Accepted Manuscripts

Joseph K. Ausserer, Marc D. Polanka, Paul J. Litke and Jacob A. Baranski
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043745
Interest is growing in converting two-stroke spark-ignition engines from motor gasoline to low-anti-knock-index fuel such as diesel and Jet A, where knock-limited operation is a significant consideration. Previous efforts have examined the knock limits for small two-stroke engines and explored the impact of engine controls such as equivalence ratio, combustion phasing, and cooling on engine operation during knock-free operation on high octane number fuel. This work culminates the research begun in those efforts, investigating the degree of knock-mitigation achievable through varying equivalence ratio, combustion phasing, and engine cooling on three small (28, 55, and 85 cm3 displacement) commercially available two-stroke spark-ignition engines operating on a 20 octane number blend of iso-octane and n-heptane. Combustion phasing had the largest impact; a 10° retardation in the CA50 mass-fraction burned angle permitted an increase in throttle that yielded a 9-11% increase in power. Leaning the equivalence ratio from 1.05 to 0.8 resulted in a 10% increase in power; enriching the mixture from 1.05 to 1.35 yielded a 6-7% increase in power but at the cost of a 25% decrease in fuel-conversion efficiency. Varying the flow rate of cooling air over the engines had a minimal impact. The results indicate that the addition of aftermarket variable spark timing and electronic fuel-injection systems offer substantial advantages for converting small, commercially available two-stroke engines to run on low-anti-knock-index fuels.
TOPICS: Two-stroke engines, Aircraft, Fuels, Engines, Cooling, Combustion, Spark-ignition engine, Heptane, Gasoline, Flow (Dynamics), Diesel, Displacement
Bing Guo and Weixiao Tang
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043718
Stability of the nuclear turbine blades is difficult to be accurately predicted because the wet steam load (WSL)) as well as its induced equivalent damping and stiffness during non-equilibrium condensation process (NECP) are hard to be directly calculated. Generally, in design, NECP is assumed as equilibrium condensation process (ECP), of which the two-phase temperature difference (PTD) between gaseous and liquid is ignored. In this paper, a novel method to calculate the WSL-induced equivalent damping and equivalent stiffness during NECP based on the combined micro-perturbation method and computational fluid dynamics method was proposed. Once the WSL-induced equivalent damping and equivalent stiffness are determined, the stability of the blade-WSL system, of which the blade was modeled by a pretwisted airfoil cantilever beam, can then be predicted based on the Lyapunov first method. Besides, to estimate the effects of PTD, comparisons between the WSL-induced equivalent damping and equivalent stiffness as well as the unstable area during NECP and ECP were presented. Results show that the WSL-induced equivalent damping and equivalent stiffness during NECP are more sensitive to the inlet boundary due to the irreversible heat transfer caused by PTD during NECP. Accordingly, the unstable area during NECP is about three times larger than during ECP.
TOPICS: Condensation, Stability, Turbine blades, Equilibrium (Physics), Steam, Stiffness, Damping, Blades, Design, Airfoils, Computational fluid dynamics, Temperature, Heat transfer, Cantilever beams, Stress
David Ritchie, Austin Click, Phillip Ligrani, Federico Liberatore, Rajeshriben Patel and Yin-Hsiang Ho
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043694
Considered is double wall cooling, with full-coverage effusion-cooling on the hot-side of the effusion plate, and a combination of impingement cooling and cross flow cooling, employed together on the cold-side of the effusion plate. Data are given for a main stream flow passage with a contraction ratio CR of 4 for main stream Reynolds numbers ReMS and ReMS,AVG of 157000-161000 and 233000-244000, respectively. For the same Reynolds number, initial blowing ratio, and streamwise location, increased thermal protection is often provided when the effusion coolant is provided by the cross flow/impingement combination configuration, compared to the cross flow only supply arrangement. In general, higher adiabatic effectiveness values are provided by the impingement only arrangement, relative to the impingement/cross flow combination configuration, when compared at the same Reynolds number, initial blowing ratio, and x/de location. Data for x/de=60 show that the highest NHFR values are produced either by the impingement/cross flow combination configuration or by the impingement only arrangement, depending upon the particular magnitude of BR which is considered. Cold-side spatially-resolved surface measurements indicate that surface regions, where local Nusselt numbers are augmented, increase in spatial extent, as the initial blowing ratio increases from 5.9 to 9.8. Such behavior is believed occur because the addition of cross flow to impingement cooling (employed with the present combination cooling arrangement) generally seems to alter and sometimes degrade associated coolant distributions, for the present experimental conditions and configurations.
