| 1. |
- Luo, Lei, et al.
(author)
-
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
-
In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 182
-
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.
|
|
| 2. |
- Zhao, Zhiqi, et al.
(author)
-
Analysis of enhanced turbulent heat transfer in a sharp turn channel having novel designed endwall with longitudinal vortex generator
- 2022
-
In: International Communications in Heat and Mass Transfer. - : Elsevier BV. - 0735-1933. ; 131
-
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.
|
|
| 3. |
- Zhao, Zhiqi, et al.
(author)
-
Effect of divider wall-to-end wall distance on the vortical structures and heat transfer characteristics of two-pass channel using topological analysis
- 2022
-
In: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 32:1, s. 219-240
-
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.
|
|
| 4. |
- Zhao, Zhiqi, et al.
(author)
-
Experimental evaluation of longitudinal and transverse vortex generators on the endwall of a serpentine passage
- 2022
-
In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 176
-
Journal article (peer-reviewed)abstract
- A 180-deg turning channel can be seen in many heat exchange systems, and a strong three-dimensional flow phenomenon will occur at the turning point. In order to strengthen the heat transfer inside the turn and considering the lack of relevant enhanced endwall heat transfer approach, the present study innovatively demonstrates the measures of enhanced internal endwall heat transfer using arrays of delta-winglet and conical vortex generators. The studied parameters of the vortex generators include aspect ratio (AR), spacing, and angle of attack, etc. The experimental results indicate that the longitudinal vortex generators have more advantages in increasing the heat transfer than the transverse conical vortex generators in certain configurations with the same order of magnitude of the increased area. Besides, transverse conical vortex generators are more helpful to improve the endwall heat transfer uniformity than longitudinal vortex generators. The array of longitudinal vortex generators with 45-deg angle of attack, 0.02 m pitch, and 2 aspect ratio provides the highest thermal performance for all studied cases, indicating that the longitudinal delta-winglet vortex generators can induce near-wall flow turbulence as well as convective heat transfer of the endwall with a modest pressure loss penalty. For this optimally designed endwall, the Nusselt number is augmented by up to 35%. From the perspective of energy saving, the present studied vortex generators provide a superior thermal performance at identical pumping power, which can be useful in the future design of endwalls in sharp-turn channels.
|
|
| 5. |
- Zhao, Zhiqi, et al.
(author)
-
Experimental study on the augmented Nusselt number of the endwall through a square-sectioned sharp-turn channel using novel heat exchanger
- 2022
-
In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 192
-
Journal article (peer-reviewed)abstract
- Aquiring higher thermal efficiency with minimum energy consumption is the continuous pursuit of heat exchanger design. This work aims to demonstrate a novel enhanced heat exchange approach for a two-pass heat exchanger surface using four forms of a novel heat exchanger (nHEX), such as high straight and low curved turbulators, etc. The nHEXs are arranged parallel and equally spaced on the internal endwall. A comparison between current design and conventionally turbulated endwalls of sharp-turn channels is presented. The results indicate that the heat transfer rate is greatly augmented by inserting an array of nHEXs compared to a smooth endwall channel. The strategically placed vortex generators can improve the lowest heat transfer region on the smooth endwall to achieve overall thermal enhancement and temperature uniformity. Among the studied geometries, higher height or straight forms of the nHEXs are prone to enhance endwall heat transfer. The high straight forms of the nHEXs provide the maximum improvement of the normalized Nusselt number and thermal enhancement factor in a sharp-turn channel, which are 32% and 18% higher than the corresponding values of the smooth endwall, respectively. This study proved the potential of an array of parallel nHEXs to provide endwall heat transfer enhancement in a sharp-turn channel with an acceptable pressure drop penalty.
|
|
| 6. |
- Zhao, Zhiqi, et al.
(author)
-
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
-
In: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 175
-
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.
|
|
