Adiabatic Surface Research Articles

Coupled CFD-FE analysis of a long-span truss beam exposed to spreading fires

This paper presents a unidirectional coupling methodology for combining fire simulation based on computational fluid dynamics (CFD) with finite element (FE) analysis to study the response of long-span steel truss beams exposed to non-uniform temperature distributions. Fire Dynamics Simulator (FDS) was used to simulate the fire scenarios, and Abaqus was used for the FE analyses. Adiabatic surface temperatures from the fire simulations were transferred to the Abaqus model using a coupling tool called FDS2FEM. The coupling methodology was validated using two experimental studies, and it was then used to analyse the response of a long-span steel truss beam inside a warehouse building exposed to two travelling fire scenarios (fire spread perpendicular to or along the truss beams) in the building. The fire simulations showed that the fire load arrangement, ignition location and ignition distance from the ventilation opening determined the severity of the thermal field, temperature heterogeneity and the fire spread behaviour. The computational efficiency of the coupling scheme enabled the structural analysis for a large-scale structure under highly time- and space-dependent thermal exposure. The FE analyses indicated that the direction of fire spread with respect to the truss beam determined if either vertical or lateral displacement at the mid-span of the girder was dominant. The analyses also showed that a long truss beam exposed to highly non-uniform temperature fields exhibits a variety of responses like thermal bowing, lateral oscillations, efficient load redistribution, local deformations, and global failure.

Incorporating Berry Force Effects into the Fewest Switches Surface-Hopping Algorithm: Intersystem Crossing and the Case of Electronic Degeneracy.

We present a preliminary surface-hopping approach for modeling intersystem crossing (ISC) dynamics between four electronic states: one singlet and one (triply degenerate) triplet. In order to incorporate all Berry force effects, the algorithm requires that, when moving along an adiabatic surface associated with the triplet manifold, one must also keep track of a quasi-diabatic index (akin to a "ms" quantum number) for each trajectory. For a simple model problem, we find that a great deal of new physics can be captured by our algorithm, setting the stage for larger, more realistic (or perhaps even ab initio) simulations in the future.

Measurement and determination of local film cooling performance along a transonic turbine blade tip with viscous dissipation

Presented are experimental and analytic procedures for the measurement and determination of film cooling performance parameters for a transonic flow environment along a turbine blade tip, which account for the influences of viscous dissipation. Such viscous dissipation magnitudes are vital to ascertain appropriate driving temperatures for convective heat transfer within high velocity, compressible environments. A key step in this procedure is the separation of adiabatic surface temperature magnitudes due to the thermal field from adiabatic surface temperature values associated with flow effects related to viscous dissipation. These latter adiabatic surface temperature magnitudes, from flow effects only, are determined with no film cooling, which, when considered relative to flow stagnation temperature, are directly related to local Mach number values within the tip gap flow along the blade tip surface. After the separation is implemented, adiabatic surface temperature variations from film cooling thermal effects only are provided relative to local baseline values with no film cooling. Resulting local adiabatic film cooling effectiveness and local heat transfer coefficients are subsequently determined which are based entirely upon locally measured temperatures (without the use of global temperature parameters), which is the most appropriate perspective for presentation of spatially-resolved surface heat transfer characteristics. Associated data are provided for the surface of a blade tip with a squealer rim and recess, with and without film cooling. An array of five film cooling holes, located along the upper pressure side of a two-dimensional turbine blade, are employed to provide the film coolant, with a local blowing ratio BR of 3.02. Ratios of tip gap to true blade span are 0.66%, and 1.16%. Experimental procedures employed to obtain the spatially-resolved surface data include infrared thermography, transient testing techniques, and the impulse response method for transient data analysis.

A Conical Intersection Influences the Ground State Rearrangement of Fulvene to Benzene.

