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Heat and Mass Transfer

, Volume 54, Issue 8, pp 2507–2520 | Cite as

Falling film evaporation of aqueous lithium bromide solution at low pressure

  • Michael Olbricht
  • Andrea Luke
Original

Abstract

Horizontal tube bundles are often used as falling film evaporators in absorption chillers, especially for systems working at low pressure with H2O/LiBr. The heat and mass transfer processes in these apparatuses are affected by several physical effects and the geometrical design of the heat exchanger, for example the material properties of the working fluid or the surface structure of the heat exchanger. Several influencing parameters are summarized and evaluated. Correlations for the prediction of the heat transfer coefficients of falling film evaporators from the literature are discussed concerning their applicability in dependence of the working conditions of the evaporator in an absorption chiller. In this work, experimental investigations are carried out in a falling film evaporator consisting of a horizontal tube bundle with eighty horizontal tubes installed in an absorption chiller because of a lack of consistent data for heat and mass transfer in the literature. The heat and mass transfer mechanisms and the flow pattern in the falling film are analysed and compared with correlations from literature. The deviations of the experimental data from those of the correlations are within a tolerance of 30%. These deviations may be explained by a change of the flow pattern at a lower Reynolds number than compared to the literature.

Nomenclature

Latin

A

Area (m2)

a

Thermal diffusivity (m2/s)

cp

Isobaric specific heat capacity (kJ/kgK)

d

Tube Diameter (m)

D*

Dimensionless diameter (−)

g

Acceleration of gravity (m/s2)

h

Specific enthalpy (J/kg)

j

Number (−)

k

Overall heat transfer coefficient (W/m2K)

l

Length (m)

\( \dot{\mathrm{m}} \)

Mass flux (kg/s)

p

Pressure (mbar)

\( \dot{\mathrm{Q}} \)

Heat flux (W)

\( \dot{\mathrm{q}} \)

Heat flux density (W/m2)

s

Inter-tube spacing (m)

T

Temperature (K)

x

Mass related concentration (mass fraction kgLiBr/kgsolution) (%)

Dimensionless numbers

Ar

Archimedes number

Ga

Galilei number

Nu

Nusselt number

Pr

Prandtl number

Re

Reynolds number

Ref

Film Reynolds number

Sc

Schmidt number

Greek

α

Heat transfer coefficient (W/m2K)

\( \dot{\Gamma} \)

Mass flow per unit length (kg/ms)

η

Dynamic viscosity (Pas)

λ

Thermal conductivity (W/mK)

ν

Kinematic viscosity (m2/s)

ξ

Friction factor (−)

ρ

Density (kg/m3)

σ

Surface tension (N/m)

Subscripts

cu

Copper

f

Film

h

Hot water

i

Inlet / inner

lat

Latent

O

Outer/outlet

s

Solution

sat

Saturation

sens

Sensible

sys

systemic

v

Vaporization

w

Wall

Superscripts

eq

Equilibrium

Notes

Acknowledgements

A part of this data was presented at the 7th European Thermal-Sciences Conference (Eurotherm 2016) from 19 to 23 June 2016 in Krakow, Poland.

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Copyright information

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

Authors and Affiliations

  1. 1.Institute of Technical ThermodynamicsUniversity of KasselKasselGermany

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