Study Of Aerodynamic Noise Of Small-Scale Ejector For Solar Jet Refrigeration

The noise of ejector has brought about disturbance and harm to human lives and work. So it is significant to study the theory and characters on how the noise take place so that the principium can be used to control the noise. Based on the CFD (Computational Fluid Dynamic) and aeroacoustics, the numerical simulation had been taken on noise of ejector and some measures were advised to reduce the unpitched sounds in details in the paper. The relevant aeroacoustics and methods of CFD were introduced and the differences between them were discussed so that the plan and procedure can be elaborated, and the specific conditions were pointed out based on FLUENT.With the foundation of previous analysis, the numerical simulation was used to analyses flow systems of three parts of ejector. For the main section, the complex structure of jet flow can be achieved by the 2D modeling of the jet with the standardκ-εturbulent model. Some phenomenon concerned the concussion of velocity and explosion of the density, pressure and temperature of steam had been analyzed particularly. For the export of gate value, the jet flow in different pressure and the opening angle decreasing of the gate valve is investigated numerically with theκ-εturbulent model. For the export of ejector, the numerical simulation was processed on steady and unsteady states. The results in steady showed that the regularity of velocity on the axis and radial directions with the standardκ-εturbulent model. The unsteady evolution of coherent structures in a turbulent jet of ejector had been reproduced by the model successfully. The Kelvin-Helmholtz instability of the shear layer in the proximal region of the jet flow and the roll-up of the vortex rings and vortex-pairing phenomena of the round jet have been numerically visualized. The coherent structures is the main source of the turbulence jet noise, so some measures can be done to The coherent structures to cut down the turbulence jet noise.According to the numerical simulation on the jet flow, the origin of noise distribution had been processed by the combination of the acoustic analogy model and broad band noise model. The main section was carried out by broad band noise model, the results showed that the main area where the turbulence jet noise being generated could be defined as the section from the spout to diverging pressure in the downstream. In this area, due to the velocity gradient increasing between the center and boundary, the internal stress became involved, the vorticity was much higher, the pressure and speed was witnessed in a sharp change, which lead to a quadrupole and a terrible noise had taken place at nozzle. For the noise at gate valve, some research was done on the pressure and gate valve with different opening height in order to obtain the distribution in different directions and the compare of sound field's reduction between upper position and backward position. From the theoretical analysis, it was known that the noise at gate valve was one of the main reasons. The FW-H model was used in a circular spout. The results showed that the spectrum graph which originated in the oscillation characteristic family that the pressure changed with time increasing was carried out by Fast Fourier Transform (FFT) and it was according with the experiments. It could be known that the sound pressure level went up and frequency characteristic went down with the pressure of steam at the export.Some experiments were tested on the noise of ejector at the export. The results showed that the largest sound pressure level was 75.4dBA and the frequency was between 20Hz and 20KHz, mostly at high frequency, which is harm for humans hearing, meanwhile, it could also reach a conclusion that the pressure level and pressure of steam had a positive correlation, and back pressure did little sense on sound pressure level, it remained nearly at the same level. Therefore, two ways were employed to cut down the noise in this paper: on the one hand, by changing the export structure to reduce the noise, the tests indicated that it was a conservative method, for the noise was only cut down 3～5dBA and the high speed flow had a strong impact on structure, which led to the secondary noise pollution; On the other hand, the measure was to use porous material could slow the gas flow velocity to reduce the noise, the results showed that circular tube made by yarns was the best ways to play down the noise, and had no secondary noise pollution, so if it was studied much more deeply in the future, it may be have better effects to reduce the noise of ejector.