Heat transfer in cross flow of gas over a smooth and modified tube

In this work flow and heat transfer are studied experimentally and numerically for a hot tube in a cross-flow of air, when the tube is placed in a relatively narrow channel. The experimental set-up features a stainless steel tube 20 mm in outer diameter, placed in a channel with blockage ratio of 0.61. A number of flow velocity values are explored, yielding the Reynolds numbers in the range 2,750 < Re < 15,240 based on the tube diameter. The wall temperature of the tube is measured at seven locations along the circumference of the tube, from the stagnation point to an angle of 180°. Simulations are performed for the flow and heat transfer in the experimental set-up. A good agreement is reached between the experiments and the numerical simulations, in terms of the tube temperature distribution, while using the k-ω SST turbulent model, especially for higher flow velocities. In addition, the model is validated using experimental results reported in the literature. In a tube of the same material and dimensions, about 2800 shallow spherical dimples were drilled in a staggered arrangement, having the depth of 0.25 mm and the radius of curvature of 4.5 mm. Comparative experiments examined the effect of the dimpled tube on the heat transfer. The experimental results show up to 10% enhancement of the heat transfer rate from the dimpled tube.