Refrigeration cycles

The CTTC group is actively researching on vapor refrigeration systems as a response to both academic and industrial needs. The study of such systems is carried out following two different but clearly related approaches: experimental tests and numerical simulations.

Numerical simulations

The simulation of vapor compression refrigeration systems is achieved from a modular numerical infrastructure based on an object-oriented tool called NEST. The whole system is modeled as a collection of different objects connected between them. Each object represents a specific part of the system (e.g. heat exchanger, compressor, expansion device, capillary tube, connecting tube, receiver, cavity, wall etc.) and can be independently solved for given boundary conditions. The global resolution procedure consists in solving all the objects iteratively, transferring information between them, until a converged solution is reached. Figure 1 shows the scheme of a simple refrigeration system and its corresponding object diagram

Fig. 1: Refrigeration system: scheme (left) and object diagram (right)

The system is easily modified by adding, subtracting or substituting any of its objects. This feature gives great flexibility to the simulation tool, not only because the configuration of the system can be altered, but also because the numerical model of each object can be straightforwardly replaced allowing different levels of simulation.

Up to now a large variety of refrigerating cycle layouts has been successfully implemented: from the simple four-element dry expansion cycle (compressor, condenser, expansion valve and evaporator) to more complex cycles including additional objects (internal heat exchangers, non-adiabatic capillary tubes, high/low pressure receivers) and to multi-loop configurations such as overfeed or gravity-fed systems (see Figure 2).

Fig. 2: Refrigeration system schemes: dry expansion (left) and overfeed (right).

The numerical infrastructure that has been developed allows the simulation of a whole refrigerating system taking into account both the refrigerating cycle itself and the cold chambers network. In addition, simulations could be carried out under steady, transient and pseudo-transient conditions (compressor on/off procedures are also considered). An illustrative example of a more complex system including all these features is shown in Figure 3.

Fig. 3: Typical household refrigerator: system scheme and transient results.

Experimental facilities

During the last decade the CTTC has significantly improve and increase its experimental facilities used to test refrigerating cycles. In fact, different experimental set-ups have been built in order to study whole systems and to validate the numerical codes (each set-up is constantly upgraded and modified to meet the required needs). These set-ups are fully equipped with measuring instruments such as temperature sensors, pressure transducers or mass flow meters, and they may include different components such as compressors, heat exchangers or expansion devices. 

Up to now a large variety of experimental tests has been carried out in order to study both the performance of the cycle components and their influence on the cycle. Both steady and transient conditions have been studied and several refrigerants have been used (e.g. R600a, CO2,  and  R134a).