Research Papers: Evaporation, Boiling, and Condensation

Heat Transfer and Boiling Crisis at Droplets Evaporation of Ethanol Water Solution

[+] Author and Article Information
S. Y. Misyura

Institute of Thermophysics Siberian Branch,
Russian Academy of Sciences,
Novosibirsk 630090, Russia
e-mail: misura@itp.nsc.ru

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 21, 2015; final manuscript received June 1, 2016; published online June 28, 2016. Assoc. Editor: Amitabh Narain.

J. Heat Transfer 138(11), 111501 (Jun 28, 2016) (8 pages) Paper No: HT-15-1290; doi: 10.1115/1.4033796 History: Received April 21, 2015; Revised June 01, 2016

Droplets evaporation and boiling crisis of ethanol water solution were studied experimentally. At intensive nucleate boiling within a droplet, most evaporation relates to an increase in the area of the wetting droplet surface and only 10–20% of evaporation relates to the effect of diffusion and a change in the thermal–physical coefficients. In alcohol solution with mass salt concentration C0 = 25–35%, maximal instability of the bubble microlayer is observed. The critical heat flux behaves nonmonotonously due to changes in mass alcohol concentration in the solution, and there are two extrema. The maximal value of sustainability coefficient at droplets evaporation of ethanol solution corresponds to C0 of 25–30%. The heat transfer coefficient of ethanol water solution of droplet in the suspended state decreases with a rise of wall overheating and spheroid diameter. Experimental dependence of the vapor layer height on wall overheating at boiling crisis was observed. The height of this layer at Leidenfrost temperature was many times higher than the surface microroughness value. The liquid–vapor interface oscillates, and this extends the transitional temperature zone associated with a droplet's boiling crisis.

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Fig. 1

Total time of evaporation of the droplets of ethanol water solution τ1 versus wall temperature Tw (V0 = 100 μl) for different initial mass concentration of ethanol C0

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Fig. 2

A change in droplet mass m with time τ for different mass alcohol concentrations C0 (V0 = 100 μl, Tw = 80 °С)

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Fig. 3

The rate of droplet mass alteration for different C0 (without consideration of the area of droplet wetting base S; V0 = 100 μl, Tw = 80 °С)

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Fig. 4

The rate of droplet evaporation for different C0 (with consideration of the area of droplet wetting base S; V0 = 100 μl, Tw = 80 °С)

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Fig. 5

A change in the droplet interface temperature (liquid–gas) Ts with time τ (V0 = 100 μl, pure water)

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Fig. 6

A change in the critical heat flux qcr for the droplets of ethanol water solution versus initial mass concentration of ethanol C0 (V0 = 100 μl): 1—experimental data; 2—calculation by formula (1)

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Fig. 7

A change in sustainability coefficient k versus mass alcohol concentration C0

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Fig. 8

Formation of liquid–gas foam at boiling the alcohol–water mixture

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Fig. 9

Thermal images of nucleate boiling in a droplet of alcohol–water solution and pure water (Tw = 115 °C, V0 = 100 μl, C0 = 30%) (scale is )

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Fig. 10

Thermal images of vapor bubbles within the droplet with multiple magnifications (scale for 5 s is ; scales for 50 s and water are )

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Fig. 11

Thermal images (τ = 1 s) for heating the droplet of (a) pure water and (b) alcohol–water solution with C0 = 30% (scaleis )

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Fig. 12

A change in the heat transfer coefficient α versus wall overheating ΔТw for different liquids: 1—water; 2—ethanol; 3—CCl4; 4—C6H6; 5—water; 6—water solution of ethanol (C0 = 92%); 7—water solution of ethanol (C0 = 27%); 8—water solution of ethanol (C0 = 64%). Point 3, 4—experimental data[1].

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Fig. 13

A generalization of heat transfer during film boiling using dimensionless coordinates A and B: curve 1—А = 7.9(B)−0.6 [1]; curve 2—generalization of experimental data of the current study А = 7(B)−0.8; 3—water solution of ethanol C0 = 92%, ΔТw = 110 °C; 4—water solution of ethanol C0 = 70%, ΔТw = 110 °C; 5—water, ΔТw = 110 °C; 6—water, ΔТw = 70 °C; 7—experimental data of Ref. [1] for water, CCl4, C6H6

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Fig. 14

A change in the heat transfer coefficient α of a liquid droplet for film boiling (V0 = 100 μl): curve 1—theoretical calculation for water by Eq. (7) according to Ref. [2]; 2—pure water; 3—water solution of ethanol C0 = 30%; 4—water solution of ethanol C0 = 70%

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Fig. 15

A change in vapor layer thickness δv depending on wall overheating ΔTw, curve 2—maximal height of microroughness of the heater wall

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Fig. 16

Surface microroughness of the heated wall

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Fig. 17

(a) Interface oscillations; (b) separation of liquid and circulation of gas; and (c) no contact between liquid and wall

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Fig. 18

Thermal image of spheroid of ethanol water solution (C0 = 30%) (scale is )




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