Heat transfer and pressure data were performed and reported on two different rigs. The first rig has an aspect ratio of (19:1) with two different inlet conditions and the second rig is composed of two different aspect ratio channels, (1:8) and (1:4). Rib turbulators were used as a flow disruptor scheme to enhance the heat transfer and friction factor. Rib aspect ratios ranging from (1:1) to (1:5) rib-height-to-width ratio were used. The first channel rib-width-to-pitch (Wr/P) ratio was kept at 1/2 where flow was kept at relatively low Reynolds numbers, between 3000 and 13000. Results from the current tests showed that existing correlations could be used for high aspect ratio channels in predicting the effectiveness of the cooling scheme. Two different inlet conditions were tested; one was arranged so that the flow was hydrodynamically fully-developed at the entrance of the heated section, while the other uses an abrupt entrance from bleeding off mass flow from a horizontal channel. The heat transfer augmentation (compared to a well known and accepted correlation proposed by Dittus-Boelter) in these channels are extremely high with an average of 350% to 400%. However, this was accompanied by a substantial increase in the pressure drop, causing the overall thermal performance to increase between twenty to thirty percent. The second channel rib-width-to-pitch ratio (Wr/P) ranges from 0.1, 0.3, and 0.5; the flow conditions are tested from 20,000 to 40,000 Reynolds number. Correlations for heat transfer and friction augmentation of the test data was also given. The test shows large rib blockage ratio does not demonstrate the best thermal performance; however it does give a high heat transfer augmentation ranging from 200 to 300 percent for both aspect ratios depending on the width of the used ribs.

Heat exchangers are used in a wide variety of industrial applications. Augmentation of heat transfer can realize a reduction in heat transfer size and increase the effectiveness and efficiency of heat exchangers. Heat transfer can be enhanced with various methods where the turbulence of the fluid flow is enhanced: by adding ribs, grooves or steps to the channel wall, using helical inserts, or by adding bluff bodies in the channel flow. By using these methods, there is also an increase in pressure drop penalty and larger pumping power is required to achieve the same flow rate. Circular cylindrical bluff bodies have been found to have smaller drag coefficients than square, rectangular or triangular cylindrical bluff bodies in the channel flow. Heat transfer and pressure drop experimental tests were done for eight different circular cylindrical cross-bar arrays at 15 different Reynolds numbers, in the range of 640 to 12 500. Eight different cross-bar configurations were tested: the cylinder diameter to pitch ratios were, d/p = 0.025, d/p = 0.05, d/pi=i0.1 and d/p = 0.2, and the angle to the flow direction, was θ = 90° and θ = 45° for each of the four different diameter-to-pitch ratios. Transient CFD simulations were done using Ansys fluent for d/p = 0.05 and d/p = 0.2, for θ = 90°, at Reynolds numbers 920 and 9 700, to analyze the secondary flow structures in the wake of the cylinders, partly responsible for the heat transfer and pressure drop increase in the channel flow in comparison to the smooth channel. The k-Ω shear stress transport (SST) model was used for the simulations. A mesh dependence study was done for spatial discretization, temporal discretization and validated against the experimental setup. The pressure drop gradient was found from the test data for the hydraulically developed part of the test section to calculate the friction factors. With an increase in Reynolds number, the friction factors decreased until reaching an asymptotic value for all the cross-bar configurations. For θi=i90° the friction factors were larger than for θ = 45° for the same d/p ratio and Reynolds number. With an increase in d/p, the friction factors increased. The largest measured friction factor was f = 0.3, for configuration d/p = 0.2, θ = 90°, at Re = 640 and the smallest measured friction factor f = 0.02, for d/pi= 0.025, θ = 45°, at Re = 12 500. The friction factor ratio was then used to quantify the pressure penalty for using cylindrical cross-bars in the channel flow to enhance heat transfer. The maximum friction factor ratio, f/f0 = 16.7 occurred at Re = 9 700, for d/pi=i0.2, θ = 90° and the minimum friction factor ratio, f/f0 = 2.1, at Re = 640, for d/pi=i0.025, θ = 45°. The average Nusselt numbers were then calculated using the spatial integral average of the local Nusselt numbers. With an increase in Reynolds number, there was an increase in the average Nusselt number for all the cylindrical cross-bar configurations. For larger d/p ratios and θ = 90° cases, the average Nusselt numbers were larger than for smaller d/p ratios and θ = 45°. The largest average Nusselt number was Nuavg = 66.3, at Re = 9 700 for d/p = 0.2, θ = 90° and the smallest average Nusselt number, Nuavg = 8.7, at Re = 640 for d/p = 0.025, θ = 45°. The Nusselt number ratio could then be used to quantify the heat transfer enhancement of the cylindrical cross-bar channel to that of the smooth channel, where the largest Nusselt number ratio was, Nuavg /Nu0,avg = 3.3, for d/p = 0.2, θ = 90°, at Rei=i3 000 and the smallest Nuavg /Nu0,avg = 1.1, for d/p = 0.025, θ = 45°, at Re = 640. The CFD results concluded that the pressure drop increase and heat transfer enhancement were caused by the flow acceleration, flow separation, eddy formation, vorticity increase, and boundary layer deformation next to and behind the cylinders. The Strouhal number for the larger d/p ratios suggested that the unsteadiness in the flow is higher for the cylinder arrays with a larger diameter, increasing both the heat transfer enhancement and friction factor in comparison with the smaller diameter cylinder arrays. Finally, the thermal performance coefficients could be calculated by using the friction factor ratios and Nusselt number ratios. The thermal performance coefficient combines the effects of the heat transfer and pressure penalty increase. The thermal performance coefficients increased from Re = 640 until Rei=i3 000 after which it decreased with an increase in Reynolds number. This is because the pressure penalty starts to outweigh the heat transfer increase caused by the turbulators. The largest thermal performance coefficient was η = 1.6, for d/p = 0.025, θ = 45°, at Re = 3 000, and the lowest, η = 0.79, for d/p = 0.05, θ = 90°, at Re = 640. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2021. / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

