Adiabatic Wall Boundary Conditions Research Articles

Analytical solution for combined heat and mass transfer in laminar falling film absorption with uniform film velocity - diabatic wall boundary

Abstract In a previous study the Laplace transform has been introduced in order to solve the combined heat and mass transfer problem in absorbing laminar falling films with constant film velocity for an isothermal as well as an adiabatic wall boundary condition. It has been stated, that the Laplace transform basically allows to apply arbitrary wall boundary conditions in contrast to the Fourier method. Therefore, in the present study a diabatic wall boundary condition is applied, which, as limiting cases, includes both the isothermal wall if the thermal resistance of the wall is zero as well as the adiabatic wall condition for an infinite thermal resistance of the wall. Temperature and mass fraction profiles across the film as well as the evolution of the absorbed mass flux with increasing flow length are presented for two different thermal resistances of the wall and modified Stefan numbers. The results are compared to the limiting cases of the isothermal and the adiabatic wall boundary condition. The present study offers an analytical solution with a more realistic boundary condition of a constant thermal resistance of the wall including the isothermal and the adiabatic wall boundary condition.

Three-Dimensional Simulation of Deflagration-to-Detonation Transition with a Detailed Chemical Reaction Model

A three-dimensional simulation of the deflagration-to-detonation transition (DDT) in a H2/O2 mixture in a rectangular tube is performed under adiabatic and isothermal wall boundary conditions. In the isothermal wall boundary case, a local explosion triggering the onset of detonation occurs near the center of the tube behind the incident shock wave, which agrees qualitatively with the two-dimensional simulation in our earlier study. In contrast, in the adiabatic case, auto-ignition is observed near the corner of the tube before the local explosion occurs on each wall behind the flame, which is accelerated by the high-temperature condition (preheated zone) caused by the generation of compression waves. The flame overtakes the incident shock and propagates toward the upper and lower walls. In some experimental studies, the local explosion occurred near the wall. Therefore, the present adiabatic case received special attention. Moreover, these phenomena are discussed in detail in terms of the flame acceleration, preheated zone, and x–t diagram.

Analytical solution for combined heat and mass transfer in laminar falling film absorption with uniform film velocity – Isothermal and adiabatic wall

In the present study the Laplace transform is applied to the partial differential equations obtained from the differential energy and absorbate balances for the combined heat and mass transfer problem in laminar falling films with uniform film velocity.By means of the inverse Laplace transform an analytical solution is provided for the isothermal as well as for the adiabatic wall boundary condition. Temperature and mass fraction profiles across the film as well as the evolution of the absorbed mass flux as a function of the flow length are presented for the adiabatic wall condition as well as for the isothermal wall with different wall temperatures. Furthermore, the influence of the Lewis number on the absorbed mass flux is discussed.In addition, the present method allows to apply other wall boundary conditions than the isothermal or the adiabatic wall boundary, which will be addressed in a subsequent study.