TOPICS: Cooling, Cross-flow, Reynolds number, Coolants, Impingement cooling, Flow (Dynamics)
Peter G Dowell, Sam Akehurst and Richard D Burke
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043700
Measuring and analyzing combustion is a critical part of the development of high efficiency and low emitting engines. Faced with changes in legislation such as Real Driving Emissions and the fundamental change in the role of the combustion engine with the introduction of hybrid-electric powertrains, it is essential that combustion analysis can be conducted accurately across the full range of operating conditions. In this work, the sensitivity of five key combustion metrics is investigated with respect to eight necessary assumptions used for single zone Diesel Combustion analysis. The sensitivity was evaluated over the complete operating range of the engine using a combination of experimental and modelling techniques. This provides a holistic understanding of combustion measurement accuracy. For several metrics, it was found that the sensitivity at the mid speed/load condition was not representative of sensitivity across the full operating range, in particular at low speeds and loads. Peak heat release rate and indicated mean effective pressure were found to be most sensitive to the determination of top dead center (TDC) and the assumption of in-cylinder gas properties. An error of 0.5° in the location of TDC would cause on average a 4.2% error in peak heat release rate. A novel method for determining TDC was proposed which improved the robustness of the metric and will enable accurate measurements of combustion data in in-service operating conditions.
TOPICS: Combustion, Engines, Diesel engines, Errors, Stress, Heat, Pressure, Accuracy and precision, Modeling, Cylinders, Diesel, Robustness, Emissions
Niklas Neupert, Janneck Christopher Harbeck and Franz Joos
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043690
In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of erosion and deposition of water on the blades leading to an increase of required torque and profile loss. Due to this detailed information about the structure and the amount of water on the surface is key for compressor performance. Experiments were conducted with a droplet laden flow in a transonic compressor cascade focusing on the film formed by deposited water. Two approaches were taken. In the first approach the film thickness on the blade was directly measured using white light interferometry. Due to significant distortion of the flow caused by the measurement system a transfer of the measured film thickness to the undisturbed case is not possible. Therefore, a film model is adapted to describe the film flow in terms of height averaged film parameters. In the second approach experiments were conducted in an undisturbed cascade setup and the water film pattern was measured using a non-intrusive quantitative image processing tool. Utilizing the measured flow pattern in combination with findings from literature the rivulet flow structure is resolved. From continuity of the water flow a film thickness is derived showing good agreement with the previously calculated results. Using both approaches a 3D reconstruction of the water film pattern is created giving first experimental results of the film forming on stationary compressor blades under overspray fogging conditions.
TOPICS: Cascades (Fluid dynamics), Drops, Flow (Dynamics), Compressors, Water, Film thickness, Blades, Cycles, Film flow, Industrial gases, Erosion, Turbines, Image processing, White light interferometry, Measurement systems, Risk, Torque
Georg Fink, Michael Jud and Thomas Sattelmayer
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043643
Natural gas as an alternative fuel in engine applications reduces both pollutant and greenhouse gas emissions. High pressure dual fuel direct injection of natural gas and Diesel pilot has the potential to minimize methane slip from gas engines and increase the fuel flexibility, while retaining the high efficiency of a Diesel engine. Speed and load variations as well as various strategies for emission reduction entail a wide range of different operating conditions. The influence of these operating conditions on the ignition and combustion process is investigated on a rapid compression expansion machine. By combining simultaneous Shadowgraphy and OH* imaging with heat release rate analysis, an improved understanding of the ignition and combustion process is established. At high temperatures and pressures the reduced pilot ignition delay and lift-off length minimize the effect of gas jet entrainment on pilot mixture formation. A simple geometrical constraint was found to reflect the susceptibility for misfiring. At the same time natural gas ignition is delayed by the early pilot ignition close to the injector tip. The shape of heat release is only marginally affected by the operating conditions and mainly determined by the degree of premixing at the time of gas jet ignition. Luminescence from the sooting natural gas flame is generally only detected after the flame extends across the whole gas jet at peak heat release rate. Termination of gas injection at this time was confirmed to effectively suppress soot formation, while a strongly sooting pilot intensifies soot formation within the gas jet.
