The Experts below are selected from a list of 26778 Experts worldwide ranked by ideXlab platform
Mohsen Torabi - One of the best experts on this subject based on the ideXlab platform.
-
Entropy Generation of Double Diffusive Forced Convection in Porous Channels with Thick Walls and Soret Effect
Entropy, 2017Co-Authors: Mohsen Torabi, Mehrdad Torabi, G P PetersonAbstract:The second law performance of double diffusive forced convection in a horizontal porous channel with thick walls was considered. The Soret effect is included in the concentration equation and the first order chemical reaction was chosen for the concentration boundary conditions at the porous-solid walls interfaces. This investigation is focused on two principal types of boundary conditions. The first assumes a constant temperature condition at the outer surfaces of the solid walls, and the second assumes a constant heat flux at the lower wall and convection heat transfer at the upper wall. After obtaining the velocity, temperature and concentration distributions, the local and Total Entropy generation formulations were used to visualize the second law performance of the two cases. The results indicate that the Total Entropy generation rate is directly related to the lower wall thickness. Interestingly, it was observed that the Total Entropy generation rate for the second case reaches a minimum value, if the upper and lower wall thicknesses are chosen correctly. However, this observation was not true for the first case. These analyses can be useful for the design of microreactors and microcombustor systems when the second law analysis is taken into account.
-
interface Entropy generation in micro porous channels with velocity slip and temperature jump
Applied Thermal Engineering, 2017Co-Authors: Mohsen Torabi, Z M Zhang, G P PetersonAbstract:Abstract In a certain size micro thermofluid systems, the temperature of the cooling fluid at the vicinity of the solid hot wall differs from the temperature of the wall. This temperature difference can be modelled by using a temperature jump parameter, which relates the temperatures of the fluid and solid at the interface, and the gradient of the temperature in the solid wall. In this investigation, two micro porous channels with asymmetric thick walls have been considered; one with constant, but different, temperature boundary conditions and the other one with heat flux and convection boundary conditions at each of the walls, in order to determine the impact of the interface Entropy generation rates on the Total Entropy generation calculation, particularly when the solid-fluid interface temperature jump has been assumed in a micro channel. The effects of the magnetic field have been addressed in both the momentum and energy equations of the porous section of the system. The slip velocity and temperature jump interface boundary conditions for both the upper and lower fluid-wall interfaces are considered. The results indicate that when the temperature jump parameter is weak, the interfaces Entropy generation rates may be neglected in the calculation of the Total Entropy generation rate. However, if the temperature jump parameter is strong enough, the Total Entropy generation rate should be calculated by considering the interfaces Entropy generation rate. It is interesting to note that depending on the micro porous channel outer boundary conditions, the Total Entropy generation rate may increase or decrease in accordance with the temperature jump parameter.
-
Temperature distribution, local and Total Entropy generation analyses in MHD porous channels with thick walls
Energy, 2015Co-Authors: Mohsen Torabi, Kaili ZhangAbstract:Entropy generation rate is an important characteristic of a thermal system. This work aims to study the temperature distribution, and local and Total Entropy generation rates within a horizontal porous channel under a uniform magnetic field with thick walls. The thermal conductivity of the walls are considered temperature-dependent and viscous dissipation effects are incorporated into the energy equation. Two types of boundary conditions are employed: Case one which has constant but different temperature boundary conditions and Case two which has heat flux boundary condition on the lower wall and convective boundary condition on the upper wall. Using a combined analytical-numerical solution procedure the temperature fields are obtained. Thereafter, the local and Total Entropy generation rates are achieved. The correctness of the analytical-numerical solution technique is checked against a completely analytical solution, for cases with temperature-independent thermal conductivities of walls. After validation, the general solution procedure, i.e., solution for cases with temperature-dependent thermal conductivities, is used to investigate the effect of various parameters such as Brinkman number, Hartmann number, Darcy number, porous medium to solid parts thermal conductivity ratio, etc. on the temperature field and Entropy generation rates. As an interesting result it was found that depending on the boundary conditions of the channel, porous medium to solid parts thermal conductivity ratio may increase or decrease the Total Entropy generation rate.
