Adiabatic Assumption Research Articles

Note on the initial conditions within the effective field theory approach of cosmic acceleration

By using the effective field theory approach, we investigate the role of initial condition for the dark energy or modified gravity models. In details, we consider the constant and linear parametrization of the effective Newton constant models. Firstly, under the adiabatic assumption, the correction from the extra scalar degree of freedom in the beyond $\Lambda$CDM model is found to be negligible. The dominant ingredient in this setup is the primordial curvature perturbation originated from inflation mechanism, and the energy budget of the matter components is not very crucial. Secondly, the iso-curvature perturbation sourced by the extra scalar field is studied. For the constant and linear model of the effective Newton constant, there is no such kind of scalar mode exist. For the quadratic model, there is a non-trivial one. However, the amplitude of the scalar field is damped away very fast on all scales. Consequently, it could not support a reasonable structure formation. Finally, we study the importance of the setup of the scalar field starting time. By setting different turn-on time, namely $a=10^{-2} $ and $a=10^{-7} $, we compare the cosmic microwave background radiation temperature, lensing deflection angle auto-correlation function as well as the matter power spectrum in the constant and linear model. We find there is an order of $\mathcal{O}(1\%)$ difference in the observable spectra for constant model, while for the linear model, it is smaller than $\mathcal{O}(0.1\%)$.

Analysis of the influence of heat transfer on the stationary operation and performance of a solid oxide fuel cell/gas turbine hybrid power plant

Hybrid power plants consisting of solid oxide fuel cells (SOFC) and a gas turbine (GT) can play an essential role in the future energy scenario due to the expected high electrical efficiency, fuel flexibility and good part-load performance. A demonstration SOFC/GT hybrid power plant is being setup in Stuttgart with state of the art, commercially available electrolyte supported cell (ESC) stacks and its operation is being simulated by means of a overall system model. However, the model used in this paper, in contrast to most models in literature, accounts for heat transfer based on actual geometries and materials. In the present study, the system model is integrated with a set of sub-models that predict the heat losses of the components of the hybrid power plant with a feasible computational speed. This allows for an improved prediction of the operating range as well as for the prevention of undesired operating conditions. The results of the simulations of the stationary operation of the hybrid power plant with varying heat losses are shown and discussed. Operating limitations are analyzed as well as system performance. It is shown that it is possible to operate the hybrid power plant from design power output to 30% of it. A system electrical efficiency higher than 0.55 considering the fuel’s higher heating value is maintained throughout the entire range. Further design choices and developments could lead to an improvement of this condition. In addition, an adiabatic assumption can lead to about 4 percentage points overestimation of electrical efficiency and reduces the high power operating range by about 10%. This approach opens up a new perspective on the simulation of this type of power plant.

Film Cooling Performances at Different Turbulence Intensities Using Conjugate Heat Transfer Analysis

This study presents a numerical investigation of cooling performances of a modified vane of the film-cooled vane reported by Timko (NASA CR-168289) at different mainstream turbulence intensities (Tus). A 3D conjugate heat transfer (CHT) analysis with SST k-ω turbulence model in FLUENT V.15 is used. Three different mechanisms in CHT analysis, i.e. fluid flow, heat convection between solid surfaces and flowing fluid in an external mainstream and internal cooling passages, and heat conduction within the vane structure, are simultaneously considered. Numerical results are conducted in terms of overall cooling effectiveness at Tu=3.3, 10, and 20%. Comparison between overall cooling effectiveness and film effectiveness under adiabatic assumption is discussed at the three Tus, also. The findings of this research indicate the following phenomena: 1) overall cooling effectiveness decreases with Tu, and this effect on the pressure side (PS) is stronger than that on the suction side (SS) in general. 2) By comparison with adiabatic film effectiveness, the level of overall cooling effectiveness in most regions is higher and more uniform than that of adiabatic film effectiveness for all three Tus. 3) In the leading edge (LE), when Tu increases, near the exits of film holes overall cooling effectiveness deteriorates, but adiabatic film effectiveness improves. Furthermore, a large area with relatively low overall cooling effectiveness is able to move with Tu in the LE region.

