| 1. |
- Luo, Lei, et al.
(author)
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An experimental investigation on the thermal augmentation of internal endwall in a two-pass duct using an array of delta-winglet vortex generator pair
- 2022
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 182
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Journal article (peer-reviewed)abstract
- The present study experimentally demonstrates a novel design for an internal endwall in a two-pass duct heat exchanger. An array of delta-winglet vortex generator pairs with different aspect ratio, angle of attack, configuration are mounted on the internal endwall in a U duct. Liquid crystal thermalgraphy and static pressure measurements are utilized in this study to evaluate the heat transfer, thermal-hydraulic performance, and pressure drop penalty. The Reynolds number is varied from 10,000 to 20,000. A comparison between the current novel design and traditional enhanced heat transfer turbulators in the open literature is also included. The results show that the endwall heat transfer is enhanced greatly as the longitudinal vortex generator array is located on the endwall. The variation of the attack angle has great influence on the endwall heat transfer, The endwall fitted with turbulators with an aspect ratio (length/height) of 2 and attack angle of 45-deg provides the highest thermal-hydraulic performance with a relatively low pressure drop penalty as long lasting longitudinal vortices might be induced at this attack angle and the heat transfer is observed to be enhanced at the downstream part of the endwall. In addition, for the temperature uniformity on the endwall, the local heat transfer at the upstream part still has the potential to be further improved for all studied cases. The optimal thermalhydraulic performance is increased by more than 24% compared with that on the endwall in the smooth U duct. This novel design presents a rather high overall thermal performance compared with the traditional design in the open literature.
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| 2. |
- Qiu, Dandan, et al.
(author)
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On heat transfer and flow characteristics of jets impingement on a concave surface with varying pin-fin arrangements
- 2021
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In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 170
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Journal article (peer-reviewed)abstract
- Numerical simulations have been conducted to investigate the effects of pin-fin arrangement and jet Reynolds number on heat transfer enhancement and flow characteristics of a concave confined jet impingement cooled surface conjugated with pin-fins. The results without a pin-fin are used as a baseline. Vortices and different sections of streamlines in the channel are employed to understand the flow structures in this cooling system. Based on this, topology pictures are obtained with skin-friction lines to discuss the fluid flow near the target and pin-fin surface. The local and averaged heat transfer characteristics are analyzed using the flow characteristics. The overall parameters (friction factor, thermal performance, pumping power) are also obtained. The results show that the appearance of the pin-fin plays a positive role in heat transfer enhancement and uniformity. The case of a 35° pin-fin arrangement angle reaches the highest overall Nusselt number and heat transfer uniformity. The far away movement of the pin-fin is beneficial to decrease pumping power at a constant heat transfer. The thermal performance is highest for 55° pin-fin arrangement angle at a jet Reynolds number of 40,000 in this work.
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| 3. |
- Qiu, Dandan, et al.
(author)
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On heat transfer and flow characteristics of jets impinging onto concave surface with varying bleeding arrangements
- 2021
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In: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 31:12, s. 3642-3660
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Journal article (peer-reviewed)abstract
- Purpose: The purpose of this study is to investigate the effects of film holes’ arrangements and jet Reynolds number on flow structure and heat transfer characteristics of jet impingement conjugated with film cooling in a semicylinder double wall channel. Design/methodology/approach: Numerical simulations are used in this research. Streamlines on different sections, skin-friction lines, velocity, wall shear stress and turbulent kinetic energy contours near the concave target wall and vortices in the double channel are presented. Local Nusselt number contours and surface averaged Nusselt numbers are also obtained. Topology analysis is applied to further understand the fluid flow and is used in analyzing the heat transfer characteristics. Findings: It is found that the arrangement of side films positioned far from the center jets helps to enhance the flow disturbance and heat transfer behind the film holes. The heat transfer uniformity for the case of 55° films arrangement angle is most improved and the thermal performance is the highest in this study. Originality/value: The film holes’ arrangements effects on fluid flow and heat transfer in an impingement cooled concave channel are conducted. The flow structures in the channel and flow characteristics near target by topology pictures are first obtained for the confined film cooled impingement cases. The heat transfer distributions are analyzed with the flow characteristics. The highest heat transfer uniformity and thermal performance situation is obtained in present work.
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| 4. |
- Zhao, Zhiqi, et al.
