Browsing by Subject "Reynolds number"
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Item A numerical study of buoyant turbulent flows using low-Reynolds number k-e model(Texas Tech University, 2001-05) Seo, Eung RyeolNumerical computation has been performed to determine the influence of buoyant eflects on convective flows with the standard k-e and the low-Reynolds number k-s models. The present study was motivated by the need to overcome the shortcoming of the standard k-e model in separating and reattaching flows because of the wall-function approach employed in the model. The low-Reynolds number k-e model is considered to be an appropriate model for recirculating flows because the model does not employ wall functions. Results of the two different models are compared against the available experimental data and direct numerical simulation (DNS) data. In this work, Kolmogorov velocity, ut=(ve), is introduced instead of shear velocity, ut=Twlp, to avoid the singularity that appears at the separating and reattaching point for both thurbulence models. QUICK differencing scheme is employed for the convective terms. Eddy-diffusivity concept is used in modeling buoyant term. Momentum equation for the velocity field and the energy equation for the temperature field are solved altematively because of the strong coupling that exists between temperature and the velocity fields in a buoyant flow. Turbulent Prandtl number was allowed to vary in the low-Reynolds number k-e model to mimic the experimental data.Item Analysis of Compressible and Incompressible Flows Through See-through Labyrinth Seals(2011-08-08) Woo, Jeng WonThe labyrinth seal is a non-contact annular type sealing device used to reduce the internal leakage of the working fluid which is caused by the pressure difference between each stage in a turbomachine. Reducing the leakage mass flow rate of the working fluid through the labyrinth seal is desirable because it improves the efficiency of the turbomachine. The carry-over coefficient, based on the divergence angle of the jet, changed with flow parameters with fixed seal geometry while earlier models expressed the carry-over coefficient solely as a function of seal geometry. For both compressible and incompressible flows, the Reynolds number based on clearance was the only flow parameter which could influence the carry-over coefficient. In the case of incompressible flow based on the simulations for various seal geometries and operating conditions, for a given Reynolds number, the carry-over coefficient strongly depended on radial clearance to tooth width ratio. Moreover, in general, the lower the Reynolds number, the larger is the divergence angle of the jet and this results in a smaller carry-over coefficient at lower Reynolds numbers. However, during transition from laminar to turbulent, the carry-over coefficient reduced initially and once the Reynolds number attained a critical value, the carry-over coefficient increased again. In the case of compressible flow, the carry-over coefficient had been slightly increased if radial clearance to tooth width ratio and radial clearance to tooth pitch ratio were increased. Further, the carry-over coefficient did not considerably change if only radial clearance to tooth width ratio was decreased. The discharge coefficient for compressible and incompressible flows depended only on the Reynolds number based on clearance. The discharge coefficient of the tooth in a single cavity labyrinth seal was equivalent to that in a multiple tooth labyrinth seal indicating that flow downstream had negligible effect on the discharge coefficient. In particular, for compressible fluid under certain flow and seal geometric conditions, the discharge coefficient did not increase with an increase in the Reynolds number. It was correlated to the pressure ratio, Pr. Moreover, it was also related to the fact that the flow of the fluid through the constriction became compressible and the flow eventually became choked. At low pressure ratios (less than 0.7), Saikishan?s incompressible model deviated from CFD simulation results. Hence, the effects of compressibility became significant and both the carry-over coefficient compressibility factor and the discharge coefficient compressibility factor needed to be considered and included into the leakage model. The carry-over coefficient compressibility factor, phi, had two linear relationships with positive and negative slopes regarding the pressure ratios. This result was not associated with the seal geometry because the seal geometry ratios for each instance were located within the nearly same ranges. Further, the phi-Pr relationship was independent of the number of teeth regardless of single and multiple cavity labyrinth seals. The discharge coefficient compressibility factor, psi, was a linear relationship with pressure ratios across the tooth as Saikishan predicted. However, in certain flow and seal geometric conditions, Saikishan?s model needed to be modified for the deviation appearing when the pressure ratios were decreased. Hence, a modified