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Research Papers

# Buoyancy Effects on Thermal Behavior of a Flat-Plate Solar Collector

[+] Author and Article Information
Jianhua Fan1

Department of Civil Engineering, Technical University of Denmark, Brovej, DK-2800 Kgs. Lyngby, Denmarkjif@byg.dtu.dk

Simon Furbo

Department of Civil Engineering, Technical University of Denmark, Brovej, DK-2800 Kgs. Lyngby, Denmark

1

Corresponding author.

J. Sol. Energy Eng 130(2), 021010 (Mar 11, 2008) (12 pages) doi:10.1115/1.2840611 History: Received June 21, 2006; Revised August 15, 2007; Published March 11, 2008

## Abstract

Theoretical and experimental investigations of the flow and temperature distribution in a $12.53m2$ solar collector panel with an absorber consisting of two vertical manifolds interconnected by 16 parallel horizontal fins have been carried out. The investigations are focused on overheating and boiling problems in the collector panel. Single-phase liquid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Differently designed collectors are investigated with different collector fluid volume flow rates. The effect of friction and the influence of the buoyancy effects are considered in the investigations. Further, experimental investigations of the solar collector panel are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the back of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good agreement between the measured and calculated temperatures. Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of collector fluid flow rate and inlet temperature on the flow and temperature distribution are shown. The flow distribution through the absorber tubes is uniform if a high flow rate of $10.0l∕min$ is used. By decreased collector fluid flow rate and by increased collector fluid inlet temperature, the flow distribution gets less uniform due to the influence of buoyancy force. If the collector fluid flow rate is small and the collector fluid inlet temperature is high enough, severe nonuniform flow distribution may happen with a small flow rate or even zero or reverse flow in the upper horizontal strips, resulting in overheating or boiling problems in the strips. The CFD calculations elucidate the flow and temperature distribution in the collector panels of different designs. Based on the investigations, recommendations are given in order to avoid overheating or boiling problems in the solar collector panel.

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## Figures

Figure 1

Design of the investigated HTU solar collector

Figure 2

A schematic illustration of the HTU solar collector configuration

Figure 3

Grid setup of the solar collector model

Figure 4

Calculated temperature and volume flow rate distribution in the strips just before the fluid enters the combining manifold

Figure 5

CFD calculated temperature distribution (°C) at the middle plane of the absorber

Figure 6

Measured and calculated tube wall temperature at the joints where the strips meet the combining manifold

Figure 7

Measurements showing boiling in the collector panel

Figure 8

CFD calculated temperature distribution (°C) at the middle plane of the collector panel showing reverse flow in the top two strips

Figure 9

Temperature and flow rate distributions among the strips just before the fluid enters or just after the fluid leaves the combining manifold

Figure 10

Photo of the collector showing bending of the absorber strips

Figure 11

Maximum fluid temperature in the collector panel at different flow rates

Figure 12

Volume flow rate and fluid temperature distributions in the strips just before the fluid enters the combining manifold with a volume flow rate of 25.0l∕min

Figure 13

Volume flow rate and fluid temperature distributions in the strips just before the fluid enters the combining manifold with a volume flow rate of 10.0l∕min

Figure 14

Volume flow rate and fluid temperature distributions in the strips just before the fluid enters the combining manifold with a volume flow rate of 4.0l∕min

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