Turbulent Convection From Deterministic Roughness Distributions With Varying Thermal Conductivities

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Steven R. Mart ◽  
Stephen T. McClain ◽  
Lesley M. Wright
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Wind Tunnel ◽  
Rough Surfaces ◽  
Roughness Element ◽  
Infrared Camera ◽  
Reynolds Numbers ◽  
Diameter Ratio ◽  
Roughness Elements ◽  
Thermal Conductivities

Many flows of engineering interest are bounded by surfaces that exhibit roughness with thermal conductivities much lower than common metals and alloys. Depending on the local roughness element convection coefficients, the low thermal conductivities of the roughness elements may create situations where temperature changes along the heights of the elements are important and must be considered in predicting the overall surface convection coefficient. The discrete-element model (DEM) for flows over rough surfaces was recently adapted to include the effects of internal conduction along the heights of ordered roughness elements. While the adapted DEM provided encouraging agreement with the available data, more data are required to validate the model. To further investigate the effects of roughness element thermal conductivity on convective heat transfer and to acquire more experimental data for DEM validation, four wind tunnel test plates were made. The test plates were constructed using Plexiglas and Mylar film with a gold deposition layer creating a constant flux boundary condition with steady state wind tunnel measurements. The four test plates were constructed with hexagonal distributions of hemispheres or cones made of either aluminum or ABS plastic. The plates with hemispherical elements had element diameters of 9.53 mm and a spacing-to-diameter ratio of 2.099. The plates with conical elements had base element diameters of 9.53 mm and a spacing-to-base-diameter ratio of 1.574. An infrared camera was used to measure the temperature of the heated plates in the Baylor Subsonic Wind Tunnel for free stream velocities ranging from 2.5 m/s to 35 m/s (resulting in Reynolds number values ranging from 90,000 to 1,400,000 based on the distance from the knife-edge to the center of the infrared camera image) in turbulent flow. At lower Reynolds numbers, the thermal conductivity of the roughness elements is a primary factor in determining the heat transfer enhancement of roughness distributions. At the higher Reynolds numbers investigated, the hemispherical distribution, which contained more sparsely spaced elements, did not exhibit a statistically significant difference in enhancement for the different thermal conductivity elements used. The results of the study indicate that the packing density of the elements and the enhancement on the floor of the roughness distribution compete with the roughness element thermal conductivity in determining the overall convection enhancement of rough surfaces.

Turbulent Convection From Deterministic Roughness Distributions With Varying Thermal Conductivities

2011 ◽  
Author(s):  
Steven R. Mart ◽  
Stephen T. McClain ◽  
Lesley M. Wright
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Wind Tunnel ◽  
Rough Surfaces ◽  
Roughness Element ◽  
Infrared Camera ◽  
Reynolds Numbers ◽  
Diameter Ratio ◽  
Roughness Elements ◽  
Thermal Conductivities

Many flows of engineering interest are bounded by surfaces that exhibit roughness with thermal conductivities much lower than common metals and alloys. Depending on the local roughness element convection coefficients, the low thermal conductivities of the roughness elements may create situations where temperature changes along the heights of the elements are important and must be considered in predicting the overall surface convection coefficient. The discrete-element model (DEM) for flows over rough surfaces was recently adapted to include the effects of internal conduction along the heights of ordered roughness elements. While the adapted DEM provided encouraging agreement with the available data, more data are required to validate the model. To further investigate the effects of roughness element thermal conductivity on convective heat transfer and to acquire more experimental data for DEM validation, four wind tunnel test plates were made. The test plates were constructed using Plexiglas and Mylar film with a gold deposition layer creating a constant flux boundary condition with steady state wind tunnel measurements. The four test plates were constructed with hexagonal distributions of hemispheres or cones made of either aluminum or ABS plastic. The plates with hemispherical elements had element diameters of 9.53 mm and a spacing-to-diameter ratio of 2.099. The plates with conical elements had base element diameters of 9.53 mm and a spacing-to-base-diameter ratio of 1.574. An infrared camera was used to measure the temperature of the heated plates in the Baylor Subsonic Wind Tunnel for free stream velocities ranging from 2.5 m/s to 35 m/s (resulting in Reynolds number values ranging from 90,000 to 1,400,000 based on the distance from the knife-edge to the center of the infrared camera image) in turbulent flow. At lower Reynolds numbers, the thermal conductivity of the roughness elements is a primary factor in determining the heat transfer enhancement of roughness distributions. At the higher Reynolds numbers investigated, the hemispherical distribution, which contained more sparsely spaced elements, did not exhibit a statistically significant difference in enhancement for the different thermal conductivity elements used. The results of the study indicate that the packing density of the elements and the enhancement on the floor of the roughness distribution compete with the roughness element thermal conductivity in determining the overall convection enhancement of rough surfaces.

