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.

Heat Transfer Correlations for Thermally Developing Flow in Rectangular Micro-Channels Subject to Four-Sided and Three-Sided H1 Boundary Conditions

In this study, heat transfer to laminar hydrodynamically fully developed flow in the thermally developing region of rectangular micro-channels is analyzed for four-wall and three-wall circumferentially uniform temperature and axially uniform heat flux (H1) boundary conditions. The temperature and wall heat flux distributions are solved numerically and used to calculate the local average Nusselt number. Based on the numerical results, new correlations capable of predicting heat transfer in the thermally developing region continuously into the fully developed region are developed. The correlations are compared with other correlations and available experimental data, and show good agreement.