Effect of Return Bend and Entrance on Heat Transfer in Thermally Developing Laminar Flow in Round Pipes of Some Heat Transfer Fluids With High Prandtl Numbers

The paper presents experimental results and analysis of heat transfer in a thermally developing region of round pipes for three fluids typically used as low temperature coolants, in the range of $0–−40°C$⁠. The experiments were performed at low Re (200–1000) and high Pr (80–140) numbers that are typically found in secondary refrigeration loop conditions. The effect of horizontal U-bend is also presented. It is shown that the positive effect of thermal development (high Nu number) lasts long because of the technically significant length of the thermally developing region. Secondary flows developed in and after the U-bend are so significant that they have almost an identical effect as the thermal development at the pipe entrance. That is a reason for the extremely good performance of the heat exchangers with secondary refrigerants in laminar flow regimes. Experimental data are presented with developed empirical correlations, which show good relationships to several existing correlations.