Large Eddy Simulation of Flow and Heat Transfer in a Staggered 45° Ribbed Duct

2004 ◽  
Author(s):  
Samer Abdel-Wahab ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Secondary Flow ◽  
Mean Flow ◽  
Cross Sectional ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy ◽  
Rib Vortex

Results from large eddy simulation (LES) of fully developed flow in a staggered 45° ribbed duct are presented with rib pitch-to-height ratio P/e = 10 and a rib height to hydraulic diameter ratio e/Dh = 0.1. The nominal Reynolds number based on bulk velocity is 47,300. Mean flow and turbulent quantities, together with heat transfer and friction augmentation results are presented. The flow is characterized by a helical vortex behind each rib and a complementary cross-sectional secondary flow, both of which result from the angle of the rib with respect to the mean flow. Averaged velocity profiles at the duct center show excellent agreement with experiments and heat transfer predictions agree well with experiments. Turbulent kinetic energy, shear stress, and heat transfer augmentation ratios show a strong correlation to the rib vortex and the secondary flow. Overall, heat transfer is augmented by a factor of 2.3 compared with a smooth duct and matches experimental data within 2%.

Large Eddy Simulation of Flow and Heat Transfer in a 90° Ribbed Duct With Rotation: Effect of Coriolis Forces

2004 ◽  
Author(s):  
Samer Abdel-Wahab ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Rotation Number ◽  
Mean Flow ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
A Value ◽  
Rotation Numbers ◽  
Large Eddy

This paper presents results from large eddy simulation (LES) of fully developed flow in a 90° ribbed duct with rib pitch-to-height ratio P/e = 10 and a rib height to hydraulic diameter ratio e/Dh = 0.1. Three rotation numbers Ro = 0.18, 0.35 and 0.67 are studied at a nominal Reynolds number based on bulk velocity of 20,000. Mean flow and turbulent quantities, together with heat transfer and friction augmentation data are presented. Turbulence and heat transfer are augmented on the trailing surface and reduced at the leading surface. The heat transfer augmentation ratio on the trailing surface asymptotes to a value of 3.7 ± 5% and does not show any further increasing trend as the rotation number increases beyond 0.2. On the other hand, augmentation ratios on the leading surface keep decreasing with an increase in rotation number with values ranging from 1.7 at Ro = 0.18 to 1.2 at Ro = 0.67. Secondary flow cells augment the heat transfer coefficient on the smooth walls by 20% to 30% over a stationary duct. An increase in rotation number from 0.35 to 0.67 decreases the frictional losses from an augmentation ratio of 9.6 to 8.75 and is a consequence of decrease in form drag and wall shear. Overall augmentation compared with a non-rotating duct ranges from +15% to +20% for heat transfer, and +10% to +15% for friction over the range of rotation numbers studied. Comparison of heat transfer augmentation with previous experimental results in the literature shows very good agreement.

Large Eddy Simulation Investigation of Flow and Heat Transfer in a Channel With Dimples and Protrusions

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Mohammad A. Elyyan ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Reynolds Numbers ◽  
Flow Acceleration ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Channel Form ◽  
Large Eddy ◽  
Dimple Surface

Large eddy simulation calculations are conducted for flow in a channel with dimples and protrusions on opposite walls with both surfaces heated at three Reynolds numbers, ReH=220, 940, and 9300, ranging from laminar, weakly turbulent, to fully turbulent, respectively. Turbulence generated by the separated shear layer in the dimple and along the downstream rim of the dimple is primarily responsible for heat transfer augmentation on the dimple surface. On the other hand, augmentation on the protrusion surface is mostly driven by flow impingement and flow acceleration between protrusions, while the turbulence generated in the wake has a secondary effect. Heat transfer augmentation ratios of 0.99 at ReH=220,2.9 at ReH=940, and 2.5 at ReH=9300 are obtained. Both skin friction and form losses contribute to pressure drop in the channel. Form losses increase from 45% to 80% with increasing Reynolds number. Friction coefficient augmentation ratios of 1.67, 4.82, and 6.37 are obtained at ReH=220, 940, and 9300, respectively. Based on the geometry studied, it is found that dimples and protrusions may not be viable heat transfer augmentation surfaces when the flow is steady and laminar.