This diploma thesis deals with optimization of propellers with the aim to reduce their noise level. Based on the measurement of noise level of serial propellers, major propeller noise sources were evaluated. Subsequent 2D simulations of the flow on the individual profiles along the propeller span revealed the locations of separation bubbles, which were identified as a potential source of the noise. Based on these findings, propellers were equipped with turbulator trips of various dimensions and their effect on the noise level and efficiency of the propellers was experimentally verified. The results did not show contribution of turbulator trips to noise reduction or efficiency.

A model of a gas turbine blade cooling channel equipped with turbulators and a backward facing step geometry was examined. Up to four turbulators oriented cross-stream and inclined 45° to the flow direction were mounted in the channel. The blockage ratio b/H of the turbulators and the height h/H of the backward facing step was 0:125 and 0:14 respectively. The number of turbulators as well as their size was varied. In a preliminary investigation, hot-wire and pressure measurements were taken for three different Reynolds numbers (5,000, 12,000, 18,000)in the center plane of the test section. Subsequently, particle image velocimetry (PIV) measurements were made on the same geometries. Results of PIV measurements for a Reynolds number range of Reb=600 to 5,000 for the turbulators and Reh=1,500 to 16,200 for the backward facing step are presented, where Reynolds numbers are based on turbulator height b and step height h, respectively. Plots of the velocity field, vorticity, reverse flow probability and RMS velocity are shown. The focus is on the steady flow behavior but also the unsteadiness of the flow is discussed in one section. Also reattachment lengths were obtained and compared among the various turbulator arrangements and the backward facing step geometry. It was found that the flow becomes periodic after three or four ribs. For one turbulator, a very large separation region was observed. The magnitude of the skin friction factor was found to be the highest for two ribs. If the first rib is replaced by a smaller rib, the skin friction factor becomes the lowest for this case. Compared to the backward facing step, the flow reattaches earlier for multiple turbulators. A dependency of reattachment length on Reynolds number was not observed.

No description available.

Gas turbine blades operate in very high temperatures to achieve a high thermal efficiency of the engine. For this reason, the blades have to be cooled to prevent degradation or even melting. The blades can be cooled using various techniques, both by cooling the inside of the blade with cooling channels, and by protecting the outside of the blade from the hot environment. One way to cool the blades from the inside is with rib turbulated channels. Straight square channels lined with 90◦, 45◦ and V-shaped ribs in a staggered configuration are investigated in this thesis.  Computational fluid dynamics (CFD), among other methods, can be used to predict important parameters such as heat transfer and pressure loss for different ribbed channel geometries. In this thesis a CFD model using RANS simulations with the turbulence models Lag Elliptic Blending k − ε, Realizable k − ε two-layer and SST k − ω is established and validated against experimental data by Taslim et al [1]. This is done by comparing the Nusselt number between a pair of ribs as well as the channel friction factor for 90◦, 45◦ and V-shape ribs. Different sensitivities are also investigated to get an understanding of the uncertainties found during the CFD implementation. These include the effect of mesh resolution, inlet turbulence intensity, rounded rib edges, wall roughness and temperature used for Reynolds number calculations. The Nusselt number and friction factor predictions of the turbulence models are also compared with existing empirical correlations.  The results of the investigation show that the CFD results for 90◦ ribs deviate the most from experimental results, while closer results are seen for the 45◦ and V-shape ribs.  In conclusion, the Lag Elliptic Blending k−ε model generally produces results closest to experimental data, especially for 90◦ ribs, but it shows some differences in Reynolds number trends. It proves to predict heat transfer and pressure loss closer to the experiment than the other models in flows where recirculation and reattachment has a significant impact. The Lag EB model is relatively stable and mesh independent. The SST k − ω model produces results rather similar to experimental data, but is unstable and sensitive to mesh resolution. The Realizable k − ε two-layer model produces results that are slightly less consistent with experimental data, but is very stable and insensitive to mesh resolution. The Nusselt number and friction factor from the investigated empirical correlations are closer to experimental results than the turbulence models for 90◦ inline ribs.

