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• 1. [Article] Experimental Study of Nucleate Boiling Bubble Dynamics and Heat Transfer Enhancement on Printed Bi-functional Surfaces

  Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface ...

  Citation × Citation Citation Abstract Copy Title: Experimental Study of Nucleate Boiling Bubble Dynamics and Heat Transfer Enhancement on Printed Bi-functional Surfaces Author: David, Michele Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface enhancement with increased surface area, varied geometry, wettability contrast and micro/nano-structures can further enhance boiling heat transfer performance through bubble nucleation augmentation. Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. Bi-functional surfaces for enhanced boiling heat transfer were fabricated and studied through investigation of bubble dynamics and pool boiling experiments. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of surfaces with distinct wettability regions. Bi-functional surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water at atmospheric pressure. Resulting boiling curves of enhanced surfaces showed up to 3X enhancement in heat transfer coefficients at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface. The surfaces show promising results for energy savings in two-phase change applications. Title: Experimental Study of Nucleate Boiling Bubble Dynamics and Heat Transfer Enhancement on Printed Bi-functional Surfaces Author: David, Michele Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface enhancement with increased surface area, varied geometry, wettability contrast and micro/nano-structures can further enhance boiling heat transfer performance through bubble nucleation augmentation. Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. Bi-functional surfaces for enhanced boiling heat transfer were fabricated and studied through investigation of bubble dynamics and pool boiling experiments. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of surfaces with distinct wettability regions. Bi-functional surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water at atmospheric pressure. Resulting boiling curves of enhanced surfaces showed up to 3X enhancement in heat transfer coefficients at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface. The surfaces show promising results for energy savings in two-phase change applications. Close

• 2. [Article] Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface

  Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured ...

  Citation × Citation Citation Abstract Copy Title: Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface Author: Graber, Christof Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured spatially- and temporally- varying fluid and surface temperatures. Unique to this study is the documentation of the effects of using a nanofluid and a nano-structured surface for dropwise cooling by comparison of heat transfer characteristics with that of water droplet impingement on a polished surface. Temperatures were determined using radiation intensities recorded using an Infrared (IR) camera. The impingement surface is either comprised of IR transparent silicon, which permits near-surface fluid temperature measurements, or an IR opaque gold coated surface, which permits surface temperature measurements. A range of surface heat fluxes, resulting in both single-phase and boiling conditions are studied. Three different impingement surfaces have been tested, including polished silicon, nano-structured porous silicon, and gold coated polished silicon. The nanofluid is a water-based carbon nanotube suspension. Five major droplet impingement and evaporation stages have been identified: initial impact, boiling (if the surface temperature was sufficiently high), approximately constant diameter evaporation, stepwise fast receding contact line evaporation, and simultaneously decaying diameter and contact angle final dryout period. The surface temperature spatial distribution shows the lowest temperature values within the contact area bulk region and increasing temperature values toward the contact line region and beyond. The basic temperature trends and evaporation behavior are similar for the polished and nano-structured surface while the nanofluid exhibits some distinction. Evaporation times are reduced up to 20% and 37% using the nanostructured surface and nanofluid, respectively. Considering the evaporation time reduction as a measure of droplet cooling performance, the nano-enhanced surface and nanofluid may improve heat transfer in droplet impingement and spray cooling applications. Title: Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface Author: Graber, Christof Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured spatially- and temporally- varying fluid and surface temperatures. Unique to this study is the documentation of the effects of using a nanofluid and a nano-structured surface for dropwise cooling by comparison of heat transfer characteristics with that of water droplet impingement on a polished surface. Temperatures were determined using radiation intensities recorded using an Infrared (IR) camera. The impingement surface is either comprised of IR transparent silicon, which permits near-surface fluid temperature measurements, or an IR opaque gold coated surface, which permits surface temperature measurements. A range of surface heat fluxes, resulting in both single-phase and boiling conditions are studied. Three different impingement surfaces have been tested, including polished silicon, nano-structured porous silicon, and gold coated polished silicon. The nanofluid is a water-based carbon nanotube suspension. Five major droplet impingement and evaporation stages have been identified: initial impact, boiling (if the surface temperature was sufficiently high), approximately constant diameter evaporation, stepwise fast receding contact line evaporation, and simultaneously decaying diameter and contact angle final dryout period. The surface temperature spatial distribution shows the lowest temperature values within the contact area bulk region and increasing temperature values toward the contact line region and beyond. The basic temperature trends and evaporation behavior are similar for the polished and nano-structured surface while the nanofluid exhibits some distinction. Evaporation times are reduced up to 20% and 37% using the nanostructured surface and nanofluid, respectively. Considering the evaporation time reduction as a measure of droplet cooling performance, the nano-enhanced surface and nanofluid may improve heat transfer in droplet impingement and spray cooling applications. Close

