Fluid borne noise, fluctuating noise, capillary tube, expansion noise
Since the beginning of the “artificial” refrigeration era, a great deal of effort has been applied on the development of high efficient systems. The efforts were primarily focused on the compressor, bringing its efficiency to record-high values. Simultaneously, the compressor noise was brought down to almost inaudible levels. As a consequence, sources of noise that were previously unnoticeable became audible and started to play a major role among refrigerator manufacturers. One of those new sources of noise, generally known as fluid borne noise, is now a growing field of interest in engineering. The aim of this work is to study the relationship between the capillary tube flow and the acoustic behavior of household refrigerators by investigating a particular type of fluid borne noise that only occurs during the freezer mode of dual-evaporator appliances, known as fluctuating noise. To this end a household refrigerator was carefully instrumented and firstly tested in a reverberant chamber. Very heavy sound power level oscillations were found, even with the system in thermodynamic steady-state conditions. Acceleration measurements were also taken at the inlet pipe of the evaporator showing that the refrigerant flow in that region is highly pulsating, i.e., that the refrigerant flow pattern at the discharge of the capillary tube is not steady, in contrast to the indications of the thermodynamic readings. Visualizations studies of the flow pattern at the inlet of the capillary tube were also carried out, showing the presence of vapor bubbles – caused by an unbalance between the compressor and capillary tube mass flow rates – for most of the time. A needle valve was then installed in series with the freezer capillary tube in order to increase restriction and match the compressor and the capillary tube mass flow rates. The restriction provided by the optimum pair needle valve-capillary tube was converted into an equivalent I.D. capillary tube. The original freezer capillary tube was replaced by a same length smaller I.D. capillary tube, and another appliance tests were carried out. It was found that the higher restriction capillary tube attenuate the pulsating characteristic of the refrigerant flow, almost averting the disturbing fluctuating noise.