Research Papers

Experimental Study With Analytical Validation of Thermally Driven Flow in Risers of Solar Water Heaters Under Varying Scale Thickness and Heat Flux

[+] Author and Article Information
U. C. Arunachala

Department of Mechanical
and Manufacturing Engineering,
Manipal institute of Technology,
Manipal 576 104, India
e-mail: arunchandavar@yahoo.co.in

M. Siddhartha Bhatt

Energy Efficiency
and Renewable Energy Division,
Central Power Research Institute,
Bangalore 560 080, India
e-mail: msb@cpri.in

L. K. Sreepathi

Department of Mechanical Engineering,
JNN College of Engineering,
Shimoga 577 204, India
e-mail: sreepathi_lk@hotmail.com

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received February 12, 2013; final manuscript received August 22, 2013; published online November 26, 2013. Editor: Gilles Flamant.

J. Sol. Energy Eng 136(2), 021018 (Nov 26, 2013) (9 pages) Paper No: SOL-13-1053; doi: 10.1115/1.4025716 History: Received February 12, 2013; Revised August 22, 2013

This paper investigates the deterioration in the performance of thermosiphon flat plate solar water heaters (SWH) with water side scaling. The study presents the analytical and experimental variation of mass flow rate of water with scale thickness in risers of conventional solar flat plate water heater for different electrical power inputs (covering the full range of solar incident radiation up to 1 kW/m2). This information is extended further to determine the drop in efficiency characteristics represented by the Hottel–Whillier–Bliss (H–W–B) constants for full–fledged SWH. To simulate scaling in risers in the absorber plate of a SWH, an artificial method of coating has been used to create single pipe riser of different uniform scale thicknesses. Four such risers are created with scale thickness of 0 mm, 0.7 mm, 1.7 mm, 2.7 mm, and 3.7 mm. The observed drop in mass flow rate through the range of risers between 0 mm and 3.7 mm scale thickness is 58.5% for the thermal input power (supplied through electric heating) of 129.5 W (corresponding to a solar incident radiation of 980 W/m2). In comparison, the analytical results show a corresponding drop of 70.12%. A comparison of the coated riser with a cut tube of an actually scaled riser indicates excellent matching of thermal conductivity. The divergence between experimental and analytical mass flow rate in the case of a riser of the highest scale thickness, viz., 3.7 mm, is the lowest because of increased pressure gain in the flow region together with higher temperature than predicted by the general equation. The experimental data of various energy parameters from the single tube scaled riser studies are matching with analytical values for the different input electrical power levels (corresponding to the different solar radiation levels). As identical conditions are used in the experimental analysis, the results for risers of various scale thicknesses and electrical power inputs are applicable to corresponding full–fledged SWH.

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Fig. 1

Various losses in SWH

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Fig. 2

Artificially coated copper tubes

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Fig. 3

Experimental set up for thermosiphon system

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Fig. 4

Arrangement to find thermally driven water flow rate

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Fig. 5

Various losses in flow through riser

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Fig. 6

Convergence for different starting value of a0

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Fig. 7

Convergence for different starting value of a1

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Fig. 8

Pressure drop/gain versus mass flow rate

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Fig. 9

Mass flow rate variation at 44 W

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Fig. 10

Mass flow rate variation at 67.5 W

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Fig. 11

Mass flow rate variation at 96 W

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Fig. 12

Mass flow rate variation at 129.5 W

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Fig. 13

Comparative mass flow rate in artificial and actually scaled riser

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Fig. 14

Flow chart to determine solar radiation level

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Fig. 15

Flow chart to determine experimental H–W–B constants

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Fig. 16

Flow chart to determine analytical H–W–B constants

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Fig. 17

Comparison of experimental and analytical mass flow rates

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Fig. 18

Comparison of experimental and analytical overall heat loss coefficient

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Fig. 19

Comparison of experimental and analytical a0

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Fig. 20

Comparison of experimental and analytical a1



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