psi-Pr relationship including Saikishan?s model was presented in order to compensate for the deviation between the simulations and his model.Item Composite expansions for active and inactive motions in the streamwise Reynolds stress of turbulent boundary layers(2008-05) McKee, Robert Joe, 1946-; Panton, Ronald L. (Ronald Lee), 1933-The proper scaling and prediction of the streamwise Reynolds stress in turbulent boundary layers has been a controversial issue for more than a decade as its Reynolds Number dependence can not be removed by normal scaling. One issue that may explain the unusual behavior of the streamwise Reynolds stress is that it is affected by both active and inactive motions per the Townsend hypothesis. The goal of this research is to develop a composite expansion for the streamwise Reynolds stress in turbulent boundary layers that considers active and inactive motions, explains various Reynolds Number dependencies, and agrees with available data. Data for the Reynolds shear stress and the streamwise Reynolds stress from six sources are evaluated and as appropriate plotted on inner and outer scales. A new asymptotic representation for the Reynolds shear stress, +, that meets the requirements for a proper composite expansion is developed and applied. This new Reynolds shear stress composite expansion agrees with data and allows predictions of + for any Reynolds Number. The streamwise Reynolds stress, +, can be separated into active and inactive parts and the Reynolds shear stress can be used to represent the active part. The inactive streamwise Reynolds stress, #, is separated from the complete + in part of this work. An outer correlation equation with the correct asymptotic limits for the inactive streamwise Reynolds stress is developed and shown to fit the outer part of the # data. A separate inner correlation equation for inner inactive streamwise Reynolds stress is developed and fit to data. Together these two equations form a composite expansion for the inactive streamwise Reynolds stress for flat plate boundary layers. This composite expansion for the inactive streamwise Reynolds stress can be combined with the Reynolds shear stress expansion to produce predictions for + that agree with data. Thus a composite expansion for predicting the streamwise Reynolds stress in turbulent boundary layers is developed and shown to reproduce the correct trends, to agree with the available data, and to explain the Reynolds Number dependence of the streamwise Reynolds stress.Item Effect of jet prandtl number on impingement cooling of rotating bodies(Texas Tech University, 1981-12) Saavedra, Jaime JoaquinThis study describes the effect of jet Prandtl number on the convective heat transfer from a rotating disk cooled by a single jet of impinging oil. Data were taken for different combinations of jet flowrates, jet temperatures, disk rotational speeds, disk temperatures, nozzle diameters and disk diameters. Three sizes of nozzle diameters were used, 0.079 in. (0.2 cm), 0.158 in. (0.4 cm) and 0.315 in. (0.8 cm). Three sizes of disk diameters were used, 3.937 in. (10.0 cm), 7.874 in. (20.0 cm) and 11.134 in. (28.28 cm). Nondimensional correlations of Nusselt number are given in terms of jet Reynolds number, Prandtl number and rotational Reynolds number. The parameter ranges that are covered in this study are:80 < jet Prandtl number < 400; 200 < jet Reynolds number < 1300; 25,000 < rotational Reynolds number < 500,000.Item Effect of Surface-to-jet Temperature Difference on Impingement Cooling of Rotating Bodies(Texas Tech University, 1982-05) Berning, Scott A.The effect of the surface-to-jet temperature difference on the convective heat transfer from rotating bodies cooled by jet impingement is investigated. The rotating bodies are circular disks, and impingement is provided by a single circular jet of oil impinging at the axis of rotation of the disk. The temperature difference between the disk surface and the oil jet is large enough to cause significant property variations in the oil flowing over the surface of the disk. The investigation is made over a range of jet flow rates, jet temperatures, and disk rotational speeds for various jet and disk diameters. A correlation of the data is presented that gives Nusselt number as a function of jet Reynolds number. Re., rotational Reynolds nijmber. Re , and jet Prandtl number, Pr. The ranges of these parameters are: jet Reynolds number, 220 < Re. < 1,300, rotational Reynolds number, 25,000 < Re < 400,000, and jet Prandtl number, 87 < Pr < 400. A comparison is made between the convective heat transfer behavior of this variable property situation with that of the constant property situation where the surface-to-jet temperature difference is small.Item Heat Transfer Enhancement in Rectangular Channel with Compound Cooling Techniques(2013-11-27) Krad, BelalVarious compound internal cooling techniques were investigated in this experiment to see which combinations can offer the greatest heat transfer. Combinations of rib turbulators as well as pin0fins were used in different