(401f) Vaporization of a Single N-Pentane Liquid Drop in a Flowing Mmicsible Liquid Media

Heat exchange in a direct contact process that involves bubbles, drops or particles as a dispersed phase is a complex phenomenon. This is due to the wide range of variables such as phase velocities, phase temperatures, the physical and thermodynamic properties of both phases and system holdup or void fraction.

Vaporization of a single drop in immiscible liquid represents the basis of the three-phase direct contact heat exchanger which has several advantages over surface or classical type heat exchangers. Generally, can operate at very low temperature differences between the fluid streams, even as low as . The absence of the internal separating wall between fluid streams leads to the elimination of fouling and corrosion problems, as well as an increase in the heat transfer rate between the fluids. It has very high heat transfer coefficient (about 20 – 100 times more than a single phase heat exchanger) because of the large heat transfer area and the latent heat effect due to phase change.

The direct contact process simply occurs when bubbles or drops are injected into a continuous phase. There is a great similarity in the heat transfer mechanism for drops and bubbles in a direct contact process. In both cases, a two-phase bubble/drop is formed just after the dispersed phase first contacts the continuous phase. This configuration, (two-phase bubble/drop) arises from two instantaneously separating parts: a vapour phase, which normally concentrates at the top of the structure, in thermal equilibrium with a liquid phase at the bottom, because of the effect of gravity.

In this paper, vaporization of a single n-pentane drop in a direct contact with another flowing immiscible liquid (warm water) has been experimentally investigated. The experiments were carried out utilising a cylindrical Perspex tube of 10 cm diameter and 150 cm height. Saturated liquid n-pentane and warm water at 45^oC were used as the dispersed phase and continuous phase, respectively. Photron FASTCAM SA 1.1, high speed camera (75,000f/s) with software V. 321 was implemented during the experiments to measure the rise and growth velocity of the two-phase bubble-droplet from the point detached the nozzle tip to the end of evaporation was recorded. Each run was studied frame by frame to give the exact time of evaporation. . Five different continuous phase flow rates (10, 20, 30, 40 and 46 L⁄h) were used in the study. The results indicated that the as evaporation proceeded, the vapour accumulated at the top and the liquid part seemed to stay at the bottom of the bubble-droplet system. The liquid part could be recognized only up to about 8%, after that it occupied a relatively small volume of the system. Before reaching this stage, the system oscillated and the liquid sloshed from side to side. In addition, increase in flow rate of the continuous phase results in increasing the two-phase bubble.