Research Papers: Heat Exchangers

Air-Side Surface Wettability Effects on the Performance of Slit-Fin-and-Tube Heat Exchangers Operating Under Wet-Surface Conditions

[+] Author and Article Information
L. Liu

Department of Mechanical Science and Engineering, University of Illinois, 1206 West Green Street, Urbana, IL 61801lliu9@illinois.edu

A. M. Jacobi

Department of Mechanical Science and Engineering, University of Illinois, 1206 West Green Street, Urbana, IL 61801a-jacobi@illinois.edu

The correction factor F was evaluated to be 0.98, which is typical to operating conditions in this study (20).

During the experiments, the accumulation of condensate changed the friction factor. The experiments were conducted at two fixed blower frequencies: 42 rpm corresponds to a dry-surface Redh250 and 50 rpm corresponds to Redh350.

J. Heat Transfer 131(5), 051802 (Mar 18, 2009) (9 pages) doi:10.1115/1.2994722 History: Received December 07, 2007; Revised June 18, 2008; Published March 18, 2009

A study of condensate retention and the attendant thermal-hydraulic effect associated with changes in air-side surface wettability is presented for a series of slit-fin-and-tube heat exchangers of identical geometry with controlled wettability covering a wide range of contact angles. An experiment in which the retained mass of air-side condensate was measured under dynamic conditions is described, and the results are analyzed using conventional thermal-hydraulic measurements of j and f. The data demonstrate that for the heat exchangers used in this study, the j factor is not strongly influenced by condensate retention, but the friction factor is significantly reduced for surfaces of increased wettability. Hydrophilic heat exchangers retain much less air-side condensate than do the hydrophobic heat exchangers, and the amount of retention is found to depend on the air-side Reynolds number (Redh) and the rate of latent heat transfer (Ql). Based on an assumption of filmwise condensation, a new model for predicting the mass of retained condensate is described and compared with the steady-sate retention data. The model is successful in predicting retained condensate over a wide range of tested conditions. The potential of this new approach and possible refinements that will add engineering value are discussed.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Schematic showing the geometry of the test slit-fin round-tube heat exchangers; dimensions are provided in Table 1

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Figure 2

Wind tunnel for thermal-hydraulic measurements and condensate retention experiments

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Figure 3

Heat exchanger test section. This design allows the real-time measurement of retained condensate, along with conventional thermal-hydraulic measurements. (a) wind tunnel; (b) electronic balance; (c) strap; (d) heat exchanger; (e) coolant supply; (f) flexible plastic film; (g) horizontal support; (h) drainage holes.

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Figure 4

Problem description: laminar film condensation on a vertical fin

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Figure 5

Film condensation under forced convection

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Figure 6

Colburn j factors versus Redh under fully wet test conditions

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Figure 7

Fanning f factors versus Redh under fully wet test conditions

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Figure 8

The mass of retained condensate on the tested heat exchangers as a function of time. These data were obtained under conditions: Ta,in=23.9 deg, Tc,in=4.4°C, DPup=18.3°C, and ṁc=0.056 kg/s: (a) air blower frequency 42 rpm or dry-surface Redh∼250; (b) air blower frequency 50 rpm or dry-surface Redh∼350

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Figure 9

With similar latent heat transfer Ql/AT∼115 W/m2, mass retention decreases with air-side Reynolds number (least-squared-error power-law fits to data shown to enhance readability)

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Figure 10

With similar air-side Reynolds number Redh∼275, mass retention increases with the amount of latent heat transfer (least-squared-error power-law fits to the data shown to enhance readability)

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Figure 11

The retained condensate mass retention predicted by the model compared with experimental data. The lines indicate ±20% from experiments, and the rms deviation of the predictions from the experiments is 14.6%.




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