• 3. [Article] Lateral fluid motion in nucleate boiling through asymmetric surface structures

  This thesis presents a feasibility study of a means to passively effect liquid motion parallel to a heated surface though surface geometrical modifications. Such a passive system is beneficial for electronics ...

  Citation × Citation Citation Abstract Copy Title: Lateral fluid motion in nucleate boiling through asymmetric surface structures Author: Kapsenberg, Florian This thesis presents a feasibility study of a means to passively effect liquid motion parallel to a heated surface though surface geometrical modifications. Such a passive system is beneficial for electronics cooling applications as it reduces the pumping equipment normally required in flow loops and is desired for space applications, where launch costs greatly restrict the weight of onboard systems. The surface geometry considered was a repeated array of asymmetric silicon ratchets with reentrant cavities located on the shallow face. A serpentine thin film heater provided heat to the surface. A complete experimental facility in which to perform experiments was designed and constructed as part of this work to comply with requirements set by microgravity flight services in anticipation of future experiments in microgravity. Experiments were performed using deionized and degassed water at atmospheric pressure at subcoolings of 5°C and 20°C. Applied area averaged heat flux was varied between 2 W/cm² and 19 W/cm² at these conditions. Magnified high-speed videos were used to resolve bubble behavior near the surface. A preferential bubble growth and departure direction was confirmed for both subcoolings and lateral fluid motion was confirmed in the high subcooling condition. Repeatability was confirmed with separate experiment performed 58 days later. Tracking of bubbles was accomplished using a custom bubble-tracking algorithm, designed to resolve only bubbles within a two-dimensional plane normal to the viewing direction. Instantaneous velocities of individual bubbles parallel to the surface were shown to be in excess of 600 mm/s immediately following departure, and liquid flows with mean velocities between 25 mm/s and 35 mm/s parallel to surface were observed in the plume farther from the surface. A simplified semi-empirical model of bubble growth phase is proposed to explain the observed mean liquid velocities. Title: Lateral fluid motion in nucleate boiling through asymmetric surface structures Author: Kapsenberg, Florian This thesis presents a feasibility study of a means to passively effect liquid motion parallel to a heated surface though surface geometrical modifications. Such a passive system is beneficial for electronics cooling applications as it reduces the pumping equipment normally required in flow loops and is desired for space applications, where launch costs greatly restrict the weight of onboard systems. The surface geometry considered was a repeated array of asymmetric silicon ratchets with reentrant cavities located on the shallow face. A serpentine thin film heater provided heat to the surface. A complete experimental facility in which to perform experiments was designed and constructed as part of this work to comply with requirements set by microgravity flight services in anticipation of future experiments in microgravity. Experiments were performed using deionized and degassed water at atmospheric pressure at subcoolings of 5°C and 20°C. Applied area averaged heat flux was varied between 2 W/cm² and 19 W/cm² at these conditions. Magnified high-speed videos were used to resolve bubble behavior near the surface. A preferential bubble growth and departure direction was confirmed for both subcoolings and lateral fluid motion was confirmed in the high subcooling condition. Repeatability was confirmed with separate experiment performed 58 days later. Tracking of bubbles was accomplished using a custom bubble-tracking algorithm, designed to resolve only bubbles within a two-dimensional plane normal to the viewing direction. Instantaneous velocities of individual bubbles parallel to the surface were shown to be in excess of 600 mm/s immediately following departure, and liquid flows with mean velocities between 25 mm/s and 35 mm/s parallel to surface were observed in the plume farther from the surface. A simplified semi-empirical model of bubble growth phase is proposed to explain the observed mean liquid velocities. Close

• 4. [Article] An experimental study of friction factors and mixed convection in the thermal entrance region of vertically narrow, horizontal rectangular channels for different heating conditions and aspect ratios

  Heat transfer and fluid mechanical behavior of water flowing horizontally in vertically-narrow rectangular channels was studied in this work. Friction factor and Nusselt number variations were determined ...

