Nicholas D. Francis

Jet impingement drying of a moist porous solid

Abstract This paper investigates the thermal characteristics of a continuous industrial drying process for semi-porous textile composites. The conservation of mass, momentum and energy are written for a partially saturate porous fiber layer attached to a solid-backing layer. The numerical solution of the one-dimensional and transient conservation equations provides the temperature, volumetric saturation and gas phase pressure distributions in the moist porous solid and the temperature distribution in the solid-backing layer. During the wet region drying period, continuous liquid exists in the pore space, the moisture transport within the solid is described by the Darcy form of the momentum equation. The moisture transport in the sorption region is described by a bound liquid diffusion and gas phase transport. For the jet impingement type dryer, it is assumed that the penetration of the flow field into the porous solid is small (assumed valid due to the presence of the solid backing). The enhanced transport coefficients at the drying surface are estimated with the use of the Kolmogoroff theory of isotropic turbulence. This theory provides correlations for the heat and mass transfer coefficients from the fluid properties and the turbulent energy dissipation rate in the fluid. The model results of the continuous industrial drying process are compared to independent experimental temperature and global moisture content measurements taken in an operational industrial dryer. From the model analysis and experimental data, the heat flux conditions at the drying surface dictate the manner in which the solid is dried. The heat transfer coefficients considered are in the range of 20–130 W m−2K−1 and necessarily affect the manner in which moisture transport occurs within the solid. It is seen that the lower heat transfer coefficients more accurately represent the internal transport phenomena occurring during the drying process and the heating of the solid. The transport coefficients are compared to previously obtained empirical results.

Thermally induced natural convection effects in Yucca Mountain drifts.

Thermally induced natural convection from the heat produced by emplaced waste packages is an important heat and mass transfer mechanism within the Yucca Mountain Project (YMP) drifts. Various models for analyzing natural convection have been employed. The equivalent porous medium approach using Darcys law has been used in many YMP applications. However, this approach has questionable fidelity, especially for turbulent flow conditions. Computational fluid dynamics (CFD), which is based on the fundamental Navier-Stokes equations, is currently being evaluated as a technique to calculate thermally induced natural convection in YMP. Data-model comparisons for turbulent flow conditions show good agreement of CFD predictions with existing experiments including YMP-specific data.

Experimental and Numerical Analysis of the Drying Characteristics of Modular Carpet Tiles

Various backing materials developed for modular carpet tiles can be heat sensitive and may be damaged if exposed to high temperatures for prolonged periods. This paper presents a model to predict the drying characteristics of carpet tiles for a batch drying process. Drying characteristics are also determined experimentally in a controlled laboratory environment. The analytical framework consists of a three-layer lumped parameter configuration to calculate temperature and moisture profiles. The capillary movement of liquid water is not explicitly calculated in the model, but a sufficient amount of moisture in the solid is assumed to be available to maintain a constant rate drying period. The three-layer model comprises a moist top fiber layer, a middle layer also containing moisture, and a backing layer. To estimate the falling rate drying period, the middle layer is incorporated into the model. In the mathematical model, the top layer is initially saturated with water, but then dries out, providing greater resistance to mass transfer. Governing equations are obtained from mass and energy balances as well as relevant rate equations. A lumped parameter approach is applied to each layer. The energy balance for each layer provides a first-order ordinary differential equation with variable coefficients. A numerical routine is constructed to solve the governing equations.

CFD Calculation of Internal Natural Convection in the Annulus between Horizontal Concentric Cylinders

The objective of this heat transfer and fluid flow study is to assess the ability of a computational fluid dynamics (CFD) code to reproduce the experimental results, numerical simulation results, and heat transfer correlation equations developed in the literature for natural convection heat transfer within the annulus of horizontal concentric cylinders. In the literature, a variety of heat transfer expressions have been developed to compute average equivalent thermal conductivities. However, the expressions have been primarily developed for very small inner and outer cylinder radii and gap-widths. In this comparative study, interest is primarily focused on large gap widths (on the order of half meter or greater) and large radius ratios. From the steady-state CFD analysis it is found that the concentric cylinder models for the larger geometries compare favorably to the results of the Kuehn and Goldstein correlations in the Rayleigh number range of about 10{sup 5} to 10{sup 8} (a range that encompasses the laminar to turbulent transition). For Rayleigh numbers greater than 10{sup 8}, both numerical simulations and experimental data (from the literature) are consistent and result in slightly lower equivalent thermal conductivities than those obtained from the Kuehn and Goldstein correlations.

