MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with num... more MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing based on direct metal laser sintering technology is capable of fabricating micro-scale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multi-row microchannels made by additive manufacturing was conducted by a scanning electron microscope at different locations along the printing direction. Channel size reduction, surface roughness and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in CAD model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.
Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooli... more Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooling jets by the counter-rotating vortices within the jets. Computations, based on the ensemble-averaged Navier-Stokes equations closed by the realizable k-ε turbulence model, were used to assess the usefulness of rectangular prisms as blockers in increasing film-cooling adiabatic effectiveness without unduly increasing surface heat transfer and pressure loss. The Taguchi’s design of experiment method was used to investigate the effects of the height of the blocker (0.2D, 0.4D, 0.8D), the thickness of the blocker (D/20, D/10, D/5), and the spacing between the pair of blockers (0.8D, 1.0D, 1.2D), where D is the diameter of the film-cooling hole. The effects of blowing ratio (0.37, 0.5, 0.65) were also studied. Results obtained show that blockers can greatly increase film-cooling effectiveness. By using rectangular prisms as blockers, the laterally averaged adiabatic effectiveness at 15D downstream of the film-cooling hole is as high as that at 1D downstream. The surface heat transfer was found to increase slightly near the leading edge of the prisms, but reduced elsewhere from reduced temperature gradients that resulted from reduced hot gas entrainment. However, pressure loss was found to increase somewhat because of the flat rectangular leading edge, which can be made more streamlined.
Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε... more Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.
Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooli... more Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooling jets by the counter-rotating vortices within the jets. Computations, based on the ensemble-averaged Navier-Stokes equations closed by the realizable k-ε turbulence model, were used to assess the usefulness of rectangular prisms as blockers in increasing film-cooling adiabatic effectiveness without unduly increasing surface heat transfer and pressure loss. The Taguchi’s design of experiment method was used to investigate the effects of the height of the blocker (0.2D, 0.4D, 0.8D), the thickness of the blocker (D/20, D/10, D/5), and the spacing between the pair of blockers (0.8D, 1.0D, 1.2D), where D is the diameter of the film-cooling hole. The effects of blowing ratio (0.37, 0.5, 0.65) were also studied. Results obtained show that blockers can greatly increase film-cooling effectiveness. By using rectangular prisms as blockers, the laterally averaged adiabatic effectiveness at 15D downstream of the film-cooling hole is as high as that at 1D downstream. The surface heat transfer was found to increase slightly near the leading edge of the prisms, but reduced elsewhere from reduced temperature gradients that resulted from reduced hot gas entrainment. However, pressure loss was found to increase somewhat because of the flat rectangular leading edge, which can be made more streamlined.
This report describes the detailed experimental study to characterize the local heat transfer coe... more This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.
Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε... more Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.
ABSTRACT CFD simulations were performed to study the flow and heat transfer in a rectangular duct... more ABSTRACT CFD simulations were performed to study the flow and heat transfer in a rectangular duct (Wd × Hd , where Wd /Hd = 3) with a staggered array of circular pin fins (D = Hd /4) mounted on the two opposite walls separated by Hd . For this array of pin fins, five different pin-fin height (H) combinations were examined, and they are (1) H = Hd = 4D (i.e., all pin fins extended from wall to wall), (2) H = 3D on both walls, (3) H = 2D on both walls, (4) H = 4D on one wall and H = 2D on the opposite wall, and (5) H = 3D on one wall and H = 2D on the opposite wall. The H values studied give H/D values of 2, 3, and 4 and C/D values of 2, 1, and 0, where C is the distance between the pin-fin tip and the opposite wall. For all cases, the duct wall and pin-fin surface temperatures were maintained at Tw = 313.15 K; the temperature and the speed of the air at the duct inlet were uniform at Tinlet = 343.15 K and U = 8.24 m/s; the pressure at the duct exit was fixed at Pb = 1 atm; and the Reynolds number based on the duct hydraulic diameter and duct inlet conditions was Re = 15,000. This CFD study is based on 3-D steady RANS, where the ensemble averaged continuity, compressible Navier-Stokes, and energy equations are closed by the thermally perfect equation of state and the two-equation realizable k-ε turbulence model with wall functions and with the low-Reynolds number model of Chen and Patel in the near-wall region. The usefulness of this CFD study was assessed by comparing predicted heat-transfer coefficient and friction factor with available experimental data. Results are presented to show how the flow induced by arrays of pin fins of different heights affects temperature distribution, surface heat transfer, and pressure loss.
This report describes the detailed experimental study to characterize the local heat transfer coe... more This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.
MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with num... more MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing based on direct metal laser sintering technology is capable of fabricating micro-scale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multi-row microchannels made by additive manufacturing was conducted by a scanning electron microscope at different locations along the printing direction. Channel size reduction, surface roughness and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in CAD model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.
Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooli... more Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooling jets by the counter-rotating vortices within the jets. Computations, based on the ensemble-averaged Navier-Stokes equations closed by the realizable k-ε turbulence model, were used to assess the usefulness of rectangular prisms as blockers in increasing film-cooling adiabatic effectiveness without unduly increasing surface heat transfer and pressure loss. The Taguchi’s design of experiment method was used to investigate the effects of the height of the blocker (0.2D, 0.4D, 0.8D), the thickness of the blocker (D/20, D/10, D/5), and the spacing between the pair of blockers (0.8D, 1.0D, 1.2D), where D is the diameter of the film-cooling hole. The effects of blowing ratio (0.37, 0.5, 0.65) were also studied. Results obtained show that blockers can greatly increase film-cooling effectiveness. By using rectangular prisms as blockers, the laterally averaged adiabatic effectiveness at 15D downstream of the film-cooling hole is as high as that at 1D downstream. The surface heat transfer was found to increase slightly near the leading edge of the prisms, but reduced elsewhere from reduced temperature gradients that resulted from reduced hot gas entrainment. However, pressure loss was found to increase somewhat because of the flat rectangular leading edge, which can be made more streamlined.
Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε... more Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.
Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooli... more Flow aligned blockers are proposed to minimize the entrainment of hot gases underneath film-cooling jets by the counter-rotating vortices within the jets. Computations, based on the ensemble-averaged Navier-Stokes equations closed by the realizable k-ε turbulence model, were used to assess the usefulness of rectangular prisms as blockers in increasing film-cooling adiabatic effectiveness without unduly increasing surface heat transfer and pressure loss. The Taguchi’s design of experiment method was used to investigate the effects of the height of the blocker (0.2D, 0.4D, 0.8D), the thickness of the blocker (D/20, D/10, D/5), and the spacing between the pair of blockers (0.8D, 1.0D, 1.2D), where D is the diameter of the film-cooling hole. The effects of blowing ratio (0.37, 0.5, 0.65) were also studied. Results obtained show that blockers can greatly increase film-cooling effectiveness. By using rectangular prisms as blockers, the laterally averaged adiabatic effectiveness at 15D downstream of the film-cooling hole is as high as that at 1D downstream. The surface heat transfer was found to increase slightly near the leading edge of the prisms, but reduced elsewhere from reduced temperature gradients that resulted from reduced hot gas entrainment. However, pressure loss was found to increase somewhat because of the flat rectangular leading edge, which can be made more streamlined.
This report describes the detailed experimental study to characterize the local heat transfer coe... more This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.
Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε... more Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.
ABSTRACT CFD simulations were performed to study the flow and heat transfer in a rectangular duct... more ABSTRACT CFD simulations were performed to study the flow and heat transfer in a rectangular duct (Wd × Hd , where Wd /Hd = 3) with a staggered array of circular pin fins (D = Hd /4) mounted on the two opposite walls separated by Hd . For this array of pin fins, five different pin-fin height (H) combinations were examined, and they are (1) H = Hd = 4D (i.e., all pin fins extended from wall to wall), (2) H = 3D on both walls, (3) H = 2D on both walls, (4) H = 4D on one wall and H = 2D on the opposite wall, and (5) H = 3D on one wall and H = 2D on the opposite wall. The H values studied give H/D values of 2, 3, and 4 and C/D values of 2, 1, and 0, where C is the distance between the pin-fin tip and the opposite wall. For all cases, the duct wall and pin-fin surface temperatures were maintained at Tw = 313.15 K; the temperature and the speed of the air at the duct inlet were uniform at Tinlet = 343.15 K and U = 8.24 m/s; the pressure at the duct exit was fixed at Pb = 1 atm; and the Reynolds number based on the duct hydraulic diameter and duct inlet conditions was Re = 15,000. This CFD study is based on 3-D steady RANS, where the ensemble averaged continuity, compressible Navier-Stokes, and energy equations are closed by the thermally perfect equation of state and the two-equation realizable k-ε turbulence model with wall functions and with the low-Reynolds number model of Chen and Patel in the near-wall region. The usefulness of this CFD study was assessed by comparing predicted heat-transfer coefficient and friction factor with available experimental data. Results are presented to show how the flow induced by arrays of pin fins of different heights affects temperature distribution, surface heat transfer, and pressure loss.
This report describes the detailed experimental study to characterize the local heat transfer coe... more This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.
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