Design and performance testing of a novel ceiling panel for simultaneous heat and moisture transfer to moderate indoor temperature and relative humidity
Date
2012-11-19
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Degree Level
Doctoral
Abstract
An important aspect in the design of buildings is the comfort of the occupants inside the building. Although many factors can affect comfort, the factor of particular interest in this research is the indoor humidity level. High indoor humidity levels can make the indoor air feel stuffy at warm temperatures and reduce the amount of moisture that can be removed from a person’s body, making a person feel warm. On the other hand, low indoor humidity levels can cause health problems such as dry eyes and sore throats. In addition to the comfort implications of improperly maintaining indoor humidity levels, the building itself can suffer from mould growth and rotting materials at high humidity levels.
This thesis presents the first research on a novel panel that can simultaneously transfer heat and moisture to/from an occupied space. The panel is referred to as a heat and moisture transfer panel (HAMP). A HAMP is similar in design to a radiant ceiling panel, but uses a liquid desiccant as the heat and moisture transfer medium and the surface of the panel is made of a semi-permeable membrane. A HAMP can be installed into a space and heat and moisture will be transferred between the liquid desiccant and the space air, through the membrane. The main objectives of this thesis are to design a prototype HAMP and to measure the performance of the HAMP under different operating conditions.
The performance of the HAMP is quantified by the sensible and latent effectivenesses, as well as the total heat and mass flux between the HAMP and the air in the test section. The results of the experiments show that the HAMP is able to simultaneously transfer heat and moisture with the air in the test section under all operating conditions. The sensible and latent effectivenesses of the HAMP are higher when the air in the test section becomes unstable, due to the natural convection in the duct (sensible effectiveness ≈ 15%, latent effectiveness ≈ 40%). These include cases of cooling and/or dehumidification. The sensible and latent effectivenesses of the HAMP are lower when forced convection is dominant in the test section (sensible effectiveness ≈ 5%, latent effectiveness ≈ 25%). This occurs during cases of heating and/or humidification. The presence of natural convection in the test section is confirmed with flow visualization photographs. The photographs show laminar boundary layer flow during the stable airflow cases, and the presence of convection roll cells during the unstable airflow cases.
The total heat flux increases with an increase in the temperature difference between the panel and the air and the mass flux increases with an increase in the humidity ratio difference between the panel and the air. For a temperature difference of 10°C, the prototype HAMP can provide ~4 W/m2 of cooling. This is small compared to the actual cooling loads that would be required in a space. The HAMP is able to provide enough moisture transfer to remove the moisture generated by one person (~70 g/hr), with ~2 m2 of panel surface area. Although the rate of heat transfer between the HAMP and the airflow is limited, the moisture transfer rates are very good.
An analysis of standard heat exchanger correlations typically used to predict the effectiveness of a heat exchanger shows that these methods are not applicable for a HAMP. Instead, new correlations must be developed to predict the sensible and latent effectivenesses of a HAMP. These correlations are not determined in this thesis, but an analysis of the experimental data is presented to show that the effectivenesses are functions of several design parameters such as the number of heat transfer units (NTUexp), the number of mass transfer units (NTUm,exp), the effective Rayleigh number of the air (Ra+), the operating condition factor (H*) and the ratio of NTUexp/NTUm,exp.
The experimental results presented in this thesis show that a HAMP can be used to simultaneously transfer heat and moisture with the air in a space, and may be used as a ceiling panel in a space to simultaneously control the temperature and relative humidity of the air. The performance of a HAMP can be determined using two parameters: NTUexp and NTUm,exp. Determining these two parameters is very complicated and involves analysis of several design parameters, such as NTUtheo, NTUm,theo, H*, Ra+ and the ratio NTUexp/NTUm,exp. The experimental data collected in this thesis was used to analyze the relationships between NTUexp and NTUm,exp and these design parameters and is a starting point for future research on developing a correlation for predicting the performance of a HAMP.
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Keywords
HVAC, coupled heat and moisture transfer, energy exchanger, liquid desiccant membrane exchanger, ceiling panel, experimental, performance testing, indoor relative humidity, flow visualization, semi-permeable membrane
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering