Showing papers in "Journal of Thermophysics and Heat Transfer in 1996"
TL;DR: In this article, the authors presented numerical solutions for natural convection heat transfer between a heated horizontal cylinder placed concentrically inside a square enclosure, where the governing elliptic conservation equations were solved in a boundary-fitted coordinate system using a control volumebased numerical procedure.
Abstract: Numerical solutions are presented for natural convection heat transfer between a heated horizontal cylinder placed concentrically inside a square enclosure. Three different aspect ratios (R/L 0.1, 0.2, and 0.3), and four different Rayleigh numbers (Ra = 10, 10, 10, and 10), are considered. The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a control volumebased numerical procedure. Results are displayed in the form of streamlines, isotherms, maximum stream function estimates, and local and average normalized Nusselt number values. At constant enclosure aspect ratio, the total heat transfer increases with increasing Rayleigh number. For constant Rayleigh number values, convection contribution to the total heat transfer decreases with increasing values of R/L. For convection-dominated flows, the average Nusselt number correlation is expressed as Nu = 0.92Ra(R/ L)*. Generated results are in good agreement with previously published experimental and numerical data.
236 citations
TL;DR: In this article, a planar Couette flow is simulated using several different interface conditions in a hybrid technique in which the direct simulation Monte Carlo (DSMC) method and the Navier-Stokes equations are coupled.
Abstract: A planar Couette flow is simulated using several different interface conditions in a hybrid technique in which the direct simulation Monte Carlo (DSMC) method and the Navier-Stokes equations are coupled. Comparison of computational times and accuracy of the different methods are made to determine the best approach for further study. It is concluded that the Marshak condition, in which the properties at the interfaces between the continuum and rarefied regions are determined from flux conservation equations, is the best technique in terms of accuracy and run-time performance. When coupling NavierStokes and DSMC solvers, the use of a Maxwellian distribution to represent the particle velocity distribution hi the Navier-Stokes region yields unacceptable errors.
102 citations
TL;DR: In this paper, the thermal conductivity and thermal contact conductance of thermoplastic and thermosetting polymers have been studied over a range of interface pressures and temperatures, and it was shown that ultra high molecular weight (UHMW) polyethylene had the highest thermal conductivities through the range of temperatures and also had a higher interface temperature of 20°C (68°F).
Abstract: The thermal conductivity and thermal contact conductance of several thermoplastic and thermosetting polymers have been studied over a range of interface pressures and temperatures. The temperature range for the thermal conductivity study varied from 10 to 100°C (50 to 212°F). The study showed that ultra high molecular weight (UHMW) polyethylene had the highest thermal conductivity through the range of temperatures and also had the highest thermal conductance values at an interface temperature of 20°C (68°F). The thermal contact conductance study was conducted over a pressure range of 510-2760 kPa (75-400 psi) and a temperature range of 20-40°C (68-104°F). The conductance values for UHMW polyethylene ranged from 1095.3 to 1659.4 W/m2 K (192.9 to 292.2 Btu/h ft2 °F), whereas the thermal conductivity remained constant at 0.45 W/m K (0.26 Btu/h ft °F) throughout the range of temperatures. Polycarbonate and Teflon® had the next highest thermal conductance values at the same interface temperature. The thermal contact conductance values for polyethylene, Teflon, and phenolic polymers were measured at an elevated temperature of 40°C (104°F). The thermal contact conductance values for both Teflon and phenolic increased with increasing temperature, whereas the values for UHMW polyethylene decreased due to their unique chain structure at the higher temperature. The polymers were chosen because of their widespread engineering interest applications.
94 citations
TL;DR: In this paper, a simple harmonic analysis of the performance of regenerators is described, based on previous one-dimensional differential equations of heat and mass flow, with the additional assumption of steadystate operation and with the key approximations that only the magnitudes and phases of the fundamental components of time-dependent variables are of interest.
