Temperature and Velocity Measurements in Sub-cooled Boiling through a Vertical Rectangular Test Channel

Abstract

Temperature and velocity measurements are performed in sub-cooled boiling experiments through a single-heated side rectangular test channel at various conditions. The temperature measurements involve the use of the Laser Induced Fluorescence (LIF) method with one heated single-phase (3.9 kw/m2 ) and two boiling conditions (30.9 and 36.6 kw/m2 ). Two flow rates are considered, for each heating level, and given by the Reynold’s number (Re) of 8121 and 20523. For the boiling cases, bubbles are allowed to slide along the heated surface and temperature fields are obtained along the bubbles sliding distance in the axial direction. Radial temperature profiles are mapped out to display the temperature distributions at various axial locations. The resultant temperature fields reveal insightful information about the temperature distribution in each condition. Boiling bubbles induce major changes to the temperature fields. In regions far from the heated wall, the temperature profiles are seen to fall within the range of the bulk fluid temperature values (21.6 ºC ± 1 ºC), as registered by the means of thermocouples. A large increase in temperature is observed near the heated wall (and at the sliding distance) to approximately 29.6 ºC ± 2 ºC. The reference temperature values obtained for this region by the infrared camera are seen to fluctuate between 30.9 and 31.9 ºC. A higher discrepancy is observed near the heated wall area due to weaker fluorescent signals can be captured in this region. The thermal boundary layers are shown to grow thicker with higher heating input and higher flow rates. The liquid velocity measurements are performed with the Particle Imaging Velocimetry (PIV) method. The liquid turbulence statistics are obtained for a heated single-phase (2.7 kw/m2 ) and two boiling conditions of varying heat flux (16.4 kw/m2 and 42.8 kw/m2 ). The same Reynold’s numbers used in temperature measurements are considered here as well (8121 and 20523). Velocity components, turbulence intensities, and Reynold’s stresses are obtained against the radial distance from the heated wall. The effect of the boiling bubbles on the flow structural behavior is clearly evident, as the liquid velocity values and turbulent intensities appear to increase near the bubble regions. This increase in liquid velocity is a function of the bubble sizes and flow rates. The sliding bubbles tend to grow in size and gain momentum as moving along the heater. The velocity boundary layer thickness is shown to grow with higher heat flux and bigger bubble’s size. This increase in the layer thickness is concomitant with the axial distance.