Miles Greiner

Simulations of Three-Dimensional Flow and Augmented Heat Transfer in a Symmetrically Grooved Channel

Navier-Stokes simulations of three-dimensional flow and augmented convection in a channel with symmetric, transverse grooves on two opposite walls were performed for 180 ≤Re≤ 1600 using the spectral element technique. A series of flow transitions was observed as the Reynolds number was increased, from steady two-dimensional flow, to traveling two and three-dimensional wave structures, and finally to three-dimensional mixing

Two-Dimensional Simulations of Enhanced Heat Transfer in an Intermittently Grooved Channel

Two-dimensional Navier-Stokes simulations of heat and momentum transport in an intermittently grooved passage are performed using the spectral element technique for the Reynolds number range 600≤Re≤1800. The computational domain has seven contiguous transverse grooves cut symmetrically into opposite walls, followed by a flat section with the same length. Periodic inflow/outflow boundary conditions are employed. The development and decay of unsteady flow is observed in the grooved and flat sections, respectively. The axial variation of the unsteady component of velocity is compared to the local heat transfer, shear stress and pressure gradient

Direct Numerical Simulation of Three-Dimensional Flow and Augmented Heat Transfer in a Grooved Channel

Direct numerical simulations of three-dimensional flow and augmented convective heat transfer in a transversely grooved channel are presented for the Reynolds number range 140 < Re < 2000. These calculations employ the spectral element technique. Multiple flow transitions are documented as the Reynolds number increases, from steady two-dimensional flow through broad-banded unsteady three-dimensional mixing. Three-dimensional simulations correctly predict the Reynolds-number-independent friction factor behavior of this flow and quantify its heat transfer to within 16 percent of measured values. Two-dimensional simulations, however, incorrectly predict laminar-like friction factor and heat transfer behaviors

Radiation Heat Transfer and Reaction Chemistry Models for Risk Assessment Compatible Fire Simulations

Risk assessment studies for hazardous material packages require fire response prediction tools that are both accurate and rapid. This article describes the theoretically based, semiempirical reaction chemistry and radiation heat transfer models for large, optically dense pool fires incorporated in the ISIS-3D CFD software. The chemistry model employs four separate reactions (two produce radiating soot). The heat transfer model divides the computational domain into the diffusely radiative fire and its nonparticipating environment. ISIS-3D simulations are performed on a 6-m square JP8 pool fire experiment in which the soot temperature and volume fraction are measured. The reaction rate and soot formation parameters of the chemistry model are determined based on a comparison of the simulation with the measured data. Simulations are then performed on an experiment that measures the temperature of a pipe calorimeter suspended over the leeside of a 19-m-diameter JP8 fuel pool fire with a 9.5 m/s crosswind. The ...

Use of Fuel Assembly/Backfill Gas Effective Thermal Conductivity Models to Predict Basket and Fuel Cladding Temperatures Within a Rail Package During Normal Transport

Two-dimensional finite element thermal simulations of a generic rail package designed to transport twenty-one spent PWR assemblies were performed for normal transport conditions. Effective thermal conductivity models were employed within the fuel assembly/backfill gas region. Those conductivity models were developed by other investigators assuming the basket wall temperature is uniform. They are typically used to predict the maximum fuel cladding temperature near the package center. The cladding temperature must not exceed specified limits during normal transport. This condition limits the number and heat generation rate of fuel assembles that can transported. The current work shows the support basket wall temperatures in the periphery of the package are highly non-uniform. Moreover the thermal resistance of those regions significantly affects the maximum fuel clad temperature near the package center. This brings the validity of the fuel/backfill gas thermal conductivity models into question. The non-uniform basket wall temperature profiles quantified in this work will be used in future numerical and experimental studies to develop new thermal models of the fuel assembly/backfill gas regions. This will be an iterative process, since the assembly/backfill model affects the predicted basket wall temperature profiles.Copyright

Three-Dimensional Simulations of Enhanced Heat Transfer in a Flat Passage Downstream From a Grooved Channel

Navier-Stokes simulations of three-dimensional flow and augmented convection in a flat passage downstream from a fully developed channel with symmetric, transverse grooves on two opposite walls were performed for 405 < Re < 764 using the spectral element technique. Unsteady flow that develops in the grooved region persists several groove-lengths into the flat passage, increasing both local heat transfer and pressure gradient relative to that in a steady flat passage. Moreover, the heat transfer for a given pumping power in the first three groove-lengths of the flat passage was even greater than the levels observed in a fully developed grooved passage.