Large Eddy Simulation in a Duct With Rounded Skewed Ribs

2005 ◽  
Author(s):  
Aroon K. Viswanathan ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Cross Section ◽  
Mean Flow ◽  
Cross Sectional ◽  
Square Duct ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Experimental Values

Results from Large Eddy Simulation (LES) of fully developed flow in a ribbed duct are presented with rib pitch-to-height ratio (P/e) is 10 and a rib height to hydraulic diameter ratio (e/Dh) is 0.1. Computations are carried out on a square duct with 45° ribs on the top and bottom walls arranged in a staggered fashion. The ribs have a rounded cross-section and are skewed at 45° to the main flow. The Reynolds number based on bulk velocity is 25,000. Mean flow and turbulent quantities, together with heat transfer and friction augmentation results are presented for a stationary case. The flow is characterized by a helical vortex behind each rib and a complementary cross-sectional secondary flow, both of which result from the angle of the rib with respect to the mean flow and result in a spanwise variation of the heat transfer. The mean flow, the turbulent quantities and the heat transfer in the duct show similar trends as in the duct with square cross-section ribs. However the results show that there is lesser friction in the ducts with rounded ribs. The overall heat transfer on the ribbed wall was augmented by 2.85 times that of a smooth duct, at the cost of friction which increases by a factor of 10. The computed values compare well with the experimental values.

Large Eddy Simulation of the Developing Region of a Stationary Ribbed Internal Turbine Blade Cooling Channel

2004 ◽  
Author(s):  
Evan A. Sewall ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Turbine Blade ◽  
Internal Cooling ◽  
Mean Flow ◽  
Heat Transfer Augmentation ◽  
The Third ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Cooling Passage

This study reports on a Large Eddy Simulation (LES) of the entrance section of a gas turbine blade internal cooling passage. The channel is fitted with in-line turbulators orthogonal to the flow, and the domain studied covers the first six ribs of the channel. The rib height-to-hydraulic diameter ratio (e/Dh) is 0.1, and the rib pitch-to-rib height ratio (P/e) is 10. A constant temperature boundary condition is imposed on the walls and the ribs, and the flow Reynolds number is 20,000. Results indicate that the mean flow is essentially fully developed by the fifth rib. Turbulent kinetic energy near the ribbed wall approaches fully developed values very quickly by the third or fourth ribs. However, turbulent intensities at the center of the duct are not fully developed by the sixth rib. As a consequence, heat transfer augmentation on the ribbed walls reaches a fully developed state quickly after the third rib, whereas, the smooth wall heat transfer augmentation shows a slight but steady increasing trend toward the fully developed value up to the sixth rib. Both augmentation ratios are to within 10% of their fully developed values after the third rib.

Large Eddy Simulation of Flow and Heat Transfer in the 180‐Deg Bend Region of a Stationary Gas Turbine Blade Ribbed Internal Cooling Duct

2005 ◽  
Vol 128 (4) ◽  
pp. 763-771 ◽  
Author(s):  
Evan A. Sewall ◽  
Danesh K. Tafti
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Friction Factor ◽  
Flow Separation ◽  
Mean Flow ◽  
Dean Vortices ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
High Heat Transfer ◽  
Large Eddy

Large eddy simulation of the 180 deg bend in a stationary ribbed duct is presented. The domain studied includes three ribs upstream of the bend region and three ribs downstream of the bend with an outflow extension added to the end, using a total of 8.4 million cells. Two cases are compared to each other: one includes a rib in the bend and the other does not. The friction factor, mean flow, turbulence, and heat transfer are compared in the two cases to help explain the benefits and disadvantages of the wide number of flow effects seen in the bend, including flow separation at the tip of the dividing wall, counter-rotating Dean vortices, high heat transfer at areas of flow impingement, and flow separation at the upstream and downstream corners of the bend. Mean flow results show a region of separated flow at the tip of the dividing region in the case with no rib in the bend, but no separation region is observed in the case with a rib. A pair of counter-rotating Dean vortices in the middle of the bend is observed in both cases. Turbulent kinetic energy profiles show a 30% increase in the midplane of the bend when the rib is added. High gradients of heat transfer augmentation are observed on the back wall and downstream outside wall, where mean flow impingement occurs. This heat transfer is increased with the presence of a rib. Including a rib in the bend increases the friction factor in the bend by 80%, and it increases the heat transfer augmentation by approximately 20%, resulting in a trade-off between pressure drop and heat transfer.

