two-phase flow patterns and flow boiling heat...
TRANSCRIPT
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 1
Two-phase flow patterns and flow
boiling heat transfer for R-245fa
in a 3.00 mm tube
at high reduced temperature
Romain CHARNAY
Rémi REVELLIN
Jocelyn BONJOUR
Université de Lyon, INSA-Lyon, CETHIL UMR5008, F-69621 VILLEURBANNE, France
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014
• Energy recovery in transport: exhaust gas temperatures : 400 – 900°C
• Refrigerant evaporation temperature > 100°C
• Working fluid : R-245fa
2
Why should we study flow boiling at high
(reduced) temperature ?
For the development of Organic Rankine Cycles
Because it may bring new insights into the physics of flow boiling
(unconventional variation of fluid properties)
flow patterns
heat transfer coefficient
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 3
Experimental setup and test conditions
Parameter Range
din [mm] 3.00
Levap [mm] 185.0
q [kW/m²] 10 – 90 ± 2-5 %
G [kg/m².s] 100 – 1500 ± 2 %
Tsat [°C] 60 – 120 ± 0.2 – 0.8
Psat [bar] 4.4 – 19.2
x [-] 0 – 1
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 4
Determination of flow patterns
Four observed flow patterns
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014
G = 300 kg/m².s, q = 50 kW/m², x = 0.30
Tsat = 60°C
Annular flow – f = 0 Hz
Tsat = 120°C
Intermittent flow– f = 41 Hz
5
Influence of Tsat on the flow pattern
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 6
Tsat
σ dbub
ρvap
bubble length Tsat σ stratification
Influence of Tsat on the flow pattern
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014
Tsat = 100°C, q = 50 kW/m² Tsat = 120°C, q = 50 kW/m²
7 29
The higher the Tsat, the narrower the range of vapor quality corresponding to annular flow whereas the larger the range of vapor quality for intermittent flow
The higher the Tsat, the lower the vapor quality corresponding to dryout flow regime inception
The higher the Tsat, the lower the vapor quality corresponding to mist flow regime inception
Influence of Tsat on the flow pattern
MIST DRY OUT
ANNULAR
INTERMITTENT
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 8 35
Two mechanisms were assumed to govern flow boiling heat transfer:
• The nucleate boiling (NB) formation of bubbles at the wall
• The convective boiling (CB) conduction and convection (liquid film)
evaporation at the liquid-vapor interface
These mechanisms were related to heat transfer coefficient (α):
• When NB is dominant, α = f(q, Tsat) & α ≠ f(G, x)
• When CB is dominant, α = f(G,x) & α ≠ f(q)
• When NB and CB are equally important, α = f(G,q,x)
Results on heat transfer
CB
NB
CB
NB
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 39 9
Results on heat transfer
36
Influence of mass velocity Tsat = 60°C
Low G
High G
NB
NB
CB
CB
G = 300 kg/m².s
G = 500 kg/m².s
G = 1000 kg/m².s
G = 700 kg/m².s
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 39 10
Results on heat transfer
36
Influence of mass velocity Tsat = 120°C
Low G
High G
NB
NB
CB
CB
37
G = 500
G = 300 kg/m².s
G = 1000 kg/m².s
G = 700
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 40
Influence of saturation temperature G = 300 kg/m².s
NB
NB
CB
CB High Tsat
Low Tsat
Tsat = 60°C
Tsat = 120°C
Tsat = 100°C
Tsat = 80°C
11
Results on heat transfer
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 12
The main conclusions on the influence of the saturation temperature on the flow
patterns and the heat transfer are:
• The higher Tsat, the smaller and shorter the bubbles
• The higher Tsat, the greater the tendency to flow stratification
• The higher Tsat, the lower the value of vapor quality for dry-out inception
• The higher Tsat, the greater the flow boiling heat transfer coefficient
• The higher Tsat, the greater the contribution of nucleate boiling to the overall heat
transfer coefficient
• The higher Tsat, the lower the contribution of convective boiling to the overall
heat transfer coefficient
Such information must be taken into account when designing evaporators for Organic
Rankine Cycles and other cycles with evaporation at high reduced temperature.
Conclusions
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 13
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 14
Results on flow patterns
Flow pattern characterization
Image processing method 1
Heat transfer coefficient behavior 2
2 R. Charnay, J. Bonjour and R. Revellin, Experimental investigation of R-245fa flow boiling in minichannels at high saturation temperatures: flow patterns and flow pattern maps, International Journal of Heat and Fluid Flow, Vol. 46, pp. 1-16 (2014).
1 R. Charnay, R. Revellin, and J. Bonjour, Flow pattern characterization for R-245fa in minichannels: optical measurement technique and experimental results, International Journal of Multiphase Flow, Vol. 57, pp. 169-181 (2013).
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 15
R245fa Critical temperature = 154 °C = 427 K Critical pressure = 36,5 bar molar mass = 134 g/mol at 60 °C P/Pcrit = 0,12 T/Tcrit = 0,78 at 120°C P/Pcrit = 0,52 T/Tcrit = 0,92
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 16
Results on flow patterns
Flow pattern characterization from heat transfer coefficient behavior
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 39 17
Results on heat transfer
36
Influence of mass velocity low T : physical explanation ?
Tsat = 60°C
Low G
High G
NB
NB
CB
CB
x
δL
ubub
Rth
convection conduction
CB
G
G = 300
G = 500
G = 1000
G = 700
MFIP – Sestri Levante (GE), Italy – 17-19 Sept. 2014 39 18
Results on heat transfer
36
Influence of mass velocity Tsat = 120°C
G
Low G
High G
NB
NB
CB
CB
37
Tsat
lbub fbub
NB
x
fbub
NB
lbub
x
NB > CB
ubub λL
convection conduction
Tsat
CB