Theoretical analysis and modeling of flow instability in a mini-channel evaporator

• Theoretical analysis and modeling of pressure drop instability are conducted.
• The pressure drop oscillation is a kind of self-excited vibration.
• The oscillation mainly consists of flow boiling stage and liquid rewetting stage.
• There is a maximum allowable inlet sub-cooling degree causing no instability.
• Mass flow rate affects the oscillation period and boiling behaviors significantly.

Pressure drop oscillations in micro/mini-channel evaporators and corresponding flow instabilities, temperature fluctuations have received copious of investigations during the last decade. This paper presents a transient lumped model and theoretical analysis for the pressure drop oscillation in a mini-channel evaporator. Based on the model, the effects of saturation temperature, heat and mass flux on the oscillation are investigated. Experimental studies of ammonia and water flow boiling instabilities are conducted. The mini-channel evaporator consists of 4 parallel 1 × 1.1 mm channels with a uniformly heated length of 250 mm. A nonlinear system stability analysis is presented. Apart from upstream compressibility, the inlet sub-cooling degree has a significant effect on the pressure drop oscillation. A maximum allowable inlet sub-cooling degree causing no pressure drop oscillation is proposed. The oscillation period is comprehensively studied, and it is found that the upstream compressible volume required sustaining the oscillation decreases with channel length/diameter ratio dramatically. Despite this, the internal compressibility of the long channel is insufficient to sustain the pressure drop oscillation. In addition, the mass flow rate of the upstream pump can greatly affect the oscillation and the flow boiling system may show different behaviors due to the variation of upstream mass flow rate.