الفهرس 
chapter 1Only 14 pages are availabe for public view
 |  | from | 205 |  |  |
| | | from | 205 | | |
Abstract
The need to increase the thermal performance of heat transfer equipment led to developing and using many of heat transfer enhancement techniques.
The present research shows the results obtained from the numerical simulation on the enhancement of heat transfer for the high viscous flow inside horizontal axially rotating tubes, using twisted tapes with the different twist ratios(defined as TR=H/D) and different width ratios (defined as WR=Wt/D). Effect of the tube aspect ratio (AR=L/D) ) on the friction factor and Nusselt number inside rotating tubes was studied first to determine which of the aspect ratio gives best thermal performance for the high viscous flow inside rotating tubes. Then, the heat transfer and flow characteristics of the high viscous fluids through the rotating tubes were analyzed by using three different viscous fluids (Engine oil, Oil (SN-500) and Ethylene glycol).
The simulation was performed with tube aspect ratio, AR = 50, 25 and 16.67. The twisted tapes with three twist ratios (TR = 5, 7.5 and 10), and three width ratios (WR = 0.7, 0.5 and 0.3) were considered. Rotation Reynolds number and Reynolds number were ranged from 0.9-5927 and 5-849, respectively.
A computational fluid dynamics (CFD) methodology using ANSYS FLUENT 14.0 was used to perform the numerical analysis by solving the Navier-Stokes equations (which represents continuity and momentum equations) and energy equation through the viscous model at all cases of rotation Reynolds numbers and Reynolds numbers.
The simulation results revealed that, the tube aspect ratio has a great influence on the thermal performance factor (TPF=(NUW/NUS ⁄(FW/FS)) of the high viscous fluids inside rotating tubes, where the reduction of the aspect ratio decreases the thermal performance factor of the high viscous fluids in the rotating tubes. The maximum thermal performance factor due to the rotation reached about 2.12, 0.97 and 0.89 for the tubes with AR = 50, 25 and 16.67, respectively.
Study of flow and heat transfer characteristics for the high viscous fluids inside rotating tubes signified that, enhancement of heat transfer in the tubes with Ethylene glycol (lower viscous fluid) increases slightly with
the further increase in rotation speed. This is because of the viscous effects which were observed significantly larger in the tubes with Ethylene glycol than those in tubes with Engine oil and Oil (SN-500) (highly viscous fluids). In tubes with Engine oil and Oil (SN-500), the maximum values of thermal performance factor were found at rotation speed, n = 1000 rpm, whereas, in tubes with Ethylene glycol were occurred at n = 100 rpm. The maximum thermal performance factor reached about 2.1, 1.6 and 1.44 for tubes with Engine oil, Oil (SN-500) and Ethylene glycol, respectively.
Thermal performance factor due to the insertion of twisted tapes in rotating tubes was strongly depended on the rotation speed. The increase in rotation speed decreases the thermal performance factor for tubes with Engine oil and Oil (SN-500), whereas, for Ethylene glycol, the thermal performance factor increases as the rotation speed increases. The influence of the twist ratio variation on the friction factor and heat transfer was small, as compared with the tape width ratio. The best twisted tape geometry, which can be selected to enhance the thermal performance was found at width ratio (WR) of 0.9 for tubes with Engine oil and Oil (SN-500), and at WR = 0.5 for tubes with Ethylene glycol. Heat transfer enhancement ratio and friction factor increase ratio correlations for rotating tubes with and without twisted tapes were developed in terms of Reynolds number, rotation Reynolds number, Prandtl number and tape width ratio with maximum deviation of 19%.