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Heat and Mass Transfer

, Volume 55, Issue 12, pp 3645–3659 | Cite as

Experimental investigation and mathematical modeling of drying of green tea leaves in a multi-tray cabinet dryer

  • Abbas ShomaliEmail author
  • Behrooz Abbasi SourakiEmail author
Original
  • 68 Downloads

Abstract

Although many experimental investigations have been carried out on the drying of foodstuffs in the tray dryers, little detailed analysis is available in the literature on the mathematical modeling of simultaneous heat and mass transfer in these types of dryers. In this work, drying behavior of green tea leaves was investigated in a multi-tray cabinet dryer. A mathematical model was developed for prediction of the moisture content and temperature of green tea leaves and also the humidity and temperature of the drying air, in the trays of the dryer. Each tray was considered as a fixed bed of tea leaves. The proposed model was solved, after dividing the bed into a series of thin layers, using a finite difference method and a trial and error technique. Equilibrium moisture contents and moisture diffusivity values, used in the mathematical model, were estimated and correlated for the green tea, used in this study. The predictions of the model showed a good agreement (MRE lower than 5%) with the experimental data, obtained from drying of green tea leaves in a domestic three-tray dryer at 60 and 70 °C. The effect of drying air velocity, temperature and the number of the green tea leaves in each tray was investigated on the drying behavior of green tea in the dryer.

Nomenclature

A

Area (m2)

C

specific heat (\( \frac{j}{kg{.}^oC\kern0.5em } \))

D

Effective diffusivity (m2/s)

hc

Convective heat transfer coefficient (\( \frac{w}{m^2{.}^oC\kern0.5em } \))

H

Height of the bed of the tea in a tray (m)

K

Thermal conductivity (\( \frac{w}{m{.}^oC\kern0.5em } \))

L

Characteristic length (m)

\( \dot{m} \)

Mass flow rate (m3/s)

m

Mass (m3)

N

Number of the particles in a tray

NR

Number of the layers inside half of a tea leaf

Nt

Number of time increments

Nz

Number of the layers in a tray

Nu

Nusselt number

P

Pressure (N/m2)

psat

Saturated pressure of water (N/m2)

\( {\dot{q}}_{conv} \)

Rate of convective heat transfer (w)

\( {\dot{q}}_{evap} \)

Rate of heat transfer in form of Latent heat of evaporation(w)

\( {\dot{q}}_{Temp} \)

Rate of accumulation of energy(w)

R

Half thickness of a tea leaf (m)

Re

Reynolds Number

Rh

Relative humidity

t

Time (s)

T

Temperature (°C)

\( {\overline{T}}_g \)

the average temperature of the drying air in the layer j (°C)

U

Apparent Velocity (m/s)

V

Volume (m3)

X

Moisture content in a particle based on dry basis (kgmoisture/kgDry_particle)

\( \overline{X} \)

The average moisture content of the tea leaves (kgmoisture/kgDry_particle)

Y

Fraction of vapor in drying gas based on dry basis (kgvapor/kgDry_gas)

Z

Height of any point in the bed of tea particle from the bottom of that bed (m)

Subscripts

0

At the beginning

amb

The ambient condition.

bs

Bone-dried Solid

eq

Equilibrium

g

Gas, drying air

in

Into a tray dryer

i

Finite deference index in r direction inside a typical particle

j

Finite difference index in z-direction of the bed of tea particles on a desired tray

p

Particle

Superscript

k

Index of the tray

n

Finite difference index of time

Greek letters

Δz

Height of an element in layer of tea leaves in a tray(m)

Δr

Length of a layer inside a tea leaf (m)

Δt

Time increment (s)

λ

Latent heat of evaporation (j/kg)

μ

Dynamics viscosity (kg.m/s)

ρ

Density (Kg/m3)

Notes

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Chemical Engineering Department, Faculty of EngineeringUniversity of GuilanRashtIran

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