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• 41. [Article] Radiation Emissions from Turbulent Diffusion Flames Burning Large Hydrocarbon Fuels

  The purpose of this study is to measure and compare radiation emissions from flames burning large hydrocarbon fuels to assist in understanding how the molecular composition changes radiation emissions. ...

  Citation × Citation Citation Abstract Copy Title: Radiation Emissions from Turbulent Diffusion Flames Burning Large Hydrocarbon Fuels Author: Zeuthen, Eric D. The purpose of this study is to measure and compare radiation emissions from flames burning large hydrocarbon fuels to assist in understanding how the molecular composition changes radiation emissions. Radiative heat transfer is significant because it is a primary mode of heat transfer for many combustion devices. This study also provides quantitative data that can be available for validation of numerical efforts. Eleven liquid fuels were burned ranging from traditional and alternative jet fuels (e.g. Jet-A) to single and multi-component large hydrocarbons. The latter is used to investigate the role of hydrocarbon classes in radiation emissions and visible soot production. The flames were stabilized using a piloted turbulent diffusion burner. The Reynolds numbers ranged from 7,500 - 45,000 for the various flames. A premixed pilot flame burning ethylene and air was used to stabilize the central jet of vaporized fuel. The vaporized fuel exited the burner at 300º C. The radiative heat flux and the radiation intensity were measured using a radiometer and a mid-infrared camera (FLIR, SC6700), respectively. The radiant fraction (χR), or the ratio of the global radiation emitted by the flames to the chemical energy released, is reported for the different fuels. For all fuels measured, χR peaks near a Reynolds number equal to 20,000. The largest radiant fractions (e.g., 0.36) were observed for fuels with relatively large aromatic content (13-31%). A maximum 20% decrease in χR was observed for fuels without aromatic content, with the exception of one single-component alkane based test fuel being 25% lower than aromatic containing fuels. This is significant because aromatic content increases soot production in flames. These findings suggest that radiative heat transfer in these flames is dominated by emissions from molecular species (i.e. carbon dioxide and water vapor) and not from soot. These findings were verified by the radiation intensity measurements. Measurements from a subset of fuels (with and without aromatic content) revealed peak emissions from CO₂ and H₂O varied less than 10% between flames while soot increased nearly 60% for aromatic containing fuels. Radiation intensity measurements further confirmed that CO₂ and H₂O emissions were evenly distributed throughout the flame while soot concentrations had peak emissions in the core of the flame. In this region, soot volume fraction and temperature are the greatest. Peak fluctuating emissions also followed this pattern, with CO₂ and H₂O emissions located close to the flame’s boundary and soot along the central flame axis. Lack of entrained oxidizer within the core of the flame enhances soot production, which is a greater issue in fuels containing aromatics. This has implications on the use of these fuels in gas turbine engines (GTE) both due to considerations of pollution and the lifespan of engines. Title: Radiation Emissions from Turbulent Diffusion Flames Burning Large Hydrocarbon Fuels Author: Zeuthen, Eric D. The purpose of this study is to measure and compare radiation emissions from flames burning large hydrocarbon fuels to assist in understanding how the molecular composition changes radiation emissions. Radiative heat transfer is significant because it is a primary mode of heat transfer for many combustion devices. This study also provides quantitative data that can be available for validation of numerical efforts. Eleven liquid fuels were burned ranging from traditional and alternative jet fuels (e.g. Jet-A) to single and multi-component large hydrocarbons. The latter is used to investigate the role of hydrocarbon classes in radiation emissions and visible soot production. The flames were stabilized using a piloted turbulent diffusion burner. The Reynolds numbers ranged from 7,500 - 45,000 for the various flames. A premixed pilot flame burning ethylene and air was used to stabilize the central jet of vaporized fuel. The vaporized fuel exited the burner at 300º C. The radiative heat flux and the radiation intensity were measured using a radiometer and a mid-infrared camera (FLIR, SC6700), respectively. The radiant fraction (χR), or the ratio of the global radiation emitted by the flames to the chemical energy released, is reported for the different fuels. For all fuels measured, χR peaks near a Reynolds number equal to 20,000. The largest radiant fractions (e.g., 0.36) were observed for fuels with relatively large aromatic content (13-31%). A maximum 20% decrease in χR was observed for fuels without aromatic content, with the exception of one single-component alkane based test fuel being 25% lower than aromatic containing fuels. This is significant because aromatic content increases soot production in flames. These findings suggest that radiative heat transfer in these flames is dominated by emissions from molecular species (i.e. carbon dioxide and water vapor) and not from soot. These findings were verified by the radiation intensity measurements. Measurements from a subset of fuels (with and without aromatic content) revealed peak emissions from CO₂ and H₂O varied less than 10% between flames while soot increased nearly 60% for aromatic containing fuels. Radiation intensity measurements further confirmed that CO₂ and H₂O emissions were evenly distributed throughout the flame while soot concentrations had peak emissions in the core of the flame. In this region, soot volume fraction and temperature are the greatest. Peak fluctuating emissions also followed this pattern, with CO₂ and H₂O emissions located close to the flame’s boundary and soot along the central flame axis. Lack of entrained oxidizer within the core of the flame enhances soot production, which is a greater issue in fuels containing aromatics. This has implications on the use of these fuels in gas turbine engines (GTE) both due to considerations of pollution and the lifespan of engines. Close

