Condensation, Refrigerant flow, Houseold refregerator, Capillary tube
Capillary tube-suction line heat exchangers (CT-SLHX) introduce complex phenomena due to simultaneous 2-phase flow expansion and heat transfer such as: reverse heat transfer, flow hysteresis and flow oscillations. Some of the negative consequences of these phenomena are: noise due to re-condensation, which is becoming an important quality issue; and reduction of the SLHX effectiveness, which also affects the global efficiency. Studies about how to solve the noise problem show that it disappears when there is enough subcooling at the capillary tube inlet. This fact also supports the idea that two-phase flow at the capillary tube inlet contributes to re-condensation phenomenon in the CT-SLHX. Different reasons may explain it, e.g. the small compressor capacity compared the expansion device capacity of the capillary tube used. Another important consequence would be wasting condenser surface due to the two-phase outlet. Therefore, the main objective of this work is to assess experimentally the actual conditions taking place at the capillary tube inlet and find a solution to the problems mentioned above. An innovative test bench has been designed in order to visualize and analyze the phenomena occurring at the condenser outlet, along the filter and at the capillary tube inlet. This experimental bench test is connected to an A+++ no-frost household refrigerator equipped with a fin-and-tube evaporator, a tube and wire condenser and a variable-speed 7.24 cm³ hermetic reciprocating compressor. The refrigerant used is isobutane (R600a). In order to determine the refrigerant temperature, a set of thermocouples has been placed along the refrigerant loop, while a pressure transducer is installed at the condenser outlet. The mass flow rate is measured with a Coriolis meter installed at the compressor discharge line. The final part of the condenser and the filters were built with PFA (Perfluoroalkoxy) transparent pipes. Three different positions of identical filters were tested to analyze their influence on the flow configuration. The first filter is horizontally oriented, second and third ones are in a vertical position but with opposite flow directions. A system composed of three manual solenoid valves enables to test each of the three configurations independently from the two others. The transparent filters make possible the visualization of the refrigerant flow pattern at the capillary inlet. An adjustable system has been designed to modify the length of the capillary inside the filters. The set of experiments were tested in steady conditions, by using electrical heaters inside the cabinets to keep the setting point constant. Results show the description of the flow pattern at the capillary tube inlet and condenser outlet with the different capillary tubes and filter arrangements. The condenser outlet conditions were analyzed and the energy efficiency of the refrigerator was compared with a system with actual subcooled outlet conditions. Once characterized the refrigerant flow at the condenser outlet, a configuration has been proposed to ensure an effective subcooling and thus an improvement of the refrigerator performance.