Thermal Stress: Evaporation and Boiling

DOE-HDBK-3010-94

3.0 Liquids; Aqueous Solutions

ligaments formed from surface disruption); droplets generated by all three

droplet generation mechanisms (i.e., bubble bursting, splashing, or foaming).

Three regions as a function of axial distance from the pool surface were identified:

near-surface region: immediate vicinity of surface; entrainment dependent on

height and gas velocity; entrainment consists of all droplets entrained.

momentum-controlled region: intermediate axial distances above pool surface;

entrainment consists of droplets with initial momentum to reach height and

droplets whose terminal velocity is equal to or less than the superficial gas

velocity; three regimes as a function of superficial gas velocity in region:

low gas flux: entrainment small and consists of very fine droplets; Efg

approximately proportional to gas flux.

intermediate gas flux: larger drops ejected from pool; Efg increases with

the 3rd or 4th power of the superficial gas velocity.

high gas flux: large gas slugs form and pool surface highly agitated;

considerable droplets formed by splashing; Efg increases with the 7th to

20th power of the superficial gas velocity.

deposition-controlled region: entrained droplets of size whose terminal velocity

is equal to or less than the superficial gas velocity.

A simple mechanistic model was developed based on the concepts presented above. Due to

the enormous number of droplets generated, the motion of individual droplets could not be

followed individually, and so droplet motion was handled statistically. Important physical

parameters and distribution functions essential to the modeling and calculations were

developed or assumed. Correlations for the height criteria and entrainment in each region

were developed as a function of:

~ jg/[σg ∆ρ/ρg2]1/4

dimensionless gas velocity, jg*

(3-2)

h/[σ/g ∆ρ]1/2

dimensionless height above surface, h* =

(3-3)

g/[ρg σ (σ/g ∆ρ)1/2]1/2

gas viscosity number, Ng

(3-4)

DH/ [σ/g ∆ρ]1/2

dimensionless vessel diameter, D*H

(3-5)

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