Comparison of the heat transfer characteristics of developing and fully developed flow in smooth tubes in the transitional flow regime

Abstract

The transitional flow regime has been mostly avoided by designers due to uncertainty and perceived chaotic behavior. However, changes in operating conditions, design constraints or additional equipment can cause the flow to operate in the transitional flow regime. Previous work done in the transitional flow regime did not focus specifically on how the heat transfer characteristics change as the flow develops across the tube length. Therefore, the purpose of this study is to investigate and compare the heat transfer characteristics of developing and fully developed flow in the transitional flow regime and is work in progress. An experimental set-up was designed, built and validated and heat transfer measurements were taken at a heat flux of 2 kW/m2 between Reynolds numbers of 700 and 10 000. The Nusselt numbers varied between 10 and 68, the Prandtl number between 4.9 and 6.8, the Grashof number between 1.89x103 and 3.14x104 and the Rayleigh number between 9.66x103 and 2.24x105. It was found that the width of the transitional flow regime decreased along the tube length as the flow approached fully developed flow. Once the flow was fully developed, the width became negligible as the flow fluctuated between the laminar and low-Reynolds-number-end regimes.