Experimental investigation on the airside heat transfer and pressure drop of the fin-and-tube heat exchangers having oval tubes

ABSTRACT

It is well known that the replacement of round tubes to oval tubes in a fin-and-tube heat exchanger reduces the air-side pressure drop as well as the low-performance region behind the tube, which improves the thermal performance of the heat exchanger. However, experimental evidences are lacking, especially for generic heat exchangers. In this study, generic heat exchangers having oval tubes were tested, which included two different oval tube dimension and two different tube pitches. Round tube samples were also tested for a comparison purpose. The effect of fin pitch on j factor was negligible, whereas f factor increased as the fin pitch increased. Furthermore, both the j and f factor decreased as the number of tube row increased. A thermal performance comparison based on the ratio of thermal conductance per unit volume (${\eta }_o{h}_o{A}_o/V_o$) to pressure drop per unit length ($\mathrm{\Delta }P/L$) revealed that the oval tube samples yielded a higher performance than the round tube samples, except at a small number of tube row. Furthermore, the best performance was obtained from the oval tube samples having a smaller diameter tube. On the other hand, relatively poor performance was obtained from the oval tube samples having a larger diameter tube, probably due to the large pressure drops. The j and f correlations were developed based on the data.

KEYWORDS:

• Fin-and-tube heat exchanger
• oval tube
• elliptic tube
• heat transfer coefficient
• friction factor
• thermal performance

Nomenclature

A	=	heat transfer area, m2
AR	=	aspect ratio
a	=	major diameter, m
b	=	minor diameter, m
C	=	heat capacity ratio
cp	=	specific heat, J/kg K
Dc	=	tube diameter including fin collar thickness, m
Dmin	=	minor tube diameter, m
f	=	friction factor
h	=	heat transfer coefficient, W/m2K
j	=	Colburn j factor
k	=	thermal conductivity, W/m K
L	=	heat exchanger length, m
m	=	mass flow rate, kg/s
N	=	number of tube row
NTU	=	number of transfer units
p	=	ratio of outer and inner diameter
Pd	=	fin depth, peak to valley excluding fin thickness, m
Pf	=	fin pitch, m
Pt	=	transverse tube pitch, m
Pl	=	longitudinal tube pitch, m
Pr	=	Prandtl number
q	=	ratio of major and minor diameter
rc	=	tube radius including fin collar, m
Req	=	equivalent radius, m
ReDc	=	Reynolds number based on Dc
ReDmin	=	Reynolds number based on Dmin
RH	=	relative humidity
t	=	tube wall thickness, m
tf	=	fin thickness, m
U	=	overall heat transfer coefficient, W/m2K
V	=	velocity, m/s
Vo	=	volume, m3
xf	=	wave pitch, m

Greek symbols

$\mathrm{\Delta }P$	=	pressure loss, Pa
$\epsilon$	=	effectiveness
$\eta$	=	fin efficiency
$\eta {\hspace{0.17em}}_o$	=	surface efficiency
$\theta$	=	corrugation angle, degree
$\rho$	=	density, kg/m3
$\sigma$	=	contraction ratio of the cross-sectional area

Subscripts

a	=	air
h	=	hydraulic
i	=	tubeside
in	=	inlet
f	=	fin
m	=	mean
max	=	maximum
min	=	minimum
o	=	airside
out	=	outlet
r	=	tube-side
t	=	tube wall
w	=	water