Experimental Investigation of Convective Heat

Loss from a Bladed Solar Receiver

Farzin Ghanadi1, Juan F. Torres2, Maziar Arjomandi1 and John Pye2

1School of Mechanical Engineering, The University of Adelaide2Research School of Engineering, The Australian National University

December 2018

Asian-Pacific Solar Research

Conference 2018

Centre for Energy Technology

2University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Introduction

Convective losses reduction for a per-

tube-area basis

Light trapping improvement, and a

smaller aperture without exceeding flux

limits at the tube surface

Project: Bladed Receiver with Active Airflow Control (BRAAC)

Heat loss reduction by converting

receivers from convex tube-banks to

finned/bladed configurations

Ho et al. (2014) and Pye et al. (2016)

3University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

To measure mixed and forced convection heat losses from a bladed

receiver

To correlate heat transfer rate and the flow behaviour between the

blades

• Receiver orientation

• Blade length

• Upcoming wind speed

• Wall temperature

Objectives

4

Heater 2

Heater 3

Heater 4

S= 72 mm

300 mm

Base ThermocouplesBackside insulation

board

Heater 1

Blades

The experimental setup in the wind tunnel at the University of Adelaide

Mixed convection regime around a tilted lab-scaled model of the bladed receiver

Bladed

receiver

𝑅𝐵𝑆:

B/S=0.5,

0.8,1, 2 & 3

2 S

B

𝜃Pitch angle

1.5m

1

3

4

Airflow

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Method

5

Temperature distribution along the blades

IR-camera images for each blade edge

Re=2.8×104, 𝜃= 70°, 𝑅𝐵𝑆=0.8, 𝑇𝑤=200°C

• The summation of measured power inputs from the four heating elements

indicates the total heat loss from the receiver

𝑄conv = 𝑃heat − 𝑄rad − 𝑄cond

50% of the total heat losses (Torres et al 2018)

Based on view factors

obtained by the MCRT model

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Method

6

Variation of heat transfer coefficient at various aspect ratios 𝑈 = 3 m/s (dotted lines) and

𝑈= 6 m/s (solid lines)

𝑇𝑤=200°C𝑇𝑤=100°C

Results

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

• Increasing wall temperature to 𝑇𝑤=200°C has low (less than 5%) influence on

the total heat loss magnitude

• Increasing flow velocity from 3 m/s to 6 m/s amplifies the heat transfer rate

by a factor of two where 𝜃= 70° and 𝑅𝐵𝑆=0.5

7University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Laser sheet

Method

Investigation of flow behaviour between the blades using Particle Image

Velocimetry (PIV)

Low-speed, circulating and open water channel: 500mmW x 500mmH x 2000mmL

Nd-YAG

Andor ZYLA sCMOS

Schematic of experimental rig in the water channel at the

University of Adelaide

8University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Total heat loss from Heater 2 (located

between the upper two blades) as a function

of the pitch angle

𝑇𝑤=100°

𝑈 = 3 m/s

𝑈 = 6 m/s

Time-averaged velocity contour and streamlines when 𝑅𝐵𝑆=3 and

Re=1.1 × 104

• Increasing pitch angle from 26° to 45° results in an increment in heat transfer rate

• For longer blades, 𝑅𝐵𝑆= 3, the

pitch angle has stronger

influence on the total heal loss

Results

9

Time-averaged velocity contour and streamlines

when 𝑅𝐵𝑆=3 and Re= 2.2 × 104Total heat loss from Heater 2 (located

between the upper two blades)

𝑇𝑤=100°

𝑈 = 3 m/s

𝑈 = 6 m/s

• Flow impingement on the trailing sidewall of the cavity downwards towards the

cavity back wall with higher velocity when Reynolds number is increased to 2.2×104 .

• For higher Reynolds number the circulation zone inside the cavity extends

downwards

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Results

10

• 𝑇∞, 𝑇𝑤 and 𝑇0: the ambient temperature, wall temperature and jet temperature

• G: gravitational acceleration

• S : blade spacing

• 𝐷𝑚𝑖𝑛

• b𝑗: jet width

• 𝑢𝑗 : jet velocity

𝑢𝑗 𝑚𝑖𝑛 =𝐷𝑚𝑖𝑛𝑆𝑇𝑗 𝑔𝑆

𝑇𝑤 − 𝑇∞𝑇∞𝑇𝑤

+0.5𝑈∞

2

𝑇𝑤𝑏𝑗

• Stable operation of the flow curtain

• Minimum bending modulus method proposed by Zhang et al (2018) & Hayes (1969),

Using water curation with the potential to reduce the convective losses

𝛼𝑗

≈ 0.15 − 0.18 (stable curtain when 𝑆 𝑏𝑗 = 20 − 25)

