Direct contact evaporation of a single two-phase bubble in a flowing immiscible liquid media. Part II: convective heat transfer coefficient

The direct contact evaporation of n-pentane volatile liquid drop in a warm flowing immiscible liquid (water) has been investigated experimentally. A Perspex column with a 10 cm internal diameter and 100 cm active height was used in the experiments. N-pentane at its saturated temperature (~36 °C) and distilled warm water were utilised as a continuous and dispersed phase. The warm water, with three different Jacobs numbers (Ja), (Ja = 6.1, 23 and 46.3), flows from the top of the column and leaves from the bottom at three different Reynolds numbers (Re = 3250, 6500 and 9750). The evaporation of the drop while rising along the column was filmed with a Photron FASTCAM high-speed camera (∼65,000 f/s). All images were analysed using AutoCAD, and the two-phase bubble, the vaporisation ratio (𝑥) and the half-opening vapour angle (β) were measured. The convective heat transfer coefficient in terms of Nusselt number (Nu) was predicted based on the measured two-phase bubble radius through the experiments. The effect of Reynolds’s number (Re), Jacobs’s number (Ja), vaporisation ratio (x), and diameter ratio (B) on Nu were investigated. The experimental results revealed that Nu increased with time. The Re and Ja significantly affected the time-dependent Nu. Although the final Nu was nearly the same for all cases (Nu = 21), the higher the continuous phase Re, the higher the Nu, especially with the progress of evaporation (τ ≥ 70). In addition, the results showed that Ja inversely influenced the average Nu, and the final value of Nu depended strongly on Ja. The higher the Ja, the lower the average Nu and the shorter the time for complete evaporation. In this regard, the dimensionless time (τ) required for complete drop evaporation was about 38, 60 and 120 for Ja of 46.3, 23 and 6.1, respectively.