The effects of different heat and mass transfer directions and different salt solution types and concentrations are experimentally investigated on the small-scale LAMEE steady-state effectiveness in this chapter which contains Manuscript #3 (Steady- state performance of a small-scale liquid-to-air membrane energy exchanger for different heat and mass transfer directions, and liquid desiccant types and concentrations: experimental and numerical data). In this manuscript, the SPEET facility is used first to investigate the effect of different heat and mass transfer directions on the steady-state performance of a LAMEE system under different operating conditions. The experimental data are compared with numerical simulation results using the small-scale LAMEE numerical model presented in Chapter 5. The results show that the performance of the LAMEE is not the same for different heat and mass transfer directions. Also, the repeatability of the experimental data for the air cooling and dehumidifying test case is studied for the first time, and repeatability results show that the experimental data are repeatable within ±2%. This chapter relates to the second step of the methodology (see Section 1.3.6) for LAMEEs which is testing the small-scale LAMEE under various test conditions.
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References for this chapter, which contains Manuscript #3, are presented in the Reference section of the thesis (see page 215) which includes all the references used in this PhD thesis.
Second, the effect of different salt solution types (i.e. LiCl and MgCl2) and concentrations are studied on the steady-state performance of the LAMEE under air cooling and dehumidifying test conditions. Different salt solutions with different concentrations have been used in membrane based energy exchangers such as the LAMEE. Salt solution properties are different for different desiccant types and concentrations, and they affect the performance of the system and air outlet conditions (Afshin, 2010).
Dr. Philip LePoudre, who worked as a researcher in our research group from September 2010 to September 2011, reviewed Manuscript #3 and gave useful advice and comments. Thus, He was added as the second author to Manuscript #3. The PhD candidate’s contributions to Manuscript #3 are: (a) testing the small-scale single-panel LAMEE under different operating conditions using LiCl solution in the SPEET facility, (b) analyzing the experimental data and presenting the experimental effectiveness results for the small-scale LAMEE, (c) experimentally and numerically investigating the effect of different heat and mass transfer directions and different salt solution types and concentrations on the small-scale LAMEE performance, (d) presenting the numerical results for the small-scale single-panel LAMEE, and (e) writing the paper and replying to reviewers comments.
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Steady-State Performance of a Small-Scale Liquid-to-Air Membrane Energy Exchanger for Different Heat and Mass Transfer Directions, and Liquid Desiccant Types and
Concentrations: Experimental and Numerical Data (ASME Journal of Heat Transfer, 2013, Volume 135)
Davood Ghadiri Moghaddam, Philip LePoudre, Robert W. Besant, and Carey J. Simonson
4.2 Abstract
A liquid-to-air membrane energy exchanger (LAMEE) is an energy exchanger that allows heat and moisture transfer between air and salt solution flows through a semi- permeable membrane. For the first time, a novel small-scale single-panel LAMEE test facility is used to experimentally investigate the effect of the direction of heat and mass transfers for the air and salt solution flows, and the effect of different salt solution types and concentrations on the LAMEE effectiveness. The data for steady-state effectiveness of the LAMEE are compared to the simulation results of a numerical model. Two studies are conducted; first, a study based on different heat and mass transfer directions (four test cases), and second, a study focused on the influence of solution types and concentration on LAMEE performance. For the first study, NTU = 3 and four different heat capacity ratios (i.e. Cr* = 1, 3, 5, 7) are used, with a LiCl salt solution in the exchanger. Mass and energy balances for all the test cases and the repeatability of the experimental data for the air cooling and dehumidifying test case show that the experimental data are repeatable and within an acceptable uncertainty range. The results show increasing effectiveness with increasing Cr*, and good agreement between the numerical and experimental results for both air cooling and dehumidifying and air heating and humidifying test cases. In the
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second study, two different salt solutions (i.e. LiCl and MgCl2), and three different concentrations for the LiCl solution (i.e. 25%, 30% and 35%) are selected to investigate the effect of different salt solution types and concentrations on the performance of the LAMEE. A maximum difference of 10% is obtained for the LAMEE total effectiveness data with the different salt solution types and concentrations. The results show that both the salt solution type and concentration affect the LAMEE effectiveness, and changing the concentration is one way to control the supply air outlet humidity ratio.
Keywords: Small-scale exchangers, LAMEE, Steady-state effectiveness, Heat and mass transfer directions, Salt solution, NTU, Heat capacity ratio.
4.3 Introduction
There are many applications for small compact heat exchangers for electronic cooling systems, aviation and aerospace applications and medical treatment and their performance is predictable for typical geometries (Jiang et al, 2001). Testing a small- scale exchanger before manufacturing a full-size exchanger can save money and time, and will give useful data provided the operating conditions and characteristic dimensionless parameters are in the same range as that for the prototype. There is much literature available related to modeling and testing of small-scale heat exchangers (Jiang
et al., 2001; Long 2008; Bejan, 2002). Ozden and Tari (2010) used a commercial CFD
package to numerically model a small-size shell and tube heat exchanger, and investigate the effect of the baffle spacing, baffle cut ratio and shell diameter on the heat transfer coefficient and pressure drop. Rogiers and Baelmans (2010) optimized the size of a small-scale counter flow parallel-plate heat exchanger with respect to maximum thermal power density. They considered the heat exchanger effectiveness, pressure drop and heat
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transfer rate density as constraints in this design and used an asymptotic solution to downsize the heat exchanger surface area.
