Este trabalho apresenta um estudo experimental do efeito da razão de bloqueio em um canal aerodinâmico no Número de Strouhal e o fenômeno da biestabilidade. É estudado o escoamento turbulento sobre um cilindro e também sobre dois cilindros dispostos lado a lado, onde fez-se uso de cilindros de vários diâmetros a fim de variar a razão de bloqueio no canal. Dois tipos de espaçamento entre os tubos são estudados. A técnica experimental consiste na medição de flutuações de velocidades em um canal aerodinâmico utilizando a técnica de anemometria de fio quente. Os dados obtidos da medição no canal aerodinâmico são tratados com o uso de ferramentas estatísticas, espectrais e de ondaletas. Para um cilindro, os resultados confirmam o aumento do número de Strouhal com o aumento da razão de bloqueio, na faixa do número de Reynolds utilizada. Razões de bloqueio mais baixas podem acarretar diminuição do número de Strouhal, também em função do número de Reynolds. Os resultados confirmam a existência do fenômeno da biestabilidade no escoamento sobre dois cilindros dispostos lado a lado, assim como a influência da razão de bloqueio sobre o fenômeno, sendo mais predominante em um dos espaçamentos estudados. Para razões de bloqueio altas, o fenômeno da biestabilidade pode não ocorrer ou pouco ocorre. Para razões de bloqueio mais baixas podemos ter também a não ocorrência do fenômeno em função das dimensões dos tubos serem mais reduzidas, fazendo com que a captação do fenômeno também seja reduzida. / This paper presents an experimental study of the effect of blockage ratio in a aerodynamic channel on Strouhal number and the bistability phenomenon. It studied the turbulent flow around a single cylinder and also on two cylinders arranged side-by-side, where use has been made of cylinders of various diameters to vary the blockage ratio of the channel. Two types of pitch to diameter ratio are studied. The experimental technique consists of measuring velocity fluctuations in aerodynamic channel using the technique of hot-wire anemometry. The data obtained from measuring the aerodynamic channel are treated with the use of statistical tools, spectral and wavelet analysis. For a single cylinder, the results confirm the increase of the Strouhal number with increasing blockage ratio in the range of Reynolds number used. Lower blockage ratio can cause a decrease of the Strouhal number, also a function of Reynolds number. The results confirm the existence of the bistability phenomenon in the flow on two cylinders arranged side-by-side, as well as the influence of blockage ratio on the phenomenon, being more predominant in one of the row spacings. For higher blockage ratios, the bistability phenomenon may not occur or occurs shortly. For lower blockage ratio, due to the smaller dimensions of the tubes the phenomenon may not occur on the capture of the phenomenon is also reduced with the present experimental technique.

To continue the development of the Boundary Layer Data System (BLDS), a constant voltage hot-wire anemometer (CVA) is implemented into the BLDS for flight-testing. The hot-wire anemometer was chosen as an alternative to the traditional pressure probe because of the ability to measure both average velocity and fluctuating velocity within the boundary layer. Previous work done on the benchtop has led to the design of miniaturization, flight-capable hardware for the BLDS. The next step in the development of the BLDS – CVA calls for quantifying the accuracy of the boundary layer measurements measured by the CVA system. To do this, numerous turbulent boundary layer velocity and fluctuating velocity profiles were taken on a flat-plate at various speeds within the Cal Poly 2x2 wind tunnel with both the traditional pressure probe and the CVA. These test resulted showed agreement between the hot-wire and pressure probe data. Once this was completed the new CVA hardware was tested along with the new software that was written for the BLDS – CVA. In addition, due to the limited memory space onboard the BLDS – CVA, an approximation had to be developed to convert the average voltage data from the BLDS – CVA to the average velocity data due to the non-linear calibration function. The approximation developed was able to match the exact values from a traditional calibration. Lastly, due to the inability to perform a laboratory calibration of the hot-wire at altitude, where the conditions differ significantly from the ground conditions, a new procedure for hot-wire calibration was developed. The method developed was validated through wind tunnel testing and a computer thermal/electric model. With the completion of this work, the BLDS – CVA is ready for flight-testing.