Abstract: A simple harmonic analysis of the performance of regenerators is described. Computer calculations based on the simple harmonic analysis run orders of magnitude faster than time-integration methods, with acceptably small reductions in accuracy. This simple harmonic analysis method is based on previous one-dimensional differential equations of heat and mass flow, with the additional assumption of steadystate operation and with the key approximations that only the magnitudes and phases of the fundamental components of time-dependent variables are of interest and that the instantaneous friction-factor and heat transfer coefficients are determined from the instantaneous velocity via steady-flow correlations. This permits a rapid computation scheme in which complex amplitudes of oscillatory variables are used, without numerical time integrations. The method is presented here using the Kays and London correlations for stacked-screen regenerators, but it can be used for any regenerator matrix for which steady friction-factor and heat transfer correlations exist.
89 citations
Abstract: Nonequilibrium vibrational distributions of N2 in the boundary layer surrounding a blunt body in hypersonic flow have been calculated by coupling the nonequilibrium vibrational kinetics, the dissociation and recombination processes, and the boundary-layer equations. The role of different energy exchange processes [vibration-vibration (V-V), vibration-translation (V-T), and recombination-dissociation] in affecting vibrational kinetics has been studied by considering each process. Then the complete kinetics are taken into account, obtaining a global view of the interplay of the different microscopic processes. The model used for the recombination/dissociation processes is such to selectively pump levels v = 25 and 45 of the N2 vibrational manifold. This vibrational energy is then redistributed by V-V and V-T processes. As a result, strongly nonequilibrium vibrational distributions are obtained, despite the thermalizing action of the V-T processes by nitrogen atoms.
85 citations
TL;DR: In this paper, a new numerical scheme for the simulation of trace species is proposed, which explicitly conserves both linear momentum and energy during collisions, and is applied to a number of problems.
Abstract: The direct simulation Monte Carlo method (DSMC) finds application to nonequilibriu m gas flows including hypersonic aerodynamics, spacecraft propulsion, materials synthesis, and flows in microma- chines. In many of these problems, the species of most interest are present in very small quantities. This presents a resolution problem for the DSMC technique in which a finite number of particles is employed. This study proposes a new numerical scheme for the simulation of trace species. Unlike an existing scheme, the new method explicitly conserves both linear momentum and energy during collisions. The scheme is applied to a number of problems to demonstrate its accuracy and utility. Reduced execution times and improved resolution of trace species properties may be obtained simultaneously with the new method. HE direct simulation Monte Carlo method (DSMC) is a physically accurate method for the computation of non- equilibrium gas flows. The technique is most useful in circum- stances where there are insufficient numbers of collisions in the flow to maintain the equilibrium forms of the distribution functions describing the various energy modes of the gas. Gen- erally, such conditions prevail when the average distance be- tween successive collisions of each particle, the mean free path, is comparable to the characteristic length scale of the flow. This type of nonequilibrium condition occurs in a variety of problems of current interest. These include hypersonic flows around vehicles flying at high altitude in planetary atmo- spheres, flows from small rockets used on satellites for control, flows involved in the synthesis of thin films, and flows in mi- cron-scale mechanical structures. The DSMC method employs particles simulated in the com- puter to represent the motions and collisions of real molecules and atoms. A typical simulation employs a few million parti- cles to represent the much larger number of real molecules. The particle weight W is the number of real molecules that each simulated particle represents. The basic assumption of the technique is that particle motion may be performed separately from particle collisions over a time step that is small compared to the average time between successive collisions of the same particle. In the DSMC method, particle motions and collisions are performed in the physical domain. Particles are moved through the length specified by the product of the time step and the velocity vector of each particle. The particles are then collected into cells, and only those particles that occupy a par- ticular cell are considered possible collision candidates. Col- lision selection is performed using statistical probability mod- els that are derived from basic kinetic theory. Macroscopic flow properties are obtained by time averaging particle prop- erties in the computational cells over several thousand itera- tions of the basic algorithm. In many of the applications of interest, the chemical species of most importance occur in very small quantities. To illustrate this point, one particular example is considered. The Bow Shock Ultra Violet (BSUV) flight experiments1'2 were designed
76 citations
TL;DR: In this paper, an experimental investigation on natural convection heat transfer in air from the outer surface of constant heat flux helicoidal pipes with vertical and horizontal orientations was performed and the temperatures along the flow direction and peripheral direction of the tube wall were measured.
