Showing papers in "Journal of Thermophysics and Heat Transfer in 1987"

Assessment of two-temperature kinetic model for ionizing air

Abstract: A two-temperatur e chemical-kinet ic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock tubes, ballistic ranges, and flight experiments. In the model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium flow in shock tubes are obtained using the computer code STRAP (shock-tube radiation program) and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code SPRAP (stagnation-point radiation program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures, the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibriu m airflow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of 6 m radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of freestream conditions. Assumptions, approximations, and limitations of the model are discussed. Nomenclature = average molecular speed ^/$kT/nm, cm s ~ ! = pre-exponential factor in reaction rate coefficient, cm3mole~ 1 s~ * - average vibrational energy per particle, erg = average vibrational energy per particle under equilibrium, erg = reaction energy, erg

Interaction of radiation with turbulence - application to a combustion system

Abstract: Turbulence/radiation interaction is examined in order to provide better fundamental understanding of temporal aspects of radiative transfer in combustion systems. Two aspects of radiative transfer in a turbulent medium are considered in this paper. In the first, transfer of radiation along a path with turbulent concentration and temperature fluctuations is calculated for the time-mean irradiance at a combustion chamber wall due to random concentration of absorbing species and emission with Gaussian probability density functions. In the second, turbulence/radiation interaction and the effect of radiation transfer on the fully coupled structure and mean properties are assessed for an industrial natural gas-fired furnace. The results of calculations based on the approximate formulation utilized here show that the effects of turbulence/radiation interaction on combustion and flow properties is relatively small for a preheated methane-air mixture. The interaction is greater when the oxidant is cold and the flame is relatively long. 24 references.

Comparison of wet and dry growth in artificial and flight icing conditions

Abstract: The heat transfer behavior of accreting ice surfaces in natural (flight test) and simulated (wind tunnel) cloud icing conditions are studied. Observations of wet and dry ice growth regimes as measured by ultrasonic pulseecho techniques are made. Observed wet and dry ice growth regimes at the stagnation point of a cylinder are compared with those predicted using a quasi steady-state heat balance model. A series of heat transfer coefficients are employed by the model to infer the local heat transfer behavior of the actual ice surfaces. The heat transfer in the stagnation region is generally inferred to be higher in wind tunnel icing tests than in natural, flight icing conditions.

Effect of thermal stratification on free convection within a porous medium

Abstract: Free convection over a vertical flat plate embedded in a thermally stratified porous medium is analyzed by exploiting the similarity transformation procedure. Numerical integration results are presented for a series of wall and ambient temperature distributions which permit similarity solutions. The conjugate conduction connection problems of a free convection fin embedded in a thermally stratified porous medium is examined, and it is shown that the influence of the thermal stratification on the heat transfer is quite significant. 8 references.

Design of a Thermocapillary Flow Experiment in Reduced Gravity

Abstract: Detailed considerations for the design of a low-gravity thermocapillary flow experiment are presented. The basic configuration is one in which a fluid contained in a circular dish is subjected to an imposed surface heat flux. Specific consideration is given to the imposed thermal signature, thermal boundary conditions, free-surface deformations, and g-jitter effects. Numerical analysis and ground-based and drop-tower tests are utilized to ensure that reasonable flows are obtained in which measurements can be made relatively easily. The configuration chosen is of further importance because it differs from others in which oscillations were observed.

Transition to oscillatory convective heat transfer in a fluid-saturated porous medium

Abstract: The transition from steady to time-periodic motion in the analog of Benard convection in a two-dimensional region of fluid-saturated porous media is studied by means of an eigenfunction expansion and a branch-tracing technique. This method leads to the location of the transition at Rayleigh number 9.9 times that at convection onset. The small-amplitude motion has a period 0.012 times shorter than the thermal diffusion time and comes into existence through a Hopf bifurcation. The structure and time progression of the destabilizing disturbance indicates that the dominant effect is an instability convected by the base flow whose strength is coupled to the steepening thermal boundary layers. The effects of truncation of the expansion on the prediction of the transition and its mechanism are discussed.