TOPICS: Natural gas, Diesel, Ignition, Heat, Fuels, Combustion, Flames, Soot, Emissions, High temperature, Ignition delay, Methane, Shapes, Pollution, Imaging, Gas engines, Luminescence, Diesel engines, Machinery, Compression, Engines, Stress, High pressure (Physics), Ejectors
Fangyuan Liu, Junkui Mao, Chao Han, Yuanjian Liu, Xingsi Han and Fengli Liang
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043445
Considering the complicated geometry in an active clearance control system, the design of an improved cooling feed pipe with a covering plate for a high pressure ribbed turbine case was investigated. Numerical calculations were analyzed to obtain the interactions between the impinging jet arrays fed by the pipe. Experimental tests were performed to explore the effect of the Reynolds number (2000–20000) and the jet-to-surface spacing ratio (6–10) on the streamwise-averaged Nusselt numbers. Additionally, the effect of the crossflow produced by the configuration was investigated. Results showed a confined curved channel was formed by the pipe and ribbed case, which resulted in crossflow. The crossflow evolved into vortices and the streamwise-averaged Nusselt number on the high ribs was subsequently increased. Furthermore, the distribution of the heat transfer on the entire surface became more uniform compared with that of traditional impinging jet arrays. A higher Nusselt number was achieved by decreasing the jet-to-surface spacing and increasing the Reynolds number. This investigation has revealed a cooling configuration for controlling the wall flow and evening the heat transfer on the case surface, especially for the ribs. Keyword: Active clearance control, high pressure turbine case, cooling feed pipe; impinging jet arrays, crossflow, heat transfer characteristics.
TOPICS: Cooling, Pipes, Turbines, Heat transfer, Reynolds number, High pressure (Physics), Clearances (Engineering), Design, Flow (Dynamics), Control systems, Vortices, Geometry
Riccardo Scarcelli, Anqi Zhang, Thomas Wallner, Sibendu Som, Jing Huang, Sameera Wijeyakulasuriya, Yijin Mao, Xiucheng Zhu and Seong-Young Lee
J. Eng. Gas Turbines Power   doi: 10.1115/1.4043397
With the engine technology moving towards more challenging (highly dilute and boosted) operation, spark-ignition processes play a key role in determining flame propagation and completeness of the combustion process. On the computational side, there is plenty of spark-ignition models available in literature and validated under conventional, stoichiometric SI operation. Nevertheless, these models need to be expanded and developed on more physical grounds since at challenging operation they are not truly predictive. This paper reports on the development of a dedicated model for the spark-ignition event at non-quiescent, engine-like conditions, performed in the commercial CFD code CONVERGE. The developed methodology leverages previous findings that have expanded the use and improved the accuracy of Eulerian-type energy deposition models. In this work, the Eulerian energy deposition is coupled at every computational time-step with a Lagrangian-type evolution of the spark channel. Typical features such as spark channel elongation, stretch, attachment to the electrodes are properly described to deliver realistic energy deposition along the channel during the entire ignition process. The numerical results are validated against schlieren images from an optical constant volume chamber and show the improvement in the simulation of the spark channel during the entire ignition event, with respect to the most commonly used energy deposition approach. Further development pathways are discussed to provide more physics-based features from the developed ignition model in the future.
TOPICS: Turbulence, Engines, Ignition, Physics, Combustion, Simulation, Computational fluid dynamics, Electrodes, Elongation, Flames
Dara Childs and Paul Esser
J. Eng. Gas Turbines Power   doi: 10.1115/1.4042721
A fixed-geometry hybrid thrust bearing is investigated with 3 different supply-orifice diameters, (1.63, 1.80, and 1.93 mm). The test rig uses a face-to-face thrust bearing design. The test bearing acts as the rotor-loading mechanism. A hydraulic shaker applies the static axial load, which is reacted by a second (slave) thrust bearing. The rotor is supported radially by two water-lubricated journal bearings and is attached to a 30.6 krpm motor via a soft high-speed coupling. Thrust bearings are tested for a range of supply pressures (5.17, 10.34, 17.34 bars), fluid-film thicknesses, and speeds (7.5, 12.5, and 17.5 krpm). The test bearings have eight pockets, with centrally-located feed orifices in each pocket. Experimental results generally agree well withpredictions from a bulk-flow model. Thrust-bearing inlet supply and inner radius flow rates all decreased with decreasing orifice diameters and bearing axial clearances. In most cases, the bearings with larger orifice diameters exhibit higher recess pressure ratios, operating clearances, and flow rates. An optimum hybrid thrust bearing orifice diameter will depend on the conditions of individual applications. Larger orifices generally provide larger operating clearances and higher stiffnesses, but also require higher flow rates. For most applications, a compromise of bearing performance parameters will be desired. The test results and comparisons presented will aid in sizing orifice diameters for future hybrid thrust bearing designs and in validating and improving models and predictions.