Abstract: An experimental investigation is reported on natural convection heat transfer in air from the outer surface of constant heat flux helicoidal pipes with vertical and horizontal orientations. The temperatures along the flow direction and peripheral direction of the tube wall were measured. The test Rayleigh numbers range from 4 x 103 to 1 x 10 s for vertical coils and 5 x 103 to 1 X 10s for horizontal coils. The local and average Nusselt numbers are evaluated and correlated. For the vertical case, the results are compared with single horizontal cylinders. It is found that the heat transfer from the first turn is almost the same as that of the single horizontal cylinder. Because of the tube curvature, the heat transfer coefficient on the outer coil wall (i/r = 77/2) is higher than that on the inner coil wall (i/r = 377/2) in the middle turns of the coil. For the horizontal orientation, the results are well correlated with the tube diameter as the characteristic length. The local heat transfer characteristics are discussed as well. The overall average Nusselt number of the horizontal coil is higher than that of the vertical coil in the laminar region.
73 citations
TL;DR: In this article, the microscopic details of fluid flow and heat transfer in the contact line region of an evaporating curved liquid film were experimentally and theoretically evaluated using null ellipsometry and image analyzing interferometry.
Abstract: The microscopic details of fluid flow and heat transfer in the contact line region of an evaporating curved liquid film were experimentally and theoretically evaluated. The evaporating film thickness profiles were measured optically using null ellipsometry and image analyzing interferometry. These thickness profiles were analyzed using the augmented Young-Laplace equation to obtain the pressure field. Using the liquid pressure field, the evaporative mass flux profile was obtained from a Kelvin -Clapeyron model for the local vapor pressure. A correlation for the local slope (apparent contact angle) at a film thickness of 6 = 20 nm as a function of a dimensionless contact line heat sink was thereby obtained for a group of completely wetting fluids. This change in local slope leads to a decrease in the maximum value of the possible capillary suction at the base of the meniscus. A complementary macroscopic interfacial force balance was also used to describe the effects of viscous losses and interfacial forces on the local values of the apparent contact angle and curvature that are functions of the film thickness and heat flux. These two perspectives give a complete description of an evaporating, nonpolar, completely wetting curved film in the contact line region.
70 citations
TL;DR: In this article, a model for heterogeneous catalysis for copper, nickel, and platinum has been devised, which simulates the heterogeneous chemical kinetics of dissociated aire ow impinging metal surfaces.
Abstract: A model for heterogeneous catalysis for copper, nickel, and platinum has been devised. The model simulates the heterogeneous chemical kinetics of dissociated aire ow impinging metal surfaces. Elementary phenomena such as atomic and molecular adsorption, Eley ‐Rideal and Langmuir ‐Hinshelwood recombinations, and thermal desorption have been accounted for. Comparisons with experimental results for nitrogen and oxygen recombination show good agreement. The e nite rate catalysis model has been used to analyze numerically the problems of heterogeneous catalysis similarity between hypersonic ground testing and reentry e ight. Therefore, the e ow around a blunt cone under these conditions has been calculated, and results for heat e uxes and for a suggested similarity parameter have been compared and discussed. Nomenclature
56 citations
TL;DR: In this article, a three-temperature thermo-chemical model was developed for analyzing the chemical composition and energy states of a hydrogen microwave plasma used for studying diamond deposition, and the chemical and energy exchange rate coefficients were determined from cross section data, assuming Maxwellian velocity distributions for electrons.
Abstract: A three-temperature thermo-chemical model is developed for analyzing the chemical composition and energy states of a hydrogen microwave plasma used for studying diamond deposition. The chemical and energy exchange rate coefficients are determined from cross section data, assuming Maxwellian velocity distributions for electrons. The model is reduced to a zero-dimensional problem to solve for the electron temperature and ion mole fraction, using measured vibrational and rotational temperatures. The calculations indicate that the electron temperature may be determined to within a few percent error even though the uncertainty in dissociation fraction is many times larger.
