Hasan Saygin

Brayton refrigeration cycles working under quantum degeneracy conditions

At sufficiently low temperatures, quantum degeneracy of gas particles becomes important and an ideal gas deviates from the classical ideal-gas behaviour. In such a case, an ideal gas is called a quantum ideal gas. For quantum ideal gases, a corrected equation of state, which considers the quantum behaviour of gas particles, is used instead of the classical one. It is valid for both quantum and classical ideal-gases and it is reduced to a classical ideal-gas equation-of-state, under the classical gas conditions. There are two types of quantum ideal-gases. One of them is the Bose type and the other is the Fermi type. Here, Brayton refrigeration cycles working with Bose and Fermi type ideal quantum gases are considered and they are called Bose and Fermi Brayton cycles respectively. Coefficients of performance and refrigeration loads of these cycles are derived by using the corrected equation of state. It is seen that refrigeration loads are different from those of the classical Brayton cycle, which works with the classical ideal gas. On the other hand, coefficients of performance of these cycles are not effected by the quantum degeneracy of the refrigerant and they are the same as that of the classical cycle. Variations of the refrigeration load with low temperature (TL) and low pressure (PL) are examined. Under the quantum degeneracy conditions, it is shown that the refrigeration load of the Bose Brayton cycle is always greater than that of the classical Brayton cycle. On the contrary, the refrigeration load of the Fermi Brayton cycle is always lower than that of the classical one. Moreover, the minimum value of TL for the Bose Brayton cycle is restricted by the Bose-Einstein condensation temperature for a given value of PL.

On the power cycles working with ideal quantum gases: I. The Ericsson cycle

The Ericsson power cycles working with ideal Bose and Fermi monoatomic gases are examined. They are conveniently called the Bose and Fermi cycles. Efficiencies of Bose and Fermi cycles are derived ( and respectively). Variations of them with the temperature ratio and pressure ratio of the cycle are examined. A comparison of the efficiencies with each other and that of the classical Ericsson cycle is made. In the degenerate gas state it is seen that , although in the classical gas state. In a Bose cycle, it is shown that there is an optimum value for the lowest temperature at which the efficiency reaches its maximum value for a given pressure ratio. Furthermore, Bose-Einstein condensation restricts the value of of a Bose cycle for a given value of . In a Fermi cycle, there is no an optimum value for . However, goes to a finite value of less than unity when goes to zero.

Quantum degeneracy effect on the work output from a Stirling cycle

The effect of quantum degeneracy on the work output from a Stirling cycle working at quantum degeneracy conditions (QDCs) is analyzed. Expressions for net work outputs of Stirling power cycles working with monatomic ideal Bose and Fermi gases are derived by using the quantum ideal gas equation of state. Ratios of net work outputs of Stirling cycles working with Bose and Fermi gases to the net work output of a classical Stirling cycle (RWB and RWF, respectively) are obtained. Variations of RWB and RWF with TH are examined for a given temperature ratio (τ=TL/TH) and a specific volume ratio (rν=νH/νL). At QDC, it is seen that RWB has a maximum value, which is greater than unity. On the other hand, there is no maximum or minimum point for RWF and RWF⩽1 for any values of TH. Consequently, the use of Bose gas as a working fluid in a Stirling cycle provides an advantage since it causes the net work output per cycle to increase by consuming more heat energy. This fact is seen to be in the opposite direction for a...

The improvement effect of quantum degeneracy on the work from a Carnot cycle

The efficiency of a Carnot ([eta]C) cycle is independent of the physical properties of the working gas. Therefore, [eta]C does not change due to quantum degeneracy of the gas. On the other hand, cycle work depends on the physical properties of the working gas since it is determined by the equation of state of the gas. Therefore, cycle work can be influenced by the quantum degeneracy of working gas. Here, Carnot power cycles working with ideal Bose and Fermi gases are examined under quantum degeneracy conditions. They are called Bose and Fermi Carnot cycles respectively. Cycle works of Bose and Fermi Carnot cycles (WB and WF) are derived. By dividing these works into the work of the classical Carnot power cycle (WC), which works with classical ideal gas, work ratios are defined as RWB=WB/WC and RWF=WF/WC. Variations of RWB and RWF with high temperature of the cycle (TH) are examined for a given temperature ratio [tau]=TL/TH and specific volume ratio rv=vH/vL. It is shown that RWB>1 for some values of TH while RWF

A time-symmetric block time-step algorithm for N-body simulations

Abstract The method of choice for integrating the equations of motion of the general N -body problem has been to use an individual time step scheme. For the sake of efficiency, block time steps have been the most popular, where all time step sizes are smaller than a maximum time step size by an integer power of two. We present the first successful attempt to construct a time-symmetric integration scheme, based on block time steps. We demonstrate how our scheme shows a vastly better long-time behavior of energy errors, in the form of a random walk rather than a linear drift. Increasing the number of particles makes the improvement even more pronounced.