-
Heat transfer and thermodynamic performance of convective–radiative cooling double layer walls with temperature-dependent thermal conductivity and internal heat generation
Energy Conversion and Management, 2015Co-Authors: Mohsen Torabi, Kaili ZhangAbstract:Abstract Composite geometries have numerous applications in industry and scientific researches. This work investigates the temperature distribution, and local and Total Entropy generation rates within two-layer composite walls using conjugate convection and radiation boundary conditions. Thermal conductivities of the materials of walls are assumed temperature-dependent. Temperature-dependent internal heat generations are also incorporated into the modeling. The differential transformation method (DTM) is used as an analytical technique to tackle the highly nonlinear system of ordinary differential equations. Thereafter, the local and Total Entropy generation rates are calculated using the DTM formulated temperature distribution. An exact analytical solution, for the temperature-independent model without radiation effect, is also derived. The correctness and accuracy of the DTM solution are checked against the exact solution. After verification, effects of thermophysical parameters such as location of the interface, convection–conduction parameters, radiation–conduction parameters, and internal heat generations, on the temperature distribution, and both local and Total Entropy generation rates are examined. To deliver the minimum Total Entropy generation rate, optimum values for some parameters are also found. Since composite walls are widely used in many fields, the abovementioned investigation is a beneficial tool for many engineering industries and scientific fields to minimize the Entropy generation, which is the exergy destruction, of the system.
-
Temperature distribution, local and Total Entropy generation analyses in asymmetric cooling composite geometries with multiple nonlinearities: Effect of imperfect thermal contact
Energy, 2014Co-Authors: Mohsen Torabi, Kaili Zhang, Guangcheng Yang, Jun WangAbstract:Entropy generation, which is available exergy destruction, is an important subject in fields of energy management and thermal engineering. With the fast-growing rate of composite media applications in both industries and academic researches, it is necessary to study these media from the second law of thermodynamics point of view. In this work, three fundamental composite media, i.e., composite walls, cylinders and spheres, are considered. The thermal contact resistance between two layers of each medium is considered to be non-zero, and the effect of the radiation heat loss from the second layer, i.e., the outer layer of the composite system, is taken into account. Thermal conductivities are assumed temperature-dependent. Temperature-independent internal heat generation within each layer is considered. The system of non-linear ordinary differential equations is solved with a combined analytical–numerical technique. Assuming temperature-independent thermal conductivities and neglecting the radiation effect, the system of ordinary equations can be solved with an exact analytical technique. Finding the solution of the temperature distribution and local Entropy generation rate with this exact analytical procedure, provides a practical tool to check the correctness and accuracy of the combined analytical–numerical solution for general problems, i.e., with the radiation effect and temperature-dependent thermal conductivities. Thereafter, temperature distribution, local and Total Entropy generation rates are plotted for number of parameters for three considered composite geometries. It is found that assuming zero thermal contact resistance overestimates the Total Entropy generation rate within these composite media. Depending on the value of parameters, it is or is not possible to find an optimum value for the radiation parameter to minimize the Total Entropy generation rate within these media.
Sergio Ciliberto - One of the best experts on this subject based on the ideXlab platform.
-
fluctuations of the Total Entropy production in stochastic systems
EPL, 2008Co-Authors: Sylvain Joubaud, Nicolas Garnier, Sergio CilibertoAbstract:Fluctuations of the Total Entropy are experimentally investigated in two stochastic systems in a non-equilibrium steady state: an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study the Total Entropy production which is the Entropy created to maintain the system in the non-equilibrium steady state. The fluctuation theorem holds for the Total Entropy production in the two experimental systems, both for all observation times and for all fluctuation magnitudes.
-
Fluctuations of the Total Entropy production in stochastic systems
EPL, 2008Co-Authors: Sylvain Joubaud, N. B. Garnier, Sergio CilibertoAbstract:Fluctuations of the excess heat in an out of equilibrium steady state are experimentally investigated in two stochastic systems : an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study excess heat that represents the difference between the dissipated heat out of equilibrium and the dissipated heat at equilibrium. Fluctuation theorem holds for the excess heat in the two experimental systems for all observation times and for all fluctuation magnitudes.
Kaili Zhang - One of the best experts on this subject based on the ideXlab platform.
