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

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.

Key words： falling film evaporation; ammonia-water; heat and mass transfer

Acknowledgements

This work was supported by the National Hi-Tech Research and Development Program of China (863) (No. 2007AA05Z442)

Notations

Cp	Specific heat/(J·kg-1·K-1)
Dm	Diffusion coefficient/(m2·s-1)
H	Specific enthalpy/(J·kg-1)
L	Tube length/m
m	Solution mass flow/(kg·h--1)
mE	Amount of evaporated ammonia vapor/(kg·h-1)
N	Sequence number of experiment
p	Evaporating pressure/kPa
PE	Percent of evaporated ammonia vapor/%
Q	Heat exchange capacity between hot water and solution/kW
T	Temperature/°C
u	Film velocity of x direction/(m·s-1)
V	Volumetric flow/(L·h -1)
v	Film velocity of y direction/(m·s-1)
Greek symbols
\xi	Mass concentration of solution/(kg·kg-1)
\delta	Film thickness/m
\mathrm{\Gamma }	Spray density/(kg·m-1·s-1)
\mu	Dynamic viscosity/(N·s·m-2)
\rho	Density/(kg·m-3)
\lambda	Coefficient of heat conductivity/(W·m-1·K-1)
Subscripts
cal	Calculated
exp	Experimental
hw	Hot water
in	Inlet
out	Outlet
s	Solution
w	Wall of evaporation tube

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