scholarly journals Large-Eddy Simulation Study of Flow and Heat Transfer in Swirling and Non-Swirling Impinging Jets on a Semi-Cylinder Concave Target

2021 ◽  
Vol 11 (15) ◽  
pp. 7167
Author(s):  
Liang Xu ◽  
Xu Zhao ◽  
Lei Xi ◽  
Yonghao Ma ◽  
Jianmin Gao ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Wall Temperature ◽  
Target Surface ◽  
Impinging Jets ◽  
Small Scale ◽  
Complex Flow ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy

Swirling impinging jet (SIJ) is considered as an effective means to achieve uniform cooling at high heat transfer rates, and the complex flow structure and its mechanism of enhancing heat transfer have attracted much attention in recent years. The large eddy simulation (LES) technique is employed to analyze the flow fields of swirling and non-swirling impinging jet emanating from a hole with four spiral and straight grooves, respectively, at a relatively high Reynolds number (Re) of 16,000 and a small jet spacing of H/D = 2 on a concave surface with uniform heat flux. Firstly, this work analyzes two different sub-grid stress models, and LES with the wall-adapting local eddy-viscosity model (WALEM) is established for accurately predicting flow and heat transfer performance of SIJ on a flat surface. The complex flow field structures, spectral characteristics, time-averaged flow characteristics and heat transfer on the target surface for the swirling and non-swirling impinging jets are compared in detail using the established method. The results show that small-scale recirculation vortices near the wall change the nearby flow into an unstable microwave state, resulting in small-scale fluctuation of the local Nusselt number (Nu) of the wall. There is a stable recirculation vortex at the stagnation point of the target surface, and the axial and radial fluctuating speeds are consistent with the fluctuating wall temperature. With the increase in the radial radius away from the stagnation point, the main frequency of the fluctuation of wall temperature coincides with the main frequency of the fluctuation of radial fluctuating velocity at x/D = 0.5. Compared with 0° straight hole, 45° spiral hole has a larger fluctuating speed because of speed deflection, resulting in a larger turbulence intensity and a stronger air transport capacity. The heat transfer intensity of the 45° spiral hole on the target surface is slightly improved within 5–10%.

Large Eddy Simulation of Flow and Heat Transfer Mechanism in Matrix Cooling Channel

2017 ◽  
Author(s):  
Yigang Luan ◽  
Lianfeng Yang ◽  
Bo Wan ◽  
Tao Sun
Keyword(s):  
Heat Transfer ◽  
Flow Field ◽  
Large Eddy Simulation ◽  
Gas Turbine ◽  
Transfer Mechanism ◽  
Heat Transfer Mechanism ◽  
Longitudinal Vortices ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy

Gas turbine engines have been widely used in modern industry especially in the aviation, marine and energy fields. The efficiency of gas turbines directly affects the economy and emissions. It’s acknowledged that the higher turbine inlet temperatures contribute to the overall gas turbine engine efficiency. Since the components are subject to the heat load, the internal cooling technology of turbine blades is of vital importance to ensure the safe and normal operation. This paper is focused on exploring the flow and heat transfer mechanism in matrix cooling channels. In order to analyze the internal flow field characteristics of this cooling configuration at a Reynolds number of 30000 accurately, large eddy simulation method is carried out. Methods of vortex identification and field synergy are employed to study its flow field. Cross-sectional views of velocity in three subchannels at different positions have been presented. The results show that the airflow is strongly disturbed by the bending part. It’s concluded that due to the bending structure, the airflow becomes complex and disordered. When the airflow goes from the inlet to the turning, some small-sized and discontinuous vortices are formed. Behind the bending structure, the size of the vortices becomes big and the vortices fill the subchannels. Because of the structure of latticework, the airflow is affected by each other. Airflow in one subchannel can exert a shear force on another airflow in the opposite subchannel. It’s the force whose direction is the same as the vortex that enhances the longitudinal vortices. And the longitudinal vortices contribute to the energy exchange of the internal airflow and the heat transfer between airflow and walls. Besides, a comparison of the CFD results and the experimental data is made to prove that the numerical simulation methods are reasonable and acceptable.

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