Heat Transfer Characteristic on Wing Pairs Vortex Generator using 3D Simulation of Computational Fluid Dynamic

Vortex generators are addition surface that can increase heat transfer area and change the fluid flow characteristics of the working fluid to increase heat transfer coefficient. The use of vortex generators produces longitudinal vortices that can increase the heat transfer performance because of the low pressure behind vortex generators. This investigation used delta winglet vortex generator that was combined with rectangular vortex generator to Reynold numbers ranging 6,000 to 10,000. The parameters of Nusselt number, friction factor, velocity vector and temperature distribution will be evaluated.

Features of air flow and heat transfer control behind a backward-facing step using a vortex generator pair

The paper presents experimental results on the study of flow dynamics and heat transfer in the separation region behind the backward-facing step with longitudinal vortex generators (VG) installed at an angle to the flow of 30° at Re = 4000. The VG installation reduces the recirculation region and the induced longitudinal vortices and rearranges the flow structure in the separation region. The influence of a VG on the local and average thermal characteristics behind the backward-facing step is investigated and their thermohydraulic efficiency is estimated.

Designing vortices in pipe flow with topography-driven Langmuir circulation

We present direct numerical simulation of a mechanism for creating longitudinal vortices in pipe flow, compared with a model theory. By furnishing the pipe wall with a pattern of crossing waves, secondary flow in the form of streamwise vortex pairs is created. The mechanism, ‘CL1’, is kinematic and known from oceanography as a driver of Langmuir circulation. CL1 is strongest when the ‘wall wave’ vectors make an acute angle with the axis, $\varphi =10^{\circ }$ – $20^{\circ }$ , changes sign near $45^{\circ }$ and is weak and of opposite sign beyond this angle. A competing, dynamic mechanism driving secondary flow in the opposite sense is also observed, created by the azimuthally varying friction. Whereas at smaller angles ‘CL1’ prevails, the dynamic effect dominates when $\varphi \gtrsim 45^{\circ }$ , reversing the flow. Curiously, the circulation strength is a faster-than-linearly increasing function of Reynolds number for small $\varphi$ . We explore an analogy with Prandtl's secondary motion of the second kind in turbulence. A transport equation for average streamwise vorticity is derived, and we analyse it for three different crossing angles, $\varphi =18.6^{\circ }, 45^{\circ }$ and $60^{\circ }$ . Mean-vorticity production is organised in a ring-like structure with the two rings contributing to rotating flow in opposite senses. For the larger $\varphi$ , the inner ring decides the main swirling motion, whereas for $\varphi =18.6^{\circ }$ , outer-ring production dominates. For the larger angles, the outer ring is mainly driven by advection of vorticity and the inner by deformation (stretching) whereas, for $\varphi =18.6^{\circ }$ , both contribute approximately equally to production in the outer ring.

Numerical and Experimental Investigations of Micro Thermal Performance in a Tube with Delta Winglet Pairs

In this research, a novel vortex generator (VG) is presented. The experimental and numerical investigations were carried out to study the micro thermal-hydraulic performance in a heated tube. The numerical results showed that the fluid in the core flow region and the near-wall region was fully mixed because of the longitudinal vortices created by the vortex generators. In addition, the experimental results showed that the heat transfer coefficient (h) decreased with the increasing pitch ratio (PR) value, while the friction coefficient exhibited the opposite trend. With the increasing ration angle (RA) numbers, the h values decreased while the f numbers increased. In addition, the maximum and minimum values of the fraction ratio were 1.66 and 4.27, while these values of the Nusselt number ratio were 1.24 and 1.83. The maximum thermal enhancement factor (TEF) was 1.21 when PR = 0.5, RA = 0° and Re = 9090. The heat transfer enhancement mechanism of the vortex generator is explained from the microscopic point of view.

