Heat Transfer Augmentation Through Wall-Shape-Induced Flow Destabilization

1990 ◽  
Vol 112 (2) ◽  
pp. 336-341 ◽  
Author(s):  
M. Greiner ◽  
R.-F. Chen ◽  
R. A. Wirtz
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Water Channel ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Reynolds Number Range ◽  
Smooth Channel ◽  
Channel Boundary ◽  
Tollmien Schlichting ◽  
Onset Location

Experiments on heat transfer augmentation in a rectangular cross-section water channel are reported. The channel geometry is designed to excite normally damped Tollmien-Schlichting modes in order to enhance mixing. In this experiment, a hydrodynamically fully developed flow encounters a test section where one channel boundary is a series of periodic, saw-tooth, transverse grooves. Free shear layers span the groove openings, separating the main channel flow from the recirculating vortices contained within each cavity. The periodicity length of the grooves is equal to one-half of the expected wavelength of the most unstable mode. The remaining channel walls are flat, and the channel has an aspect ratio of 10:1. Experiments are performed over the Reynolds number range of 300 to 15,000. Streakline flow visualization shows that the flow is steady at the entrance, but becomes oscillatory downstream of an onset location. This location moves upstream with increasing Reynolds numbers. Initially formed traveling waves are two dimensional with a wavelength equal to the predicted most unstable Tollmien-Schlichting mode. Waves become three dimensional with increasing Reynolds number and distance from onset. Some evidence of wave motion persists into the turbulent flow regime. Heat transfer measurements along the smooth channel boundary opposite the grooved wall show augmentation (65 percent) over the equivalent flat channel in the Reynolds number range 1200 to 4800. The degree of enhancement obtained is shown to depend on the channel Reynolds number, and increases with the distance from the onset location.

Heat Transfer And Pressure Drop Of An Angled Discrete Turbulator At Elevated Reynolds Numbers Up To 900,000

2021 ◽  
pp. 1-29
Author(s):  
Sam Ghazi-Hesami ◽  
Dylan Wise ◽  
Keith Taylor ◽  
Peter Ireland ◽  
Étienne Robert
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Friction Factor ◽  
Rectangular Channel ◽  
Reynolds Numbers ◽  
Enhance Heat Transfer ◽  
Near Surface ◽  
Number Range ◽  
Smooth Channel

Abstract Turbulators are a promising avenue to enhance heat transfer in a wide variety of applications. An experimental and numerical investigation of heat transfer and pressure drop of a broken V (chevron) turbulator is presented at Reynolds numbers ranging from approximately 300,000 to 900,000 in a rectangular channel with an aspect ratio (width/height) of 1.29. The rib height is 3% of the channel hydraulic diameter while the rib spacing to rib height ratio is fixed at 10. Heat transfer measurements are performed on the flat surface between ribs using transient liquid crystal thermography. The experimental results reveal a significant increase of the heat transfer and friction factor of the ribbed surface compared to a smooth channel. Both parameters increase with Reynolds number, with a heat transfer enhancement ratio of up to 2.15 (relative to a smooth channel) and a friction factor ratio of up to 6.32 over the investigated Reynolds number range. Complementary CFD RANS (Reynolds-Averaged Navier-Stokes) simulations are performed with the κ-ω SST turbulence model in ANSYS Fluent® 17.1, and the numerical estimates are compared against the experimental data. The results reveal that the discrepancy between the experimentally measured area averaged Nusselt number and the numerical estimates increases from approximately 3% to 13% with increasing Reynolds number from 339,000 to 917,000. The numerical estimates indicate turbulators enhance heat transfer by interrupting the boundary layer as well as increasing near surface turbulent kinetic energy and mixing.

Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

2009 ◽  
Author(s):  
Sunil Patil ◽  
Santosh Abraham ◽  
Danesh Tafti ◽  
Srinath Ekkad ◽  
Yong Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Convective Heat Transfer ◽  
Gas Turbine ◽  
Convective Heat ◽  
Swirl Number ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Peak Location

Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared to fully-developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.

Wavy-Channel for Microscale Heat Transfer in Macro Geometries

2017 ◽  
Author(s):  
Kai Xian Cheng ◽  
Zi Hao Foo ◽  
Kim Tiow Ooi
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Performance Index ◽  
Heat Transfer Performance ◽  
Cnc Machining ◽  
Hydrodynamic Performance ◽  
Working Fluid ◽  
Transfer Performance ◽  
Number Range ◽  
Reynolds Number Range

Microscale heat and fluid flow in macro geometries have been made practical in terms of cost and fabrication, by superimposing two macro geometries which are fabricated using readily-available CNC machining methods. Wavy-profile has been proposed to enhance heat transfer performance in the microchannel owing to the simplicity of geometry and feasibility to be fabricated using simple turning process. Experimental studies were conducted on single-phase, forced convective heat transfer using water as the working fluid for the Reynolds number range of 1300 to 4600, for a constant heat flux of 53.0 W/cm2. Three sinusoidal waves with different wavelength and same amplitude are studied to examine the effect of the total number of waves on the heat transfer and hydrodynamic performance within constant microchannel length. The maximum performance index, which evaluates heat transfer performance per unit pumping power, is 1.39, achieved by wavy profile with the shortest wavelength at Reynolds number of 2800. The performance index for all the enhanced microchannels peaks at the Reynolds number range of 2500 to 2800. Beyond that, the performance index is not a strong function of the wavelength. At lower Reynolds numbers, profile with the shortest wavelength achieves substantially higher performance indices, as the increment in pressure drop is accompanied by a comparable increment in heat transfer. Future work includes the introduction of correlations for the implementation of such geometries in industrial heat exchangers.