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangement─a unimolecular path and a hydrogen-atom-assisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

Method for determining the thermal diffusivity and thermal conductivity coefficient by temperatures of plate surface as a semi-bounded body

The studied numerical and analytical model of a semi-bounded body is used to simultaneously determine the thermophysical characteristics (TFC): thermal diffusivity at and thermal conductivity coefficient λt of the material which make it easy to determine the volumetric heat capacity сt . Temperature distribution over the plate cross-section at the end of the calculated time interval τ is described by a power function, its exponent n depends on the Fourier number Fo. The values of TFC were calculated from the dynamics of changes in surface temperatures T(xp = Rp , τ) and T(xp = 0, τ) of the plate with a thickness Rp heated under boundary conditions of the second kind q = const. The temperature T(xp = 0, τ) was used to determine the time moment τe , at which the temperature perturbation reached the adiabatic surface xp = 0 (T(Rp , τe ) – Tb (0, τe = 0) = 0.1 K). Calculations of TFC (at and λt ) were performed using formulas whose parameters were found by solving a nonlinear system of three algebraic equations by selecting the Fourier number corresponding to τe . The author studied the complexity and accuracy of TFC calculation using the test (initial) temperature fields of a plate made of refractory material by the finite difference method. Dependences of TFC on the temperature ai (T ), λi (T ) and ci (T ) were set by polynomials. Temperatures of the plate with a thickness of Rp = 0.04 m with initial conditions Tb = T(xp , τ = 0) = 300, 900, 1200, 1800 K (0 ≤ xp ≤ Rp ) were calculated for a specific heat flow q = 5000 W/m2. The heating time to τe was 105 – 150 s. The average mass temperature Tm, pl of the plate during the τe increased by 5 – 11 K. The TFC values were restored by solving the inverse thermal diffusivity problem for 10 time points τi + 1 = τi + Δτ. The arithmetic mean deviations of TFC (Tm, pl ) from the initial values for calculations at Tb = 300, 900, 1200, 1800 K were less than 2.5 %. It was established that the values of at and λt obtained for the time moments ti are practically constant, therefore, a simplified calculation of at, o and λt, o is possible only from the values of temperatures T(Rp , τe ) and T(0, τe ) at the end of heating. The values of at, o and λt, o , which were calculated immediately for the entire heating time, differed from the initial values of the accepted heat exchange conditions by about 2 %. The parameters of simple algebraic formulas for calculating at, o and λt, o were found by solving a system of three nonlinear equations n = n( Fo), at, o = a(Tb , T(Rp , τe ), Rp , n, τe ), Fo = Fo(at, o , Rp , τe ) and expressions for λt, o = λ(Rp , q, n, Tb , T(Rp , τe )). The proposed method significantly simplifies the solution of the inverse problem of thermal conductivity.

Measurement and determination of local film cooling performance along a transonic turbine blade tip with viscous dissipation

Abstract Presented are experimental and analytic procedures for the measurement and determination of film cooling performance parameters for a transonic flow environment along a turbine blade tip, which account for the influences of viscous dissipation. Such viscous dissipation magnitudes are vital to ascertain appropriate driving temperatures for convective heat transfer within high velocity, compressible environments. A key step in this procedure is the separation of adiabatic surface temperature magnitudes due to the thermal field from adiabatic surface temperature values associated with flow effects related to viscous dissipation. These latter adiabatic surface temperature magnitudes, from flow effects only, are determined with no film cooling, which, when considered relative to flow stagnation temperature, are directly related to local Mach number values within the tip gap flow along the blade tip surface. After the separation is implemented, adiabatic surface temperature variations from film cooling thermal effects only are provided relative to local baseline values with no film cooling. Resulting local adiabatic film cooling effectiveness and local heat transfer coefficients are subsequently determined which are based entirely upon locally measured temperatures (without the use of global temperature parameters), which is the most appropriate perspective for presentation of spatially-resolved surface heat transfer characteristics. Associated data are provided for the surface of a blade tip with a squealer rim and recess, with and without film cooling. An array of five film cooling holes, located along the upper pressure side of a two-dimensional turbine blade, are employed to provide the film coolant, with a local blowing ratio BR of 3.02. Ratios of tip gap to true blade span are 0.66 percent, and 1.16 percent. Experimental procedures employed to obtain the spatially-resolved surface data include infrared thermography, transient testing techniques, and the impulse response method for transient data analysis.

