Showing papers in "Journal of Thermophysics and Heat Transfer in 1995"

Experimental support for the lagging behavior in heat propagation

Abstract: This work examines the lagging behavior for heat transport in small-scale and fast-transient processes. The experimental result by Qiu et al. for the femtosecond transient response in gold films and that by Bertman and Sandiford for the temperature pulse traveling through superfluid liquid helium are re-examined with emphasis on the lagging behavior. The model employing the phase-lag concept provides as competent or even better results when describing the special response observed in these experiments.

Theory and validation of the physically consistent coupled vibration-chemistry-vibration model

Abstract: A consistent thermochemical relaxation model is presented that has been derived on the basis of 7\ ib,Boltzmann populated vibrational energy modes and truncated harmonic oscillators. A persistent application of these two assumptions to all types of chemical reactions with molecular reactants leads to the coupled vibrationchemistry-vibration model (CVCV model), which specifies multiple temperature rate constants and vibrational energies transferred due to chemical reactions in a consistent way. The simple analytic expressions obtained for both rate constants and transferred vibrational energies enable to account for thermal nonequilibrium not only in dissociation, but also in exchange and associative ionization reactions. The model assumptions as well as the introduced model parameters are evaluated by comparisons with a state-selectiv e model calculation and an experimental result.

Analysis of Forced Convection Heat Transfer in Microencapsulated Phase Change Material Suspensions

Abstract: B = Bi, = C = C* = C = E= = e = Fo = h, = K, = k = L = rn = Nu, = Pe = 4 = R = Rd = Re = r = r* = r,. = r,, = Is1. = Ste = S = T= t = u = 47, = X = X = a = A numerical solution of laminar forced convection heat transfer of a microencapsulated phase change material suspension in a circular tuhe with constant heat flux has been presented in this article. Melting in the microcapsule was solved by a temperature transforming model instead of a quasisteady model. The effects of the microcapsules crust, the initial subcooling, and the width of the phase change temperature range on the variation of the dimensionless tuhe wall temperatures, along the axial direction, were also considered in the present model. The agreement between the present numerical results and the experimental results is very good.

Comparison of discrete ordinates formulations for radiative heat transfer in multidimensional geometries

Abstract: This article compares predictions of radiative heat transfer in two-dimension al enclosures for several formulations of the discrete ordinates method. The discrete ordinates equations are formulated for an absorbing, isotropically scattering, and re-emitting medium enclosed by gray walls. Control volume based finite element formulations of the radiative transport equation (RTE) are presented for the primitive variable (PV) and even parity (EP) equations. These formulations are compared to the finite element formulation of the EP equations, to the control volume formulation of the PV radiative transport equation, and to exact solutions. Several test enclosures are modeled, including enclosures with either absorbing or isotropically scattering media. Solution accuracy is investigated for two angular discretization schemes: 1) the Sn discrete ordinates approximation and 2) the piece wise constant angular approximation. The PV formulations of the RTE appear to be more accurate than EP formulations. Nomenclature Eh = emissive power, aT4, W/m2 G = incident energy, J477/ dil, W/m2 G* = nondimensional incident energy, GIEh I = intensity, /(r, ft), W/m2- sr M = total number of discrete ordinate directions N = total number of global nodes NJ = basis function N6 = number of divisions in polar direction Nfi = number of divisions in azimuthal direction n = surface normal Q* = nondimensional net wall heat flux, qlEh q =

Comparison of coupled radiative flow solutions with Project Fire II flight data

Abstract: A nonequilibrium, axisymmetric, Navier-Stokes flow solver with coupled radiation has been developed for use in the design or thermal protection systems for vehicles where radiation effects are important. The present method has been compared with an existing now and radiation solver and with the Project Fire 2 experimental data. Good agreement has been obtained over the entire Fire 2 trajectory with the experimentally determined values of the stagnation radiation intensity in the 0.2-6.2 eV range and with the total stagnation heating. The effects of a number of flow models are examined to determine which combination of physical models produces the best agreement with the experimental data. These models include radiation coupling, multitemperature thermal models, and finite rate chemistry. Finally, the computational efficiency of the present model is evaluated. The radiation properties model developed for this study is shown to offer significant computational savings compared to existing codes.