Few studies have been done on small-scale energy exchangers until now. Zhang (2011a) developed an analytical solution for coupled heat and mass transfer problems in a hollow fiber membrane energy exchanger used for air dehumidification with counter flow configuration for air and liquid. The analytical results compared favorably with experimental data for a small-scale shell and tube hollow fiber membrane energy exchanger. Zhang et al. (2012a; 2012b) numerically investigated the coupled heat and mass transfer problem in a cross flow and a counter flow hollow fiber membrane contactors. They calculated the actual Nusselt and Sherwood number for the air and solution sides of the small-scale hollow fiber membrane contactors using a free surface model. In other work, Zhang and Huang (2011) used the cross flow and counter flow hollow fiber membrane contactors for air humidification using water in the system. The performance of the small-scale hollow fiber membrane module was investigated numerically and was validated with experimental data. The results showed that the cross flow configuration has a lower pressure drop in the hollow fiber membrane contactors, and the packing density is the dominant factor influencing the performance of the cross flow contactors. Ge et al. (2012) used Zhang’s analytical solution (Zhang, 2011a) for liquid-to-air membrane energy exchangers (LAMEE) and compared the results with experimental data for a small-scale single-panel LAMEE. Good agreement was obtained between analytical and experimental results and the maximum discrepancy in exchanger effectiveness was 10%. Abe et al. (2006c) and Wang et al. (2005) experimentally investigated the transient response of temperature and humidity sensors in the presence of
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an energy wheel with step changes in temperature and no change in humidity, and step changes in humidity and no change in the inlet temperature. A new test facility had been developed for these experiments which included a small part of a stationary energy wheel in a test section. Abe et al. (2006d) and Abe et al. (2006e) used the time constants that they measured for the energy wheel to analytically predict the performance of the full- size counter-flow energy wheel and compared the results with steady-state experimental performance data for the same energy wheel.
The LAMEE is a novel membrane energy exchanger that transfers heat and moisture between air and a salt solution (Namvar et al., 2012; Ghadiri Moghaddam et al., 2013b). The air and liquid desiccant are separated using a semi-permeable membrane inside the LAMEE, which is permeable for water vapor, but impermeable for liquid water. The novel test apparatus used in this research was introduced in Ghadiri Moghaddam et al. (2013b). Some preliminary numerical and experimental results were presented for the steady-state small-scale LAMEE effectiveness, when water was used as a transfer medium in the exchanger mainly to humidify the air. Mass and energy balance results for the test facility showed that the system was balanced during the experiments. The current chapter will build on this research by using different salt solutions, rather than water in the LAMEE; investigating a wider range of test cases, including air cooling and humidifying; and will present results for the repeatability of the experiments. In previous work (Ghadiri Moghaddam et al., 2013b), the authors used water which is different than salt solution especially in a numerical model.
The test facility will be used firstly to investigate the effect of different heat and mass transfer directions on the steady-state performance of a LAMEE system under
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different operating conditions. Experimental data will be compared with numerical simulation results. The performance of the LAMEE is not the same for different heat and mass transfer directions. This has been shown for a run-around membrane energy exchangers (RAMEE) that the direction of heat and mass transfer are important, where the RAMEE includes two LAMEEs located one in the supply and the other in the exhaust air stream (Seyed-Ahmadi et al., 2009a; Seyed-Ahmad et al., 2009b). The performance of the supply and exhaust LAMEEs are not the same while operating conditions and heat and mass transfer directions are different in those two LAMEEs in the RAMEE system. Also, the effect of different heat and mass transfer directions on the performance of energy exchangers was shown by Simonson and Besant (1999) for energy wheels. In this chapter the effect of different heat and mass transfer directions on the steady-state effectiveness of the LAMEE system will be investigated for various combinations of the heat and mass transfer directions. As well, the mass and energy balances and the repeatability of the experimental data for summer AHRI test conditions (AHRI STANDARD 1060, 2005) as a standard test condition will be studied. The repeatability of the experimental data for a LAMEE will be investigated to show how much the experimental data are repeatable and trust worthy.
The second main objective of this chapter is to study the effect of different salt solution types and concentrations on the steady-state performance of the LAMEE. Different salt solutions (i.e. LiCl, LiBr and MgCl2) with different concentrations have been used in membrane based energy exchangers such as the LAMEE (Fan et al., 2006; Erb et al., 2009). Salt solution properties are different for different desiccant types and concentrations, and they affect the performance of the system and air outlet conditions
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(Afshin, 2010). The main question is - what is the effect of different salt solutions types and concentrations on the LAMEE performance? This chapter will address this question using the small-scale single-panel LAMEE test apparatus. Lithium chloride (LiCl) salt solution will be used for this purpose with three different concentrations. Moreover, Magnesium Chloride (MgCl2) liquid desiccant will be used as a second option for salt solution in the system. The MgCl2 inlet conditions are selected at the same temperature and humidity ratio as the 25% concentration LiCl to have better comparison between the results for these two desiccant types. There are no numerical results or experimental data available related to the second objective in literature.