TOPICS: Hydrostatics, Thrust bearings, Bearings, Flow (Dynamics), Rotors, Orifices, Fluid films, Geometry, Water, Journal bearings, Pressure, Design, Engines, Motors, Stress
Thomas Hagemann and Hubert Schwarze
J. Eng. Gas Turbines Power   doi: 10.1115/1.4041026
Flooded lubrication of tilting-pad journal bearings provides safe and robust operation for many applications due to a completely filled gap at the leading edge of each pad. While flooded conditions can be ensured by restrictive seals on the lateral bearing ends for any conventional bearing design, direct lubrication by leading edge grooves (LEG) placed on the pads represents an alternative to produce completely filled gaps at the entrance to the convergent lubricant film. Moreover, this design is flexible to apply different axial sealing baffles in order to influence the thermal equilibrium within the entire bearing. A theoretical model is presented that describes the specific influences of LEG design on the operating characteristics. First, in opposite to conventional tilting-pad journal bearing designs the LEG is a self-contained lube oil pocket which is generally connected to an outer annular oil supply channel. Consequently, each leading edge groove can feature a specific speed and load dependent effective pocket pressure and flow rate. As a consequence of this and the fact that the LEG is part of the pad, it directly influences its tilting angle. Secondly, the thermal inlet mixing model must consider the specific flow conditions depending on the main flow direction within the film as well as the one between outer annular channel and pocket. The novel LEG model is integrated into a comprehensive bearing code and validated with test data from high performance journal bearing test rig for a four tilting-pad bearing in load between pivot orientation.
TOPICS: Experimental analysis, Journal bearings, Bearings, Flow (Dynamics), Lubrication, Design, Stress, Thermal equilibrium, Bearing design, Lubricants, Sealing (Process), Pressure
Zihan Shen, Benjamin Chouvion, Fabrice Thouverez, Aline Beley and Jean-Daniel Beley
J. Eng. Gas Turbines Power   doi: 10.1115/1.4041024
In this paper, the Co-Rotational (C-R) finite element method derived from a rotating reference frame is proposed to investigate the nonlinear vibration of rotating 2D beams with large displacement. This method has been widely applied for static analysis with very large displacement. However, at present, the application of the C-R method in the non-linear dynamic analysis is relatively limited, especially in rotating machinery simulations. The consideration of C-R method provides us with the possibility to treat geometrical nonlinearity directly with pre-extracted rigid body motion displacements. Moreover, it re-activates the existing linear finite element library, as the pure deformational displacements are also extracted. In this work, the Euler-Bernoulli beam hypotheses are used but the extension to other beam theory should not be an issue. The accuracy of the C-R formulation is examined by a convergence theoretical study by comparing the approximated strains in local C-R frames with exact ones derived in Total Lagrangian formulations. In our proposed C-R formulation, starting from the consistent expression of kinematic energy, the governing equations for nonlinear vibration are obtained by using Lagrange's equations, in which the constant angular velocity is taken into consideration. By this way, the geometrical nonlinearity of large displacement is perfectly dealt with. To enhance the numerical simulations, the mass, the Coriolis, and the tangent stiffness matrices are derived analytically. The proposed formulations are used in modal and temporal simulations comparing with results from Total-Lagrangian formulations.
Klaus Brun, Sarah Simons, Kelsi Katcher, Ryan Cater, Brandon Ridens and Rainer Kurz
J. Eng. Gas Turbines Power   doi: 10.1115/1.4041023
Gas property prediction is necessary for proper design of compressors. Equations of state are utilized to predict the thermo-physical gas properties needed for such calculations. These are semi-empirical models that allow the calculation of thermodynamic properties such as density, enthalpy, and speed of sound of gas mixtures for known pressures and temperature. Currently, there is limited or no data publically available to verify the results of these equation of state calculations for the range of pressures, temperatures and gas compositions relevant to many oil & gas applications. Especially for isentropic enthalpy head (i.e., the enthalpy rise along constant entropy lines), which is a critical parameter required to accurately design and performance test compressors, limited public domain data is available for equation of state validation. In this paper a method and test apparatus is described to measure compression enthalpy rise directly. In this apparatus a test gas is compressed using a fast acting piston inside an adiabatic autoclave. Test results are then corrected using calibration efficiencies from a known reference gas compression process at a similar Reynolds number. The paper describes the test apparatus, calibration, measurement methodology, and test results for one complex hydrocarbon gas composition at elevated temperatures and pressures. An uncertainty analysis of the new measurement method is also presented and results are compared to several equations of state. The results show that commonly used equations of state significantly under-predicted the compression enthalpy rise for the test gas case by more than 6%.