The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind power farms. In order to better estimate the total power output of a wind power farm, knowledge about the development and stability of wind turbine wakes is crucial. In the present thesis, the wake behind a small-scale model turbine was investigated experimentally in a wind tunnel. The velocity in the wake was measured with hot-wire anemometry, for different free stream velocities and tip speed ratios. To characterize the behaviour of the model turbine, the power output, thrust force and rotational frequency of the model were also measured. These results were then compared to calculations using the Blade Element Momentum (BEM) method. New turbine blades for the model was constructed using the same method, in order to get an estimate of the distribution of the lift and drag forces along the blades. This information is needed for comparisons with certain numerical simulations, which however remains to be performed.By placing the turbine at different heights in a turbulent boundary layer, the effects of forest turbulence on wind turbine outputs (power and thrust) could also be investigated.The evolution of the tip vortices shed from the turbine blades was studied by performing velocity measurements around the location of the tip vortex breakdown. The vortices' receptivity to disturbances was then studied by introducing a disturbance in the form of two pulsed jets, located in the rear part of the nacelle. In order to introduce a well-defined disturbance and perform phase-locked measurements, a new experimental setup was constructed and successfully tested for two different disturbance frequencies. The mean streamwise velocity and the streamwise turbulence intensity was found to scale well with the free stream velocity and the spreading of the wake was found to be proportional to the square root of the downstream distance.  The comparison for power and thrust between measurements and BEM calculations showed good agreement in some cases but worse agreement when the pitch angle of the blades was small.The velocity measurements showed that the tip vortices can be susceptible to disturbances and an earlier breakdown could be detected. However, more measurements need to be made to fully investigate the dependance on disturbance frequency and amplitude. / QC 20120504

This work studied the hot wire method in measuring thermal conductivity at room temperature. The purpose is to find the optimized experimental conditions to minimize natural convection in liquid for this method, which will be taken as reference for high temperature thermal conductivity measurement of slag. Combining room temperature experiments and simulation with COMSOL Multiphysics 4.2a, the study on different experimental parameters which may influence the accuracy of the measured thermal conductivity was conducted. The parameters studied were the diameter of crucible, the position of wire in the liquid, including z direction and x-y plane position, diameter of the hot wire, and current used in the measurement. In COMSOL simulations, the maximum natural convection velocity value was used to evaluate the natural convection in the liquid. The experiment results showed after 4~5 seconds of the measuring process, the natural convection already happened. Also when current was fixed, the thinner the hot wire, the larger convection it would cause. This is because thinner wire generates more heat per unit surface area. Using higher current in measuring, more heat generation improved accuracy of result but also had earlier and larger effect on convection. Both simulation and experiments showed that with the height of the liquid fixed, the smaller diameter of the crucible (not small to the level which is comparable with hot wire diameter), the higher the position in z direction (still covered by liquid), the less natural convection effect existed. But the difference was not significant. The radius-direction position change didn’t influence the result much as long as the wire was not too close to the wall.

The hot wire method, with pH and temperature sensors, was evaluated to determine its usefulness and application for cheese production automation. Coagulation of milk substrate was measured with the hot wire instrument and by four other methods: Formagraph, Brookfield®, vixcometer, Omnispec™ bioactivity monitor, and Sommer and Matsen rolling bottle method. The hot wire, using the time at maximum slope, detected coagulation before methods that measure resistance to shear, and after methods that measure light reflectance. Coagulation time was not significantly different from the industry standard rolling bottle method used by Sommer and Matsen. the hot wire instrument was also used to distinguish samples that formed curd at different rates. This was accomplished by measuring the rate of temperature change of the hot wire probe during curd formation. Milk samples of varying protein, fat, and calcium concentrations were prepared to determine if the instrument could be used to predict a consistent curd cut-point. The pH level was also adjusted, and rennet additions were varied. Coagulation was monitored simultaneously with the hot wire system and a Formagraph. All five factors (pH, calcium, fat, protein, and rennet) had significant effects on cut time estimations (CT20) on the Formagraph. Linear correlations (R2) ranging from .74 to .94 were obtained using stepwise regressions when comparing hot wire and compositional data with the Formagraph. A Formagraph was used to measure effects of calcium, pH, and rennet changes on the coagulation properties of late lactation milk. Calcium, pH, and rennet treatments significantly affected the coagulation parameters measured by the Formagraph. However, response among the poor coagulating samples to treatments to improve coagulation was sample dependent. General composition and SDS-PAGE fractionation data could not be used as an indicator of poor or good coagulability of samples. The hot wire method worked well for monitoring coagulation time and curd firming rate, but did not measure maximum curd firmness well. Curd firming rates determined from the hot wire data are acceptable for estimation of a curd cut time. Added benefits of the hot wire method for monitoring cheese manufacture are that stirring, coagulation, and healing of curd can also be measured. Therefore, the rates of change of important parameters, such as pH, temperature, and coagulation during critical processing steps, can easily be determined by a computer and displayed, printed out, or saved for future evaluation.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Robert Erik Grip. / M.S.

