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Research Papers

Effects of Heat Loss/Gain on the Transient Testing of Heat Wheels

[+] Author and Article Information
Farhad Fathieh

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

Robert W. Besant

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

Richard W. Evitts

Department of Chemical & Biological Engineering,
University of Saskatchewan,
57 Campus Drive,
Saskatoon, SK S7N 5A9, Canada
e-mail: Richard.Evitts@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 THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received October 3, 2015; final manuscript received February 4, 2016; published online April 5, 2016. Assoc. Editor: Pedro Mago.

J. Thermal Sci. Eng. Appl 8(3), 031003 (Apr 05, 2016) (12 pages) Paper No: TSEA-15-1283; doi: 10.1115/1.4032762 History: Received October 03, 2015; Revised February 04, 2016

Heat wheels are used in ventilation systems to provide indoor thermal comfort by recovering considerable amount of sensible energy from exhaust airstream. The transient single step test is a new testing method developed to determine the sensible effectiveness of heat wheels. In practice, heat loss/gain may create large uncertainty in the sensible effectiveness obtained through the transient testing. In this study, the transient analytical model in the literature is extended to account for heat loss/gain effects in the transient testing. The results state that in particular operating conditions, the sensible effectiveness can be affected by more than 10% due to heat loss/gain. A new testing facility is developed to investigate the effects of heat loss/gain on the sensible effectiveness through transient testing of a small-scale heat exchanger. After decoupling heat loss/gain effects from transient test data, less than 2% difference was observed in the sensible effectiveness while supply and exhaust flow rate was small (Re < 209) and the temperature difference between them was ΔTst < 7.0 °C. However, the sensible effectiveness decreased more than 9% while ΔTst > 37.5 °C or Re > 600. An empirical correlation was proposed based on the transient test data that correlates the sensible effectiveness with the heat capacity rate ratio. Comparing the results of proposed correlation with literature, less than 2% difference was observed at the heat capacity rate ratio of greater than 0.5 after the heat loss/gain effects were decoupled from transient test data.

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Figures

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Fig. 1

The small-scale heat exchanger geometry with the supporting frame

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Fig. 2

Schematic of the testing facility with the airflow lines, small-scale exchanger, and the test section

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Fig. 3

Test section, sliding and supporting plates, and air ducts

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Fig. 7

The upper and lower limits for the sensible effectiveness of a counterflow heat wheel with the equal supply and exhaust airflow rate with respect to the wheel time constant for different weighting factors and wheel angle ratio ψw

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Fig. 6

Normalized temperature profile (θ) of a parallel-flow heat wheel with the equal supply and exhaust airflow rate for different values of loss weighting factors: (a) γ′ = 0.1 and (b) γ′ = 0.4

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Fig. 5

Normalized temperature for a parallel-flow heat wheel with the equal supply and exhaust airflow rate at different angle ratios

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Fig. 4

Sensible effectiveness versus angle ratio (ψw) for (a) parallel-flow and (b) counterflow heat wheel with the equal supply and exhaust airflow rates

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Fig. 12

Sensible effectiveness with respect to Reynolds number for different values of wheel angular speeds obtained through transient testing of the small-scale heat exchanger (ΔTst = 7.0 °C): (a) ω  = 1.00 rpm, (b) ω  = 0.50 rpm, (c) ω  = 0.25 rpm, and (d) 0.10 rpm

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Fig. 8

The critical ratio of wheel to heat loss/gain time constant (τw/τ′), above which less than 2% uncertainty is expected in the sensible effectiveness

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Fig. 9

Normalized temperature profile with respect to time obtained through the transient testing of the small-scale heat exchanger at different step change amplitudes (Vf = 0.34 m/s): (a) ΔTst = 4.8 °C, (b) ΔTst = 15.8 °C, (c) ΔTst = 20.1 °C, and (d) ΔTst = 37.5 °C

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Fig. 10

The difference in the sensible effectiveness with respect to the wheel angular speed obtained through transient testing of the small-scale heat exchanger (Vf = 0.34 m/s, Redh = 174)

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Fig. 11

Normalized temperature profile with respect to time for different flow rates obtained through transient testing of the small-scale heat exchanger (ΔTst = 7.0 °C): (a) Redh = 26, (b) Redh = 87, (c) Redh = 261, and (d) Redh = 608

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Fig. 13

Comparison between the sensible effectiveness of a heat wheel calculated from the proposed correlation (obtained by SEM and DEM) and the correlations in literature for (a) Ntu0 = 13.1 and (b) Ntu0 = 2.3

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Fig. 14

The difference between the sensible effectiveness of a heat wheel obtained through literature correlations and the proposed correlation according to (a) SEM (b) DEM

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