Constrained Large-Eddy Simulation of Compressible Flow Past a Circular Cylinder

AbstractCompressible flow past a circular cylinder at an inflow Reynolds number of 2x105is numerically investigated by using a constrained large-eddy simulation (CLES) technique. Numerical simulation with adiabatic wall boundary condition and at a free-stream Mach number of 0.75 is conducted to validate and verify the performance of the present CLES method in predicting separated flows. Some typical and characteristic physical quantities, such as the drag coefficient, the root-mean-square lift fluctuations, the Strouhal number, the pressure and skin friction distributions around the cylinder,etc.are calculated and compared with previously reported experimental data, finer-grid large-eddy simulation (LES) data and those obtained in the present LES and detached-eddy simulation (DES) on coarse grids. It turns out that CLES is superior to DES in predicting such separated flow and that CLES can mimic the intricate shock wave dynamics quite well. Then, the effects of Mach number on the flow patterns and parameters such as the pressure, skin friction and drag coefficients, and the cylinder surface temperature are studied, with Mach number varying from 0.1 to 0.95. Non-monotonic behaviors of the pressure and skin friction distributions are observed with increasing Mach number and the minimum mean separation angle occurs at a subcritical Mach number of between 0.3 and 0.5. Additionally, the wall temperature effects on the thermodynamic and aerodynamic quantities are explored in a series of simulations using isothermal wall boundary conditions at three different wall temperatures. It is found that the flow separates earlier from the cylinder surface with a longer recirculation length in the wake and a higher pressure coefficient at the rear stagnation point for higher wall temperature. Moreover, the influences of different thermal wall boundary conditions on the flow field are gradually magnified from the front stagnation point to the rear stagnation point. Moreover, the influences of different thermal wall boundary conditions on the flow field are graduallymagnified from the front stagnation point to the rear stagnation point. It is inferred that the CLES approach in its current version is a useful and effective tool for simulating wall-bounded compressible turbulent flows with massive separations.

Numerical Simulation of Single- and Dual-media Thermocline Tanks for Energy Storage in Concentrating Solar Power Plants

A single molten-salt thermocline tank is a low-cost alternative to conventional multiple-tank systems for concentrating solar power thermal energy storage. Thermocline tanks are typically composed of molten salt and a filler material that provides sensible heat capacity at reduced cost; such tanks are referred to as a dual-media thermocline (DMT). However, inclusion of quartzite rock filler introduces the potential for mechanical ratcheting of the tank wall during thermal cycling. To avoid this potential thermomechanical mode of failure, the tank can be operated solely with molten salt, as a single-medium thermocline (SMT) tank. In the absence of a filler material to suppress formation of tank-scale convection eddies, the SMT tank may exhibit undesirable internal fluid flows in the tank cross-section. The performance of DMT and SMT tanks is compared under cyclic operation, assuming adiabatic external wall boundary conditions. A computational fluid dynamics model is used to solve for the spatial temperature and velocity distributions within the tank. For the DMT tank, a two-temperature model is used to account for the non-thermal equilibrium between the molten salt and the filler material, and Forchheimer's extension of Darcy's Law is added to the porous-medium formulation of the laminar momentum equation. The governing equations are solved numerically using a finite volume approach. For adiabatic external boundaries, the SMT tank yields a percentage point increase in the first and second law efficiencies relative to the DMT tank. Future work is needed to compare the thermocline tank designs with respect to capital cost and storage performance under non-adiabatic wall boundaries.

Large Eddy Simulation-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions

The influence of the thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large eddy simulations are carried out using the dynamically thickened flame combustion model. Then system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with a 90 mm × 90 mm combustor cross section and nonadiabatic walls: (1) a combustor cross section similar to the reference case with adiabatic combustor walls, and (2) a different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in the flame shape and flow field. In particular, the FTF computed with an adiabatic wall boundary condition which produces a flame with a significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in the flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in the FTF on stability limits is analyzed with a low-order thermoacoustic model.

Detailed Unsteady Simulation of a Counterrotating Aspirated Compressor with a Focus on the Aspiration Slot and Plenum

An unsteady analysis of the MIT counterrotating aspirated compressor (CRAC) has been conducted using the Numeca FINE/Turbo 3D viscous turbulent flow solver with the Nonlinear Harmonic (NLH) method. All three blade rows plus the aspiration slot and plenum were included in the computational domain. Both adiabatic and isothermal solid wall boundary conditions were applied and simulations with and without aspiration were completed. The aspirated isothermal boundary condition solutions provide the most accurate representation of the trends produced by the experiment, particularly at the endwalls. These simulations provide significant insight into the flow physics of the aspiration flow path. Time histories and spanwise distributions of flow properties in the aspiration slot and plenum present a flow field with significant temporal and spatial variations. In addition, the results provide an understanding of the aspiration flow path choking mechanism that was previously not well understood and is consistent with experimental results. The slot and plenum had been designed to aspirate 1% of the flow path mass flow, whereas the experiment and simulations show that it chokes at about 0.5% mass flow.

