Extended surface on heat transfer is the most method adopted. To acknowledge the key factor in heat transfer, the mechanism of laminar convective heat transfer enhancement has been experimentally investigated by measuring heat transfer data in spirally integral bent fin tubes at various Graetz numbers. To maintain laminar flow status, lube oil is used as the working fluid and water as the coolant. Graetz number ranged from 50 to 400. For the smooth annular tube, the result of the analysis is different from Sieder-Tate equation as expected. The influence of different cross sections using hydraulic diameter is no longer suitable for heat transfer in annular pipe. The effects of fin height and pitch on heat transfer and friction factor are discussed in detail. The experimental results indicate that the effect of fin height appears to be more crucial on heat transfer and high fins are not recommended at low Graetz number. There is an inflection point along the curve of thermohydraulic performance which is proved a maximum point and tends to be affected by geometric parameters. A novel heat transfer formula is proposed based upon collecting data, above geometry parameters are taken into account.
