Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

FORMING A STRUCTURED SURFACE OF A NEW TYPE SOLAR ABSORBER WITH

HYDROFORMING

Libor MRŇA1, Jan ŘIHÁČEK2

(1) doc. RNDr. Libor Mrňa, Ph.D., VUT v Brně, FSI, ÚST, Technická 2, 619 69 Brno, mrna@fme.vutbr.cz

a Ústav přístrojové techniky AV ČR, Královopolská 147,612 64 Brno, mrna@isibrno.cz

(2) Ing. Jan Řiháček, VUT v Brně, FSI, ÚST, Technická 2, 619 69 Brno, y116018@stud.fme.vutbr.cz

Abstrakt

One of the ways to increase efficiency of the solar absorber is modification of absorption area in the system

of pyramidal cavities, where the incident radiation is absorbed by multiple reflections. The paper deals with

the technology of production of the flow solar absorber with a structured surface in a single technological

operation with hydroforming. Two laser-welded sheets are inserted into the forming chamber. Then the

space between them is pressurized with hydraulic fluid which causes their form to the desired shape. The

first part of this article deals with theoretical simulations of forming a different structured surface in the

programming environment of ANSYS. The second section describes hydroforming box. Follows practical test

of hydroforming. As a material for the production of samples is used austenitic stainless steel X5CrNi18-10.

Keywords: Solar absorber, hydroforming, austenitic steel, ANSYS, numeric simulation

1. INTRODUCTION

Currently the design of plate solar collectors for water heating is flat aluminum sheet coated with an

absorbent layer and register from pipes which is connected on the reverse side. This construction has two

principal drawbacks. The first drawback is the transfer of heat from the absorber sheet in a pipe only at their

linear contact. A related technological problem joints between aluminum sheet and copper tube. A second

drawback is the requirement principled perpendicular incidence of solar radiation on the absorber plate for

maximum thermal efficiency. This situation occurs only in a small period of time; therefore, the total daily

(annual) thermal efficiency is low.

It was designed a new type of flat solar absorber eliminates the above-mentioned drawbacks. It is based on

the so-called plate design solar absorber with meander structure providing controlled heat transfer medium

circulation over the entire surface of the absorber. In addition, the absorption area comprises the set of

pyramidal cavities which absorbing solar radiation through multiple reflections. This will reduce the

dependence on the impact angle and increases the heat transfer surface. But this solution creates the

difficulties of technological nature [1].

2. TECHNOLOGY OF FABRICATION STRUCTURE SURFACE

Technology of laser welding and technology of hydroforming was proposed for the production. The

manufacturing process is as follows: first weld two thin sheets using a laser deep penetration welding and

these welds creates the required meander structure. Thereafter, this weldment inserted into the

hydroforming device whose internal surface is formed by the desired structure (set of pyramids). Then

forming liquid under high pressure is begun pump to the space between the plates, see fig.1. This fabrication

requires solutions solve some structural nodes. It is primarily a question of the outlet, because solar absorber

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

is formed by sheet thickness 0.5 to 1.0 mm. These outlets are formed technology Flow-drill when the friction

tool formed the collar by the excess material in the plate [3]. In this collar creates a thread for screwing the

flanges for inlet forming fluid. Also, additional technical

measures are made to prevent the rapture high pressure

flanges of the thread. The actual realization of the

hydroforming device is in Figure 2. Hand pressure two-

stage pump with a maximum pressure of 700 bar is

connected via quick connectors. Two types of dies with

an apex angle of 900 and 600 were used for experiments.

Further, combination of stainless steel sheet was

selected to realize the sample size 150x150 mm. Sheet

1.0 mm thick for the rear plate, which was created inlet

and outlet port G1 / 4 ". The second section with of 0.5

mm and 0,8 mm was chosen for the front plate, which is

to create a textured surface thickness [4], [5].

Figure 2 Hydroforming device. Left – cross section, right – realization include hydraulic pump

3. THE VERIFICATION OF MANUFACTURABILITY USING FEM

Manufacturability of these pyramids on the surface of the absorber of course depend on several parameters.

