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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
Questions?
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