Flow distortion in an S-duct inlet with simulated icing effect and heat transfer

AbstractA diffusing S-duct aircraft intake system is computationally studied for the effects of inlet icing and wall heat transfer on engine face distortion. A Reynolds-Averaged Navier-Stokes (RANS) code with k -ω SST turbulence model is used to simulate the compressible viscous flow in the duct. The glaze ice accretion on the inlet lip is simulated as a fixed protrusion from NASA LEWICE3D code. The shape and size of the ice on the inlet lip are assumed to remain constant even in the case of the heated wall. The case of zero external heat transfer is modeled by the adiabatic wall boundary condition, and constant-wall temperature is used to simulate the heated wall effect. The freestream Mach number of 0·85 and glaze ice condition on the inlet lip produce massive flow separation at the lip and internal shock cell structure in the S-duct. The heated wall creates additional vorticity and thus increases the engine face distortion level. The area-averaged total pressure distortion at the engine face in a freestream Mach number of 0·85 is increased by ~7% in the heated wall case as compared to the adiabatic flow.

Load characteristics of steel and concrete tubular members under jet fire: An experimental and numerical study

The aim of this study was to evaluate the load characteristics of steel and concrete tubular members under jet fire, with the motivation to investigate the jet fire load characteristics in FPSO topsides. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) ( Paik and Czujko, 2009; Paik, 2010). To obtain reliable load values, jet fire tests were carried out in parallel with a numerical study. Computational fluid dynamics (CFD) simulation was used to set up an adiabatic wall boundary condition for the jet fire to model the heat transfer mechanism. A concrete tubular member was tested under the assumption that there is no conduction effect from jet fire. A steel tubular member was tested and considered to transfer heat through conduction, convection, and radiation. The temperature distribution, or heat load, was analyzed at specific locations on each type of member. ANSYS CFX, (2008) and KFX, 2007 codes were used to obtain similar fire action in the numerical and experimental methods. The results of this study will provide a useful database to determine design values related to jet fire.

Compressible Laminar Boundary-Layer Flows with Statistical Bhatnagar-Gross-Krook Approaches

DOI: 10.2514/1.45699 In this work, we study the well-known, fundamental viscous fluid problem of compressible, laminar boundarylayer flowovera flatplate.Thepaperinvestigatesthemodelingofsuch flowsusingtheBhatnagar–Gross–Krookand ellipsoidal statistical Bhatnagar–Gross–Krook model kinetic equations for the supersonic flow of argon gas over a flat plate in the semirarefied regime. The Bhatnagar–Gross–Krook and ellipsoidal statistical Bhatnagar–Gross– Krook solutions are compared with the exact, well-known direct simulation Monte Carlo method as well as the theoretical solution of the boundary-layer equations that include velocity slip and temperature jump boundary conditions for adiabatic and isothermal wall boundary conditions. It is found that the solutions obtained by the statistical Bhatnagar–Gross–KrookandellipsoidalstatisticalBhatnagar–Gross–Krookmethodsagreewell withthe theoretical and the benchmark direct simulation Monte Carlo solutions. Both approaches capture the shock wave appearing at the leading edge of the flat plate, but the ellipsoidal statistical Bhatnagar–Gross–Krook approach is shown to be in better agreement with the benchmark direct simulation Monte Carlo method solutions as well as the theoreticalresults.Inaddition,thestatistical Bhatnagar–Gross–Krookandellipsoidalstatistical Bhatnagar–Gross– Krook methods are shown to be numerically more efficient than the direct simulation Monte Carlo method.