One of the most significant indicators is prediction of thinning of the sheet thickness in the area with the

greatest deformation of indents and finding amount of pressure needed to die-stamping, ie. identification of

critical points with maximum thinning of the wall, which of course must not exceed the permissible limit.

Described problems in complexity can be solving besides analytical calculation also experimental verification

of numerical simulations using Finite Element Method (FEM). It also allows users to easily change the

conditions of simulating and therefore helps to easier and faster optimization of stamping process.

Figure 1 Principle of hydroforming structure

surface

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

0

100

200

300

400

500

600

700

800

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Engin

eering s

tress σ

[M

Pa]

→

Engineering strain ε [-] →

3.1 The Model of Material

The main point of pre-processing, ie.

preparing data before the actual numerical

calculation, is the correct description of the

behavior of formed material. As previously

stated, the pre-defined criteria for the

production of absorbers was chosen

austenitic chromium-nickel stainless steel

X5CrNi18-10. The main characteristic of the

material experimentally obtained is the

tensile test (Fig. 3). Of course it’s possible

converted according to the classical

equations (1) and (2) to obtain the

dependence of the strength on the true (logarithmic) strain, which has been primarily in the area of

reinforcement material used as material input for numerical simulation [6].

(1)

(2)

where σTRUE [MPa] is the true stress, σENG [MPa] is the engineering tensile stress, ε [-] is the engineering

strain, φ [-] is the true (logarithmic) strain.

Then the material properties was approximated entered to ANSYS Workbench 14 software by using bilinear

material model, which is characterized by the values shown in the table. 1.

Table 1 Material properties of X5CrNi18-10 steel

Yield strength Rp0,2 [MPa] 282,85

Tensile modulus E [MPa] 1,99 ∙ 105

Plastic hardening modulus Epl [MPa] 1 727,50

Poisson ratio µ [ - ] 0,30

Density at 20 °C ρ [kg ∙ m3] 7,90 ∙ 103

3.2 The Model of Geometry

In order to simplify and therefore speed up the

computation forming process was used to simulate

a simplified geometric model. It assumes the role of

partial symmetry and the possibility of neglecting

those parts that do not have direct involvement in

the pressing process. Software ANSYS has full

compatibility with that CAD modeler - Autodesk

Inventor Professional 2010, allowing not only the

direct import of CAD system in the simulation

environment, but also adaptive update the model

when its changes. It was also necessary to make

a full continuum discretization of stamped material

and tools using finite element mesh.

Detail of some discretization parts is presented in Fig. 4. Emphasis was placed on the requirement greatest

possible uniformity FEM mesh due later correctness of the results. Fully defined model, which is ready to

calculate was obtained after discretization of individual macro elements and applying the boundary

conditions, ie. stamping pressure, contacts and other bounding conditions.

Fig. 4 Discretization of die and sheet

)1ln(

)1(

ENGTRUE

Fig. 3 Tensile test record for X5CrNi18-10 steel

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

It is possible to focus on the evaluation after the

simulation. The simulation by finite element method

gets primarily required pressing force result. At

a pressure of 400 bar bulges are formed to a depth

of about 3.5 mm, at 600 bar are already about

4 mm. Fig. 5 shows a comparison of formed

structures with the prediction of shape by simulation

using a forming pressure of 600 bar.

4. EXPERIMENTAL HYDROFORMING

The simulation shows that the required forming pressure is in the range of 500 bar. It follows that, on both

surfaces of the hydroforming device will cause the force size of about 1125 kN. Although the walls are

composed of structural steel thickness of 15 mm, the resulting force causes the swelling of tens of

millimeters. Therefore, the entire hydroforming device is compressed in a hydraulic press before the

experiment - see Figure 6 - to applicate a counterforce to the forming pressure of the hydraulic oil. Fig. 7

shows the shape of the indents after hydroforming.

5. COMPARISON OF THINNING BULGES WITH THE THEORETICAL MODEL

One of the results of numerical simulations using finite element analysis is also changing the thickness of the

forming. This was subsequently confronted with experimental way - by measuring the profile of the bulges.

