Showing papers in "Journal of Thermophysics and Heat Transfer in 2001"

Chemical-Kinetic Parameters of Hyperbolic Earth Entry

Abstract: Chemical-kinetic parameters governing the e ow in the shock layer over a heat shield of a blunt body entering Earth’ s atmosphere from a hyperbolic orbit are derived. By the use of the assumption that the heat shield is made of carbon phenolic and by allowing for an arbitrary rateof pyrolysis-gasinjection, chemical reactions occurring in the shock layer are postulated, and the collision integrals governing the transport properties, the rate coefe cients of the reactions, and the parameters needed for the bifurcation model and for the e nite-rate kinetic wall boundary conditions are determined using the best available techniques. Sample e owe eld calculations are performed using this set of parameters to show that the heating and surface removal rates are substantially smaller than calculated using theexisting setofsuch parameters and traditionalassumptionsof gas ‐surfaceequilibrium and quasi-steadystate ablation.

Heat transfer in a channel with dimples and protrusions on opposite walls

Abstract: Local and spatially averaged Nusselt-number data are presented forthe dimpled surface of a channel, both with and without protrusions (with the same shapes as the dimples ) on the opposite wall. Channel aspect ratio is 16, ratio of channel height to dimple print diameter is 0.5, Reynolds numbers based on channel height ranges from 5 ££ 10 3 to 3:5 ££ 10 4 , and ratio of channel inlet stagnation temperature to wall temperature ranges from 0.73 to 0.94. Because of the added vortical, secondary e ow structures and e ow unsteadiness induced by the protrusions, local and spatially averaged Nusselt numbers are augmented considerably and have greater Reynolds-number dependence, compared to a channel with a smooth top wall and dimples on one opposite wall. Nusselt-number variations arealso observed asthelocation ofthearray of protrusionsis changed with respect to thedimples. Form drag and channel friction factors are augmented, and thermal performance factors are then generally lower when protrusions are employed, compared to a channel with a smooth top surface and a dimpled bottom surface. Nomenclature D = dimple print diameter f = channel friction factor f0 = baseline friction factor in channel measured with smooth top and bottom channel surfaces H = channel height k = thermal conductivity Nu = Nusselt number based on channel hydraulic diameter Nu0 = baseline Nusselt number based on channel hydraulic diameter and measured with smooth top and bottom channel surfaces ReDh = Reynolds number based on bulk mean channel velocity and channel hydraulic diameter ReH = Reynolds number based on bulk mean channel velocity and channel height Toi = spatially averaged stagnation temperature at test section inlet Tw = surface temperature X = streamwise coordinate measured from upstream edge of test surface Z = spanwise coordinate measured from spanwise centerline of test surface

Self-Consistent Model of Chemical, Vibrational, Electron Kinetics in Nozzle Expansion

Abstract: A self-consistent model to describe vibrational, electronically excited states (master equations) and free electron kinetics (Boltzmann equation) has been applied to study N 2 expansion through a converging-diverging conic nozzle. Strong departures from equilibrium can be observed for both vibrational, electronically excited states and electron energy distributions. In particular, the role of electronically excited states of nitrogen molecules and free electrons has been investigated. The strong interaction between these two systems, by means of inelastic and superelastic collisions, influences not only the internal state kinetics, but also the macroscopic quantities such as Mach number and gas temperature profile

Vibration-Dissociation Coupling Using Master Equations in Nonequilibrium Hypersonic Blunt-Body Flow

Abstract: Numerical simulations are presented of a steady-state hypersonic flow past a hemisphere cylinder. Two types of models, one a lumped Landau-Teller vibrational relaxation model (Landau, L., and Teller, E.) and the other a discrete state kinetic relaxation model (DSKR), were used to study effects of vibration-dissociation coupling on the flow physics. The widely used Park's dissociation model was used as baseline for coupling vibration and dissociation processes (Park, C.). For a Mach 8.6 flow, both relaxation models matched experimental data. At Mach 11.18, however, the underprediction of shock-standoff distance by both relaxation models using Park's model for dissociation coupling provided the motivation to implement a new master equation-based (DSKR) depletion model. The new model was used to study the effect of dissociation on population depletion in the vibrational states of the nitrogen molecule. The new model helps explain the restricted success of Park's dissociation model in certain temperature ranges of hypersonic flow past a blunt body. In the range of 5000-15,000 K, the new model yielded a substantial rate reduction relative to Park's equilibrium rate at lower temperatures and a consistent value at the high end

Aerothermal analysis of the Project Fire II afterbody flow

Abstract: Computational fluid dynamics (CFD) is used to simulate the wake flow and afterbody heating of the Project Fire II ballistic reentry to Earth at 11.4 km/sec. Laminar results are obtained over a portion of the trajectory between the initial heat pulse and peak afterbody heating. Although non-catalytic forebody convective heating results are in excellent agreement with previous computations, initial predictions of afterbody heating were about a factor of two below the experimental values. Further analysis suggests that significant catalysis may be occurring on the afterbody heat shield. Computations including finite-rate catalysis on the afterbody surface are in good agreement with the data over the early portion of the trajectory, but are conservative near the peak afterbody heating point, especially on the rear portion of the conical frustum. Further analysis of the flight data from Fire II shows that peak afterbody heating occurs before peak forebody heating, a result that contradicts computations and flight data from other entry vehicles. This result suggests that another mechanism, possibly pyrolysis, may be occurring during the later portion of the trajectory, resulting in less total heat transfer than the current predictions.