• 42. [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

• 43. [Article] Effect of vibration on forced convection to water from a cylinder at Reynolds numbers in the range of stable vortex shedding

  An experimental investigation has been made of the effect of vibration on heat transfer from a cylinder in crossflow. The cylinder used was 3/16-inch in diameter and was sinusoidally oscillated in the ...

  Citation × Citation Citation Abstract Copy Title: Effect of vibration on forced convection to water from a cylinder at Reynolds numbers in the range of stable vortex shedding Author: Graves, Donald Larry An experimental investigation has been made of the effect of vibration on heat transfer from a cylinder in crossflow. The cylinder used was 3/16-inch in diameter and was sinusoidally oscillated in the vertical direction by a mechanical drive mechanism while immersed in an open water channel. Nominal amplitude ratios, a/d, varied from 0.125 to 1.43 while frequency varied from 0 to 6.5 cycles per second. Data was taken at three flow Reynolds numbers: 64; 103 and 144. Temperature difference between the cylinder and the water was held at approximately 10°F. A hydrogen bubble technique was used in conjunction with dye studies to observe the flow near the cylinder. It was found that the heat transfer could be increased or decreased by vibration, depending on the amplitude ratio and the vibrational Reynolds number, (N[subscript Re])[subscript v] = awρ d/√2μ. Below a critical vibrational Reynolds number, which increased with the flow Reynolds number, oscillation had no effect on the heat transfer. After this critical point was passed, the data for the amplitude ratios 0.125, 0.250, and 0.500 showed a decrease in the heat transfer while the data for higher amplitude ratios (a/d = 1.00; 1.43) gave a steady increase. After reaching a minimum, the data for a/d = 0.500 increased to converge with the data of higher amplitudes; this trend was also suggested by the lower amplitude ratios but was not completely substantiated because of the frequency limitation of the drive mechanism. The maximum decrease in the heat transfer below stationary conditions was about 20 percent; the maximum increase in the heat transfer was 180, 125, and 90 percent for flow Reynolds numbers of 64; 103 and 144 respectively. The vortex shedding frequencies were in the range of frequency used for mechanical oscillation; noting this, an attempt was made to oscillate the cylinder through the immediate frequency range of vortex shedding to see if any special heat transfer effects would result. No change occurred other than the general increase due to increasing the vibrational intensity. The increases in heat transfer are attributed to vibrationally induced turbulence. An adequate explanation of the decreases in heat transfer was not provided by this investigation; however, it is thought that the phenomenon occurring may be similar to the observations of Kubanskii (11) who imposed acoustical vibrations on a cylinder in crossflow. By appropriately locating the cylinder with respect to the nodes of the sound field he found that the point of flow separation shifted toward the upstream side of the cylinder resulting in lower heat transfer rates. A critical (N[subscript Re])[subscript v]/(N[subscript Re])[subscript f] required to alter the heat transfer was established for this investigation and although it had a weak dependence on the amplitude ratio, a value of 0.35 serves as a