Numerical analysis of laminar flow in a double-walled annular heat pipe

Abstract: The parabolic and elliptic representations of the governing equations of fluid motion for steady two-dimensional laminar flow in a double-walled annular heat pipe with various distributions of heating and cooling loads have been analyzed numerically. The results show that the flow reverses at very high condensation rates and that the parabolic representation provides a sufficiently accurate picture of the vapor pressure variations at both low and high evaporation and condensation rates. An approximate analytical method is also presented that can predict the pressure drop and velocity variation accurately along the double-walled annular heat pipe, as well as the location at which the flow reverses in the condenser zone.

Radiative heat transfer in emitting-absorbing-scattering spherical media

Abstract: Radiative heat transfer in spherical media that emit, absorb, and scatter radiation is analyzed. Linear anisotropic scattering is considered. The spherical harmonics method is used to solve the equation of transfer. The P-1, P-2, P-3, P-7, and P-11 solutions are presented and their accuracy is discussed. Numerical results presented include the radiant heat fluxes at the inner and outer walls of the spherical enclosure and the distribution of the blackbody emissive power in the medium. The effect of scattering on the transfer of radiant energy is illustrated. 6 references.

Local heat-transfer performance and mechanisms in radial flow between parallel disks

Abstract: An experimental study is conducted to determine local heat-transfer performance and mechanisms in radial flow through two parallel heated disks. Three distinct heat-transfer mechanisms are disclosed along the flow passage: steady laminar, periodic laminar, and turbulent. The heat-transfer performance is enhanced proportional to the 0.5 and 0.8 powers of Reynolds number, in the steady periodic and turbulent ranges, respectively. The local and average heat-flow maps are constructed to define the domain or range of the transfer mechanisms. The locations for the onset of both steady laminar and turbulent mechanisms shift upstream with an increase in the flow rate.

Aerothermodynamic study of slender conical vehicles

Abstract: A numerical study was performed to assess the applicability of some current techniques which can be used for aerothermal predictions over slender spherically blunted cones Predictions using a viscous-shock-layer method and several engineering approaches were compared with experimental results from flight and ground-based tests, with each other, and with other detailed results Good agreement was obtained in comparisons with laminar and turbulent heating data from the Reentry F flight vehicle and with the wind-tunnel data In particular, the viscous-shock-layer method was shown to yield excellent comparisons and should be useful in providing detailed flowfield and surface values for slender blunted cones Additional predictions were obtained with these methods for two 5-deg half-angle cones with different nose radii to illustrate the effects of nose bluntness and angle of attack on drag and heat transfer These results demonstrate the benefit of nose blunting with respect to heating and drag for laminar and transitional flow at zero-lift conditions and the benefit of heating reduction at angle of attack Detailed comparison of the engineering code predictions with the viscous-shock-layer results for these additional cases generally showed good agreement except for the laminar prodictions at angle of attack on the forward cone surface Nonequilibrium calculations at 0-deg angle of attack showed that substantial benefits of low surface catalysis existed only in the nose-dominated region

Local endwall heat/mass-transfer distributions in pin fin channels

Abstract: Naphthalene sublimination experiments were conducted to study the effects of the pin configuration, the pin length-to-diameter ratio, and the entrance length on local endwall heat/mass transfer in a channel with short pin fins (pin length-to-diameter ratios of 0.5 and 1.0). The detailed distributions of the local endwall heat/mass-transfer coefficient were obtained for staggered and aligned arrays of pin fins, for the spanwise pin spacing-to-diameter ratio of 2.5, and for streamwise pin spacing-to-diameter ratios of 1.25 and 2.5. The Reynolds numbers were kept at about 3.3 X 104. Overall- and row-averaged Nusselt numbers compared very well with those from previous heat-transfer studies.

