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Research Papers: Heat and Mass Transfer

Extension of the Concepts of Heat Capacity Rate Ratio and Effectiveness-Number of Transfer Units Model to the Coupled Heat and Moisture Exchange in Liquid-to-Air Membrane Energy Exchangers

[+] Author and Article Information
Houman Kamali

Department of Mechanical Engineering,
University of Saskatchewan,
57 Campus Drive,
Saskatoon, SK S7N 5A9, Canada
e-mail: hok051@mail.usask.ca

Gaoming Ge

Department of Mechanical Engineering,
University of Saskatchewan,
57 Campus Drive, Saskatoon,
SK S7N 5A9, Canada
e-mail: gag827@mail.usask.ca

Robert W. Besant

Department of Mechanical Engineering,
University of Saskatchewan,
57 Campus Drive,
Saskatoon, SK S7N 5A9, Canada
e-mail: rwb729@mail.usask.ca

Carey J. Simonson

Department of Mechanical Engineering,
University of Saskatchewan,
57 Campus Drive,
Saskatoon, SK S7N 5A9, Canada
e-mail: carey.simonson@usask.ca

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 30, 2015; final manuscript received April 12, 2016; published online May 17, 2016. Assoc. Editor: Andrey Kuznetsov.

J. Heat Transfer 138(9), 092001 (May 17, 2016) (12 pages) Paper No: HT-15-1312; doi: 10.1115/1.4033418 History: Received April 30, 2015; Revised April 12, 2016

The underlying concept of the standard effectiveness-number of transfer units (NTU) model is that the effectiveness of an exchanger can be correlated to two dimensionless parameters, namely, heat capacity ratio (Cr) and NTU. However, a limitation of this model is that it cannot account for the changes in effectiveness due to changes in operating temperature and humidity of simultaneous heat and moisture exchangers, specifically liquid-to-air membrane energy exchangers (LAMEEs). The purpose of this paper is to explain the reason for this limitation and also to explore the extension of the aforementioned concept of the effectiveness-NTU model to LAMEEs. The first contribution of this paper is to demonstrate that the reason for this limitation is that one of the simplifying assumptions of the standard effectiveness-NTU model, i.e., that Cr represents the ratio between the changes in the temperatures of the two fluid streams across an exchanger, is not applicable to LAMEEs. Further analysis in this paper yields two new fundamental dimensionless parameters that are analogous to Cr, termed effective Cr and effective m*, which represent the actual ratios between the changes in the temperatures and humidity ratios of the fluid streams. Then, it is shown that models analogous to the standard effectiveness-NTU model can be used to correlate the dependency of the effectiveness of LAMEEs on the operating temperature and humidity to effective Cr and effective m*.

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References

Figures

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Fig. 1

Schematics of a counterflow single-panel LAMEE with an air channel and two adjacent solution channels. Hdx represents a differential area of a membrane, through which heat and moisture are exchanged between two adjacent air and solution streams.

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Fig. 2

Equilibrium humidity ratio curves of MgCl2 and LiCl  desiccant solutions at select concentrations

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Fig. 3

Inlet operating conditions of air and solution streams for dataset M1 [9]

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Fig. 4

(a) Heat exchange effectiveness ϵs and (b) moisture exchange effectiveness ϵm plotted against the dimensionless parameters Cre and me* for the experiments of dataset M1, compared with the effectiveness curves obtained using the simplified-extended ϵ−NTU  and ϵm−NTUm models, Eqs. (37) and (42), with NTU and NTUm equal to the originally reported values of 3.0 and 1.7 [9], and also the updated values of 3.8 and 2.9 [20]

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Fig. 5

(a) Heat exchange effectiveness ϵs and (b) moisture exchange effectiveness ϵm plotted against the dimensionless parameters Cr and m* for the experiments of dataset M1, compared with the effectiveness curves obtained using the standard ϵ−NTU  and ϵm−NTUm models, Eqs. (3) and (43), with NTU and NTUm equal to the originally reported values of 3.0 and 1.7 [9], and also the updated values of 3.8 and 2.9 [20]

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Fig. 8

Schematics of the HFMC by Zhang [12]

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Fig. 6

(a) Heat exchange effectiveness ϵs and (b) moisture exchange effectiveness ϵm plotted against the dimensionless parameters Cr and m* for the experiments of dataset M2, compared with the effectiveness curves obtained using the simplified-extended ϵ−NTU  and ϵm−NTUm models

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Fig. 7

(a) Heat exchange effectiveness ϵs and (b) moisture exchange effectiveness ϵm plotted against the dimensionless parameters Cr and m* for the experiments of dataset M2, compared with the effectiveness curves obtained using the standard ϵ−NTU  and ϵm−NTUm models

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Fig. 9

(a) Change in Tair estimated using the standard and the simplified-extended ϵ−NTU  models compared with the experimental data and (b) change in Wair estimated using the standard and the simplified-extended ϵm−NTUm models compared with the experimental data for dataset Z [12]

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