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|Numerical Analysis of Refrigerant Flow in Adiabatic Straight Capillary Tube
|Mittal, Madhup Kumar
|In the present work a mathematical model based on homogenous two phase flow is developed to predict the flow characteristics of refrigerants in adiabatic straight capillary tube. The mathematical model has been developed by using equations of conservation of mass, momentum, and energy. The refrigerant usually enters the capillary tube in liquid state and remains in liquid state for certain length. At a certain point during the flow, the flashing of refrigerant takes place and beyond this point, the flow becomes two-phase flow till the exit of capillary tube. During the single phase flow, the refrigerant properties remains almost constant, therefore conservation equations are solved analytically in single phase region. However, during the two phase flow, the refrigerant properties keep on changing as the flow progresses, therefore the conservation equations are solved by using finite difference method in order to take into account the variation of refrigerant properties in the two-phase region. Friction factor also plays an important role during the flow of refrigerant through capillary tube. Churchill (1994) correlation is used to calculate the friction factor. Duklers et al. (1964) viscosity correlation has been used to evaluate the two phase viscosity of the refrigerant. A computer program coded in MATLAB has been developed to solve the mathematical model. The developed model for straight capillary tube geometries have been validated with the experimental data of Melo et al.(1999), Fiorelli et al. (2002) and Jabaraj et al. (2006). Finally after results of validation, simulation has been done with different refrigerants to analyse their flow characteristics for different geometries at different operating conditions. The results of simulation reveal the following points. 1. The flow characteristics of R-12 and R-134a are close to each other at same condenser pressure and same degree of subcooling. 2. The flow characteristics of R-22 and R-407C are close to each other at same condenser pressure and same degree of subcooling. 3. The pressure drop for refrigerants R-410A, R-407C, M-20 and R-22 are close to each other at same condenser pressure and same degree of subcooling. 4. By varying the model input parameters it has been found that for all refrigerants, the mass flow rate increases with increase in degree of subcooling, increases as diameter increases, increases as condenser temperature increases, decreases as roughness increases and decreases as length increases.
|M.E. (Thermal Engineering)
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