  Citation × Citation Citation Abstract Copy Title: An experimental study of friction factors and mixed convection in the thermal entrance region of vertically narrow, horizontal rectangular channels for different heating conditions and aspect ratios Author: Hong, Seung-Ho Heat transfer and fluid mechanical behavior of water flowing horizontally in vertically-narrow rectangular channels was studied in this work. Friction factor and Nusselt number variations were determined experimentally for four different wall heating conditions, and for aspect ratios(height to width) of 5, 4, 3, 2, and 1. Wall heating conditions examined were: all surfaces heated; three surfaces heated, the top adiabatic; one side surface heated the others adiabatic; and the bottom surface heated, the others adiabatic. Friction factors, in laminar flow, varied as predicted from analysis. The critical Reynolds number varied linearly with 1n(D[subscript]). Local Nusselt numbers were determined, at each aspect ratio and heating condition, as functions of Reynolds and Rayleigh numbers. Mixed convection was the result of buoyancy-induced secondary flows. Local Nusselt numbers decreased in a manner common to pure forced convection, reaching minimum values some distance from the entrance, then increased due to the presence of the secondary flows. For given aspect ratio, local Nusselt numbers were found to increase and the thermal entrance lengths decreased, with increasing Rayleigh numbers. In the thermal entry region, for all heating conditions except the bottom-heating case, local Nusselt number behavior showed minor dependence on aspect ratio. Title: An experimental study of friction factors and mixed convection in the thermal entrance region of vertically narrow, horizontal rectangular channels for different heating conditions and aspect ratios Author: Hong, Seung-Ho Heat transfer and fluid mechanical behavior of water flowing horizontally in vertically-narrow rectangular channels was studied in this work. Friction factor and Nusselt number variations were determined experimentally for four different wall heating conditions, and for aspect ratios(height to width) of 5, 4, 3, 2, and 1. Wall heating conditions examined were: all surfaces heated; three surfaces heated, the top adiabatic; one side surface heated the others adiabatic; and the bottom surface heated, the others adiabatic. Friction factors, in laminar flow, varied as predicted from analysis. The critical Reynolds number varied linearly with 1n(D[subscript]). Local Nusselt numbers were determined, at each aspect ratio and heating condition, as functions of Reynolds and Rayleigh numbers. Mixed convection was the result of buoyancy-induced secondary flows. Local Nusselt numbers decreased in a manner common to pure forced convection, reaching minimum values some distance from the entrance, then increased due to the presence of the secondary flows. For given aspect ratio, local Nusselt numbers were found to increase and the thermal entrance lengths decreased, with increasing Rayleigh numbers. In the thermal entry region, for all heating conditions except the bottom-heating case, local Nusselt number behavior showed minor dependence on aspect ratio. Close

• 5. [Article] Local heat transfer rate and bubble dynamics during jet impingement boiling

  Characterization of local boiling trends, in addition to the typically reported area-averaged trends, is essential for the robust design and implementation of phase change technologies to sensitive heat ...

