AN INVESTIGATION OF TEMPERATURE AND HUMIDITY VARIATIONS DURING SILICA GEL-MOISTURE INTERACTIONS
Date
2003-08
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Degree Level
Masters
Abstract
Recent successes in heat and moisture recovery from ventilation exhaust air from
buildings using energy wheels have caused extensive interest in the HVAC design
community. Anew method of heat and moisture recovery using desiccant particles to
transfer heat and moisture in an energy recovery system is explored in this thesis. The
research of this thesis is the first step of this investigation. The objective is to study
temperature and moisture variations during transient flow of humid air passing through a
silica gel particle bed.
A one-dimensional theoretical/numerical model was developed to simulate the process of
heat transfer and moisture adsorption when an
initially dry granular silica gel bed is
subject to a sudden air flow passing through it at selected temperature and humidity.
Eleven governing equations, with initial and boundary conditions were established to
describe this complicated problem. The equations reflect the effect of moisture diffusion
inside each particle. This diffusion coefficient was found to be very much smaller than
the diffusion coefficient for water vapour in air. Theoretical equations were discretized
by using the control volume method and coded to simulate the problem. Before the
application of the code, some
parameters of the silica gel particles, such as permeability,
porosity, specific surface area and isothermal moisture adsorption capacity, were
measured or calculated.
An experimental test facility was set up to measure the temperature, humidity and
moisture content in a silica gel particle bed. The initially dry silica gel particles were
subject to a sudden air flow passing through them at selected inlet temperature and
relative humidities. In total, six experiments were completed for two sizes of silica gel
particles.
The numerical results were
compared with the experimental data of measured outlet
temperature, humidity ratio and moisture accumulation in the silica gel bed.
Comparisons were made for three conditions of inlet air humidity and two sizes of silica
gel particles. A sensitivity study of the numerical model was completed.
From the numerical and experimental studies, it was concluded that silica gel particles
have very high moisture adsorption capacity. As well, a large amount of heat is released
during the moisture adsorption process. This heat release is most evident during the first
1.5 hours of a 10-hour test period. It was also found that, at higher inlet relative
humidities and with smaller particle sizes, silica gel particles adsorb more moisture. The
values of uncertainty for the bed properties such as specific surface area (S), density of
silica gel particles ( Per)' specific heat of silica gel particles (Cper) and moisture-silica gel
heat effect (Q) strongly affect the simulation results.
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Degree
Master of Science (M.Sc.)
Department
Mechanical Engineering
Program
Mechanical Engineering