Results of experimental investigation of heat transfer with emulsions with low-boiling disperse phase

Abstract

The results are given of a theoretical investigation of heat transfer from thin wires to emulsions in which the disperse phase is low-boiling compared to the dispersion medium. A special feature of heat transfer to such emulsions is observed in the mode of nucleate boiling, when only the droplets of the disperse phase of emulsion are boiling. The transition from the convective mode of heat transfer to nucleate boiling requires high values of temperature gradient ΔT = T W -T 0 at which the temperature of the heat-transfer surface T W is higher than the temperature T S of saturated vapors of the disperse phase by 100 °C and more. Here, T 0 is the temperature of the heat-transfer agent away from the heat-transfer surface. Note that, for pure liquids and solutions, the value of delay of the beginning of boiling ΔT i = T W -T S as a rule does not exceed 1-5 °C. Another feature of heat transfer to the emulsions being treated is a wide temperature range of the mode of nucleate boiling (over 100 °C); the upper limit of this temperature range is defined by the temperature of transition to film boiling of the dispersion medium of the emulsion being investigated that has already boiled by this instant of time. The value of delay of the beginning of boiling is affected significantly by additions of surfactants and adsorbents.

Effect of the droplet size of an emulsion dispersion phase in nucleate boiling and emulsion boiling crisis

“…Experiments were conducted in the temperature range from 23 to 25°C on the experimental setup presented in Ref. [4]. The onset of a nucleate boiling crisis was accompanied by the burnout of a wire heater.…”

“…The boiling of emulsions with a low-boiling disperse phase is accompanied by a number of peculiarities [1][2][3][4][5][6]. These are high superheating of droplets, wide, as compared with pure liquids, intervals of nucleate boiling, and high values of heat transfer coefficients.…”

“…Since the emulsion under investigation was opaque, a photoflash was used during the filming. The investigations were conducted on the experimental setup described in [4]. A coarse-grained emulsion with an average diameter of disperse-phase droplets to 20-30 lm was prepared with a propeller mixer, and fine-grained emulsion with an average droplet diameter of 1-2 lm in ultrasound field with a frequency of 22 kHz.…”

Boiling of small droplets

“…Droplet radii are chosen in the range 2 6 R d 6 15 lm, which are typical of emulsions used in heat transfer research [3,23,24]. Three combinations of fluids are simulated.…”

“…Pentane is typical of light hydrocarbons that have little or no solubility in water and are used in emulsion boiling research [24]. FC-72 is a 3M TM Fluorinert TM electronic liquid that has properties similar to refrigerants such as R-113 and is taken as typical of heat transfer liquids.…”