Transient three-dimensional startup side load analysis of a regeneratively cooled nozzle

The objective of this effort is to develop a computational methodology to capture the side load physics and to anchor the computed aerodynamic side loads with the available data by simulating the startup transient of a regeneratively cooled, high-aspect-ratio nozzle, hot-fired at sea level. The computational methodology is based on an unstructured-grid, pressure-based, reacting flow computational fluid dynamics and heat transfer formulation, and a transient inlet history based on an engine system simulation. Emphases were put on the effects of regenerative cooling on shock formation inside the nozzle, and ramp rate on side load reduction. The results show that three types of asymmetric shock physics incur strong side loads: the generation of combustion wave, shock transitions, and shock pulsations across the nozzle lip, albeit the combustion wave can be avoided with sparklers during hot-firing. Results from both regenerative cooled and adiabatic wall boundary conditions capture the early shock transitions with corresponding side loads matching the measured secondary side load. It is theorized that the first transition from free-shock separation to restricted-shock separation is caused by the Coanda effect. After which the regeneratively cooled wall enhances the Coanda effect such that the supersonic jet stays attached, while the hot adiabatic wall fights off the Coanda effect, and the supersonic jet becomes detached most of the time. As a result, the computed peak side load and dominant frequency due to shock pulsation across the nozzle lip associated with the regeneratively cooled wall boundary condition match those of the test, while those associated with the adiabatic wall boundary condition are much too low. Moreover, shorter ramp time results show that higher ramp rate has the potential in reducing the nozzle side loads.

Large-eddy simulation of aero-optical effects in a spatially developing turbulent boundary layer

Large eddy simulations (LESs) of aero-optical effects in a turbulent boundary layer have been carried out at two Mach numbers (0.9 and 2.3) for an adiabatic wall boundary condition. This study is the continuation of previous work by the present authors using the temporal approximation. However, these temporal simulations have to cope with several drawbacks (thickening in time of the boundary layer, no temporal average and under-resolved statistics). In order to compensate these limits, spatially evolving simulations are performed by means of an extension to compressible flows of the rescaling method of Lund et al. Within this configuration, a blur image caused by phase distortion is the main aero-optical aberration undergone by the wave front. This aberration is due to density variations in turbulent flow. First, aerodynamic fields are proved to compare favourably with theoretical and experimental results. Once validated, the characteristics of the boundary layer allow us to obtain information concerning optical beam degradation. The link between index-of-refraction fluctuations and phase distortion fluctuations is then discussed. Also, the density field is used to compute variance of phase distortion, on the one hand, directly and, on the other hand, by means of the optical models. Therefore, LESs allow us to study these models and the validity of their assumptions. Furthermore, contrary to the temporal approximation, spatially evolving simulations enable us to perform a spectral analysis of phase distortion fluctuations. Finally, LES is proved to be considered as a reference tool for evaluating aero-optical effects.

Direct numerical simulation of nonpremixed flame–wall interactions

The objective of the present study is to use detailed numerical modeling to obtain basic information on the interaction of nonpremixed flames with cold wall surfaces. The questions of turbulent fuel–air–temperature mixing, flame extinction, and wall-surface heat transfer are studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene–air diffusion flame stabilized in the near-wall region of a chemically inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling and convective heat transfer rates up to 90 kW/m 2. The structure of the simulated wall flames is studied in terms of a classical mass-mixing variable, the fuel–air based mixture fraction, and a less familiar heat loss variable, the excess enthalpy variable, introduced to provide a measure of nonadiabatic behavior due to wall cooling. In addition to the flame structure, extinction events are also studied in detail and a modified flame extinction criterion that combines the concepts of mixture fraction and excess enthalpy is proposed and then tested against the DNS data.

