Two-phase heat transfer and bubble dynamics in a microchannel array

Heat transfer coefficients and bubble dynamics are reported for two-phase water flow in an array of 13 equally spaced microchannels over an area of 1 cm². Each channel has D_h = 451 ± 3 8 μm, W/H = 0.8, and L/D_h = 22.2. Uniform heat flux is applied through the base, and wall temperatures are determined from thermocouple readings corrected for heat conduction effects. The upper surface is insulated and transparent. Single-phase heat transfer coefficients are obtained for 216 ≤ Re ≤ 2530 and 216 ≤ G ≤ 4100 kg/m²s and are in good agreement with comparable trends of existing correlations for developing flow and heat transfer, although a difference is seen due to the insulated upper surface. Two-phase experiments are run to determine overall heat transfer coefficients and bubble dynamics for a mass flux of 221 ≤ G ≤ 466 kg/sm² and heat flux of 25 ≤ q ≤ 178 W/cm². Heat transfer coefficients normalized with mass flux exhibit a trend comparable to that of available studies that use similar thermal boundary conditions. Two-phase flow visualization via shows expanding vapor slug flow as the primary flow regime, but bubbly flow and nucleation leading to elongated bubble flow are also observed. Analysis of bubble dynamics reveals a t^1/3 dependence for bubble growth, and flow reversal is observed and quantified. Different speeds of the phase fronts are observed at the leading and trailing edges of elongated slugs once a bubble diameter equals the channel width. Bubble formation, growth, coalescence and detachment at the outlet of the array are characterized by the Weber number.