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Review for prediction of evaporation rate at natural convection

  • Tibor PoósEmail author
  • Evelin Varju
Original
  • 47 Downloads

Abstract

The aim of the present research is to analyse evaporation intensity induced by natural convection. Simultaneous heat and mass transfer is observed in the case of evaporation from open water tanks as well as during the evaporation of surface moisture when drying materials. Relationships to determine the evaporation rate at natural convection were collected within this work. The equations for evaporation rate from these studies are valid in different ranges of interpretation (different gas and liquid temperature, surface area) and under certain conditions, which reduces their usefulness. New measuring apparatus has been installed and equations for determination of evaporation rate were validated. As a result of the validation, the equations of Leven, Boelter et al. and Bower and Saylor proved to be the most precise (±5% deviation) and equations of Box showed ±10% deviation to our measurement results.

Nomenclature

a, b, n

Constants in Eq. (1) and (2)

A

Evaporating surface [m2]

c

Constant in Eq. (2)

c

Concentration [kg of moisture/m3 of air]

C

Coefficient in equation of Sparrrow

d

Diameter of the evaporating tank [m]

D

Diffusion coefficient [m/s]

g

Gravitational acceleration [m/s2]

Gr

Grashof number [1]

i

Index of summation

k

Mass transfer coefficient [m/s]

L

Characteristic length [m]

m

Evaporated water mass [kg]

M

Molecular weight [kg/kmol]

n

Upper bound of summation

N

Evaporation rate [kg/m2s]

P

Total pressure [Pa]

p

Partial pressure [Pa]

r

Latent heat of vaporization [J/kg]

R

Radius of evaporation pan [m]

RG

Specific gas constant for dry air [287.05 J/kgK]

Rw

Specific gas constant for water vapour [461.495 J/kgK]

RaM

Rayleigh number for mass transfer [1]

Sh

Sherwood number [1]

Sc

Schmidt number [1]

t

Evaporation time [s]

T

Temperature [°C]

v

Air velocity [m/s]

V

Volume [m3]

X

Parameter

Y

Absolute humidity of air [kg of moisture/kg of dry air]

Greek letters

δ

Deviation [%], measuring error

Δ

Difference

Δ

Mean step height in equation of Sparrow [m]

ν

Kinematic viscosity [m2/s]

ρ

Density [kg/m3]

σ

Evaporation factor [kg/m2s]

φ

Relative humidity [1]

Subscripts and superscripts

c

Critical

cal

Calculated

f

Liquid surface

G

Gas

L

Liquid

m

Measured

sat

Saturated

v

Vapor

w

Water

Notes

Acknowledgements

This paper was supported by Gedeon Richter’s Talentum Foundation (H-1103 Budapest, Gyömrői str. 19-21, Hungary), and by the Higher Education Excellence Program of the Ministry of Human Capacities in the frame of Water science & Disaster Prevention research area of Budapest University of Technology and Economics (BME FIKP-VÍZ). Special thanks to Dr. Róbert Goda for his help in this work.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Building Services and Process EngineeringBudapest University of Technology and EconomicsBudapestHungary

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