Paloma Paleo Cageao, Kathy Simmons, Arun Prabhakar and Budi Chandra
J. Eng. Gas Turbines Power   doi: 10.1115/1.4040812
Experimental research was conducted into a scooped rotor system that captures oil from a stationary jet and directs it through passages within the shaft to another axial location. Such a system has benefits for delivering oil via under-race feed to aeroengine bearings where direct access is limited. Oil capture efficiency was calculated for three jet configurations, a range of geometric variations relative to a baseline and a range of operating conditions. Flow visualization techniques yielded high-speed imaging in the vicinity of the scoop leading edge. Overall capture efficiency depends on the amount of oil initially captured by the scoop that is retained. Observation shows that when the jet hits the tip of a scoop element, it is sliced and deflected upwards in a `plume'. Ligaments and drops formed from this plume are not captured. In addition, some oil initially captured is flung outwards as a consequence of centrifugal force. Although in principle capture of the entire supply is possible over most of the shaft speed range, as demonstrated by a simplified geometric model, in practice 60% to 70% is typical. Significant improvement in capture efficiency was obtained with a lower jet angle (more radial) compared to baseline. Higher capture efficiencies were found where the ratio of jet to scoop tip speed was lower. This research confirms the capability of a scoop system to capture and retain delivered oil. Additional numerical and experimental work, is recommended to further optimise the geometry and increase the investigated temperature and pressure ranges.
TOPICS: Pressure, Temperature, Centrifugal force, Flow visualization, Plumes (Fluid dynamics), Bearings, Gas turbines, Rotors, Geometry, Imaging
Review Article  
David A. Shifler
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038037
It has been conjectured that if sulfur in fuel is removed, engine materials will cease to experience attack from hot corrosion, since this sulfur has been viewed as the primary cause of hot corrosion and sulfidation. Historically, hot corrosion has been defined as an accelerated degradation process that generally involves deposition of corrosive species (e.g., sulfates) from the surrounding environment (e.g., combustion gas) onto the surface of hot components, resulting in destruction of the protective oxide scale. Most papers in the literature, since the 1970s, consider sodium sulfate salt as the single specie contributing to hot corrosion. Recent Navy standards for Navy F-76 and similar fuels have dropped the sulfur content down to 15 parts per million (ppm). Most observers believe that the removal of sulfur will end hot corrosion events in the Fleet. However, the deposit chemistry influencing hot corrosion is known to be much more complex consisting of multiple sulfates and silicates. Sulfur species may still enter the combustion chamber via ship's air intake, which may include seawater entrained in the air. In addition to sodium sulfate, seawater contains magnesium, calcium and potassium salts, and atmospheric contaminants that may contribute to hot corrosion. This paper will cover some of the revised understanding of hot corrosion and consider other possible contaminants that could further complicate a full understanding of hot corrosion.
TOPICS: Corrosion, Sulfur, Seawater, Sodium, Navy, Fuels, Engines, Combustion gases, Combustion chambers, Potassium, Chemistry, Magnesium (Metal)
Klaus Brun, Rainer Kurz and Sarah Simons
J. Eng. Gas Turbines Power   doi: 10.1115/1.4034314
Pressure pulsations into a centrifugal compressor can move its operating point into surge. This is concerning in pipeline stations where centrifugal compressors operate in series/parallel with reciprocating compressors. Sparks (1983), Kurz et al., (2006), and Brun et al., (2014) provided predictions on the impact of periodic pressure pulsation on the behavior of a centrifugal compressor. This interaction is known as the “Compressor Dynamic Response” (CDR) theory. Although the CDR describes the impact of the nearby piping system on the compressor surge and pulsation amplification, it has limited usefulness as a quantitative analysis tool, due to the lack of prediction tools and test data for comparison. Testing of compressor mixed operation was performed in an air loop to quantify the impact of periodic pressure pulsation from a reciprocating compressor on the surge margin of a centrifugal compressor. This data was utilized to validate predictions from Sparks' CDR theory and Brun's numerical approach. A 50 hp single-stage, double-acting reciprocating compressor provided inlet pulsations into a two-stage 700 hp centrifugal compressor. Tests were performed over a range of pulsation excitation amplitudes, frequencies, and pipe geometry variations to determine the impact of piping impedance and resonance responses. Results provided clear evidence that pulsations can reduce the surge margin of centrifugal compressors and that geometry of the piping system immediately upstream and downstream of a centrifugal compressor will have an impact on the surge margin reduction. Surge margin reductions of <30% were observed for high centrifugal compressor inlet suction pulsation.
