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Determination of drying kinetics and convective heat transfer coefficients of ginger slices

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Abstract

In the present work, the effects of some parametric values on convective heat transfer coefficients and the thin layer drying process of ginger slices were investigated. Drying was done in the laboratory by using cyclone type convective dryer. The drying air temperature was varied as 40, 50, 60 and 70 °C and the air velocity is 0.8, 1.5 and 3 m/s. All drying experiments had only falling rate period. The drying data were fitted to the twelve mathematical models and performance of these models was investigated by comparing the determination of coefficient (R 2), reduced Chi-square (χ 2) and root mean square error between the observed and predicted moisture ratios. The effective moisture diffusivity and activation energy were calculated using an infinite series solution of Fick’s diffusion equation. The average effective moisture diffusivity values and activation energy values varied from 2.807 × 10−10 to 6.977 × 10−10 m2/s and 19.313–22.722 kJ/mol over the drying air temperature and velocity range, respectively. Experimental data was used to evaluate the values of constants in Nusselt number expression by using linear regression analysis and consequently, convective heat transfer coefficients were determined in forced convection mode. Convective heat transfer coefficient of ginger slices showed changes in ranges 0.33–2.11 W/m2 °C.

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Abbreviations

a, b, c, g, h, n :

Empirical constants in the drying models

A t :

Area of tray (m2)

C :

Constant

C v :

Specific heat of humid air (J/kg °C)

D eff :

Effective diffusivity (m2/s)

D 0 :

Constant in Arrhenius equation (m2/s)

E a :

Activation energy (kJ/mol)

h c :

Convective heat transfer coefficient (W/m2 °C)

k, k o , k 1 :

Empirical coefficients in the drying models (s−1)

K v :

Thermal conductivity of humid air (W/m °C)

K :

Slope

L :

Slab thickness (m)

\( \dot{m}_{ev} \) :

Moisture evaporated (kg)

M :

Moisture content (dry basis)

M e :

Moisture content in equilibrium state (dry basis)

M o :

Moisture content at t = 0 (dry basis)

M t :

Moisture content at t (dry basis)

MR :

Moisture ratio (dimensionless)

MR exp :

Experimental moisture ratio (dimensionless)

MR pre :

Predicted moisture ratio (dimensionless)

n :

Number constants, constant

N :

Number of observations

Nu :

Nusselt number (Nu = h c X/K v )

Pr :

Prandtl number (Pr = μ v C v /K v )

P(T):

Partial vapour pressure at temperature T (N/m2)

\( \dot{Q}_{e} \) :

Rate of heat utilized to evaporate moisture (J/m2 s)

r :

Diffusion path (m)

R :

Gas consant (kJ/mol K)

R 2 :

Determination coefficient

Re :

Reynolds number (Re = ρvvd/μv)

RMSE :

Root mean square error

t :

Time (sec, h, min)

T :

Temperature (°C)

T c :

Product temperature (°C)

T e :

Exit air temperature (°C)

T i :

Average of product and humid air temperature (°C)

X :

Characteristic dimension (m)

Β :

Coefficient of volumetric expansion (1/°C)

γ:

Relative humidity (°)

λ:

Latent heat of vaporization (J/kg)

\( \mu_{v} \) :

Dynamic viscosity of humid air (kg/m s)

\( \rho_{v} \) :

Density of humid air (kg/m3)

χ 2 :

Chi-square

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Acknowledgments

Authors thank Firat University Research Foundation (FUBAP) for financial support, under Project Number 943.

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Authors and Affiliations

  1. Mechanical Engineering Department, Firat University, 23279, Elazig, Turkey

    Ebru Kavak Akpinar & Seda Toraman

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  1. Ebru Kavak Akpinar
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  2. Seda Toraman
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Correspondence to Ebru Kavak Akpinar.

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Akpinar, E.K., Toraman, S. Determination of drying kinetics and convective heat transfer coefficients of ginger slices. Heat Mass Transfer 52, 2271–2281 (2016). https://doi.org/10.1007/s00231-015-1729-6

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