ICE SLURRY PRODUCTION IN A TUBULAR HEAT EXCHANGER

International audienceOne of the challenges of ice slurry generation using a heat exchanger is to overcome the risk of choking. The nucleation usually appears after a supercooling state that remains a key aspect of this technology. In order to focus on this crucial problem, the crystallization of a water ethanol solution was studied in a transparent tubular heat exchanger. Our experiments consisted in cooling the solution until ice slurry formation. Flow visualization permitted mainly to determine the conditions of crystallization according to the flow rate. A limited range of flow rate permitted to obtain the desired ice slurry quality. The heterogeneous crystallization appeared on the wall and dendrites began to develop before being carried away by the solution. Experiments also indicated that dendrites growth from the freezing front upstream in the counter flow direction. The velocity of this retro-propagation phenomenon was estimated using an image analysis tool. Temperature measurements also provided important information on this retro-propagation. Selected process parameters have to be properly adjusted (flow rate, wall properties …). This technology has been patented by the CNRS in selected European countries and in the USA

Finding a subdivision of a prescribed digraph of order 4

The problem of when a given digraph contains a subdivision of a fixed digraph F is considered.Bang-Jensen et al. [2] laid out foundations for approaching this problem from the algorithmic pointof view. In this paper we give further support to several open conjectures and speculations about algorithmiccomplexity of finding F-subdivisions. In particular, up to 5 exceptions, we completely classify forwhich 4-vertex digraphs F, the F-subdivision problem is polynomial-time solvable and for which it is NPcomplete.While all NP-hardness proofs are made by reduction from some version of the 2-linkage problemin digraphs, some of the polynomial-time solvable cases involve relatively complicated algorithms

Complexity of greedy edge-colouring

International audienceThe Grundy index of a graph G = (V, E) is the greatest number of colours that the greedy edge-colouring algorithm can use on G. We prove that the problem of determining the Grundy index of a graph G = (V, E) is NP-hard for general graphs. We also show that this problem is polynomial-time solvable for caterpillars. More specifically, we prove that the Grundy index of a caterpillar is (G) or (G) + 1 and present a polynomial-time algorithm to determine it exactly

Lower Bounds for the Graph Homomorphism Problem

The graph homomorphism problem (HOM) asks whether the vertices of a given $n$-vertex graph $G$ can be mapped to the vertices of a given $h$-vertex graph $H$ such that each edge of $G$ is mapped to an edge of $H$. The problem generalizes the graph coloring problem and at the same time can be viewed as a special case of the $2$-CSP problem. In this paper, we prove several lower bound for HOM under the Exponential Time Hypothesis (ETH) assumption. The main result is a lower bound $2^{\Omega\left( \frac{n \log h}{\log \log h}\right)}$. This rules out the existence of a single-exponential algorithm and shows that the trivial upper bound $2^{{\mathcal O}(n\log{h})}$ is almost asymptotically tight. We also investigate what properties of graphs $G$ and $H$ make it difficult to solve HOM$(G,H)$. An easy observation is that an ${\mathcal O}(h^n)$ upper bound can be improved to ${\mathcal O}(h^{\operatorname{vc}(G)})$ where $\operatorname{vc}(G)$ is the minimum size of a vertex cover of $G$. The second lower bound $h^{\Omega(\operatorname{vc}(G))}$ shows that the upper bound is asymptotically tight. As to the properties of the "right-hand side" graph $H$, it is known that HOM$(G,H)$ can be solved in time $(f(\Delta(H)))^n$ and $(f(\operatorname{tw}(H)))^n$ where $\Delta(H)$ is the maximum degree of $H$ and $\operatorname{tw}(H)$ is the treewidth of $H$. This gives single-exponential algorithms for graphs of bounded maximum degree or bounded treewidth. Since the chromatic number $\chi(H)$ does not exceed $\operatorname{tw}(H)$ and $\Delta(H)+1$, it is natural to ask whether similar upper bounds with respect to $\chi(H)$ can be obtained. We provide a negative answer to this question by establishing a lower bound $(f(\chi(H)))^n$ for any function $f$. We also observe that similar lower bounds can be obtained for locally injective homomorphisms.Comment: 19 page

New apparatus to provide repeatable surface temperature-time treatments on inoculated food samples

The final stage of the BUGDEATH project (James and Evans, 2005) was to further develop the surface heating and cooling apparatus so it would be suitable for commercial exploitation. The new apparatus can be accommodated on a bench top and produces faster more uniform heating and cooling cycles than that previously described. It can heat the surface of a food from 8 to 120 °C in 14 s, cool from 120 to 40 °C in 28 s and then to 8 °C in 5 min. There is an average control error of approximately ±1 °C and a temperature variation over the surface of the sample of only 1.2 °C. Wet heating, using steam at 100 °C, was achieved using a portable steam generator. The price of the parts which made up the apparatus were approximately €10 000 and it takes approximately two person-weeks to build. © 2005 Elsevier Ltd. All rights reserved

Design and development of apparatus to provide repeatable surface temperature-time treatments on inoculated food samples

The majority of data relating heat treatments to thermal death kinetics has previously been obtained by carrying out in vitro tests on small samples of microorganisms in growth medium or food slurries. However, strong evidence exists that suggests microbial death on heat-treated food surfaces cannot be predicted accurately from such systems. An apparatus was therefore designed to enable microbial death on food surfaces to be quantified. The apparatus uses hot air to provide a ‘dry’ heat treatment to raise the surface of a food sample to a given temperature, up to 100 °C, hold and then cool it. The surface temperature was measured using an infra-red (IR) thermometer and the air heater was controlled to give a specified surface temperature history. Steam was also used to provide a ‘wet’, but less controlled heating cycle

Contraception and screening for cervical and breast cancer in neuromuscular disease: A retrospective study of 50 patients monitored at a clinical reference centre

AbstractObjectiveTo analyse contraceptive methods and the extent of screening for breast and cervical cancer in women with neuromuscular disease, compare these results with data and guidelines for the general population and determine the environmental and attitudinal barriers encountered.Patients and methodsA retrospective, descriptive study in a population of female neuromuscular disease patients (aged 20 to 74) monitored at a clinical reference centre.ResultsComplete datasets were available for 49 patients. Seventy percent used contraception (hormonal contraception in most cases). Sixty-eight percent had undergone screening for cervical cancer at some time in the previous 3 years and 100% of the patients over 50 had undergone a mammography. Architectural accessibility and practical problems were the most common barriers to care and were more frequently encountered by wheelchair-bound, ventilated patients.ConclusionsIn general, the patients had good access to contraceptive care and cervical and breast cancer screening. However, specific measures may be useful for the most severely disabled patients