Surface effects of satellite material outgassing products

Abstract: : Optical property measurements of condensed contaminant films on cryogenically cooled substrates were completed for 18 additional materials to those reported in AEDC-TR-87-8, 'Surface Effects of Satellite Outgassing Products'. The materials studied were either being considered for use in space or in ground test chambers at Arnold Engineering Development Center (AEDC). Infrared spectral transmittance measurements were made for thin films of contaminant deposited on a 77 K germanium window under vacuum conditions (. 000001 torr). Material temperatures were controlled at 125 C for 24 hr to provide the outgassing source. The objective of this report is to present the refractive (n) and absorptive (k) indices of the condensed outgassing products of satellite materials which have been determined from the experimental measurements of the infrared transmittance spectra.

Vapor-deposited emittance-catalysis coatings for superalloys in heat-shield applications

Abstract: Statically oxidized Inconel 617 and MA 956 superalloys were prepared with vapor deposited aluminosilicate layers and exposed to temperatures which simulated reentry conditions. The aluminosilicate layers were 1-2 microns thick. The trials consisted of exposures to 1500-2300 F heat while surface temperatures were monitored with pyrometers. Analyses were then performed of the catalytic activity, oxidation phenomena, and radiative properties after thermal cycling the specimens for 8 hr. Both alloys were found to be catalytic to the recombination processes of dissociated species in the reentry environment. The coatings reduced the catalytic activity by 40 percent for both alloys but did not change the emittance. An enhanced Cr depletion zone was detected in the Inconel samples, implying that the coating did not prevent diffusion loss of Cr. The coated MA 956, on the other hand, gained weight over the course of the trials.

Heat Transfer Analysis of Fiberglass Insulations With and Without Foil Radiant Barriers

Abstract: This work compares analytical results with experimental data for the total heat transfer through typical fiberglass insulations subjected to a time-varying incident radiative heat flux and with time-varying temperature boundaries. The insulation configurations and thermal environments analyzed are consistent with conditions that exist in residential attics during the summer months. A heat transfer analysis was performed, which treats the problem as one of coupled transient conduction and radiative heat transfer in a fully participating medium (absorbing, emitting, and scattering). Computations were performed for both isotropic and anisotropic phase functions and for both gray and nongray radiative transport. Temperature data recorded for summer-time conditions in FL and MS were used to describe the boundary conditions for the heat transfer analysis. The effect of foil radiant barriers on the overall heat transfer through the insulation also was studied. Calculations are presented that compare the foil radiant barrier and no-foil cases; the foil barrier is shown to reduce the total heat transfer by about 42%. All computations used the discrete ordinates solution method to solve the radiative transport equation and a control-volume based finite difference technique to solve the energy equation.

Analysis of radiation-induced natural convection in rectangular enclosures

Abstract: A model has been developed for predicting the volumetric deposition rate of radiant energy from an external source in a vertical fluid layer and the subsequent buoyancy-driven flow and heat transfer. The model calls for solution of the coupled two-dimensional equations of continuity, momentum, and energy with a one-dimensional radiative transfer model. After experimental validation of the model, parametric calculations were performed to determine the effect of the modified Rayleigh number, fluid Prandtl number, fluid layer opacity, cavity aspect ratio, opaque wall reflectivity, and convective heat loss from the transmitting wall. Natural convective motion and heat transfer are quite different from that found in cavities with differentially heated side walls. Results are presented in the form of temperature distributions, local heat transfer, contours of the stream function, and profiles of the predicted radiative flux divergence. 20 references.

Direct simulation of hypersonic flows over blunt wedges

Abstract: The results of a numerical study of low-density hypersonic flow about cylindrically blunted wedges with body half-angles of 0, 5, and 10 deg are presented. Most of the transitional flow regime encountered during entry between the free molecule and continuum regimes is simulated for a re-entry velocity of 7.5 km/s by including freestream conditions of 70 to 100 km. The bodies are at zero angle of incidence and have diffuse and finite catalytic surfaces. Translational, thermodynamic, and chemical nonequilibrium effects are considered in the numerical simulation by utilizing the direct simulation Monte Carlo (DSMC) method. The numerical simulations show that noncontinuum effects such as surface temperature jump and velocity slip are evident for all cases considered. The onset of chemical dissociation occurs at a simulated altitude of 96 km for the configurations considered.