  Citation × Citation Citation Abstract Copy Title: Local heat transfer rate and bubble dynamics during jet impingement boiling Author: Mani, Preeti Characterization of local boiling trends, in addition to the typically reported area-averaged trends, is essential for the robust design and implementation of phase change technologies to sensitive heat transfer applications such as electronics cooling. Obtaining the values of heat fluxes corresponding to locally varying surface temperatures has been a challenge limiting most investigations to area-averaged results. This thesis illustrates the importance of a spatially local heat transfer analysis during boiling. Pool and submerged jet impingement boiling scenarios on a silicon surface are considered at the macroscale (27.5 mm heater with multiple nucleation sites) and microscale (1000 μm heater for isolated bubble generation), by the use of two thin film serpentine heater geometries. The macroscale heater highlights the effect of spatial variations in imposed heat flux on boiling heat transfer with a circumferentially uniform but radially non-uniform heat flux distribution. The microscale heater simulates a local hot-spot for spot cooling on an electronic device. Spatial variation in boiling heat transfer and bubble dynamics with and without a jet flow are documented using thin film voltage sensors along with qualitative and quantitative high speed imaging and infra-red thermography. Unique to this study is the documentation of local boiling curves for different radial locations on the heat transfer surface and their comparison with the corresponding area-averaged representations. It is shown here that sectionally averaged representations of boiling curves over regions of like-imposed heat flux can substantially simplify the interpretation of data while retaining important information of the local variations in heat transfer. The radial influence of the convective jet flow on the bubble dynamics and boiling heat transfer is assessed for a single circular submerged jet configuration. Varied parameters include jet exit Reynolds numbers, nozzle geometry, test fluid (deionized water and FC-72), fluid subcooling and the supplied heat flux. Distinct modifications of the surface temperature distribution imposed by the impinging jet flow are highlighted by comparing radial temperature profiles during pool and jet impingement boiling. It is demonstrated that in contrast with pool boiling, thermal overshoots during jet impingement boiling for a highly wetting fluid like FC-72 are highest in regions farthest from the impingement point. The effect of jet inertia on bubble departure characteristics are compared with pool boiling under subcooled conditions for FC-72. Qualitative high speed visualization indicates the presence of two modes of bubble generation during jet impingement boiling (a) bubble departure from the surface and (b) bubble separation from the source resulting in sliding bubbles over the surface. The effect of jet flow on bubble entrainment is depicted. Quantitative results indicate that in general departure diameters for pool and jet impingement boiling increase and plateau at a maximum value with increasing power input while no notable trends were observed in the corresponding departure frequencies. The largest departure diameters for jet impingement boiling at fixed fluid subcoolings of 10°C and 20°C were found to be smaller than that for the corresponding pool boiling test by a factor of 1.6 and 2.3, respectively. Title: Local heat transfer rate and bubble dynamics during jet impingement boiling Author: Mani, Preeti Characterization of local boiling trends, in addition to the typically reported area-averaged trends, is essential for the robust design and implementation of phase change technologies to sensitive heat transfer applications such as electronics cooling. Obtaining the values of heat fluxes corresponding to locally varying surface temperatures has been a challenge limiting most investigations to area-averaged results. This thesis illustrates the importance of a spatially local heat transfer analysis during boiling. Pool and submerged jet impingement boiling scenarios on a silicon surface are considered at the macroscale (27.5 mm heater with multiple nucleation sites) and microscale (1000 μm heater for isolated bubble generation), by the use of two thin film serpentine heater geometries. The macroscale heater highlights the effect of spatial variations in imposed heat flux on boiling heat transfer with a circumferentially uniform but radially non-uniform heat flux distribution. The microscale heater simulates a local hot-spot for spot cooling on an electronic device. Spatial variation in boiling heat transfer and bubble dynamics with and without a jet flow are documented using thin film voltage sensors along with qualitative and quantitative high speed imaging and infra-red thermography. Unique to this study is the documentation of local boiling curves for different radial locations on the heat transfer surface and their comparison with the corresponding area-averaged representations. It is shown here that sectionally averaged representations of boiling curves over regions of like-imposed heat flux can substantially simplify the interpretation of data while retaining important information of the local variations in heat transfer. The radial influence of the convective jet flow on the bubble dynamics and boiling heat transfer is assessed for a single circular submerged jet configuration. Varied parameters include jet exit Reynolds numbers, nozzle geometry, test fluid (deionized water and FC-72), fluid subcooling and the supplied heat flux. Distinct modifications of the surface temperature distribution imposed by the impinging jet flow are highlighted by comparing radial temperature profiles during pool and jet impingement boiling. It is demonstrated that in contrast with pool boiling, thermal overshoots during jet impingement boiling for a highly wetting fluid like FC-72 are highest in regions farthest from the impingement point. The effect of jet inertia on bubble departure characteristics are compared with pool boiling under subcooled conditions for FC-72. Qualitative high speed visualization indicates the presence of two modes of bubble generation during jet impingement boiling (a) bubble departure from the surface and (b) bubble separation from the source resulting in sliding bubbles over the surface. The effect of jet flow on bubble entrainment is depicted. Quantitative results indicate that in general departure diameters for pool and jet impingement boiling increase and plateau at a maximum value with increasing power input while no notable trends were observed in the corresponding departure frequencies. The largest departure diameters for jet impingement boiling at fixed fluid subcoolings of 10°C and 20°C were found to be smaller than that for the corresponding pool boiling test by a factor of 1.6 and 2.3, respectively. Close

• 6. [Article] Performance and flow stability characteristics in two-phase confined impinging jets

  Advances in electronics fabrication, coupled with the demand for increased computing power, have driven the demand for innovative cooling solutions to dissipate waste heat generated by these devices. To ...

