A thermodynamic analysis of forced convection through porous media using pore scale modeling

The flow thorough porous media is analyzed from a thermodynamic perspective, with a particular focus on the entropy generation inside the porous media, using a pore scale modeling approach. A single representative elementary volume was utilized to reduce the CPU time. Periodic boundary conditions were employed for the vertical boundaries, by re-injecting the velocity and temperature profiles from the outlet to the inlet and iterating. The entropy generation was determined for both circular and square cross-sectional configurations, and the effects of different Reynolds numbers, assuming Darcy and Forchheimer regimes, were also taken into account. Three porosities were evaluated and discussed for each cross-sectional configuration, and streamlines, isothermal lines and the local entropy generation rate contours were determined and compared. The local entropy generation rate contours indicated that the highest entropy generation regions were close to the inlet for low Reynolds flows and near the central cylinder for high Reynolds flows. Increasing Reynolds number from 100 to 200 reveals disturbances in the dimensionless volume averaged entropy generation rate trend that may be due to a change in the fluid flow regime. According to Bejan number evaluation for both cross-section configurations, it is demonstrated that is mainly provoked by the heat transfer irreversibility. A performance evaluation criterion parameter was calculated for different case-studies. By this parameter, conditions for obtaining the least entropy generation and the highest Nusselt number could be achieved simultaneously. Indeed, this parameter utilizes both the first and the second laws of thermodynamics to present the best case-study. According to the performance evaluation criterion, it is indicated that the square cross-section configuration with o=0.64 exhibits better thermal performance for low Reynolds number flows. A comparison between the equal porosity cases for two different cross-sectional configurations indicated that the square cross-section demonstrated a higher performance evaluation criterion than the circular cross-section, for a variety of different Reynolds numbers

Ethnic differences in transition to first marriage in Iran

This paper, using data from the 2000 Iran Demographic and Health Survey and a range of time-varying district-level contextual information derived from the 1986 and 1996 censuses of Iran, applies a discrete time hazard model to study ethnic differences in women’s transition to first marriage. The model specification accounts for both spatial and temporal changes in the socio-economic context of transition to marriage. We found ethnic-specific responses on women’s marriage timing to changes in the socio-economic context between the mid 1970s and 2000. Some ethnic groups appear to be more resistant to change despite sharing similar changes in their socio-economic context.development, education, ethnicity, Iran, marriage, marriage market, women status

Small loop spaces and covering theory of non-homotopically Hausdorff spaces

In this paper we devote to spaces that are not homotopically hausdorff and study their covering spaces. We introduce the notion of small covering and prove that every small covering of $X$ is the universal covering in categorical sense. Also, we introduce the notion of semi-locally small loop space which is the necessary and sufficient condition for existence of universal cover for non-homotopically hausdorff spaces, equivalently existence of small covering spaces. Also, we prove that for semi-locally small loop spaces, $X$ is a small loop space if and only if every cover of $X$ is trivial if and only if $\pi_1^{top}(X)$ is an indiscrete topological group.Comment: 7 page

Heat transfer and second law analyses of forced convection in a channel partially filled by porous media and featuring internal heat sources

This paper provides a comprehensive study on the heat transfer and entropy generation rates in a channel partially filled with a porous medium and under constant wall heat flux. The porous inserts are attached to the walls of the channel and the system features internal heat sources due to exothermic or endothermic physical or physicochemical processes. Darcy-Brinkman model is used for modelling the transport of momentum and an analytical study on the basis of LTNE (local thermal non-equilibrium) condition is conducted. Further analysis through considering the simplifying, LTE (local thermal equilibrium) model is also presented. Analytical solutions are, first, developed for the velocity and temperature fields. These are subsequently incorporated into the fundamental equations of entropy generation and both local and total entropy generation rates are investigated for a number of cases. It is argued that, comparing with LTE, the LTNE approach yields more accurate results on the temperature distribution within the system and therefore reveals more realistic Nusselt number and entropy generation rates. In keeping with the previous investigations, bifurcation phenomena are observed in the temperature field and rates of entropy generation. It is, further, demonstrated that partial filling of the channel leads to a substantial reduction of the total entropy generation. The results also show that the exothermicity or endothermicity characteristics of the system have significant impacts on the temperature fields, Nusselt number and entropy generation rates