Coupled CFD-FEM Simulation of Steel Box Bridge Exposed to Fire

Increasing fire-induced bridge failures are demanding more precise behavior prediction for the bridges subjected to fires. However, current numerical methods are limited to temperature curves prescribed for building structures, which can misestimate the fire impact significantly. This paper developed a framework coupling the computational dynamics (CFD) method and finite element method (FEM) to predict the performance of fire-exposed bridges. The fire combustion was simulated in CFD software, Fire Dynamic Simulator, to calculate the thermal boundary required by the thermomechanical simulation. Then, the adiabatic surface temperatures and heat transfer coefficient were applied to the FEM model of the entire bridge girder. A sequential coupled thermomechanical FEM simulation was then carried out to evaluate the performance of the fire-exposed bridge, thermally and structurally. The methodology was then validated through a real fire experiment on a steel beam. The fire performance of a simply supported steel box bridge was simulated using the proposed coupled CFD-FEM methodology. Numerical results show that the presented method was able to replicate the inhomogeneous thermomechanical response of box bridges exposed to real fires. The girder failed due to the buckling of a central diaphragm after the ignition of the investigated tanker fire in no more than 10 min. The framework presented in this study is programmatic and friendly to researchers and can be applied for the estimation of bridges in different fire conditions.

Energy separation in a channel with permeable wall

The present paper is an experimental and numerical analysis of a new method of energy separation. By energy separation, we mean the spontaneous separation of the gas flow into two flows with stagnation temperatures above and below the initial (“hot” and “cold”). One of the most famous devices using the phenomenon of energy separation is the Rank-Hilsch vortex tube. The considered method is based on the well-known effect of the stagnation temperature profile curvature over the boundary layer thickness, which occurs when a high-speed gas flows around an adiabatic surface. It is known that the higher the flow velocity and the more the Prandtl number differs from unity, the higher the energy separation within the boundary layer. In a channel with a permeable wall, part of the high-speed flow can be sucked out through the wall due to the natural pressure drop. As a result, the flow's stagnation temperatures at the outlet of the channel and the flow sucked out through the wall are different: one is hotter, the other is colder compared to the initial stagnation temperature. Based on the obtained experimental data, validation of one- and two-dimensional mathematical models was carried out. The influences of the initial Mach number, the initial stagnation pressure, the relative channel length, and the Prandtl number on the energy separation were investigated. It was found that as the initial Mach number changes from Mis = 1 to Mis = 3, the air flow's cooling increases from −5 to −15 K. When the Prandtl number changes from Pr = 0.7 to Pr = 0.2, the flow's cooling is increased by factor of 2.25 from −20 to −45 K. It is shown that at a certain ratio of parameters, the heating and the cooling of flows pass through an extremum.The obtained temperature difference due to the suction of a part of the flow can find application in the analysis of flows with variable mass flow rates in power systems, such as heat pipes, boundary layer control devices, etc.

Heat transfer model for calculation of the temperature field inside thin-walled sections in fire

This paper introduces the heat transfer model for simple calculation of the temperature field inside thin-walled sections in fire. It is dedicated for various shapes of a cross-section of structural elements made of metal. It utilizes the concept of an adiabatic surface temperature (AST) for determination of thermal boundary conditions. Due to the non-uniform heat exposure and arbitrary shape of the cross-section, the resulted temperature field is non-uniform. The given solution is valid for a transient heat transfer problem. The heat transfer model takes into account the practical abilities for measuring heat exposures both in experiments and numerical fire simulations and takes into account the inherent features of beam finite elements with stiffness integration at each step of an analysis, which is important for analyses of skeletal structures in fire. Finally, the heat transfer model is verified by comparison of results to the 2D finite element method solution and validated using data obtained from experiments.

