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The present work reports an analysis of forced convective heat transfer data
for air flowing through straight circular pipes with twisted-tape inserts.
Twisted-tape insert is a well-recognized augmentative device used for enhancement
of heat transfer rates in forced convection. The twisted tape has been in use
in industries for six decades. Fabrication is simple and is usually accomplished
by twisting a metallic strip and inserting it in the flow channel. However,
the superiority of the twisted-tape insert as an augmentative device is not
well-established in comparison with other types such as rough
surfaces—natural or artificial, finned tube
assemblies—continuous or discrete, longitudinal
or transverse, etc. Much effort has been directed to gain an insight into the
tape-generated swirl flow heat transfer (Sarma et
al., 2005; Naphon, 2006; Chang et al., 2007;
Eiamsa-ard et al., 2009; Sharma et
al., 2009; and Rahimi et al., 2009).
However, it is to be noted that no universally applicable equation is available for the
prediction of heat transfer coefficients in forced convection in the presence of
twisted-tape inserts. The performance characteristics of twisted-tape assemblies on equal
pumping power basis have shown diverse trends for the data from different
investigations. However, it is well-recognized that the
twisted-tape inserts can be gainfully employed in cases where non-uniform heat fluxes are involved.
The twisted-tape assemblies are usually categorized as `tight fit', `snug fit'
and `loose fit' depending on the gap between the tape and tube wall. With no
measurable gap, the assembly is considered to be a `tight fit'. A `snug fit' is with
a gap less than 0.254 mm. A gap of more than 0.254 mm is usually considered to be a `loose fit'.
The pitch, y, of the twisted tape is generally expressed as the number of tube
diameters per 180° twist. |