Heat Transfer From Protuberances and Simulated Ice Accretion Roughness Elements

2009 ◽  
Author(s):  
Steven R. Mart ◽  
Stephen T. McClain
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Surface Roughness ◽  
Wind Tunnel ◽  
Roughness Element ◽  
New Technique ◽  
Gold Deposition ◽  
Ice Accretion ◽  
Roughness Elements ◽  
A New Technique

The heat transfer phenomena occurring during the initial phases of in-flight ice formation and accumulation on aircraft surfaces are not completely understood. The aim of this investigation was to quantify the local apparent heat transfer enhancement on surface roughness elements intended to mimic the early stages of ice accretion. In order to achieve this objective, a new technique for mounting gold-deposition Mylar film in a gold-side down orientation was developed and tested. Gold deposited Mylar film is commonly used to establish a constant heat flux convective boundary condition for wind-tunnel test surfaces. However, the accepted mounting technique causes problems if the films are used to explore convective heat transfer from surfaces with high thermal conductivity protuberances and surface roughness. To overcome the problems with roughness-element attachment, a new technique for mounting gold-deposition Mylar film in a gold-side down orientation was developed and tested. After validating the new mounting procedure, a large test plate was created following the same technique and was mounted in a wind tunnel. Using infrared thermometry to acquire temperature profiles of a gold Mylar heated flat plat prepared with three hemi-spherical roughness elements of varying thermal conductivity, the apparent enhancement of the elements was evaluated for varying tunnel velocities. Flow characteristics at and behind the roughness elements were also examined using the infrared images. This study presents the results of the new Mylar film mounting procedure and the apparent enhancement and flow results.

The Effect of Element Thermal Conductivity on Turbulent Convective Heat Transfer From Rough Surfaces

2009 ◽  
Author(s):  
Stephen T. McClain ◽  
B. Keith Hodge ◽  
Jeffrey P. Bons
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Convective Heat Transfer ◽  
Rough Surfaces ◽  
Element Model ◽  
Discrete Element ◽  
Discrete Element Model ◽  
Experimental Measurements ◽  
Fin Efficiency ◽  
Roughness Elements

The discrete-element model for flows over rough surfaces considers the heat transferred from a rough surface to be the sum of the heat convected from the flat surface and the heat convected from the individual roughness elements to the fluid. In previous discrete-element model development, heat transfer experiments were performed using metallic or high-thermal conductivity roughness elements. Many engineering applications, however, exhibit roughness with low thermal conductivities. In the present study, the discrete element model is adapted to consider the effects of finite thermal conductivity of roughness elements on turbulent convective heat transfer. Initially, the boundary-layer equations are solved while the fin equation is simultaneously integrated so that the full conjugate heat transfer problem is solved. However, a simpler approach using a fin-efficiency is also investigated. The results of the conjugate analysis and the simpler fin-efficiency analysis are compared to experimental measurements for turbulent flows over ordered cone surfaces. Possibilities for extending the fin-efficiency method to randomly-rough surfaces and the experimental measurements required are discussed.

The Influence of Element Thermal Conductivity, Shape, and Density on Heat Transfer in a Rough Wall Turbulent Boundary Layer With Strong Pressure Gradients

2021 ◽  
Vol 143 (8) ◽  
Author(s):  
Christoph Gramespacher ◽  
Holger Albiez ◽  
Matthias Stripf ◽  
Hans-Jörg Bauer
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Boundary Layer ◽  
Reynolds Numbers ◽  
Formation Mechanisms ◽  
Pressure Gradients ◽  
Pressure Distributions ◽  
Roughness Elements ◽  
The Difference ◽  
Study Heat Transfer

Abstract Formation mechanisms for turbine roughness are manifold, including erosion, corrosion, deposition, and spallation or more recently additive manufacturing processes. Consequently, the resulting surfaces differ remarkably not only in roughness shape, height, and density but also in element thermal conductivity. Because the roughness elements extend into the boundary layer, their temperature distribution has a direct influence on the thermal boundary layer and thus on the resulting convective heat transfer. In the current study, heat transfer distributions along a flat plate with more than 20 deterministic rough surface topographies that differ in element eccentricity, height and density are measured. For each surface roughness, measurements are conducted using two different element thermal conductivities (0.2 W/(mK) and 30 W/(mK)), two pressure distributions, four Reynolds numbers between 3 × 105 and 7.5 × 105 and various inlet turbulence intensities in the range of 1.5 % to 8 %. The pressure distributions resemble a typical suction and pressure side, respectively. Results show a heat transfer increase of up to 60 % for the high thermal conductivity surfaces and up to 50 % for the low conductivity ones. While heat transfer on the high conductivity surfaces is always higher than on the low conductivity ones, the difference becomes smaller with decreasing element density.