Predictions of ablating hypersonic vehicles using an iterative coupled fluid/thermal approach

Abstract: A new iterative approach for the prediction of the ablation of hypersonic vehicles is presented. The solution technique is achieved by an iterative coupling of a computational e uid dynamics code and a material thermal response code through mass and energy balances at a common interface. The iterative approach proved necessary due to severe numerical instabilities that resulted when an explicit technique was employed involving a loose coupling approach between the two codes. The results presented are for an axisymmetric carbon ‐carbon nosetip on a reentry vehicle e ying a ballistic trajectory. Details of the computational technique and the computed e owe eld and materialthermalresponsearepresented.Comparisonsweremadewith thewidelyusedABRES ShapeChange Code (ASCC), and favorable results were obtained.

Thermal Contact Conductance of Metal/Polymer Joints: An Analytical and Experimental Investigation

Abstract: The heat e ow across a metal/polymer interface is a very important problem in many modern engineering applications. A thermal joint conductance model that employs the surface mechanics of a contact interface in conjunction with an existing elastic thermal contact conductance model was developed. In developing the model, an elastic contact hardness term was derived to predict the actual contact area of a metal/polymer interface under loading. The model predicts a microscopic resistance region where the interface resistance is dominant and a bulk resistance region where the thermal conductivity of the polymer is dominant. An experimental apparatus was fabricated, and a successful experimental program was conducted. New experimental data were gathered on different polymeric specimens over a pressure range of 138 ‐2758 kPa (20‐400 psi). The experimental data were compared to the proposed thermal joint conductance model. It was found that the proposed model predicted the data quite well. The data followed the predicted trends for both the microscopic and bulk resistance regions.

Effect of Film-Hole Shape on Turbine-Blade Film-Cooling Performance

Abstract: The detailed heat transfer coefficient and film cooling effectiveness distributions as well as tile detailed coolant jet temperature profiles on the suction side of a gas turbine blade A,ere measured using a transient liquid crystal image method and a traversing cold wire and a traversing thermocouple probe, respectively. The blade has only one row of film holes near the gill hole portion on the suction side of the blade. The hole geometries studied include standard cylindrical holes and holes with diffuser shaped exit portion (i.e. fanshaped holes and laidback fanshaped holes). Tests were performed on a five-blade linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on cascade exit velocity was 5.3 x 10(exp 5). Upstream unsteady wakes were simulated using a spoke-wheel type wake generator. The wake Strouhal number was kept at 0 or 0.1. Coolant blowing ratio was varied from 0.4 to 1.2. Results show that both expanded holes have significantly improved thermal protection over the surface downstream of the ejection location, particularly at high blowing ratios. However, the expanded hole injections induce earlier boundary layer transition to turbulence and enhance heat transfer coefficients at the latter part of the blade suction surface. In general, the unsteady wake tends to reduce film cooling effectiveness.

Experimental Investigation of Oscillating Heat Pipes

Abstract: Operation requirements of oscillating heat pipes (OHPs) are proposed. Based on the requirements, OHPs with nonflammable fluorocarbon fluids, FC-72 and FC-75, as the working fluid are developed. The OHPs have an inner diameter of 1.75 mm, a total length of 446 mm, and 40 tubing turns. There are two condensers on both outer sides and one evaporator in the middle of the OHPs. Thermal performance tests are conducted at various operating temperatures and heat rates. The working fluid fill ratio is varied. A high-performance OHP with FC-72 has been indicated for the first time. The FC-72 OHP can transport a 2040-W heat rate without dryout. The gravitational acceleration does not have a noticeable influence on the performance of the fluorocarbon OHP. The thermal performances of the fluorocarbon OHPs are compared with the case of an acetone OHP.

