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Timeline of Ben Akih-Kumgeh

2018
Jul
13
Published new article




Article

Numerical investigation of the impact of tailored driver gases and driver inserts on shock tube flows

Published: 13 July 2018 by Springer-Verlag in Shock Waves

doi: 10.1007/s00193-018-0851-z

Presented is a numerical study of the effects of imperfect driver gas tailoring and the use of a driver section insert on the flow field within the shock tube. Two-dimensional axisymmetric simulations of a shock tube are conducted with pure argon as the driven gas and four \(\hbox {He/N}_2\) mixtures representing different tailoring conditions. The second part of the study investigates the use of a profiled driver insert with two \(\hbox {He/N}_2\) driver gas mixtures to minimize the pressure rise in the test section such as that resulting from over-tailored driver mixtures. The chemical kinetic implications of imperfectly tailored driver gas mixtures on the interpretation of shock tube experiments are explored through simulations of propanol ignition with pressure and temperature changes prescribed based on the simulation results. The simulations show that imperfect tailoring toward under-tailored mixtures is more detrimental to kinetic studies in the post-reflected shock region than over-tailoring. The least compromising situation in test conditions is observed for a mildly over-tailored condition with 25% of nitrogen as opposed to the tailored value of 20%. The numerical results further show that the combination of the driver insert and slightly over-tailored driver mixture leads to longer test times. The study seeks to address problems of available test times and accuracy of shock tube reactor conditions that are crucial to the use of shock tube data for validation of chemical kinetic models.

1 Reads | 0 Citations
2018
Jul
13
Published new article




Article

Thermomechanics of laser-induced shock waves in combustible mixtures

Published: 13 July 2018 by Springer-Verlag in Shock Waves

doi: 10.1007/s00193-018-0850-0

In this study, a combined experimental and numerical approach is taken to further investigate the thermomechanics of shock wave formation in combustible mixtures. This study focuses on several gas mixtures including air, methane/\(\hbox {N}_2\), methane/air, and biogas/air. The lean and stoichiometric combustible mixtures are studied at three different laser energy levels. The Jones blast wave theory is used to calculate the energy involved in development of the resulting shock waves and used to specify the initial conditions for the plasma in simulations. In addition, interferometry is used to gain insights into the thermodynamic conditions within the plasma kernel, for validation of the numerical model results. The energies calculated for the different mixtures studied show that shock waves generated in the combustible mixture likely gain energy early in the breakdown process from exothermic chemical reactions. Shock wave results from experiment are shown to be replicated by the model, and the variations of the density inside of the hot core region are in agreement between the simulation and experimental results.

0 Reads | 0 Citations
2017
Nov
20
Published new article






Beyond the arrow of time: can there be a relation between measurement of entropy and time?

Published: 20 November 2017 by MDPI AG in Proceedings

doi: 10.3390/ecea-4-05018

<p>The tendency of the entropy of isolated to increase is considered to be directly linked to the direction of the flow of time. This raises the question whether a quantitative relation can be established such that a time interval can be measured by measuring entropy change and vice versa. The existence or absence of such a link also calls for further consideration of the nature of time. Prigogine argued that the true nature of time can only be discovered by investigating this phenomenon using scientific and philosophical methods. If this is true, then ongoing debates in the metaphysics of time and progress in the scientific study of entropy can be brought together to shed light on this fascinating but elusive concept. In this paper, starting from my recent modified definition of entropy change as a non-dimensional measure of energy change, a direct link between entropy and time duration is presented. It draws from steady energy transfer processes such as heat transfer and shows that a measure of time can be found to be associated with a measure of entropy change. In the absence of other driving forces, the passage of time in an isolated system can therefore be tracked with a well calibrated entropy change meter. When other forces are allowed to interfere and there is no external point from which the system can be considered to be isolated, then the measure of time is non-monotonous since an isolated system can be restored to an earlier state of non-equilibrium.</p>