The Effect of Element Thermal Conductivity on Turbulent Convective Heat Transfer From Rough Surfaces

2010 ◽  
Vol 133 (2) ◽  
Author(s):  
Stephen T. McClain ◽  
B. Keith Hodge ◽  
Jeffrey P. Bons
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Convective Heat Transfer ◽  
Rough Surfaces ◽  
Element Model ◽  
Discrete Element ◽  
Discrete Element Model ◽  
Experimental Measurements ◽  
Fin Efficiency ◽  
Roughness Elements

The discrete-element model for flows over rough surfaces considers the heat transferred from a rough surface to be the sum of the heat convected from the flat surface and the heat convected from the individual roughness elements to the fluid. In previous discrete-element model developments, heat transfer experiments were performed using metallic or high-thermal conductivity roughness elements. Many engineering applications, however, exhibit roughness with low thermal conductivities. In the present study, the discrete-element model is adapted to consider the effects of finite thermal conductivity of roughness elements on turbulent convective heat transfer. Initially, the boundary-layer equations are solved while the fin equation is simultaneously integrated so that the full conjugate heat transfer problem is solved. However, a simpler approach using a fin efficiency is also investigated. The results of the conjugate analysis and the simpler fin efficiency analysis are compared to experimental measurements for turbulent flows over ordered cone surfaces. Possibilities for extending the fin efficiency method to randomly rough surfaces and the experimental measurements required are discussed.

The Influence of Element Thermal Conductivity, Shape, and Density on Heat Transfer in a Rough Wall Turbulent Boundary Layer With Strong Pressure Gradients

2020 ◽  
Author(s):  
Christoph Gramespacher ◽  
Holger Albiez ◽  
Mattias Stripf ◽  
Hans-Jörg Bauer
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Boundary Layer ◽  
Reynolds Numbers ◽  
Formation Mechanisms ◽  
Pressure Gradients ◽  
Pressure Distributions ◽  
Roughness Elements ◽  
The Difference ◽  
Study Heat Transfer

Abstract Formation mechanisms for turbine roughness are manifold, including erosion, corrosion, deposition, and spallation or more recently additive manufacturing processes. Consequently, the resulting surfaces differ remarkably not only in roughness shape, height, and density, but also in element thermal conductivity. Because the roughness elements extend into the boundary layer, their temperature distribution has a direct influence on the thermal boundary layer and thus on the resulting convective heat transfer. In the current study, heat transfer distributions along a flat plate with more than 20 deterministic rough surface topographies that differ in element eccentricity, height and density are measured. For each surface roughness, measurements are conducted using two different element thermal conductivities (0.2 W/(mK) and 30 W/(mK)), two pressure distributions, four Reynolds numbers between 3 × 105 and 7.5 × 105 and various inlet turbulence intensities in the range of 1.5% to 8%. The pressure distributions resemble a typical suction and pressure side, respectively. Results show a heat transfer increase of up to 60% for the high thermal conductivity surfaces and up to 50% for the low conductivity ones. While heat transfer on the high conductivity surfaces is always higher than on the low conductivity ones, the difference becomes smaller with decreasing element density.

Reduced Rough-Surface Parametrization for Use With the Discrete-Element Model

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Stephen T. McClain ◽  
Jason M. Brown
Keyword(s):  
Heat Transfer ◽  
Rough Surface ◽  
Surface Characterization ◽  
Rough Surfaces ◽  
Roughness Element ◽  
Three Dimensional ◽  
Element Model ◽  
Discrete Element ◽  
Diameter Distribution ◽  
Discrete Element Model

The discrete-element model for flows over rough surfaces was recently modified to predict drag and heat transfer for flow over randomly rough surfaces. However, the current form of the discrete-element model requires a blockage fraction and a roughness-element diameter distribution as a function of height to predict the drag and heat transfer of flow over a randomly rough surface. The requirement for a roughness-element diameter distribution at each height from the reference elevation has hindered the usefulness of the discrete-element model and inhibited its incorporation into a computational fluid dynamics (CFD) solver. To incorporate the discrete-element model into a CFD solver and to enable the discrete-element model to become a more useful engineering tool, the randomly rough surface characterization must be simplified. Methods for determining characteristic diameters for drag and heat transfer using complete three-dimensional surface measurements are presented. Drag and heat transfer predictions made using the model simplifications are compared to predictions made using the complete surface characterization and to experimental measurements for two randomly rough surfaces. Methods to use statistical surface information, as opposed to the complete three-dimensional surface measurements, to evaluate the characteristic dimensions of the roughness are also explored.