Validation of Multitemperature Nozzle Flow Code

Abstract: A computer code nozzle in n-temperatures (NOZNT), which calculates one-dimensional flows of partially dissociated and ionized air in an expanding nozzle, is tested against three existing sets of experimental data taken in arcjet wind tunnels. The code accounts for the differences among various temperatures, i.e., translational-rotational temperature, vibrational temperatures of individual molecular species, and electron-electronic temperature, and the effects of impurities. The experimental data considered are (1) the spectroscopic emission data; (2) electron beam data on vibrational temperature; and (3) mass-spectrometric species concentration data. It is shown that the impurities are inconsequential for the arcjet flows, and the NOZNT code is validated by numerically reproducing the experimental data.

Estimation of Thermal Properties and Surface Heat Flux in Carbon-Carbon Composite

Abstract: This article discusses a laboratory method of measurement of the thermal properties of a carbon matrix carbon fiber composite material. Another important objective is the determination of the surface heat flux from measured temperatures within the body. This article is unique because the thermal properties are being estimated with known heat flux and temperature histories; then the opposite problem of recovering the heat flux history is solved using the estimated values for the thermal properties and temperature history. This calculated heat flux history can then be compared with the measured input heat flux. Results were obtained for a temperature range of 30-600 C. The thermal properties demonstrate a presumed quadratic relationship with temperature, and a good agreement between the estimated and measured heat flux histories is demonstrated. 18 refs.

Heat Transfer Measurement on a Waverider at Mach 10 Using Fluorescent Paint

Abstract: The principles of the fluorescent paint technique for global surface temperature measurement are presented. This technique is used to measure the surface temperature and heat transfer on the windward side of a waverider model at Mach 10. The model used for calculation of heat transfer for hypersonic flow over an insulating layer is also discussed. Quantitative comparisons between heat transfer measured by the fluorescent paint and data obtained by thermocouples are made with good agreement. The heat transfer maps are able to clearly show not only transition from laminar to turbulent flow, but also movement of the transition line. Nomenclature c = specific heat at constant pressure / = fluorescence intensity k = thermal conductivity L = insulating-layer thickness p = pressure q = heat flux T = temperature a = thermal diffusivity 0 = T - T-m p = density Subscripts b = / in r s w 0 base insulating layer initial reference surface wall total freestream

Radiative Properties of Fibrous Insulations: Theory Versus Experiment

Abstract: The validity of an exact theoretical model for calculation of the radiative properties of high-porosity, randomly oriented fiber media is examined by comparing experimental measurements with theoretical predictions for spectral reflectance and transmittance. Spectral hemispherical reflectances and spectral normal transmittances were calculated at wavelengths from 1.5 to 10.0 /tm for several thicknesses of three types of thermal insulating materials having randomly oriented silica fibers of different size distributions. Theoretical results are compared with experimental measurements made on each of the three materials. The theoretical methodology, analytical results, and experimental characterizations of geometric parameters and radiative properties of the test materials are presented. The comparison shown between theory and experiment is excellent. Thus, the model is considered to be a valid tool for prediction of radiative properties of high-porosity, randomly oriented fibrous media. Nomenclature d2F = fiber orientation distribution function Fv = fiber volume fraction / = intensity i(rj, ) = isolated fiber scattering intensity distribution Ke = extinction coefficient L = thickness N = number of fiber sizes p = phase function Q = single fiber efficiency R = reflectance r = fiber radius T = transmittance

Dependence of Heat Transfer to a Pulsating Stagnation Flow on Pulse Characteristics

Abstract: A detailed boundary-layer model is implemented to ascertain the influence of pulse shape, frequency, and amplitude on instantaneous and time-averaged convective heat transfer in a planar stagnation region formed beneath an incident periodic, pulsating flow with temperature-dependent kinematic viscosity and thermal conductivity. Interactions between low-frequency/high-amplitude flow pulsations and the nonlinearities in the governing equations lead to reductions in time-averaged Nusselt numbers up to 16%. Predictions are in good agreement with experimental results. A means to suppress heat transfer is thus suggested that may have practical application to gas turbines where blades are exposed to a periodic flow. Phase portraits, Poincare maps, Fourier spectra, and Lyapunov exponents are used to elucidate the most complex solutions.

Cooling panel optimization for the active cooling system of a hypersonic aircraft

Abstract: Optimization of cooling panels for an active cooling system of a hypersonic aircraft is explored. The flow passages are of rectangular cross section with one wall heated. An analytical fin-type model for incompressible flow in smooth-wall rectangular ducts with coupled wall conduction is proposed. Based on this model, the a flow rate of coolant to each design minimum mass flow rate or coolant for a single cooling panel is obtained by satisfying hydrodynamic, thermal, and Mach number constraints. Also, the sensitivity of the optimal mass flow rate of coolant to each design variable is investigated. In addition, numerical solutions for constant property flow in rectangular ducts, with one side rib-roughened and coupled wall conduction, are obtained using a k-epsilon and wall function turbulence model, these results are compared with predictions of the analytical model.

