Ali Siahpush

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.

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

Preliminary Design for Conventional and Compact Secondary Heat Exchanger in a Molten Salt Reactor

In this study, the heat transfer coolant utilized in the heat exchanger is a molten salt, which transfers thermal energy to water (steam) for power production by a supercritical Rankine (25MPa) or subcritical Rankine (17MPa) cycle. Molten salts are excellent coolants, with 25% higher volumetric heat capacity than pressurized water, and nearly five times that of liquid sodium. The greater heat capacity of molten salts results in more compact components like pumps and heat exchangers. However, the use of a molten salt provides potential materials compatibility issues. After studying a variety of individual molten salt mixtures, chlorides and fluorides have been given the most serious consideration because of their heat transport and transfer characteristics.In this study thermal designs of conventional (shell and tube), and compact (printed circuit) heat exchangers are carried out and compared for a given thermal duty. There are a couple of main issues that need to be addressed before this technology could be commercialized. The main issue is with the material compatibility of molten salts (especially fluoride salts) and secondarily, with the pressure difference across the heat exchanger. The heat exchanger’s primary side pressure is nearly atmospheric and the secondary side (power production) is pressurized to about 25MPa for supercritical cycle and 17MPa for subcritical cycle. Further in the analysis, the comparison of both the cycles will be carried out with recommendations.© 2012 ASME

ASME Material Challenges for Advance Reactor Concepts

This study presents the material challenges associated with Advanced Reactor Concepts (ARC) such as the Advanced High Temperature Reactor (AHTR). ARCs are the next generation concepts focusing on power production and providing thermal energy for industrial applications. The efficient transfer of energy for industrial applications depends on the ability to incorporate cost-effective heat exchangers between the nuclear heat transport system and industrial process heat transport system.The heat exchanger required for AHTR is subjected to a unique set of conditions that introduce several design challenges not encountered in standard heat exchangers. The corrosive molten salts, especially at higher temperatures, require materials throughout the system to avoid corrosion, and adverse high-temperature effects, such as creep. Given the very high steam generator pressure of the supercritical steam cycle, it is anticipated that water tube and molten salt shell steam generators heat exchanger will be used.In this paper, the American Society of Mechanical Engineers (ASME) Section III and Section VIII requirements (acceptance criteria) are discussed. Also, the ASME material acceptance criteria (ASME Section II, Part D) for high temperature environments are presented. Finally, lack of ASME acceptance criteria for thermal design and analysis are discussed with potential benefit.Copyright

Numerical Investigation of Melting With Internal Heat Generation in a Vertical Cylindrical Geometry

There have been significant efforts by the heat transfer community to investigate the melting phenomenon of materials. These efforts have included the analytical development of equations to represent melting, the numerical development of computer codes to assist in the modeling, and the collection of experimental data. The understanding of the melting phenomenon has application in several areas of interest, for example, the melting of a phase change material used as a thermal storage medium as well as the melting of the fuel bundle in a nuclear power plant during an accident scenario. The objective of this paper is to present a numerical investigation, using computational fluid dynamics (CFD), of melting with internal heat generation for a vertical cylindrical geometry. As a precursor to the development of this numerical model, two classical configurations were also modeled. The first configuration consists of pure convection (no phase change) of a liquid with an external heat source and the second is melting with an externally applied heat source. For both of these two configurations, the numerical results were compared with experimental data from previous work.Copyright

Experimental and Scale Analysis of a Solid/Liquid Phase Change Thermal Energy Storage System

ABSTRACT A detailed experimental study has been carried out to evaluate the heat transfer performance of a solid/liquid phase-change thermal energy storage system. The phase-change material, 99% pure eicosane with a melting temperature of 36.5°C, was contained in a vertically oriented test cylinder that was cooled or heated at its outside boundary, resulting in radially inward freezing or melting, respectively. Detailed quantitative time-dependent temperature distributions and melt-front motion and shape data were obtained. In the freezing case study, a mathematical model was developed based on a one-dimensional analysis, which considered heat conduction as the only mode of heat transfer. In the melting case study, a heat transfer scale analysis was used to help interpret the data and development of heat transfer correlations. In the melting scale analysis, conduction heat transfer in the solid and natural convection heat transfer in liquid were considered. Comparison of experimental data with scale analysis predictions of the solid-liquid interface position and temperature distribution was performed. The analytical results agreed, in the worst case, within 10% of the experimental results in both melting and freezing cases. In the case of melting, scale analysis results agreed within 5% (after initial superheat disappeared in 50 minutes) with experimental results, and experimental results confirm the existence of four melting regions.

