Abstract:
Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers
are known to have an effect on its convective heat transfer capabilities. Much literature
exists for predicting the inner tube’s heat transfer coefficients, however, limited research
has been conducted for the annulus and some of the existing correlations are known to
have large errors.
Linear and nonlinear regression models exist for determining the heat transfer coefficients,
however, these are complex and time consuming methods and require much experimental data in order to obtain accurate solutions. A direct solution to obtain the heat transfer
coefficients in the annulus is sought after.
In this study a large dataset of experimental measurements on heat exchangers with
annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 was gathered. The annular
diameter ratio is defined as the ratio of the outer diameter of the inner tube to the inner
diameter of the outer tube. Using various methods, the data was processed to determine
local and average Nusselt numbers in the turbulent flow regime. These methods included
the modified Wilson plot technique, a nonlinear regression scheme, as well as the log mean
temperature difference method. The inner tube Reynolds number exponent was assumed
to be a constant 0.8 for both the modified Wilson plot and nonlinear regression methods.
The logarithmic mean temperature difference method was used for both a mean analysis on
the full length of the heat exchanger, and a local analysis on finite control volumes. Friction
factors were calculated directly from measured pressure drops across the annuli.
The heat exchangers were tested for both a heated and cooled annulus, and arranged in a
horizontal counter-flow configuration with water as the working medium. Data was
gathered for Reynolds numbers (based on the hydraulic diameter) varying from 10 000 to 28
000 for a heated annulus and 10 000 to 45 000 for a cooled annulus. Local inner wall
temperatures which are generally difficult to determine, were measured with
thermocouples embedded within the wall. Flow obstructions within the annuli were
minimized, with only the support structures maintaining concentricity of the inner and outer
tubes impeding flow.
