EXPERIMENTAL INVESTIGATION FOR THE FLOW OF R32 IN A CAPILLARY TUBE

Abstract

The present research work has been carried out to investigate the flow characteristics of the refrigerant R32 while flowing through a straight and a spiral capillary tube under adiabatic conditions. The experiments were carried out for refrigerant R32 flowing through the capillary tube. The investigation for partially condensed (0.05 < x < 0.30) flow is carried out for a straight capillary, and the subcooled (2 0C to 8 0C) flow is carried out for a straight and spiral capillary tube. The capillary tube test-sections were prepared with capillary tube diameter and length combinations. A total of three capillary tube diameters (1.12 mm, 1.27 mm, and 1.52 mm) were combined with three capillary tube lengths (1000 mm, 1500 mm, and 2000 mm) to prepare a testsection. Nine test-sections were prepared for a straight capillary tube. The spiral capillary testsections were prepared by considering the above dimensions combined with three spiral pitches (32 mm, 48 mm, and 64 mm). Therefore, a total of twenty-seven spiral test-sections were prepared by considering parameters such as (diameters, lengths, and spiral pitches). In fact, thirty-six testsections were prepared. All the test-sections were tested for three capillary inlet pressures (1586 kPa, 1655 kPa, and 1724 kPa). The experimental investigation was carried out for the partial condensed and subcooled condition of refrigerant R32 at the entry of the capillary tube. The parametric study was conducted by varying the operating conditions and geometric parameters of the capillary testsections. The data of the partial condensed flow of refrigerant show that the mass flow rate is higher for the refrigerant quality of 0.05. The increase in refrigerant quality decreases the mass flow rate. The mass flow rate increases by increasing the capillary tube diameter and the capillary inlet pressure. The mass flow rate of refrigerant R32 decreases with the increases in the capillary length and L/d ratio. The results also show that the mass flow rate increases when the degree of subcooling of refrigerant increases from 2 0C to 8 0C. The higher diameter capillary tube has a higher refrigerant mass flow rate for the constant inlet pressure and given capillary tube length. For the spiral pitch geometry of the capillary tube, the mass flow rate is higher for the higher spiral pitch. Further, as the length of a capillary tube is cut short, the refrigerant mass flow rate through the capillary tube increases. The experimental data obtained from the present work have also been used to develop a nondimensional correlation for predicting the mass flow rate of R32 through a capillary tube. The nondimensional equation was established by considering the geometry and operating parameters of a test-section, vaporization, and the bubble's growth in the flow of refrigerants; by viewing all these effects and using the Buckingham’s-π theorem for the Non-dimensional analysis was carried out with the Multiple variable regression method. As stated by π theorem, if the total parameters for any physical phenomenon involve ‘n’ numbers of dependent and independent variables and ‘m’ fundamental dimensions, the quantity of the parameters are arranged in the form of (n-m) dimensionless π-groups. Consider the X1 variable depends on the independent variables X2, X3, X4…….Xn. The mathematical representation of a functional relationship between them may be expressed as X1 =f1 {X2, X3, X4………..Xn} The quantities X1, X2, X3, X4………..Xn was set into separate groups known as π- groups. The π- groups can be written as π1 =f2 { π2, π3, π4………… π n-m} The non-dimensional correlations for the partial condensed, subcooled, and spiral subcooled flow are given below: π1, partial condensed = 0.78 (π2) 2.23 (π3) -0.85 (π4) -0.89 (π5) -3.54; 𝜋􀬵, 􀯦􀯨􀯕􀯖􀯢􀯢􀯟􀯘􀯗 = 0.0506 (𝜋􀬶)􀬵.􀬶􀬹􀬵(𝜋􀬷)􀬿􀬴.􀬵􀬺(𝜋􀬸)􀬴.􀬵􀬼(𝜋􀬺)􀬿􀬵.􀬶􀬺􀬽 ; 𝜋􀬵, 􀯦􀯣􀯜􀯥􀯔􀯟 􀯦􀯨􀯕􀯖􀯢􀯢􀯟􀯘􀯗 = 0.45 (𝜋􀬶)􀬵.􀬶􀬺(𝜋􀬷)􀬿􀬴.􀬵􀬺(𝜋􀬸)􀬴.􀬵􀬽(𝜋􀬹)􀬿􀬵.􀬵􀬽 (𝜋􀬺)􀬴.􀬶􀬹(𝜋􀬻)􀬿􀬴.􀬴􀬵(𝜋􀬼)􀬿􀬴.􀬴􀬶(𝜋􀬽)􀬴.􀬵􀬶 The above correlation shows a good agreement with the experimental mass flow rate. The equation for the flow through a partially condensed straight capillary tube predicts 96% of the experimental data within an error band of ±15%. The correlation equation for the subcooled flow through a straight capillary tube predicts almost all the data within an error band of ±10%. The correlation equation for the spiral subcooled flow through a different spiral pitch of the capillary tube predicts all data within an error band of ±12.5%.