Conjugate heat transfer analysis has been carried out on an 89kN thrust chamber in order to evaluate whether combined discrete film cooling and regenerative cooling in a rocket nozzle is feasible. Several cooling configurations were tested against a baseline design of regenerative cooling only. New designs include combined cooling channels with one row of discrete film cooling holes near the throat of the nozzle, and turbulated cooling channels combined with a row of discrete film cooling holes. Blowing ratio and channel mass flow rate were both varied for each design. The effectiveness of each configuration was measured via the maximum hot gas-side nozzle wall temperature, which can be correlated to number of cycles to failure. A target maximum temperature of 613K was chosen. Combined film and regenerative cooling, when compared to the baseline regenerative cooling, reduced the hot gas side wall temperature from 667K to 638K. After adding turbulators to the cooling channels, combined film and regenerative cooling reduced the temperature to 592K. Analysis shows that combined regenerative and film cooling is feasible with significant consequences, however further improvements are possible with the use of turbulators in the regenerative cooling channels.

No description available.

Vid konstruktion av flygplan uppkommer svårigheter att efterlikna de tänktadriftsförhållandena vid vindtunnelprovning av mindre skalmodeller. Dessa svårigheter berorfrämst på att ekonomiska och skalbara begränsningar gör att tillräckligt stora vindtunnlar intekan byggas för att generera de strömningshastigheter som ger samma Reynoldstal som för detänkta driftsförhållandena. För att komma runt detta problem kan så kallade störelementanvändas vilka inducerar en övergång mellan laminär och turbulent strömning i skalmodellensgränsskikt och möjliggör vindtunnelprovning i lägre strömningshastigheter. Detta arbete ämnardärför att ta fram ett kostnadseffektivt och flexibelt designkoncept för störelement ämnade förvindtunnelprovning av en skalmodell. Under arbetets gång undersöktes olika typer avstörelement som sedan togs fram med hjälp av CAD och additiv tillverkning. Dessa testadessedan på en skalmodell i en vindtunnel i syfte att hitta anfallsvinkeln för vilken skalmodellenöverstegrade. Mätdata jämfördes sedan med tidigare simuleringar gjorda för skalmodellen ochslutsatsen som drogs var att de framtagna störelementen hade en otillräcklig förmåga i att fåskalmodellen att överstegra, men att ytterligare modifikation av störelementens höjd och breddskulle göra modellering i CAD och additiv tillverkning till ett attraktivt framtagningskonceptför störelement ämnade för vindtunnelprovning av en skalmodell.

Die Effektivität keramischer Wärmeübertrager kann durch eine feinere Strukturierung der Oberflächen gesteigert werden. Dies kann durch die Integration textiler Urformen anstatt der konventionell im Schlickguss hergestellten gröberen Geometrien erfolgen. Für Strukturierungen in Form von wandgebundenen Halbbögen werden die Ergebnisse umfangreicher experimenteller und numerischer Untersuchungen zu den wärmetechnischen und strömungsmechanischen Eigenschaften vorgestellt. Basierend auf den Erkenntnissen der mittels numerischer Simulation durchgeführten Parameterstudie werden verschiedene Empfehlungen für eine optimierte Anordnung der Halbbögen gegeben, um das Verhältnis von Wärmeübergang zur Druckverlust zu verbessern. Die experimentellen Ergebnisse belegen die Richtigkeit der gewählten Randbedingungen und Vereinfachungen im numerischen Modell. Des Weiteren wurden die Strömungsstrukturen mit laserdiagnostischen Messmethoden umfangreich charakterisiert.