No description available.

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on April 9, 2009) Includes bibliographical references.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1991. / Vita. Abstract. Includes bibliographical references (107-110). Also available via the Internet.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains xv, 183 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 133-134).

Heavy water moderator surrounding each fuel channel is one of the important safety features in CANDU reactors since it provides an in-situ passive heat sink for the fuel in situations where other engineered means of heat removal from fuel channels have failed. In a critical break LOCA scenario, fuel cooling becomes severely degraded due to rapid flow reduction in the affected flow pass of the heat transport system. This can result in pressure tubes experiencing significant heat-up during early stages of the accident when coolant pressure is still high, thereby causing uniform thermal creep strain (ballooning) of the pressure tube (PT) into contact with its calandria tube (CT). The contact of the hot PT with the CT causes rapid redistribution of stored heat from the PT to CT and a large heat flux spike from the CT to the moderator fluid. For conditions where subcooling of the moderator fluid is low, this heat flux spike can cause dryout of the CT. This can detrimentally affect channel integrity if the CT post-dryout temperature becomes sufficiently high to result in continued thermal creep strain deformation of both the PT and the CT. A comprehensive mechanistic model is developed to predict the critical heat flux (CHF) variations along the downward facing outer surface of calandria tube. The model is based on the hydrodynamic model of \cite{Cheung/Haddad1997} which considers a liquid macrolayer beneath an elongated vapor slug on the heated surface. Local dryout is postulated to occur whenever the fresh liquid supply to the macrolayer is not sufficient to compensate for the liquid depletion within the macrolayer due to boiling on the heating surface. A boundary layer analysis is performed, treating the two phase motion as an external buoyancy driven flow, to determine the liquid supply rate and the local CHF. The model takes into account different types of flow regime or slip ratio. It is applicable for a calandria vessel as well, under a sever accident condition where a thermal creep failure is postulated to occur if sustained CHF is instigated in the surrounding shield tank water. Model shows good agreement with the available experimental CHF data. The model has been modified to take into account the effect of subcooling and has been validated against the empirical correction factors. / Dissertation / Doctor of Philosophy (PhD)

The ability to predict and control laminar-to-turbulent transition in high-speed flow has a substantial effect on heat transfer and skin friction, thus improving the design and operation of hypersonic vehicles. The control of transition on blunt bodies is essential to improve the performance of lifting and control surfaces. The objective of this Ph.D. research is to develop efficient and accurate algorithms for the detection of high-frequency heat flux fluctuations supported by hypersonic flow in transitional boundary layers. The focus of this research is on understanding the mathematical properties of the reconstruction such as regularity, sensitivity to noise, multi-resolution, and accuracy. This research is part of an effort to develop small-footprint heat flux sensors able to measure high-frequency fluctuations on test articles in a hypersonic wind tunnel with a small curvature radius. In the present theoretical/numerical study a multi-resolution formulation for the direct and inverse reconstruction of the heat flux from temperature sensors distributed over a multidimensional solid in a hypersonic flow was developed and validated. The solution method determines the thermal response by approximating the system Green's function with the Galerkin method and optimizes the heat flux distribution by fitting the distributed surface temperature data. Coating and glue layers are treated as separate domains for which the Green's function is obtained independently. Connection conditions for the system Green's function are derived by imposing continuity of heat flux and temperature concurrently at all interfaces. The solution heat flux is decomposed on a space-time basis with the temporal basis a multi-resolution wavelet with arbitrary scaling function. Quadrature formulas for the convolution of wavelets and the Green's function, a reconstruction approach based on isoparametric mapping of three-dimensional geometries, and a boundary wavelet approach for inverse problems were developed and verified. This approach was validated against turbulent conjugate heat transfer simulations at Mach 6 on a blunted wedge at 0 angle of attack and wind tunnel experiments of round impinging jet at Mach 0.7 It was found that multidimensional effects were important near the wedge shoulder in the short time scale, that the L-curve regularization needed to be locally corrected to analyze transitional flows and that proper regularization led to sub-cell resolution of the inverse problem. While the L2 regularization techniques are accurate they are also computationally inefficient and lack mathematical rigor. Optimal non-linear estimators were researched both as means to promote sparsity in the regularization and to pre-threshold the inverse heat conduction problem. A novel class of nonlinear estimators is presented and validated against wind tunnel experiments for a flat-faced cylinder also at Mach 6. The new approach to hypersonic heat flux reconstruction from discrete temperature data developed in this thesis is more efficient and accurate than existing techniques. / Doctor of Philosophy / The harsh environment supported by hypersonic flows is characterized by high-frequency turbulent bursts, acoustic noise, and vibrations that pollute the signals of the sensors that probe at high frequencies the state of the boundary layers developing on the walls. This research describes the search for optimal estimators of the noisy signal, i.e., those that lead to the maximum attenuation of the risk of error pollution by non-coherent scales. This research shows that linear estimators perform poorly at high-frequency and non-linear estimators can be optimized over a sparse projection of the signal in a discrete wavelet basis. Optimal non-linear estimators are developed and validated for wind tunnel experiments conducted at Mach 6 in the Advanced Propulsion and Power Laboratory at Virginia Tech.

No description available.