(author)
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Analysis of enhanced turbulent heat transfer in a sharp turn channel having novel designed endwall with longitudinal vortex generator
- 2022
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In: International Communications in Heat and Mass Transfer. - : Elsevier BV. - 0735-1933. ; 131
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Journal article (peer-reviewed)abstract
- This work provides a new method to enhance endwall heat transfer of a sharp turn channel. Several longitudinal vortex generators (LVGs) at various spacing and aspect ratio are placed in parallel on the inner side of endwall in a sharp turn channel. Liquid crystal thermography, pressure difference measurements, and a statistical method are applied to analyze the thermal behavior, friction factor, and temperature uniformity on the endwall mounted with an array of LVGs. The Reynolds number are in the range of 10,000 to 20,000. A comparison between the novel designed endwall and conventional turbulators in the same U bend channel is also conducted. Results indicate that the local Nusselt number is greatly augmented with the application of LVGs on the endwall, especially at the downstream part of the endwall. The LVGs with an aspect ratio of 2 and a spacing of 20 mm (dense case) provides the optimal thermal characteristics. The optimal design provides augmentation of heat transfer rate and thermal performance factor by up to 35.1% and 25.5%, respectively. The endwall fitted with delta-winglet vortex generators is a promising enhanced heat transfer method compared with the conventional design available for the same U-duct model.
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| 5. |
- Zhao, Zhiqi, et al.
(author)
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Effect of divider wall-to-end wall distance on the vortical structures and heat transfer characteristics of two-pass channel using topological analysis
- 2022
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In: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 32:1, s. 219-240
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Journal article (peer-reviewed)abstract
- Purpose: This study aims to explore the 3 D separated flow fields and heat transfer characteristics at the end wall of a serpentine channel with various turn clearances using topological analysis and critical points principles of three-dimensional vortex flow. Design/methodology/approach: This aims to explore the 3 D separated flow fields and heat transfer characteristics at the end wall of a serpentine channel with various turn clearances using topological analysis as well as critical points principles of three-dimensional vortex flow. Findings: The endwall heat transfer in the narrow spacing passage is significantly stronger than that in a wide spacing channel. As the gap clearance is kept at 0.87 times of the hydraulic diameter, the endwall heat transfer and thermal performance can be accordingly enhanced with low pressure drops, which is because a relatively strong concentrated impingement flow for the medium gap clearance helps to restrain the downstream fluid flow and enhance the shear effect of the secondary flow. Practical implications: The numerical results can be applied in designing sharp turn of serpentine channel in heat exchangers, heat sinks, piping system, solar receiver and gas turbine blades. Originality/value: The evolution mechanism of the vortices in the turning region under different gap clearance was analyzed, and thermal enhancement characteristics were predicted innovatively using topological analysis method.
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| 6. |
- Zhao, Zhiqi, et al.
(author)
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Influence of spacing of a delta-winglet vortex generator pair on the flow behavior and heat transfer at the internal tip of gas turbine blades
- 2022
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In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 175
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Journal article (peer-reviewed)abstract
- The hot erosion of turbine tip and the maintenance of tip reliability and service times have always been great challenges for researchers. This work presents a novel design of a tip wall with the internal surface fitted with delta-winglet vortex generator pairs (DVGP). Various spacing of DVGP is considered to study the influence of spacing on the internal tip heat transfer and flow behavior in the turn region of a serpentine cooling channel inside a blade. A smooth internal cooling channel is regarded as the Baseline. Five spacings of the leading edge of the DVGP, i.e., 0.03Dh(hydraulic diameter), 0.07Dh, 0.13Dh, 0.2Dh, 0.27Dh, respectively, are considered. Topological analysis in the skin friction field is introduced to better illustrate the 3D fluid flow. The inlet Reynolds number is set in the range of 10,000 to 50,000. Results show that the variation of the spacing between the DVGPs has a remarkable influence on the thermal performance of the internal blade tip. The heat transfer on each side of the DVGP is weakened increaseing spacing, which is caused by the movement of the attachment line on the surface of the DVGP and the weakening of the vortex energy. The optimal thermal performance appears for the narrow spacing (0.03Dh) of the leading edge of the DVGP, and the heat transfer augmentation is approximately 7.90% compared with the two-pass channel with a smooth tip. The delta-winglet vortex generator arranged narrowly on the internal tip is aimed for enhancement of heat transfer. The analysis and results of this work are beneficial to the tip design of internal passages in gas turbine blades.
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| 7. |
- Zhao, Zhiqi, et al.
