CONDENSATION AND FLOW BOILING HEAT TRANSFER OF HYDROCARBONS IN MINICHANNELS
Stefano Bortolin, Marco Azzolin,Arianna Berto, Davide Del Col
Department of Industrial EngineeringUniversity of Padova
Padova, Italy
Workshop on Heat Pumps for Low-GWP Refrigerants – December 7, 2020
Outline
2
▪ Introduction
▪ Description of the test facility
▪ Pressure drop measurements
▪ Condensation heat transfer coefficients
▪ Condensation performance and condenser charge evaluation
▪ Flow boiling heat transfer coefficients
▪ Applications: minichannels shell-and-tube condenser and evaporator
▪ Conclusions
Introduction
3
• In the last years, international organizations undertook some actions aimed at
HFCs phase-down:➢ F-gas Regulation (No 517/2014) of the European Union
➢ The Kigali Amendment to the Montreal Protocol (United Nations, 2016)
• The heat pump market continues to prosper in EU countries:
(19th EurObserv’ER Report):➢ At least 4 million HPs were sold in 2018
➢ Reversible air-air HPs still dominate EU market with 3.5 million systems sold in 2018
➢ The air-water HP market increased by 21.5% between 2017 and 2018
• Alternatives to the high GWP fluids currently employed in heat pumps (e.g.
R134a, R410A) must be investigated.
• The GWP100-years is lower than 4 for propane and propylene.
• The major issue related to a wide utilization of HCs is due to their flammability.
Introduction
• Hydrocarbons (HCs) present favorable thermodynamic and transport propertiesthat make them attractive for use in heat pumps.
• Refrigerant charge in heat pumps accumulates in the components (e.g. heatexchangers) where the liquid phase is present.
• Because of HCs flammability it is therefore essential to decrease the volume ofheat exchangers.
• Minichannel technology is a viable solution to reduce refrigerants charge.
• In the scientific literature, it is possible to find only few experimental data takenduring condensation/vaporization of HC refrigerants and the majority of them weretaken inside conventional channels (6-8 mm) or inside plate heat exchangers.
• Propane heat pumps can be used to produce hot water in residential buildings.
• Propane (R290) and propylene (R1270) can be also considered alternatives toR404A in commercial refrigeration and to R134a in R134a/CO2 cascaderefrigeration plants.
Experimental test rig
5
Pressure drop tests – Experimental technique
6
R290 - R1270
,
, ,
water water water PS
in MS in PSref
m c Th h
m
= −
, ( , )in PSh f p T=( ),, in MSx f p h=
ADIABATIC
enters as subcooled liquid
enters as superheated vapor
0.96 mm internal diameter
7
Adiabatic pressure drop
0
50
100
150
200
0 50 100 150 200
CA
LC
. P
RE
SS
UR
E G
RA
DIE
NT
[kP
a m
-1]
EXP. PRESSURE GRADIENT [kPa m-1]
Del Col et al. (2013a)
Friedel (1979)
Müller-Steinhagen and Heck (1986)
Zhang and Webb (2001)
-20%
+20%
Propane Propylene
Cavallini et al.
(2009)
40 °C saturation temperature
0.96 mm diameter channel
Del Col et al. IJR 47, 2014
Del Col et al. IJR 83, 2017
R290
R1270
Test section for HTC measurement
8
0.96 mm internal diameter
REFRIGERANT
Hole for wall
thermocouple
WATER
Heat transfer – Experimental technique
9
• Local heat transfer coefficient HTC
( )( )
in
r LG
q zx z x
m h= −
• Local vapor quality x(z)
( )( )
( ) ( )HTC
sat wall
q zz
d T z T z=
− ( )
( ),
d
d
w
w p w
T zq z m c
z= −
( )satT z by pressure measurement ( )wallT z directly measured
inx from hin,MS (p,T)
by heat balance
Condensation HTC
10
Propane Propylene
40 °C saturation temperature
0.96 mm diameter channel
Cavallini et al.
(2006)Cavallini et al.
(2006)
11
Fluid psat [bar] ρl [kg m-3] ρv [kg m-3] µl [µPa s] λl [W m-1 K-1] GWP100-yearsTemp.
Glide [K]
R1270 16.48 478.6 35.7 82.99 0.105 2 -
R290 13.69 467.5 30.2 82.84 0.087 3 -
R32 24.78 893.0 73.3 94.99 0.115 677 -
R134a 10.17 1146.7 50.1 161.45 0.075 1300 -
R452BR32/R1234yf/R125
67/26/7 %22.85 924.9 82.7 94.78 0.090 676 1.1
R32/R1234ze(E)
75/25%21.78 947.5 70.9 104.3 0.106 507 3.1
Fluid properties at 40°C (mean) saturation temperature
Performance in condensation
Performance in condensation
12
• Penalty Factor (PF) is employed to compare the
potential heat transfer performance of different
refrigerants.
• The parameter PF is a function of:➢ the driving temperature difference ΔTdr;
➢ the refrigerant temperature decrease due to
pressure drop ΔTsr.
• Considering PF=5 K2 at x=0.5, the mass velocity
that gives the same energy penalization is different
for each of the four fluids.
• R1270 displays HTC 26% higher than that of
propane and R134a.
