RESEARCH ARTICLE

Heat and mass transfer of ammonia-water in falling film evaporator

  • Xianbiao BU ,
  • Weibin MA ,
  • Huashan LI
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  • Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Sciences, Guangzhou 510640, China

Received date: 15 Apr 2011

Accepted date: 08 Jun 2011

Published date: 05 Dec 2011

Copyright

2014 Higher Education Press and Springer-Verlag Berlin Heidelberg

Abstract

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x<100 mm, while they vary slightly with the tube length in the fully developed region when x>100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimalβrelationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate.

Cite this article

Xianbiao BU , Weibin MA , Huashan LI . Heat and mass transfer of ammonia-water in falling film evaporator[J]. Frontiers in Energy, 2011 , 5(4) : 358 -366 . DOI: 10.1007/s11708-011-0161-y

Acknowledgements

This work was supported by the National Hi-Tech Research and Development Program of China (863) (No. 2007AA05Z442)
Notations
CpSpecific heat/(J·kg-1·K-1)
DmDiffusion coefficient/(m2·s-1)
HSpecific enthalpy/(J·kg-1)
LTube length/m
mSolution mass flow/(kg·h--1)
mEAmount of evaporated ammonia vapor/(kg·h-1)
NSequence number of experiment
pEvaporating pressure/kPa
PEPercent of evaporated ammonia vapor/%
QHeat exchange capacity between hot water and solution/kW
TTemperature/°C
uFilm velocity of x direction/(m·s-1)
VVolumetric flow/(L·h -1)
vFilm velocity of y direction/(m·s-1)
Greek symbols
ξMass concentration of solution/(kg·kg-1)
δFilm thickness/m
ΓSpray density/(kg·m-1·s-1)
μDynamic viscosity/(N·s·m-2)
ρDensity/(kg·m-3)
λCoefficient of heat conductivity/(W·m-1·K-1)
Subscripts
calCalculated
expExperimental
hwHot water
inInlet
outOutlet
sSolution
wWall of evaporation tube
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