Numerical Investigation of Capillary Tube Diameter and Thermostatic Expansion Valve Effects on the Performance of an R-134a Automotive Air Conditioning System

Abstract

Automotive air conditioning (AC) systems require an effective expansion device to regulate refrigerant mass flow and achieve high cooling capacity and energy efficiency. This study aims to investigate the effects of capillary tube diameter and compare its performance with a Thermostatic Expansion Valve (TXV) in an R-134a automotive air conditioning system using MATLAB-based numerical simulation. The mathematical model was developed based on the vapor-compression refrigeration cycle by incorporating mass and energy conservation, the Darcy–Weisbach equation for pressure drop, the Blasius correlation for friction factor, a compression-ratio-based compressor model, and an evaporator effectiveness approach. Capillary tube diameters of 0.8, 1.0, 1.2, and 1.4 mm were evaluated at an evaporator temperature of 5 °C and a condenser temperature of 45 °C. The simulation results show that increasing the capillary tube diameter increases the refrigerant mass flow rate from 0.00268 to 0.01212 kg/s while reducing the pressure drop from 694.22 to 606.63 kPa. Consequently, the cooling capacity increases from 0.330 to 1.604 kW, whereas the compressor power increases from 0.088 to 0.320 kW, resulting in an improvement in the Coefficient of Performance (COP) from 3.751 to 5.013. Under the same operating conditions, the TXV model produced a COP of 2.955. The difference is mainly attributed to the assumptions adopted in the numerical model; therefore, the proposed model is more appropriate for parametric analysis and preliminary design optimization than for direct representation of actual operating conditions.