The unusual behavior of local heat transfer coefficient in a circular tube with a bell-mouth inlet

The local heat transfer characteristics for ethylene glycol water mixtures flowing in a horizontal circular straight tube with a bell-mouth inlet have been determined experimentally over a flow Reynolds number range of 1500–27,000. A wall-boundary heating condition of uniform heat flux was imposed. The variation of local heat transfer coefficient with length in the transition and turbulent flow regimes is very unusual. For the bell-mouth inlet, the boundary layer along the tube wall is at first laminar and then changes through a transition region to the turbulent condition causing a dip in the Nu– x/ D curve. The length of the dip in the transition region is much longer than that in the turbulent region. For our experiments with a fixed inside diameter of 1.584 cm, the length of the dip in the transition region varied from x/ D=100–175 in comparison to an x/ D