Detached Eddy Simulation of Film Cooling Performance on the Trailing Edge Cutback of Gas Turbine Airfoils

The present study deals with the unsteady flow simulation of trailing edge film cooling on the pressure side cut back of gas turbine airfoils. Before being ejected tangentially on the inclined cut-back surface, the coolant air passes a partly converging passage that is equipped with turbulators such as pin fins and ribs. The film mixing process on the cut back is complicated. In the near slot region, due to the turbulators and the blunt pressure side lip, turbulence is expected to be anisotropic. Furthermore, unsteady flow phenomena like vortex shedding from the pressure side lip might influence the mixing process (i.e., the film cooling effectiveness on the cut-back surface). In the current study, three different internal cooling designs are numerically investigated starting from the steady RaNS solution, and ending with unsteady detached eddy simulations (DES). Blowing ratios M=0.5; 0.8; 1.1 are considered. To obtain both, film cooling effectiveness as well as heat transfer coefficients on the cut-back surface, the simulations are performed using adiabatic and diabatic wall boundary conditions. The DES simulations give a detailed insight into the unsteady film mixing process on the trailing edge cut back, which is indeed influenced by vortex shedding from the pressure side lip. Furthermore, the time averaged DES results show very good agreement with the experimental data in terms of film cooling effectiveness and heat transfer coefficients.

Direct numerical simulation of non-premixed flame-wall interactions

The overall objective of this paper is to illustrate how detailed numerical modelling may be used to bring basic information on fundamental problems in combustion science. We consider in the following the interaction of non-premixed flames with cold solid wall surfaces. Flame-wall interactions are an important feature in many combustion systems, that result in significant changes in the flame and wall dynamics: the flame strength is reduced near cold wall surfaces, leading possibly to (partial or total) quenching, while the gassolid heat flux takes peak values at flame contact. The questions of turbulent fuel-air-temperature mixing, flame extinction and wall surface heat transfer are here studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene-air diffusion flame stabilized in the near-wall region of a chemically-inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions, and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling, and convective heat transfer up to 90 kW/m2. The structure of the simulated wall flames is studied in terms of a classical mass mixing variable, i.e. the fuel-airbased mixture fraction, and a less familiar heat loss variable, i.e. the excess enthalpy variable, introduced to provide a measure of non-adiabatic behavior due to wall cooling.

Numerical approach to film cooling effectiveness over a plate surface with coolant impingement

The aim of the present study is conducting the numerical approach to a combination of internal jet impingement and external film cooling over a flat plate. A multi-block three-dimensional Navier-Stokes code, CFX 4.4, with k-e turbulence model is used to simulate this complicated thermal-flow structure induced by the interaction of coolant jet and hot cross mainstream. By assuming the adiabatic wall boundary condition on the tested film-cooled plate, both the local and the spanwise-averaged adiabatic film cooling effectiveness are evaluated for comparison of the cooling performance at blowing ratios of Br=0.5, 1.0, and 1.5. Film flow data were obtained from a row of five cylindrical film cooling holes, inclined in angle of 35° and 0° in direction of streamwise and spanwise, respectively. The film cooling hole spacing between adjacent holes is 15 mm for all the holes. Before the coolant flow being injected through individual cooling hole then encountered with the mainstream, an impingement chamber containing an impingement plate with 43 holes is located on the path of coolant flow. Present study also focused on the effect of impingement spacing, 10mm, 20mm, and 30mm. Compare the results, we find the impingement jet has a significant effect on the adiabatic film cooling effectiveness. As the coolant impingement spacing is fixed, results indicated that higher blowing ratio would enhance the local and the spanwise-averaged adiabatic film cooling effectiveness.

Hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy Simulations of Supersonic Turbulent Mixing

