This study analyzes instability due to pressure drop oscillation occurring in a closed system comprising of a vapor compression cycle. The system includes a microchannel evaporator, a condenser, an accumulator, an electronic expansion valve and a compressor. Using a spatially lumped dynamic model, this study investigates how various system parameters, such as the valve setting, the accumulator heat load, the compressor speed and the amount of compressible volume in the system, affect the amplitude and the frequency of oscillations. The model also predicts the system response to dynamic evaporator heat loads. It shows that the system stability can be predicted based on the system demand and supply pressure curves. For an unstable system, an increase in the evaporator heat load and the opening of the valve increases the amplitude and the period of oscillations by altering the demand pressure curve. On the other hand, while a larger compressible volume in the system does not affect the amplitude, it increases the period of oscillations. Small variations in the compressor speed and the accumulator heat load do not affect the oscillation characteristics significantly. However, a sufficiently large change in these parameters can help stabilize the system by altering the supply pressure curve. For a given evaporator heat load, the analysis shows that the system controllable parameters, which include the valve setting, the accumulator heat load and the compressor speed can be chosen judiciously to avoid pressure drop oscillation. This study provides general guidelines to select the appropriate combination of system parameters to stabilize the system while ensuring efficient operation.
Applied Thermal Engineering, Volume 160, September, 2019.