Monte Carlo Simulation of Rarefied Flow Along a Flat Plate

Abstract: The direct simulation Monte Carlo technique is applied to a diverging flowfield along a flat plate. The model accounts for a plate of finite thickness with a nonzero leading edge bevel angle, and permits the specification of surface accommodation coefficients ranging from diffuse to specular reflection. The computed density, velocity, and temperature profiles at several axial stations along the plate are compared with experimental data. These comparisons indicate good agreement when the leading edge bevel is included in the calculation, and when the surface accommodation coefficients are each assigned a value of 0.8. Results are also presented that indicate the sensitivity of the computed skin friction to the ratio of the computational cell size to molecular mean free path. The results indicate that the skin friction is insensitive to the cell size if the local Knudsen number based on the cell size is maintained larger than unity everywhere in the flowfield. The results also indicate that the skin friction is underpredicted toward the plate leading edge and overpredicted at downstream axial positions when the cell size exceeds the molecular mean free path.

Heat Transfer by Combined Conduction and Radiation in Axisymmetric Enclosures

Abstract: in the area of atmospheric radiation and that of Hyde and Truelove5 and Fiveland6'7 in the area of engineering heat transfer comprise the few studies undertaken in this

Successive improvement of the modified differential approximation in radiative heat transfer

Abstract: A modified differential approximation proposed by Olfe is extended to the treatment of a medium with isotropic scattering and boundary reflection. The method can be used to readily generate successive improvement of the solutions. In this work, the basic procedure is demonstrated by computing the transmissivity and hemispherical reflectivity of an absorbing, isotropically scattering slab with a reflecting boundary. Comparison of present computational results with existing exact solutions shows that, in general, the modified differential approximation is superior to the unmodified differential approximation, and that one or two applications of the improvement of the modified differential approximation produces results very close to the exact solution. The present method also gives accurate solutions for an optically-thin medium. The general concept of the current method appears to be adaptable to the treatment of multidimensional cases.

Combined conduction and radiation heat transfer in concentric cylindrical media

Abstract: The exact radiative transfer expressions for gray and nongray gases which are absorbing, emitting and nonscattering, contained between infinitely long concentric cylinders with black surfaces, are given in local thermodynamic equilibrium. Resulting energy equations due to the combination of conduction and radiation modes of heat transfer, under steady state conditions for gray and nongray media, are solved numerically using the undetermined parameters method. A single 4.3-micron band of CO2 is considered for the nongray problems. The present solutions for gray and nongray gases obtained in the plane-parallel limit (radius ratio approaches to one) are compared with the plane-parallel results reported in the literature.

Experimental study of natural convection in a partially porous enclosure

Abstract: This paper presents the experimental results for steady-state natural convection heat transfer in a rectangular enclosure. The enclosure was divided vertically with an impermeable partition into a fluid-saturated porous region and a fluid-filled region. The horizontal boundaries of the enclosure were adiabatic and the vertical boundaries were isothermal. Experimental runs were carried out for different porous materials, Rayleigh numbers, and thicknesses of the porous region. The results verified an earlier prediction that the heat transfer could be minimized by partially filling the enclosure with a porous material rather than filling it entirely. A correlation has been developed expressing the Nusselt number in terms of the relevant governing parameters.

