Transient and Steady-state Performance of A Liquid-to-Air Membrane Energy Exchanger (LAMEE)
Date
2014-06-20
Authors
Journal Title
Journal ISSN
Volume Title
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ORCID
Type
Degree Level
Masters
Abstract
The main objective of this thesis is to investigate the transient response and steady-state performance of a counter-cross flow liquid-to-air membrane energy exchanger (LAMEE). The LAMEE is constructed from several semi-permeable membranes which separate the air and liquid streams. In addition to heat transfer, moisture transfer occurs between the air and liquid streams since the membranes are permeable to water vapor. The LAMEE performance is assessed experimentally and the results are used to verify a numerical model. The verified numerical model is also used to extrapolate the transient and steady-state performance parameters to other test conditions.
The transient response of the LAMEE is important since there are times when the LAMEE operates under transient conditions due to daily start-up or changing operating conditions such as flow rates, temperatures or humidities. The transient response of the LAMEE is investigated experimentally and numerically. The number of heat transfer units (NTU), and the ratio of solution and air heat capacity rates (Cr*) are two important parameters that affect the LAMEE performance. The results show that the transient sensible, latent and total effectivenesses increases with time after a step change in the conditions of the inlet liquid desiccant. The experimental and numerical transient effectiveness values and trends are compared for different NTU and Cr* values under summer and winter test conditions and the results show satisfactory agreement.
In addition to the transient effectiveness, the time constant of the LAMEE is assessed as an important transient parameter. The time constant represents the time it takes for the LAMEE to reach 63.2% of the steady-state conditions after a step change in inlet conditions. The transient response of the outdoor air temperature and humidity ratio are normalized and used to determine the sensible and latent time constants. It is found that time constant depends on NTU, Cr* and thermal mass capacity of the LAMEE. The experimental and numerical results show that time constant increases as Cr* decreases or NTU increases. Furthermore, the verified numerical model is used to study the effect of outdoor air conditions on the LAMEE time constant. The numerical results reveal that the latent time constant is influenced by outdoor air conditions and the time constant decreases as H* increases, but the sensible time constant is almost constant for various outdoor air conditions. However, the outdoor air conditions affect the transient response of the LAMEE considerably since the total transient response of the LAMEE is closer to the latent transient response for the conditions studied.
The steady-state performance of the LAMEE is studied for different NTU and Cr* values under summer test conditions. The experimental data are compared to numerical and analytical results and acceptable agreement is achieved. It is found that the steady-state effectiveness of the LAMEE increases with NTU and Cr*. The maximum total effectiveness reaches 88% for NTU=10 and Cr*=6.3. The verified numerical model is also used to investigate the effect of outdoor air conditions on the steady-state sensible and latent effectiveness of the LAMEE. The sensible effectiveness is significantly influenced by outdoor air conditions variation while the latent effectiveness is only slightly influenced by these variations. The sensible effectiveness decreases as the operating condition factor (H*) increases, but the latent effectiveness increases with H*.
Description
Keywords
Liquid-to-air membrane energy exchanger, Transient, Steady-state
Citation
Degree
Master of Science (M.Sc.)
Department
Mechanical Engineering
Program
Mechanical Engineering