Film Cooling Effectiveness Enhancement Using Multi-Longitudinal Vortex Generated by Alternating Elliptical Film Holes

This paper proposed an alternating elliptical film hole for gas turbine blade to restrain kidney vortex and enhance film cooling effectiveness, based on the multi-longitudinal vortexes generated in alternating elliptical tube. The detailed flow structures in film hole delivering tube and out of the film hole, adiabatic film cooling effectiveness distributions as well as the total pressure loss coefficient were investigated. The delivering tube of alternating elliptical film hole consists of two straight sections and a transition section. In the straight sections, the cross section of the film hole is elliptical, and in the transition section, along flow direction, the major axis gradually shortened into the minor axis, and the minor axis gradually expanded to the major axis. But, the cross-section area of the film hole kept constant. Numerical simulations were performed by using 3D steady flow solver of Reynolds-averaged Navier-Stokes equations (RANS) with the SST k-ω turbulence model. To reveal the mechanism of kidney vortex suppression and film cooling effectiveness enhancement, the simulation results were compared with the cylindrical film hole set as the baseline at different mass flow ratios (MFR). Besides, the aerodynamic characteristics of these two kinds of film holes were also investigated. The results showed that obvious jet effect could be found in the cylindrical film hole, and the coolant mainly flowed along the upper wind wall, then interacted with the main flow, forming a strong kidney vortex after flowing out, which made the coolant to lift away from the wall surface and reduced the cooling effectiveness. The alternating elliptical film hole had a good inhibition impact on the jet effect in the hole due to the longitudinal vortices, which made the film adhere to the wall surface better after the coolant flowed out. The longitudinal vortices generated by alternating elliptical film hole have the opposite rotation direction to the vorticity of the kidney vortices, thus the kidney vortices were restrained to a certain extent. The height of kidney vortices is lower, and the size of kidney vortices is also smaller. As a result, the film cooling effectiveness of alternating elliptical film hole is distinctly higher than that of the cylindrical film hole, and the enhancement effect is more significant at higher mass flow ratio. In addition, the total pressure loss coefficient of alternating elliptical film hole is only slightly higher than the cylindrical film hole at the mass flow ratio of 1%, 2% and 3%, and is even lower at the mass flow ratio of 4%, thus inducing an excellent comprehensive performance.

Aerodynamic Fields inside S-Shaped Baffled-Channel Air-Heat Exchangers

A numerical study of an especial heat exchanger (HE) equipped with complicated geometry baffles was performed in this research study. This shell-and-tube HE could be applied in various engineering applications like solar collectors. It can be acknowledged that generating longitudinal vortices in the flow results in enhancing the turbulent convective heat transfer. In order to generate these vortices, S-shaped baffles can be applied. It should be noted that computational analysis of shell-and-tube HEs is considered a challenging task due to these vortices. So, in this study, a commercial CFD software has been used for solving the problem and important equation and numerical approach used in the simulation have been explained. The aerodynamic aspect with respect to stream function, mean, axial, and transverse velocities, dynamic pressure, turbulent dissipation rate, turbulence kinetic energy, turbulent viscosity, and turbulence intensity fields was included in this research. This study reports many physical phenomena, such as the turbulence, instability, flow separation, and the appearance of reverse secondary currents. The average speed changed in different areas, where it is low next to the baffles. Velocity amounts are paramount around the upper channel’s wall, starting from the upper left side of the last baffle to the exit. This increase in velocity can be justified by a reduction in flow area and pressure augmentation.

Effects of alternating elliptical chamber on jet impingement heat transfer in vane leading edge under different cross-flow conditions

ABSTRACT This paper proposes an alternating elliptical impingement chamber in the leading edge of a gas turbine to restrain the cross flow and enhance the heat transfer, and investigates the detailed flow and heat transfer characteristics. The chamber consists of straight sections and transition sections. Numerical simulations are performed by solving the three-dimensional (3D) steady Reynolds-Averaged Navier–Stokes (RANS) equations with the Shear Stress Transport (SST) k– $\omega$ turbulence model. The influences of alternating the cross section on the impingement flow and heat transfer of the chamber are studied by comparison with a smooth semi-elliptical impingement chamber at a cross-flow Velocity Ratio (VR) of 0.2 and Temperature Ratio (TR) of 1.00 in the primary study. Then, the effects of the cross-flow VR and TR are further investigated. The results reveal that, in the semi-elliptical impingement chamber, the impingement jet is deflected by the cross flow and the heat transfer performance is degraded. However, in the alternating elliptical chamber, the cross flow is transformed to a pair of longitudinal vortices, and the flow direction at the centre of the cross section is parallel to the impingement jet, thus improving the jet penetration ability and enhancing the impingement heat transfer. In addition, the heat transfer in the semi-elliptical chamber degrades rapidly away from the stagnation region, while the longitudinal vortices enhance the heat transfer further, making the heat transfer coefficient distribution more uniform. The Nusselt number decreases with increase of VR and TR for both the semi-elliptical chamber and the alternating elliptical chamber. The alternating elliptical chamber enhances the heat transfer and moves the stagnation point up for all VR and TR, and the heat transfer enhancement is more obvious at high cross-flow velocity ratio.

