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Numerical modeling of falling-film plate generator and rectifier designed for NH3—H2O absorption machines

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Abstract

This study presents the numerical development of a generator–rectifier combined component (called a “combined-generator”) composed of plate heat exchangers, designed and meant to be integrated in a single-stage ammonia–water absorption cooling system. Investigations are made to find the most compact and efficient design. Numerical simulations are presented describing parameters such as the ammonia fraction in the vapor produced and the combined generator efficiency as a function of the inlet temperatures or mass flow rate of the heat transfer fluid. The combined generator produces vapor with a high ammonia mass fraction and high mass efficiency for a solution inlet temperature range of [315–320 K] and a mass flow rate of the heat transfer fluid between [0.4 and 0.6 kg.s−1]. The impacts of the length and number of plates as well as the adiabatic/heated ratio of the plates are also examined. The ammonia fraction increases with the increase in the adiabatic/heated ratio, and the combined generator efficiency increases with the increase in the plate aspect ratio of length/width. The proposed system is developed to be operated in compact and medium-capacity absorption chillers (approximately 5 kW cold) for air-conditioning.

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Abbreviations

AHR:

Adiabatic/heated ratio [ -]

AR:

Aspect ration [ -]

Cp:

Calorific capacity [J.kg−1 K−1]

dAi:

Exchange surface [m2]

dM:

Mass flux transferred [kg.s−1]

H:

Enthalpy [J.kg−1]

h:

Thermal transfer coefficient [W.m−2.K−1]

k:

Mass transfer coefficient [m.s−1]

Lv:

Latent heat of vaporization [kJ.kg−1]

P:

Pressure [bar]

Q:

Heat [W]

R:

Radius [m]

Re:

Reynolds number [ -]

Req:

Equilibrium factor [ -]

T:

Temperature [K]

U:

Global thermal transfer coefficient [W.m−2.K−1]

x:

Liquid ammonia fraction [ -]

y:

Vapor ammonia fraction [ -]

\(\dot{\text{m}}\)  :

Mass flow rate [kg.s−1]

z:

Mass flow fraction [ -]

\(\varepsilon_m\) :

Mass efficiency [ -]

δ:

Thickness of the film [m]

ΔP:

Pressure losses [bar]

Γ:

Mass flow rate per unit width [kg.m−1.s−1]

μ:

Viscosity [kg.m−1.s−1]

ρ:

Density [kg.m−3]

σ:

Surface tension [kg.s-2]−2

1 :

Initial state

2 :

State after transfers

H 2 O :

Water

NH 3 :

Ammonia

cav :

Cavity radius

crit :

Critical

L :

Liquid

V :

Vapor

I :

Internal

In :

Inlet

Out :

Outlet

max :

Maximum

m,des :

Mass desorption

sat :

Saturation

w :

Wall

∞ :

Limit

HTF :

Heat transfer fluid

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Authors and Affiliations

  1. CEA, LITEN, DTBH. INES, 50 avenue du Lac Léman, 73370, Le Bourget du Lac, France

    Mathilde Wirtz, Hai Trieu Phan & François Boudehenn

  2. LOCIE, Univ. Savoie Mont Blanc, CNRS UMR5271, Savoie Technolac, 73370, Le Bourget du Lac , France

    Mathilde Wirtz & Benoit Stutz

Authors
  1. Mathilde Wirtz
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  2. Benoit Stutz
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  3. Hai Trieu Phan
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  4. François Boudehenn
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Correspondence to Mathilde Wirtz or Benoit Stutz.

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Highlights

∙ A numerical model simulating the behavior of a combined generator in an NH3-H2O absorption chiller was developed.

∙ The combined generator has a double function of ammonia vapor generation and purification.

∙ Impact of design and operating parameters is investigated.

∙ The combined generator enables one to produce vapor with a high ammonia fraction and high mass efficiency.

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Wirtz, M., Stutz, B., Phan, H.T. et al. Numerical modeling of falling-film plate generator and rectifier designed for NH3—H2O absorption machines. Heat Mass Transfer 58, 431–446 (2022). https://doi.org/10.1007/s00231-021-03111-z

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  • DOI: https://doi.org/10.1007/s00231-021-03111-z

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