Temperature-dependent Specific Heats Research Articles

Model for Chapman-Jouguet deflagrations in open ended tubes with varying vent ratios

We present a gas dynamic model for Chapman-Jouguet deflagrations driving a shock in tubes or congested configurations with rear venting. The model extends the previous work of Chue, Clarke and Lee (Proc. Roy. Soc. 1993) to include rear venting. Two models are formulated, one for a perfect gas with constant heat of combustion, and the second by assuming chemical equilibrium in the product gases and temperature dependent specific heats. In the presence of rear venting, much slower flames and shock strengths are generated; these are controlled by the vent ratio, which reduces the post shock strength and speed of the gas convecting the flame forward. Calculation results are presented for small hydrocarbon-air and hydrogen-air deflagrations. The results of the model are found in good agreement with the critical deflagration speed measured experimentally in large scale weakly confined congested experiments for auto-ignition insensitive mixtures. This suggests that flames in these experiments have reached their maximum Chapman-Jouguet deflagration speed prior to transition. We present a time scale estimate for flame acceleration and auto-ignition behind reflected shocks. For sensitive mixtures for which the flame acceleration time is longer than the auto-ignition time (hydrogen, acetylene), limited experimental evidence indicate transition to detonation at critical speeds lower than the CJ deflagration condition. The time scale ratio thus provides the conditions for which reaching the CJ deflagration condition can be taken as a necessary condition for DDT.

Navier–Stokes Equations and Bulk Viscosity for a Polyatomic Gas with Temperature-Dependent Specific Heats

A system of Navier–Stokes-type equations with two temperatures is derived, for a polyatomic gas with temperature-dependent specific heats (thermally perfect gas), from the ellipsoidal statistical (ES) model of the Boltzmann equation extended to such a gas. Subsequently, the system is applied to the problem of shock-wave structure for a gas with large bulk viscosity (or, equivalently, with slow relaxation of the internal modes), and the numerical results are compared with those based on the ordinary Navier–Stokes equations. It is shown that the latter equations fail to describe the double-layer structure of shock profiles for a gas with large bulk viscosity.

Kinetic description of polyatomic gases with temperature-dependent specific heats

We present the continuous kinetic approach to modeling of thermally perfect (non-polytropic) gases with temperature dependent molecular internal degrees of freedom. Starting from the Boltzmann collision operator and a specific choice of the cross section enriched with free parameters, models are developed for transport coefficients, bulk and shear viscosities, and thermal conductivity. A procedure is proposed for simultaneously fitting parameters to match experimental data for temperature dependence of the specific heat, shear viscosity, and Prandtl number. Results for the dynamic pressure relaxation time, or equivalently, the bulk viscosity, are compared with previously existing estimates.

Oscillation-free nodal discontinuous spectral element method for the simulation of compressible multicomponent flows

The nodal discontinuous spectral element method (DSEM) is improved to avoid pressure and velocity oscillations from material interfaces in the solution of multi-component compressible flows. A novel double flux method compatible with the DSEM scheme is developed to account for variations in the specific heats ratio at material interfaces. The double flux method consists of a modified Harten-Lax-van Leer-Contact (HLLC) Riemann solver and an averaging technique to freeze the temperature-dependent specific heats ratio during the solution update to remove oscillations. The double flux model is combined with a conservative method of calculating the convective flux function to avoid unnecessary energy conservation error at shock fronts. Numerical diffusion terms are also introduced for capturing a shock wave, contact discontinuity, and material interface, without inducing oscillations at the material interface. A series of one- and two-dimensional tests are conducted to evaluate the efficacy of the numerical scheme in capturing sharp discontinuities, as well as vortices, without introducing significant conservation error. According to the benchmark results, the new approach maintains the pressure, velocity, and temperature equilibriums across material interfaces. Artificial diffusion is added to the equation in a controlled manner that enables a higher Courant-Friedrichs-Lewy (CFL) number for high polynomial orders. The present scheme's performance is validated by comparing the spatiotemporal evolution of characteristic points in the shock-bubble interaction and Richtmyer-Meshokov's instability problems.