Determining Temperature of Adiabatic Surfaces in Rooms with Radiant Heating

Determining Temperature of Adiabatic Surfaces in Rooms with Radiant Heating

Numerical modeling of localized fire exposures on structures using FDS-FEM and simple models

This paper investigates different numerical modeling strategies for evaluating temperatures in structures subjected to localized fires. Three simple fire models and two methods to interface computational fluid dynamics models with finite element thermal analyses are reviewed. Based on the field of application of each model, a decision flowchart is proposed to select the fire model and strategy suited to the problem. Validation is conducted against three experiments on steel members subjected to localized fires. Results show that the coupling between computational fluid dynamics and finite element analysis through the adiabatic surface temperature is a versatile method able to capture the member temperature in the different experiments in this study. The use of an automatic transfer interface using a transfer file with gas temperatures and radiant intensities is an efficient alternative when the member is outside the fire area and far enough that it does not significantly influence the fire development. The member should not be present in the FDS model when using the interface method. The simple models tend to yield more conservative predictions compared with the experimental results.

Aero-Thermal Aspects of Film Cooled Nozzle Guide Vane Endwall—Part 2: Thermal Measurements

Abstract Flow over gas turbine endwalls is complex and highly three-dimensional. As boundaries for modern engine designs are pushed, this already-complex flow is affected by aggressive application of film cooling flows that actively interact. This two-part study describes, experimentally, the aero-thermal interaction of cooling flows near the endwall of a first-stage nozzle guide vane passage (NGV). The approach flow conditions represent flow exiting a low-NOx combustor. The test section includes geometric and cooling details of a combustor-turbine interface in addition to endwall film cooling flows injected upstream of the passage. The first part of this study describes in detail, the passage aerodynamics as affected by injection of cooling flows. It reveals a system of secondary flows, including the newly discovered Impingement Vortex, which redefines our understanding of the aerodynamics of flow in a modern, film-cooled, first-stage vane row. The second part investigates, through thermal measurements, the distribution, mixing, and disruption of cooling flows over the endwall. Measurements are made with and without active endwall film cooling. Descriptions are made through adiabatic surface effectiveness measurements and correlations with in-passage velocity (presented in part one) and thermal fields. Results show that the newly discovered impingement vortex has a positive effect on coolant distribution through passage vortex suppression and by carrying the coolant to hard-to-cool regions in the passage, including the pressure surface near the endwall.

A numerical investigation to analyze the effect of changing ambient conditions on accurate and stable identification of thermal conductivity of ground source heat pump system using thermal response test

This study aimed to investigate the effect of changing ambient conditions on the accuracy and stability of ground thermal conductivity, λs, using the thermal response test (TRT). First, a detailed numerical model considering the heat fluxes on the field surface of TRT was developed. Then, the numerical model was compared with that with the adiabatic surface boundary in two representative test seasons. After that, the impacts of three parameters associated with the surface heat fluxes, including the surface absorptivity, evaporation coefficient, and surface infrared emissivity on the λs were analyzed. Lastly, statistical analyses with various ambient conditions were employed to evaluate the degree of estimated λs deviating from its actual value in different specifications of TRT and actual ground thermal conductivities. Results show that the surface absorptivity can be generally seen as the most significant contributory factor to the uncertainty of λs. The λs is overestimated and the uncertainty of TRT is aggravated by considering the changing ambient conditions, especially when the depth of drilling test well is short and the actual λs is high. For such conditions, the heat insulation on the test site surface is highly recommended considering the acceptable accuracy of λs with the adiabatic boundary.

Enabling complete multichannel nonadiabatic dynamics: A global representation of the two-channel coupled, 1,21A and 13A states of NH3 using neural networks.

