Evaluation of the flow at the contraction of a heat exchanger. Pt. 2. Effect of thermal-hydraulic factors on scale deposition at the contraction

In heat exchangers used in power plants, scale may deposit on the tube support plates of heat transfer tubes, especially at the leading edge where the flow passes a sudden contraction. This phenomenon can lead to flow path blockage, which in turn can affect plant performance. As a result, the mechanism of scale deposition and growth needs to be clarified. This phenomenon is assumed to be caused by a complex of thermal-hydraulic and electrochemical factors. In this study, flashing induced by pressure drop and turbulence at the leading edge of a contraction were assumed to be the main factors from the thermal-hydraulic point of view. And these factors in two different type of contractions were evaluated with a High Pressure / High Temperature steam-water two-phase flow experiment and 3D numerical analysis. Considerable differences in amount of steam caused by flashing and turbulence magnitude were observed between the two contractions which have same flow path area but different hydraulic diameter. It was also found that the size of bubbles passing the leading edge of contraction were smaller than 1 mm, while the bubbles in the upstream part were more than 10 times larger than those of the leading edge. (author)