Yakıt olarak benzin ve LPG kullanılan buji ateşlemeli bir motorun simülasyonu ve performans analizi

Bu çalışmada sonlu zaman termodinamiği metodu kullanılarak buji ateşlemeli motorlarda kullanılmak üzere, Otto çevrimi için bir simülasyon modeli oluşturulmuştur. Simülasyon modelinde, logaritmik bir fonksiyona göre özgül ısıların sıcaklığa bağlı değiştiği, çevrim başlangıç sıcaklığının artık gaz sıcaklığının etkisi ile ortam sıcaklığından yüksek olduğu, sıkıştırma genişleme süreçlerinde tersinmezliklerin olduğu ve ısı, yanma ve sürtünme kayıplarının olduğu, çalışma maddesinin yakıt-hava-artık gaz karışımından oluştuğu kabul edilmiştir. Sıkıştırma oranı, eşdeğerlik oranı ve strok/çap oranının motor performansına etkisi detaylı olarak incelenmiştir. Performans analizi için ısıl verim, özgül yakıt tüketimi, özgül yakıt maliyeti ve güç yoğunluğu kullanılmıştır. Ayrıca motorda LPG kullanıldığında hacimsel verimin azaldığı kabul edilmiştir. Motor ısı balansı için performans kaybı faktörleri kullanılmıştır. Tek silindirli buji ateşlemeli bir motorun özellikleri referans olarak simülasyonda kullanılmıştır. Yapılan kapsamlı sayısal çalışma neticesinde, LPG kullanımında hacimsel verimin %10 azaldığı kabul edildiğinde güç yoğunluğunun %12 azaldığı görülmüştür. LPG’nin özgül yakıt tüketimi benzine göre yüksek olmasına rağmen LPG/benzin fiyat oranı nedeniyle LPG’nin özgül yakıt maliyeti benzine göre oldukça düşük olmaktadır. Sıkıştırma oranının artmasıyla ve strok/çap oranının azalmasıyla birlikte performans kayıpları azalmaktadır. Ayrıca eşdeğerlik oranı 1’den büyük olduğunda performans kayıpları artmaktadır. LPG’nin performans düşüklüğü özellikle hacimsel verimin artırılması veya sıkıştırma oranının artırılmasıyla bertaraf edilebileceği görülmektedir. LPG/benzin fiyat oranı 0,54 olduğunda LPG ile çalışan motorun benzin ile çalışmasına kıyasla %24 daha ekonomik olduğu belirlenmiştir. LPG/benzin fiyat oranı 0,67 olduğunda ise LPG’nin ekonomik avantajının olmadığı görülmüştür. Bu çalışmayla birlikte özellikle motor tasarımcıları için önemli sonuçlar elde edilmiştir.

Optimization of the phase-change wallboard test method: Experimental and numerical investigation

Abstract The application of phase change materials (PCMs) within building envelopes has been a topic of significant research in recent years. Conventionally, determination of the thermophysical properties of PCM-containing wallboard is achieved using the differential scanning calorimetry (DSC) and guarded hot plates method. The thermophysical parameters (phase change temperature, specific heat (Cp), and enthalpy-temperature curves (H(t))), however, of PCM wallboard (PCMW) determined using different methods present certain deviations. This has a significant impact on the reliability of corresponding numerical simulation investigations (e.g., EnergyPlus). Taking this into consideration, this study compares the temperature-dependent specific heats - Cp(T) - from both the DSC and guarded hot plates methods (in dynamic mode, steady mode, and step mode) via experimental investigation. Numerical simulation (using FLUENT software) was then employed to deduce the optimal test parameters for the phase change wallboard test, applying the guarded hot plates method (including dynamic/linear mode, steady mode, and step mode). These parameters include the heating/cooling rate, PCM content, wallboard thickness, and recommended thermal stability time (RST). Key results show that, using the dynamic mode, the heating/cooling rate should be set to 0.5°C/min or less, while for the step mode, the RST function was established. The purpose of this study was to propose an optimal phase-change wallboard test mode, using the guarded hot plates method, to accurately measure the thermophysical properties of PCMs within a full-scale specimen.