Molten salts are high-temperature heat transfer fluids intended for cooling and/or storage purposes in a variety of energy applications. The current work seeks to ultimately study the thermophysical properties of fluoride and chloride salts, which are commonly considered for use in advanced nuclear reactors. Thermophysical properties like thermal conductivity are fundamental to ensuring safe, efficient, and competitive designs for advanced commercial nuclear reactors. Measurement challenges with liquid salts such as electrical conduction, corrosion, convection, and thermal radiation have hindered traditional approaches in their attempts to accurately quantify these properties at high temperatures. Here, a needle probe is developed, which modifies principles from existing instrumental techniques in order to experimentally measure the thermal conductivity of molten salts with reduced error. An analytical heat transfer model is developed to characterize 1D radial heat flow in a multilayered cylindrical system. This includes a thin layer of salt located between the needle probe and a crucible to limit natural convection. After being validated with finite-element methods, the needle probe is used to measure the thermal conductivity of several reference liquids, whose thermophysical properties are well-established at low temperatures. These seven samples are water, sodium nitrate (molten salt), potassium nitrate (molten salt), toluene, ethanol, propylene glycol, and galinstan. The needle probe was able to accurately measure thermal conductivity between 0.40-0.66W/mK for these samples with 3.5-10% uncertainty. Three eutectic halide molten salts (presented by molar composition) were selected for high-temperature testing. These include the ternary fluorides LiF(46.5%)-NaF(11.5%)-KF(42%) and NaF(34.5%)-KF(59%)-MgF2(6.5%), as well as the binary chloride NaCl(58.2%)-KCl(41.8%). Because testing temperatures range between 500-750C, the governing model is adapted to account for radiative heat transfer through the salt sample in parallel with conductive heat transfer. Improvements to the experimental apparatus are also made. For all three salts, the needle probe accurately measured thermal conductivity between 0.490-0.849W/mK with total uncertainty generally being less than 20%. A linear fit to the data demonstrates a clear negative relationship between thermal conductivity and an increase in temperature, which agrees with theoretical and computational predictions. These results indicate that the needle probe successfully handles the assortment of measurement challenges associated with high-temperature molten salts and provides reliable data to create correlations for thermophysical property databases.

This thesis documents research done for a transient hot wire system that will be used in future thermal conductivity measurements of molten salts. Research done with molten salts have been limited because of erroneous measurement capabilities, but the current research strives to introduce a new technique to accurately record thermal conductivity over a wide range of temperatures. This work follows up on past transient hot wire researchers whose designs and tests produced an instrument that can measure the thermal conductivity of molten metals up to 750 K. The transient hot wire (THW) technique has been selected to be used in molten salt to derive thermal conductivity values. While running a THW test in molten salts is outside the scope of this thesis, a modular system has been created for the use of running transient hot wire test that allows for a robust and repeatable testing. A PEGDA/galinstan sensor is used for the validation of the system. A robust GUI has been created to automate the experimental procedure in a glovebox environment. The inverse finite element method has been paired with a non linear fit script to optimize calculations and reduce run times. Test have been done to determine the thermal conductivity of PEGDA. The overall uncertainty of the thermal conductivity measured with the PEGDA sensor is estimated to be ±5% at a 95% confidence level. With a THW system implemented and validated a sensor has been designed to work in molten salts. A model has been created in two separate FEA programs to validate design changes and material properties. The sensor is made up of a chemical vapor deposition (CVD) diamond substrate and tungsten wires to overcome corrosion and heat challenges introduced when measuring molten salts. New manufacturing processes have been designed to allow the technique to use these materials in the THW sensor design. The selected material properties of the sensor and extensive finite element work have laid down the ground work for future experimentation and understanding of the thermal properties of molten salts. It is predicted that the CVD diamond (carbide) apparatus design will use the THW techniques to operate with an estimated accuracy of ±3% over a wide range of temperatures, from ambient up to 1200 K. Manufacturing of the diamond-tungsten sensor have proven the viability of depositing tungsten wire onto CVD diamond and growing a secondary layer of CVD diamond over the tungsten wire.

