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1
Content available Numerical analysis of structure, stability and entropy generation in biogas coflow diffusion flames
Kumar R. Nivethana, Kumaran S. Muthu, Raghavan Vasudevan
Archive of Mechanical Engineering
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2022
|
LXIX, nr 1
99--128
EN
Biogas is a gaseous biofuel predominantly composed of methane and carbon-dioxide. Stability of biogas flames strongly depend upon the amount of carbon-dioxide present in biogas, which varies with the source of biomass and reactor. In this paper, a comprehensive study on the stability and flame characteristics of coflow biogas diffusion flames is reported. Numerical simulations are carried out using reactive flow module in OpenFOAM, incorporated with variable thermophysical properties, Fick’s and Soret diffusion, and short chemical kinetics mechanism. Effects of carbon-dioxide content in the biogas, temperatures of the fuel or coflowing air streams (preheated reactant) and hydrogen addition to fuel or air streams are analyzed. Entropy generation in these flames is also predicted. Results show that the flame temperature increases with the degree of preheat of reactants and the flames show better stability with the preheated air stream. Preheating the air contributes to increased flame stability and also to a significant decrease in entropy generation. Hydrogen addition, contributing to the same power rating, is seen to be relatively more effective in increasing the flame stability when added to the fuel stream. Results in terms of flow, temperature, species and entropy fields, are used to describe the stability and flame characteristics.
2
Content available Entropy generation and squeezing flow past a Riga plate with Cattaneo-Christov heat flux
Atlas M., Hussain S., Sagheer M.
Bulletin of the Polish Academy of Sciences. Technical Sciences
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2018
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Vol. 66, nr 3
291--300
EN
In this article, we investigate the convective heat transfer of the two-dimensional unsteady squeezing flow past a Riga plate. To examine the heat transfer, Cattaneo-Christov heat flux model is used. Influence of entropy generation on heat transfer has been investigated numerically. With the help of suitable similarity transformation, the governing partial differential equations (PDEs) are converted into ordinary differential equations (ODEs). The obtained system of non-linear ordinary differential equations subject to the convective boundary conditions is solved by the shooting method using the computational software MATLAB. To strengthen the reliability of the results obtained by the shooting method, the MATLAB built-in function bvp4c has been used. The graphs show the effect of different physical parameters for velocity, temperature, concentration and tables are presented to observe the behaviour of skin friction and sherwood number under the influence of certain physical parameters. It is observed that for increasing values of thermal relaxation parameter, the temperature profile increases and an opposite behaviour is shown for the concentration profile. Moreover, with an increase in the Brinkman number, the entropy generation increases.
3
Content available Thermal and hydraulic phenomena in boundary layer of minijets impingement on curved surfaces
Kura T., Fornalik-Wajs E., Wajs J.
Archives of Thermodynamics
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2018
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Vol. 39, no 1
147--166
EN
Presented work considers flow and thermal phenomena occurring during the single minijet impingement on curved surfaces, heated with a constant heat flux, as well as the array of minijets. Numerical analyses, based on the mass, momentum and energy conservation laws, were conducted, regarding single phase and two-phase simulations. Focus was placed on the proper model construction, in which turbulence and boundary layer modeling was crucial. Calculations were done for various inlet parameters. Initial single minijet results served as the basis for the main calculations, which were conducted for two jet arrays, with flat and curved heated surfaces. Such complex geometries came from the cooling systems of electrical devices, and the geometry of cylindrical heat exchanger. The results, regarding Nusselt number, heated surface temperature, turbulence kinetic energy, production of entropy and vorticity, were presented and discussed. For assumed geometrical parameters similar results were obtained.
4
Content available Entropy analysis of third-grade MHD convection flows from a horizontal cylinder with slip
Madhavi K., Prasad V. R., Gaffar S. A., Venkatadri K.
