215,438 research outputs found
Simple constructions for balanced incomplete block designs with block size three
AbstractLet S be a finite set with v elements. It is known that there exists a sequence of three-element subsets of S such that each two-element subset of S is contained in exactly λ terms of the sequence if and only if λ(v − 1)2 and λv(v − 1)6 are integers. The known proof is somewhat complicated when v ≡ 2 (mod 6), and this paper provides a simpler proof for this case. Proofs are also given for all other values of v by reviewing known constructions or providing new ones
Experimental study of the effects of flameholder geometry on emissions and performance of lean premixed combustors
Emissions of NOx, CO, and unburned hydrocarbons (UHC) are reported for a lean premixed propane-air system at inlet conditions of 800K and 1MPa using twelve flameholder designs. The flameholders tested represent six design concepts with two values of blockage for each concept. Data were obtained at reference velocities of 35 m/s, 25 m/s and 20 m/s at combustor stations 10 cm and 30 cm downstream of the flameholders. Flameholder pressure drop was found to be a principal determinant of emissions performance. Designs producing larger pressure drops also produced less NOx, CO, and UHC emissions. The lean stability limit equivalence ratio was found to be approximately 0.35 for all designs. Flashback velocities (axial components in the flameholder passages) varied between 30 m/s and 40 m/s. A perforated plate flameholder was operated with a velocity as low as 23 m/s through the perforations at equivalence ratio 0.7 without producing flashback
Mixed group divisible designs with three groups and block size 4
AbstractA group divisible design (GDD) with three groups and block size 4 is called even, odd, or mixed if the sizes of the non-empty intersections of any of its blocks with any of the three groups are always even, always odd, or always mixed. It has been shown that the necessary conditions for the existence of GDDs of these three types are also sufficient except possibly for the minimal case of mixed designs for group size 5t (t>1). In this paper, we complete the undetermined families of mixed GDDs using two constructions based on idempotent self-orthogonal Latin squares and skew Room squares
Large Cross-free sets in Steiner triple systems
A {\em cross-free} set of size in a Steiner triple system
is three pairwise disjoint -element subsets such that
no intersects all the three -s. We conjecture that for
every admissible there is an STS with a cross-free set of size
which if true, is best possible. We prove this
conjecture for the case , constructing an STS containing a
cross-free set of size . We note that some of the -bichromatic STSs,
constructed by Colbourn, Dinitz and Rosa, have cross-free sets of size close to
(but cannot have size exactly ).
The constructed STS shows that equality is possible for in
the following result: in every -coloring of the blocks of any Steiner triple
system STS there is a monochromatic connected component of size at least
(we conjecture that equality holds for every
admissible ).
The analogue problem can be asked for -colorings as well, if r-1 \equiv
1,3 \mbox{ (mod 6)} and is a prime power, we show that the answer is the
same as in case of complete graphs: in every -coloring of the blocks of any
STS, there is a monochromatic connected component with at least points, and this is sharp for infinitely many .Comment: Journal of Combinatorial Designs, 201
Modified group divisible designs with block size four
AbstractThe existence of modified group divisible designs with block size four is settled with a handful of possible exceptions
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