John Crepeau

Phase Change Heat Transfer Enhancement Using Copper Porous Foam

A detailed experimental and analytical study has been performed to evaluate how copper porous foam (CPF) enhances the heat transfer performance in a cylindrical solid/liquid phase change thermal energy storage system. The CPF used in this study had a 95% porosity and the phase change material (PCM) was 99% pure eicosane. The PCM and CPF were contained in a vertical cylinder where the temperature at its radial boundary was held constant, allowing both inward freezing and melting of the PCM. Detailed quantitative time-dependent volumetric temperature distributions and melt/freeze front motion and shape data were obtained. As the material changed phase, a thermal resistance layer built up, resulting in a reduced heat transfer rate between the surface of the container and the phase change front. In the freezing analysis, we analytically determined the effective thermal conductivity of the combined PCM/CPF system and the results compared well to the experimental values. The CPF increased the effective thermal conductivity from 0.423 W/m K to 3.06 W/mK. For the melting studies, we employed a heat transfer scaling analysis to model the system and develop heat transfer correlations. The scaling analysis predictions closely matched the experimental data of the solid/liquid interface position and Nusselt number.

Options Extending the Applicability of High-Temperature Irradiation-Resistant Thermocouples

Abstract Several options have been identified that could further enhance the reliability and extend the applicability of high-temperature irradiation-resistant thermocouples (HTIR-TCs) developed by the Idaho National Laboratory (INL) for in-pile testing, allowing their use in temperature applications as high as 1800%C.The INL and the University of Idaho (UI) investigated these options with the ultimate objective of providing recommendations for alternate thermocouple designs that are optimized for various applications. This paper reports results from INL/UI investigations. Results are reported from tests completed to evaluate the ductility, resolution, transient response, and stability of thermocouples made from specially formulated alloys of molybdenum and niobium,not considered in initial HTIR-TC development. In addition, this paper reports insights gained by comparing the performance of HTIR-TCs fabricated with various heat Ntreatments and alternate geometries.

On the spectral entropy behavior of self-organizing processes

Using the spectral entropy techniques of Powell and Percival, we demonstrate a significant decrease in the spectral entropy of systems defined by both the logistic and Lorenz equations, as the control parameter increases past the initial bifurcation. This decrease in entropy may be related to self-organizing processes and the formation of coherent structures in the laminar to turbulence transition

The effect of variable viscosity in double diffusion problem of MHD from a porous boundary with Internal Heat Generation

The steady, laminar boundary layer, two-dimensional Magnetohydrodynamics (MHD) flow past a continuously moving (with constant velocity) semi-infinite vertical porous plate is studied taking into account the Dufour and Soret effects (Double diffusion) on variation of fluid viscosity with temperature. The effect of an exponential form of Internal Heat Generation (IHG) is also considered. The fluid viscosity is assumed to vary as a linear function of temperature. The governing fundamental equations of the problem are obtained by using the usual similarity technique. The local similarity solutions of the transformed dimensionless equations are solved numerically. The effects of the governing parameters within the boundary layer of the flow field are studied and the corresponding set of numerical results for the non-dimensional velocity, temperature and concentration profiles as well as the skin friction parameter, Nusselt number and Sherwood number are illustrated and displayed with the aid of graphs and tables to show typical trends of the solutions considering with and without IHG. It has been found that the results of the present study are completely different from similar problems in the absence of double diffusion and IHG.

Extension wire for high temperature irradiation resistant thermocouples

In an effort to reduce production costs for the doped molybdenum/niobium alloy high temperature irradiation resistant thermocouples (HTIR-TCs) recently developed by the Idaho National Laboratory, a series of evaluations were completed to identify an optimum compensating extension cable. Results indicate that of those combinations tested, two inexpensive, commercially-available copper–nickel alloy wires approximate the low temperature (0 °C to 500 °C) thermoelectric output of KW–Mo (molybdenum doped with tungsten and potassium silicate) versus Nb–1%Zr in HTIR-TCs. For lower temperatures (0 °C to 150 °C), which is the region where a soft extension cable is most often located, results indicate that the thermocouple emf is best replicated by the Cu–3.5%Ni versus Cu–5%Ni combination. At higher temperatures (300 °C to 500 °C), data suggest that the Cu–5%Ni versus Cu–10%Ni combination may yield data closer to those obtained with KW–Mo versus Nb–1%Zr wires.

