Heat Transfer

Heat transfer is induced by the difference in temperature between the air stream and the surface of the desiccant soaked membrane, assumed to be equal to the bulk temperature of the desiccant solution.

For forced convection heat transfer in a closed conduit, the air-side heat transfer, dq, in J is determined using Eq. 3.1 (Welty et al. 2008).

Ta,in and Tsol,in are the respective inlet temperatures of the air and desiccant solution in °C. kH and &S0i are the respective convective heat transfer coefficients of the air and solution in W m-2 K. &4m is the surface area available for heat and mass exchange between the air stream and desiccant solution in m2.

The solution side heat transfer in J is defined in Eq. 3.2 (Liu 2008).The heat transferred to the solution includes the sensible heat due to the temperature difference between the air and desiccant plus the latent heat of water vapour absorption.

dm is the mass of water vapour condensed from the air to the desiccant solution in kg s-1, and is defined in Eq. 3.8. The convective heat transfer coefficient in W m-2 K of the air and desiccant is defined in Eq. 3.3 (?engel and Ghajar 2011).

Re is the Reynolds number of the fluid. pb and are the dynamic viscosity in Pa s of the bulk air moving through the exchanger, and that of the desiccant solution at the exchanger wall respectively. k is the thermal conductivity of the fluid in W m-1 K. Dh is the hydraulic diameter of the channel in metres, and is defined in Eq. 3.6.

The Reynolds number is defined in Eq. 3.4 as:

p is the bulk density of the fluid in kg m-3, u is the velocity of the fluid in m s-1. The Prandtl (Pr) is defined in Eq. 3.5 as:

cp is the specific heat capacity of the fluid in J kg-1 K The hydraulic diameter, Dh, is defined in Eq. 3.6 as:

Hc and wc are the height and width in metres of the channel, respectively.

The overall heat transfer coefficient ки) of the investigated heat and mass exchanger is calculated using Eq. 3.7 (Ge et al. 2014).

 
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