Spray cooling of power electronics at cryogenic temperatures

Abstract: The operation of power electronics at liquid nitrogen temperature (LNT) is a very attractive possibility. However, a high heat flux (over 1.0 x 10 6 W/m2) cooling technique, like spray cooling, will have to be used to realize all the advantages of low-temperature operation. This study details the results from experiments conducted to study the heat transfer characteristics during spray cooling with liquid nitrogen. Four different nozzles at various pressures were used to study the variation in spray cooling heat transfer at LNT. The effect of nozzle and flow rate on the critical heat flux and overall heat transfer characteristics are presented. Heat fluxes close to 1.7 x 10 6 W/m2 were realized at temperatures below 100 K. The mass flow rate range was from 6.1 x 10 4 kg/h m2 to 3.2 x 10 s kg/h m2. Nomenclature q" = heat flux, W/m2 rsat = saturation temperature, K Tw = surface temperature, K

Thermal contact resistance between balls and rings of a bearing under axial, radial, and combined loads

Abstract: The thermal contact resistance between the balls and the inner and outer rings of a space-use deep groove ball bearing is investigated assuming that heat transfer between smooth contacting elements occurs through the elastic contact areas. It is also assumed that the stationary bearing sustains axial, radial, or combined loads under a steady-state temperature condition. The shapes and sizes of the contact areas are calculated using the Hertzian theory. In particular, the contact force for the axial load is determined with careful consideration of the change in the contact angle induced by elastic deformation at the contact area. The correlation between the experimental data and the calculated values confirms the validity of the prediction method for the thermal contact resistances between the elements of a dry bearing with a surface roughness of less than 0.5 x 10~ m under the mean temperature of less than 353 K, and temperature differences across the rings of less than 35 K.

Approximate calculation of transport coefficients of Earth and Mars atmospheric dissociating gases

Abstract: High-temperature transport properties (viscosity, thermal conductivity, binary and multicomponent mass diffusion, and thermal diffusion) of dissociating gases of the Earth and Martian atmospheres have been calculated within the framework of the Chapman -Enskog method. The exponential repulsion potentials of interactions between atoms (N, O, and C) and molecules (N2, O2, NO, C2, CN, CO, and CO2) were studied. The collision cross sections were calculated from numerous experimental molecular-beam scattering data. The bifurcation approximation to the binary diffusion coefficients and the Wilke-Saxena approximate techniques were effectively used to simplify numerical algorithms of heat transfer calculations.

Charge optimization for a triangular-shaped etched micro heat pipe

Abstract: An analytical model considered to be the first analytical micro heat pipe model to predict the capillary limit of operation as well as the radius of curvature in the evaporator section and the optimal value of liquid charge was developed for a triangular-shaped, etched micro heat pipe This model predicted optimal liquid prime values between 16-25% for various power levels and also indicated an optimal charge value which would not exceed 25% Micro heat pipe arrays with liquid prime values ranging from 10 to 50% were used The model was validated by comparing the predicted values of maximum heat transport capacity with experimental data at various liquid fill ratios Although the model correctly predicted the trends associated with the variations in the fill ratio, significant differences were evident in the predicted and measured dryout level 8 refs

Inverse determination of thermal conductivity for one-dimensional problems

Abstract: Two finite difference procedures are presented for the inverse determination of the thermal conductivity in a one-dimensional heat conduction domain. The thermal conductivity is reconstructed from the inverse analysis based on the assumption that the temperature measurements are either available continuously over the entire domain or at discrete grid points. The convergence and stability of the computational algorithms are investigated. It is concluded that both procedures are first-order accurate methods. A comparison of the exact thermal conductivity with the one estimated was made to confirm the validity of the numerical procedures. The close agreement between the two results confirms that the proposed finite difference techniques are effective procedures for the inverse determination of thermal conductivity in a one-dimensional heat conduction domain. The methods are applicable for linear and nonlinear spatially- as well as temperature-d ependent thermal conductivities. Additionally, the special feature of the present techniques is that a priori knowledge of the functional form for the thermal conductivity is not mandatory. Nomenclature C = arbitrary constant F = function defined by Eq. (10) / = function defined at the boundary G = function defined by Eq. (10) g — heat generation, W/nr k = thermal conductivity, W/m-°C kM = initial thermal conductivity calculated at dT/dx — 0 in the discrete formulation, W/m-°C k(} = initial thermal conductivity calculated at dT/dx = 0 in the continuous formulation, W/m-°C q = heat flux, W/m2 T = temperature, °C t = time, s / = selected time in the continuous formulation, s /,- — selected time in the discrete formulation, s A' = spatial coordinate, m XM = spatial coordinate where dT/dx = 0 in the discrete formulation, m A',, = spatial coordinate where dT/dx = 0 in the continuous formulation, m Subscripts /, /, k = indices Superscript = approximated value

