Transient Leading to Periodic Fluid Flow and Heat Transfer in a Cavity With Constant Temperature Walls Due to an Isothermal Rotating Object

2007 ◽  
Author(s):  
Y.-C. Shih ◽  
J. M. Khodadadi ◽  
K.-H. Weng
Keyword(s):  
Heat Transfer ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
High Heat ◽  
Sliding Mesh ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
High Heat Transfer ◽  
Transient Variations ◽  
Rotating Objects

Computational analysis of transient phenomenon followed by the periodic state of laminar flow and heat transfer due to a rotating object in a square cavity is investigated. A finite-volume-based computational methodology utilizing primitive variables is used. Various isothermal rotating objects (circle, square and equilateral triangle) with different sizes are placed in the middle of the cavity. A combination of a fixed computational grid with a sliding mesh was utilized for the square and triangle shapes. The motionless object is set in rotation at time t = 0 and its temperature is maintained constant but different from the temperature of the walls of the cavity. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta < 1750). The evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. Similarities and differences of the flow and thermal fields for various shapes is discussed. Transient variations of the average Nusselt numbers on the surface of the rotating object and cavity walls show that for high Re numbers, a quasi-periodic behavior due to the onset of Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt number of the cavity is correlated to the rotational Reynolds number and shape of the object. The triangle object clearly gives rise to high heat transfer followed by the square and circle objects.

Transient Leading to Periodic Fluid Flow and Heat Transfer in a Differentially-Heated Cavity Due to an Insulated Rotating Object

2007 ◽  
Author(s):  
Y.-C. Shih ◽  
J. M. Khodadadi ◽  
K.-H. Weng ◽  
H. F. Oztop
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
High Heat ◽  
Thermal Fields ◽  
Flow And Heat Transfer ◽  
High Heat Transfer ◽  
Transient Variations ◽  
Rotating Objects

Computational analysis of transient phenomenon followed by the periodic state of laminar flow and heat transfer due to an insulated rotating object in a square cavity is investigated. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square and equilateral triangle) with different sizes are placed in the middle of the cavity. A combination of a fixed computational grid with a sliding mesh was utilized for the square and triangle shapes. The cavity is maintained as a differentially-heated enclosure and the motionless insulated object is set in rotation at time t = 0. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta < 1750). The evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. At the same time, similarity of the thermal fields for various shapes for the same Reynolds number varifies the appropriate selection of the hydraulic diameter. Transient variations of the average Nusselt numbers on the two walls show that for high Re numbers, a quasi-periodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt number of the cavity is correlated to the rotational Reynolds number and shape of the object. The triangle object clearly gives rise to high heat transfer followed by the square and circle objects.

Periodic Fluid Flow and Heat Transfer in a Square Cavity Due to an Insulated or Isothermal Rotating Cylinder

2009 ◽  
Vol 131 (11) ◽  
Author(s):  
Y.-C. Shih ◽  
J. M. Khodadadi ◽  
K.-H. Weng ◽  
A. Ahmed
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
Rotating Cylinder ◽  
Flow Fields ◽  
Square Cavity ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Rotating Objects

The periodic state of laminar flow and heat transfer due to an insulated or isothermal rotating cylinder object in a square cavity is investigated computationally. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square, and equilateral triangle) with different sizes are placed in the middle of a square cavity. A combination of a fixed computational grid and a sliding mesh was utilized for the square and triangle shapes. For the insulated and isothermal objects, the cavity is maintained as differentially heated and isothermal enclosures, respectively. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr=5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta<1750). The periodic flow field, the interaction of the rotating objects with the recirculating vortices at the four corners, and the periodic channeling effect of the traversing vertices are clearly elucidated. The simulations of the dynamic flow fields were confirmed against experimental data obtained by particle image velocimetry. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to the conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. Transient variations of the average Nusselt numbers of the respective systems show that for high Re numbers, a quasiperiodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt numbers of the insulated and isothermal object systems were correlated with the rotational Reynolds number and shape of the object. For high Re numbers, the performance of the system is independent of the shape of the object. On the other hand, with lowering of the hydraulic diameter (i.e., bigger objects), the triangle and the circle exhibit the highest and lowest heat transfers, respectively. High intensity of the periodic channeling and not its frequency is identified as the cause of the observed enhancement.

Heat Transfer in a Surfactant Drag-Reducing Solution—A Comparison With Predictions for Laminar Flow

2005 ◽  
Vol 128 (6) ◽  
pp. 557-563 ◽  
Author(s):  
Paul L. Sears ◽  
Libing Yang
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Reynolds Numbers ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Nusselt Numbers ◽  
Drag Reducing Additive ◽  
Good Agreement

Heat transfer coefficients were measured for a solution of surfactant drag-reducing additive in the entrance region of a uniformly heated horizontal cylindrical pipe with Reynolds numbers from 25,000 to 140,000 and temperatures from 30to70°C. In the absence of circumferential buoyancy effects, the measured Nusselt numbers were found to be in good agreement with theoretical results for laminar flow. Buoyancy effects, manifested as substantially higher Nusselt numbers, were seen in experiments carried out at high heat flux.