-
Temperature distribution, local and Total Entropy generation analyses in MHD porous channels with thick walls
Energy, 2015Co-Authors: Mohsen Torabi, Kaili ZhangAbstract:Entropy generation rate is an important characteristic of a thermal system. This work aims to study the temperature distribution, and local and Total Entropy generation rates within a horizontal porous channel under a uniform magnetic field with thick walls. The thermal conductivity of the walls are considered temperature-dependent and viscous dissipation effects are incorporated into the energy equation. Two types of boundary conditions are employed: Case one which has constant but different temperature boundary conditions and Case two which has heat flux boundary condition on the lower wall and convective boundary condition on the upper wall. Using a combined analytical-numerical solution procedure the temperature fields are obtained. Thereafter, the local and Total Entropy generation rates are achieved. The correctness of the analytical-numerical solution technique is checked against a completely analytical solution, for cases with temperature-independent thermal conductivities of walls. After validation, the general solution procedure, i.e., solution for cases with temperature-dependent thermal conductivities, is used to investigate the effect of various parameters such as Brinkman number, Hartmann number, Darcy number, porous medium to solid parts thermal conductivity ratio, etc. on the temperature field and Entropy generation rates. As an interesting result it was found that depending on the boundary conditions of the channel, porous medium to solid parts thermal conductivity ratio may increase or decrease the Total Entropy generation rate.
-
Heat transfer and thermodynamic performance of convective–radiative cooling double layer walls with temperature-dependent thermal conductivity and internal heat generation
Energy Conversion and Management, 2015Co-Authors: Mohsen Torabi, Kaili ZhangAbstract:Abstract Composite geometries have numerous applications in industry and scientific researches. This work investigates the temperature distribution, and local and Total Entropy generation rates within two-layer composite walls using conjugate convection and radiation boundary conditions. Thermal conductivities of the materials of walls are assumed temperature-dependent. Temperature-dependent internal heat generations are also incorporated into the modeling. The differential transformation method (DTM) is used as an analytical technique to tackle the highly nonlinear system of ordinary differential equations. Thereafter, the local and Total Entropy generation rates are calculated using the DTM formulated temperature distribution. An exact analytical solution, for the temperature-independent model without radiation effect, is also derived. The correctness and accuracy of the DTM solution are checked against the exact solution. After verification, effects of thermophysical parameters such as location of the interface, convection–conduction parameters, radiation–conduction parameters, and internal heat generations, on the temperature distribution, and both local and Total Entropy generation rates are examined. To deliver the minimum Total Entropy generation rate, optimum values for some parameters are also found. Since composite walls are widely used in many fields, the abovementioned investigation is a beneficial tool for many engineering industries and scientific fields to minimize the Entropy generation, which is the exergy destruction, of the system.
-
Temperature distribution, local and Total Entropy generation analyses in asymmetric cooling composite geometries with multiple nonlinearities: Effect of imperfect thermal contact
Energy, 2014Co-Authors: Mohsen Torabi, Kaili Zhang, Guangcheng Yang, Jun WangAbstract:Entropy generation, which is available exergy destruction, is an important subject in fields of energy management and thermal engineering. With the fast-growing rate of composite media applications in both industries and academic researches, it is necessary to study these media from the second law of thermodynamics point of view. In this work, three fundamental composite media, i.e., composite walls, cylinders and spheres, are considered. The thermal contact resistance between two layers of each medium is considered to be non-zero, and the effect of the radiation heat loss from the second layer, i.e., the outer layer of the composite system, is taken into account. Thermal conductivities are assumed temperature-dependent. Temperature-independent internal heat generation within each layer is considered. The system of non-linear ordinary differential equations is solved with a combined analytical–numerical technique. Assuming temperature-independent thermal conductivities and neglecting the radiation effect, the system of ordinary equations can be solved with an exact analytical technique. Finding the solution of the temperature distribution and local Entropy generation rate with this exact analytical procedure, provides a practical tool to check the correctness and accuracy of the combined analytical–numerical solution for general problems, i.e., with the radiation effect and temperature-dependent thermal conductivities. Thereafter, temperature distribution, local and Total Entropy generation rates are plotted for number of parameters for three considered composite geometries. It is found that assuming zero thermal contact resistance overestimates the Total Entropy generation rate within these composite media. Depending on the value of parameters, it is or is not possible to find an optimum value for the radiation parameter to minimize the Total Entropy generation rate within these media.