Dimensionless analytical solutions for steady-state fire smoke spread through high-rise shaft

Analytical solutions in terms of dimensionless numbers for the smoke spread through high-rise shafts during fires are essential to provide a fundamental understanding of smoke transport physics, which is a complex coupled heat and mass transfer problem. Existing solutions are often dimensional based on simplification of the problem such as assuming adiabatic conditions. In order to obtain the dimensionless analytical solutions, energy balance equation, mechanical energy equation and mass balance equation were established for smoke spread in high-rise buildings under both mechanical and natural venting conditions. Experiments were designed and conducted on two scaled shafts with different sizes and materials, and the measured results were compared to the dimensionless analytical solutions. It was found that the dimensionless analytical solutions could predict temperature profiles, mass flow rate and neutral plane level accurately. The effect of the adiabatic assumption on the accuracy was also discussed. For example, due to the adiabatic assumption, the error of the calculated mass flow rate required during mechanical venting to maintain a high-rise shaft smoke free was found to increase with a dimensionless number, ω, defined by the geometrical and thermal properties of the shaft.

Comparing parameterized versus measured microphysical properties of tropical convective cloud bases during the ACRIDICON–CHUVA campaign

Abstract. The objective of this study is to validate parameterizations that were recently developed for satellite retrievals of cloud condensation nuclei supersaturation spectra, NCCN(S), at cloud base alongside more traditional parameterizations connecting NCCN(S) with cloud base updrafts and drop concentrations. This was based on the HALO aircraft measurements during the ACRIDICON–CHUVA campaign over the Amazon region, which took place in September 2014. The properties of convective clouds were measured with a cloud combination probe (CCP), a cloud and aerosol spectrometer (CAS-DPOL), and a CCN counter onboard the HALO aircraft. An intercomparison of the cloud drop size distributions (DSDs) and the cloud water content (CWC) derived from the different instruments generally shows good agreement within the instrumental uncertainties. To this end, the directly measured cloud drop concentrations (Nd) near cloud base were compared with inferred values based on the measured cloud base updraft velocity (Wb) and NCCN(S) spectra. The measurements of Nd at cloud base were also compared with drop concentrations (Na) derived on the basis of an adiabatic assumption and obtained from the vertical evolution of cloud drop effective radius (re) above cloud base. The measurements of NCCN(S) and Wb reproduced the observed Nd within the measurements uncertainties when the old (1959) Twomey's parameterization was used. The agreement between the measured and calculated Nd was only within a factor of 2 with attempts to use cloud base S, as obtained from the measured Wb, Nd, and NCCN(S). This underscores the yet unresolved challenge of aircraft measurements of S in clouds. Importantly, the vertical evolution of re with height reproduced the observation-based nearly adiabatic cloud base drop concentrations, Na. The combination of these results provides aircraft observational support for the various components of the satellite-retrieved methodology that was recently developed to retrieve NCCN(S) under the base of convective clouds. This parameterization can now be applied with the proper qualifications to cloud simulations and satellite retrievals.

Numerical Modeling of a Simplified Ground Heat Exchanger Coupled with Sandbox

The ground heat exchanger (GHE) system is an energy-efficient application employing the geothermal energy in residential and commercial buildings. Compared with field tests or laboratory measurements, computational fluid dynamics and heat transfer offers a cost-effective approach to give accurate prediction on thermal performance assessment of the GHE system. In this study, the effects of different boundary configurations were numerically investigated based on a referred laboratory GHE experiment. First, both recorded input water temperature profile and the given heat load were used as heat input for numerical simulations, and all facets of the ground domain were set as adiabatic condition. Through comparing the numerically predicted output water temperature profile with the experimental recorded one, the first approach using the recorded input water temperature profile as heat input gave a close prediction with the experimental data, while the numerical results based on the second approach using the given heat load showed a considerable discrepancy compared with experimental data. This comparison discrepancy can be attributed to the adiabatic assumption for the ground domain facets through heat balance analysis, and can be addressed by introducing a dynamic thermal boundary configuration for side- and end-walls. This study demonstratedthe significance of boundary conditions consideration and provided a solution by utilizing dynamic thermal boundary treatments compared with the adiabatic assumption. The numerical modeling methods can contribute towards improved GHE numerical simulations based on finite ground domains.

