An Experimental Comparison of the Refrigerant Flow through Adiabatic and Non-Adiabatic Helical Capillary Tubes (2014)

Capillary tubes are used as refrigerant controlling devices, expansion devices and also as heart of a small vapor compression refrigeration cycle. It connects outlet condenser to the inlet evaporator and balances the refrigeration cycle pressure and controls the refrigerant mass flux. Capillary tubes are relatively cheap, resulting in extensive implementations in small household refrigerators and freezers with nearly constant refrigeration load. In general, the inner diameter and length of a capillary tube ranges are from 0.5-2.0 mm, and 2-6 m, respectively. In some vapor compression refrigeration cycle applications (VCRC), capillary tubes are coiled to minimize the space. The fluid flow in coiled capillary tubes is subjected to the centrifugal force which causes secondary flow effect. Some researchers have referred to Dean effect to describe this secondary flow. Dean Number is defined as De = Re (d/D)0.5 that affects the amount of heat transfer, momentum, and mass flux in both kinds of coiled tubes. Generally, in domestic refrigerators, the capillary tube and suction line is soldered to each other in order to increase the heat transfer between them, which results in increasing cooling capacity, and avoid the entering of liquid into the compressor. The refrigerant flow through capillary tube flow is notably affected by the heat transfer to the suction line. The adiabatic flow follows a path that is close to an isenthalpic line, whilst the non-adiabatic flow is projected toward the line of saturated liquid, increasing the amount of liquid in the two-phase mixture and decreasing the vapor quality at the evaporator inlet. As a consequence, the refrigerating effect in the evaporator is increased. This paper presents experimental study of refrigerant flow through helical non-adiabatic (diabatic) capillary tubes. This model is validated by previously published experimental data and also by test results performed and presented in this work for R-134a.The effect of capillary tube inner diameter, length, relative roughness and coil diameter, and also various test conditions such as inlet pressure, inlet temperature, and sub-cooling degree of refrigerants is investigated. Pressure distribution and temperature variation are obtained experimentally. Furthermore, main subject of this study is experimental investigation of diabatic helical capillary tubes. Therefore this experimental set up is used for investigation of metastable flow, re-condensation, choked and hysteresis phenomenon. The experimental results illustrated that critical ratio (Gc/Gs) is main parameters that affect metastable flow through coiled diabatic capillary tube. If mass flux in the coiled capillary tube (Gc) is constant, the rate of heat transfer from the capillary tube to the suction decreases with a decrease of the mass flux in the suction line (Gs). Therefore, an increase of the mass flux ratio represents a decrease of the heat transfer rate between the capillary tube and suction line. The results show that metastable flow in the diabatic coiled capillary tube with 1.397 mm inner diameter, 30 mm coil diameter,4360 mm length, 4 mm inner diameter of suction line, exists when the heat transfer rate between the coiled capillary tube and the suction line is weak with Gc/Gs>343.