Heat transfer and pressure drop characteristics of smooth tubes at a constant heat flux in the transitional flow regime

Abstract

Due to constraints and changes in operating conditions, heat exchangers are often forced to operate under conditions of transitional flow. However, the heat transfer and flow behaviour in this regime is relatively unknown. By describing the transitional characteristics it would be possible to design heat exchangers to operate under these conditions and improve the efficiency of the system. The purpose of this study was to experimentally measure the heat transfer and pressure drop characteristics of smooth tubes at a constant heat flux in the transitional flow regime. The measurements were used to describe the flow behaviour of this regime and attempt to develop a correlation that can be used in the design of a heat exchanger. An experimental set-up was developed, consisting of an overall set-up, a removable test section as well as a controller, which ensured a uniform heat flux boundary. The test section allowed for the measurement of the temperature along the length of the test section, the pressure drop across the test section, the heat flux input and the flow rate. The measurements were used to determine the heat transfer coefficients and friction factor of the system. Three test sections were developed with outer diameters of 6, 8 and 10 mm in order to investigate the influence of heat exchanger size. Each test section was subject to four different heat flux cases of approximately 1 500, 3 000, 4 500 and 6 000 W/m2. The experiments covered a Reynolds number range of 450 to 10 300, a Prandtl number range of 4 to 7, a Nusselt number range of 2.3 to 67, and a Grashoff number range of 60 to 23 000. Good comparison was found between the measurements of this experiment and currently available literature. The experiments showed a smooth transition from laminar to turbulent flow with the onset of transition dependent on the heat flux of the system and with further data capturing, a correlation can be found to describe the Nusselt number in the transitional flow regime.