Reacting Unsteady Reynolds-Averaged Navier–Stokes with the Tabulated Premixed Conditional Moment Closure Method

The tabulated premixed conditional moment closure model has shown the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable run times in a Reynolds-averaged Navier–Stokes formulation. The tabulated premixed conditional moment closure model is a table lookup combustion model that tabulates species, reaction rates, and thermodynamic data for use by the computational-fluid-dynamics code during run time. In this work, the tabulated premixed conditional moment closure model is extended to unsteady Reynolds-averaged Navier–Stokes. The new model is validated against particle image velocimetry and laser Raman measurements of an enclosed turbulent reacting methane jet from the German Aerospace Center. The flame’s reaction progress variable, its variance, and the scalar dissipation rate are calculated by the computational fluid dynamics in three dimensions. These three parameters are used to index detailed species information from the table for use by the computational-fluid-dynamics code. The scalar dissipation is used to account for the effects of the small-scale mixing, whereas a presumed shape beta function probability density function is used to account for the effects of large-scale turbulence on the reaction rates. Velocity, temperature, and major species are compared to the experimental data. Accurate predictions of the velocity fields were obtained, but accurate predictions of scalar quantities were limited by the adiabatic assumption of the tabulated premixed conditional moment closure model.

CFD simulations of Rapid Compression Machines using detailed chemistry: Impact of multi-dimensional effects on the auto-ignition of the iso-octane

In Rapid Compression Machines (RCM), several phenomena can induce inhomogeneities inside the reaction chamber. The benefits of using a creviced piston have been largely demonstrated through good agreement with the widely used adiabatic core assumption. Still, temperature inhomogeneities due to wall heat transfer cannot be avoided. These induce spatial variations in terms of chemical composition, potentially affecting the auto-ignition process. Mass transfer to the crevices during two-stage ignition is also a phenomenon that can influence the ignition process. In this study, we quantify the impact of multi-dimensional effects on the auto-ignition of the iso-octane by comparing 0-D and RANS simulations of the Argonne RCM. A detailed kinetic mechanism is employed, which makes this study the first to couple an accurate description of both the physical and chemical phenomena in an RCM context for such a complex fuel. It is found that the influence of the inhomogeneities on the ignition delay is globally marginal except for the lowest temperature condition explored where the diffusive transport of intermediate species and radicals plays a key role. The effect of mass transfer to the crevices does not affect significantly the auto-ignition delay under the test conditions. The sensitivity of the results to the turbulence level is also assessed and the results indicate that turbulence may only exert a minor influence on the auto-ignition delay. Comparison with the experimental data is good, and RANS simulation results are similar to those of the 0-D simulations.

The influence of n-butanol blending on the ignition delay times of gasoline and its surrogate at high pressures

The influence of blending n-butanol at 20% by volume on the ignition delay times for a reference gasoline was studied in a rapid compression machine (RCM) for stoichiometric fuel/air mixtures at 20bar and 678–858K. Delay times for the blend lay between those of stoichiometric gasoline and stoichiometric n-butanol across the temperature range studied. At lower temperatures, delays for the blend were however, much closer to those of n-butanol than gasoline despite n-butanol being only 20% of the mixture. Under these conditions n-butanol acted as an octane enhancer over and above what might be expected from a simple linear blending law. The ability of a gasoline surrogate, based on a toluene reference fuel (TRF), to capture the main trends of the gasoline/n-butanol blending behaviour was also tested within the RCM. The 3-component TRF based on a mixture of toluene, n-heptane and iso-octane was able to capture the trends well across the temperature range studied. Simulations of ignition delay times were also performed using a detailed blended n-butanol/TRF mechanism based on the adiabatic core assumption and volume histories from the experimental data. Overall, the model captured the main features of the blending behaviour, although at the lowest temperatures, predicted ignition delays for stoichiometric n-butanol were longer than those observed. A brute-force local sensitivity analysis was performed to evaluate the main chemical processes driving the ignition behaviour of the TRF, n-butanol and blended fuels. The reactions of fuel+OH dominated the sensitivities at lower temperatures, with H abstraction from n-butanol from α and γ sites being key for both the n-butanol and the blend. At higher temperatures the decomposition of H2O2 and reactions of HO2 and that of formaldehyde with OH became critical, in common with the ignition behaviour of other fuels. Remaining uncertainties in the rates of these key reactions are discussed.

