Abstract
D.Ing.
Many studies have been conducted in order to establish the respective influence of
geometric parameters such as fins number, fin shape (apex angle), spiral angle, fin
height, fin pitch etc. on the condensation heat transfer performance of the spiralled
micro-fin tubes. However, the effect of the spiral angle could not be clearly established
in those investigations, because other geometric parameters affecting the heat transfer
performance such as fin height, fin thickness, apex angle were also varied. The
influence of the spiral angle on the heat transfer performance during condensation
inside spiralled micro-fin tubes having all other geometric parameters the same was
experimentally investigated in this study. A new experimental-based predictive
correlation was developed for practical design of this specific class of micro-fin tubes.
Tests were conducted for condensation of R22, R134a and R407c inside a smooth and
three micro-fin tubes having spiral angles of 10, 18 and 37 degrees. Experimental
results indicated a heat transfer augmentation due to heat transfer area increase. As the
spiral angle was increased, the heat transfer area increased causing a substantial heat
transfer augmentation. Condensation inside the 10° spiralled micro-fins produced a
heat transfer augmentation of about 170% for a heat transfer area increase of 1.87 when
compared to condensation in ~he correspondent smooth tube while the 18° spiralled
"
micro-fins produced an augmentation of 180% for a heat transfer area increase of 1.94.
The 37° spiralled micro-fins produced the highest enhancement of 220% for a heat
transfer area increase of 2.13. Additional heat transfer augmentation was produced by:
(a) the turbulence in the condensate film due to the presence of spiralled micro-fins
(stronger effect at lower mass velocities and vapor qualities) and (b) the effect of
surface tension forces (at higher vapor qualities). The proposed new correlation
predicted the majority of experimental results of the present study within a deviation
zone of ± 20 percent.