  Citation × Citation Citation Abstract Copy Title: Performance and flow stability characteristics in two-phase confined impinging jets Author: Sabo, Michael D. Advances in electronics fabrication, coupled with the demand for increased computing power, have driven the demand for innovative cooling solutions to dissipate waste heat generated by these devices. To meet future demands, research and development has focused on robust and stable two-phase heat transfer devices. A confined impinging jet is explored as means of utilizing two-phase heat transfer while minimizing flow instabilities observed in microchannel devices. The test configuration consists of a 4 mm diameter jet of water that impinges on a 38 mm diameter heated aluminum surface. Experimental parameters include inlet mass flow rates from 150 to 600 g/min, nozzle-to-surface spacing from 1 to 8 mm, and input heat fluxes from 0 to 90 W/cm2. Results were used to assess the influence of the testing parameters on the heat transfer performance and stability characteristics of a two-phase confined impinging jet. Stability characteristics were explored using power spectral densities (PSDs) of the inlet pressure time series data. Confined impinging jets, over the range of conditions tested, were found to be stable and an efficient means of removing large amounts of waste heat. The radial geometry of the confined jet allows the fluid to expand as it flows radially away from the nozzle, which suppresses instabilities found in microchannel array geometries. Conditions of the heater surface were found to strongly influence two-phase performance. Analysis of PSDs, for stable operation, showed dominate frequencies in the range of 1-4 Hz, which were attributed to generated vapor expanding in the outlet plenum and the subsequent collapse as it condensed. A stability indicator was developed by inducing artificial instabilities into the system by varying the amount of cross sectional area available for outlet vapor removal and compared to the results for stable operation. Title: Performance and flow stability characteristics in two-phase confined impinging jets Author: Sabo, Michael D. Advances in electronics fabrication, coupled with the demand for increased computing power, have driven the demand for innovative cooling solutions to dissipate waste heat generated by these devices. To meet future demands, research and development has focused on robust and stable two-phase heat transfer devices. A confined impinging jet is explored as means of utilizing two-phase heat transfer while minimizing flow instabilities observed in microchannel devices. The test configuration consists of a 4 mm diameter jet of water that impinges on a 38 mm diameter heated aluminum surface. Experimental parameters include inlet mass flow rates from 150 to 600 g/min, nozzle-to-surface spacing from 1 to 8 mm, and input heat fluxes from 0 to 90 W/cm2. Results were used to assess the influence of the testing parameters on the heat transfer performance and stability characteristics of a two-phase confined impinging jet. Stability characteristics were explored using power spectral densities (PSDs) of the inlet pressure time series data. Confined impinging jets, over the range of conditions tested, were found to be stable and an efficient means of removing large amounts of waste heat. The radial geometry of the confined jet allows the fluid to expand as it flows radially away from the nozzle, which suppresses instabilities found in microchannel array geometries. Conditions of the heater surface were found to strongly influence two-phase performance. Analysis of PSDs, for stable operation, showed dominate frequencies in the range of 1-4 Hz, which were attributed to generated vapor expanding in the outlet plenum and the subsequent collapse as it condensed. A stability indicator was developed by inducing artificial instabilities into the system by varying the amount of cross sectional area available for outlet vapor removal and compared to the results for stable operation. Close

• 7. [Article] Turbulent heat transfer in annuli at large diameter ratios

  Heat transfer from the inner wall of an annulus to subcooled water in turbulent parallel flow was studied. The inner core of the annulus consisted of wires varying from 0.0019 to 0.0197 inch in diameter. ...

