Droplet EvaporationJie Bao

10/23/2007ME 3007 Energetic

Outline

General introduction of evaporation

Applications

Experiment observation of droplet evaporation on heated surface

Numerical method for droplet evaporation of heated surface

Heat transfer between the droplet and heated surface

Tests of evaporation rate

Evaporation

For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic to overcome liquid-phase intermolecular forces.

Factors influencing the rate of evaporation

Concentration of the substance evaporating in the airFlow rate of airTemperature of the substance Inter-molecular forces Surface area Concentration of other substances in the liquid Concentration of other substances in the air

http://en.wikipedia.org/wiki/Evaporation

Applications

Internal combustion engineEngine

Higher efficiencyVolkswagen Lupo3 litres of fuel per 100 kilometres (78 miles per US gallon or 94 miles per Imperialgallon)

The Diesel cycle uses compression ignition: the fuel ignites upon beinginjected into the highly compressed air in the combustion chamber

Diesel engine operates using the Diesel cycle

The fuel and air are pre-mixed before compression.

Petrol engine or Gasoline engine operates using Tow-stroke cycle or Four-stroke cycle (Otto cycle)

Higher speedLimited compressionLower efficiency

Otto cycle

ApplicationsEngine

Jet engine

A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton’s third law of motion

ApplicationsInstant coffeeInstant coffee was invented in 1901 by Satori Kato

The Nescafé brand, which introduced a more advanced coffee refining process, was launched in 1938

Roasting, Grinding, Extraction and DryingFreeze Drying or Spray Drying

Spray drying is a commonly used method of drying a liquid feed through a hot gas

Cost effectiveness Short drying time

Complete in 5-30 secondsSpherical particles of size roughly equal to 300 µm with a density of 0.22 g/cm³

Droplet 500 to 1,000 µm in diameter

Applications

Dropwise evaporative coolingDropwise evaporative cooling is the process by which a single fluid droplet impacts a heated surface and cools by latent heat absorption which transforms the liquid into the vapor phase

The heat transfer rate is much greater than traditional fluids cooling techniques

Metallurgical applicationsNuclear thermal managementElectronic industriesFire suppression systems

G. F. Hewitt, G. L. Shires, T. R. Bott, Process Heat Transfer, p. 431, 1994

Applications

Dropwise evaporative coolingThe microelectronics industry has been a major motivating influence on the research in the spray-cooling field

The compatibility of cooling system limits the speed of computers

Liquid nitrogen

ApplicationsDropwise evaporative cooling

Spray evaporative cooling system for Cray X12005

Research Areas Relating to Droplet Evaporation

Droplet

Heated surfaceHeat flux on heated surfaceTemperature distribution on heated surfaceExperiment technique

Droplet shape, size and deformationConduction of the dropletConvection and flow motion in the dropletProperty of different droplet during evaporationExperiment technique

Evaporation and Explosion of Liquid Drops on a Heated Surface

Total vaporization time

a-b: liquid-film type vaporization regimeb-c: nucleate-boiling type vaporization regimec-d: transition regimed-e: spheroidal (film-boiling type) vaporization regime

A typical curve for the evaporation of benzene drops with the initial diameter of 2.14 mm

Boiling point: 80.1 °C (353.2 K)

Efficiency

Smaller drag force, lower energy cost

Greater heat transferConfucius(551 BCE -479 BCE)

Ancient philosophyDoctrine of the Mean

Structure at the liquid-air interface

Evaporation and Explosion of Liquid Drops on a Heated Surface

Stable-type: smooth, outer rings in concentric circular

Ethyl, cyclohexane and carbon tetrachloride

Unstable-type: rippled, outer rings form irregular radiant stripes within a sawtooth-like circle

Acetone, methanol or ethyl alcohol

Sub-stable-type: outer rings distorted from a circular shape and spiked

Ethyl acetate, benzene, chloroform and methylene chloride

liquid-film type vaporization regime (a-b)

Evaporation and Explosion of Liquid Drops on a Heated Surface

A schematic diagram of flow pattern in a stable or sub-stable drop under liquid-film type evaporation process

I: intensive natural convectionII: cellular structureIII: stagnation regionIV: weakly circulating region

During evaporation: region I shrink, region II moves to region IIIIncrease surface temperature: region I increase, finally, only region I and III

Unstable-type acetone drop under liquid-film type vaporization process

Nucleate-boiling type vaporization regime (b-c)