Aturbulentmixinglayerformedfromtwosupersonicstreamsinitially separatedbyasplitterplateisinvestigated withahybridcomputationalmethodusingaReynolds-averagedNavier ‐Stokes(RANS)approachforwall-bounded regions and a large-eddy simulation (LES) approach for the turbulent mixing region. Simulations of the mixing layer predict a vortex shedding originating from the splitter base and then a rapid transition to turbulence, which is in agreement with experimental observations. As a result, the potential of the hybrid method is demonstrated for e ows in which a geometric feature, such as the blunt splitter plate considered here, provides the dominant unsteady feature leading to turbulence, without imposing additional ine ow disturbances. Parametric studies of LES subgrid modeling settings, variations in the spanwise computational domain, and wall temperature settings in the RANS region were conducted. The subgrid model cases, using a baseline computational grid with small spanwise computational domain, overpredicted the mixing layer turbulencelevels but only showed small variation in the mixing layer predictions with large changes in the model parameters. The cases examining wider spanwise domains enabled more turbulent energy to be released in the spanwise direction, which in turn reduced axial and vertical turbulence levels. Finally, prescribing the wall temperatures in the RANS regions instead of using the more traditional adiabatic wall boundary conditions further reduced turbulence levels and enabled reasonable agreement with experimental data.

Analysis of the effect of viscous dissipation for laminar flow in stadium-shaped ducts

The fully-developed laminar forced convection of a Newtonian fluid in a duct with stadium-shaped cross section has been analyzed. The effect of viscous dissipation has been taken into account. Three different thermal boundary conditions have been considered: (T) uniform wall temperature distribution; (H1) axially uniform wall heat flux distribution with peripherally uniform wall temperature distribution; (H2) axially and peripherally uniform wall heat flux distribution. The adiabatic-wall boundary condition has also been analyzed as a special case of the H2 boundary condition. The velocity and temperature distributions in the fluid, as well as the Fanning friction factor and the Nusselt number, have been evaluated numerically, by employing a Galerkin finite element method. As expected, the numerical evaluation of the dimensionless temperature distribution and of the Nusselt number reveals that increasing discrepancies between the H1 and H2 boundary conditions exist if the stadium-shaped duct is gradually flattened.

Morkovin Hypothesis and the Modeling of Wall-Bounded Compressible Turbulent Flows

Modelingofcompressiblewall-boundedturbulente owsreliesonthehypothesisofMorkovin,who suggestedthat compressibility effects on turbulence could be accounted for by the mean density variations alone. This hypothesis has been shown to yield good results for the mean velocity and mean temperature e elds when the incompressible turbulence models are extended directly to calculate compressible turbulent boundary layers. However, its applicability for the turbulence e eld has been less closely scrutinized. The reason is the lack of sufe ciently detailed compressible turbulence data for comparison. Such data are now becoming available. Therefore, the purpose here is to assess the applicability of the Morkovin hypothesis to the turbulence e eld using direct numerical simulation data of a supersonic, e at plate boundary layer. A near-wall Reynolds-stress closure based on a quasi-linear pressure-strain model is used to calculatethis supersonic, boundary-layer e ow. Comparisons between calculations and direct numerical simulation data show that the Morkovin hypothesis is just as applicable for the turbulence e eld and there is a dynamic similarity between the near-wall turbulence e eld of an incompressible and a compressible wall-bounded turbulent e ow. In addition, the validation of this model is reported for compressible e ow calculations covering a wide range of Mach numbers with adiabatic and constant-temperature wall boundary conditions. These results show that the model yields good predictions of e at-plate turbulent boundary layers up to a Mach number of 10.31.

Transport By Gravity Currents In Building Fires

Gravity currents (GC) are important physical phenomena which transport smoke and hot gases id corridors of buildings. In this paper, they are studied using l q e eddy simulations (LES). The transient Navier-Stokes (N-S) equations are numerically integrated with very high resolution, in two dimensions and with high resolution in three dimensions. The LES computations require no adjustable parameters, and are found to agree well with available experimental results in the absence of heat transfer. Fresh-water/salt-water experimental results are compared with computational results with adiabatic boundary conditions to demonstrate the validity of the model. Whereas, a high-resolution two-dimensional computation simulates very well the gravity current with adiabatic wall boundary conditions, significant wall heat transfer requires the consideration of three-dimensional effects. Heat transfer to the ceiling of a corridor from an underlying GC produces longitudinal convection rolls which significantly enhance heat losses to the ceiling and reduce the speed of progression of the GC down the corridor.