Effect of novel short-length wavy-tape turbulators on fluid flow and heat transfer: experimental study

ABSTRACT

The heat transfer characteristics of turbulent flow regime through a circular tube with short- and full-length wavy-tape inserts were experimentally investigated. Air was used for Reynolds numbers between 6,000 and 20,000 with different combinations of wave, tape thickness, and diameter ratios. It was found that wavy-tape inserts enhanced the heat transfer by up to 67% higher than smooth tubes. For approximately the same pumping power, the heat transfer for full-length wavy tape was approximately 25% higher than for short-length wavy tape. However, the pumping power of full-length tape was 26% higher than for short-length tape.

KEYWORDS:

• Turbulent flow
• wavy tape
• swirl flow
• heat transfer
• heat exchanger
• forced convection

Nomenclature

A	=	: heat transfer area, = $\pi DL$, m2
A0	=	plain duct flow cross-sectional area, m2
Cp	=	: constant pressure specific heat, J/kgK
D	=	: internal diameter of the plain tube, m
Dh	=	hydraulic diameter of the test duct = 4A0/P, m
$f$	=	: fully developed Fanning friction factor, dimensionless
g	=	: gravitational acceleration, m/s2
Gr	=	: Grashof number =$g\beta {\rho }^2{D}_h^3\mathrm{\Delta }{T}_w/{\mu }^2$, dimensionless
H	=	: pitch of the spring tape, m
hz	=	: axially local heat transfer coefficient, W/(m2K).
K	=	: fluid thermal conductivity, W/(mK).
k	=	: twist ratio
L	=	: length of spring tape, m
m	=	: mass flow rate, kg/min
Num	=	: axially averaged Nusselt number = $\frac{1}{L}\underset{0}{\overset{L}{\int }}\frac{h_z{D}_{h}dz}{k}$, dimensionless
$\mathrm{\Delta }{P}_z$:	=	pressure drop, mm
P	=	: wetted perimeter in the particular cross-section of the duct, m
Pr	=	: fluid Prandtl number = $\mu C_p/k$, dimensionless
Ra	=	: Rayleigh number = $Gr\cdot Pr$
Re	=	: Reynolds number based on plain tube diameter, dimensionless
T	=	: temperature, K
∆Tw	=	: wall to fluid bulk temperature difference, K
X	=	: Prn, the value of n depends on the exponent of Pr in the correlation
Y	=	: ${\left(\frac{{\mu }_b}{{\mu }_w}\right)}^{-0.14}\times \frac{1}{5.172}$
y	=	: spring ratio, dimensionless

Greek Symbols

β	=	: coefficient of isobaric thermal expansion, K−1
δ	=	: tape thickness ratio, t/D, m
µ	=	: fluid dynamic viscosity, kg/ms.
ρ	=	: density of the fluid, kg/m3.

Subscripts

ax	=	: at axial flow condition
b	=	: at bulk fluid temperature
m	=	: axially averaged
FLST	=	: full length spring tape
SLST	=	: short length spring tape
SLTT	=	: short length twisted tape
FLTT	=	: full length twisted tape
sw	=	: at swirl flow condition
w	=	: at duct wall temperature, with
z	=	: local value

Additional information

Funding

This work was supported by the MCKV Institute of Engineering [MCVKIE/01/2014].