Evaporation and Explosion of Liquid Drops on a Heated Surface

The nucleation and bubble growth phenomena occur inside the drop, when Tw takes a value between Tb and Tc, the maximum boiling rate point

Stable-type, cyclohexanedrop under nucleate-boiling type vaporation process (81 °C)

substable-type, benzenedrop under nucleate-boiling type vaporization process (80.5 °C)

Unstable-type, acetone drop under nucleate-boiling type

vaporization process (1) 57.1 °C and (2) 62 °C

Transition-boiling type vaporization regime (c-d)

Evaporation and Explosion of Liquid Drops on a Heated Surface

Td: the Leidenfrost point

A drop exploded immediately on contact with the heating surface

Film-boiling type vaporization regime (d-e)The drop taking a spherical form never exploded but evaporated rather slowly

A schematic of drop behavior under various

heating regimes

Laser shadowgraphic system

Evaporation and Explosion of Liquid Drops on a Heated Surface

Direct photographic systems for drops evaporating on (a) a thin glass

plate and (b) a concavcsurfaced metal block

Experiment techniques

Numerical model

Evaporation and Explosion of Liquid Drops on a Heated Surface

Mass conservation

Momentum conservation

Energy conservation

Axisymmetric coordinate system

Evaporation and Explosion of Liquid Drops on a Heated Surface

Numerical modelBoundary condition

Evaporation and Explosion of Liquid Drops on a Heated Surface

Numerical modelBoundary condition

Mass conservation

Force balance (normal direction)

Force balance (tangential direction)

Energy balance

nv⋅−∇=κ

m ′′

σ

Snchfghlocal mean curvature

viscous stress tensor

evaporation mass flux

surface tension

latent heat of vaporization

Natural convection heat transfer coefficient

Evaporation and Explosion of Liquid Drops on a Heated Surface

Numerical model

Long time solution: Isotherm and Streamfunction contours with velocity vectors superimposed for a 0.1 mm initial contact diameter water drop on a 100°C heated surface. Initial contact angle=90 deg. Environment: T=20°C, dry air: a)5 ms, b)20 ms, c)40 ms, and d)60 ms

Evaporation and Explosion of Liquid Drops on a Heated Surface

Numerical modelThermal lattice Boltzmann equation model

t=4000

t=2000t=0

t=500

Evaporation and Explosion of Liquid Drops on a Heated Surface

Numerical modelThermal lattice Boltzmann equation model

t=43000

t=33000t=20000

t=23000

Evaporation and Explosion of Liquid Drops on a Heated Surface

Heat transfer on the heated surfacePrevious researches all assume the temperature of the wall is constant or the conductivity of surface’s material is very big

Temperature distribution of the surface for single droplet

a) at 1s; b) at 10s; c) at 30s;d) at 50s; e) at 70s; f) at 90s;e) g) at 100s h) At 110s; i) 130s

V0=30 lμ

T0=124 Co Macor

Evaporation and Explosion of Liquid Drops on a Heated Surface

Heat transfer on the heated surfaceSparse spray on a solid heated surface

Typical surface temperature distribution for water on Macor (after 60s)

CT o1400 =lV μ100 =

smgG 2/5.1=

Transient average surface temperatureCT o1600 =

smgG 2/97.0=

Typical droplet distribution

Evaporation rate

Evaporation rate (Ke)

Schematic diagram of the experimental apparatus

Evaporation rate of water droplet in each vapor mole

Explosion of ethanol (Vapor mole 40%)Evaporation rate of ethanol droplet in each vapor mole

ReferencesOrlando E. Ruiz and William Z. Black, Evaporation of water droplets placed on a heated horizontal surface, Journal of heat transfer, Vol. 124, 2002

M. J. Lee, Y. W. Kim, J. Y. HA and S. S. Chung, Effects of watery vapor concentration on droplet evaporation in hot environment, International journal of automotive technology, Vol. 2, No. 3, pp. 109-115, 2001

Alfred M. Moyle, Paul M. Smidansky and Dennis Lamb, Laboratory studies of water droplet evaporation kinetics

J. J. Hegseth, N. Rashidnia and A. Chai, Natural convection in droplet evaporation, Physical review E, Vol. 54, No. 2, 1996

Marino di Marzo, Dropwise evaporative cooling, National heat transfer conference, 22-23 Giugno, Bologna, Italy, 1995

N. Zhang and W. J. Yang, Evaporation and explosion of liquid drops on a heated surface, Experiments in fluids 1, 101-111, 1983

Droplet evaporation

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