Heat transfer and friction factor characteristics of annuli formed by the smooth inner tube and corrugated outer tube – An experimental study

ABSTRACT

The heat transfer characteristic in the annulus of a double pipe heat exchanger using the corrugated outer tube is analyzed with three corrugation pitches (10, 20, and 30 mm), three diameter ratios (0.53, 0.44, and 0.40), and Reynolds number range 3500–10,500. Tests are conducted for air as a working fluid. The heat exchanger with least pitch and diameter ratio is found to exhibit the highest Nusselt number and pressure drop. The maximum enhancement in the Nusselt number and friction factor is found to be 71% and 81%, respectively. In addition, correlations have been developed for Nusselt number, friction factor, and thermal performance factor.

KEYWORDS:

• Corrugated tubes
• heat exchanger
• Nusselt number
• thermal performance factor
• friction factor

Nomenclature

A	=	Heat transfer area, m2
Cp	=	Specific heat of fluid, J/kg-K
D	=	Hydraulic mean diameter of annulus, m
DR	=	Diameter ratio
Di	=	Inner diameter of outer tube, m
di	=	Inner diameter of inner tube, m
Do	=	Outer diameter of outer tube, m
do	=	Outer diameter of inner tube, m
DPHE	=	Double pipe heat exchanger
e	=	Height of corrugation, mm
$\bar{f}$	=	Average friction factor
h	=	Heat transfer coefficient, W/m2-K
k	=	Thermal conductivity, W/m-K
L	=	Length of test section, m
Lt	=	Length of inner tube, m
Ls	=	Length of outer tube, m
m	=	Mass flow rate of fluid, kg/s
Nu	=	Average Nusselt number
P	=	Pitch, mm
Pr	=	Prandtl number
Q	=	Heat duty, W
R	=	Resistance, m2-K/W
Re	=	Reynolds number
T	=	Temperature, K
U	=	Overall heat transfer coefficient, W/m2-K
V	=	Mean velocity of fluid, m/s

Greek Symbols

Ρ	=	Density kg/m3
µ	=	Dynamic viscosity, kg/m-s
ΔP	=	Pressure difference across test section, Pa
ɳ	=	Thermal performance factor

Subscripts

a	=	Augmented tube (corrugated tube)
avg	=	Average
c	=	Cold
h	=	Hot
in	=	Inlet
LMTD	=	Logarithmic mean temp. difference
out	=	Outlet
s	=	Surface
sm	=	Smooth
w	=	Wall