In this study, a numerical model is developed for determining coupled heat and moisture transfer in a run-around membrane energy exchanger (RAMEE) using two counter/cross flow exchangers and with a salt solution of MgCl2 as the coupling fluid. The counter/cross flow exchanger is a counter-flow exchanger with cross-flow inlet and outlet headers. The model is two-dimensional, steady-state and based on the physical principles of conservation of momentum, energy, and mass. The finite difference method is used in this model to discretize the governing equations.<p>
The heat transfer model is validated with effectiveness correlations in the literature. It is shown that the difference between the numerical model and correlations is less than ¡À2% and ¡À2.5% for heat exchangers and run around heat exchangers (RAHE), respectively. The simultaneous heat and moisture transfer model is validated with data from another model and experiments. The inter-model comparison shows a difference of less than 1%. The experimental validation shows an average discrepancy of 1% to 17% between the experimental and numerical data for overall total effectiveness. At lower NTUs the numerical and experimental results show better agreement (e.g. within 1-4% at NTU=4).<p>
The model for RAHE is used to develop new effectiveness correlations for the geometrically more complex counter/cross flow heat exchangers and RAHE systems. The correlations are developed to predict the response of the exchangers and overall system to the change of different design characteristics as it is determined by the model. Discrepancies between the simulated and correlated results are within ¡À2% for both the heat exchangers and the RAHE systems.<p>
It is revealed by the model that the overall effectiveness of the counter/cross flow RAMEE depends on the entrance ratio (the ratio of the length of the inlet and outlet headers to the length of the exchanger, xi/x0), aspect ratio (the ratio of the height to the length of the exchanger, y0/x0), number of heat transfer units (NTU), heat capacity rate ratio (Cr*), number of mass transfer units (NTUm), and the mass flow rate ratio of pure salt in desiccant solution to dry air (m*). Beside these dimensionless parameters, the performance of the RAMEE system is affected by the liquid-air flow configuration and the operating inlet temperature and humidity.<p>
This study concludes that the maximum effectiveness of the RAMEE system with two counter/cross flow exchangers occurs when NTU and NTUm are large (e.g. greater than 10). At any NTU, the overall effectiveness of the RAMEE system increases with Cr* until it reaches a maximum value when Cr*= . Increasing Cr* above causes the overall effectiveness to decrease slightly. Therefore, to achieve the maximum overall effectiveness of the system, Cr* must be close to . is a function of NTU and operating conditions e.g., with NTU=10, and under AHRI summer and winter operating conditions, respectively. The exchangers in the RAMEE system are needed to have a small aspect ratio (e.g. y0/x0<0.2) and small entrance ratio (e.g. xi/x0<0.1) to get the maximum overall effectiveness of a RAMEE system using two counter/cross flow exchangers. Such a RAMEE system has a total effectiveness 6% higher and 1.5% lower compared to the same cross-flow and counter-flow RAMEE, respectively (at NTU=10, Cr*¡Ö3, y0/x0=0.2 and xi/x0=0.1).
| Identifer | oai:union.ndltd.org:USASK/oai:usask.ca:etd-06032009-154644 |
| Date | 29 June 2009 |
| Creators | Vali, Alireza |
| Contributors | Torvi, David A., Soltan, Jafar, Simonson, Carey J., Bugg, James D., Besant, Robert W. |
| Publisher | University of Saskatchewan |
| Source Sets | University of Saskatchewan Library |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://library.usask.ca/theses/available/etd-06032009-154644/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Saskatchewan or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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