| Analysis of fluid friction and heat transfer in low Reynolds number flow heat exchangers is undertaken. Three configurations typically utilized in compact heat exchangers are examined. These are: the plain non-circular duct of constant cross-sectional area, the offset or interrupted strip fin, and the turbulator strip. Analytical models for each of these geometries are developed by combining asymptotic solutions using simple non-linear superposition.; Models for predicting the friction factor---Reynolds number product, f Re, and Nusselt number, Nu, in non-circular ducts for hydrodynamically fully developed flow (HFDF), hydrodynamically developing flow (HDF), thermally fully developed flow (TFDF), thermally developing flow (TDF), and simultaneously developing flow (SDF) are developed. Thermal and hydrodynamic entrance models are developed by combining the asymptotic solutions for small and large values of the dimensionless duct length. Through the use of a novel characteristic length, the square root of the cross-sectional flow area, scatter in the dimensionless data for fully developed laminar flows is considerably reduced. Most numerical and analytical data are predicted within +/-10% for HFDF and TFDF, +/-12% for HDF and TDF, and +/-15% for SDF for most non-circular ducts.; Simple analytic models for predicting the Fanning friction factor, f, and Colburn j factor of two common enhancement devices, the offset strip fin and the turbulator strip are developed from fundamental solutions of fluid dynamics and heat transfer. Models for the offset strip fin are valid over the full range of Reynolds numbers for rectangular and other non-circular sub-channel cross-sections. Model predictions for the offset strip fin agree with published experimental data within +/-20%. Models for the turbulator strip are valid over the full Reynolds number range for both straight and curved turbulator profiles. Model predictions for the turbulator strip agree with new experimental data to within +/-20%.; Finally, a detailed experimental study of the thermal and hydraulic characteristics of turbulator strips is undertaken. Simple design correlations are presented along with a performance evaluations of each device using the constant mass flow rate and constant pumping power criteria. |