Integral Solutions of Phase Change With Internal Heat Generation

This paper presents solutions to a one-dimensional solid-liquid phase change problem using the integral method for a semi-infinite material that generates internal heat. The analysis assumed a quadratic temperature profile and a constant temperature boundary condition on the exposed surface. We derived a differential equation for the solidification thickness as a function of the internal heat generation (IHG) and the Stefan number, which includes the temperature of the boundary. Plots of the numerical solutions for various values of the IHG and Stefan number show the time-dependant behavior of both the melting and solidification distances and rates. The IHG of the material opposes solidification and enhances melting. The differential equation shows that in steady-state, the thickness of the solidification band is inversely related to the square root of the IHG. The model also shows that the melting rate initially decreases and reaches a local minimum, then increases to an asymptotic value.Copyright

Unstable vortices in the near region of an internal flow cavity

This paper presents experimental data taken in the forward region of a separated internal free-shear layer produced in an internal cavity flowfield. It has been found that in the region very near the forward restrictor, experimental velocity profiles agree closely with the exact Stuart instability velocity profile with various values of a steepness parameter. Reynolds shear-stress profiles suggest the presence of counter-rotat ing longitudinal vortices. Spectral analysis by the maximum entropy method of the time samples within the vortices indicates subharmonic and harmonic components of the fundamental frequency, with a weak indication of the fundamental frequency itself. NTERNAL flow cavity configurations occur in many engineering applications, from segmented solid propellant rocket motors to dump combustors in ramjet engines. A recent review1 indicates that information concerning the characteristics of the shear layers at the leading edge of flow cavities is not sufficiently detailed to identify clearly the physical mechanisms involved in shear layer instability and transition. The objective of this paper is to present experimental measurements of the flow field in the near region of an internal cavity flow. The results presented suggest the occurrence of unstable spiral vortices in the steep shear layers very near the forward restrictors in an internal flow cavity. The near region of the free-shear layer in an internal cavity flow plays an important role in the development of the downstream characteristics of the flow, especially in regard to the formation of axially directed spiral vortices and large, spanwise roll vortices. There is an important need to understand the processes that occur very near the forward restrictors in an internal cavity flow, a region in which the velocity profiles are very steep, and in which three-dimensional instabilities are in evidence. For the flow configuration used here, as indicated in Figs. 1 and 2, the presence of a beveled edge on the upstream side of each of the upper and lower forward restrictors produces a flow along separation lines away from the forward bevel lines, and over the sharp edges of the restrictors. The flow over each of the restrictors appears to be similar to the flow of parallel streams as studied by Stuart.2 Comparison of measured velocity profiles with the profiles of Stuart show excellent agreement. Profiles of Reynolds shear stress in the forward region of the cavity flow show evidence of axially directed, counter-rotating vortices produced along the shear layer that are time-dependent and that rapidly move apart in the downstream direction. We have characterized these flow structures as unstable velocity bursts. Time samples of the axial velocity signals within the bursts were obtained, and the maximum entropy

SCALE ANALYSIS OF CONVECTIVE MELTING WITH INTERNAL HEAT GENERATION

Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.Copyright

Entropy generation in bypass transitional boundary layer flows

The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics (CFD) turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation (DNS) data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Reθ, skin-friction coefficient, Cf, approximate entropy generation rates, S‴, dissipation coefficient, Cd, and the intermittency, γ, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes (Rans) turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition Sst k- ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other Rans models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.

A Brief History of the T4 Radiation Law

Since the 1700s, natural philosophers understood that heat exchange between two bodies was not precisely linearly dependent on the temperature difference, and that at high temperatures the discrepancy became greater. Over the years, many models were developed with varying degrees of success. The lack of success was due to the difficulty obtaining accurate experimental data, and a lack of knowledge of the fundamental mechanisms underlying radiation heat exchange. Josef Stefan, of the University of Vienna, compiled data taken by a number of researchers who used various methods to obtain their data, and in 1879 proposed a unique relation to model the dependence of radiative heat exchange on the temperature: the T4 law. Stefan’s model was met with some skepticism and was not widely accepted by his colleagues. His former student, Ludwig Boltzmann, who by then had taken a position at the University of Graz in Austria, felt that there was some truth to the empirical model proposed by his mentor. Boltzmann proceeded to show in 1884, treating electromagnetic radiation as the working fluid in a Carnot cycle, that in fact the T4 law was correct. By the time that Boltzmann published his thermodynamic derivation of the radiation law, physicists became interested in the fundamental nature of electromagnetic radiation and its relation to energy, specifically determining the frequency distribution of blackbody radiation. Among this group of investigators was Wilhelm Wien, working at Physikalisch-Technische Reichsanstalt in Charlottenburg, Berlin. He proposed a relation stating that the wavelength at which the maximum amount of radiation was emitted occurred when the product of the wavelength and the temperature was equal to a constant. This became known as Wien’s Displacement Law, which he deduced this by imagining an expanding and contracting cavity, filled with radiation. Later, he combined his Displacement Law with the T4 law to give a blackbody spectrum which was accurate over some ranges, but diverged in the far infrared. Max Planck, at the University of Berlin, built on Wien’s model but, as Planck himself stated, “the energy of radiation is distributed in a completely irregular manner among the individual partial vibrations...” This “irregular” or discrete treatment of the radiation became the basis for quantum mechanics and a revolution in physics. This paper will present brief biographies of the four pillars of the T4 radiation law, Stefan, Boltzmann, Wien and Planck, and outline the methodologies used to obtain their results.Copyright