Radiation Transfer in Isotropically Scattering, Rectangular Enclosures

Abstract: Radiative heat transfer in absorbing, emitting, isotropically scattering, gray, two-dimensional rectangular regions having an emitting boundary and spacially distributed energy sources is solved by both the collocation and the Galerkin methods. Using the expressions given in this work, the quantities, such as the incident radia- tion, the radiation heat flux, and the divergence of the radiation heat flux anywhere in the medium, can be deter- mined to a high degree of accuracy for all values of the single scattering albedo, from small to moderately large values of horizontal and vertical optical dimensions of the enclosure. Results are presented for representative cases, to illustrate the application of the method. A parameter study is made to show the effects of the single scattering albedo on the incident radiation and the heat flux. HERE are numerous situations in which radiation heat transfer in absorbing, emitting, isotropically scattering, gray, two-dimensional rectangular media is important. They include, among others, modeling of heat transfer in fur- naces, fire protection, manufacturing of glass, studies of in- sulation properties of various types of fibrous and foam materials, cryogenics, and heat transfer through partially transparent materials. In order to study radiation heat transfer in any one of these situations, it is necessary to solve the equation of radiative heat transfer. Numerous approximate and exact methods of analysis have been reported for solving the equa- tion of radiative transfer in a one-dimensional medium; but for the two-dimensional situation, the problem of an absorb- ing and emitting medium has been studied with approximate methods of analysis by several investigators including Modest,1 Razzaque et al., 2 and Fiveland.3 In the case of a scattering medium, the source function expansion technique has been used by Sutton and Ozisik4; a numerical integration scheme has been employed by Crosbie and Schrenker5 to study the problem of diffuse incident radiation and no energy sources in the medium; and the same numerical technique has been applied by Crosbie and Schrenker6 to solve the problem of collimated incident radiation and no energy sources in the medium. Further information, with regard to solution methods and areas of applications of radiative heat transfer, is given in a recent survey by Viskanta.7 We present a highly accurate method for solving two- dimensional radiation transfer in an absorbing, emitting, isotropically scattering, two-dimensional rectangular en- closure. In the following sections, the method of analysis is described first. Results are then presented that can be used to check the accuracy of various approximate methods of analysis. Finally, a parameter survey is made to examine the effects of single scattering albedo on the radiation heat transfer for various optical dimensions of the medium.

Thermal boundary layer on a continuous moving plate with freezing

Abstract: The growth of a solidified layer or "freeze coat" on the surface of a chilled continuous plate traveling steadily through a bath of warm liquid is investigated analytically. The behavior of the thermal boundary layer in the liquid flowfield that is induced by the motion of the plate is modeled along with the process of heat conduction in the solid phase to determine the location of the freezing front. Using the method of similarity, axial variations of the freeze-coat thickness and the coefficient of local convective heat transfer from the liquid to the solid phase are obtained as functions of various controlling parameters of the system. It is found that, while the shape of the freeze coat depends strongly on the local convective heat flux, the flow is, in turn, heavily influenced by the variation of the solid /liquid interface location. Because of this mutual interaction between the phase change process and the flow, the local convective heat-transfer coefficient at the freezing front is considerably larger than the corresponding value for the case of forced convection over a continuous moving plate without freezing. The effect of flow/freezing interaction is found to be quite pronounced, especially when the liquid Prandtl number is large and the freeze coat grows rapidly in the axial direction.

New Data on Two-Phase, Two-Component Heat Transfer and Hydrodynamics in a Vertical Tube

Abstract: In forced-convec tive, two-phase, two-component (gas-liquid) flow, experimental data for mean heat-transfer coefficients, pressure drop and flow patterns were taken simultaneously for the flow in a 0.46-in. (1.17-cm) i.d. electrically heated vertical tube using three liquids: water, a glycerine-wate r solution (58-42% by weight), and silicone h'quid (Dow Corning 200, 5 cS viscosity grade) with air as the gas phase. The combination of silicone liquid and the glycerine-water solution provided a set of data, appearing for the first time in the literature, in which the surface tension changed by a factor of 3.4 (being lower for the silicone liquid) with a rough matching of other hydrodynamic properties and a precise matching of the Prandtl number (63 at 25°C). The flow-pattern results showed a significant change in the bubble-slug boundary for the silicone liquid compared with the glycerine-water solution, whereas the total pressure drop for silicone/air in the range of superficial liquid velocity VSL of 0.910 ^ VSL ^ 2.26 ft/s (0.277 ^ VSL ^ 0.690 m/s) at high gas flow rates showed a sudden drop followed by a subsequent increase. The behavior of the mean heat-transfer coefficient KTP for silicone/air in the range 0.260 ^ VSL <*2.26 ft/s (0.080 ^ VSL ^ 0.690 m/s) at high gas flow rates has not been seen with other liquids (KTP, after a maximum decrease, goes through a minimum, and then increases again).