This study documents the integration and testing of a new variety of unsteady surface temperature gages designed to operate in high speed flow. Heat flux through the surface of the test article was determined from the unsteady temperature by applying a 3D reconstruction algorithm based on a Green's function approach. The surface temperature gages used in this work were 1.59 mm inserts designed to maximize material matching with the test article, in this case 316 stainless steel. A series of benchtop experiments were first performed to understand the individual properties of the gage and determine measurement uncertainty. Prior to testing, all temperature gages are calibrated using an environmental chamber. Gages were installed into slugs of several materials and subjected to a heated jet with a total temperature of 620 K to examine the effects of material mismatch. A shock tube with a notional operating Mach of 2.6 was used to determine the thermal response of the gages as a function of time. In both tests, reference Medtherm Schmidt-Boelter gages ensure consistent heat fluxes are applied across all runs. The time response of the entire electrical system was determined by subjecting the gage to a nanosecond scale laser pulse. Two experimental campaigns were conducted in Virginia Tech's Hypersonic Wind Tunnel. First, gages were integrated into a flat plate test article and subjected to a notionally 2D Mach 3 flow. Tunnel total pressures and temperatures ranged from 793-876 kPa and 493-594 K, respectively. A reference 3.18 mm Medtherm Schmidt-Boelter gage was also installed for comparison. All temperature data are reconstructed using the algorithm to determine heat flux. The second test campaign utilized a flat-faced cylindrical test article in a notionally axisymmetric Mach 6 flow environment. Flow total pressures and temperatures ranged from 8375-8928 kPa and 485.5-622 K. respectively. The Fay-Riddell analytical method was applied to the resulting temperature traces in order to infer the heat flux at the stagnation point for comparison with the reconstructed heat flux. This experiment was complimented with steady, 3D CFD in order to understand the temperature variation across the test article. Both campaigns demonstrate good agreement between the heat flux reconstructed from surface temperatures measured using the new gage, reference measurements, and simulations/analytical methods. The importance of material matching is highlighted during this study. The performance of this gage is shown to exceed the current state-of-the-art, opening the possibility for future analysis of phenomenon present in high-speed flow. / Doctor of Philosophy / At very fast speeds, it is important to understand how the temperatures of surfaces change with time. Traditional devices which can measure surface temperatures have a number of weaknesses, and to address these a new type of surface temperature device has been designed. By using computational methods, one can determine how much energy is being transferred through the surface by measuring how the surface temperature changes over time. A series of laboratory experiments were conducted to understand how this new instrument compares to the current state-of-the-art. Two experimental campaigns were then conducted to test the temperature gages. The first experiment used a simple flat plate geometry in a flow 3 times the speed of sound to serve as a benchmark test case, as the flow over a flat plate is well understood. The second test utilized a flat-faced cylindrical test article in a flow 6 times the speed of sound. The results of this test was compared to exact solutions and flow simulations. The result of this study is a well quantified tool to study how energy flows through a body subjected to very high speed flow, which will enable further study of the complicated thermal environments experienced at high speeds.

The effects of using an insert Heat Flux Microsensor (HFM) versus an HFM deposited directly on a turbine blade to measure heat flux in a transonic cascade are investigated. The HFM is a thin-film sensor, 6.35 mm (0.250") in diameter (for an insert gage, including the housing) which measures heat flux and surface temperature. The thermal time response of both gages was modeled using a 1-D, finite difference technique and a 2-D, finite element solver. The transient response of the directly deposited gage was also tested against insert gages using an unsteady shock wave in a bench test setup and using a laser of known output. The effects of physical gage offset from the blade surface were also investigated. The physical offset of an insert HFM near the stagnation point on the suction side of a turbine blade was intentionally varied and the average heat transfer coefficient measured. Turbulence grids were used to study how offset affects the heat transfer coefficient with freestream turbulence added to the flow. The time constant of the directly deposited gage was measured to be 856 ms compared to less than 30 ms for the insert gages. Model results predict less than 20 ms for both gages and rule out the anodization layer (used for electrical isolation of the directly deposited gage from the blade) as the cause for the directly deposited gage's much slower time response. Offsets of ± 0.254 mm (0.010") at the gage location with an estimated boundary layer thickness of 0.10 mm (0.004") produced a higher average heat transfer coefficient than the 0.000" offset case. Using an insert HFM resulted in a higher average heat transfer coefficient than using the directly deposited gage and reduced the effects of freestream turbulence. To accurately measure heat transfer coefficients and the effects of freestream turbulence, the disruption of the flow caused by a gage must be minimized. Depositing a gage directly on the blade minimizes the effects of offset, but the cause of the slow time response must first be resolved if high speed data is to be taken. / Master of Science

The effects of grid generated freestream turbulence on surface heat transfer to turbine blades were measured experimentally. Time-resolved and unsteady heat flux measurements were made with Heat Flux Microsensors at two positions on the suction side of turbine blades. The experiments were conducted on a stationary cascade of aluminum turbine blades for heated runs at transonic conditions. Non-dimensional flow parameters were matched to actual engine conditions including the design exit Mach number of 1.26 and the gas-to-wall temperature ratio of 1.4. Methods for determining the adiabatic wall temperature and heat transfer coefficient are presented and the results are compared to computer predictions for these blades. Heat transfer measurements were taken with a new, directly deposited HFM gage near the trailing edge shock on nitrogen cooled blades. The average heat transfer coefficient for Mach 1.26 was 765 W/(m² °C) and matched well with a predicted value of 738 W/(m² °C). Freestream turbulence effects were studied at a second gage location 1.0 cm from the stagnation point on uncooled blades. Results at this location show an increase in freestream turbulence from 1 % to 8% led to a 15% increase of the average heat transfer coefficient and also matched well with predictions. The fast response time of the HFM illustrated graphically the increase in energy spectra due to freestream turbulence at the 0 - 10kHz range. The heat flux turbulence intensity (Tu_q) was defined as another physical quantity important to turbine blade heat transfer. / Master of Science