A Kinetic Model for a Polyatomic Gas with Temperature-Dependent Specific Heats and Its Application to Shock-Wave Structure

The ellipsoidal statistical (ES) model of the Boltzmann equation for a polyatomic gas with constant specific heats (calorically perfect gas), proposed by Andries et al. (Eur J Mech B Fluids 19:813, 2000), is extended to a polyatomic gas with temperature-dependent specific heats (thermally perfect gas). Then, the new model equation is applied to investigate the structure of a plane shock wave with special interest in $$\hbox {CO}_2$$ gas, which is known to have a very large bulk viscosity, and in the case of relatively strong shock waves. A numerical analysis, as well as an asymptotic analysis for large bulk viscosity, is performed in parallel to the previous paper by two of the present authors (Kosuge and Aoki, in: Phys Rev Fluids 3:023401, 2018), where the structure of a shock wave in $$\hbox {CO}_2$$ gas was investigated using the ES model for a polyatomic gas with constant specific heats. From the numerical and analytical results, the effect of temperature-dependent specific heats on the structure of a shock wave is clarified.

APPLICATION OF A NOVEL THERMO-ECOLOGICAL PERFORMANCE CRITERION: EFFECTIVE ECOLOGICAL POWER DENSITY (EFECPOD) TO A JOULE-BRAYTON CYCLE (JBC) TURBINE

This study presents an application of a new performance analysis criterion named as Effective Ecological Power Density (EFECPOD) to a Joule-Brayton cycle (JBC) turbine. The turbine performance is expressed a single value by the proposed criterion using effective efficiency, effective power, cycle temperature ratio and volume. NOx formation and turbine dimensions are considered by the cycle temperature ratio and turbine volume, respectively. The turbine volume is also related to production cost of the heat engine. Therefore, the proposed criterion is essential for multi purpose optimization. Furthermore, this criterion can be developed and applied to the other gas cycle and heat engines. Also, the influences of engine design parameters such as cycle temperature ratio, pressure ratio, turbine speed, and equivalence ratio on the EFECPOD have been examined based on Finite-Time Thermodynamics Modelling (FTTM). In order to obtain realistic results, temperature-dependent specific heats for working fluid have been used and heat transfer and exhaust output losses have been taken into consideration. The results presented could be an essential tool for JBC turbine designers.

Exergetic and ecological performance analyses of a gas turbine system with two intercoolers and two re-heaters

In this study, performance analyses of a gas turbine with two intercoolers and two re-heaters based on energetic, exergetic and ecological concepts such as power, power density, second-law efficiency (exergy efficiency), exergy destruction, ecological coefficient of performance (ECOP) and effective ecological power density (EFECPOD). The effects of turbine design parameters such as turbine speed, air mass flow rate, equivalence ratio, pressure ratio, turbine diameter, turbine length, heat transfer coefficient, residual gas fraction, turbine wall temperature, inlet pressure and inlet temperature on the performance characteristics have been investigated by taking temperature-dependent specific heats and heat transfer losses into consideration.

Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo2Ga2C are investigated using density functional theory (DFT) within the generalized gradient approximation (GGA). The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states (DOS) is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per eV. Technologically important optical parameters (e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function) are calculated for the first time. The study of dielectric constant (ɛ1) indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic. The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of Tc expression using available parameter values (DOS, Debye temperature, atomic mass, etc.) suggests that the compound is less likely to be superconductor.