Radiative transfer in thermal insulations of hollow and coated fibers

Abstract: This paper presents a numerical study of radiative transfer in thermal insulation made of hollow cylindrical fibers and of solid fibers coated with thin dielectric films. The radiative properties of the packed fibrous insulation are evaluated from the single-fiber absorption and scattering characteristics that are based on electromagnetic theory. The results show that, for cases of practical interest, hollow fibers have higher backscattered radiation and higher extinction efficiency than solid fibers of the same outside diameter. Thus, insulations made of such are not only of low weight and low heat capacity, but are also more effective in preventing radiative heat loss. Coating solid fibers with thin dielectric films does not appreciably enhance the radiative extinction characteristics, unless the coating is thick or has a high index of refraction.

Heat and mass transfer in partial enclosures

Abstract: The present investigation is about the condensation and phase change processes in an enclosure partially filled with a porous insulation. The effect of variations in the porous insulation thickness on the moisture, relative humidity, temperature, and condensation rate fields is investigated. The problem is modeled as a transient, multiphase flow in a composite slab consisting of a porous portion followed by an air gap with impermeable, adiabatic horizontal boundaries and permeable vertical boundaries. The thickness of the porous insulation is varied between 60 and 100 percent of the overall thickness of the enclosure. For some typical conditions in a building insulation, it is found that the condensation rate and the resultant liquid accumulation do not increase significantly as the thickness of the insulation is decreased in the aforementioned range. 15 references.

Transient, stratified, enclosed gas and liquid behavior with concentrated heating from above

Abstract: A computational study has been made of transient heat transfer and fluid flow in a cylindrical enclosure con- taining a two-layer gas-and-liquid system. The geometric configuration and the boundary conditions on the pro- blem are relevant to the analysis of the prevaporization and preignition processes during the fire accident situa- tion involving a pool of liquid fuel in the vicinity of an ignition source. It is demonstrated that the effects of the natural and thermocapillary convection, radiative transfer, and thermal inertia and conduction of the walls bounding the enclosure and the magnitude of the gravity field play important roles in the development of the temperature and velocity fields in the container. HEN a concentrated source of heat is placed above a pool of liquid fuel, a number of complex ther- mophysical processes are induced preceding the ignition of the fuel vapors. If the initial liquid fuel temperature is below the flash point, a substantial amount of thermal energy may be re- quired to evaporate some portion of the fuel and to create a combustible vapor air mixture. This energy is supplied from the hot source by several heat-transfer mechanisms, including heat conduction through the gas phase, natural convection in- duced in the gas near the source, and thermal radiation. The heat flux to the liquid surface is largest directly under the source and decreases monotonically with the distance from the hot source. The nonuniform heating of the fuel induces both buoyancy and surface-tension-driven currents in the subsur- face liquid layer. The latter effect is caused by the gradient in surface tension due to the temperature variation along the gas liquid interface. The first stage of the preignition period essentially consists of heating the liquid fuel near the surface. The second stage starts when the partial pressure of the fuel vapor becomes suf- ficient to yield a combustible mixture with the air above the fuel. At this stage (and during flame spreading), evaporation, diffusion, and chemical reactions play an important role.. An experimental study of liquid pool ignition has been reported by Murad et al.1 In particular, it was found that the heating of a fuel to the point of ignition is retarded con- siderably by the surface-tension-driven currents induced in the pool. Sirignano and Glassman2 found that the same subsur- face flows also govern the process of the flame spreading over liquid fuels. The latter phenomenon has been extensively studied by many authors. A comprehensive review on the sub- ject is presented in the recent paper by Furita et al. 3 These authors performed also a finite-differe nce study of a steady laminar flame spread over the shallow fuel pool. Momentum, energy, and mass conservation equations were solved simultaneously both in the gas and liquid phases. The combus- tion process was modeled using the flame sheet approxima- tion. Radiative transport was neglected. In contrast to steady-state flame spread, transient ignition has not been treated theoretically in the literature. However,