ISSN Online: 2377-424X
ISBN Print: 978-1-56700-421-2
International Heat Transfer Conference 15
Analysis of Two-Phase Flow Behavior in a Sharp Return Bend Using Capacitive Measurements
Abstract
Fin-and-tube heat exchangers are often used as evaporators in HVAC&R systems. To enhance the compactness, the tubes of these heat exchangers are folded up in a sequence of hairpin tubes. These hairpins consist of two straight tubes interconnected by a sharp U-bend. In this work the evolution the two-phase flow behavior up- and downstream of a sharp U-bend is studied using a capacitive sensor. On 3 upstream
locations and 5 downstream locations on the hairpin, the capacitance of the flow is measured. The
capacitance time traces are analyzed to determine whether there is an effect of the return bend present in the
flow behavior. The hairpin tube has an inner diameter of 8 mm and a bend radius of 11 mm. The refrigerant
used in this study is R134a, the mass flux was varied between 200 and 400 kg/m2s. The tubes up-and
downstream of the return bend are horizontal and the plane of the bend is placed vertical. Downward, as well
as upward flow in the bend was tested. Three flow regimes were observed: slug flow, intermittent flow and
annular flow. Downstream of the bend, a disturbance of the flow behavior was observed up to 22 tube diameters downstream. For each flow regime the effect of the bend on the flow regime was slightly different. However, for each flow regime the effect was the same for up-and downward flow. Upstream of the return bend, the disturbance was found to be mostly negligible.