Computer Simulation

By applying heat and mass conservation, as shown in Eq. 3.17, it is possible to determine the outlet conditions of both the air and desiccant solution based upon the inlet boundary conditions, geometry of the heat and mass exchanger and calculated heat and mass transfer coefficients.

m_a and m_s0 are the respective air and solution mass flow rates in kg s^-1. The left- hand side of Eq. 3.17 represents the total enthalpy change (temperature and mass) of the air and the right-hand side represents the total enthalpy change of the desiccant solution. Engineering Equation Solver (EES) has been used to complete a one dimensional computer simulation to determine the thermal performance of the dehumidifier and regenerator. Some simplifying assumptions were made, and are as follows:

• • The heat and mass transfer processes are considered steady-state.
• • The dehumidification/regeneration processes are adiabatic; heat loss to surroundings is negligible.
• • There is perfect wetting and coverage of the membrane material with desiccant solution, with ideal contact between the air and desiccant solution.
• • The membrane material poses no resistance to heat and mass transfer i.e. direct contact is assumed.
• • Air and desiccant solution physical and thermal properties are consistent between channels.

• • No desiccant carry-over to the airstream occurred.
• • The latent heat of sorption is absorbed by the desiccant solution.

The above assumptions have been adopted by previous researches in numerical studies, and have been found to have negligible effect on the overall desiccant system performance (Liu 2008; Jradi and Riffat 2014a).

The outlet air temperature is determined for each channel using Eq. 3.18.

dq_air is the heat transfer on the air side in J, and is defined in Eq. 3.1 and &>_a is air absolute humidity in kgvapour/kgdryair.

The outlet desiccant solution temperature is determined for each channel using Eq. 3.19.

dq_soiuti_on is the heat transfer on the solution side in J, and is defined in Eq. 3.2. The heat transferred to the solution includes the sensible heat due to the temperature difference plus the latent heat of water vapour absorption. dq_air and dq_soluti_on will be positive during dehumidification and negative during regeneration.

The outlet air absolute humidity in kg_vapour/kg_drya;_r is determined for each channel using Eq. 3.20

The outlet desiccant solution mass concentration is determined using Eq. 3.21

dm is the mass of water vapour absorbed/desorbed in kg s^-1 and is defined in Eq. 3.8. dm will be positive during dehumidification and negative during regeneration.

The liquid desiccant simulation work has been carried out using EES where air and water property routines are in-built validated functions. Appendix B provides the equations used to determine the thermodynamic properties of moist air as a reference. The thermophysical properties of the desiccant solution are determined from linear regression curve fits to published empirical data (James 1998; Melinder 2007). Section 3.3.3 provides the evaluation metrics used to analyse the performance of the dehumidifier and regenerator.