Progress in Favré-Reynolds Stress Closures for Compressible Flows

A closure for the compressible portion of the pressure-strain covariance is developed. It is shown that, within the context of a pressure-strain closure assumption linear in the Reynolds stresses, an expression for the pressure-dilatation can be used to construct a representation for the pressure-strain. Additional closures for the unclosed terms in the Favré-Reynolds stress equations involving the mean acceleration are also constructed. The closures accommodate compressibility corrections depending on the magnitude of the turbulent Mach number, the mean density gradient, the mean pressure gradient, the mean dilatation, and, of course, the mean velocity gradients. The effects of the compressibility corrections are consistent with current DNS results. Using the compressible pressure-strain and mean acceleration closures in the Favré-Reynolds stress equations an algebraic closure for the Favré-Reynolds stresses is constructed. Noteworthy is the fact that, in the absence of mean velocity gradients, the mean density gradient produces Favré-Reynolds stresses in accelerating mean flows. Computations of the mixing layer using the compressible closures developed are described. Full Reynolds stress closure and two-equation algebraic models are compared to laboratory data. The mixing layer configuration computations are compared to laboratory data; since the laboratory data for the turbulence stresses is inconsistent, this comparison is inconclusive. Comparison

Premium Fuel Production From Mining and Timber Waste Using Advanced Separation and Pelletizing Technologies

The Commonwealth of Kentucky is one of the leading states in the production of both coal and timber. As a result of mining and processing coal, an estimated 3 million tons of fine coal are disposed annually to waste-slurry impoundments with an additional 500 million tons stored at a number of disposal sites around the state due to past practices. Likewise, the Kentucky timber industry discards nearly 35,000 tons of sawdust on the production site due to unfavorable economics of transporting the material to industrial boilers for use as a fuel. With an average heating value of 6,700 Btu/lb, the monetary value of the energy disposed in the form of sawdust is approximately $490,000 annually. Since the two industries are typically in close proximity, one promising avenue is to selectively recover and dewater the fine-coal particles and then briquette them with sawdust to produce a high-value fuel. The benefits are i) a premium fuel product that is low in moisture and can be handled, transported, and utilized in existing infrastructure, thereby avoiding significant additional capital investment and ii) a reduction in the amount of fine-waste material produced by the two industries that must now be disposed at a significant financial and environmental price. As such, the goal of this project was to evaluate the feasibility of producing a premium fuel with a heating value greater than 10,000 Btu/lb from waste materials generated by the coal and timber industries. Laboratory and pilot-scale testing of the briquetting process indicated that the goal was successfully achieved. Low-ash briquettes containing 5$8 per ton of the coal-sawdust blend. REAX/lime and a second multi-component formulation were identified as the most cost-effective for the production of briquettes targeted for the stoker-coal market. Various sources of sawdust generated from different wood types were also investigated to determine their chemical properties and to evaluate their relative performance when briquetted with clean coal to form a premium fuel. The highest heating values, approaching 7,000 Btu/lb, were obtained from oak. Sawdusts from higher-density, red oak, white oak, hickory, and beech trees provided higher quality briquettes relative to their lower-density counterparts. In addition to sawdust type, a number of other parameters were evaluated to characterize their impact on briquette properties. The parameters that exhibited the greatest impact on briquette performance were binder concentration; sawdust concentration and particle size; cure temperature; and ash content. Parameters that had the least impact on briquette properties, at least over the ranges studied, were moisture content, briquetting force, and briquetting dwell time. The continuous production of briquettes from a blend of coal and sawdust was evaluated using a 200 lbs/hr Komarek Model B-100 briquetter. The heating values of briquettes produced by the unit exceeded the goal of the project by a large margin. A significant observation was the role of feed moisture on the stability of the mass flow rate through the briquetter and on briquette strength. Excessive feed moisture levels caused inconsistent or stoppage of material flow through the feed hopper and resulted in the production of variable-quality briquettes. Obviously, the limit on feed moisture content has a significant impact on the economics of coal-sawdust briquetting since it will ultimately dictate dewatering costs. Interestingly, the maximum feed moisture was found to be dependent to some extent on the binder type with molasses-containing blends being difficult to feed when the moisture content approached 12% while guar gum blends flowed consistently at moisture levels as high as 15% by weight. Due to the low ash and moisture contents of the coal-sawdust briquettes, a production increase of about 50 tons/hr would potentially be realized at a 1,400 ton/hr preparation plant. The overall capital cost of a 50 ton/hr flotation and briquetting addition was estimated to be around $8 million. Based on a conservative briquetting operating cost of$12/ton, the annual profit before taxes was approximated to be $4 million thereby indicating a return on investment in about 2 years. The internal rate of return based on a 10 year life was an attractive 43%