Thermo-ecological performance analysis of a Joule-Brayton cycle (JBC) turbine with considerations of heat transfer losses and temperature-dependent specific heats

This study presents a new performance analysis criterion called Effective Ecological Power Density (EFECPOD) and its application to a Joule-Brayton cycle (JBC) turbine. Effective efficiency (EE), effective power (EP), cycle temperature ratio and turbine volume are considered together and the performance is stated a unique value by the developed criterion. emissions formation and turbine dimensions are considered by the cycle temperature ratio and turbine volume, respectively. The turbine volume is also related to production cost of the heat engine. Therefore, the proposed criterion is essential for multipurpose optimization. Furthermore, this criterion can be developed and applied to the other gas cycles and heat engines. Also, a performance investigation for the JBC gas turbine in terms of EE, EP and effective power density (EPD) has been carried out using a new finite-time thermodynamics modeling (FTTM). The influences of engine design parameters such as cycle temperature ratio, cycle pressure ratio, turbine speed, intake temperature, intake pressure and equivalence ratio on the EE, EP, EPD and EFECPOD have been examined. The results presented can be an essential tool for JBC turbine designers.

The influence of the electronic specific heat on swift heavy ion irradiation simulations of silicon

The swift heavy ion (SHI) irradiation of materials is often modelled using the two-temperature model. While the model has been successful in describing SHI damage in metals, it fails to account for the presence of a bandgap in semiconductors and insulators. Here we explore the potential to overcome this limitation by explicitly incorporating the influence of the bandgap in the parameterisation of the electronic specific heat for Si. The specific heat as a function of electronic temperature is calculated using finite temperature density functional theory with three different exchange correlation functionals, each with a characteristic bandgap. These electronic temperature dependent specific heats are employed with two-temperature molecular dynamics to model ion track creation in Si. The results obtained using a specific heat derived from density functional theory showed dramatically reduced defect creation compared to models that used the free electron gas specific heat. As a consequence, the track radii are smaller and in much better agreement with experimental observations. We also observe a correlation between the width of the band gap and the track radius, arising due to the variation in the temperature dependence of the electronic specific heat.

Thermodynamic analysis and performance maps for the irreversible Dual–Atkinson cycle engine (DACE) with considerations of temperature-dependent specific heats, heat transfer and friction losses

A comprehensive performance analysis depending on the non-dimensional power output, effective power, non-dimensional power density, effective power density, thermal efficiency, effective efficiency, maximum power output (MP), maximum power density (MPD) and maximum thermal efficiency (MEF) criteria has been conducted for the irreversible Dual–Atkinson cycle engine (DACE) which includes internal irreversibilities by virtue of the irreversible-adiabatic compression process, expansion process, heat transfer and friction losses. In the analyses, Classical Thermodynamics Modeling (CTM) and a new realistic Finite-Time Thermodynamics Modeling (FTTM) have been used. The power output, power density and thermal efficiency are obtained with respect to the variation of the pressure ratio, cut-off ratio, stroke ratio, Atkinson cycle ratio, cycle pressure ratio and cycle temperature ratio. The effects of the engine design and operating parameters on the general and maximum performances of the DACE have been investigated with respect to the variation of the cycle pressure ratio and cycle temperature ratio in the CTM section. The influences of the other engine design and operating parameters such as engine speed, mean piston speed, stroke length, equivalence ratio, compression ratio and bore–stroke length ratio on the engine performance have been investigated in the FTTM section. In addition, the energy losses depending on incomplete combustion, friction, heat transfer and exhaust output have been described as fuel input energy. In order to obtain realistic results, temperature-dependent specific heats for working fluid have been used. The DACE is a new concept for internal combustion engines and just a few studies have been carried out. This study presents new contributions to the analysis of the Dual–Atkinson cycle engines in terms of the effects of engine design and operating parameters on the engine performance. Because CTM, FTTM, energy losses, power density and MPD analyses, for the DACE are newly presented just in this study.

