Research Papers

Thermomechanical Modeling of Laser Spot Welded Solar Absorbers

[+] Author and Article Information
L. A. Spyrou

Centre for Research and Technology
Hellas (CERTH),
Institute for Research and Technology—Thessaly,
Volos 38333, Greece
e-mail: lspyrou@ireteth.certh.gr

N. Aravas

Fellow ASME
Department of Mechanical Engineering,
University of Thessaly,
Volos 38334, Greece
International Institute for Carbon Neutral
Energy Research (WPI-I2CNER),
Kyushu University,
744 Moto-oka, Nishi-ku,
Fukuoka 819-0395, Japan
e-mail: aravas@mie.uth.gr

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received March 14, 2014; final manuscript received July 30, 2014; published online November 26, 2014. Assoc. Editor: Wayne Cai.

J. Manuf. Sci. Eng 137(1), 011016 (Feb 01, 2015) (15 pages) Paper No: MANU-14-1117; doi: 10.1115/1.4028197 History: Received March 14, 2014; Revised July 30, 2014; Online November 26, 2014

A finite element (FE) approach is developed to investigate the laser spot welding (LSW) of flat-plate solar absorbers and the stress and distortion fields that develop after fabrication and during operation. Numerical calculations at two different levels are carried out. At a microscopic scale, the details of a spot weld are analyzed. At a macroscopic level, a global approach is used to simulate the joining of the pipeline to the absorber plate and the “restoration” (flattening) process of the absorber. The simulated welding-induced distortion is compared with experimental measurements. The thermomechanical behavior of a solar absorber under working conditions is also studied and operational stresses and the critical locations for structural failure are reported.

Copyright © 2015 by ASME
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Fig. 1

(a) A laser spot welded Al–Cu flat-plate solar absorber, (b) detail of LSWs, and (c) 3D sketch of the structure's components

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Fig. 2

(a) 2D FE model of a two-sided spot welded solar fin (half of the geometry was modeled due to symmetry), (b) transverse cross section of a typical weld macrostructure, and (c) FE mesh in the near-weld area

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Fig. 3

(a) Schematic diagram of the heat flow into the model; d = 0.6 mm is the diameter of laser's beam circular cross section and d2=23d. (b) Outward unit normals on surfaces.

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Fig. 4

Temperature-dependent thermal–physical properties used in the simulations for (a) 1050 aluminum and (b) DHP copper. The dashed lines show properties used during the simulation of the solidification process when the supercooling effect is taken into account.

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Fig. 5

Contours of temperature (a) at the start of the analysis, (b) after the application of the heat source (0.3 ms), (c) after the addition of the weld joint in the model (0.3 ms), and (d) after cooling down to room temperature

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Fig. 6

Temperature histories on surface points of Al-sheet (point A in Fig. 5(b)) and Cu-tube at the spot weld area (point B in Fig. 5(b)): (a) Al-sheet heating, (b) Al-sheet cooldown, (c) Cu-tube heating, and (d) Cu-tube cooldown

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Fig. 7

Temperature-dependent mechanical properties used in the simulations. The dashed lines show properties used during the simulation of the solidification process when the supercooling effect is taken into account.

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Fig. 8

Contours of (a) von Mises equivalent stress, (b) maximum principal stress, and (c) minimum principal stress

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Fig. 9

Geometric representation of (a) flat sheet, (b) pipe network, (c) flat-plate solar absorber, and (d) solar fin

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Fig. 10

Welding sequences. (a) Continuous sequence starting from one lateral side and ending at the other lateral side of the collector, (b) sequence starting from the middle part of the collector and ending at the lateral sides. Letters S and E in the figures stand for start and end of the welding sequence.

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Fig. 11

(a) Regions in red indicate the locations of the spot welds and (b) and (c) temperature contours showing the spot welding pattern at time t = 0.37 s and t = 0.96 s, respectively

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Fig. 12

Welding-induced longitudinal bending distortion (5×) of (a) solar fin and (b) full-size solar absorber. The maximum vertical displacements at the lateral sides of the absorbers are defined as dfin and dfull for the solar fin and the full-size absorber, respectively.

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Fig. 13

Distortion restoration process: (a) rotation is applied at the two opposite boundary edges of the panel, (b) inverse bending distortion occurs, and (c) the initial flat shape of the panel is restored when the rotation is released

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Fig. 14

Contours of von Mises equivalent stress at the solar collector (a) after the welding process is completed and (b) after the restoration of the welding-induced longitudinal bending distortion

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Fig. 15

Contours of the final normal residual stresses at the solar collector after the manufacturing process is completed. (a) Transversal residual stress σ11 and (b) longitudinal residual stress σ22.

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Fig. 16

Typical daily distribution of solar flux on a flat surface during summer periods

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Fig. 17

Under operating conditions the solar absorber is fixed in four points at its two opposite boundary edges. The points are depicted with red color.

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Fig. 18

Contours of von Mises equivalent stress σeq after 10 days of continuous operation. (a) σeq distribution at morning and night hours and (b) σeq distribution at noon.

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Fig. 19

Contours of von Mises equivalent stress σeq when the solar absorber faces stagnation temperatures for 10 days. (a) σeq distribution at morning and night hours and (b) σeq distribution at noon.




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