We report on an experimental study conducted to investigate the flow behaviour in a heat exchanger pipe submerged in a hot stagnant fluid. Particle Image Velocimetry (PIV) was used to measure the two-dimensional velocity field in the mid-vertical plane of the tube. Fluid temperatures in the cross-sectional plane were also measured using thermocouples. The mode of heat transfer into the pipe was mixed convection where both inertia and buoyancy contributed to the convection. The results show that when the contribution of buoyancy-driven flow (natural convection) was smaller than that of the inertia-driven flow (forced convection), in an originally turbulent flow, the shear-induced turbulence dominated the flow and the turbulent velocity profile was not influenced by the heat input. In an originally laminar flow, the role of buoyancy was primarily limited to the initiation of instabilities in the laminar flow to trigger the turbulence transition. The temperature profiles indicate the presence of stably stratified layer inside the pipe in originally laminar flow regime that suppressed the heat transfer rate. In originally turbulent regime, the fluid temperature field was nearly uniform indicating efficient flow mixing.

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