Global coupled three-state two-channel potential energy and property/interaction (dipole and spin-orbit coupling) surfaces for the dissociation of NH3(Ã) into NH + H2 and NH2 + H are reported. The permutational invariant polynomial-neural network approach is used to simultaneously fit and diabatize the electronic Hamiltonian by fitting the energies, energy gradients, and derivative couplings of the two coupled lowest-lying singlet states as well as fitting the energy and energy gradients of the lowest-lying triplet state. The key issue in fitting property matrix elements in the diabatic basis is that the diabatic surfaces must be smooth, that is, the diabatization must remove spikes in the original adiabatic property surfaces attributable to the switch of electronic wavefunctions at the conical intersection seam. Here, we employ the fit potential energy matrix to transform properties in the adiabatic representation to a quasi-diabatic representation and remove the discontinuity near the conical intersection seam. The property matrix elements can then be fit with smooth neural network functions. The coupled potential energy surfaces along with the dipole and spin-orbit coupling surfaces will enable more accurate and complete treatment of optical transitions, as well as nonadiabatic internal conversion and intersystem crossing.

Towards photoassociation processes of ultracold rubidium trimers

We theoretically investigate the prospects for photoassociation (PA) of Rb$_3$, in particular at close range. We provide an overview of accessible states and possible transitions. The major focus is placed on the calculation of equilibrium structures, the survey of spin-orbit effects and the investigation of transition dipole moments. Furthermore we discuss Franck-Condon overlaps and special aspects of trimers including the (pseudo) Jahn-Teller effect and the resulting topology of adiabatic potential-energy surfaces. With this we identify concrete and suitable PA transitions to potentially produce long-lived trimer bound states. Calculations are performed using the multireference configuration-interaction method together with a large-core effective core potential and a core-polarization potential with a large uncontracted even-tempered basis set.

Effects of Endwall Film Coolant Flowrate on Secondary Flows and Coolant Mixing in a First Stage Nozzle Guide Vane

AbstractModern gas turbines are subjected to very high thermal loading. This leads to a need for aggressive cooling to protect components from damage. Endwalls are particularly challenging to cool due to a complex system of secondary flows near them that wash and disrupt the protective coolant films. This highly three-dimensional flow not only affects but is also affected by the momentum of film cooling flows, whether injected just upstream of the passage to intentionally cool the endwall or as combustor cooling flows injected further upstream in the engine. This complex interaction between the different cooling flows and passage aerodynamics has been recently studied in a first stage nozzle guide vane. The present paper presents a detailed study on the sensitivity of aero-thermal interactions to endwall film cooling mass flow to mainstream flow ratio. The test section represents a first stage nozzle guide vane with a contoured endwall and endwall film cooling injected just upstream of it. The test section also includes an engine-representative combustor–turbine interface geometry with combustor cooling flows injected at a constant rate. The approach flow conditions represent flow exiting a low-NOx combustor. Adiabatic surface thermal measurements and in-passage velocity and thermal field measurements are presented and discussed. The results show the dynamics of passage vortex suppression and the increase of impingement vortex strength as MFR changes. The effects of these changes of secondary flows on coolant distribution are presented.

A Periodic Heating into Different Layers of a Semi-Infinite Solid

This paper deals with a periodic heating into different layers of a semi-infinite solid in order to examine the thermal parameters influence of the different layers on the development of the temperature oscillations. This influence is introduced by a physical model that shows the temperature evolution in every location into the solid layers as a function of time. It was shown that the structure with the larger thermal conductivity in the semi-infinite layer has larger absolute gradient of temperature on the external surface (the system left surface). Received at certain times temperature distributions with extreme, maximum or minimum temperatures values. These values actually represent adiabatic surfaces at the relevant times, meaning that heat transfer, entry or exit occurs in the limited volume between the constraint surface and those adiabatic surfaces. These temperature distributions also show receiving of identical, positive or negative values along all the medium depth. At the start time of the time period and at the half time of the time period, linear temperatures distributions are obtained. These times can be seen as weakening times in terms of supporting the transient heat transfer. The temperature distribution in the structure of a wall made up of real building materials shows that the wall is a retainer for temperature fluctuations and not just is a basic resistance to heat transfer.

