Masters Theses

Date of Award

5-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Roy J. Schulz

Committee Members

Firouz Shahrokhi, Basil Antar

Abstract

This study investigates the heat transfer effects of swirl flow on heat transfer within the annulus of a modular bayonet tube heat exchanger. Currently, the flow of fluids through bayonet tube heat exchangers has not been extensively studied. particularly under the presence of swirl. The active heat transfer length was primarily focused upon in the majority of the computational models. Attempts were made to effectively model the entire flow domain within the inner annulus, to observe what effect, if any, swirl flow and undesired recirculation in the momentum flow field would have on the development of the thermal boundary layer. The computational approach utilized a computer numerical code, which solved turbulent incompressible Navier- Stokes equations, in addition to the equations of turbulent kinetic energy (k), eddy dissipation (e), and thermal energy (T). Several modifications were made to the numerical code, in an effort to properly develop Nusselt number correlations for the inner and outer tube walls. Modified wall functions which have proven very effective in modeling near wall regions in an annular geometry, were used in place of the standard laws of the Van Driest hypothesis, and planar mixing-length model. The computational source code was adapted to handle multi-fluid flow capabilities, thus producing simultaneous velocity, thermal, and heat flux profiles for counter flowing fluids. It was found that a swirl angle of 75°, produced the highest values of the average heat transfer coefficient on the inner and outer tube walls of the inner annulus. At this respective angle, the average heat transfer coefficients on the outer and inner tube walls were computed at 1.93 to 3 times that of fully developed non-swirling flow, for approximately 5 to 10 hydraulic diameters. The local Nusselt number distributions demonstrate a similar trend by rising to approximately 2.5 times the value of the fully developed Dittus-Boelter correlation value. The aforementioned numerical characteristics strongly support implications that swirl has a positive effect on heat transfer enhancement, and its uses are beneficial in the perpetuation of advanced thermal boundary layer growth in bayonet tube heat exchangers. To account for the accuracy of the numerical model, computational results for laminar and turbulent flow in a pipe are compared with the experimental results of previous investigators. Local Nusselt number distributions along the inner annulus-outer wall of a bayonet tube-type heat exchanger, possessing a 20° helix-swirl vane are predicted, and a comparison is made to previous experiments. In the comparing the computational results of this present study with previous experiments it is noticed the local Nusselt number distributions along the inner annulus-outer wall of both studies are reasonably close. It is worth mentioning that the Nusselt number distributions for the experiment illustrated a faster decline than the present study’s values, but the asymptotic decline of the profile is well predicted by the modified SAINTS source code, thus reflecting the effectiveness of the annular wall law modifications. The numerical solutions obtained from this quantitative study rely directly upon empirical laws and well tested algorithms, thus it is fairly safe to expect that the numerical solution should agree reasonably well with future experimental studies pertaining to the Modular Bayonet Tube Heat Exchanger.

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