Heat transfer augmentation in a radial curved microchannel

2021 ◽  
pp. 095440892110412
Author(s):  
Nitin Kumar Mamidi ◽  
Karthik Balasubramanian ◽  
Kiran Kumar Kupireddi ◽  
V P Chandramohan ◽  
Poh Seng Lee ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Characteristics ◽  
Heat Removal ◽  
Heat Transfer Analysis ◽  
Cooling Systems ◽  
Number Range ◽  
Cooling Performance ◽  
Curvature Ratio ◽  
Reynolds Number Range

Rapid advancement toward miniaturization has emerged with confront for superior heat dissipation techniques. Of all the available cooling systems, microchannel-based cooling systems stand out to provide better cooling performance through superior heat removal abilities. In the present study, the cooling performance and hydraulic flow characteristics of a radial curved microchannel with three curvature ratios were numerically investigated and compared with a radial straight microchannel. Unlike the conventional straight microchannels, curved channels possess better fluid mixing as a result of the centrifugal force caused due to curvature. This phenomenon has a significant effect on heat transfer and fluid flow characteristics. Work on radial curved microchannels has been scarce and there is a lot of potential to augment the heat transfer with lower pumping power particularly with a central inlet. A three-dimensional conjugate heat transfer analysis was carried out for three radial curved microchannels and a radial straight microchannel using the ANSYS Fluent commercial software with the Reynolds number range of 125–275. The results showed a Nusselt number increment of 36.38% for radial curved microchannels when compared to the radial straight microchannel. Further, the lowest average wall temperature was noted for the radial curved microchannel with a curvature ratio of 0.17 which was 15.63 °C lower when compared to that in a radial straight microchannel for the same Reynolds number. Contours of velocity and temperature are presented at various locations along the stream to aid the results. The overall performance of all three radial curved microchannels was found to be higher than that of the radial straight microchannel in the Reynolds number range considered, out of which the maximum performance factor of 1.245 was obtained for the radial curved microchannel with a curvature ratio of 0.17 as compared to the radial straight microchannel.

CFD analysis for thermal performance augmentation of solar air heater using deflector ribs

2020 ◽  
Vol 14 (3) ◽  
pp. 7282-7295
Author(s):  
R. Venkatesh ◽  
Nitesh Kumar ◽  
N. Madhwesh ◽  
Manjunath M.S.
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Enhancement Factor ◽  
Air Gap ◽  
Solar Air Heater ◽  
Number Range ◽  
Reynolds Number Range ◽  
Thermal Enhancement ◽  
Gap Height ◽  
Thermal Enhancement Factor

This paper presents the effect of deflector ribs on the thermal performance of flat plate solar air heater using Computational Fluid Dynamics (CFD) methodology. The analysis is carried out using two-dimensional computational domain for the Reynolds number range of 6000-18000. RNG k-є turbulence model is used to capture the turbulence characteristics of the flow. The deflector rib has a cross-section of isosceles triangle and is placed transversely with respect to the flow. The distance between consecutive ribs is varied as 40mm, 80mm, 160mm and 320mm while the air gap height is varied as 2mm, 3mm, 5mm and 10mm. The numerical model is validated against the well-known correlation of Dittus-Boelter for smooth duct. The simulation results reveal that the presence of deflector ribs provide augmented heat transfer through flow acceleration and enhanced turbulence levels. With reference to smooth duct, the maximum achieved heat transfer improvement is about 1.39 times for the inter-rib distance of 40mm and an air gap height of 3mm while the maximum fiction factor achieved was about 3.82 times for pitch value of 40mm and air gap height of 3mm. The highest thermal enhancement factor is achieved for the pitch value of 320mm and an air gap height of 3mm at Re=6000. The air gap height value of 10mm exhibits thermal enhancement factor values lesser than 1.0 and hence is not recommended for use as heat transfer enhancement device for the entire Reynolds number range used in the analysis. The pitch value of 320 mm exhibits thermal enhancement factor greater than 1.0 for almost all the Reynolds number range used in the analysis and varies between 0.93 and 1.07.

Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Sunil Patil ◽  
Santosh Abraham ◽  
Danesh Tafti ◽  
Srinath Ekkad ◽  
Yong Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Convective Heat Transfer ◽  
Gas Turbine ◽  
Convective Heat ◽  
Swirl Number ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Peak Location

Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow, which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared with fully developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer, which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.

scholarly journals Flow and heat transfer characteristics around an elliptic cylinder placed in front of a curved plate

2014 ◽  
Vol 18 (2) ◽  
pp. 465-478
Author(s):  
Mahmoud Mostafa ◽  
Radwan Kamal ◽  
Mohamed Gobran
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Angle Of Attack ◽  
Elliptic Cylinder ◽  
Heat Transfer Characteristics ◽  
Number Range ◽  
Transfer Characteristics ◽  
Reynolds Number Range

An experimental investigation has been conducted to clarify heat transfer characteristics and flow behaviors around an elliptic cylinder. Also, flow visualization was carried out to clarify the flow patterns around the cylinder. The elliptic cylinder examined has an axis ratio of 1:2.17, was placed in the focus of parabolic plate. The test fluid is air and the Reynolds number based on the major axis length, c, ranged from 5 x 103 to 3 x 104. The angle of attack (?) was changed from 0? to 90? at 15? interval. It is found that the pressure distribution, form drag, location of separation point, and heat transfer coefficient depend strongly upon the angle of attack. Over the Reynolds number range examined, the mean heat transfer coefficient is at its highest at ? = 60? - 90?. The values of heat transfer coefficient in the case of free cylinder are higher than those for cylinder/plate combination at all angles of attack and Reynolds number range examined.

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