• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.8
• /
• pp.783-789
• /
• 2012

In this study, turbulent flow and heat transfer characteristics in a helically coiled tube have been numerically investigated. Helically coiled tubes are commonly used in heat exchange systems to enhance the heat transfer rate. Accordingly, they have been widely studied experimentally; however, most studies have focused on the pressure drop and heat transfer correlations. The centrifugal force caused by a helical tube increases the wall shear stress and heat transfer rate on the outer side of the helical tube while decreasing those on the inner side of the tube. Therefore, this study quantitatively shows the variation of the local Nusselt number and friction factor along the circumference at the wall of a helical tube by varying the coil diameter and Reynolds number. It is seen that the local heat transfer rate and wall shear stress greatly decrease near the inner side of the tube, which can affect the safety of the tube materials. Moreover, this study verifies the previous experimental correlations for the friction factor and Nusselt number, and it shows that the correlation between the two in a straight tube can be applied to a helical tube. It is expected that the results of this study can be used as important data for the safety evaluation of heat exchangers and steam generators.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.34 no.1
• /
• pp.32-41
• /
• 2006

In this study, the thrust generation by simultaneous flapping airfoils in tandem configuration is parametrically studied with respect to flapping frequency, amplitude and relative location. Navier-Stokes solver with overset grid topology is employed to calculate the unsteady flowfields. The computation results indicate that when the two airfoils stroke in-phase - flapping phase lag is zero - the maximum propulsive efficiency and thrust can be obtained for most frequency and amplitude range. At a flapping amplitude of 0.2 chord and a reduced frequency of 0.75, the propulsive efficiency of aft airfoil is enhanced by about 37 % compared with that of forward airfoil. However, if flapping frequency exceeds some critical value, the strength of the leading edge vortex of aft airfoil is fortified by the trailing edge vortex of the forward airfoil, resulting in poor propulsive efficiency. It is also found that out-of-phase flapping has relatively low propulsive efficiency and thrust since vortical wake of the forward airfoil interacts with the leading edge vortex of aft airfoil in the unfavorable fashion. The total thrust and propulsive efficiency are shown to decrease with the horizontal miss distance of the aft airfoil. On the contrary, the vertical miss distance has little effect on the overall aerodynamic performance.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.50 no.1
• /
• pp.1-12
• /
• 2022

Wind tunnel test for a small scale electric ducted fan with a 104mm diameter was conducted to analyze the aerodynamic characteristics when it was used as a propulsion system of tilt-propeller UAV. Experimental conditions were derived from flight conditions of a sub-scaled OPPAV. Forces and moments of the ducted fan model were measured by a 6-axis balance and 3-dimensional wake vectors which could induce an aerodynamic influence in the vehicle were measured by 5-hole probes. Thrust and torque on hover and cruise conditions were measured and analyzed to drive out the operating conditions when it was applied in the sub-scaled OPPAV. On transition conditions, thrust keep its value with tilt angle variation below 40° and increase after that. But, sideforce increase constantly until 75°. The maximum axial velocity in the wake on hover and cruise conditions was around 60m/s and tangential velocity was around 12m/s. The position of the maximum axial velocity and vortex center move off the fan rotation center line as the tilt angle increases.