Archive of Mechanical Engineering
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2018
|
Vol. LXV, nr 3
417--440
EN
In thermos fluid dynamics, free convection flows external to different geometries, such as cylinders, ellipses, spheres, curved walls, wavy plates, cones, etc., play major role in various industrial and process engineering systems. The thermal buoyancy force associated with natural convection flows can play a critical role in determining skin friction and heat transfer rates at the boundary. In thermal engineering, natural convection flows from cylindrical bodies has gained exceptional interest. In this article, we mathematically evaluate an entropy analysis of magnetohydrodynamic third-grade convection flows from permeable cylinder considering velocity and thermal slip effects. The resulting non-linear coupled partial differential conservation equations with associated boundary conditions are solved with an efficient unconditionally stable implicit finite difference Keller-Box technique. The impacts of momentum and heat transport coefficients, entropy generation and Bejan number are computed for several values of non-dimensional parameters arising in the flow equations. Streamlines are plotted to analyze the heat transport process in a two-dimensional domain. Furthermore, the deviations of the flow variables are compared with those computed for a Newtonian fluid and this has important implications in industrial thermal material processing operations, aviation technology, different enterprises, energy systems and thermal enhancement of industrial flow processes.
5
Content available Statistical analysis of entropy generation in longitudinally finned tube heat exchanger with shell side nanofluid by a single phase approach
Konchada P. K., Pv V., Bhemuni V.
Archives of Thermodynamics
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2016
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Vol. 37, no. 2
3--22
EN
The presence of nanoparticles in heat exchangers ascertained increment in heat transfer. The present work focuses on heat transfer in a longitudinal finned tube heat exchanger. Experimentation is done on longitudinal finned tube heat exchanger with pure water as working fluid and the outcome is compared numerically using computational fluid dynamics (CFD) package based on finite volume method for different flow rates. Further 0.8% volume fraction of aluminum oxide (Al2O3) nanofluid is considered on shell side. The simulated nanofluid analysis has been carried out using single phase approach in CFD by updating the user-defined functions and expressions with thermophysical properties of the selected nanofluid. These results are thereafter compared against the results obtained for pure water as shell side fluid. Entropy generated due to heat transfer and fluid flow is calculated for the nanofluid. Analysis of entropy generation is carried out using the Taguchi technique. Analysis of variance (ANOVA) results show that the inlet temperature on shell side has more pronounced effect on entropy generation.
6
Content available Entropy generation in a condenser and related correlations
Laskowski R., Jaworski M., Smyk A.
Archives of Thermodynamics
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2015
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Vol. 36, no. 2
27--48
EN
The paper presents an analysis of relations describing entropy generation in a condenser of a steam unit. Connections between entropy generation, condenser ratio, and heat exchanger effectiveness, as well as relations implied by them are shown. Theoretical considerations allowed to determine limits of individual parameters which describe the condenser operation. Various relations for average temperature of the cold fluid were compared. All the proposed relations were verified against data obtained using a simulator and actual measurement data from a 200 MW unit condenser. Based on data from a simulator it was examined how the sum of entropy rates, steam condenser effectiveness, terminal temperature difference and condenser ratio vary with the change in the inlet cooling water temperature, mass flow rate of steam and the cooling water mass flow rate.
7
Energia czy egzergia
Szargut J.
Rynek Energii
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2010
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nr 5
3-5
PL
W języku potocznym słowem "energia" określa się zwykle tylko energię podwyższonej jakości. Jakość tę określa egzergia, nie podlegająca prawu zachowania. Straty egzergii są proporcjonalne do entropii generowanej. Należy dążyć do ich zmniejszania w granicach opłacalności ekonomicznej i możliwości technologicznych. Znajomość analizy egzergetycznej jest niezbędna przy projektowaniu i udoskonalaniu procesów energetycznych.
EN
In a colloquial speech the word "energy" is used for the kinds of energy characterized with a higher quality. In a scientitic speech the quality of energy is expressed by means of exergy, exempt from the law of conservation. The losses of exergy are proportional to the generated entropy. For the reduction of these losses it should be striven in the limits of economic profitability and technological possibilities. The knowledge of the exergy analysis is indispensable in the design and improvement of energy processes.
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