Modeling bubble evolution in air-oil mixture with a simplified method

This work addresses the problem of bubble evolution arising from gas cavitation in hydraulic oils. Two significant aspects, including the interphase mass transfer represented by air release and absorption phenomena and different thermodynamic considerations, are currently taken into account using a simplified method. In particular, three new models in progressive relationship are proposed on the basis of Rayleigh–Plesset equation which describes bubble dynamics. They are Model A in which air content is assumed to be constant, Model B in which the interphase mass transfer is introduced with the air undergoing an isothermal transformation, and Model C assuming an adiabatic process for the bubble evolution. With the goal of investigating the effects of these aspects, comparisons of the three models for two typical cases are presented with regard to the practical circumstances in which the oil pressure is set to increase linearly or oscillate sinusoidally. Results show a consistent trend for both cases concerning Model B and Model C compared to Model A. Although its speed relates to many factors, air release and absorption has a relevant impact on gas bubble radius. By the reason of adiabatic assumption, Model C provides a slower response regarding the oil pressure change. However, Model B and Model C may be both inaccurate if considering the actual interfacial heat transfer. In this viewpoint, the oil temperature in fluid power system could be affected.

Measurement of species concentration and estimation of temperature in the wake of evaporating n-heptane droplets at trans-critical conditions

Rising pressures in modern combustion systems lead to conditions that can be supercritical for the injected liquid fuels. Quantitative measurement data is required for a better understanding of this complex process and for validation of corresponding simulations. The present study focusses on the evaporation of single free-falling droplets to reduce the complexity of the problem. The species concentration field in the wake of n-heptane droplets is measured with 2D Raman imaging. The atmosphere consists of nitrogen at conditions that are sub- to super-critical with regards to pure n-heptane. The concentration fields of all droplets at one set of operation conditions are overlaid to allow statistical interpretation. Additionally, data points for the correlation between n-heptane concentration and the relative number density are determined from the Raman signals. These data points are fitted by a model based on the adiabatic mixing assumption and a Virial equation of state for mixtures. The fitted model is used to calculate the correlation between fuel concentration and temperature. As suggested by the temperature results the droplet is cooled due to evaporation rather than being heated up towards its critical temperature. The data is available for the validation of numerical simulations.

The Efficacy of Hydrodynamic Compression Using a Conical Cavity

A technique for controlled hydrodynamic compression of gases using a conical cavity driven by liquid metal was proposed by Lei (2016) as a convenient, low-cost, repetitive instrument to achieve a very high compression ratio of up to \(10^9\). If the compression is fully adiabatic, such compression will raise the gas temperature by a factor of \(10^6\) for monoatomic gases. In this paper, we develop a 0-D semi-analytic model to examine in detail the adiabatic assumption, taking into account in particular the physical processes of dissociation, ionization, radiation, and energy exchange between the compressed gas and the wall, and provide an assessment of the temperature of the gas or plasma that may be attainable using the technique. The numerical results show that these physical processes drastically lower the temperature attainable. Nevertheless, the results still indicate that final gas temperature of more than 10 eV, and volume-compression-ratio more than \(10^6\), are still attainable that might enable a number of interesting applications. The modeling results also revealing some interesting effects of partially ionized gases under high ratio of compression.

Measurements of slip velocity and frictional heating in the capillary extrusion of linear‐low‐density polyethylene with a fluoropolymer processing aid

The slip velocity and frictional or slip heating of linear‐low density polyethylene with a fluoropolymer processing aid in capillary flow were measured by rheo‐particle image velocimetry and thermal imaging. The pure polymer did not show slip before the stick‐slip regime but exhibited strong slip when blended with the processing additive. However, for shear stresses beyond the stick‐slip regime, the pure polymer and the blend exhibited the same flow behavior with slip. The slip velocity increased with the shear stress at two different rates before and after the stick‐slip and the contribution of slip to the total flow rate exhibited a minimum. Significant rises in temperature were measured under slip and no slip conditions, being these much higher than the values predicted by the adiabatic flow assumption. Clear difference was made between viscous and frictional heating before the stick‐slip regime, even though they could not be distinguished from one another at higher stresses. Overall, in the presence of slip, frictional and viscous heating act synergistically producing higher temperature rises in the melt. Finally, in contrast to predictions by numerical simulations of viscous heating, measured velocity profiles did not evidence the heating effects in the shear stress range analyzed in this work. POLYM. ENG. SCI., 56:837–845, 2016. © 2016 Society of Plastics Engineers

