Mixed convection around two vertically aligned horizontal cylinders: A numerical, experimental, and modeling investigation on the effect of local conditions on heat transfer

Experimental and numerical studies were performed to assess the effects of buoyancy driven flows from lower cylinders in a tube array on the heat transfer of higher-elevation cylinders. First, PIV and LIF experiments were performed for a single 13.4 mm diameter cylinder submerged in water at atmospheric conditions to measure velocity and temperature fields. Then the effect of buoyancy from a lower heater was studied by arranging two horizontal cylinders at different vertical displacements from each other (P/D = 1.5, 2.15, 6.45). Wall temperatures were measured as a function of pitch and power. CFD analysis was performed for the same configurations using an LES model. Using a combination of CFD and experimental measurements, mechanistic models for velocity and temperature behaviour in the plume were developed. These models are used to determine the impact of buoyant plumes from cylinders at lower elevations on heat transfer from heated cylinders at higher elevations. Since the plumes from cylinders at lower elevation are imposed on the natural convection from higher-elevation cylinders there is a combined effect from the momentum and temperature of the lower plume as it interacts with the natural convection phenomena from the upper-elevation heater. A model to predict the Nu_D of the upper cylinder in the case of horizontal vertically aligned pair of tubes was developed based on superposition of the convection caused by the lower cylinder and the natural convection from the higher-elevation cylinder. This model was found to predict the cylinder Nu_D with an RMS error of 2.8 and an absolute deviation of 2.31 as compared to the experimental data obtained in this study. It was also found to agree well with independent data available in literature.