Validation of the Isis-3D Computer Code for Simulating Large Pool Fires Under a Variety of Wind Conditions

The Isis -3D computational fluid dynamics/radiation heat transfer code was developed to simulate heat transfer from large fires. It models liquid fuel evaporation, fuel vapor and oxygen transport, chemical reaction and heat release, soot and intermediate species formation/destruction, diffuse radiation within the fire, and view factor radiation from the fire edge to nearby objects and the surroundings. Reaction rate and soot radiation parameters in Isis -3D have been selected based on experimental data. One-dimensional transient conduction modules calculate the response of simp le objects engulfed in and near the flames. In this work, Isis -3D calculations were performed to simulate the conditions of three experiments that measured the temperature response of a 4.66-m-diameter culvert pipe located at the leeward edge of 18.9-m and 9.45-m diameter pool fires in crosswinds with average speeds of 2.0, 4.6 and 9.5 m/s. The measured wind conditions were used to formulate time-dependent velocity boundary conditions for a rectangular Isis -3D domain with 16,500 nodes. Isis -3D accurately calculated characteristics of the time-dependent temperature distributions in all three experiments. Accelerated simulations were also performed in which the pipe specific heat was reduced compared to the measured value by a factor of four. This artificially increased the speed at which the pipe temperature rose and allowed the simulated fire duration to be reduced by a factor of four. A 700 sec fire with moderately unsteady wind conditions was accurately simulated in 10 hours on a 2.4 GHz LINUX workstation with 0.5 GB of RAM.

USE OF FUEL ASSEMBLY/BACKFILL GAS EFFECTIVE THERMAL CONDUCTIVITIES TO PREDICT BASKET AND FUEL CLADDING TEMPERATURES WITHIN A RAIL PACKAGE DURING NORMAL TRANSPORT

Two-dimensional finite element thermal simulations of large rail casks designed to transport spent nuclear fuel assemblies were performed for normal conditions. Two different effective thermal conductivity models, developed by other investigators, were implemented within the basket openings that support the fuel assemblies. The effective thermal conductivity models affect the peak cladding temperature directly by influencing the temperature difference between the hottest cladding at the cask center and the walls that surround it. It also affects it indirectly by influencing the center basket wall temperature. The fuel assembly heat generation rates that cause the peak cladding temperature to reach the allowed limit were determined for both effective thermal conductivity models. At those generation rates the basket wall temperatures in the periphery of the package were highly nonuniform. The basket wall temperatures determined in this work will be used in future studies to develop improved thermal models of fuel assemblies.

Measurements of Heat Transfer to a Massive Cylindrical Calorimeter Engulfed in a Circular Pool Fire

A large-scale experiment was performed to measure heat transfer to a massive cylindrical calorimeter engulfed in a 30 minute circular-pool fire. This test simulated the conditions of a truck-sized nuclear waste transport package in a severe fire. The calorimeter inner surface temperature and the flame environment emissive power were measured at several locutions us functions of time. An inverse heat conduction technique was used to estimate the net heat flux to the calorimeter. Tall porous fences surrounded the test facility to reduce the effect of wind on the fire. Outside the fences, 2.9 m/s winds blew across the calorimeter axis at the beginning of the test but decreased with time. The wind tilted and moved the fire so that the initial flame environment emissive power was substantially less on the windward side than the leeward side. The calorimeter became more uniformly engulfed as the winds decreased. The maximum heat flux to the calorimeter was 150 MW/m 2 on the leeward side at the beginning of the fire, and generally decreased with time. The local variations of calorimeter temperature and heat flux were closely related to the local flame environment emissive power.

Numerical Simulations of Resonant Heat Transfer Augmentation at Low Reynolds Numbers

The effect of flow rate modulation on low Reynolds number heat transfer enhancement in a transversely grooved passage was numerically simulated using a two-dimensional spectral element technique. Simulations were performed at subcritical Reynolds numbers of Rem = 133 and 267, with 20% and 40% flow rate oscillations. The net pumping power required to modulate the flow was minimized as the forcing frequency approached the predicted natural frequency. However, mixing and heat transfer levels both increased as the natural frequency was approached. Oscillatory forcing in a grooved passage requires two orders of magnitude less pumping power than flat passage systems for the same heat transfer level. Hydrodynamic resonance appears to be an effective method of increasing heat transfer in low Reynolds number systems where pumping power is at a premium, such as micro heat transfer applications.