good approximation for the data of this experiment. Title: Effect of vibration on forced convection to water from a cylinder at Reynolds numbers in the range of stable vortex shedding Author: Graves, Donald Larry An experimental investigation has been made of the effect of vibration on heat transfer from a cylinder in crossflow. The cylinder used was 3/16-inch in diameter and was sinusoidally oscillated in the vertical direction by a mechanical drive mechanism while immersed in an open water channel. Nominal amplitude ratios, a/d, varied from 0.125 to 1.43 while frequency varied from 0 to 6.5 cycles per second. Data was taken at three flow Reynolds numbers: 64; 103 and 144. Temperature difference between the cylinder and the water was held at approximately 10°F. A hydrogen bubble technique was used in conjunction with dye studies to observe the flow near the cylinder. It was found that the heat transfer could be increased or decreased by vibration, depending on the amplitude ratio and the vibrational Reynolds number, (N[subscript Re])[subscript v] = awρ d/√2μ. Below a critical vibrational Reynolds number, which increased with the flow Reynolds number, oscillation had no effect on the heat transfer. After this critical point was passed, the data for the amplitude ratios 0.125, 0.250, and 0.500 showed a decrease in the heat transfer while the data for higher amplitude ratios (a/d = 1.00; 1.43) gave a steady increase. After reaching a minimum, the data for a/d = 0.500 increased to converge with the data of higher amplitudes; this trend was also suggested by the lower amplitude ratios but was not completely substantiated because of the frequency limitation of the drive mechanism. The maximum decrease in the heat transfer below stationary conditions was about 20 percent; the maximum increase in the heat transfer was 180, 125, and 90 percent for flow Reynolds numbers of 64; 103 and 144 respectively. The vortex shedding frequencies were in the range of frequency used for mechanical oscillation; noting this, an attempt was made to oscillate the cylinder through the immediate frequency range of vortex shedding to see if any special heat transfer effects would result. No change occurred other than the general increase due to increasing the vibrational intensity. The increases in heat transfer are attributed to vibrationally induced turbulence. An adequate explanation of the decreases in heat transfer was not provided by this investigation; however, it is thought that the phenomenon occurring may be similar to the observations of Kubanskii (11) who imposed acoustical vibrations on a cylinder in crossflow. By appropriately locating the cylinder with respect to the nodes of the sound field he found that the point of flow separation shifted toward the upstream side of the cylinder resulting in lower heat transfer rates. A critical (N[subscript Re])[subscript v]/(N[subscript Re])[subscript f] required to alter the heat transfer was established for this investigation and although it had a weak dependence on the amplitude ratio, a value of 0.35 serves as a good approximation for the data of this experiment. Close

• 44. [Article] Analysis of heat transfer, including axial conduction, for laminar tube flow with arbitrary circumferential wall heat flux

  This thesis is concerned with the analysis of heat transfer in a tube with forced flow under conditions of an arbitrary variation of wall heat flux both axially and circumferentially. This total study ...

