INFLUENCE OF FLUID FLOW ON NUCLEATE BOILING FROM A TUBE

The mechanisms of nuclease boiling on the outside of a horizontal tube differ fundamentally from those on a flat plate. In this experimental work the variation of heal transfer coefficient around the periphery of a tube is measured with the aim of clarifying these mechanisms. A specially designed tube is used in which local variations are not masked by conduction through the metal surface. The tube diameter is 27 mm, and the working fluid is RII3 under saturated conditions at I atm. When there is no imposed velocity the peripheral variations are typically 10-20% with the maximum heal transfer coefficient at the base of the tube. At very low velocity of upward flow (similar to 0.1 m/s) there is a marked change in the variation, with the maximum coefficient occurring at a point about 70 degrees from the base. At higher velocities there is a slight increase in angle to the maximum point with maximum peripheral variations in heat transfer coefficient of around 25%. The variations are explained in terms of nucleate boiling at the base and top and flow boiling at the sides. The thin layer of two-phase bubbly flow at the sides leads to the predominance of heal transfer caused by sliding bubbles over other mechanisms in this region. The complex mix of mechanisms involved in boiling on tubes implies an inherent limit to the accuracy of predictive correlations.