A Numerical Study of Heat Transfer and Flow Structure in Channels With Miniature V Rib-Dimple Hybrid Structure on One Wall

2018 ◽  
Author(s):  
Peng Zhang ◽  
Yu Rao ◽  
Yanlin Li
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Hybrid Structure ◽  
Transfer Enhancement ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Flow Mixing

This paper presents a numerical study on turbulent flow and heat transfer in the channels with a novel hybrid cooling structure with miniature V-shaped ribs and dimples on one wall. The heat transfer characteristics, pressure loss and turbulent flow structures in the channels with the rib-dimples with three different rib heights of 0.6 mm, 1.0 mm and 1.5 mm are obtained for the Reynolds numbers ranging from 18,700 to 60,000 by numerical simulations, which are also compared with counterpart of a pure dimpled and pure V ribbed channel. The results show that the overall Nusselt numbers of the V rib-dimple channel with the rib height of 1.5 mm is up to 70% higher than that of the channels with pure dimples. The numerical simulations show that the arrangement of the miniature V rib upstream each dimple induces complex secondary flow near the wall and generates downwashing vortices, which intensifies the flow mixing and turbulent kinetic energy in the dimple, resulting in significant improvement in heat transfer enhancement and uniformness.

Heat Transfer in a “Cover-Plate” Preswirl Rotating-Disk System

1999 ◽  
Vol 121 (2) ◽  
pp. 249-256 ◽  
Author(s):  
R. Pilbrow ◽  
H. Karabay ◽  
M. Wilson ◽  
J. M. Owen
Keyword(s):  
Heat Transfer ◽  
Rotating Disk ◽  
Reynolds Numbers ◽  
Turbine Disk ◽  
Cover Plate ◽  
Blade Cooling ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Cooling Air ◽  
Plate System

In most gas turbines, blade-cooling air is supplied from stationary preswirl nozzles that swirl the air in the direction of rotation of the turbine disk. In the “cover-plate” system, the preswirl nozzles are located radially inward of the blade-cooling holes in the disk, and the swirling airflows radially outward in the cavity between the disk and a cover-plate attached to it. In this combined computational and experimental paper, an axisymmetric elliptic solver, incorporating the Launder–Sharma and the Morse low-Reynolds-number k–ε turbulence models, is used to compute the flow and heat transfer. The computed Nusselt numbers for the heated “turbine disk” are compared with measured values obtained from a rotating-disk rig. Comparisons are presented, for a wide range of coolant flow rates, for rotational Reynolds numbers in the range 0.5 X 106 to 1.5 X 106, and for 0.9 < βp < 3.1, where βp is the preswirl ratio (or ratio of the tangential component of velocity of the cooling air at inlet to the system to that of the disk). Agreement between the computed and measured Nusselt numbers is reasonably good, particularly at the larger Reynolds numbers. A simplified numerical simulation is also conducted to show the effect of the swirl ratio and the other flow parameters on the flow and heat transfer in the cover-plate system.

Heat Transfer in a “Cover-Plate” Pre-Swirl Rotating-Disc System

1998 ◽  
Author(s):  
Robert Pilbrow ◽  
Hasan Karabay ◽  
Michael Wilson ◽  
J. Michael Owen
Keyword(s):  
Heat Transfer ◽  
Reynolds Numbers ◽  
Cover Plate ◽  
Swirl Ratio ◽  
Rotating Disc ◽  
Blade Cooling ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Turbine Disc ◽  
Cooling Air

In most gas turbines, blade-cooling air is supplied from stationary pre-swirl nozzles that swirl the air in the direction of rotation of the turbine disc. In the “cover-plate” system, the pre-swirl nozzles are located radially inward of the blade-cooling holes in the disc, and the swirling air flows radially outwards in the cavity between the disc and a cover-plate attached to it. In this combined computational and experimental paper, an axisymmetric elliptic solver, incorporating the Launder-Sharma and the Morse low-Reynolds-number k-ε turbulence models, is used to compute the flow and heat transfer. The computed Nusselt numbers for the heated “turbine disc” are compared with measured values obtained from a rotating-disc rig. Comparisons are presented, for a wide range of coolant flow rates, for rotational Reynolds numbers in the range 0.5 × 106 to 1.5 × 106, and for 0.9 < βp < 3.1, where βp is the pre-swirl ratio (or ratio of the tangential component of velocity of the cooling air at inlet to the system to that of the disc). Agreement between the computed and measured Nusselt numbers is reasonably good, particularly at the larger Reynolds numbers. A simplified numerical simulation is also conducted to show the effect of the swirl ratio and the other flow parameters on the flow and heat transfer in the cover-plate system.