A model for predicting temperature effect on flammability limits

A model for predicting the variation of flammability limits by the initial temperature was derived from the energy balance equation. All parameters of this proposed model can be sourced from the literature and do not need to be regressed by any experimental data. The proposed model was validated using 20 sets of experimental data consisting of 11 compounds, including alkanes, alkenes, alcohols, a ketone, an ester and inorganic fuels. Some samples were measured experimentally and other sample data were adopted from the literature. Overall, the proposed model predicted the temperature variation data of the flammability limits well when based on the adiabatic assumption. Application of this model is not restricted to organic fuels, but also inorganic fuels, such as carbon monoxide and hydrogen.

Investigation of the adiabatic assumption for estimating cloud micro- and macrophysical properties from satellite and ground observations

Abstract. Cloud properties from both ground-based as well as from geostationary passive satellite observations have been used previously for diagnosing aerosol–cloud interactions. In this investigation, a 2-year data set together with four selected case studies are analyzed with the aim of evaluating the consistency and limitations of current ground-based and satellite-retrieved cloud property data sets. The typically applied adiabatic cloud profile is modified using a sub-adiabatic factor to account for entrainment within the cloud. Based on the adiabatic factor obtained from the combination of ground-based cloud radar, ceilometer and microwave radiometer, we demonstrate that neither the assumption of a completely adiabatic cloud nor the assumption of a constant sub-adiabatic factor is fulfilled (mean adiabatic factor 0.63 ± 0.22). As cloud adiabaticity is required to estimate the cloud droplet number concentration but is not available from passive satellite observations, an independent method to estimate the adiabatic factor, and thus the influence of mixing, would be highly desirable for global-scale analyses. Considering the radiative effect of a cloud described by the sub-adiabatic model, we focus on cloud optical depth and its sensitivities. Ground-based estimates are here compared vs. cloud optical depth retrieved from the Meteosat SEVIRI satellite instrument resulting in a bias of −4 and a root mean square difference of 16. While a synergistic approach based on the combination of ceilometer, cloud radar and microwave radiometer enables an estimate of the cloud droplet concentration, it is highly sensitive to radar calibration and to assumptions about the moments of the droplet size distribution. Similarly, satellite-based estimates of cloud droplet concentration are uncertain. We conclude that neither the ground-based nor satellite-based cloud retrievals applied here allow a robust estimate of cloud droplet concentration, which complicates its use for the study of aerosol–cloud interactions.

Three-dimensional adiabatic mode parabolic equation method and its applications

Complex zone of the ocean is often characterized by horizontal variations of environmental parameters(bathymetry, sound speed profile, bottom properties etc.), initiating redistribution of the sound field in horizontal plane, which is the so-called three-dimensional (3D) effect. Based on the adiabatic mode parabolic equation method, modeling of 3D effects is carried out, in which the eigenvalues and eigenfunctions are calculated by the standard normal mode model KRAKEN, and the amplitude corresponding to each mode is computed by the wide-angle parabolic equation model RAM. The present 3D model is very efficient and can give clear physical meaning, but it can be only applied to a waveguide whose properties vary gradually with horizontal range due to the adiabatic assumption between different modes. This model is then used to analyze the horizontal refraction caused by internal waves and by a coastal wedge. The numerical results show that there are several areas in the horizontal plane, corresponding to different structures of intensity distributions. Moreover, the redistribution of the sound field in horizontal plane depends on source frequency and mode number. Frequency and modal dependences lead to variations of spectrum, distortion of signal with some spectrum, and spatiotemporal fluctuations of the sound field.

