Abstract
Effective heat transfer coefficients were measured using an aluminum test body and compared with the results obtained from a Gnielinski correlation for air-blast freezing of a fruit pulp model in multilayer boxes, with the internal airflow through rectangular ducts and the hydraulic diameter as characteristic dimensions. The quantities of products inside the boxes were varied, and the inlet air velocities and temperature profiles during freezing were measured. The inlet air velocities were applied in dimensionless Gnielinski correlations to estimate the local heat transfer coefficient values. The experimental and predicted heat transfer coefficient values were used to determine an average convective heat transfer coefficient weighted by the heat transfer area. The results from this methodology were used in an analytically derived procedure for freezing-time estimates and then compared with experimental results. The average effective heat transfer coefficient underestimated freezing times and demonstrated a higher level of accuracy than the Gnielinski correlation when applied to boxes containing smaller product amounts. For experiments with greater quantity of products, the use of average heat transfer coefficients from the Gnielinski correlation yielded errors lower than 20%. Based on boundary layer theory, the Gnielinski correlation can be used to explain the isotherm behaviors observed during freezing. Many of the results satisfy the standards of accuracy used in engineering, and the procedure does not require extra computational effort.
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Abbreviations
- A :
-
area, m2
- Bi s :
-
Biot number = h2L/k
- C f :
-
friction factor
- Cp :
-
specific heat Jkg−1 °C−1
- Cm :
-
layer number
- D 1 :
-
width of polyethylene bags m
- D 2 :
-
length of polyethylene bags m
- D 3 :
-
height of polyethylene bags m
- D h :
-
channel hydraulic diameter \( {D_h} = \frac{{2{H_c}W}}{{\left( {{H_c} + W} \right)}}\left( {\text{m}} \right) \)
- e :
-
error between experimental and predicted values
- E AN :
-
equivalent heat transfer dimensionality analytically derived
- \( {\overline h_i} \) :
-
local convective heat transfer coefficient Wm−2 °C−1
- \( heff \) :
-
effective heat transfer coefficient Wm−2 °C−1
- H :
-
test section height m
- k :
-
total area defined in equation (16), m2
- K :
-
thermal conductivity Wm−1 °C−1
- L :
-
surface length of the test section m; duct length m
- Nu :
-
Nusselt number
- \( {\overline {Nu}_i} \) :
-
local Nusselt number, Eqs. (6 and 7)
- \( {\overline {Nu}_{{_{\infty }}}} \) :
-
theoretical average Nusselt number, Eqs. (4 and 6)
- Pr :
-
Prandtl number
- Re Dh :
-
Reynolds number based on the hydraulic diameter
- SD :
-
standard deviation
- S 2 :
-
cooling coefficient defined in Eq. (2)
- t :
-
time (s min, h)
- t exp :
-
experimental freezing time h
- t f :
-
freezing time h
- t f,slab :
-
freezing time of an infinite slab with the same basic dimension h
- \( \overline u \) :
-
average air velocity ms−1
- \( \hat{V} \) :
-
estimator of variance
- W :
-
width m
- x :
-
distance from entrance of the test section m
- z :
-
distance from wall of the test section m
- Z n , Z m :
-
roots of transcendental equation of type C = αtanα
- Z nm :
-
defined by the Eq. (13).
- βi :
-
ratio of dimension to characteristic dimension i = 1, 2
- ΔHi :
-
change in relative enthalpy content (J/m3) i = 1 (pre-cooling) and i = 2 (freezing and tempering)
- ΔTi :
-
change in temperature (K) i = 1 (pre-cooling) and i = 2 (freezing and tempering)
- ρ :
-
density (kgm−3)
- Al :
-
aluminum
- air :
-
air
- c :
-
channel
- corr :
-
Gnielinski correlation
- i :
-
initial
- pred :
-
predicted
- exp :
-
experimental
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Acknowledgments
The authors wish to thank the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG- Brazil), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq - Brazil) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES – Brazil) for financial support for this research.
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de Resende, J.V., Prado, M.E.T. & Junior, V.S. Non-uniform Heat Transfer During Air-Blast Freezing of a Fruit Pulp Model in Multilayer Boxes. Food Bioprocess Technol 6, 146–159 (2013). https://doi.org/10.1007/s11947-011-0757-6
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DOI: https://doi.org/10.1007/s11947-011-0757-6