Numerical and experimental analysis of microchannel heat transfer for nanoliquid coolant using different shapes and geometries

2014 ◽  
Vol 229 (11) ◽  
pp. 2056-2065 ◽  
Author(s):  
Harry Garg ◽  
Vipender Singh Negi ◽  
Nidhi Garg ◽  
AK Lall
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Fluid Flow ◽  
Transfer Coefficient ◽  
Experimental Analysis ◽  
High Heat ◽  
Heat Transfer Analysis ◽  
Flow And Heat Transfer ◽  
High Heat Transfer ◽  
High Heat Transfer Coefficient

As part of the liquid cooling, most of the work has been done on fluid flow and heat transfer analysis for flow field. In the present work, the experimental and numerical studies of the microchannel the fluid flow and heat transfer analysis using nanoliquid coolant have been discussed. The practical aspects for increasing the high heat transfer coefficient from conventional studies and the different geometries and shapes of the microchannel are studied. The Aspect Ratio has significant effect on the microchannels and has been varied from AR 2, 4 and 8 to choose the optimum one. Three different fluids, i.e. de-ionized water, ethylene glycol, and a custom nanofluid are chosen for study. The proposed nanofluid almost interacts as another solid and has reduced thermal resistance, friction effect, and thus it almost vanishes high hot spots. Experimental analysis shows that the proposed nanofluid is excellent fluid for high rate heat removals. Moreover, the performance of the overall system is excellent in terms of high heat transfer coefficient, high thermal conductivity, and high capacity of the fluid. It has been reported that the heat transfer coefficient can be increased to 2.5 times of the water or any other fluid. It was also reported that the AR 4 rectangular-shaped channels are the optimum geometry in the Reynolds number ranging from 50 to 800 considering laminar flow. Examination and identification is based upon the practical result that includes fabrication constraints, commercial application, sealing of the system, ease of operation, and so on.

A Convection Heat Transfer Correlation for a Binary Air-Helium Mixture at Low Reynolds Number

2007 ◽  
Vol 129 (11) ◽  
pp. 1494-1505 ◽  
Author(s):  
Arindam Banerjee ◽  
Malcolm J. Andrews
Keyword(s):  
Heat Transfer ◽  
Mixing Layer ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
Gas Mixtures ◽  
Correlation Technique ◽  
Hot Wire ◽  
Nusselt Numbers ◽  
Low Reynolds ◽  
Constant Temperature Anemometer

The results of experiments investigating heat transfer from a hot wire in a binary mixture of air and helium are reported. The measurements were made with a constant temperature anemometer at low Reynolds numbers (0.25<Re<1.2) and correlated by treating the data in terms of a suitably defined Reynolds and Nusselt numbers based on the wire diameter. The correlation was obtained by taking into account the temperature dependency of gas properties, properties of binary gas mixtures, and the fluid slip at the probe surfaces as well as gas accommodation effects. The correlation has been used to measure velocity and velocity-density statistics across a buoyancy driven Rayleigh–Taylor mixing layer with a hot wire. The measured values obtained with the correlation agree well with measurements obtained with a more rigorous and extensive calibration technique (at two different overheat ratios). The reported correlation technique can be used as a faster and less expensive method for calibrating hot wires in binary gas mixtures.

The Thickness of the Liquid Microlayer Between a Cap-Shaped Sliding Bubble and a Heated Wall: Experimental Measurements

2006 ◽  
Vol 128 (9) ◽  
pp. 934-944 ◽  
Author(s):  
Xin Li ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Heat Transfer ◽  
Reynolds Numbers ◽  
High Heat ◽  
Heated Wall ◽  
Transfer Coefficients ◽  
Data Set ◽  
Laser Probe ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Spherical Bubbles

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20–100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2 deg to 15 deg from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor were injected near the lower end of a uniformly heated aluminum plate. The laser probe yielded microlayer thicknesses of 22–55 μm for cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers from 0.9 to 1.7, and Weber numbers from 2.6 to 47. The microlayer thickness above cap-shaped bubbles was correlated to a function of inclination angle and a bubble shape factor. The successful correlation suggests that this data set can be used to validate the results of detailed models of the microlayer dynamics.

Natural Convection Heat Transfer From a Cylinder With High Conductivity Permeable Fins

2003 ◽  
Vol 125 (2) ◽  
pp. 282-288 ◽  
Author(s):  
Bassam A/K Abu-Hijleh
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Convection Heat Transfer ◽  
High Heat ◽  
Horizontal Cylinder ◽  
High Conductivity ◽  
Transfer Rates ◽  
High Heat Transfer ◽  
Wide Range

The problem of laminar natural convection from a horizontal cylinder with multiple equally spaced high conductivity permeable fins on its outer surface was investigated numerically. The effect of several combinations of number of fins and fin height on the average Nusselt number was studied over a wide range of Rayleigh number. Permeable fins provided much higher heat transfer rates compared to the more traditional solid fins for a similar cylinder configuration. The ratio between the permeable to solid Nusselt numbers increased with Rayleigh number, number of fins, and fin height. This ratio was as high as 8.4 at Rayleigh number of 106, non-dimensional fin height of 2.0, and with 11 equally spaced fins. The use of permeable fins is very advantageous when high heat transfer rates are needed such as in today’s high power density electronic components.

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