10 Reads | 0 Citations
2016
Nov
07
Published new article






Black Holes and Entropy: A Skeptical Perspective

Published: 07 November 2016 by MDPI AG in 3rd International Electronic and Flipped Conference on Entropy and Its Applications

doi: 10.3390/ecea-3-A006

<p>Black holes are objects of significant interest in modern cosmology. From what initially looked like a superficial analogy between black hole mechanics and thermodynamics, a new epistemological framework has emerged according to which far-reaching conclusions about black hole can be reached through thermodynamic analysis. An example of this is the view that the temperature of a black hole is inversely proportional to its mass. This paper raises doubts about the currently accepted connection between black holes and entropy. It does so by first reviewing the principles of thermodynamics and the properties a system must have in order to admit of proper thermodynamic analysis. It is argued that the current view of black holes preclude their distinct classification either as closed or open systems, a fact which has a bearing on the formulation of the First and Second Laws. From a mechanistic view of temperature and heat, combined with my recent work on the physical meaning of classical entropy, I show that the generalized Second Law of black hole thermodynamics is probably in error. The notion of heat transfer (which is central to entropy definition) is not explicit in the black hole energy equation. To address the challenges raised, black hole mechanics must either commit to a phenomenological approach and therefore only invoke thermodynamics in the classical sense or accept a microscopic view of black hole matter in order to readily draw from established results of statistical mechanics. It is argued that a proper connection to classical&nbsp; thermodynamics would lead to the view that the temperature of a black hole increases with its mass, as a result of which a positive specific heat capacity is to be expected, contrary to the prevailing doctrine.</p>

9 Reads | 0 Citations
2015
Nov
13
Published new article






Toward improved understanding of the physical meaning of entropy in classical thermodynamics

Published: 13 November 2015 by MDPI AG in 2nd International Electronic Conference on Entropy and Its Applications

doi: 10.3390/ecea-2-A014

<p>This year marks the 150th anniversary of the concept of entropy, introduced into thermodynamics by Rudolf Clausius. Despite its central role in the mathematical formulation of the Second Law and most of classical thermodynamics, its physical meaning continues to be elusive and confusing. This is particularly the case when one invokes the connection between the classical thermodynamics of a system and the statistical behavior of its constituent microscopic particles.</p> <p>This paper sketches Clausius approach to its definition and offers a modified mathematical definition that is still in the spirit of Clausius&rsquo; derivation. In the modified version, the differential of specific entropy appears as a non-dimensional energy term that captures the invigoration or reduction of microscopic motion upon addition or withdrawal of heat from the system. It is also argued that heat transfer is a better thermodynamic model process to illustrate the concept of entropy instead of the canonical heat engines and refrigerators that are not relevant to new areas of thermodynamics (e.g. thermodynamics of biological systems). In this light, it is emphasized that entropy changes, as invoked in the Second Law, are necessarily related to the non-equilibrium interactions of two or more systems that might have initially been in thermal equilibrium but at different temperatures. The overall direction of entropy increase indicates the direction of naturally occurring heat transfer processes in an isolated system of internally interacting (non-isolated) sub systems.</p> <p>We discuss the implication of the proposed modification on the interpretation of entropy in statistical thermodynamics as well as the formulation of the most common thermodynamic potentials.</p>

4 Reads | 0 Citations
2014
Sep
16
Published new article




Article

Comparative Analysis of Chemical Kinetic Models Using the Alternate Species Elimination Approach

Published: 16 September 2014 by ASME International in Journal of Engineering for Gas Turbines and Power

doi: 10.1115/1.4028388

A major thrust in combustion research is the development of chemical kinetic models for computational analysis of various combustion processes. Significant deviations can be seen when comparing predictions of these models against experimentally determined combustion properties over a wide range of operating conditions and mixture strengths. However, these deviations vary from one model to another. It would be insightful in such circumstances to elucidate the species and subchemistry models which lead to the varying prediction ability in various models. In this work, we apply the alternate species elimination (ASE) method to selected mechanisms in order to analyze their predictive ability with respect to propane and syngas combustion. ASE is applied to a homogeneous reactor undergoing ignition. The ranked species of each model are compared based on their normalized changes. We further provide skeletal versions of the various models for propane and syngas combustion analysis. It is observed that this approach provides an easy way to determine the chemical species which are central to the predictive performance of a model in their order of importance. It also provides a direct way to compare the relative importance of chemical species in the models under consideration. Further development and in-depth analysis could provide more information and guidance for model improvement.

3 Reads | 0 Citations
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