Effect of bubble motion on local heat transfer around a tube across horizontal in-line and staggered tube bundles in bubbly and intermittent flows

Abstract

Cross-flow boiling in horizontal tube bundles occurs in kettle type evaporators. Convective heat transfer due to the motion of vapor bubbles is an important factor for boiling heat transfer in the evaporator under low heat flux conditions. To clarify the liquid agitation effect on heat transfer, the local heat transfer around a tube in in-line and staggered tube bundles was investigated in two-phase flows under adiabatic and atmospheric pressure conditions. Air and tap water were used as the working fluids. The test section was a vertical duct with inner dimensions of 90 × 90 mm2. In-line and staggered tube bundles each containing eight rows and five columns, were used as test sections. For both bundles, the tube diameter, d, was 18 mm, and the tube pitch, p, was 22.5 mm (p/d = 1.25). The local heat transfer had the highest values around θ = ± 90° where the liquid velocities were high in single-phase, bubbly and intermittent flows for both in-line and staggered arrays. A significant improvement in the heat transfer caused by the bubble motion was present for the in-line array as compared to the staggered array. Owing to the fluctuation of the liquid velocity, the heat transfer coefficient fluctuated significantly under intermittent flow conditions. In the bubbly flow at p/d = 1.25, the average heat transfer coefficient around a tube for the in-line array was higher than that for the staggered array. In contrast, the heat transfer coefficient of the staggered array was high under intermittent flow at a lower gas flow rate. This tendency is different from the results at p/d = 1.5. With a decrease in the relative size between the bubble diameter and the tube gap, there was a high improvement in the heat transfer coefficient due to the liquid agitation in the bubbly flow.