Joule-Thomson coefficients of quantum ideal-gases

The temperature drop of a gas divided by its pressure drop under constant enthalpy conditions is called the Joule-Thomson coefficient (JTC) of the gas. The JTC of an ideal gas is equal to zero since its enthalpy depends on only temperature. On the other hand, this is only true for classical ideal gas which obeys the classical ideal gas equation of state, pV=mRT. Under sufficiently low-temperature or high-pressure conditions, the quantum nature of gas particles becomes important and an ideal gas behaves like a quantum ideal gas instead of a classical one. In such a case, enthalpy becomes dependent on both temperature and pressure. Therefore, JTC of a quantum ideal gas is not equal to zero. In this work, the contribution of purely quantum nature of gas particles on JTC is examined. JTCs of monatomic Bose and Fermi type quantum ideal gases are derived. Their variations with temperature are examined for different pressure values. It is shown that JTC of a Bose gas is always greater than zero. Minimum value of temperature is limited by the Bose-Einstein condensation phenomena under the constant enthalpy condition. On the other hand, it is seen that JTC of a Fermi gas is always lower than zero and there is not any limitation on its temperature. For high temperature values, JTCs of Bose and Fermi gases go to zero since the quantum nature of gas particles becomes negligible. Moreover, variation of temperature versus pressure under the constant enthalpy condition is also examined. Consequently, it is understood that the quantum nature of a Bose-type gas contributes to the positive values of JTC while the quantum nature of a Fermi type gas contributes to the negative values of JTC. Therefore, a Bose-type gas is more suitable for cryogenic refrigeration systems.

Interaction of PM2.5 airborne particulates with ZnO and TiO 2 nanoparticles and their effect on bacteria

A significant knowledge gap in nanotechnology is the absence of standardized protocols for examining and comparison the effect of metal oxide nanoparticles on different environment media. Despite the large number of studies on ecotoxicity of nanoparticles, most of them disregard the particles physicochemical transformation under real exposure conditions and interaction with different environmental components like air, soil, water, etc. While one of the main exposure ways is inhalation and/or atmosphere for human and environment, there is no investigation between airborne particulates and nanoparticles. In this study, some metal oxide nanoparticle (ZnO and TiO2) transformation and behavior in PM2.5 air particulate media were examined and evaluated by the influence on nanoparticle physicochemical properties (size, surface charge, surface functionalization) and on bacterium (Gram-positive Bacillus subtilis, Staphylococcus aureus/Gram-negative Escherichia coli, Pseudomonas aeruginosa bacteria) by testing in various concentrations of PM2.5 airborne particulate media to contribute to their environmental hazard and risk assessment in atmosphere. PM2.5 airborne particulate media affected their toxicity and physicochemical properties when compared the results obtained in controlled conditions. ZnO and TiO2 surfaces were functionalized mainly with sulfoxide groups in PM2.5 air particulates. In addition, tested particles were not observed to be toxic in controlled conditions. However, these were observed inhibition in PM2.5 airborne particulates media by the exposure concentration. These observations and dependence of the bacteria viability ratio explain the importance of particulate matter-nanoparticle interaction.

A NEW MULTI-FUNCTIONAL HALF MODE SUBSTRATE INTEGRATED WAVEGUIDE SIX-PORT MICROWAVE COMPONENT

By attention to price of microwave components and need to use of them in many applications, the creation of an integrated component which can incorporate the performances of several components in one structure is a necessity. Therefore, in this paper a novel symmetric six-ports multifunctional microwave component is designed and realized. The proposed component consists of two modified half mode substrates integrated waveguide couplers which are joined and a slot which is attained from joined two mentioned couplers. Despite the slot prevents the exciting of higher order modes in proposed component, it divides signal in two parts by exciting middle SIW ports. By exciting each of the ports as input, the component can act as an equal and an unequal 90-degree couplers or power dividers. The proposed component with mentioned conditions covers 23.5% frequency bandwidth with maximum magnitude and phase error of ±0.7 dB and ±0.63 degree, respectively.

Numerical simulation of Kelvin-Helmholtz instability using an implicit, non-dissipative DNS algorithm

An in-house, fully parallel compressible Navier-Stokes solver was developed based on an implicit, non-dissipative, energy conserving, finite-volume algorithm. PETSc software was utilized for this purpose. To be able to handle occasional instances of slow convergence due to possible oscillating pressure corrections on successive iterations in time, a fixing procedure was adopted. To demonstrate the algorithms ability to evolve a linear perturbation into nonlinear hydrodynamic turbulence, temporal Kelvin-Helmholtz Instability problem is studied. KHI occurs when a perturbation is introduced into a system with a velocity shear. The theory can be used to predict the onset of instability and transition to turbulence in fluids moving at various speeds. In this study, growth rate of the instability was compared to predictions from linear theory using a single mode perturbation in the linear regime. Effect of various factors on growth rate was also discussed. Compressible KHI is most unstable in subsonic/transonic regime. High Reynolds number (low viscosity) allows perturbations to develop easily, in consistent with the nature of KHI. Higher wave numbers (shorter wavelengths) also grow faster. These results match with the findings of stability analysis, as well as other results presented in the literature.

Conduction Heat Transfer Problem Solution Using the Method of Fundamental Solutions With the Dual Reciprocity Method

The method of fundamental solutions (MFS) is first proposed in 1964 by Kupradze and theoretical basis of this method is constructed at the end of 1980’s. As a meshless method, no domain meshing is required for MFS. Fundamental solutions are used to solve problems without coping with the singularity on the boundary because of the fictitious boundary defined containing the domain of the problem. In this paper effectiveness of the MFS will be introduced by two test problem for the homogeneous and inhomogeneous modified helmholtz equations. In-homogeneous terms are approximated by using the method of particular solutions through the dual reciprocity method. The conduction heat transfer problem is defined and transformed to the corresponding elliptic partial differential equation by using finite difference and the method of lines method which gives an inhomogeneous helmholtz equation. Then the problem is solved iteratively by using the MFS. Two test problem are solved by both the finite element method (FEM) and MFS and compared in the figures. It can be seen that as a meshless method, MFS gives very good results for the test problems. The thermal shock problem presented here also gives accurate solutions by using MFS and agrees well with the FEM solution.Copyright