Vibration Effect on Natural Convection Heat Transfer in an Inclosed Cubic Cavity

An experimental study was performed to clarify the effect of vertical mechanical vibration on natural convection at normal gravity in the air filled cubic enclosure (L=120mm) (Pr=0.71) . In the enclosure, there were two vertical and opposing surfaces. The right wall was heated to a standardized heat flux surrounded by four other adiabatic surfaces, where the left wall cooledTc. Vibration stress was added to this heat transfer cell by vertically mounting it on the armature of electrodynamics vibratory (shaker). The experimental work was performed on a built rig which was mainly composed of a cubic enclosure cavity provided with a vibrator exciter as well as the necessary measurement instrumentation to fulfill the required investigations. At Ra= 7* 107 the frequencies shedded to the enclosure (2,4&8) Hz and at Ra=4* 108 the frequencies shedded to the enclosure (3,6&9)Hz. Three type of tests for an experimental were carried out. The first one reached to steady state and then shedded the effect of vibration to the cubic enclosure (Interrupted Vibrations), while the second shedded the vibration (Continuous Vibrations) from the start at ascending frequencies and the third shedded the vibration (Continuous Vibrations) from the start at descending frequencies. From the results of the experimental investigations two main conclusions may be raised, in the case of Rayleigh number (Ra=4*108), the gravitational thermal convection is dominant. And the motion of vibration does not enhance the transfer of heat exceptionally well. On the other side, in Rayleigh (Ra=7*), the thermal vibration convection is dominant, and the vibration greatly increased the rate of heat transfer..Also, the results show that an increase in the average Nusselt number with time as the vibrational Rayleigh number Ravib will increase as resulting of increasing the vibration frequencies. In addition, the higher the frequency of vibration reaches, the faster the steady states is achieved. And for that, two cases of Rayleigh number, the increasing frequencies are usually higher than those of downward frequencies. Finally, the results show reasonable agreement between the theoretical and the experimental results and the present results and available previous work.

Permutationally Restrained Diabatization by Machine Intelligence.

Simulations of electronically nonadiabatic processes may employ either the adiabatic or diabatic representation. Direct dynamics calculations are usually carried out in the adiabatic basis because the energy, force, and state coupling can be evaluated directly by many electronic structure methods. However, although its straightforwardness is appealing, direct dynamics is expensive when combined with quantitatively accurate electronic structure theories. This generates interest in analytically fitted surfaces to cut the expense, but the cuspidal ridges of the potentials and the singularities and vector nature of the couplings at high-dimensional, nonsymmetry-determined intersections in the adiabatic representation make accurate fitting almost impossible. This motivates using diabatic representations, where the surfaces are smooth and the couplings are also smooth and-importantly-scalar. In a recent previous work, we have developed a method called diabatization by deep neural network (DDNN) that takes advantage of the smoothness and nonuniqueness of diabatic bases to obtain them by machine learning. The diabatic potential energy matrices (DPEMs) learned by the DDNN method yield not only diabatic potential energy surfaces (PESs) and couplings in an analytic form useful for dynamics calculations, but also adiabatic surfaces and couplings in the adiabatic representation can be calculated inexpensively from the transformation. In the present work, we show how to extend the DDNN method to produce good approximations to global permutationally invariant adiabatic PESs simultaneously with DPEMs. The extended method is called permutationally restrained DDNN.

Comparative study on convection loss of different shape solar dish collector cavity receivers at different positions using CFD

In this work, the performance of the five solar concentrating dish receiver configurations were studied for convection heat loss. Flat, spherical, dome, cylindrical and frustum of cone shape receivers are modeled using SolidWorks 16 and analyzed with FLUID SIMULATION software under no wind condition. Receiver inner side is considered as isothermal surface and outer side is considered as adiabatic surface. Temperature variation and flow air in and around the receiver are factor influencing convection heat loss. For spherical, receiver, air flow is almost stagnant inside cavity, temperature affected region is comparatively less and flow velocity and flow affected region are also comparatively lesser. Hence, spherical cavity receiver is efficient receiver compared all other cavity receivers.