52 citations
TL;DR: A simplified fluid dynamic model of the boundary layer in hypersonic flow has been coupled with complete vibrational kinetics for pure nitrogen in this article, focusing on the electron-molecule collisions and calculate the rate coefficients of such processes by solving the stationary Boltzmann equation for electron kinetics.
Abstract: A simplified fluid dynamic model of the boundary layer in hypersonic flow has been coupled with complete vibrational kinetics for pure nitrogen. We focus our attention on the electron-molecule collisions and calculate the rate coefficients of such processes by solving the stationary Boltzmann equation for electron kinetics. The role of ionization degree in affecting vibrational and electron energy distributions has also been investigated.
TL;DR: In this paper, the extinction coefficient and the single scattering albedo of selected cellular (reticulated) ceramics that are candidates for use in radiant burners were determined in the 1200-1400 K temperature range.
Abstract: The extinction coefficient and the single scattering albedo of selected cellular (reticulated) ceramics that are candidates for use in radiant burners were determined in the 1200-1400 K temperature range. Total radiation intensities leaving layers of the reticulated material heated to steady state in a tube furnace with two different boundary conditions were measured. An inverse radiation approach involving the two-flux approximation using a gray, isotropically scattering model was used to obtain the radiative properties from the measured intensities. Above an optical thickness of unity, this procedure was not sufficiently sensitive to yield the extinction coefficient. For such cases, the extinction coefficient was estimated from the geometric optics limit. The single scattering albedos varied from 0.68 to 0.88 with an uncertainty of ±7%, whereas the extinction coefficients varied from 81 to 270 m"1 with an uncertainty of ±11%, depending on the number of pores per centimeter and the sample material. In the range of temperature investigation the variations in the radiation properties were not significant. Nomenclature b = backscattering fraction d = pore diameter, m / = forward scattering fraction fv = solid volume fraction / = intensity, W/m2-sr
TL;DR: In this article, the thermal conductivity of composite laminates was measured at various resin degrees of cure and fiber volume fractions while using various ply lay-up angles, and various analytical models were researched, and those that compare most favorably with the experimental data were presented.
Abstract: The thermal conductivity of a thermosetting advanced composite material during its cure was experimentally investigated and compared to analytical models. The thermal conductivity of graphite/epox y was experimentally measured using a guarded hot plate apparatus. The through-thickness transverse thermal conductivity of composite laminates was measured at various resin degrees of cure and fiber volume fractions while using various ply lay-up angles. The in-plane thermal conductivity was measured using unidirectional laminates, as well as laminates with various ply lay-up angles. Techniques for manufacturing composite laminates with these various properties, as well as the guarded hot plate apparatus necessary for accurate measurements, are described. Finally, various analytical models were researched, and those that compare most favorably with the experimental data are presented. Nomenclature Hc = thermal contact conductance of the experiment, W/m2°C Hr = resin heat of reaction for AS4/3501-6, J/g K = thermal conductivity, W/m°C L = laminate dimension in the x direction, m N = number of plies in a laminate p = function of the fiber volume fraction, Eq. (6) Q = total heat flux, W q = heat flux/area, W/m2 q = resin energy generation rate per unit volume, W/m3 qln = heat flux/area applied by the guarded hot plate,
TL;DR: Although all methods improve convergence, the mesh rebalance method improves the RTE convergence best, and a general solution strategy is discussed, which aims to reduce RTE iterations by an order of magnitude.
Abstract: Discrete ordinates (DO) methods have been developed to solve the multidimensional radiative transport equation (RTE) for applications including combustion processes and other combined-mode heat transfer cases. The convergence of DO methods is known to degrade for optical thicknesses greater than unity, which occur for example in a flame. Acceleration schemes have been developed for use in neutron transport applications, but little work has been done to accelerate convergence of the RTE for radiative heat transfer applications. This article presents several acceleration schemes for the RTE, including successive overtaxation, synthetic acceleration, and mesh rebalance methods. Solution convergence is discussed and demonstrated using two- and three-dimensional examples. Although all methods improve convergence, the mesh rebalance method improves the RTE convergence best. For some conditions, the rebalance method improves convergence dramatically, reducing RTE iterations by an order of magnitude. However, the mesh rebalance method fails to produce convergence of the RTE for large optical thicknesses and fine mesh discretizations. Examples are used to demonstrate that unproved convergence can be obtained by solving the rebalance equation on a coarser grid, which is determined by regrouping the base RTE grid, until an optical thickness of near unity is obtained on the coarse grid. Based on these findings, a general solution strategy is discussed. / /
TL;DR: In this article, a two-dimensional axisymmetric ablation problem with unstructured grids is solved using the control volume finite element method, and the exterior shape of the ablating body is determined from a surface energy balance with the interior mesh displaced in response to the surface ablation as if the body was a linear elastic solid.