  Citation × Citation Citation Abstract Copy Title: Analysis of heat transfer, including axial conduction, for laminar tube flow with arbitrary circumferential wall heat flux Author: Faghri, Mohammad This thesis is concerned with the analysis of heat transfer in a tube with forced flow under conditions of an arbitrary variation of wall heat flux both axially and circumferentially. This total study is separated into two distinct problems which are presented separately. The first is the case of a Newtonian fluid in laminar flow with allowance made for the inclusion of axial heat conduction, viscous heat dissipation and heat generation. Secondly, the problem of laminar flow of a non-Newtonian fluid is considered. Axial conduction is not included in this problem since it is likely negligible in those cases where non-Newtonian effects are significant. Heretofore, no general method has been available for obtaining solutions to these problems. Analytical results are given in such generality and completeness that many of the previously reported work in the heat transfer literature in laminar tube flow are limiting cases of the present work. In the first problem, the solution is expanded in a power series form that accounts for any arbitrary variation of wall heat flux around the circumference that can be expressed in terms of a Fourier series expansion. Substitution of this series into the energy equation leads to an eigenvalue problem. The first 12 eigenvalues and eigenfunctions have been obtained numerically. The resulting eigenfunctions are not orthogonal and therefore the power series expansion coefficients cannot be obtained by the usual analytical schemes. A least squares method was used to determine these coefficients. For the limiting problem of uniform wall heat flux around the circumference with the inclusion of axial conduction, the eigenfunctions and eigenvalues are in excellent agreement with previously reported work; however, two additional considerations were made to correct errors made in the heat transfer literature. The first was the determination of coefficients of the non-orthogonal power series expansion and, second was the inclusion of the nonvanishing axial conduction term at the tube entrance which was not included in earlier asymptotic expressions for the temperature. Both of these considerations are included in the numerical procedures in this work. The problem where wall heat flux varies circumferentially but axial fluid conduction is neglected is another limiting case of the present work. For the special case of uniform wall heat flux, the eigenfunctions, eigenvalues, and expansion coefficients agree well with those in the existing literature. The same analytical techniques were employed for the second problem. The resulting eigenfunctions for this problem are orthogonal, therefore the power series expansion coefficients were determined by utilizing the orthogonality property of the eigenfunctions. For the special case of power-law pseudo-plastic fluids with uniform wall heat flux the eigenfunctions, eigenvalues, and the expansion coefficients are in excellent agreement with previously reported values. Finally, by an illustrative example, it was concluded that circumferential wall heat flux variation has a pronounced effect in both Newtonian and non-Newtonian heat transfer results. Title: Analysis of heat transfer, including axial conduction, for laminar tube flow with arbitrary circumferential wall heat flux Author: Faghri, Mohammad This thesis is concerned with the analysis of heat transfer in a tube with forced flow under conditions of an arbitrary variation of wall heat flux both axially and circumferentially. This total study is separated into two distinct problems which are presented separately. The first is the case of a Newtonian fluid in laminar flow with allowance made for the inclusion of axial heat conduction, viscous heat dissipation and heat generation. Secondly, the problem of laminar flow of a non-Newtonian fluid is considered. Axial conduction is not included in this problem since it is likely negligible in those cases where non-Newtonian effects are significant. Heretofore, no general method has been available for obtaining solutions to these problems. Analytical results are given in such generality and completeness that many of the previously reported work in the heat transfer literature in laminar tube flow are limiting cases of the present work. In the first problem, the solution is expanded in a power series form that accounts for any arbitrary variation of wall heat flux around the circumference that can be expressed in terms of a Fourier series expansion. Substitution of this series into the energy equation leads to an eigenvalue problem. The first 12 eigenvalues and eigenfunctions have been obtained numerically. The resulting eigenfunctions are not orthogonal and therefore the power series expansion coefficients cannot be obtained by the usual analytical schemes. A least squares method was used to determine these coefficients. For the limiting problem of uniform wall heat flux around the circumference with the inclusion of axial conduction, the eigenfunctions and eigenvalues are in excellent agreement with previously reported work; however, two additional considerations were made to correct errors made in the heat transfer literature. The first was the determination of coefficients of the non-orthogonal power series expansion and, second was the inclusion of the nonvanishing axial conduction term at the tube entrance which was not included in earlier asymptotic expressions for the temperature. Both of these considerations are included in the numerical procedures in this work. The problem where wall heat flux varies circumferentially but axial fluid conduction is neglected is another limiting case of the present work. For the special case of uniform wall heat flux, the eigenfunctions, eigenvalues, and expansion coefficients agree well with those in the existing literature. The same analytical techniques were employed for the second problem. The resulting eigenfunctions for this problem are orthogonal, therefore the power series expansion coefficients were determined by utilizing the orthogonality property of the eigenfunctions. For the special case of power-law pseudo-plastic fluids with uniform wall heat flux the eigenfunctions, eigenvalues, and the expansion coefficients are in excellent agreement with previously reported values. Finally, by an illustrative example, it was concluded that circumferential wall heat flux variation has a pronounced effect in both Newtonian and non-Newtonian heat transfer results. Close

