Experimental And Mechanism Investigation Of Flow And Heat Transfer In The Mixing Zone Of T-Junction Filled With Porous Media

T-junction is widely used in various pipeline system of industrial production such as in the energy and power fields. Temperature fluctuations are caused by cold and hot fluids mixing with temperature differences in T-junction. Temperature fluctuations of the mixing fluids may cause cyclical thermal stresses and subsequent thermal fatigue cracking of the pipe wall. Thermal fatigue of T-junction is a very important technical issue affecting the safety and service life of a piping system in the petrochemical, nuclear power and other industries. Therefore, to investigate characteristics and mechanism of hot and cold fluids mixing flow and heat transfer in T-junction, to accurately predict velocities and temperature fluctuations in the mixing process, to seek to control and reduce the temperature fluctuations, which have important scientific and practical significance for reducing thermal stresses in order to avoid the pipeline thermal fatigue damage and improving the safety and service life of the piping system. Recently, many researchers are keen to arrange porous media in the channel using turbulent flow enhancement characteristics of porous media. That is proved to be an effective method to change the flow field distribution and achieve the effect of heat transfer enhancement in the channel. In this thesis, experimental and numerical investigations of hot and cold fluids mixing flow and heat transfer in the mixing zone of T-junction filled with porous media made up of orthogonal arrangement spherical particles were presented. The mathematical model of flow and heat transfer of fluids mixing process in T-junction filled with porous media was established and solved by the Large Eddy Simulation(LES) numerical method using the FLUENT software platform. The experimental device of porous media pipeline was designed and manufactured. The experiments of hot and cold water mixing in T-junction filled with porous media were carried out at57kinds of working conditions. The instantaneous temperature in the mixing zone was measured and recorded by the multi-point thermocouple temperature sensor designed specially and the data acquisition system. Comparisons of the normalized mean and root mean square temperatures at different conditions of the temperature difference (Richardson number), the flow ratio (momentum ratio), and Reynolds number in the mixing zone, the effect mechanism of these factors on temperature fluctuations in mixing zone of T-junction filled with porous media were analyzed. The LES numerical method was validated by the comparison of the numerical results and experimental measurement results. And on this basis, hot and cold fluids mixing flow and heat transfer in the mixing zone of T-junction filled with porous media was further numerically investigated at several influence factors of porous media structural effect (particle diameter and porosity), and the skeleton material (thermal conductivity). The main conclusions were drawn as follows:(1) The calculated values using the LES numerical method were in agreement with the experimental results for the normalized mean temperature and the normalized root mean square temperature in the mixing zone of T-junction filled with porous media. The mathematical model and the LES numerical method were validated.(2) In both the effect of flow disturbance from pore channel and the impact of the branch tube fluid, the flow in the mixing zone is mainly affected by the inertial force, and the effect of buoyancy caused by the temperature difference on flow is weakened. Therefore, as the buoyancy and the temperature difference between the main tube and the branch tube increase, the pressure drop, velocities and velocity fluctuations, and the temperature distribution have no significant changes in the mixing zone of T-junction filled with porous media, but the temperature fluctuation increase slightly.(3) With the flow ratio or the momentum ratio between the main tube and the branch tube increasing, the impact action of cold fluid from branch tube decreases and the distribution of hot and cold fluids in the mixing zone changes. In the mixing zone of T-junction filled with porous media, as a result, the temperature and temperature fluctuations of mixing fluids increase, the region of the larger temperature fluctuations decreases and moves to the branch tube inlet. When the flow ratio or the momentum ratio remains unchanged, Reynolds number of the mixing zone is increased with the flow of the main tube and the branch tube increasing. However, the distribution of hot and cold fluids in the mixing zone has not fundamentally changed so that the temperature and temperature fluctuations of mixing fluid increase slightly.(4) For the same uniform porosity porous media, with the reduction of the particle diameter and the pore channel size in the mixing zone of T-junction, the turbulent mixing scale of hot and cold fluids decreases. It causes that the velocity, the amplitude and region of velocity fluctuations decrease, while the pressure drop increases in mixing zone. Meanwhile, the phenomenon of temperature stratification becomes obvious in mixing zone because of the convective heat transfer weakened and thermal conductivity enhanced. The amplitude and region of temperature fluctuations decrease.(5) For uniform porous media in the same arrangement, as the porosity decreases, the velocity and velocity fluctuations decrease, while the pressure drop increases sharply in mixing zone. Meanwhile, the effective thermal conductivity of porous media increases resulted in the heat conduction between the solid skeleton and fluids enhancement. The mixing temperature of hot and cold fluids in the mixing zone is accelerated uniform, and temperature fluctuations also decrease.(6) In the mixing zone of T-junction filled with porous media, the thermal conductivity of skeleton materials increases resulted in the heat conduction between the solid skeleton and fluids enhancement. Therefore, the fluid temperature rises in the mixing zone, and temperature fluctuations at the middle and upper of the tube increase. But the thermal conductivity of skeleton materials has no significant influence on the pressure drop, the velocity and velocity fluctuations.