Abstract: This article presents the solution of two-dimensional axisymmetric ablation problems with unstructured grids. The energy equation is formulated and solved using the control volume finite element method. The exterior shape of the ablating body is determined from a surface energy balance with the interior mesh displaced in response to the surface ablation as if the body was a linear elastic solid. Boundary condition matrices are formulated for both heat of ablation and generalized thermochemical ablation boundary conditions. Computed recession rates are compared to an analytical solution to demonstrate the accuracy and convergence of this approach. Additionally, the practical application of this approach to the ablation of a re-entry vehicle nose tip is presented.
TL;DR: In this paper, the effect of radiation on combustion liners, turbine vanes, and rotating blades of an aircraft engine was analyzed using the two-flux method for the radiative contribution.
Abstract: Using thermal barrier coatings on combustor liners, turbine vanes, and rotating blades is important for reducing metal temperatures in current and advanced aircraft engines. Zirconia is a common coating material, and it is partially transparent to thermal radiation. Radiation becomes more significant as temperatures are raised for higher efficiency in advanced engines. Calculations are often made with radiation effects neglected inside the coating. The effect of radiation is illustrated here, where an analytical procedure is provided by using the two-flux method for the radiative contribution. A detailed study was made of ceramic thermal barrier coatings for diesel engines, and a two-flux analysis was developed for radiation in semitransparent multilayer composites. These efforts provide the basis for the present analysis where illustrative solutions are obtained for typical conditions in an aircraft engine. The formulation and solution of the exact spectral radiative transfer equations including large scattering, as is characteristic of zirconia, are rather complicated. The two-flux method is used here to provide a simplified method.
TL;DR: In this paper, a two-temperature chemical nonequilibrium model is developed for nitrogen/hydrogen (N 2/H2) arcjet thrusters, where all viscous flow properties are considered assuming steady, laminar, continuum, and axisymmetric flow.
Abstract: A two-temperature chemical nonequilibrium model is developed for nitrogen/hydrogen (N 2/H2) arcjet thrusters. All viscous flow properties are considered assuming steady, laminar, continuum, and axisymmetric flow. A seven-species N2/H2 plasma composition of molecules, atoms, ions, and electrons is assumed, and a finite rate chemistry model is employed to model collisional processes among the species. Separate energy equations are formulated for the electrons and heavy species. The anode temperature distribution is included, and propellant electrical conductivity is coupled to the plasma properties, allowing for a selfconsistent current distribution. The numerical solution employs the compressible form of the pressureimplicit with splitting of operators algorithm to solve the continuity and momentum equations. Numerical results are presented for a low-power simulated hydrazine thruster. The centerline constrictor region of the arcjet flowfield is predicted to be near thermal equilibrium, whereas a high degree of thermal nonequilibrium is predicted in the near-anode region of the arcjet nozzle. Strong electric fields near the anode produce elevated electron temperatures that enhance ionization levels and electrical conduction through the arcjet boundary layer. Radial diffusion of electrons from the arc core also enhances the near-anode ionization levels. Thus, the nonequilibrium approach is required to accurately model the plasma current distribution.
TL;DR: In this article, a theory and its validation by experiment for the radiative properties of high-porosity silica aerogels that contain randomly oriented and uniformly dispersed fibers are presented.
Abstract: This paper presents a theory and its validation by experiment for the radiative properties of high-porosity silica aerogels that contain randomly oriented and uniformly dispersed fibers. The formulations for the fiber radiative properties are based on fundamental principles, for which parameters defining the material composition are the only required inputs. The predicted spectral transmittance and reflectance for collimated irradiation from the solution of the radiative transfer equation are compared with the measured values. The validity of the theoretical model is established by the good agreement between the predictions and experimental data for various specimens that contain different fiber types, fiber and aerogel solid volume fractions, and specimen thicknesses.
