Optimal plate spacing for mixed convection from an array of vertical isothermal plates

International audienceNumerical simulations of mixed convection of air between vertical isothermal surfaces were conducted in order to determine the optimum spacing corresponding to the peak heat flux transferred from an array of isothermal, parallel plates cooled by mixed convection. Comparisons between approximate analytical solutions for natural and forced convection are first discussed. It is shown that the agreement is fairly good. From the computations carried out for aiding mixed convection by assuming a pressure drop at the outlet section rather than a constant flow rate, it is numerically predicted that the optimum spacing is smaller than those for pure natural or pure forced convection. This spacing is determined according to the pressure drop. As a sample, we considered an array of 10 cm -height, isothermal surfaces at temperature T-h = 340 K with air as the working fluid entering into the channels at T-0 = 300 K. The increases in heat flux corresponding to the optimal spacing are discussed for outlet pressure drops ranging from -0.1 Pa to -1 Pa. Such a range covers the entire laminar mixed convection regime for this specific application

Analysis and Functional Consequences of Increased Fab-Sialylation of Intravenous Immunoglobulin (IVIG) after Lectin Fractionation

It has been proposed that the anti-inflammatory effects of intravenous immunoglobulin (IVIG) might be due to the small fraction of Fc-sialylated IgG. In this study we biochemically and functionally characterized sialic acid-enriched IgG obtained by Sambucus nigra agglutinin (SNA) lectin fractionation. Two main IgG fractions isolated by elution with lactose (E1) or acidified lactose (E2) were analyzed for total IgG, F(ab’)2 and Fc-specific sialic acid content, their pattern of specific antibodies and anti-inflammatory potential in a human in vitro inflammation system based on LPS- or PHA-stimulated whole blood. HPLC and LC-MS testing revealed an increase of sialylated IgG in E1 and more substantially in the E2 fraction. Significantly, the increased amount of sialic acid residues was primarily found in the Fab region whereas only a minor increase was observed in the Fc region. This indicates preferential binding of the Fab sialic acid to SNA. ELISA analyses of a representative range of pathogen and auto-antigens indicated a skewed antibody pattern of the sialylated IVIG fractions. Finally, the E2 fraction exerted a more profound anti-inflammatory effect compared to E1 or IVIG, evidenced by reduced CD54 expression on monocytes and reduced secretion of MCP-1 (CCL2); again these effects were Fab- but not Fc-dependent. Our results show that SNA fractionation of IVIG yields a minor fraction (approx. 10%) of highly sialylated IgG, wherein the sialic acid is mainly found in the Fab region. The tested anti-inflammatory activity was associated with Fab not Fc sialylation

Heating and partial cooling problems in unsteady forced convection

The study of thermoconvective problems in a laminar external boundary layer flow is theoretically presented for both partial heating and cooling situations after that the fluid/wall system has already reached a first steady heating state. The 4th order Karman-Pohlhausen integral approach is used under the hypothesis that the thermal boundary layer thickness is assumed to be steady during the transient phase. The final mathematical equation for both cooling and heating problems is proposed and exemples of space and time corresponding thermograms are given

Laminar boundary-layer flow over a semi-infinite plate impulsively heated or cooled

The unsteady forced convection involved in incompressible laminar boundary-layer over a semi-infinite plate is investigated. Initially, a steady state convective heat transfer with constant plate temperature or constant plate heat flux is established between the plate and the flow. By differential method, we study the thermal effect on this first state of a positive or a negative sudden change in either the plate temperature or plate heat flux. Numerical solution of differential equations is obtained for the whole transient from the initial steady state to the final one. Detailed results of the effect of the wall conditions change on the transient process are given for the fluids encountered in the usual applications (0.6 ≤ Pr ≤ 100). An exponential law of the instantaneous heat coefficient is deduced as function of Pr in the said Prandtl number range

Comportement du sous-sol soumis à l'influence d'un puits où règne une température imposée. Méthodes de calcul, approche expérimentale

Dans le cadre d'une étude sur le stockage intersaisonnier de la chaleur en sous-sol par échangeur vertical, nous nous proposons d'établir l'expression du champ de température dans un milieu infini limité intérieurement par un cylindre de longueur également infinie. Nous développons pour cela trois méthodes dites : asymptotique, intégrale et de Gaver. Nous comparons d'abord ces méthodes entre elles pour vérifier leur équivalence, puis nous calons la méthode de Gaver sur une expérimentation grandeur nature. Cette étude nous permet, d'une part, d'obtenir une modélisation bidimensionnelle simple du phénomène dans un cas assez général, et d'autre part, de justifier expérimentalement cette approche bidimensionnelle du problème

Simulation du fonctionnement des échangeurs thermiques soumis à des conditions aux limites variables

An unsteady state model is proposed for the simulation of heat exchangers with time dependent inlet temperature of a fluid. The results obtained are in accordance with experimental measures. The precision is much better than the one obtained after a steady state model. The influence of the exchanger parameters on the time constant and on the static transfer function is also studied ; it is then possible to predict easily the system working in unsteady state.Un modèle théorique est proposé pour simuler le fonctionnement des échangeurs thermiques, avec évolution quelconque de la température d'entrée de l'un des fluides. Ce modèle suit avec une bonne précision les résultats expérimentaux (précision nettement supérieure à celle que l'on obtient à l'aide d'un modèle de régime permanent). Une étude de l'influence des principaux paramètres de l'échangeur sur la constante de temps et la transmittance statique complète ce travail et permet de prévoir le comportement général du système en régime variable

Heat exchanger effectiveness in unsteady state

A method is proposed to determine the thermal effectiveness of heat exchangers in situ, when one of the fluids is submitted to any kind of flow rate variations. It leads to the definition of the average effectiveness in unsteady state, which forms an extension of the classical effectiveness used in steady state. It requires an unsophisticated equipment of measurement and should lead to an easy and continuous control of the fouling of heat exchangers