Analysis Of The Influence Of Outlet Position On Mixing Characteristics In The Square Hot Water Storage Tank

Solar hot water heating system has been widely used in industrial,agricultural and civil fields.As the key component of the solar hot water heating system,it is not only to consider the performance of water discharge when using hot water,so that the heat in the tank can be taken out as much as possible,but also to consider the convenience of construction,installation and maintenance and fully integrated with building.The shape and structure of the heat storage tank are the key factors affecting the discharging performance of the tank.The square storage tank is preferred by the architectural design industry for its easy making,installation,and integration with building.In order to understand the mixing characteristics caused by negative buoyancy jet in the discharge stage in square hot water storage tanks,and to analyze the thermal stratification and mixing degree in the tanks,in this paper,the phenomenon of horizontal negative buoyancy jet at the discharging stage in the square water tank is intuitively and accurately understood by using PIV（Particle Image Velocimetry）and LIF（Laser Induced Fluorescence）experimental technique,and the characteristics of velocity and temperature field are gained.Combined with numerical simulation method,the water mixing characteristics in the hot water storage tank under different conditions in the discharging stage are analyzed qualitatively and quantitatively,to provide reference for the hot water storage tank design and engineering application.In this thesis,a transparent square steel glass water tank with different outlet tubes of a common square water tank is made as a prototype.Under the conditions of different control parameters which is Reynolds number Re（940 ≤ Re ≤ 3290）,the horizontal circular negative buoyancy jet in uniform environmental fluid（hot water）is studied by using PIV/LIF technology.It is found that:（1）With the continual increase of Re,the greater the entrainment range of the flow field,the higher the entrainment height,the stronger the mixing between the jet and the environmental fluid.（2）The axial Reynolds normal stress is positively correlated with the velocity distribution.（3）In the case of the same Re number,the relative error is not more than1% with the average speed of the turning point of each operation condition.The initial of the jet in the tank at different outlet locations is approximately the same length and its flow pattern is similar.（4）According to the analysis of transient temperature: with the Re number increases,the temperature diffusion range of the jet flow is larger,the more,finer,unequal in size and unevenly distributed enrolling vortices appeared,showing that under the action of strong horizontal negative buoyancy jet,disordered entrainment and mixing are constantly occurring in the water tank.By using PIV experimental results to verify the square storage tank’s numerical simulation model and methodes,and a geometric model is established based on the practical tank size.Numerical simulation and analysis are carried out in the hot water storage tank under different conditions in the discharging stage.It is found that:（1）when the inlet velocity is 0.25 m/s,0.5 m/s,1 m/s respectively,the mixing of hot and cold water in the tank is not obvious,the horizontal negative buoyancy jet is weak and the temperature stratification is obvious under weak inlet velocity.With the increase of inlet velocity,the greater the mixing degree in the tank,the easier it is to destroy the thermal stratification level,leading to the obvious weakening of the temperature stratification effect in the water tank,and the increase of the thermocline thickness.（2）For different hot water outlet position,the mixed flow patterns of cold and hot water are consistent when the inlet velocity is low in a short period of time.The lower the position of the hot water outlet,the more obvious the decrease of the discharge efficiency in the tank.（3）With the increase of discharge time,the overall mixing degree in the tank has increased and the thermal stratification level has decreased.The average dimensionless thermocline thickness is increased gradually in the tank when the inlet diameter is 5 cm,8 cm and 10 cm respectively when the inlet velocity is 0.25 m/s.（4）The average thickness of the dimensionless thermocline in the square tank and cylindrical tank are 0.1542 and 0.1285 respectively when two tanks have same height-diameters ratio,volumes,and inlet velocity with inlet and outlet diameter of 5 cm.The average dimensionless exergy of the square tank is 1.14 times that of the cylindrical tank.It shows that the mixing in square tank is stronger than that in cylindrical tank under the same working conditions.（5）Dimensionless exergy decreases gradually with the increase of dimensionless time in the tank.The greater exergy value means the higher mixing degree in the tank,the less useful heat can be taken out,that is,the less hot water can be taken out.