Medios De Comunicacion

Applied Thermal Engineering 25 (2005) 45–60
www.elsevier.com/locate/apthermeng

Heat transfer with freezing in a scraped surface heat exchanger
Mohamed Ben Lakhdar a, Rosalia Cerecero b, Graciela Alvarez b,
Jacques Guilpart b, Denis Flick c,*, Andr Lallemand d
e
a

b

LGL France Refrigerating Division, 42, rue Roger Salengro BP 205, 69741 Genas, France
UMR Gnie industrielalimentaire––Food process engineering Ensia/Cemagref/INAPG/INRA, Cemagref,
e
UR Gnie des procds frigorifiques, BP 44, 92163 Antony cedex, France
e
ee
c
UMR Gnie industriel alimentaire––Food process engineering, Ensia/Cemagref/INAPG/INRA,
e
Institut National Agronomique, Paris-Grignon, 16, rue Claude Bernard, 75231 Paris cedex 05, France
d
Centre de Thermique––UMR A CNRS 5008, Institut Nationaldes Sciences Appliques de Lyon, 20,
e
avenue Albert Einstein, 69621 Villeurbanne cedex, France
Received 12 February 2004; accepted 9 May 2004
Available online 25 June 2004

Abstract
An experimental study was carried out on a scraped surface heat exchanger used for freezing of water–
ethanol mixture and aqueous sucrose solution. The influence of various parameters on heat transferintensity was established: product type and composition, flow rate, blade rotation speed, distance between
blades and wall. During starting (transient period) the solution is first supercooled, then ice crystals appear
on the scraped surface (heterogeneous nucleation) and no more supercooling is observed. It seems that,
when blades are 3 mm far from the surface, a constant ice layer is formed having thisthickness and acting
as a thermal resistance. But when the blades rotate at 1 mm from the surface, periodically all the ice layer is
removed despite the surface is not really scraped. This could simplify ice generator technology. An internal
heat transfer coefficient was defined; it depends mainly on rotation speed. Correlations were proposed for
its prediction, which could be applied, at leastas a first approach, for the most common freezing applications of scraped surface heat exchanger i.e. ice creams (which are derived from sucrose solutions) and
two-phase secondary refrigerants (which are principally ethanol solutions).
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Scraped surface heat exchanger; Freezing; Sucrose; Ethanol; Correlation; Heat transfer coefficient

*Corresponding author. Tel.: +33-1-4408-7239; fax: +33-1-4408-1666.
E-mail address: flick@inapg.inra.fr (D. Flick).

1359-4311/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.applthermaleng.2004.05.007

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M.B. Lakhdar et al. / Applied Thermal Engineering 25 (2005) 45–60

Nomenclature
A
cp
D
Dr
Dh
e
h
h
Lf
_
N
Nu ¼ hD
k
Pr ¼
_
Q

heat exchangearea, m2
specific heat capacity, J kgÀ1 KÀ1
scraped wall inner diameter, m
rotor diameter, m
external hydraulic diameter, m
gap between the blades extremity and the wall, m
heat transfer coefficient, W mÀ2 KÀ1
specific enthalpy, J kgÀ1 KÀ1
specific latent heat of fusion for ice at 0 °C, J
rotor rotation speed, sÀ1
Nusselt number

lCp
k

Prantdl number
heat flux, W
À
Rea ¼ quðDl Dr Þaxial Reynolds number
_

2

Rer ¼ qNlD rotational Reynolds number
Reext ¼ qvDh external Reynolds number
l
R
heat transfer resistance K m2 WÀ1
T
temperature, K or °C
_
V
u ¼ pðD2 ÀD2 Þ=4 mean axial velocity, m sÀ1
r
v
mean velocity of pure ethanol
_
V
volume flow rate, m3 sÀ1

Greek symbols
k
l
q
um
uv
x
x0

thermal conductivity, W mÀ1 KÀ1
dynamic viscosity, Pa sdensity, kg mÀ3
mass fraction of ice
volume fraction of ice
solute mass fraction in the solution; mass of solute/mass of solution
global mass fraction of solute; mass of solute/mass of solution and ice

Subscripts
eth
ext
f
i
int
0
p
s
w
cri

pure ethanol
external
freezing point
ice
internal
initial
product
solution
wall
critical value

M.B. Lakhdar et al. / Applied...