  Citation × Citation Citation Abstract Copy Title: Turbulent heat transfer in annuli at large diameter ratios Author: Unzicker, William Frank Heat transfer from the inner wall of an annulus to subcooled water in turbulent parallel flow was studied. The inner core of the annulus consisted of wires varying from 0.0019 to 0.0197 inch in diameter. The outer tube consisted of a 1.01 inch diameter glass tube 24 inches long. Diameter ratios therefore varied from 530:1 to 51:1. The water velocity varied from 1.8 to 13.5 feet per second. Rectified alternating current was supplied to the wires. The current, wire surface area, wire resistance, and bulk water temperature were measured and from this data the average convective heat transfer coefficient calculated. The heat transfer coefficient varied from 1840 to 4990 BTU /hr. ft²[superscript .]F for the 0.0197 inch diameter wire to 7210 to 13000 BTU /hr. ft. ²[superscript .]F for the 0.0019 inch diameter wire. The data agreed well with that of Mueller who had worked with air in annuli of large diameter ratios. The data correlated when plotted as N[subscript Nu]/N[subscript Prf]¹[superscript /]³ versus N[subscript Re] when the equivalent diameter was based on experimentally determined radii of maximum velocity in turbulent annular flow. Over the range of operating conditions studied the heat transfer coefficients were considerably lower than predicted by the usual empirical equations. Title: Turbulent heat transfer in annuli at large diameter ratios Author: Unzicker, William Frank Heat transfer from the inner wall of an annulus to subcooled water in turbulent parallel flow was studied. The inner core of the annulus consisted of wires varying from 0.0019 to 0.0197 inch in diameter. The outer tube consisted of a 1.01 inch diameter glass tube 24 inches long. Diameter ratios therefore varied from 530:1 to 51:1. The water velocity varied from 1.8 to 13.5 feet per second. Rectified alternating current was supplied to the wires. The current, wire surface area, wire resistance, and bulk water temperature were measured and from this data the average convective heat transfer coefficient calculated. The heat transfer coefficient varied from 1840 to 4990 BTU /hr. ft²[superscript .]F for the 0.0197 inch diameter wire to 7210 to 13000 BTU /hr. ft. ²[superscript .]F for the 0.0019 inch diameter wire. The data agreed well with that of Mueller who had worked with air in annuli of large diameter ratios. The data correlated when plotted as N[subscript Nu]/N[subscript Prf]¹[superscript /]³ versus N[subscript Re] when the equivalent diameter was based on experimentally determined radii of maximum velocity in turbulent annular flow. Over the range of operating conditions studied the heat transfer coefficients were considerably lower than predicted by the usual empirical equations. Close

• 8. [Article] The local natural convection heat transfer coefficient on a heated horizontal cylinder oscillating in water

  An experimental study has been made of the local natural convective heat transfer coefficient around the circumference of a heated horizontal cylinder oscillating vertically in water. The heat transfer surface ...

