Experimental identification of evaporator dynamics for vapor compression refrigeration cycle during phase transition

Experimental identification of the evaporator dynamics in a vapor compression cycle (VCC) subjected to imposed heat flux in the evaporator is studied. The imposed heat flux boundary condition represents a specific application of the VCC for electronics cooling. However, this application requires different models and control algorithms than traditional VCCs with fluid-to-fluid heat exchanges because of the faster time response of the imposed heat flux boundary condition and because the refrigerant flow at the exit of the evaporator is expected to be a two-phase mixture. First principle models are highly nonlinear and, hence, are not practical for system control. To obtain a simplified model, a dynamic identification of component response characteristics is performed by applying a pulse change in the system operating conditions (i.e., the heat load to the evaporator, the expansion valve opening, or the compressor speed). It is shown that for changes in expansion valve opening, the temperature of the refrigerant at the exit of the evaporator has opposite trends when the flow is initially in the two-phase region and when the flow is in the superheated region. This phenomena represents a challenge for controller design, and further investigation is required.