With regard to complexity of the shape measurement was performed in a straight direction but also

diagonally, as indicated in Figure 9. Measurement was carried out from the edge formed area just behind the

weld bead across three bulges. The first analysis was carried out variations in thickness in the forward

direction.

Fig.5 Comparison of model and formed structure

Fig. 7 Hydroformed structure surface, apex angle is 900

Fig. 6 Hydroforming device

inside hydraulic press

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

Schematically measurement path is shown in Fig. 8, respective cross section of formed sheet is shown in

Fig. 9. Thickness of formed sheet was measured by tip micrometer Mitutoyo. Evaluation for hydroforming

test with 900 die is on figure 10 and 11.

Fig. 10 Thickness of indents - direct Fig. 11 Thickness of indents - diagonal

In the both groups can be observed variable thickness values from forming about 0.47 mm to 0.36 mm. In

view of comparing simulation software ANSYS and experimental measurement can be stated very good

level of agreement in the results with a maximum deviation of 4.5%.

Similar evaluation was performed for the die 600, where hydroforming is more problematic.

Fig. 12 Thickness of indents - direct Fig. 13 Thickness of indents - diagonal

In this experiment violation drawn material before reaching the calculated pressure. It will be necessary to

increase the wall thickness of 0.8 mm. Nevertheless, the difference of the measured and modeled values, in

accordance to 10%.

0,36

0,38

0,40

0,42

0,44

0,46

0,48

0,50

0 5 10 15 20 25 30

Th

ickn

ess [m

m] →

Measured distance [mm] →

0,36

0,38

0,40

0,42

0,44

0,46

0,48

0,50

0 5 10 15 20 25 30 35

Th

ickn

ess [m

m] →

Measured distance [mm] →

0,3

0,32

0,34

0,36

0,38

0,4

0,42

0,44

0,46

0,48

0,5

0 10 20 30

Th

ickn

ess [m

m] →

Measured distance [mm] →

ExperimentalmeasureFEM simulation

0,3

0,32

0,34

0,36

0,38

0,4

0,42

0,44

0,46

0,48

0,5

0 10 20 30 40

Th

ickn

ess [m

m] →

Measured distance [mm] →

ExperimentalmeasureFEMsimulation

Fig. 8 Measurement path Fig. 9 Cross section of formed sheet

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

6. CONCLUSION

For verification technology textured surface of the solar absorber with a controlled circulation has been

designed and implemented hydroforming device and practical tests were conducted with a sample, which

was shaped wall made of a material X5CrNi18-10 thickness of 0.5 mm. It was also structured surface

simulated by ANSYS. During the experiment it was shown that in the manufactured hydroforming device can

create a structured surface of the solar absorber of the austenitic stainless steel sheet of 0.5 mm thickness

for dies with an apex angle of 900 and 600. Comparison of theoretical simulations thinning of the formed

material showed very good agreement and therefore can be used for further modeling simulations.

ACKNOWLEDGEMENTS

The contribution was made in the framework of the project VUT FSI Brno no. FSI-S-11-26 "The use of

progressive technologies in the production of solar absorber" (ev. no: 1390) and no. FSI-S-12-05

"Advanced solar absorber with a structured surface and controlled circulation" (ev. no: 1664) and

with the support of the European Commission and the Ministry of education (project no CZ

1.05/2.1.00/01.0017) and NPU LO1212.

Further with support of Technological Agency of Czech Republic, project No.: TA04020456

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(Velká Británie): Cambridge University Press, 2014, 344 s. ISBN 978-1-107-00452-8.

[3] Flow drill. Www.flowdrill.com [online]. [cit. 2014-05-14]. Dostupné z: www.flowdrill.com

[4] MRŇA, L., FOREJT M., LIDMILA Z., PODANÝ K. a KUBÍČEK J. VUT Brno, CZ. Solární absorbér se

strukturovaným povrchem. ČR. Užitný vzor, 24053. Uděleno 11.7.2012. Zapsáno 2.7.2012.

[5] MRŇA, L. VUT Brno, CZ. Zařízení pro pneumatické přidržování součástí u velkoplošného průvarového svařování.

ČR. Užitný vzor, 25889. Uděleno 2.10.2013. Zapsáno 24.9.2013

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