configurations in order to analyze heat transfer and pressure loss characteristics to determine which configuration had the overall best performance. Two different flow configurations were considered, a uniform channel flow setup as well as a jet impingement setup. There were a total of sixteen cases performed for the experiment, eight for the channel flow and eight for the jet impingement. The types of cases that were performed were: a smooth surface case, two cases of only copper rib turbulators (P/e ratios of 5 and 10), two cases of only copper pin0fins (P/e ratios of 5 and 10), and three cases of a combinations of copper rib turbulators and pin0fins (P/e ratios of 2.5, 5, and 10). All of the cases were performed at four different Reynolds numbers to explore the effect of Reynolds number on the heat transfer. In terms of the channel flow experiment, the results indicate that the all ribs case with a P/e ratio of 5 had the highest heat transfer coefficients but also produced the highest friction factor. If the total area is considered and not just the projected area, than the case of all pins P/e ratio of 10 is the best candidate due to its extensively low pressure drop and moderate heat transfer. In terms of the jet impingement experiment, none of the cases significantly enhanced heat transfer and many of them had results lower than the smooth case. The case of all pins P/e ratio of 5 performed the best out of all the rough cases but the case of all pins P/e ratio of 10 perform the best when taking into account the total surface area. Cross0flow contributed to the jet impingement results, lowering the local Nusselt number due to the bending of the jet beams in the low x/d regions but started increasing the local Nusselt number at further x/d due to the cross flow heat transfer.Item The cumulative effects of roughness and Reynolds number on NACA 0015 airfoil section characteristics(Texas Tech University, 1984-05) Lewis, Kevin WayneIn this study, wind tunnel tests were made on a NACA 0015 airfoil section to examine the combination of scale and surface roughness effects. Reynolds numbers of 110,000 and 220,000 were used to determine scale effects. Roughness effects were obtained by applying roughness elements with heights of 0.111, 0.282, and 0.564 percent of the airfoil chord length. it was found that each combination of Reynolds number and roughness results in a unique set of lift and drag coefficients for a given angle of attack.Item Turbine Vane Film Cooling and Internal Rotating Coolant Passage Heat Transfer(2014-12-17) Yang, ShangfengThe first part of this dissertation experimentally studies the effect of transonic flow velocity on local film cooling effectiveness distribution of turbine vane suction side. Pressure Sensitive Paint (PSP), a conduction-free method is used to determine local film cooling effectiveness. Tests were performed in a five-vane annular cascade at TAMU Turbolab blow-down flow loop facility. The exit Mach numbers are controlled to be 0.7, 0.9, and 1.1, from subsonic to transonic flow conditions. Three foreign gases N2, CO2 and Argon/ SF6 mixture are selected to study the effects of three coolant-to-mainstream density ratios, 1.0, 1.5, and 2.0 on film cooling. Four averaged coolant blowing ratios in the range, 0.7, 1.0, 1.3 and 1.6 are investigated. The test vane features 3 rows of radial-angle cylindrical holes around the leading edge, and 2 rows of compound-angle shaped holes on the suction side. Results suggest that the PSP is a marvelous technique capable of producing clear and detailed film cooling effectiveness contours at transonic condition. The effects of coolant to mainstream blowing ratio, density ratio, and exit Mach number on the vane suction-surface film cooling distribution can be obtained, and the consequence results can be presented and explained in this research. The second part of this dissertation experimentally investigates the effect of rotation on heat transfer in typical turbine blade serpentine coolant passage with ribbed walls at low Mach numbers. To achieve the low Mach number (around 0.01) condition, pressurized Freon R-134a vapor is utilized as the working fluid. The flow in the first passage is radial outward, after the 180? tip turn the flow is radial inward to the second passage, and after the 180? hub turn the flow is radial outward to the third passage. The effects of rotation on the heat transfer coefficients were investigated at rotation numbers up to 0.6 and Reynolds numbers from 30,000 to 70,000. Heat transfer coefficients were measured using the thermocouples-copper-plate-heater regional average method. Heat transfer results are obtained over a wide range of Reynolds numbers and rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Regional heat transfer coefficients are correlated with Reynolds numbers for non-rotation and with rotation numbers for rotating condition, respectively. The results can be useful for understanding real rotor blade coolant passage heat transfer under low Mach number, medium-high Reynolds number and high rotation number conditions.