Function estimation in predicting temperature-dependent thermal conductivity without internal measurements

Abstract: The conjugate gradient method of minimization with an adjoint equation is used successfully to solve the inverse problem in estimating the temperature-dependent thermal conductivity of the homogeneous as well as nonhomogeneous solid material. It is assumed that no prior information is available on the functional form of the unknown thermal conductivity in the present study, thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using simulated exact and inexact measurements obtained within the medium. Results show that an excellent estimation on the thermal conductivity can be obtained with any arbitrary initial guesses by using just boundary measurements (i.e., internal measurements are unnecessary) within 1 s CPU time in a VAX-9420 computer. The advantages of applying this algorithm in inverse analysis can greatly simplify the experimental setup, diminish the sensitivity to the measurement errors, and reduce the CPU time in inverse calculation, while the reliable predictions can still be achieved.

Boiling heat transfer in the quenching of a hot tube under microgravity

Abstract: Quenching of a hot tube by injection of a subcooled liquid was investigated under microgravity conditions. Liquid Freon (R-113) was injected at mass fluxes between 160-850 kg/ms into an initially hot, thin-walled stainless steel tube, 11.3 mm i.d. and 914 mm long. Data collected in microgravity aboard NASA's KC-135 aircraft were compared with quenching tests in a horizontal tube under normal gravity. The re wetting temperatures were found to be 15°-25°C lower in microgravity than those obtained under 1 g, and the film boiling heat transfer coefficients in microgravity were only 20-50% of the values obtained in 1 g. This resulted in much longer precursory cooling periods in microgravity, and hence, the time to totally quench the initially hot tube in microgravity was greatly extended. However, once the tube was cooled sufficiently to allow axial propagation of the quench front, the re wetting velocity was found to be slightly greater in microgravity. The boiling curves showed that the nucleate and transition boiling curves in microgravity were shifted to lower wall superheats as compared to 1 g as a result of the lower re wetting temperature. The heat flux profiles were otherwise quite similar, indicating that these boiling regimes are little affected by reduction in gravity.

Characterization of arcjet flows using laser-induced fluorescence

Abstract: A sensor based on laser-induced fluorescence has been installed at the 20-MW NASA Ames Aerodynamic Heating Facility. The sensor has provided new, quantitative, real-time information about properties of the arcjet flow in the highly dissociated, partially ionized, nonequilibrium regime. Number densities of atomic oxygen, flow velocities, heavy particle translational temperatures, and collisional quenching rates have been measured. These results have been used to test and refine computational models of the arcjet flow. The calculated number densities, translational temperatures, and flow velocities are in moderately good agreement with experiment

Heat Transfer Variation on Protuberances and Surface Roughness Elements

Abstract: In order to determine the effect of surface irregularities on local convective heat transfer, the variation in heat transfer coefficients on small (2-6 mm diam) hemispherical roughness elements on a flat plate has been studied in a wind funnel using IR techniques. Heat transfer enhancement was observed to vary over the roughness elements with the maximum heat transfer on the upstream face. This heat transfer enhancement increased strongly with roughness size and velocity when there was a laminar boundary layer on the plate. For a turbulent boundary layer, the heat transfer enhancement was relatively constant with velocity, but did increase with element size. When multiple roughness elements were studied, no influence of adjacent roughness elements on heat transfer was observed if the roughness separation was greater than approximately one roughness element radius. As roughness separation was reduced, less variation in heat transfer was observed on the downstream elements. Implications of the observed roughness enhanced heat transfer on ice accretion modeling are discussed.

Spray cooling characteristics under reduced gravity

Abstract: bution in the medium and much higher radiative wall flux along the plates than the correlated formulations. This difference comes from the statistical relationship for determining the absorption location. The R{ calculated from the noncorrelated formulation, Eq. (9), is greater than that from the correlated formulations, Eqs. (8) and (13). Therefore, for the noncorrelated formulation, an energy bundle travels a long distance and is likely to be absorbed on the wall. This also explains why the CPU time required for the noncorrelated solution is larger than that required for the corresponding correlated solution.

