Pellet heat transfer coefficients in packed beds: global and local values

Pellet heat transfer coefficients in a packed bed have been obtained, both for specific individual pellets and for the entire bed. They are referred to as local and global values, respectively. It appears that the local values are scattered around the global value. This is due to the heat transfer coefficient of individual pellets being statistically distributed, as a result of the randomnes of the packing. At low Reynolds numbers, both global and local values fall well below Nu = 2, which is the lower limit for a single sphere in absence of convection. In the literature, this behaviour has been attributed i.a. to axial dispersion and fluid maldistribution. However, these phenomena cannot explain why the same behaviour is observed in slurries. The fact that the local values fall below Nu = 2 would suggest that neither of these explanations is valid for packed beds

Mathematical modelling and design of an advanced once-through heat recovery steam generator

The once-through heat recovery steam generator (HRSG) design is ideally matched to very high temperature and pressure, well into the supercritical range. Moreover this type of boiler is structurally simpler than a conventional one, since no drum is required. In a conventional design, each tube plays a well-defined role: water preheating, vaporisation, superheating. Empirical equations are available to predict the average beat transfer coefficient for each region. For once-through applications, this is no more the case and mathematical models have to be adapted to account for the disappearance of the conventional economiser, boiler and superheater. General equations have to be used for each tube of the boiler, and the actual heat transfer condition in each tube has to be identified. The mathematical complexity as well as the number of equations is increased. A thermodynamic model has been selected and implemented to suit very high pressure (up to 240 bar), sub- and supercritical steam properties. Model use is illustrated by two case studies: a 180 bar once-through boiler (OTB) and a conventional boiler superheater and reheater. (C) 2004 Elsevier Ltd. All rights reserved

Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C

Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at.%. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at.% value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of one Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the magnetization provides an estimate Tc = 385K of the Curie temperature that exceeds the experimental value, Tc = 270K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330K is attributed to disorder and proximity to a metal-insulating transition.Comment: 4 pages, 4 figures (RevTex4

Temperature response functions (G-functions) for single pile heat exchangers

Foundation piles used as heat exchangers as part of a ground energy system have the potential to reduce energy use and carbon dioxide emissions from new buildings. However, current design approaches for pile heat exchangers are based on methods developed for boreholes which have a different geometry, with a much larger aspect (length to diameter) ratio. Current methods also neglect the transient behaviour of the pile concrete, instead assuming a steady state resistance for design purposes. As piles have a much larger volume of concrete than boreholes, this neglects the significant potential for heat storage within the pile. To overcome these shortcomings this paper presents new pile temperature response functions (G-functions) which are designed to reflect typical geometries of pile heat exchangers and include the transient response of the pile concrete. Owing to the larger number of pile sizes and pipe configurations which are possible with pile heat exchangers it is not feasible to developed a single unified G-function and instead upper and lower bound solutions are provided for different aspects ratios

Dynamic thermal simulation of horizontal ground heat exchangers for renewable heating and ventilation of buildings

A ground heat exchanger is used to transfer thermal energy stored in soil in order to provide renewable heating, cooling and ventilation of a building. A computer program has been developed for simulation of the dynamic thermal performance of horizontally coupled earth-liquid heat exchanger for a ground source heat pump and earth-air heat exchanger for building ventilation. Neglecting the dynamic interactions between a heat exchanger and environments would significantly over predict its thermal performance and in terms of the amount of daily heat transfer the level of over-prediction could be as much as 463% for an earth-liquid heat exchanger and more than 100% for an earth-air heat exchanger. The daily heat transfer increases with soil moisture and for an earth-liquid heat exchanger the increase is between 3% and 35% with increase in moisture from 0.22 to 0.3 m3/m3 depending on the magnitude of heat transfer. Heat transfer through a plastic earth-liquid heat exchanger can be increased by 10%–12% if its thermal properties are improved to the same as surrounding soil. The increase is smaller between 2% and 4% for an earth-air heat exchanger. In addition, an earth-liquid heat exchanger is more efficient than an earth-air heat exchanger

Comments on analogies for correlated heat and mass transfer in turbulent flow

No abstract.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34252/1/10146_ftp.pd