TL;DR: In this article, the infrared radiation of nitric oxide (NO) behind a shock wave in O2-N2 mixtures has been calculated by two different techniques, and compared with recent shock-tube experiments.
Abstract: The infrared radiation of nitric oxide (NO) behind a shock wave in O2-N2 mixtures has been calculated by two different techniques, and compared with recent shock-tube experiments. The first technique (model I) utilizes the Park model. This model incorporates the vibrational relaxation of O2 and N2 and assumes a Boltzmann distribution of vibrational energy during the relaxation process. Model II uses a master equation solution, employing recently published state-to-state vibration-translation and vibration-vibration transition probabilities. Vibration-chemistry coupling is provided through the MacheretFridman-Rich model (MFR). The calculations are compared with experimental results for shock waves in the range of 3-4 km/s. Results of the two model calculations are compared at speeds up to 9 km/s, for both normal shocks and bow shocks. The two models predict nearly the same NO production rates behind all of the normal shocks, and show the prominent effect of N2 vibrational coupling in the reaction N2 + O —> NO + N. For high-altitude bow shocks, where extreme vibrational nonequilibrium is present, there are large differences in the results calculated by the Park and MFR coupling techniques.
TL;DR: In this paper, a computational method for calculating supersonic two-dimensional nozzle flows with nonequilibrium phase change is developed using a fully implicit, finite volume formulation with Steger-Warming flux vector splitting and Jacobi point relaxation.
Abstract: A computational method for calculating supersonic two-dimensional nozzle flows with nonequilibrium phase change is developed. The code uses a fully implicit, finite volume formulation with Steger-Warming flux vector splitting and Jacobi point relaxation. Source terms for finite rate condensation and evaporation are constructed from the classical theories of nucleation and droplet growth. Calculations of the NASA Langley Research Center 8-ft High Temperature Tunnel nozzle are performed and compared with previously reported quasi-one-dimensional calculations and experimental data.
TL;DR: In this paper, the authors investigated thermal stress convection in rarefied gases and in a continuum flow regime under zero gravity conditions using the direct simulation Monte Carlo (DSMC) method.
Abstract: Benard and thermal stress instabilities in a rarefied gas are investigated by the direct simulation Monte Carlo (DSMC) method. Particular emphasis is given to the numerical aspects of DSMC (accuracy, convergence, etc.), that were not investigated in the previous studies and to the refining of the simulation results by using a special data processing (filtering) procedure. In the stratified rarefied gases with hightemperature gradients the Boussinesq approximation is not valid and it is shown that the onset of instabilities in the non-Boussinesq fluid (rarefied gas) is not determined by a single nondimensional parameter (Rayleigh number). Benard convection in the stratified rarefied gas is analyzed in different geometries, including the case with curvilinear boundaries. The occurrence of thermal stress convection in rarefied gases and in a continuum flow regime under zero gravity conditions was demonstrated using the DSMC method.
TL;DR: In this article, the authors used the Navier-Stokes equations to investigate the radiative effects in chemically reacting compressible flows of premixed hydrogen and air in an expanding nozzle.
Abstract: The two-dimensional spatially elliptic Navier-Stokes equations have been used to investigate the radiative effects in chemically reacting compressible flows of premixed hydrogen and air in an expanding nozzle. The radiative heat transfer is simulated using the Monte Carlo method. The nongray model employed is based on the statistical narrow-band model with an exponential-tailed inverse intensity distribution. The spectral correlation has been considered in the Monte Carlo formulations. Results obtained demonstrate that the radiative effects on the wall heat transfer may be significant. Extensive parametric studies are conducted to investigate the effects of equivalence ratio, wall temperature, inlet flow temperature, and the nozzle size on the radiative and convective heat fluxes on the walls.
TL;DR: In this paper, a theoretical model for simulating heat and mass transfer in the vicinity of the liquid meniscus edge in the evaporator of a groove heat pipe is proposed, which consists of macro-and microregions.