1 1 dPF
4 d
satdr sr
fL V
G d T pT T
z
= = −
0
2
4
6
8
10
12
14
16
18
20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
HE
AT
TR
AN
SF
ER
CO
EF
FIC
IEN
T [
kW
m-2
K-1
]
VAPOR QUALITY [/]
R32 G797
R1270 G375
R290 G300
R134a G420
R452B G822
R32/R1234ze(E)75%/25% G664
Condenser charge evaluation
13
• Minichannels condenser with 1 mm internal diameter.
• 40°C saturation temperature and condensation from
x=1 to x=0.
• The mass velocity G is calculated with PF = 5 K2 at
x=0.5.
• A fixed wall-to-saturation temperature difference ΔTdr
has been assumed.
• Considering the condition for minimum Total
Temperature Penalization (TTP)
minimum TTP 0.5 2srdr sr
dr opt
TT T
T
= + =
• For each element, the HTC has been evaluated
using the Cavallini et al. (2006) correlation.
0.5PF
2drT
=
14
Flow boiling HTC
Sun and Mishima
(2009)
Sun and Mishima
(2009)
30 °C saturation temperature
0.96 mm diameter channel
15
Performance in flow boiling
R1270 and R290 HTC calculated using the model by
Sun and Mishima (2009) at G = 400 kg m-2 s-1 and two
different heat fluxes.
Saturation temperature drop for R1270 and R290
calculated with the Del Col et al. (2013) model.
16
Propane heat pump
• 100 kW water-to-water heat pump
• Two commercial PHEs can be used as an
evaporator and a condenser.
• Two prototypes heat exchangers using
minichannels can be used as a condenser and
as an internal heat exchanger.
• A semi-hermetic reciprocating compressor is
installed in the heat pump.
• The internal volume is 2.9 L in the case of the
minichannel prototype and 8.4 L in the case of
the PHE; 65% volume reduction has been
obtained.Cavallini et al. IJR 33, 2010
17
Minichannels condenser
• When using the minichannel condenser instead of the plate
condenser, a very small COP reduction (around 2%) is observed.
• By using the minichannel condenser instead of the plate
condenser, 0.8 kg refrigerant charge reduction can be obtained,
corresponding to 25% of the total mass.
• Segmentally baffled shell-and-tube
heat exchanger using 2 mm i.d. copper
minichannels.
• Propane flows inside the tubes and
water flows on the shell side.
18
Minichannels evaporator
• Segmentally baffled shell-and-tube evaporator using 2 mm i.d. copper
minichannels.
• Refrigerant flows inside the tubes and water flows on the shell side.
• A single shell pass has been adopted with two tube passes by using a U-tube
bundle.
• The design capacity of the prototype is 80 kW when operating with propane
• A perforated plate having 624 holes with 1 mm diameter is installed inside the
header.
• In the design of the evaporator, particular attention has been paid to minimize the
internal volume of the cylinder head.
Del Col et al. STBE 21, 2015
19
0
2
4
6
8
10
12
70 80 90 100 110 120
Ou
tle
t E
va
po
rati
ng
Te
mp
era
ture
[°C
]
Heat Flow Rate [kW]
BPHE (0.33 inlet quality, 7 K superheat)BPHE (0.25 inlet quality, 5 K superheat)PROTOTYPE (0.33 inlet quality, 7 K superheat)PROTOTYPE (0.25 inlet quality, 5 K superheat)
R290
WATER TEMPERATURE 12-7 C
0
0.05
0.1
0.15
0.2
0.25
0.3
Pro
pa
ne
Ch
arg
e In
ve
nto
ry [kg
]
ROUHANI NIÑO LOCKHART
MARTINELLI
CISE ZIVI BAROCZY HOMOGENEOUS
Minichannels evaporator
Estimated total charge in the microchannel evaporator
when using propane (80 kW, 12 °C – 7 °C water
temperature, 0.33 inlet quality, 7 K superheat).
• Calculated outlet evaporating temperature vs heat flow
rate when using propane in the microchannel evaporator
and in a commercial BPHE.
• The refrigerant-side internal volume is 5.8 L in the case of
the prototype and 8.4 L in the case of the BPHE.
Conclusions
20
• Condensation HTCs and adiabatic two-phase pressure drop have been measured with
propane (R290) and propylene (R1270) inside a 0.96 mm diameter channel at 40°C
saturation temperature.
• The Del Col et al. (2013) model for frictional pressure gradient and the Cavallini et al.
(2006) model for condensation well predict experimental data.
• A comparative analysis has been conducted using the parameter Penalty Factor as
performance evaluation criterion: propylene displays higher heat transfer performance
as compared to propane and R134a.
• An evaluation of the refrigerant charge inside the condenser has been done: when using
HCs in minichannels, the specific charge is found to less than half the charge of R134a.
• HTCs have been measured during flow boiling at 30 °C saturation temperature.
Considering also pressure drop, propylene shows better performance with respect to
R290.
• A shell-and-tube minichannel condenser and a shell-and-tube minichannel evaporator
operating with propane lead to a reduction of the refrigerant charge compared with a
commercial BPHXs without loss of performance.
Thank you for your
attention!
http://stet.dii.unipd.it/
Sustainable Thermal Energy Technologies LAB
Department of Industrial Engineering
University of Padova
Stefano Bortolin