The Gravity Funnels, formed by the longitudinal vortices, according to the new Axioms and Laws

In the age of information, it is no secret that the modern science is in a very difficult position. On the one hand, it has high hopes for solving the problems of modern humanity and very practical tasks. On the other hand, science shows limited potential and difficulty in carrying out the tasks. Beyond scientific theory remain such phenomena as gravity and gravitational waves and other unexplored and very useful phenomena. Obviously, the reason for these limited capabilities of modern science is its limited foundation. The foundation of science is determined by its basic axioms. If we expand the foundation of science, we will be able to build a more comprehensive, perfect and voluminous theory. In two monographs and a series of articles the author offers a system of extended axioms (with two new axioms) and a more extended theory (with eight new laws). To the great surprise of even the author, this new theory turned out to be extensive enough to cover and explain and the gravity. Moreover, the extended axioms and theory directly and naturally outlined the algorithm in the explanation of the so-called Gravity Funnels. According to the new axioms and laws, Gravity Funnels are both for suction (accelerating) and for expansion (decelerating). Expansion Gravity Funnel decelerates along its longitudinal direction as emits the matter in the transverse direction. In this way it consumes energy and generates matter. Suction Gravity Funnel accelerates along its longitudinal direction as sucks the matter in transverse direction. In this way it consumes matter and generates energy. The both of Funnels are situated in a new Space-time. The Space-time of decelerating and accelerating Funnels is packed by longitudinal vortices, in which the Space (S) is constant. It is radically different of the Space-Time where we live now. The Space-time where we live now is packed by cross vortices, where the time (T) is constant. According the new Axioms and Laws the two described Space- times are mutually orthogonal.

Heat transfer augmentation characteristics of a fin punched with curve trapezoidal vortex generators at the rear of tubes

The thermal-hydraulic characteristics of a novel fin punched with curve trapezoidal vortex generators (CTVGs) are investigated numerically. The effects of multi-parameters including the geometry of CTVG, the location of CTVGs, and working condition on thermal performance are considered. On one hand, CTVGs can availably lessen the size of tube wake zone, decrease the mechanical energy consumption and heighten the fin heat transfer ability in this area. On the other hand, the secondary flow strength is strengthened because the longitudinal vortices generated by CTVGs, which efficiently enhances the heat transfer on the fin downstream CTVGs. Close relationship exists between the volume-averaged secondary flow strength and the mean Nusselt number. For studied cases, the optimal circumferential location angle of ? = 90? is found, while the optimal radial location Dg is about 1.8 times the tube outside diameter. The smaller is the height or base length of CTVGs, the better the thermal performance of the enhanced fin punched with CTVGs. Better thermal performance is achieved as the fin spacing is about 0.24 times the tube outside diameter.

Enhancement in Film Cooling Effectiveness Using Delta Winglet Pair

A simulation study is performed to inspect the influence of delta winglet pair for improving the film cooling effectiveness of gas turbine blade. Incompressible continuity, momentum, energy and two equations - SST model have been used for investigating the nature of flow field, temperature field and turbulent statistics. Reynolds number based on the jet velocity and film cooling hole diameter is 4232. The jet to cross-flow blowing ratio has been varied as 0.5, 1.0 and 1.5. The corresponding Reynolds numbers based on cross-flow velocity and film-hole diameter are equal to 6462, 4229 and 3231 respectively. It is observed that common flow down configuration augments the film cooling effectiveness which attributed to the development of secondary longitudinal vortices. Longitudinal vortices annihilate the counter rotating vortex structures present in the baseline flow. The generation of hairpin vortices and boost of shear layer vortices are modified due to the implementation of Delta winglet pair. The overall turbulence intensity and vorticity get reduced due to the presence of Delta winglet pair. A maximum of 97.46% and a minimum of 61.50% enhancement in film cooling effectiveness is observed at blowing ratio of 1.5 and 0.5 respectively.Wake region of film cooling jet is modified due to Delta winglet pair leading to formation of stagnation region and lower mixing resulting in higher film cooling effectiveness.