International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 05 Issue: 06 | June 2018 www.irjet.net p-ISSN: 2395-0072 © 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1361 Helical Screw-Tape Influence on Swirl Flow Profile in a Diffuser Ehan Sabah Shukri Askari Department of electronic technology, Engineering Technical College-Baghdad, Middle Technical University (MTU) Baghdad, Iraq ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract – In this research, a numerical study is performed to model the airflow behavior in an annular diffuser with a swirl generator. For this purpose, helical screw-tape inserted around the cylindrical hub with different swirl number (Sn) is used. Three swirl numbers (2.8, 3.9 and 5.8) are analyzed. The simulations are carried out with air as the working fluid and Reynolds number (Re) 1.2 x10 5 . The analysis of the flow structure shows that swirling flow is induced in the diffuser. This swirling flow leads to acceptable air mixing between the core flow and the near diffuser wall regions. The numerical results indicated that swirling flow induced by swirl number 5.8 enhanced the velocity distribution better than the other tested swirl numbers. Key Words: CFD, annular diffuser, helical screw-tape, swirl number, velocity distribution. 1. INTRODUCTION Swirling flows, which make fluid mixing possible, are widely used in different engineering applications such as combustors, hydro-cyclones or inline swirl element separators [1]. The use of the swirling flow is to provide and to increase the centrifugal force. These days, the analysis of swirl flow show great interest. There are in the literature, countless works on swirl flow that generated by different methods [2 - 3]. The influence of the inlet swirl on the flow development has been confirmed by the researchers [4 - 5]. Therefore, results revealed the improvement of pressure recovery coefficient due to the swirling flow insertion. Most attention has been given to strong recirculating swirling flow in diffusers, for their extensively used in fluid mechanical devices. The intensity of swirl flow is characterized by swirl number (Sn) which is known as the ratio of the axial flux of tangential momentum to the axial flux of axial momentum [6]. As accomplished study by Singh et al. [5] the range of the inlet swirl intensity was investigated to get the best performance of annular diffuser with different geometries but having the same equivalent cone angle. The optimum development was to increase the pressure recovery coefficient that considered as the measure of diffusers performance and to decrease the pressure loss coefficient. Findings reached a good agreement with the study target. Abdalla et al. [7] introduced an experimental study for the effect of swirling inlet flow on the performance of annular diffusers. Five annular diffusers were tested with different casing wall angles. The performance of the tested diffusers was experimentally obtained in the presence of free-swirl and forced-swirl flows. In the aforementioned cases of swirl generators, the pressure coefficient of annular diffusers increased with increasing the inlet swirl angle. Swirling flows are widely used in industrial combustion devices such as gas turbine combustors, furnaces, burners to provide power generation. It has a dominant effect on the mixing process in gas turbine combustion chamber. Eldrainy et al. [8 - 9] examined the effect of different swirler geometries on the flow dynamics inside a micro gas turbine combustion chamber model. Designing air swirler to produce stable and efficient combustion with low-pressure losses was a challenge. Axial flat vane swirler with different vane angles was tested to show the effect of vane angle on the internal flow field. The simulated results confirmed that weak swirl with low swirl number was not sufficient to produce a strong centrifugal force. Consequently, if the swirler vane angle increased, both swirl number and flow deflection angles were increased. Moreover, extra pressure reduction occurred, and the reversed flow was increased as well, so as a conclusion, with high deflection angles, turbulence intensity increased and the fuel-air mixing would be promoted [8]. Furthermore, Eldrainy et al. [9] confirmed that swirl number under 0.4, recirculation would not be observed. Therefore, mixing applications should be designed for swirl number more than 0.6. Thus, controlling such parameters, the mixing process can be improved. Swirling flow is one of the passive techniques. Since it is usually accompanied with high tangential velocity and turbulence intensity, which provides an additional mechanism to increase the heat transfer [10 - 12], it has been studied to enhance heat transfer in many industrial applications. Many types of swirl generators have been studied to improve heat transfer and temperature distribution inside flow passages. This include fixed vanes with different blade angles in the inlet flow [13 - 15], twisted tapes [16 - 18], helical screw- tapes [19 - 21], struts [22 - 23], pins [25] and conical rings [26 - 27] These types of turbulators are the most favorable passive techniques because they are inexpensive and can be easily used in the existing system. Most of these works mentioned above mainly concentrated on the flow characteristic in pipes and tubes rather than in diffusers. In this study, a numerical study was performed to investigate the flow behavior in an annular diffuser. A circular hub equipped with inserted helical screw-tape is adapted to act as a swirl generator.