Mechanism Of Helmholtz Oscillator And Its Application

This dissertation is devoted to the investigation of the influence of self-excited oscillation jet on heat transfer enhancement. Firstly, the mechanism of self-excited oscillation was introduced. When the water jets from the inlet nozzle flows into the chamber, the momentum exchange between the jet and the liquid in the chamber occurs, thus to form an unstable shearing layer with certain thickness. The shearing layer is carried by the jet and the eddy is formed because of the instability and selective amplification of shearing layer which will propagate to the downstream. As the eddy impacts the impinging wall, a wave of pressure disturbance with certain frequency is induced. This wave then propagates to the upstream with high speed, and results in the overlap and amplification of the waves when the frequencies of them are close with each other. The repetition of the above process leads to the occurrence of the self-excited oscillation pulsating jet at the exit of the outlet of the back nozzle. When the pulsating jet was introduced in the heat exchanger, pulsating flow leads to the formation of an amount of vortices near the tube wall. The boundary layer was destroyed and the mixing of liquid was improved, so heat transfer enhancement was achieved.Secondly, the Helmholtz oscillator was designed. Based on the flow network theory, the expression of intrinsic frequency was deduced. The main parameter of Helmholtz oscillator was calculated.Finally, the experiment was performed in order to exam the effect. The pulsating liquid flows through a single pipe heated electrically. The heat transfer coefficients were measured under different conditions. Several important conclusions based on the experiment are drawn. (1) The performance of the oscillator is very sensitive to the geometric structure and the size of its parts. If the geometric structure and flow rate or pressure difference of oscillator are suitable, the expected oscillation can be obtained. (2) A remarkable enhancement of heat transfer was observed when the oscillator was installed. Experiments show that the heat transfer coefficient can increase 5-30%in comparing with the normal case when the oscillator was removed.