Bachelor's thesis Kanghua Zhang

 

Numerical investigation of entrainment properties of multi jet applications using scale adaptive turbulence models

Transient Velocity of the LES Simulation Copyright: EBC Transient Velocity of the LES Simulation

The German environmental objectives for the year 2050 require energy savings in the building sector, which nowadays make up for more than one third of the energy consumption in Germany. In the building sector a substantial factor in the energy consumption consists of the climate control within public buildings. In order to lower the energy consumption while maintaining the same air quality, active chilled diffusers are developed which reduce the amount of required fresh air while simultaneously dissipating the identical heat load. For that the entrainment effect of surrounding ambient air created by free jets of centrally conditioned fresh air are used. The magnitude of entrainment is mainly determined by the degree of turbulence occurring at the free jets boundary layers which are examined with numerical methods.

This thesis compares the SST k-omegamodel, the LES model and the DES model by validating the results with PIV data taken from a testing bench. The validation process investigates free jet properties, such as potential core length and deflection, and general flow properties, such as velocities and velocity fluctuation which are used to describe the occurring turbulence. Additionally the mixing point of the free jets is investigated. This study aims to find the model which best describes the PIV data with justifiable computational load. This model is then used to compare the properties of a flow through cross shaped nozzles.

The results show that the LES model predict the PIV data best. To note are the different modelling methods of the secondary opening of the turbulence models and the testing bench, which is the cause of slight variations of the flow properties. In terms of computational cost the SST k-omegea model needed the least amount of time at the expense of accuracy. The DES model required the highest computational cost and failed to deliver any improvement in regards to accuracy. Therefore, the comparison between the two nozzles are conducted using the LES model. The results of the comparison show that the induction ratio, which to a certain degree determines the efficiency of the system, is enhanced by the cross shaped nozzle.