Numerical investigation of transient heat and mass transfer in a parallel-flow liquid-desiccant absorber

Liquid desiccant systems have received significant attention as a way to reduce latent loads. Tests of liquid desiccant systems in humid climates have shown significant reductions in energy consumption. As moisture in the air is absorbed at the dehumidifier, the temperature of the liquid desiccant increases due to the addition of heat from the enthalpy of condensation of the water vapor. Thus, the coupled effects of heat and mass transfer are relevant for these applications. A two-dimensional mathematical model of the transient heat and mass transfer for an absorber where a thin film of liquid desiccant flows down its walls and dehumidifies the air in parallel-flow configuration is developed and the dynamics of the modeled system are analyzed. List of symbols B Depth of the channel (m) D Diffusion coefficient (m s) Fo Fourier number fs Height of the channel (m) g Acceleration of gravity (m s) H Channel length (m) ifg Enthalpy of condensation (J kg ) k Thermal conductivity (W m K) Le Lewis number _ m Mass flow rate (kg s) _ m0 Mass flow rate per unit of length (kg s m) Nu Nusselt number P Pressure (Pa) Pe Peclet number Pr Prandtl number Re Reynolds number Sh Sherwood number T Temperature ( C) t Time (s) U Dimensionless velocity u Velocity (m s) x Direction along the channel (m) y Direction across the channel (m) Greek symbols a Thermal diffusivity (m s) d Thickness (m) g Dimensionless coordinate across the channel l Dynamic viscosity (kg m s) m Kinematic viscosity (m s) x Air humidity ratio (kgwv kg -1 da ) q Density (kg m) s Time constant (s) h Dimensionless temperature n Dimensionless coordinate along the channel f Water concentration in liquid desiccant (kgwater/kgsol) Subscripts and superscripts AVG Average a Air d Desiccant da Dry air 0 Initial int Interface sol Solution w Wall wv Water vapor G. Diaz (&) School of Engineering, University of California-Merced, 5200 North Lake Road, Merced, CA 95343, USA e-mail: [email protected] 123 Heat Mass Transfer (2010) 46:1335–1344 DOI 10.1007/s00231-010-0665-8