Corrosion of a stainless steel waste heat recuperator

Waste heat recuperation has significant potential for saving energy in fossil-fuel-fired industrial furnaces. Preheating the air used to burn the fuel can significantly reduce fuel consumption. The US Department of Energy is contracting several high-temperature waste heat recuperation demonstrations with the objective of using successful efforts to stimulate the industrial utilization of these devices. One of the recuperator demonstration contracts has as an objective the successful operation of a concentric-shell radiation recuperator of a new design on aluminum-scrap-remelting furnaces. The design employs type 309 stainless steel reradiant inserts within the type 309 stainless steel inner shell to increase heat radiation to the recuperator partition, thereby increasing the heat exhanger's effectiveness. The first demonstration recuperator in this program was installed on a furnace fired with No. 2 oil and melting about 60 Mg (66 tons) of aluminum per 24-h day. The unit operated for about 30 d and provided air to the burner at 540/sup 0/C. during this period, a burner control misoperation provided very fuel-rich gases to the base of the recuperator. This fuel combined with safety dilution air at the recuperator base and burned within the recuperator. Also, during this period, air flow loss was detected at the burner. An inspection revealed that this was caused by failure of the partition wall separating the primary and secondary sides of the recuperator. Extensive corrosion of the partition wall and reradiant inserts was also observed. The recuperator was removed from the furnace for an analysis of the failure

Effects of alternate fuels report No. 7: analysis of failure of a mullite-based refractory brick in an industrial oil-fired burner

Industrial conversion from natural gas to alternate fuels, such as residual oils and coal, often results in accelerated degradation of refractory materials due to chemical reactions with the metal impurities in the alternate fuels. The cause of failure of a refractory brick used in an industrial burner firing an alternate fuel is described. The burner, which was used to calcine CaSO/sub 4/ in a lime-type kiln, was fired with No. 6 residual oil. The refractory lining in the burner was constructed of aluminosilicate brick, castable, and mortar in contact with one another. The lining deteriorated after about 1000 h, during which the maximum hot-face temperature was about 1750/sup 0/C. The degraded refractories were subjected to chemical analyses, ceramography, x-ray diffraction, scanning electron microscopy, and electron microprobe analysis. Liquid phases that formed in the castable and mortar during operation of the burner at temperatures above about 1600/sup 0/C reacted with the brick, resulting in decomposition of mullite. Contamination of the original refractory with CaO and V/sub 2/O/sub 5/ resulted in the formation, during cooling, of compounds which are less refractory than the original castable and mortar. It was concluded that failure was initiated by melting in the castable and mortar. Large concentrations of aggressive oxide liquid were in the burner lining at the service temperature. The liquid phase eventually advanced into the refractory from the hot face to the extent that the brick grossly deteriorated. Therefore, rapid degradation of the refractory system was due to a combination of excess temperature and fluxing by process carry-over and impurities from the fuel oil

Effects of alternate fuels report No. 8: analysis of degradiation of magnesia-based refractory bricks from a residual oil-fired rotary cement kiln

Residual oil was used as an alternate fuel to natural gas to supply heat in a rotary cement kiln. Principal impurities in the residual oil were Ca, Fe, Mg, Na, Ni, P.S. and V. the kiln operators were concerned about the effects of these oil impurities on observed degradation of the magnesia-based bricks used as a liner in the burning zone of the kiln. Two degraded bricks, which had been in service for six to nine months, were analyzed to determine the role of fuel impurities on the observed degradation. The maximum hot-face temperature of the refractory during service was about 1500/sup 0/C. One brick had decreased in thickness about 45%, the about 15%. Various analytical measurements on these samples failed to reveal the presence of fuel impurities at or near the hot face of the bricks, and therefore it is concluded that the relatively short service life of these refractories was not due to use of residual oil as the fuel in the kiln. The observed degradation, therefore, was attributed to other reactions and to thermal mechanical conditions in the kiln, which inevitably resulted in extensive erosion of the bricks

The geodesic rule for higher codimensional global defects

We generalize the geodesic rule to the case of formation of higher codimensional global defects. Relying on energetic arguments, we argue that, for such defects, the geometric structures of interest are the totally geodesic submanifolds. On the other hand, stochastic arguments lead to a diffusion equation approach, from which the geodesic rule is deduced. It turns out that the most appropriate geometric structure that one should consider is the convex hull of the values of the order parameter on the causal volumes whose collision gives rise to the defect. We explain why these two approaches lead to similar results when calculating the density of global defects by using a theorem of Cheeger and Gromoll. We present a computation of the probability of formation of strings/vortices in the case of a system, such as nematic liquid crystals, whose vacuum is $\mathbb{R}P^2$.Comment: 17 pages, no figures. To be published in Mod. Phys. Lett.