  Citation × Citation Citation Abstract Copy Title: The local natural convection heat transfer coefficient on a heated horizontal cylinder oscillating in water Author: Martin, Timothy William An experimental study has been made of the local natural convective heat transfer coefficient around the circumference of a heated horizontal cylinder oscillating vertically in water. The heat transfer surface consisted of a 1 3/8-inch diameter cylinder with a small test section imbedded in its surface. This enabled data to be taken so that the local and overall values of the heat; transfer coefficient could be determined. The cylinder was oscillated sinusoidally in a tank of distilled water at a frequency of 0 to 25-cps with an amplitude of 0 to 0.100-inch. The temperature difference between the water bath and the test cylinder was held at approximately twenty degrees. Observations of the flow patterns around the cylinder were made using a shadowgraph technique and a dye stream visualization, The local heat transfer coefficient versus position data were taken at six different conditions of frequency and amplitude. These conditions were: (1) stationary, (2) n = 500 rpm, a = 0.100-inch, (3) n = 750 rpm, a = 0.0667-inch, (4) n = 1000 rpm, a = 0.100-inch, (5) n = 1500 rpm, a = 0.0667-inch, and (6) n = 1500 rpm, a = 0.100-inch. The overall cylinder results were similar to the results found by V. H. Swanson and by Martinelli and Boelter in similar work. The maximum increase in the overall cylinder heat transfer rate was of the order of 200 percent. The data for the local heat transfer coefficient showed that the maximum increase in the heat transfer coefficient occurred at the top of the cylinder and was on the order of 290 percent. At the same condition of oscillation the coefficient at the side increased 230 percent while the coefficient at the bottom increased 72 percent. In comparing the shapes of the distributions of local Nusselts number with the shapes Fand, Roos, Cheng, and Kaye found by imposing a sound field on a air-cylinder system, a difference was noted which can be attributed to the difference in the direction of oscillation between the two investigations. In the present investigation the cylinder was oscillated vertically while Fand, Roos, Cheng, and Kaye used a horizontal oscillation of the fluid particles. The resulting differences in the acoustic streaming pattern account for the differences noted in the shapes of the local heat transfer coefficient versus position curves. The shapes did show that the effect of mechanical oscillation and the effect of a sound field on the convective heat transfer rate were similar. A dye stream visualization of the flow pattern indicated Fand, Roos, Cheng, and Kaye were correct when they concluded that the shape of the distribution of Nusselt number was caused by the interaction of a natural convection flow pattern and acoustic streaming. This study sheds some light on the mechanism causing the increase in the natural convection heat transfer coefficient when oscillation is introduced, and it shows the need for more experimental investigation into the distribution of the local heat transfer coefficient around cylinders. Title: The local natural convection heat transfer coefficient on a heated horizontal cylinder oscillating in water Author: Martin, Timothy William An experimental study has been made of the local natural convective heat transfer coefficient around the circumference of a heated horizontal cylinder oscillating vertically in water. The heat transfer surface consisted of a 1 3/8-inch diameter cylinder with a small test section imbedded in its surface. This enabled data to be taken so that the local and overall values of the heat; transfer coefficient could be determined. The cylinder was oscillated sinusoidally in a tank of distilled water at a frequency of 0 to 25-cps with an amplitude of 0 to 0.100-inch. The temperature difference between the water bath and the test cylinder was held at approximately twenty degrees. Observations of the flow patterns around the cylinder were made using a shadowgraph technique and a dye stream visualization, The local heat transfer coefficient versus position data were taken at six different conditions of frequency and amplitude. These conditions were: (1) stationary, (2) n = 500 rpm, a = 0.100-inch, (3) n = 750 rpm, a = 0.0667-inch, (4) n = 1000 rpm, a = 0.100-inch, (5) n = 1500 rpm, a = 0.0667-inch, and (6) n = 1500 rpm, a = 0.100-inch. The overall cylinder results were similar to the results found by V. H. Swanson and by Martinelli and Boelter in similar work. The maximum increase in the overall cylinder heat transfer rate was of the order of 200 percent. The data for the local heat transfer coefficient showed that the maximum increase in the heat transfer coefficient occurred at the top of the cylinder and was on the order of 290 percent. At the same condition of oscillation the coefficient at the side increased 230 percent while the coefficient at the bottom increased 72 percent. In comparing the shapes of the distributions of local Nusselts number with the shapes Fand, Roos, Cheng, and Kaye found by imposing a sound field on a air-cylinder system, a difference was noted which can be attributed to the difference in the direction of oscillation between the two investigations. In the present investigation the cylinder was oscillated vertically while Fand, Roos, Cheng, and Kaye used a horizontal oscillation of the fluid particles. The resulting differences in the acoustic streaming pattern account for the differences noted in the shapes of the local heat transfer coefficient versus position curves. The shapes did show that the effect of mechanical oscillation and the effect of a sound field on the convective heat transfer rate were similar. A dye stream visualization of the flow pattern indicated Fand, Roos, Cheng, and Kaye were correct when they concluded that the shape of the distribution of Nusselt number was caused by the interaction of a natural convection flow pattern and acoustic streaming. This study sheds some light on the mechanism causing the increase in the natural convection heat transfer coefficient when oscillation is introduced, and it shows the need for more experimental investigation into the distribution of the local heat transfer coefficient around cylinders. Close

• 9. [Article] Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics

  The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an ...