Sylvain Joubaud - One of the best experts on this subject based on the ideXlab platform.
-
fluctuations of the Total Entropy production in stochastic systems
EPL, 2008Co-Authors: Sylvain Joubaud, Nicolas Garnier, Sergio CilibertoAbstract:Fluctuations of the Total Entropy are experimentally investigated in two stochastic systems in a non-equilibrium steady state: an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study the Total Entropy production which is the Entropy created to maintain the system in the non-equilibrium steady state. The fluctuation theorem holds for the Total Entropy production in the two experimental systems, both for all observation times and for all fluctuation magnitudes.
-
Fluctuations of the Total Entropy production in stochastic systems
EPL, 2008Co-Authors: Sylvain Joubaud, N. B. Garnier, Sergio CilibertoAbstract:Fluctuations of the excess heat in an out of equilibrium steady state are experimentally investigated in two stochastic systems : an electric circuit with an imposed mean current and a harmonic oscillator driven out of equilibrium by a periodic torque. In these two linear systems, we study excess heat that represents the difference between the dissipated heat out of equilibrium and the dissipated heat at equilibrium. Fluctuation theorem holds for the excess heat in the two experimental systems for all observation times and for all fluctuation magnitudes.
Ken R. Duffy - One of the best experts on this subject based on the ideXlab platform.
-
Guesswork Subject to a Total Entropy Budget
arXiv: Information Theory, 2017Co-Authors: Arman Rezaee, Ahmad Beirami, Ali Makhdoumi, Muriel Medard, Ken R. DuffyAbstract:We consider an abstraction of computational security in password protected systems where a user draws a secret string of given length with i.i.d. characters from a finite alphabet, and an adversary would like to identify the secret string by querying, or guessing, the identity of the string. The concept of a "Total Entropy budget" on the chosen word by the user is natural, otherwise the chosen password would have arbitrary length and complexity. One intuitively expects that a password chosen from the uniform distribution is more secure. This is not the case, however, if we are considering only the average guesswork of the adversary when the user is subject to a Total Entropy budget. The optimality of the uniform distribution for the user's secret string holds when we have also a budget on the guessing adversary. We suppose that the user is subject to a "Total Entropy budget" for choosing the secret string, whereas the computational capability of the adversary is determined by his "Total guesswork budget." We study the regime where the adversary's chances are exponentially small in guessing the secret string chosen subject to a Total Entropy budget. We introduce a certain notion of uniformity and show that a more uniform source will provide better protection against the adversary in terms of his chances of success in guessing the secret string. In contrast, the average number of queries that it takes the adversary to identify the secret string is smaller for the more uniform secret string subject to the same Total Entropy budget.
-
Allerton - Guesswork subject to a Total Entropy budget
2017 55th Annual Allerton Conference on Communication Control and Computing (Allerton), 2017Co-Authors: Arman Rezaee, Ahmad Beirami, Ali Makhdoumi, Muriel Medard, Ken R. DuffyAbstract:We consider an abstraction of computational security in password protected systems where a user draws a secret string of given length with i.i.d. characters from a finite alphabet, and an adversary would like to identify the secret string by querying, or guessing, the identity of the string. The concept of a “Total Entropy budget” on the chosen word by the user is natural, otherwise the chosen password would have arbitrary length and complexity. One intuitively expects that a password chosen from the uniform distribution is more secure. This is not the case, however, if we are considering only the average guesswork of the adversary when the user is subject to a Total Entropy budget. The optimality of the uniform distribution for the user's secret string holds when we have also a budget on the guessing adversary. We suppose that the user is subject to a “Total Entropy budget” for choosing the secret string, whereas the computational capability of the adversary is determined by his “Total guesswork budget.” We study the regime where the adversary's chances are exponentially small in guessing the secret string chosen subject to a Total Entropy budget. We introduce a certain notion of uniformity and show that a more uniform source will provide better protection against the adversary in terms of his chances of success in guessing the secret string. In contrast, the average number of queries that it takes the adversary to identify the secret string is smaller for the more uniform secret string subject to the same Total Entropy budget.