Simple Heat Transfer Experiment to Evaluate the Solid/Liquid Phase Change Thermal Energy Storage System

A detailed experimental freezing study, designed for undergraduate students, has been carried out to evaluate the heat transfer performance of a solid/liquid phase-change thermal energy storage system. The test vessel system, experimental procedure and results, and analytical solutions are discussed. The phase-change material (PCM) is contained in a vertically oriented test cylinder that is cooled at its outside boundary, resulting in radially inward freezing. Detailed quantitative time-dependent volumetric temperature distributions and freeze-front motion and shape data were experimentally obtained. To fully understand the behavior of the eicosane, four freezing tests were performed with different temperature set points as low as 10°C.In the analysis, results of a test in which molten eicosane, initially at 50°C, was solidified and brought to a final temperature of 10°C are presented. In the freezing case study, a mathematical model based on a one-dimensional analysis, which considered heat conduction as the only mode of heat transfer was developed. The phase-change medium, 99% pure eicosane (C20H42) was chosen as the PCM. Eicosane is desirable because its fusion temperature is just slightly higher than ambient temperature (36.5°C), which is convenient for phase-change experimentation. Low-temperature heating can be used to melt the PCM and ambient-temperature cooling can be used to re-freeze it.To evaluate the inward radius of fusion, several analytical and experimental approaches were considered. These approaches were (1) experimental method; (2) conduction model; (3) integral method; and (4) cumulative heat transfer method. Comparison of these methods reveals excellent agreement. In most cases, the heat transfer estimated from the freezing-front analysis was slightly higher than the heat transfer evaluated from the time-series data. The largest discrepancy occurs at fifty minutes into the experiment (10.7%).Copyright

Scale/Analytical Analyses of Freezing and Convective Melting with Internal Heat Generation

Using a scale/analytical analysis approach, we model phase change (melting) for pure materials which generate constant internal heat generation 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. The analysis also consider constant heat flux (in a cylindrical geometry). We show the time scales in which conduction and convection heat transfer dominate.Copyright

Conceptual Design of a MEDE Treatment System for Sodium Bonded Fuel

Unirradiated sodium bonded metal fuel and casting scrap material containing highly enriched uranium (HEU) is stored at the Materials and Fuels Complex (MFC) on the Idaho National Laboratory (INL). This material, which includes intact fuel assemblies and elements from the Fast Flux Test Facility (FFTF) and Experimental Breeder Reactor-II (EBR-II) reactors as well as scrap material from the casting of these fuels, has no current use under the terminated reactor programs for both facilities. The Department of Energy (DOE), under the Sodium-Bonded Spent Nuclear Fuel Treatment Record of Decision (ROD), has determined that this material could be prepared and transferred to an off-site facility for processing and eventual fabrication of fuel for commercial nuclear reactors. A plan is being developed to prepare, package and transfer this material to the DOE High Enriched Uranium Disposition Program Office (HDPO), located at the Y-12 National Security Complex in Oak Ridge, Tennessee. Disposition of the sodium bonded material will require separating the elemental sodium from the metallic uranium fuel. A sodium distillation process known as MEDE (Melt-Drain-Evaporate), will be used for the separation process. The casting scrap material needs to be sorted to remove any foreign material or fines that are not acceptable to the HDPO program. Once all elements have been cut and loaded into baskets, they are then loaded into an evaporation chamber as the first step in the MEDE process. The chamber will be sealed and the pressure reduced to approximately 200 mtorr. The chamber will then be heated as high as 650 oC, causing the sodium to melt and then vaporize. The vapor phase sodium will be driven into an outlet line where it is condensed and drained into a receiver vessel. Once the evaporation operation is complete, the system is de-energized and returned to atmospheric pressure. This paper describes the MEDE process as well as a general overview of the furnace systems, as necessary, to complete the MEDE process.

COMPARISON OF COMPUTATIONAL AND QUASI-STATIC SOLUTIONS OF PHASE CHANGE WITH HEAT GENERATION

In this paper, we investigate the effects that the volumetric heat generation has on the movement and steady-state location of a solid-liquid phase change front in melting and freezing processes. Volumetric heat generation enhances melting and impedes freezing. This phenomenon occurs in nuclear, geologic, cryogenic and material processing applications. We compare the results from a FLUENT computational model with analytical results of a quasi-static solution of the governing equations. These models are applied for constant surface temperature boundary conditions and various volumetric heat generation values in cylindrical plane wall and spherical geometries. The quasi-static method results in an exact steadystate solution which shows that the location of the phase change front is inversely proportional to the square root of the volumetric heat generation. This method is valid for Stefan numbers less than one and the computational results for this regime give excellent agreement with the analytical model, thereby validating the technique and solutions. For the sake of comparison, we also plot the analytical model and computational results for Stefan numbers of one and greater. The quasi-static analytical solution converges more rapidly to the steady-state value than the computational solution. As expected, at longer time intervals, both the analytical and computational solutions converge to the exact steady-state solution.