Numerical simulation of the temperature rise in intermediate and high strain rate experiments

Intermediate and high strain rate experiments are of a limited duration. Thus, the specimen temperature cannot be assumed constant. In this work, we investigated the adiabatic assumption in intermediate and high strain experiments. A two-step one-dimensional model was developed to simulate the temperature rise in Hopkinson bar experiments for strain rates ranging between 1 and 5000/s. The model is applied to predict temperature rise in an aluminum alloy. The adiabatic assumption is shown to be valid for strain rates higher than 500/s. However, the isothermal assumption is not valid even at 1/s of strain rate. These conclusions are very important for the interpretation of the stress-strain curves that are measured at medium and high strain rates.

Quench Analysis of High-Current-Density Nb3Sn Conductors in Racetrack Coil Configuration

The luminosity upgrade of the large hadron collider (HL-LHC) requires the development of new type of superconducting cables based on advanced Nb 3 Sn strands. In the framework of the FP7 European project EUCARD, the cables foreseen for the HL-LHC project have been tested recently in a simplified racetrack coil configuration, the so-called Short Model Coil (SMC). In 2013-2014, two SMCs wound with 40-strand (RRP 108/127) cables, with different heat treatment processes, reached during training at 1.9 K a current and peak magnetic field of 15.9 kA, 13.9 T, and 14.3 kA, 12.7 T, respectively. Using the measured signals from the voltage taps, the behavior of the quenches is analyzed in terms of transverse and longitudinal propagation velocity and hot-spot temperature. These measurements are compared with both analytical and numerical calculations from adiabatic models. The coherence of the results from the presented independent methods helps in estimating the relevance of the material properties and the adiabatic assumption for impregnated Nb 3 Sn conductor modeling.

Steady and unsteady experimental analysis of a turbocharger for automotive applications

The paper describes the steady and unsteady performance characteristics of a small size turbocharger typically employed in automotive downsized engine applications. The analysis is carried out by experimental means using an innovative hot gas generator system specifically designed for turbocharger testing which is capable of delivering a wide range of flow rates with adequate thermodynamic characteristics. More in detail, the gas generator consists of a medium size direct injection compression ignition Internal Combustion Engine (ICE) feeding the turbine of the test article. To independently set the hot gas mass flow rate and the turbine inlet temperature, the operating parameters of the aforementioned ICE are specified through an electronic control unit in a fully automated manner.Compared to previously presented data [1] (Energy Procedia, vol. 45, pp 1116-1125, 2014), those reported herein have been collected with the help of newly installed equipment and controlling software allowing for the estimation of the thermal power transferred from the turbocharger to the environment. In particular, thanks to a first law analysis, the collected measurements have shown that the algebraic sum of the thermal power transferred to the lubricating oil as well as to the environment is roughly speaking 20–30% of the compressor total enthalpy change per unit time. Moreover, it has been shown that evaluating the compressor efficiency through classical expression based on the adiabatic assumption leads to a 5–10% relative error.The improved experimental set up also allows for higher precision transient analysis both on the cold and hot side branch of the test article. While the steady-state performance maps of the turbocharger are readily obtained with the semi-automated testing procedure, the detailed analysis of the unsteady phenomena related for instance to the occurrence of mild and deep compressor surge events, are reproduced and thoroughly analysed using the rig in more advanced operating modes.

The effect of thermal stratification on HCCI combustion: A numerical investigation

The present study focuses on a numerical investigation of thermal stratification in HCCI combustion. The simulation is conducted with a multi-zone model, which incorporates heat and mass transfer between zones and to the combustion chamber walls. The multi-zone model is used to study the effect of three different initial thermal stratifications on the combustion duration and the pressure rise rate, while considering heat transfer effects. Subsequently, the assumption of adiabatic zones and combustion chamber is applied in the model, while maintaining the initial thermal stratifications. The results obtained are compared to the ones in which heat transfer is included, thereby elucidating the consequences of the adiabatic assumption when examining thermal stratification effects on HCCI combustion. An investigation is also conducted to determine the effect of engine speed on the thermal stratification of the charge, for the closed part of the engine cycle. Since engine speed directly affects the time available for all rate processes, including heat transfer and combustion, the investigation is conducted under motoring conditions. Lastly, the effect of wall temperature on the thermal stratification of an originally homogeneous mixture is examined under firing conditions, for the closed part of the engine cycle.