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Two-Phase Fluid Refrigeration

This example models a vapor-compression refrigeration cycle using two-phase fluid components. The compressor drives the R-134a refrigerant through a condenser, an expansion valve, and an evaporator. The hot gas leaving the compressor condenses in the condenser via heat transfer to the environment. The pressure drops as the refrigerant passes through the expansion valve. The drop in pressure lowers the saturation temperature of the refrigerant. This enables it to boil in the evaporator as it absorbs heat from the refrigerator compartment. The refrigerant then returns to the compressor to repeat the cycle. The controller turns the compressor on and off to maintain the refrigerator compartment temperature within a band around the desired temperature.

Model

Simulation Results from Scopes

Simulation Results from Simscape Logging

This figure plots the refrigeration cycle performance over time including the pressures, temperatures, energy flows, and mass flows. It shows that this refrigeration cycle operates at a compressor pressure ratio of about 5.5. The coefficient of performance, which is the ratio of the heat extracted to the compressor power input, is approximately 4.

This figure plots the vapor quality at each of the four points on the refrigeration cycle. It shows that when the compressor is turned on, the evaporator absorbs enough heat from the refrigerator compartment to completely vaporize the refrigerant. The condenser then brings the vapor quality down to about 0.02. Flash evaporation occurs in the expansion valve such that the refrigerant enters the evaporator at a vapor quality of about 0.4.

Animation of Simscape Logging Results

This figure shows the evolution of the fluid states in the refrigeration cycle over time. The four points on the refrigeration cycle (compressor inlet, condenser inlet, expansion valve inlet, and evaporator inlet) are plotted on the pressure-enthalpy diagram. The dotted contour lines indicates the temperature and the gray curve represents the saturation dome.

Fluid Properties

The following two figures plot the fluid properties of refrigerant R-134a as a function of pressure (p) and specific internal energy (u) and as a function of pressure (p) and normalized internal energy (unorm), respectively. The fluid is a

  • subcooled liquid when -1 <= unorm < 0;

  • two-phase mixture when 0 <= unorm <= 1;

  • superheated vapor when 1 < unorm <= 2.

The fluid property data is provided as a rectangular grid in p and unorm. Therefore, the grid in terms of p and u is non-rectangular.

The R-134a fluid property data can be found in r134aPropertyTables.mat.

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