Thermodynamic Analysis and Comparison of the Air-Standard Atkinson and Dual-Atkinson Cycles with Heat Loss, Friction and Variable Specific Heats of Working Fluid

Employing finite-time thermodynamics, the effects of irreversibility factors, i.e., friction, heat transfer of cylinder wall, compression and expansion efficiencies and variable specific heats (temperature-dependent specific heats of the working fluid) on the air-standard Atkinson and Dual-Atkinson cycles are analyzed. In addition, a numerical comparison between the Atkinson and Dual-Atkinson cycles is made. Moreover, numerical examples show the relations between the thermal efficiency and compression ratio and between power and compression ratio. As mentioned, compression ratio, rc, initial temperature, T1, specific heat defined by ap, bv and k1, friction defined by b and heat transfer defined by \({\alpha}\) and \({\beta}\) are some of the key parameters of the internal combustion engines. The effects of these on the performances of the Atkinson and Dual-Atkinson cycles are presented in this article. According to the findings, the thermal efficiency and the output power of the Dual-Atkinson are higher than those of the Atkinson cycle at the same condition. Also, these irreversibility factors must be considered to design and analyze the Atkinson and Dual-Atkinson cycles. The obtained results also will provide guidance for the design of internal combustion engines.

Numerical investigation of a single-mode chemically reacting Richtmyer-Meshkov instability

We report on high-resolution, numerical simulations of a single-mode, chemically reacting, Richtmyer-Meshkov (RM) instability, at different interface thicknesses. The gases on either side of the diffuse interface were Hydrogen (H $$_2)$$ and Oxygen (O $$_2)$$ , with a pre-shock Atwood number $$(A_t\equiv \frac{\rho _{h}-\rho _{l}}{\rho _{h}+\rho _{l} })$$ of $$\sim $$ 0.5. An incident shock with a Mach number of 1.2 is allowed to traverse from the light (H $$_2)$$ to the heavy (O $$_2)$$ medium in the 2D numerical shock tube. The simulations were performed using the astrophysical FLASH code developed at the University of Chicago, with extensive modifications implemented by the authors to describe detailed H $$_2$$ –O $$_2$$ chemistry, temperature-dependent specific heats, and multi-species equation of state. The interface thickness was systematically varied in the simulations to study the effect of the total mass of fuel burnt and heat added on the hydrodynamic instability growth rates. In the absence of an incident shock, burning results in the formation of so-called combustion waves, which spontaneously trigger RM and Rayleigh-Taylor like instability growth of the interface. We are able to obtain the resulting growth rates of an imposed sinusoidal perturbation, and compare them with the predictions of an impulsive model, with simple modifications to account for the finite thickness of the interface, density changes due to heat addition, and compression of the material line due to the combustion wave. When additionally an incident shock is present, we observe complex interactions between the shock and the aforementioned combustion waves, resulting in significant non-planar distortions of each. When the unstable interface is subjected to a reshock, significant mixing enhancement is observed, accompanied by a dramatic increase in combustion product formation, and combustion efficiency.

Zirconium metal-based MAX phases Zr2AC (A = Al, Si, P and S): A first-principles study

We have investigated theoretical Vickers hardness, thermodynamic and optical properties of four zirconium metal-based MAX phases Zr 2 AC (A = Al , Si , P and S ) for the first time in addition to revisiting the structural, elastic and electronic properties. First-principles calculations are employed based on density functional theory (DFT) by means of the plane-wave pseudopotential method. The theoretical Vickers hardness has been estimated via the calculation of Mulliken bond populations and electronic density of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and volume thermal expansion coefficient of all the compounds are derived from the quasi-harmonic Debye model. Further, the optical properties, e.g., dielectric functions, indices of refraction, absorption, energy loss function, reflectivity and optical conductivity of the nanolaminates have been calculated. The results are compared with available experiments and their various implications are discussed in detail. We have also shed light on the effect of different properties of Zr 2 AC as the A-group atom moves from Al to S across the periodic table.