TOPICS: Compressors, Surges, Pressure, Pipes, Geometry, Piping systems, Testing, Dynamic response, Suction, Pipelines, Excitation, Resonance
shilpi agarwal, Puneet Rana and B. S. Bhadauria
J. Eng. Gas Turbines Power   doi: 10.1115/1.4028491
In this paper, we study the effect of local thermal non-equilibrium on the linear thermal instability in a horizontal layer of a Newtonian nanofluid. The nanofluid layer incorporates the effect of Brownian motion along with thermophoresis. A two-temperature model has been used for the effect of local thermal non-equilibrium among the particle and fluid phases. The linear stability is based on normal mode technique and for nonlinear analysis, a minimal representation of the truncated Fourier series analysis involving only two terms has been used. We observe that for linear instability, the value of Rayleigh number can be increased by a substantial amount on considering a bottom heavy suspension of nano particles. The effect of various parameters on Rayleigh number has been presented graphically. A weak nonlinear theory based on the truncated representation of Fourier series method has been used to find the concentration and the thermal Nusselt numbers. The behavior of the concentration and thermal Nusselt numbers is also investigated by solving the finite amplitude equations using a numerical method.
TOPICS: Equilibrium (Physics), Nanofluids, Rayleigh-Benard convection, Fourier series, Rayleigh number, Nanoparticles, Numerical analysis, Stability, Temperature, Fluids, Particulate matter, Brownian motion
Corey E. Clifford and Mark Kimber
J. Eng. Gas Turbines Power   doi: 10.1115/1.4028492
Natural convection heat transfer from a horizontal cylinder is of importance in a large number of applications. Although the topic has a rich history for free cylinders, maximizing the free convective cooling through the introduction of sidewalls and creation of a chimney effect is considerably less studied. In this study, a numerical model of a heated horizontal cylinder confined between two, vertical adiabatic walls is employed to evaluate the natural convective heat transfer. Two different treatments of the cylinder surface are investigated: constant temperature (isothermal) and constant surface heat flux (isoflux). To quantify the effect of wall distance on the effective heat transfer from the cylinder surface, 18 different confinement ratios are selected in varying increments from 1.125 to 18.0. All of these geometrical configurations are evaluated at seven distinct Rayleigh numbers ranging from 102 to 105. Maximum values of the surface-averaged Nusselt number are observed at an optimum confinement ratio for each analyzed Rayleigh number. Relative to the pseudo-unconfined cylinder at the largest confinement ratio, a 74.2% improvement in the heat transfer from an isothermal cylinder surface is observed at the optimum wall spacing for the highest analyzed Rayleigh number. An analogous improvement of 60.9% is determined for the same conditions with a constant heat flux surface. Several correlations are proposed to evaluate the optimal confinement ratio and the effective rate of heat transfer at that optimal confinement level for both thermal boundary conditions. One of the main application targets for this work is spent nuclear fuel, which after removal from the reactor core is placed in wet storage and then later transferred to cylindrical dry storage canisters. In light of enhanced safety, many are proposing to decrease the amount of time the fuel spends in wet storage conditions. The current study helps to establish a fundamental understanding of the buoyancy-induced flows around these dry cask storage canisters to address the anticipated needs from an accelerated fuel transfer program.
TOPICS: Heat, Natural convection, Cylinders, Storage, Heat transfer, Rayleigh number, Heat flux, Fuels, Safety, Computer simulation, Cooling, Temperature, Flow (Dynamics), Buoyancy, Spent nuclear fuels, Convection, Boundary-value problems

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