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Numerical study of heat and mass transfer of ammonia-water in falling film evaporator

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Abstract

To investigate the performance of the 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 developed and solved based on stream function. Then an experimental study of falling film evaporation was carried out in order to validate the mathematical model. A series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. The calculated results show that the average velocity and the film thickness change sharp at the entrance region when x < 100 mm, while they vary slightly in the fully developed region when x > 100 mm. The film thickness depends largely on the flow rate of solution. It is observed that the heating power and mass flow of solution significantly affect the concentration difference between the inlet and outlet of evaporation tube. The calculated results reveal that the tube length has a significant impact on the amounts of ammonia vapor evaporated. It is suggested that the roll-worked enhanced tube should be used in order to decrease the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate. Furthermore, the experimental and calculated results indicate that the inlet solution concentration has a great influence on the heat exchange capacity, the amounts of ammonia vapor evaporated and the evaporation pressure.

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Abbreviations

C P :

Specific heat, J kg−1K−1

D :

Tube diameter, m

D m :

Diffusion coefficient, m2/s

H :

Specific enthalpy, J/kg

L :

Tube length, m

m :

Solution mass flow, kg/h

mE :

The amount of evaporated ammonia vapor, kg/h

\( \dot{m} \) :

Evaporation rate, kg/s/m2

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

v :

Film velocity of y direction, m/s

V :

Volumetric flow of water, L/h

ΔH :

Enthalpy difference, J/kg

ξ:

Mass concentration of solution, kg/kg

δ:

Film thickness, m

Г:

Spray density, kg m−1 s−1

µ:

Dynamic viscosity, N s/m2

ρ:

Density, kg/m3

λ:

Thermal conductivity, W m−1K−1

cal:

Calculated

exp:

Experimental

in:

Inlet

out:

Outlet

hw:

Hot water

s:

Solution

W:

The wall of evaporation tube

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Acknowledgments

The authors are thankful for the financial support by the High Technology Research and Development (863) Program of China under Grant No: 2007AA05Z442 and 2009AA05Z433.

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Authors and Affiliations

  1. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 510640, Guangzhou, People’s Republic of China

    Xianbiao Bu, Weibin Ma & Yuanfeng Huang

  2. Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Sciences, 510640, Guangzhou, People’s Republic of China

    Xianbiao Bu, Weibin Ma & Yuanfeng Huang

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Correspondence to Xianbiao Bu.

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Bu, X., Ma, W. & Huang, Y. Numerical study of heat and mass transfer of ammonia-water in falling film evaporator. Heat Mass Transfer 48, 725–734 (2012). https://doi.org/10.1007/s00231-011-0923-4

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