Hydrides of YPd3: Electronic structure and dynamic stability

Ternary intermetallic YPd3 is known to exhibit superior hydrogen storage capacity compared to pure palladium. To understand the characteristics of YPd3 on hydrogenation, the ground state electronic and dynamical properties were computed by two computational methods, the full potential linearized plane wave and projector augmented wave methods within the density functional theory. Hydrogen can be inserted in YPd3 at various octahedral sites, giving rise to model structures YPd3H and YPd3H4 which retain the L12 crystal structure. The calculated energy bands confirm the metallic nature of YPd3 and also exhibit greater dispersion of bands with increase in hydrogen content. Large variations in the optical constants such as transmittance is observed (by ∼40% in the violet region) with insertion of hydrogen, YPd3 may have thus have applications as a sensing device for monitoring hydrogen for using hydrogen safely. The electronic component γ obtained from the temperature dependent specific heats, is related to the density of states at the Fermi level which may be co-related to instability of hydrides.The modes at Γ-point in YPd3H and at X and M-points in YPd3H4 give rise to high peaks in the imaginary frequency regime which could drive the dynamical instabilities. From the formation energies and phonon modes it is found that the monohydride YPd3H is more stable, thus occupation of the octahedral sites at 2Y4Pd by hydrogen atoms results in greater dynamic instability in YPd3.

Thermal analysis of the melting process in a nuclear fuel rod

Melting of nuclear fuel rods is a crucial issue that must be addressed when conducting simulation analyses of severe accidents in nuclear reactors. The present work aims to develop a mathematical model and carry out numerical simulation of one-dimensional heat conduction with phase change in a nuclear fuel rod including two regions: uranium dioxide fuel pellet and zirconium alloy cladding. The temperature-dependent specific heats, thermal conductivities and densities are considered in the present investigation. The enthalpy formulation of the melting process in the nuclear rod during full reactor power operation is solved by using a finite-difference method. The thermal analysis is divided into four different phases: (1) transient phase before the cladding melting, (2) cladding melting phase, (3) transient phase before the fuel melting and (4) fuel melting phase. Then, the effect of heat transfer coefficient between coolant and fuel rod on the temperature histories of the fuel pellet and the cladding is investigated. Finally, the melting process of a nuclear rod with decay reactor power after shutdown is also simulated.

BAND STRUCTURE, HARDNESS, THERMODYNAMIC AND OPTICAL PROPERTIES OF SUPERCONDUCTING Nb2AsC, Nb2InC AND Mo2GaC

First-principles investigation of the geometry, electronic band structure, Vickers hardness, thermodynamic and optical properties of three superconducting MAX compounds Nb 2 AsC , Nb 2 InC and Mo 2 GaC have been carried out by the plane-wave pseudopotential method based on density functional theory (DFT) implemented in the CASTEP code. The theoretical Vickers hardness has been studied by means of Mulliken bond population analysis and electronic densities of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and thermal expansion coefficient of the three 211 MAX phases are derived from the quasi-harmonic Debye model with phononic effect for the first time. Furthermore, all the optical properties are determined and analyzed for the first time for two different polarization directions. The theoretical findings are compared with relevant experiments (where available) and the various implications are discussed in details.

Effects of Heat Transfer and Friction on the Performance of a Miller Cycle Diesel Engine with Considerations of Variable Specific Heats of the Working Fluid

The objective of this study is to analyze the effects of friction and heat transfer with considerations of variable specific heats of working fluid on the performance of a Miller cycle Diesel engine. The variations in power output and thermal efficiency with compression ratio, and the relations between the power output and the thermal efficiency of the Miller cycle Diesel engine are presented. The results show that the power output as well as the efficiency where maximum power output occurs will decrease with the increase of heat loss. The temperature-dependent specific heats of working fluid have a significant influence on the performance. The power output and the working range of the Miller cycle Diesel engine increase with the increase of specific heats of working fluid, while, the efficiency decreases with the increase of specific heats of working fluid. The influence of the parameter b related to the friction loss has a negative effect on the performance. Therefore, the power output and efficiency of the cycle decrease with increasing b. Note that the effects of heat transfer with considerations of variable specific heats of working fluid and friction loss on the performance are significant and should be considered in practice cycle analysis.