  Citation × Citation Citation Abstract Copy Title: Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics Author: Wang, Zhefu The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an experimental technique based on the thermal wave approach is developed. A stainless steel strip functions as both a heating element and a sealing cover for a chamber containing a test liquid. A periodic current passing through this metal strip generates a periodic Joule heating source. An infrared detector measures the temperature response at the front surface of the stainless steel strip. The phase and magnitude of the temperature response were measured by a lock-in amplifier at various frequencies. A one-dimensional, two-layered transient heat conduction model is developed to predict the temperature response. The phase information of this temperature response shows high sensitivity to the change of thermal properties of the liquid layer and is employed to match experimental data to find the thermal properties of the test liquid. The measured thermal conductivities of water and ethylene glycol agree quite well with data from the literature and support the validity of this measurement technique. An aqueous fluid consisting of gold nanoparticles was also tested and anomalous thermal conductivity enhancement was observed. Our measurement results also showed a divergence of thermal transport behavior between nanofluids and pure liquids. This suggests a need to carefully examine the role of the measurement technique in the heat transfer experimental studies of nanofluids. For the study of flow property diagnostics, a heat transfer system with a periodic boundary condition in a steady flow field is examined. Due to linearity and homogeneity of the heat transfer system under certain conditions, a thermal wave field generated by a periodic heating flux at the boundary of the flow field may be used to detect important flow field parameters, such as the velocity gradient at the wall, and therefore, the wall shear stress. A wall shear stress sensor design with a heater and two temperature sensors on a silicon dioxide substrate is analyzed. The heater with a periodic heating source generates an oscillating temperature field which interacts with the flow field. The temperature sensors pick up the temperature response that contains information on the velocity gradient at the wall. Based on the above sensor design, a two-dimensional conjugate heat convection model is developed with a periodic heating flux at the solid/fluid interface and a linear velocity profile in the fluid domain. Two designs are studied, one with a silicon heat sink under the silicon dioxide substrate and another without the heat sink. The effects of the two main design parameters, the operating frequency and the distance between the heater and the temperature sensor, are discussed. A reasonable sensitivity of the phase information to the velocity gradient suggests a practical sensor design. A preliminary experimental test on a water channel flow has been conducted to support the concept of applying the thermal wave method to wall shear stress measurements. Title: Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics Author: Wang, Zhefu The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an experimental technique based on the thermal wave approach is developed. A stainless steel strip functions as both a heating element and a sealing cover for a chamber containing a test liquid. A periodic current passing through this metal strip generates a periodic Joule heating source. An infrared detector measures the temperature response at the front surface of the stainless steel strip. The phase and magnitude of the temperature response were measured by a lock-in amplifier at various frequencies. A one-dimensional, two-layered transient heat conduction model is developed to predict the temperature response. The phase information of this temperature response shows high sensitivity to the change of thermal properties of the liquid layer and is employed to match experimental data to find the thermal properties of the test liquid. The measured thermal conductivities of water and ethylene glycol agree quite well with data from the literature and support the validity of this measurement technique. An aqueous fluid consisting of gold nanoparticles was also tested and anomalous thermal conductivity enhancement was observed. Our measurement results also showed a divergence of thermal transport behavior between nanofluids and pure liquids. This suggests a need to carefully examine the role of the measurement technique in the heat transfer experimental studies of nanofluids. For the study of flow property diagnostics, a heat transfer system with a periodic boundary condition in a steady flow field is examined. Due to linearity and homogeneity of the heat transfer system under certain conditions, a thermal wave field generated by a periodic heating flux at the boundary of the flow field may be used to detect important flow field parameters, such as the velocity gradient at the wall, and therefore, the wall shear stress. A wall shear stress sensor design with a heater and two temperature sensors on a silicon dioxide substrate is analyzed. The heater with a periodic heating source generates an oscillating temperature field which interacts with the flow field. The temperature sensors pick up the temperature response that contains information on the velocity gradient at the wall. Based on the above sensor design, a two-dimensional conjugate heat convection model is developed with a periodic heating flux at the solid/fluid interface and a linear velocity profile in the fluid domain. Two designs are studied, one with a silicon heat sink under the silicon dioxide substrate and another without the heat sink. The effects of the two main design parameters, the operating frequency and the distance between the heater and the temperature sensor, are discussed. A reasonable sensitivity of the phase information to the velocity gradient suggests a practical sensor design. A preliminary experimental test on a water channel flow has been conducted to support the concept of applying the thermal wave method to wall shear stress measurements. Close