct numerical simulation method and scaling laws are used to study the transient flow behavior of fountains in homogeneous fluid combined the effect temperature and concentration.This study will provide insightful new knowledge for fluid mechanics,turbulence theory,engineering hydraulics and environmental sciences.