Massflow fluctuation in capillary tubes of household refrigerators.

The preferred cooling process for household refrigeration appliances is the vapor compression cycle. The refrigerant commonly used in Europe and other parts of the world for those appliances is isobutane (R600a). A capillary tube is usually used as expansion device. The refrigerant’s state changes during the expansion from saturated or subcooled liquid to the two phase liquid-vapor state. This phase change is accompanied by an increase of vapor quality towards the capillary tube outlet. The flow pattern of the refrigerant as well as the pressure drop along the capillary tube is mainly influenced by the vapor quality. Additionally the dimensions of the capillary tube and the thermodynamic conditions of the refrigerant at capillary tube inlet and outlet have an influence on the resulting mass flow rate. Due to transient changes of the thermodynamic inlet and outlet conditions it can be expected that the capillary tube mass flow rate is fluctuating. Accordingly a periodical change of the refrigerant flow pattern at the capillary tube outlet is resulting and the occurrence of refrigerant flow induced noises can be observed. The present paper describes the experimental investigation on the mass flow characteristics of a capillary tube installed in a refrigeration test cycle. Furthermore the reasons for the mass flow rate fluctuation in the capillary tube will be discussed with respect to the refrigerant flow pattern and refrigerant flow induced noise effects at the capillary tube outlet.