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Experimental study on the droplet formation around pins of different geometry for the design of a compact falling-droplet absorber

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Abstract

Absorber downsizing for the development of compact absorption chillers is a known challenge of this type of refrigerator. Past studies have revealed how a droplet flow regime can increase the interface area and enhance absorption rates, especially during the droplet formation. This study proposes a space-efficient design for an adiabatic absorber based on a bank of solid pins coupled with a droplet flow regime. Manufacturing through 3D printing technique is used to study the effect of different fin shapes during droplet formation. Droplet behavior is firstly studied analytically through a variational approach. Experiments on pure water are then carried out to validate the model and produce design guidelines for a H2O-LiBr absorber. Results show that the analytical model is more accurate in the regions close to the droplet bottom. The rhomboidal geometry with 120° returned the smallest droplet volume without allowing coalescence of more droplets, ensuring the maintenance of droplet flow and a high surface area for mass transfer. Disturbances in the droplet profiles were observed, caused by the pin-droplet interaction. A map has been then created to allow a quick sizing of the absorber and find its main geometrical and operational features.

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Abbreviations

A:

Orifice cross section area (m2)

B:

Curvature radius (non-dimensional)

Bo = ρgr2/σ:

Bond number

C:

Normalizing factor

Cd :

Discharge coefficient

d:

Orifice diameter (m)

dm :

Droplet diameter (m)

g:

Gravitational acceleration (m/s2)

H:

Static head (m)

L:

Pin length (m)

Oh = μ/(ρrσ)1/2 :

Ohnesorge number

P:

Pressure (Pa)

Q:

Volume flow rate (m3/s)

r:

Orifice radius (m)

S:

Arc length (non-dimensional)

u:

Speed (m/s)

V:

Volume (non-dimensional)

We = ρru2/σ:

Weber number

X:

Radial coordinate (non-dimensional)

x:

Radial coordinate (m)

Z:

Height (non-dimensional)

z:

Height (m)

β:

Contact point angle

λ:

Taylor wavelength

ρ:

Density (kg/m3)

σ:

Surface tension (N/m2)

θ:

Angle (rad)

μ:

Dynamic viscosity (Pa s)

cr:

Critical

d:

Droplet

eq:

Equilibrium

exp:

Experimental

L:

Surrounding fluid

o:

Boundary

th:

Theoretical

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Authors and Affiliations

  1. Nanyang Technological University, 50 Nanayag Ave, Singapore, 639798, Singapore

    Fabrizio Cola & Alessandro Romagnoli

  2. Singapore Institute of Manufacturing Technology (SIMTech), 73 Nanyang Drive, Singapore, 637662, Singapore

    Jonathan Hey

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  1. Fabrizio Cola
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  2. Alessandro Romagnoli
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Correspondence to Jonathan Hey.

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Appendix – Validation of droplet formation model for different pin shapes

Appendix – Validation of droplet formation model for different pin shapes

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Cola, F., Romagnoli, A. & Hey, J. Experimental study on the droplet formation around pins of different geometry for the design of a compact falling-droplet absorber. Heat Mass Transfer 54, 3599–3616 (2018). https://doi.org/10.1007/s00231-018-2381-8

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  • DOI: https://doi.org/10.1007/s00231-018-2381-8

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