The study demonstrates the systematic investigation of thermophysical properties of synthetic slags and commercial mold fluxes in a wide temperature range. Focal points of the work are (i) the development and the construction of the transient hot-wire method for the thermal conductivity measurement of solid and molten slags and (ii) the investigation of the thermal conductivity of all layers of casting powders being in the mold. The work includes viscosity, density and surface tension measurements as well as the investigation of characteristic temperatures. The crystallization behavior of mold fluxes was characterized using a SHTT/DHTT (single hot and double hot thermocouple technique). The study discusses the temperature dependence, the influence of the basicity and the non-bridging oxygen per tetrahedra (NBO/T) on the slag properties. The novelty of the work is the systematic characterization of properties of two commercial mold fluxes and the thermal conductivity measurement in the glass transition temperature range.

The unsteady behavior of a tip leakage flow downstream of a simulated axial compressor rotor has been studied. The Virginia Tech low speed linear cascade wind tunnel was adapted to model the unsteady tip leakage flow produced by a rotor operating in the vortical wakes of a set of stator vanes. The cascade, consisting of 8 GE rotor B blades, has adjustable tip gap, inlet angle of 65.1 degrees, turning angle of 11.8 degrees and solidity of 1.076. The cascade Reynolds number, based on blade chord, was 393,000. A moving end wall was used to simulate the relative motion between rotor and casing, and vortex generators attached to the moving end wall were used to produce an idealized periodic unsteady vortical inflow similar to that shed by the junction of a row of inlet guide vanes. Measurements of the vortical inflow to the cascade produced by the generators and of the mean blade loading at the mid span are presented. The periodic and aperiodic behavior of the tip leakage flow downstream of the cascade, produced by this vortical disturbance, is also presented using phase and time averaged 3-component turbulence and pressure fluctuation measurements. These measurements are made for tip gap from 0.83% to 3.3% chord and streamwise locations from 0.772% to 1.117% blade spacing axially downstream of the cascade. The phase averaged inflow measurements reveal that the inflow produced by the vortex generators consists of a pair asymmetric counter-rotating vortices embedded in a thin (4.6% chord) endwall boundary layer. The vortices extend some 7.4% chord from the end wall. Their strength is about two orders smaller than the typical circulation of the tip leakage vortices produced by the cascade. Phase averaged single point three component hot-wire measurements downstream of the cascade reveal that the vortical inflow is, however, capable of producing significant large scale fluctuations in the size, strength, structure and position of the tip leakage vortex. These effects increase in magnitude with increase of tip gap. For small tip gaps these effects appear to be due to simple superposition between the inflow vortices and the tip leakage vortex. However for larger tip gaps these effects appear primarily a consequence of the inflow vortices interfering with the shedding of circulation from the blade tip. The fact that the circulation fluctuation is consistent with the inviscid unsteady loading prediction suggests that the inviscid response may be a major mechanism for generating the tip leakage unsteadiness. Although there is large periodic fluctuation in the tip leakage flow disturbed by the inflow, there is a larger aperiodic component. Two point correlation measurements and linear stochastic estimation are used to reveal the structure of this aperiodic part for a tip gap of 3.3% chord. The aperiodic fluctuation, containing most of the turbulence energy, is found appearing to be organized structures in large scale, and making the estimated instantaneous velocity field significantly different from the phase averaged periodic velocity field. Phase averaged pressure fluctuation measurements made using a microphone in the tip leakage vortex downstream of the cascade reveal that there are significant periodic fluctuating pressure waves and intense mean square fluctuation of the aperiodic fluctuating pressure. They are consistent with the measured periodic flow and aperiodic flow field respectively. These microphone measurements are validated using fluctuating pressure gradient estimates determined from the hot-wire measurements. / Ph. D.