Use of axially graded burnable boron for hot-spot temperature reduction in a pressurized water reactor core

Shortly after the loading of a pressurized water reactor (PWR) core, the axial power distribution in fresh fuel has already attained the characteristic, almost flat shape, typical of burned fuel. At beginning of cycle (BOC), however, the axial distribution is centrally peaked. In assemblies hosting uniform burnable boron rods, this BOC peaking is even more pronounced. A reduction in the axial peaking is today often achieved by shortening the burnable boron rods by some 30 cm at each edge. It is shown that a two-zone grading of the boron rod leads, in a representative PWR cycle, to a reduction of the hot-spot temperature of approximately 70 °C, compared with the nongraded case. However, with a proper three-zone grading of the boron rod, an additional 20 °C may be cut off the hot-spot temperature. Further, with a slightly skewed application of this three-zone grading, an additional 50 °C may be cut off. The representative PWR cycle studied was cycle 11 of the Indian Point 2 station, with a simplification in the number of fuel types and in the burnup distribution. The analysis was based on a complete three-dimensional burnup calculation. The code system was ELCOS, with BOXER as an assembly code for the generation of burnup-dependent cross sections and SILWER as a three-dimensional core code with thermal-hydraulic feedback.

The prediction of hot streak migration in a high-pressure turbine

Accurate predictions of combustor hot streak migration enable the turbine designer to identify high-temperature regions that can limit component life. It is therefore important that these predictions are achieved within the short time scales of a design process. This article compares temperature measurements of a circular hot streak through a turning duct and a research turbine with predictions using a three-dimensional Reynolds-averaged Navier-Stokes solver. It was found that the mixing length turbulence model did not predict the hot streak dissipation accurately. However, implementation of a very simple model of the free stream turbulence (FST) significantly improved the exit temperature predictions on both the duct and research turbine. One advantage of the simple FST model described over more complex alternatives is that no additional equations are solved. This makes the method attractive for design purposes, as it is not associated with any increase in computational time.

Thermal characterization of SOI CMOS micro hot-plate gas sensors

This work reports on thermal characterization of SOI (silicon on insulator) CMOS (complementary metal oxide semiconductor) MEMS (micro electro mechanical system) gas sensors using a thermoreflectance (TR) thermography system. The sensors were fabricated in a CMOS foundry and the micro hot-plate structures were created by back-etching the CMOS processed wafers in a MEMS foundry using DRIE (deep reactive ion etch) process. The calibration and experimental details of the thermoreflectance based thermal imaging setup, used for these micro hot-plate gas sensor structures, are presented. Experimentally determined temperature of a micro hot-plate sensor, using TR thermography and built-in silicon resistive temperature sensor, is compared with that estimated using numerical simulations. The results confirm that TR based thermal imaging technique can be used to determine surface temperature of CMOS MEMS devices with a high accuracy. © 2010 EDA Publishing/THERMINIC.

Simulations of autoignition and laminar premixed flames in methane/air mixtures diluted with hot products

This article considers constant-pressure autoignition and freely propagating premixed flames of cold methane/air mixtures mixed with equilibrium hot products at high enough dilution levels to burn within the moderate to intense low oxygen dilution (MILD) combustion regime. The analysis is meant to provide further insight on MILD regime boundaries and to identify the effect of hot products speciation. As the mass fraction of hot products in the reactants mixture increases, autoignition occurs earlier. Species profiles show that the products/reactants mixture approximately equilibrates to a new state over a quick transient well before the main autoignition event, but as dilution becomes very high, this equilibration transient becomes more prominent and eventually merges with the primary ignition event. The dilution level at which these two reactive zones merge corresponds well with that marking the transition into the MILD regime, as defined according to conventional criteria. Similarly, premixed flame simulations at high dilutions show evidence of significant reactions involving intermediate species prior to the flame front. Since the premixed flame governing equations system demands that the species and temperature gradients be zero at the "cold" boundary, flame speed cannot be calculated above a certain dilution level. Up to this point, which again agrees reasonably well with the transition into the MILD regime according to convention, the laminar burning velocity was found to increase with hot product dilution while flame thickness remained largely unchanged. Some comments on the MILD combustion regime boundary definition for gas turbine applications are included. Copyright © Taylor & Francis Group, LLC.

Design and control of a climbing robot based on hot melt adhesion

Robust climbing in unstructured environments has been one of the long-standing challenges in robotics research. Among others, the control of large adhesion forces is still an important problem that significantly restricts the locomotion performance of climbing robots. The main contribution of this paper is to propose a novel approach to autonomous robot climbing which makes use of hot melt adhesion (HMA). The HMA material is known as an economical solution to achieve large adhesion forces, which can be varied by controlling the material temperature. For locomotion on both inclined and vertical walls, this paper investigates the basic characteristics of HMA material, and proposes a design and control of a climbing robot that uses the HMA material for attaching and detaching its body to the environment. The robot is equipped with servomotors and thermal control units to actively vary the temperature of the material, and the coordination of these components enables the robot to walk against the gravitational forces even with a relatively large body weight. A real-world platform is used to demonstrate locomotion on a vertical wall, and the experimental result shows the feasibility and overall performances of this approach. © 2013 Elsevier B.V. All rights reserved.

Physical connection and disconnection control based on hot melt adhesives

Physical connection and disconnection control has practical meanings for robot applications. Compared to conventional connection mechanisms, bonding involving a thermal process could provide high connection strength, high repeatability, and power-free connection maintenance, etc. In terms of disconnection, an established bond can be easily weakened with a temperature rise of the material used to form the bond. Hot melt adhesives (HMAs) are such material that can form adhesive bonds with any solid surfaces through a thermally induced solidification process. This paper proposes a novel control method for automatic connection and disconnection based on HMAs. More specifically, mathematical models are first established to describe the flowing behavior of HMAs at higher temperatures, as well as the temperature-dependent strength of an established HMA bond. These models are then validated with a specific type of HMA in a minimalistic robot setup equipped with two mechatronic devices for automated material handling. The validated models are eventually used for determining open parameters in a feedback controller for the robot to perform a pick-and-place task. Through a series of trials with different wooden and aluminum parts, we evaluate the performance of the automatic connection and disconnection methods in terms of speed, energy consumption, and robustness. © 1996-2012 IEEE.