• 45. [Article] Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks

  The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management ...

  Citation × Citation Citation Abstract Copy Title: Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks Author: Rasouli, Erfan The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management techniques. Thermal management using heat sinks with microscale features is amongst the prominent techniques developed over the past two decades. In this dissertation, single and phase change heat transfer and pressure drop through one such heat sink, namely microscale pin fin heat sinks (µPFHS), is examined experimentally. In particular, effects of pitch-to-diameter and aspect ratio variations are studied on the thermofluidic performance of studied µPFHSs. Single phase heat transfer and pressure drop of two distinct fluids, liquid nitrogen and Performance Fluid (PF5060) are characterized experimentally through the µPFHSs with staggered diamond shape pin fins. The LN₂ and PF5060 experiments' Reynolds number (Re_Dh, based on pin fin hydraulic diameter) is in range of 108-570 and 8-462, respectively. Results are presented in a non-dimensional form in terms of the friction factor (f), Nusselt (Nu), and Reynolds numbers and are compared with the predictions of existing correlations in the literature for micro pin fin heat sinks. Heat sinks with the higher pitch ratio (coarser array) not only show lower pressure drops at a fixed Re_Dh, but also enhance significantly heat transfer rate when compared against the heat sink of the same pin fin size but denser arrangement. Flow visualization experiments using an infrared camera on PF5060 single phase tests are performed to understand the counter-intuitive trends seen in the global results. Flow through heat sinks with the same aspect ratio but larger pitch ratio exhibit unsteady vortex shedding in the wake region of pin fins, which markedly enhances convective heat transfer rate. Existing correlations developed for µPFHSs (such as that by Prasher et al. [1] and Koşar and Peles [2]) are capable of predicting the f and Nu data with good agreement only in the absence of vortex shedding, while the unsteady flow past the transition Re_Dh results in poor comparison of correlations with experimental data. A comparison of the experimental Nu data of PF5060 (Pr≅12.2) with the data of LN₂ (Pr≅1.9) shows significant change between the slopes of the curves of two fluids only in the heat sinks without vortex shedding. In the heat sinks with unsteady vortex shedding, the Nu_Dh curves show significantly decreased dependency on Pr number. Consequently, separate correlations are developed for predicting Nu in the case with and without unsteady vortex shedding using data from two distinct fluids and four PFHS geometries over a range of Re_Dh from 8 to 643. Given the clear heat transfer enhancement that occurs for certain pitch ratio designs of PFHSs in single phase flows, flow boiling experiments with PF5060 are performed to clarify whether additional changes to the pressure drop and two-phase heat transfer coefficient occur upon the introduction of the unsteady vortex shedding. Subcooled (∆T_sub=12.5℃) and saturated flow boiling of PF5060 through the micro pin fins are investigated. The heat sinks are tested at three constant mass fluxes of 30, 60, and 100 kg/m².s with heat fluxes ranging from 1.1 to 17.8 W⁄(cm² ) based on the planform area of the heat sinks. Flow regimes are studied with high speed imaging. Nucleate boiling heat transfer is the dominant mechanism for exit vapor qualities less than 0.5; at higher qualities annular film evaporation becomes dominant. The salient effect of unsteady vortex shedding is in elimination of wall temperature overshoot. In nucleate boiling regime, the heat sinks with unsteady flow flapping show higher two-phase heat transfer coefficients. The predictions of existing correlations for h_tp in literature are not in good agreement with the