The effects of surface roughness on fully developed trans TRANSITIONAL FLOW FRICTION FACTORS IN HORIZONTAL CIRCULAR TUBES

Abstract

Clean energy is currently one of the largest research topics in modern society due to the increasing levels of pollution being produced by power cycles. Increasing the efficiency of these cycles is one of the methods which can be used to decrease the number of harmful by-products produced by such cycles. The efficiency of these cycles may be increased by optimizing the heat exchangers used within them. Designers of heat exchangers must compromise between heat transfer and pressure drop, where higher heat transfer is desirable whilst minimising the pressure drop. Previous research has shown that the best solution or compromise may be found within the transitional flow regime. These studies have been performed for developing and fully developed flow within smooth tubes and enhanced tubes. Although enhanced tubes have been investigated through micro-finned tubes and twisted tape inserts, limited work has been performed in roughened tubes which investigated simultaneous heat transfer and pressure drop characteristics. Therefore, the purpose of this study is to experimentally investigate the transitional and quasi-turbulent flow regimes, seeking characteristics through roughened tubes. Experiments were performed and validated with literature for smooth tubes as well as rough tubes to attempt to bridge the gap left within the available literature. The experiments were performed for Reynolds numbers between 500 and 10 000 whilst varying the constant surface heat flux boundary condition through a 5.14 mm inside diameter tube. The tubes which were used are that of one smooth tube which was then sand blasted to obtain two separate relative surface roughnesses of 0.0003 and 0.0006. The influence of the increase in surface roughness was investigated through pressure drop characteristics, simultaneous heat transfer and pressure drop characteristics and flow regime boundaries. A new schematic was developed which defines new regions where the influence of relative surface roughness changes, depending on the magnitude of the relative surface roughness.