≈ 3 mm

≈ 1.6𝑈∞

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Results (Active flow control)

11

Results (Active flow control)

Top view Side view

Arrangement of experimental rig in the water

channel at the University of Adelaide

Schematic of the mini-water tunnel to generate the planner jet

• Flush mounted water jet on

the top vertex of the cavity

parallel to its aperture plane

• Uniform flow generated by

a mini-water tunnel

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

1212

Time-averaged velocity contour and streamlines for 𝑈∞ = 0.14 𝑚/𝑠 (𝑈𝑗/𝑈∞=5)

No jet 𝛼𝑗 = 20°𝛼𝑗 = 0°

15 cm

𝑼∞

Jet

No jet 𝛼𝑗 = 0° 𝛼𝑗 = 20°

Jet

𝑼∞

Time-averaged velocity contour and streamlines for 𝑈∞ = 0.28 𝑚/𝑠 (𝑈𝑗/𝑈∞=2.5)

• Increasing the freestream velocity reduces the stability of the jet flow

• Using a planner jet flow with 𝑈𝑗/𝑈∞= 5 and 𝛼𝑗= 20° increases the stagnation zone inside

the cavity

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Results (Active flow control)

𝛼𝑗 = 0° 𝛼𝑗 = 20°No jet

Time-averaged velocity contour and streamlines for 𝑈∞ = 0.28 𝑚/𝑠 (𝑈𝑗/𝑈∞=2.5)

No jet 𝛼𝑗 = 0° 𝛼𝑗 = 20°

Time-averaged velocity contour and streamlines for 𝑈∞ = 0.14 𝑚/𝑠 (𝑈𝑗/𝑈∞=5)

𝑼∞

𝑼∞

Jet

Jet

5 cm

13

Use of an air curtain to reduce heat loss

Arrangement of test rig located at the test section of the

wind tunnel, University of Adelaide

Results (Active flow control)

Schematic diagram of a cross-section of

the test rig and wind direction

• A mini-wind tunnel was designed and 3D printed

- b𝑗: jet width ≈ 5 mm

- 𝑢𝑗 : jet velocity≈ 7.7m/s (for stable air curtain 𝑢𝑗 ≈ 2.2 𝑢∞ )

• Wall temperature Tw = 1000C

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

𝜶𝒋 = 𝟏𝟓°𝜶𝒋 = 𝟎°

𝑄𝑤𝑖𝑡ℎ𝑎𝑖𝑟𝑐𝑢𝑟𝑡𝑎𝑖𝑛/ 𝑄𝑤𝑖𝑡ℎ𝑜𝑢𝑡𝑎𝑖𝑟𝑐𝑢𝑟𝑡𝑎𝑖𝑛

𝑄𝑤𝑖𝑡ℎ𝑎𝑖𝑟𝑐𝑢𝑟𝑡𝑎𝑖𝑛/ 𝑄𝑤𝑖𝑡ℎ𝑜𝑢𝑡𝑎𝑖𝑟𝑐𝑢𝑟𝑡𝑎𝑖𝑛

Results (Active flow control)

• A significant reduction in the heat transfer ratio when the wind velocity is

close to the jet velocity

• 10% reduction in the heat transfer ratio when the jet angel increases from

0 to 15°

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver 14

15

• Up to 75% reduction in the convective heat transfers as the blade length

becomes 6 times longer,

• Up to 40% reduction in the convective heat transfer by increasing the

pitch angle from 26° to 70°,

• Linear relationship between the convective heat loss and the freestream

velocity,

• Increasing wall temperature from 100°C to 200°C enhances convective

heat transfer coefficient by less than 5,

• The ideal receiver; longer blade, 𝑅𝐵𝑆 = 3, and steep tilt, 70°,

• The jet flow with an angle of 20° and jet-to-cross-stream velocity ratios

(R) ≈5, can bridge the cavity opening without a strong interaction with the

circulation zone inside the cavity.

Conclusion

University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

To do: Critical parameters of “air curtain” which have significant impacts on the mixed convective heat loss.

16University of Adelaide, Australian National University │ Experimental Investigation of Convective Heat Loss from a Bladed Solar Receiver

Acknowledgements

Centre for Energy Technology at the University of Adelaide

Australian Renewable Energy Agency (ARENA).

Mechanical and Electronic workshops at the University of Adelaide

Life Impact | The University of Adelaide

Delivering innovative technologies

for a clean energy future

Thank You