Numerical Prediction and Analysis of Surface Transfer Coefficients on Moist Object During Heat and Mass Transfer Application

Abstract

Convective surface transfer coefficients data for a moist object are not easily available from the literature and this coefficient is usually assumed to be constant in most studies. A three-dimensional (3D) numerical model is developed for prediction of heat and mass transfer coefficients on the surface of the rectangular moist object. A computational fluid dynamics tool is used for flow analysis at three different higher velocities. The spatial distribution of heat and mass transfer coefficients is estimated at the surfaces of the object. The effect of constant and variable surface transfer coefficient is analyzed through transient 3D simultaneous heat and mass transfer solutions of convective drying. The numerical results are compared with experimental results. Good agreement was found with the experimental results when the numerical solution is considered with variable heat transfer coefficient. The numerical solutions with constant heat transfer coefficient gives overestimated results. A new set of Nusselt number and Sherwood number correlations is developed from a numerical model, and these correlations are expected to be useful for drying industries.

NOMENCLATURE

a, b, c, and d	=	constants
B	=	breadth of the rectangular sample object, m
db	=	dry basis
D	=	moisture diffusivity, m2 s−1
DAB	=	moisture diffusivity of gas phase, m2 s−1
D0	=	preexponential factor, m2 s−1
h	=	heat transfer coefficient, W m−2 K−1
H	=	width of the rectangular sample object, m
hm	=	mass transfer coefficient, m s−1
k	=	thermal conductivity, W m−1 K−1
L	=	length of the rectangular sample object, m
Le	=	Lewis number
M	=	moisture content, kg kg−1 of db
Nu	=	Nusselt number
p	=	pressure, N m−2
Re	=	Reynolds number
R2	=	coefficient of determination
Sh	=	Sherwood number
T	=	temperature, °C
t	=	time, s
u, v, w	=	velocities in X, Y, and Z directions, m s−1
X, Y, Z	=	coordinate axes/directions

Greek Symbols

α	=	thermal diffusivity, m2 s−1
ρ	=	density, kg m−3
μ	=	dynamic viscosity, kg m−1 s−1
Φ	=	nondimensional moisture content

Subscripts

a	=	air
cr	=	critical
eq	=	equilibrium
w	=	surface
m	=	mass, mean
n	=	normal to surface
s	=	coordinate along the surface
0	=	initial condition
∞	=	supplied air condition

Additional information

Notes on contributors

V. P. Chandramohan

V. P. Chandramohan is an assistant professor in the Department of Mechanical Engineering at the National Institute of Technology Warangal, India. He received his Ph.D. degree from the Indian Institute of Technology Delhi, India, in 2012. He was awarded the Dr. S. Sathik Prize for First Rank in the batch of 2002 of M.E. degree from Annamalai University, Tamilnadu. He obtained his bachelor's degree in 2001 from Noorul Islam College of Engineering. His research areas are experimental heat transfer, convective drying, simultaneous solution of heat and mass transfer, and computational fluid dynamics.