Presentazione dell'Ing. Davide Del Col (Università di Padova) al XVI Convegno Europeo il 12 giugno 2015 - 1a sessione

1. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Davide DEL COL, Marco AZZOLIN, Stefano BORTOLIN
HEAT TRANSFER PERFORMANCE
OF THE BLEND R32/R1234ze(E)
Dipartimento di Ingegneria Industriale
Università degli Studi di Padova

2. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Outline
 Refrigerant mixtures
 Mixtures as possible low-GWP alternatives
 Mixture R32/R1234ze(E)
Pressure drop
Heat transfer in condensation
Heat transfer during flow boiling
Heat transfer penalization due to mass transfer
 Conclusions

3. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Refrigerant mixtures
Azeotropic mixtures
 At the vapor-liquid equilibrium state, the vapor phase and the
liquid phase display the same composition.
 Condensation and vaporization take place at a constant
temperature (for isobaric process), without temperature
glide.
 They behave like pure components.
 Not additional mass transfer resistance in the heat transfer.
 Some HFC mixtures proposed in the past years (e.g. R410A)
display a low temperature glide (near azeotropic mixtures).
R410A at p = 20 bar
tbubble = 39.56°C; tdew = 39.68 °C; temperature glide = 0.12 K

4. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Refrigerant mixtures
Zeotropic mixtures
 At the vapor-liquid equilibrium, the vapor phase and the liquid
phase have different compositions.
 At a fixed pressure, condensation and vaporization take place
with a temperature glide.
 Better matching of temperature profiles can reduce the
thermodynamic irreversibility in counter-current heat exchangers.
 The heat transfer is influenced by an additional mass transfer
resistance.
 Heat transfer coefficients are more difficult to predict.
 Possible shift of composition in case of leakages.
 New HFC/HFO blends are zeotropic mixtures.

5. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Refrigerant characteristics
 Stability within the refrigeration system
 Low GWP
 No ozone depletion potential (ODP)
 Low flammability and toxicity
 Low cost
 Compatibility with the materials of construction.
 Few single-component low-GWP refrigerants are well developed so
far that can replace HFCs in refrigeration systems.
 Without considering the possibility of synthesizing new compounds
in the future, the choice is at the moment limited to few promising
fluids. (McLinden et al., Int. J. Refrig. , 2014).

6. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Why mixtures?
Legislative acts recently adopted by the European Union to control emissions from
fluorinated greenhouse gases (F-gases), including HFCs :
• The MAC Directive prohibits the use of F-gases with a global warming potential of more
than 150 times greater CO2 in new types of cars and vans introduced from 2011, and in all
new cars and vans produced from 2017.
• The original F-gas Regulation (2006) is replaced by a new Regulation adopted in 2014
which applies from 1 January 2015 (Regulation (EU) No 517/2014)
Thanks to the new F-gas Regulation, the EU’s F-gas emissions will be cut by two-thirds by
2030 compared with 2014 levels.
Natural
Refrigerants
(flammable, toxic)
HFOs
(few are well developed,
poor HT characteristics
if compared to HFC)
Mixtures
(reliable alternative,
easy and fast)

7. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
R32/R1234ze(E) blends
Properties of pure R32 and R1234ze(E) at 40°C saturation temperature. Data from NIST Refprop v. 9.1.
Fluid
psat
[bar]
ρl
[kg m-3]
ρg
[kg m-3]
µl
[µPa s]
λl
[W m-1 K-1]
tdew
[°C]
tbub
[°C]
Glide
[K]
R32 24.8 893.0 73.3 95.0 0.115 / / /
R1234ze(E) 7.7 1111.3 40.6 167.0 0.069 / / /
23/77% 13.1 1075.3 56.3 143.8 0.083 45.5 34.5 11
46/54% 17.4 1017.7 64.6 121.9 0.094 44.3 36.1 8.2
50/50% 18 1008.3 65.5 119.2 0.096 43.7 36.3 7.4

8. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Condensation of zeotropic mixtures
Additional mass transfer resistance during the condensation process of a non–
azeotropic mixture

9. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Experimental apparatus
 1 mm diameter circular channel
 wall and water thermocouples for
HTC measurements
 pressure drop test section
 condensation and vaporization test
section

10. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Two-phase pressure drop
Del Col et al. (2013) model compared
against frictional pressure gradient data for
the mixture R32/R1234ze(E).
Experimental frictional pressure gradient
vs. vapor quality at G = 400 kg m-2s-1.

11. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Condensation heat transfer coefficients
HTC experimental data Cavallini et al. (2006) model with the
Silver-Bell-Ghaly correction.
0
2000
4000
6000
8000
10000
12000
14000
16000
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
HEATTRANSFERCOEFFICIENT[Wm-2K-1]
VAPOR QUALITY []
G800 46/54 G800 23/77
G600 46/54 G600 23/77
G400 46/54 G400 23/77
G300 46/54 G300 23/77
G200 46/54 G200 23/77
G150 46/54 G150 23/77

12. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
0
5000
10000
15000
20000
25000
30000
0 5000 10000 15000 20000 25000 30000
HTCCalculated[Wm-2K-1]
HTC Experimental [W m-2 K-1]
G300
G400
G550
G600
+30%
-30%
0
5000
10000
15000
20000
25000
30000
0 5000 10000 15000 20000 25000 30000
HTCCalculated[Wm-2K-1]
HTC Experimental [W m-2 K-1]
G300
G400
G550
G600
+30%
-30%
Comparison with the models
12
Sun and Mishima (IJHMT, 2009)Gungor and Winterton (IJHMT, 1986)
CalculatedHTC[Wm-2K-1]
Experimental HTC [W m-2 K-1] Experimental HTC [W m-2 K-1]
CalculatedHTC[Wm-2K-1]
HTC models for pure fluid are corrected to account for the mass transfer resistance.
(correction by Thome, IJR 1996)

13. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Comparison with pure fluids
Dashed lines refer to an ideal linear behavior. A mass composition equal to 0 corresponds to pure R1234ze(E).
0
2000
4000
6000
8000
10000
12000
14000
16000
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
HEATTRANSFERCOEFFICIENT[Wm-2K-1]
MASS COMPOSITION (fraction of R32)
G800 x=0,7 G800 x=0,4
G400 x=0,7 G400 x=0,4
G200 x=0,6 G200 x=0,4
Condensation
heat transfer
coefficient

14. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Comparison with pure fluids
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
HEATTRANSFERCOEFFICIENT[Wm-2K-1]
MASS COMPOSITION (fraction of R32)
vaporization x=0,4
vaporization x=0,2
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
HEATTRANSFERCOEFFICIENT[Wm-2K-1]
MASS COMPOSITION (fraction of R32)
condensationx=0,74
condensationx=0,46
Mass composition (fraction of R32) Mass composition (fraction of R32)
Heattransfercoefficient[Wm-2s-1]
Heattransfercoefficient[Wm-2s-1]
-23%
-12%
-50%
Penalization from an ideal linear behavior of the pure fluids at G = 400 kg m-2 s-1
Condensation Vaporization

15. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Conclusions
 For many applications, an alternative to high GWP synthetic refrigerants
would rely on refrigerant mixtures.
 Condensation and vaporization heat transfer of R32/R1234ze(E) mixture
inside a 1 mm diameter channel is investigated.
 Condensation HTCs are well predicted by the Cavallini et al. (2006)
model corrected for mixtures. Frictional two-phase pressure drop can be
estimated with Del Col et al. (2013) model. For vaporization, still there are
models providing sufficient accuracy.
 The effect of the additional mass transfer resistance can be seen
comparing the HTCs of the mixture with those of the pure fluid
components
 As an approximation, the heat transfer coefficient can be penalized by
20% in condensation and by 50% in flow boiling as compared to the pure
components. The global HTC also depends on the secondary fluid.

16. Latest Technologies in Refrigeration and Air Conditioning - XVI European Conference Milano, 12th - 13th June 2015
Thank you for
your attention
davide.delcol@unipd.it
http://www.dii.unipd.it/en/sustainable-thermal-energy-technologies
Università degli Studi di Padova
Dipartimento di Ingegneria Industriale