Abstract: Evaporative heat and mass transfer phenomena in the vicinity of the liquid meniscus edge in the evaporator of a groove heat pipe is investigated theoretically and experimentally. A theoretical model for simulating the phenomena is proposed, which consists of macro- and microregions. The former represents the part of the liquid meniscus region where conventional heat and mass transfer take place, whereas the latter is a narrow meniscus edge region close and contacted to the solid wall where transport phenomenon is affected by the intermolecular forces or disjoining pressure. The numerical results obtained for ammonia as a working fluid indicate that a large heat flux on the order of megawatt per square meters is transported in the narrow microregion, and accordingly more than one-third of the total heat energy supplied from outer surface is transported through the microregion that is less than one-hundredth of the total meniscus area. An optical measurement was conducted at the meniscus edge to confirm the existence of the thin nonevaporative liquid film and to identify its thickness on the order of several tens of nanometers. These magnitudes of nonevaporative film thickness can be explained by the proposed theory.
TL;DR: In this article, the authors used two-photon laser-induced fluorescence for detection of O and N atom loss in a diffusion tube to measure surface catalytic activity, and their associated errors were reported for N and O atoms on a quartz surface at room temperature.
Abstract: Advanced laser-based diagnostics have been developed to examine catalytic effects and atom/surface interactions on thermal protection materials. This study establishes the feasibility of using two-photon laser-induced fluorescence for detection of O and N atom loss in a diffusion tube to measure surface catalytic activity. The experimental apparatus is versatile in that it allows fluorescence detection to be used for measuring species selective recombination coefficients and for performing diffusion tube and microwave discharge diagnostics. Many of the potential sources of error in measuring atom recombination coefficients by this method have been identified and taken into account. These include scattered light, detector saturation, sample surface cleanliness, reactor design, gas pressure and composition, and selectivity of the laser probe. Recombination coefficients and their associated errors are reported for N and O atoms on a quartz surface at room temperature.
TL;DR: In this paper, the present understanding of shock-layer radiation in the low-density regime, as appropriate to hypersonic vehicles, is discussed, and the NONEQ and GENRAD computer programs are compared with experimental spectra recorded at NASA Ames's electric arc-driven shock-tube facility.
Abstract: The present understanding of shock-layer radiation in the low-density regime, as appropriate to hypersonic vehicles, is discussed. Calculated spectra using the NONEQ and GENRAD computer programs are compared with experimental spectra recorded at NASA Ames's electric arc-driven shock-tube facility. The computations predict the intensity of the Nj(1~) system very well, but overpredict the intensities of various atomic O and N transitions and underpredict the intensities of the N2(2) band system. To compute the correct electronic populations, it appears that the quasi-steady-state formulation must be expanded to include more individual electronic states. However, this may cause some computational difficulties and will require excitation rate data for many states, which are not well known.
TL;DR: In this article, a numerical and theoretical study is performed to analyze the steady-state natural convection fluid flow and heat transfer in a rectangular porous cavity filled with a non-Newtonian fluid and bounded by impermeable surfaces.
Abstract: A numerical and theoretical study is performed to analyze the steady-state natural convection fluid flow and heat transfer in a rectangular porous cavity filled with a non-Newtonian fluid and bounded by impermeable surfaces. The flow is modeled by utilizing the modified Darcy equations. Isothermal boundary conditions are considered where two opposite vertical walls are kept at constant but different temperatures, and the horizontal walls are insulated. The external parameters are the AR, /?«, and powerlaw index. Employing pure scaling arguments, four heat transfer modes are identified: 1) pure conduction, 2) high-/ta convection, 3) distinct horizontal boundary-layer convection, and 4) distinct vertical boundarylayer convection. The results obtained from the scaling arguments are verified numerically. The agreement between the results obtained from the scaling arguments and the numerical model is good. The numerical results are presented in terms of theoretical streamlines and isotherms, the average Nu at the hot wall, the horizontal velocity at the vertical midplane, the vertical velocity at the horizontal midplane, and the temperature distribution at the horizontal and the vertical midplanes.