Turbulence and passive scalar transport in a free-slip surface

We consider the two-dimensional (2D) flow in a flat free-slip surface that bounds a three-dimensional (3D) volume in which the flow is turbulent. The equations of motion for the two-dimensional flow in the surface are neither compressible nor incompressible but strongly influenced by the 3D flow underneath the surface. The velocity correlation functions in the 2D surface and in the 3D volume scale with the same exponents. In the viscous subrange the amplitudes are the same, but in the inertial subrange the 2D one is reduced to 2/3 of the 3D amplitude. The surface flow is more strongly intermittent than the 3D volume flow. Geometric scaling theory is used to derive a relation between the scaling of the velocity field and the density fluctuations of a passive scalar advected on the surface.Comment: 11 pages, 10 Postscript figure

On the Hausdorff volume in sub-Riemannian geometry

For a regular sub-Riemannian manifold we study the Radon-Nikodym derivative of the spherical Hausdorff measure with respect to a smooth volume. We prove that this is the volume of the unit ball in the nilpotent approximation and it is always a continuous function. We then prove that up to dimension 4 it is smooth, while starting from dimension 5, in corank 1 case, it is C^3 (and C^4 on every smooth curve) but in general not C^5. These results answer to a question addressed by Montgomery about the relation between two intrinsic volumes that can be defined in a sub-Riemannian manifold, namely the Popp and the Hausdorff volume. If the nilpotent approximation depends on the point (that may happen starting from dimension 5), then they are not proportional, in general.Comment: Accepted on Calculus and Variations and PD

On non-local variational problems with lack of compactness related to non-linear optics

We give a simple proof of existence of solutions of the dispersion manage- ment and diffraction management equations for zero average dispersion, respectively diffraction. These solutions are found as maximizers of non-linear and non-local vari- ational problems which are invariant under a large non-compact group. Our proof of existence of maximizer is rather direct and avoids the use of Lions' concentration compactness argument or Ekeland's variational principle.Comment: 30 page

Ruelle-Perron-Frobenius spectrum for Anosov maps

We extend a number of results from one dimensional dynamics based on spectral properties of the Ruelle-Perron-Frobenius transfer operator to Anosov diffeomorphisms on compact manifolds. This allows to develop a direct operator approach to study ergodic properties of these maps. In particular, we show that it is possible to define Banach spaces on which the transfer operator is quasicompact. (Information on the existence of an SRB measure, its smoothness properties and statistical properties readily follow from such a result.) In dimension $d=2$ we show that the transfer operator associated to smooth random perturbations of the map is close, in a proper sense, to the unperturbed transfer operator. This allows to obtain easily very strong spectral stability results, which in turn imply spectral stability results for smooth deterministic perturbations as well. Finally, we are able to implement an Ulam type finite rank approximation scheme thus reducing the study of the spectral properties of the transfer operator to a finite dimensional problem.Comment: 58 pages, LaTe

Prescribing the Jacobian in critical spaces

International audienceWe consider the Sobolev space $X=W^{s,p}({\mathbb S}^m ; {\mathbb S}^{k-1})$. We prove the existence of a robust distributional Jacobian $Ju$ for $u\in X$ provided $sp\ge k-1$. This generalizes a result of Bourgain, Brezis and the second author (Comm. Pure Appl. Math. 2005), where the case $m=k$ is considered. In the critical case where $sp=k-1$, we identify the image of the map $X\ni u\mapsto Ju$. This extends a result of Alberti, Baldo and Orlandi (J. Eur. Math. Soc. 2003) for $s=1$ and $p=k-1$. We also present a new, analytical, dipole construction method

Local invertibility in Sobolev spaces with applications to nematic elastomers and magnetoelasticity

We define a class of deformations in W^1,p(\u3a9,R^n), p>n 121, with positive Jacobian that do not exhibit cavitation. We characterize that class in terms of the non-negativity of the topological degree and the equality between the distributional determinant and the pointwise determinant of the gradient. Maps in this class are shown to satisfy a property of weak monotonicity, and, as a consequence, they enjoy an extra degree of regularity. We also prove that these deformations are locally invertible; moreover, the neighbourhood of invertibility is stable along a weak convergent sequence in W^1,p, and the sequence of local inverses converges to the local inverse. We use those features to show weak lower semicontinuity of functionals defined in the deformed configuration and functionals involving composition of maps. We apply those results to prove existence of minimizers in some models for nematic elastomers and magnetoelasticity