Study On The Flow Behavior Of Fountains And The Mixing Characteristics In A Thermally Stratified Water Tank

Fountain(negatively buoyant jet)is a common flow phenomenon in nature and engineering applications.In the field of energy utilization,a heat water storage tank with a mantle heat exchanger is a key component in a solar water heating system attached to the balcony,which is widely used in the northern part of China.Nevertheless,the operation of the tank involves complex fountain flow and heat/mass transfer which are not fully understood,making this a challenging issue to improve the thermal performance of the solar water heating system.In this thesis,the Particle Image Velocimetry(PIV)experimental technique and the direct numerical simulation method are used to study the transient flow behavior of fountains in both homogeneous and linearly stratified ambient,along with their effects on the temperature stratification in the heat water storage tank when it is operated under the charging,discharging and simultaneous charging/discharging modes.This study improves our understanding of the transient flow behavior of fountains and their entrainment.The major work and outcomes can be summarized as follows:1.The fountain behavior is analyzed with the entrainment framework,the three mechanisms for the development of the thermal energy transfer in the stratified water of the tank are explored,and the entrainment features of the temperature stratification are found.2.A PIV experimental rig is constructed and used to carry out the experimental study on the fountain behavior in homogeneous ambient over the ranges of 0.5?Fr?10,50?Re?820,where Fr and Re are the Froude and Reynolds numbers.The results show:(1)Axial Reynolds stress is the main mechanism of entrainment and mixing;entrainment rate?increases with the increase of Fr;As the vertical height increases,the entrainment rate?increases gradually,which further indicates the enhancement of entrainment degree;When Re is large enough,the degree of asymmetry and the intensity of irregular turbulence increase.(2)Shear stress,vortex dynamics,velocity field etc.have significant effect on the entrainment in fountain flows,and an analysis of the spatial and temporal characteristics of these parameters can also recover the mechanisms of entrainment and its quantification.(3)The velocity field,vortex field and shear stress on the section of the fountain flow are analyzed using the Proper Orthogonal Decomposition(POD)method,and the effect of vortexes with different sizes on the entrainment and the development of vortex characteristics are discussed.3.A PIV experimental rig is constructed and used to carry out the experimental study on the fountain behavior in linearly-stratified ambient over the ranges of 1?Fr?14,62?Re?579,0.001?Sp?0.005,where Sp is the stratification number.The transient fountain behavior is analyzed and compared to that in the homogeneous ambient.The results show:(1)the improved“twin-tank”method can be used to produce excellent linearly-stratified ambient.(2)Compared to the fountains behavior in homogeneous ambient,stratification significantly weakens and restricts the fountain flow effect,leading to a reduced maximum penetration height and a smaller fountain width,which significantly change the entrainment and mixing features of the fountains in stratified ambient.(3)When the stratification is weak,the difference between the fountain density and the ambient density is small,the injected fountain is relatively strong,the fountain in stratified ambient can become neutrally-buoyant.4.Direct numerical simulation and experiments are carried out to study the thermal performance of a solar water storage tank with a mantle heat exchanger operated under the charging,discharging and simultaneous charging/discharging modes with real application conditions,focusing on the effect of the fountain flows on the temperature stratification in the tank.The results show that:(1)In the three modes,due to the negative buoyancy jet at the inlet,the low temperature zone,thermocline and high temperature zone will be formed in the water tank.The temperature distribution in thermocline is linear,and the average temperature gradient is not affected by Re_inumber.The thermocline thickness Z_th^* varies with time and charging rate.(2)In charging mode,the motion behavior of negative buoyancy jet at the interface of density stratification is consistent with the PIV Experimental results.The results showed that the local Ri_e can directly evaluate the stratification performance of the water tank,and the extent of entrainment and mixing of the negative buoyant jet in the thermally stratified water layer increases with the increase of Re_inumber.When t^*?0.4 and Re_i<1090,the flow state of negative buoyancy jet appears.The maximum rising height of the negative buoyant jet entering the water tank is affected by the position and thickness of the thermocline,and its mixing is mainly located in the lower part of the thermocline and the top-hat of the jet.(3)When operated in the simultaneous charging/discharging mode,when the dimensionless time is 0.4?t^*?0.8,the thermocline thickness Z_th^* keeps stable,which can prolong the service time of hot water.The Ri number is always lower than that of the drainage mode,which can optimize the structural parameters of the water tank and improve the heat release level.(4)In the straight pipe,small and large number of holes can be used to realize the function of diffuser,which can reduce the mixing of water tank,greatly improve the output rate of hot water and take out more hot water.