Preliminary thermomechanical results of a heater test in welded tuff

Abstract The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Comparison of CFD Calculations With Experimental Results for the YMP Scaled Natural Convection Tests

The Yucca Mountain Project (YMP) is currently designing a geologic repository for high level nuclear waste. The design encompasses two distinct phases, the pre-closure period where temperatures within the repository will be controlled by active ventilation, and the post-closure period where the repository will be sealed. A prerequisite for designing the repository is the ability to both understand and control the heat generated from the decay of the nuclear waste. This decay heat affects the performance of both the waste packages and the emplacement drift. The ability to accurately model the complex heat transfer within the repository is critical to the understanding of the repository performance. Currently, computational fluid dynamics codes are being used to model the post-closure performance of the repository. Prior to using the codes on the project they were required to be thoroughly validated. Eight pilot-scale tests were performed at the Department of Energy North Las Vegas Atlas Facility to evaluate the processes that govern thermal transport in an environment that scales to the proposed repository environment during the post closure period. The tests were conducted at two geometric scales (25 and 44% of full scale), with and without drip shields, and under both uniform and distributed heat loads. The tests provided YMP specific data for model validation. A separate CFD model was developed for each of the four test configurations. The models included the major components of the experiment, including the waste packages (heated steel canisters), invert floor, and emplacement drift (insulated concrete pipe). The calculated model temperatures of the surfaces and fluids, and velocities, are compared with experimental data.Copyright

In-drift natural convection analysis of the low temperature operating mode design.

Abstract Yucca Mountain has been designated as the nation’s high-level radioactive waste repository, and the U.S. Department of Energy has been approved to apply to the U.S. Nuclear Regulatory Commission for a license to construct a repository. The temperature and humidity inside the emplacement drift will affect the degradation rate of the waste packages and waste forms as well as the quantity of water available to transport dissolved radionuclides out of the waste canister. Thermal radiation and turbulent natural convection are the main modes of heat transfer inside the drift. This paper presents the result of three-dimensional computational fluid dynamics simulations of a segment of emplacement drift. The model contained the three main types of waste packages and was run at the time that the peak waste package temperatures are expected. Results show that thermal radiation is the dominant mode of heat transfer inside the drift. Natural convection affects the variation in surface temperature on the hot waste packages and can account for a large fraction of the heat transfer for the colder waste packages. The paper also presents the sensitivity of model results to uncertainties in several input parameters. The sensitivity study shows that the uncertainty in peak waste package temperatures due to in-drift parameters is <3°C.

Two-Dimensional CFD Calculations for YMP Natural Convection Tests

Processes important to the performance of a nuclear waste repository include cooling of spent nuclear fuel casks emplaced in tunnels bored into volcanic tuff. The geometry consists of an emplacement drift (tunnel), waste package, and a layer of gravel invert providing a flow barrier at the bottom of the drift. During the postclosure period, a drip shield, which is a thin metal sheet that covers the waste package, is also included. The geometry is in essence an enclosed annulus where the heated inner cylinder represents the waste package and the outer cylinder represents the emplacement drift. The waste package is below the centerline of the drift, so the geometry is eccentric. The invert floor impedes the flow in the lower portion of the annulus. Yucca Mountain Project (YMP) has developed two natural convection tests (25% and 44%-scale) in order to understand the heat transfer and fluid flow processes associated with this geometry. Measurements of temperature and fluid velocity are the primary results of the tests. Computational fluid dynamics (CFD) is used to determine the heating characteristics associated with the natural convection tests. The CFD analysis described in this report is two-dimensional. Steady-state annulus temperature distributions and flow fields are presented for different experimental heating conditions. Maximum heat source temperatures from the CFD models range from 37C to 50C for cases without a drip shield and from 40C to 56C for cases with a drip shield. Hand calculations for a simplified geometry without a drip shield resulted in a temperature of 42.8C for the 25%-scale configuration and 43.4C for the 44%-scale configuration.

CFD Calculation of Internal Natural Convection in the Annulus Between Horizontal Concentric Cylinders

Thermally-induced natural convection heat transfer in the annulus between horizontal concentric cylinders has been studied using the commercial code Fluent. The boundary layers are meshed all the way to the wall because forced convection wall functions are not appropriate. Various oneand two-equation turbulence models have been considered. Overall and local heat transfer rates are compared with existing experimental data.Copyright