Spectral element-Fourier method for unsteady conjugate heat transfer in complex geometry flows

Abstract: A spectral-element Fourier method (SEFM) is presented for the direct numerical simulation of forced convective heat transfer and conjugate conduction/convection in transitional internal flows in complex geometries. The SEFM is employed for the spatial discretization of the unsteady, incompressible, three-dimensional Navier— Stokes and energy equations. The resulting discrete equations are solved by a semi-implicit method in time treating explicitly the convection operator and implicitly the remaining pressure and viscous contributions. This methodology is illustrated by performing direct numerical simulations to investigate forced convective heat transfer in supercritical self-sustained oscillatory flows and conjugate effects in multimaterial domains. Highly unsteady flows in complex geometries are considered, including modified channels with periodic inhomogeneities such as spanwise rectangular and triangular grooves encountered in electronic equipment and compact heat exchangers.

Cumulative variable formulation for transient conductive and radiative transport in participating media

Abstract: A new mathematical formulation is proposed for transient conductive and radiative transport in a participating gray, isotropically scattering plane-parallel medium. The methodology can be easily extended to include numerous additional effects. A systematic and unified treatment is presented using cumulative variables that allows for high-order integration using standard initial-value methods in the temporal variable while allowing for an effective orthogonal collocation method to be implemented in the spatial variable. A spectral approach is incorporated in the present context where Chebyshev polynomials of the first kind are used as the basis functions. This article illustrates the methodology and presents some comparisons with previously reported works.

Atomic species radiation from air modeled with direct simulation Monte Carlo method

Abstract: This article deals with the evaluation of thermal radiation from atmospheric atomic species with the direct simulation Monte Carlo method under conditions of nonequilibrium. Models to calculate bound-bound, freebound, and free-free radiation are introduced. This new scheme attempts to improve and extend an earlier approach towards a more detailed modeling of the mechanism of radiation. The results are compared against experimental results and the differences are discussed and evaluated.

Nose cavity effects on blunt body pressure and temperatures at Mach 5

Abstract: Surface temperatures on a hemisphere-cylinder body with a nose cavity in a Mach 4.9 airflow have been measured using an infrared camera. Fluctuating surface pressures have also been measured at the cavity base. The cavity diameter D was fixed at one-half the cylinder diameter and the length L of the cavity was varied. If the cavity lip is sharp and the cavity is "shallow" (0.15 1) an axisymmetric, nominally steady "cool ring" forms on the external surface downstream of the lip. Flow visualization shows that the cool ring is caused by separation at the lip. Rounding the cavity lip eliminates or reduces separation and temperatures return to levels characteristic of the model without the cavity. For "intermediate deep" cavities (0.40 < LID < 0.70) the cavity pressure signals switch from a low-amplitude to high-amplitude level at random intervals resulting in an unstable, nonaxisymmetric temperature field downstream of the lip. Changes in cavity base shape from spherical to flat have little effect on the temperature history for shallow and very deep cavities, whereas for intermediate depth cavities the effects are more significant.

Analytical method for forced convection from flat plates, circular cylinders, and spheres

Abstract: A simple general method is developed to predict forced convection heat transfer from isothermal body shapes such as flat plates, infinite circular cylinders, and spheres for a wide range of both Reynolds number and Prandtl number. The proposed method is based on linearization of the thermal energy equation that is accomplished by the introduction of an effective velocity that is related to the freestream velocity. Next, the linear energy equation is transformed to an equivalent transient heat conduction equation that has existing solutions. These solutions are retransformed to the final expression as a function of the effective velocity that is defined in the limits of Pr + m and Pr + 0 using scaling analysis. The approximate analytic solutions are in closed form, they are simple and quite accurate when compared with previous experimental and analytical studies.

Transient conductive and radiative heat transfer in a silica window

Abstract: A detailed thermal analysis for transient conduction and nongray radiation of glass subjected to high levels of convection is discussed. Only a small variation of results due to the best available refractive index data was found. A peak value that does not coincide with the peak temperature was created by the radiation heat flux. This effect in turn produced a hump in the results for dimensional conduction heat flux, but has little effect on the temperature distribution. However, the highest temperature always occurred at the first boundary, which allows the prediction of the onset of softening and subsequent distortion optics. 5 refs.