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.
Nomenclature
A | = | : heat transfer area, = |
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 |
= | : fully developed Fanning friction factor, dimensionless | |
g | = | : gravitational acceleration, m/s2 |
Gr | = | : Grashof number = |
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 = |
= | pressure drop, mm | |
P | = | : wetted perimeter in the particular cross-section of the duct, m |
Pr | = | : fluid Prandtl number = |
Ra | = | : Rayleigh number = |
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 | = | : |
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 |