(author)
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On the topology of vortex structures and heat transfer of a gas turbine blade internal tip with different arrangement of delta-winglet vortex generators
- 2021
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In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 160
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Journal article (peer-reviewed)abstract
- This paper aims to provide a novel enhanced heat transfer method for the internal tip surface of a U bend channel of relevance for a gas turbine rotor blade. The DWVGs (delta-winglet vortex generators) pair is arranged at multiple locations on the tip surface. Two types of vortex generators are studied, including common-flow-up and common-flow-down configurations. The inlet channel Reynolds number varies from 10,000 to 50,000. The topological analysis method is used to determine the formation and evolution of the vortices and to better understand the mechanism of the heat transfer enhancement. Results of skin-friction lines, topological portrait, Nusselt number, friction factor, thermal performance are included. The results show that due to the interaction among the vortices surrounding the vortex generators, the DWVGs pair in common-flow-up configuration has a slight heat transfer improvement, and is not sensitive to the tip location. However, the DWVGs pair in common-flow-down configuration placed at the downstream of the tip surface improved the heat transfer significantly as the induced vortices between the turbulator pairs effectively reduce the thickness of the boundary layer. Compared with the smooth U bend channel, the optimal design shows that the heat transfer and overall thermal performance can be increased by up to 7.4% and 6.8%, respectively. This study elaborates the flow and heat transfer processes from the perspective of topology, which is helpful in the design of cooling procedures of turbine blades.
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| 8. |
- Zhao, Zhiqi, et al.
(author)
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Vortical structures and heat transfer augmentation of a cooling channel in a gas turbine blade with various arrangements of tip bleed holes
- 2021
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In: Numerical Heat Transfer; Part A: Applications. - : Informa UK Limited. - 1040-7782 .- 1521-0634. ; 79:1, s. 40-67
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Journal article (peer-reviewed)abstract
- This study investigates the internal cooling processes affected by the tip bleed holes in gas turbine blades. Double bleed holes are fixed at the center of the blade tip near the pressure side and suction side, respectively. Five different arrangements of the holes along the center line of the tip are studied. The purely double holes are set as the Baseline. The purpose of the present study is to provide a new perspective of the tip film cooling to understand the internal flow processes, vorticity evolution and the mechanism of the heat transfer augmentation. A topological analysis and the boundary layer analysis methods are introduced to better understand the tip heat transfer. The total extraction area and volume is kept at the same level for all the studied cases. The results show that the Dean vortices and the near-wall vortices induced by the secondary flow contribute to the high heat transfer coefficient on the tip surface. The mixing effect of the Dean vortices and the hole extraction helps to enhance heat transfer upstream of the tip. Different arrangement of the bleed holes can affect the internal flow processes and heat transfer performance. The suction effect of the center-line bleed hole can accelerate the near-hole flow and reduce the thickness of the boundary layer. The center-line hole fitted at the middle of the tip affects significantly the rear side of the hole. Thus, the holes aligned in the middle of the tip provide the highest heat transfer and thermal performance. The thermal performance is enhanced by up to 4.7% compared with the Baseline.
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| 9. |
- Liu, Yi, et al.
(author)
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Reactive consolidation of layered-ternary Ti(2)AlN ceramics by spark plasma sintering of a Ti/AlN powder mixture
- 2011
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In: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 31:5, s. 863-868
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Journal article (peer-reviewed)abstract
- A reactive consolidation process for preparing ternary Ti(2)AlN ceramics was investigated by spark plasma sintering (SPS). A Ti/AlN powder mixture with a molar ratio of 2:1 was consolidated at temperatures ranging from 800 to 1450 degrees C. The phase composition and microstructure evolution during the process were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS). A series of intermediate phases, namely TiN, Ti(3)Al, Ti(3)AlN and TiAl were indentified, which revealed a reaction pathway towards the formation of Ti(2)AlN.
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| 10. |
- Luo, Lei, et al.
(author)
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Surface temperature reduction by using dimples/protrusions in a realistic turbine blade trailing edge
- 2018
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In: Numerical Heat Transfer; Part A: Applications. - : Informa UK Limited. - 1040-7782 .- 1521-0634. ; 74:5, s. 1265-1283
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Journal article (peer-reviewed)abstract
- In this article, numerical simulations have been conducted on the heat transfer effect of dimple/protrusion layouts of a pin-finned wedge duct. Conjugate heat transfer calculations are further performed to investigate the cooling effect of modified schemes with dimples and protrusions added. Comparisons are carried out with a turbine second stage guide vane employed as the prototype. The dimple/protrusion-pin fin arrangement is set as the optimum one obtained above, and dimple depth/protrusion height varies from 0.2 to 0.3 times the structure diameter. It is found that the side-by-side arrangement and protrusion structure is more beneficial for the wedge duct endwall heat transfer. Comparison with the prototype blade shows that the addition of both dimples and protrusions are helpful in enhancing the trailing edge cooling effect. The cooling effect is increased with an increase in dimple depth/protrusion height. The results also show that the modified blade with protrusions attached at 0.3 height saves 0.48 g/s cooling mass flow and reaches the most positive performance with a 17 K, 14 K average temperature reduction, 0.022, 0.018 cooling effect increasing for pressure, suction side, respectively.
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