An experimental study on the pressure drop and heat transfer through straight and curved small diameter tubes

A tube type heat exchanger is often the only solution when minimum pressure loss is a requirement. In addition, small diameter tubes are preferable because of an increased heat transfer area within an acceptable pressure loss limit. The present work reports on both an analytic model and experimental results with regards to the pressure drop and heat transfer characteristics of compact straight, C-curved, and U-curved tubes. The inner diameter of the tube (D) for our selected heat exchanger type was 1.26 mm with a thickness of 0.12 mm and a total length of 150.8 D. For the experiment, pressurized nitrogen gas bottles were used rather than an air compressor system in order to simplify the test facility. Hence the pressure conditions were easily set at 10, 30, and 50 bar corresponding to a range of Reynolds numbers from 10000 to 50000. To elevate the air temperature outside the tube (from 100°C to 400°C), an electric furnace was installed around the “test tube”. An analytic model to determine the pressure loss through curved tubes-referred to as the modified friction factor- is proposed. Good agreement was found between the modified friction factor and existing correlations, thus confirming the suitability of this model for determining pressure losses for different shape of tubes. The average measured Nusselt numbers were within 10- 15% of the Dittus-Boelter and Gnielinski correlations.