Experimental investigations were conducted to study the effect of micro-bubbles injected into water flowing through straight and helical pipes with different curvature ( 0.025, 0.05 and 0.1). The micro-bubbles were produced using two hydrocyclone micro-bubble generators positioned face to face with opposite direction of swirl flow. The mean diameter of the micro-bubbles generated was about 60 μm, while the maximum diameter observed was 174 μm, with air fraction, which will be defined in Eq.(4), ranging from 0.21% to 0.44%. The effect of the Reynolds number on the drag reduction is also presented. The experimental results show that, the micro-bubble has an effect of drag reduction on both straight and helical pipes, though the drag reduction in a straight pipe is much higher than that in a helical pipe, because of the secondary flow in a helical pipe. It is found that the drag reduction increases with the decrease of curvature and the increase of air fraction. The maximum value of drag reduction ratio was 51% in case of a straight pipe, while that for a helical pipe was 16%.

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The vortical flow at the blade tip area of a turbine rotor has a great effect on the aerothermal performance of the blade tip of turbine rotor however its physics is not fully understood. The present paper is a numerical study to investigate the physics behind vortex formation at the blade tip area. The blade under investigation is a linear model of the tip section of the GE-E3 high-pressure turbine first stage rotor blade. Calculations were carried out for three tip geometries, namely, conventional double squealer, thick pressure side squealer and thick suction side squealer. The code used in this study is an in-house, unstructured, finite volume, multiblock, 3D, compressible, viscous solver. The turbulent viscosity was calculated using the Delayed Detached Eddy Simulation (DDES) model. Computational results show that the vortex formation depends on the vorticity imparted with the incoming flow. Therefore, the flow with high velocity gradient caused larger vortex than that with low velocity gradient. This result is valid for both the cavity vortex as well as the leakage vortex.

This study presents the numerical investigation of the fluid flow and heat transfer characteristics for three different high viscous fluids (Engine oil, Oil (SN-500) and Ethylene glycol) flowing inside horizontal rotating tubes. A computational fluid dynamics (CFD) methodology using ANSYS FLUENT 14.0 is used to perform the numerical analysis by solving the Navier-Stokes and energy equations through the viscous model at all cases of rotation Reynolds numbers and Reynolds numbers. The investigation is conducted at rotation speed of 25, 50, 100, 500, 1000 and 2000 rpm and Reynolds number ranged between 5 and 10 for Engine oil, 54 and 109 for Oil (SN-500) and 425 and 849 for Ethylene glycol. The results revealed 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 are observed significantly larger in the tubes with Ethylene glycol than those in the tubes with Engine oil and Oil (SN-500). These effects weakened growing of the tangential velocity component in the flow. In the tubes with Engine oil and Oil (SN-500), the maximum values of thermal performance factor are found at rotation speed of 1000 rpm, whereas, in the tubes with Ethylene glycol are occurred at rotation speed of 100 rpm. The value of the maximum thermal performance is about 2.1 for tube with Engine oil at a Reynolds number of 10, 1.6 for tube with Oil (SN-500) at a Reynolds number of 109 and 1.44 for tube with Ethylene glycol at a Reynolds number of 849.

The present work shows the results obtained from the numerical simulation of the heat transfer enhancement for the high viscous flow inside horizontal axially rotating tubes, using twisted tapes with the different twist and width ratios. The simulation is performed with the twisted tapes of three twist ratios (TR = 5, 7.5 and 10) and four width ratios (WR = 0.9, 0.7, 0.5 and 0.3). Rotation Reynolds number and Reynolds number are ranged from 0.9 to 5927 and 5 to 849, respectively. A computational fluid dynamics (CFD) methodology using ANSYS FLUENT 14.0 is used to perform the numerical analysis by solving the Navier-Stokes and energy equations through the viscous model at all cases of rotation Reynolds numbers and Reynolds numbers. The results revealed that, thermal performance factor due to the insertion of the twisted tapes in rotating tubes is 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, in the tubes with 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 is small, as compared with the tape width ratio. The best twisted tape geometry, to achieve best thermal performance is found at WR of 0.9 for tubes with Engine oil and Oil (SN-500), and 0.5 for tubes with Ethylene glycol.