experimental data (MAE>30%) and show a systematic deviation depending on the µPFHSs dimensions. Title: Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks Author: Rasouli, Erfan The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management techniques. Thermal management using heat sinks with microscale features is amongst the prominent techniques developed over the past two decades. In this dissertation, single and phase change heat transfer and pressure drop through one such heat sink, namely microscale pin fin heat sinks (µPFHS), is examined experimentally. In particular, effects of pitch-to-diameter and aspect ratio variations are studied on the thermofluidic performance of studied µPFHSs. Single phase heat transfer and pressure drop of two distinct fluids, liquid nitrogen and Performance Fluid (PF5060) are characterized experimentally through the µPFHSs with staggered diamond shape pin fins. The LN₂ and PF5060 experiments' Reynolds number (Re_Dh, based on pin fin hydraulic diameter) is in range of 108-570 and 8-462, respectively. Results are presented in a non-dimensional form in terms of the friction factor (f), Nusselt (Nu), and Reynolds numbers and are compared with the predictions of existing correlations in the literature for micro pin fin heat sinks. Heat sinks with the higher pitch ratio (coarser array) not only show lower pressure drops at a fixed Re_Dh, but also enhance significantly heat transfer rate when compared against the heat sink of the same pin fin size but denser arrangement. Flow visualization experiments using an infrared camera on PF5060 single phase tests are performed to understand the counter-intuitive trends seen in the global results. Flow through heat sinks with the same aspect ratio but larger pitch ratio exhibit unsteady vortex shedding in the wake region of pin fins, which markedly enhances convective heat transfer rate. Existing correlations developed for µPFHSs (such as that by Prasher et al. [1] and Koşar and Peles [2]) are capable of predicting the f and Nu data with good agreement only in the absence of vortex shedding, while the unsteady flow past the transition Re_Dh results in poor comparison of correlations with experimental data. A comparison of the experimental Nu data of PF5060 (Pr≅12.2) with the data of LN₂ (Pr≅1.9) shows significant change between the slopes of the curves of two fluids only in the heat sinks without vortex shedding. In the heat sinks with unsteady vortex shedding, the Nu_Dh curves show significantly decreased dependency on Pr number. Consequently, separate correlations are developed for predicting Nu in the case with and without unsteady vortex shedding using data from two distinct fluids and four PFHS geometries over a range of Re_Dh from 8 to 643. Given the clear heat transfer enhancement that occurs for certain pitch ratio designs of PFHSs in single phase flows, flow boiling experiments with PF5060 are performed to clarify whether additional changes to the pressure drop and two-phase heat transfer coefficient occur upon the introduction of the unsteady vortex shedding. Subcooled (∆T_sub=12.5℃) and saturated flow boiling of PF5060 through the micro pin fins are investigated. The heat sinks are tested at three constant mass fluxes of 30, 60, and 100 kg/m².s with heat fluxes ranging from 1.1 to 17.8 W⁄(cm² ) based on the planform area of the heat sinks. Flow regimes are studied with high speed imaging. Nucleate boiling heat transfer is the dominant mechanism for exit vapor qualities less than 0.5; at higher qualities annular film evaporation becomes dominant. The salient effect of unsteady vortex shedding is in elimination of wall temperature overshoot. In nucleate boiling regime, the heat sinks with unsteady flow flapping show higher two-phase heat transfer coefficients. The predictions of existing correlations for h_tp in literature are not in good agreement with the experimental data (MAE>30%) and show a systematic deviation depending on the µPFHSs dimensions. Close