Endwall Heat Transfer Measurements in a Staggered Pin Fin Array With an Adiabatic Pin

2007 ◽  
Author(s):  
Forrest E. Ames ◽  
Chad A. Nordquist ◽  
Lindsay A. Klennert
Keyword(s):  
Heat Transfer ◽  
Turbulent Transport ◽  
Infrared Camera ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Skin Friction Coefficient ◽  
Vortex System ◽  
Flow Acceleration ◽  
Pin Fin ◽  
Pin Fin Array

Full surface endwall heat transfer distributions have been acquired in a staggered pin fin array with the use of an infrared camera. Values are presented at Reynolds numbers of 3000, 10,000 and 30,000 based on pin diameter and average velocity through adjacent pins. Average endwall Nusselt numbers agree closely with archival values at each Reynolds number. Locally averaged heat transfer levels show a substantial increase from the inlet through the first few rows and finally a nearly streamwise periodic condition in the second half of the eight row geometry. Increasing levels of heat transfer in the inlet region can be attributed to the leading edge vortex system, flow acceleration around pins, and the generation of turbulence. Distributions of turbulence intensity and turbulent scale are shown to help document the turbulent transport conditions through the array. Detailed endwall Nusselt number distributions are presented and compared at the three Reynolds numbers for the first four and last four rows. These detailed heat transfer distributions highlight the influence of the horseshoe vortex system in the entrance region and the wake generated turbulence throughout the pin fin array. Local velocity and turbulence distributions are presented together with local Stanton number and skin friction coefficient data to examine the aggressive nature of the turbulent mixing.

An Experimental Investigation of Structured Roughness Effect on Heat Transfer During Single-Phase Liquid Flow at Microscale

2012 ◽  
Vol 134 (10) ◽  
Author(s):  
Ting-Yu Lin ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Transfer Enhancement ◽  
Enhancement Factor ◽  
Roughness Element ◽  
Hydraulic Diameter ◽  
Transfer Enhancement ◽  
Roughness Elements ◽  
The Heat Transfer Coefficient

The effect of structured roughness on the heat transfer of water flowing through minichannels was experimentally investigated in this study. The test channels were formed by two 12.7 mm wide × 94.6 mm long stainless steel strips. Eight structured roughness elements were generated using a wire electrical discharge machining (EDM) process as lateral grooves of sinusoidal profile on the channel walls. The height of the roughness structures ranged from 18 μm to 96 μm, and the pitch was varied from 250 μm to 400 μm. The hydraulic diameter of the rectangular flow channels ranged from 0.71 mm to 1.87 mm, while the constricted hydraulic diameter (obtained by using the narrowest flow gap) ranged from 0.68 mm to 1.76 mm. After accounting for heat losses from the edges and end sections, the heat transfer coefficient for smooth channels was found to be in good agreement with the conventional correlations in the laminar entry region as well as in the laminar fully developed region. All roughness elements were found to enhance the heat transfer. In the ranges of parameters tested, the roughness element pitch was found to have almost no effect, while the heat transfer coefficient was significantly enhanced by increasing the roughness element height. An earlier transition from laminar to turbulent flow was observed with increasing relative roughness (ratio of roughness height to hydraulic diameter). For the roughness element designated as B-1 with a pitch of 250 μm, roughness height of 96 μm and a constricted hydraulic diameter of 690 μm, a maximum heat transfer enhancement of 377% was obtained, while the corresponding friction factor increase was 371% in the laminar fully developed region. Comparing different enhancement techniques reported in the literature, the highest roughness element tested in the present work resulted in the highest thermal performance factor, defined as the ratio of heat transfer enhancement factor (over smooth channels) and the corresponding friction enhancement factor to the power 1/3.

Reduced Rough-Surface Parameterization for Use With the Discrete-Element Model

2007 ◽  
Author(s):  
Stephen T. McClain ◽  
Jason M. Brown
Keyword(s):  
Heat Transfer ◽  
Rough Surface ◽  
Surface Characterization ◽  
Rough Surfaces ◽  
Roughness Element ◽  
Three Dimensional ◽  
Element Model ◽  
Discrete Element ◽  
Diameter Distribution ◽  
Discrete Element Model

The discrete-element model for flows over rough surfaces was recently modified to predict drag and heat transfer for flow over randomly-rough surfaces. However, the current form of the discrete-element model requires a blockage fraction and a roughness-element diameter distribution as a function of height to predict the drag and heat transfer of flow over a randomly-rough surface. The requirement for a roughness element-diameter distribution at each height from the reference elevation has hindered the usefulness of the discrete-element model and inhibited its incorporation into a computational fluid dynamics (CFD) solver. To incorporate the discrete-element model into a CFD solver and to enable the discrete-element model to become a more useful engineering tool, the randomly-rough surface characterization must be simplified. Methods for determining characteristic diameters for drag and heat transfer using complete three-dimensional surface measurements are presented. Drag and heat transfer predictions made using the model simplifications are compared to predictions made using the complete surface characterization and to experimental measurements for two randomly-rough surfaces. Methods to use statistical surface information, as opposed to the complete three-dimensional surface measurements, to evaluate the characteristic dimensions of the roughness are also explored.

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