Heat Transfer Engineering, 27(5):23–38, 2006 Copyright C  Taylor and Francis Group, LLC ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457630600559538 Transitional Heat Transfer in Plain Horizontal Tubes LAP MOU TAM Department of Electromechanical Engineering, University of Macau, Taipa, Macau, China AFSHIN J. GHAJAR School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma, USA In this study, the heat transfer behavior in the transition region for plain horizontal tubes under a uniform wall heat flux boundary condition is discussed in detail. In particular, the influence of inlet configuration and free convection superimposed on the forced convection (or mixed convection) at the start and end of the transition region and the magnitude of heat transfer are addressed. The available correlations to predict the heat transfer coefficient in the transition region are reviewed, and their performance are evaluated based on 1290 experimental data points obtained under a wide range of experimental conditions. Appropriate correlations for the mixed and forced convection transition regions are recommended. Finally, a flow regime map for determination of the boundary between forced and mixed convection in horizontal tubes with different inlets is presented. INTRODUCTION An important design problem in industrial heat exchangers arises when flow inside the tubes falls into the transition region. In practical engineering design, the usual recommendation is to avoid design and operation in this region; however, this is not always feasible under design constraints. The usually cited transitional Reynolds number range of about 2300 (the onset of turbulence) to 10,000 (fully turbulent condition) applies, strictly speaking, to a very steady and uniform entry flow with a rounded entrance. If the flow has the disturbed entrance typical of heat exchangers, in which there is a sudden contraction and possi- bly even a re-entrant entrance, the transitional Reynolds number range will be much different. This in turn influences the magni- tude of heat transfer coefficient in this region. The selection or development of an appropriate heat transfer correlation for the transition region in horizontal tubes requires an understanding of the factors that influence the start and end of the transition region and the behavior of heat transfer in this region. The objective of this paper is to first gain an understanding of the factors influencing heat transfer in the transition region Address correspondence to Dr. Afshin J. Ghajar, School of Mechanical and Aerospace Engineering, Oklahoma State University, 218 Engineering North, Stillwater, OK 74078. E-mail: ghajar@ceat.okstate.edu and then analyze the available heat transfer correlations for this region. HEAT TRANSFER BEHAVIOR IN THE TRANSITION REGION To gain a complete understanding of the heat transfer behav- ior in the transition region, the experimental results of Ghajar and his coworkers [1–3] will be examined. This is the most complete and systematic heat transfer experimental data set in the transi- tion region available in the open literature. They experimentally investigated the inlet configuration effects on developing and fully developed transitional forced and mixed convection heat transfer in a circular tube under a uniform wall heat flux bound- ary condition. Their experiments were conducted in a circular tube with a maximum length-to-inside diameter ratio of 385. In their experiments, distilled water and mixtures of distilled wa- ter and ethylene glycol were used. They collected 1290 exper- imental data points, and their experiments covered a local bulk Reynolds number range of 280 to 49000, a local bulk Prandtl number range of 4 to 158, a local bulk Grashof number range of 1000 to 2.5 × 10 5 , and a local bulk Nusselt number range of 13 to 258. The wall heat flux for their experiments ranged from 4 to 670 kW/m 2 . In their investigations, they used three types of inlet configurations, as shown in Figure 1: re-entrant 23