Construction and Validation of a Dense Gas Shock Tube

Abstract: Results from initial tests of a shock tube designed to verify the nonclassical dynamics of Bethe-Zel'dovich-Thompson (BZT) fluids are presented. These tests employed nitrogen at pressures and temperatures required for shock-tube initial conditions that will produce nonclassical phenomena in a BZT fluid. A single-diaphragm, stainless steel shock tube is enclosed in a resistance-heated tube furnace. Each of the 16 segments of the furnace is controlled by a proportional integral differential loop to sustain a uniform temperature along the tube. Water-cooled static and dynamic pressure transducers and thermocouples are used to monitor the static initial conditions and the dynamic wave field. The nitrogen test cases are compared to the ideal gas Riemann problem. The pressure differential of the incident expansion wave matches the theoretical magnitude to within 2%, whereas the average wave speed agrees within 7%

Numerical Simulation of Gas Flow over Microscale Airfoils

Abstract: Flows over microscale airfoils are investigated using both particle and continuum approaches. An implementation of the information preservation technique based on the direct simulation Monte Carlo method is used to simulate flows over a flat plate of zero thickness at low Reynolds number (Re < 1 x 10 2 ). The aerodynamics of a flat plate with thickness ratio of 5% at Re = 4 is quite different from that at Re = 4 x 10 3 that were measured experimentally. A continuum approach with slip boundary conditions predicts a similar basic flow pattern as the information preservation method with differences in details, which may indicate that continuum approaches are not suitable for this kind of flow because of rarefied effects

Inverse Method for Estimating Thermal Conductivity in One-Dimensional Heat Conduction Problems

Abstract: An inverse analysis is provided to determine the spatial- and temperature-dependent thermal conductivities in several one-dimensional heat conduction problems. A e nite difference method is used to discretize the governing equations, and then a linear inverse model is constructed to identify the undetermined thermal conductivities. The present approach is to rearrange the matrix forms of the differential governing equations so that the unknown thermal conductivity can be represented explicitly. Then, the linear least-squares-error method is adopted to e nd the solutions. The results show that only a few measuring points at discrete grid points are needed to estimate the unknown quantitiesofthethermalconductivities, evenwhenmeasurementerrorsareconsidered.Incontrasttothe traditional approach, the advantages of this method arethatno prior information is needed on thefunctional form oftheunknown quantities,no initialguessesarerequired,andno iterations in thecalculating processarenecessary and that the inverse problem can be solved in a linear domain. Furthermore, the existence and uniqueness of the solutions can be easily identie ed. Nomenclature A = coefe cient matrix of vector T B = coefe cient matrix of vector C C = vector constructed from the unknown thermal conductivities D = vector constructed from the functions of the unknown thermal conductivities E = productof A i1 and B F = error function g = heat generation, W/m 3 k = thermal conductivity, W/m ¢ ± C q = heat e ux, W/m 2 R = reverse matrix T = temperature, ± C T = temperature vector t = time, s x = spatial coordinate, m 1t = increment of time domain, s 1x = increment of spatial coordinate, m ae = standard deviation ! = random variable

Experimental and Computational Study of Infrared Emission from Underexpanded Rocket Exhaust Plumes

Abstract: Underexpanded rocket exhaust plumes generated in a test-bed facility are studied experimentally. Infrared (IR) images of the plume have been obtained with an IR camera equipped with narrow bandpass e lters. The spectral distribution of the IR emission has been measured with a spectroradiometer. Numerical simulations of the exhaust plume have been conducted. The gas-dynamic and thermodynamic properties of the e owe eld were computed by the General Aerodynamic Simulation Program computational e uid-dynamic code. On the basis of thesimulatede owe eld, theIRradiationwasnumerically calculated byusing a computerprogram INFRAD,which has been developed for this purpose. The program calculates the radiation transfer through the plume using the statistical narrowband model technique. It allows simulation of the IR plume images as well as the local and the space-integrated plume spectra. The simulated and experimental results agree well.

Homogenization of Temperature-Dependent Thermal Conductivity in Composite Materials

Abstract: Of the various homogenization approaches, the asymptotic expansion homogenization (AEH) approach for homogenizing nonlinear composite material properties continues to grow in prominence due to its ability to handle complex microstructural shapes while relating continuum fields of different scales. The objective is to study the AEH approach for nonlinear thermal heat conduction with temperature-dependent conductivity. First, two approaches are proposed to investigate the sensitivity of the homogenized conductivity to higher-order terms of the asymptotic series. Under conditions of symmetry such as in unidirectional composites, the two approaches give the same homogenized properties. Then validations are shown for unidirectional composites for changing volume fraction and temperature. The validations are performed using measurements and analytical formulas available in the literature

Heat Transfer in Adhesively Bonded Honeycomb Core Panels

Abstract: The Swann and Pittman semi-empirical relationship has been used as a standard in aerospace industry to predict the effective thermal conductivity of honeycomb core panels. Recent measurements of the effective thermal conductivity of an adhesively bonded titanium honeycomb core panel using three different techniques, two steady-state and one transient radiant step heating method, at four laboratories varied significantly from each other and from the Swann and Pittman predictions. Average differences between the measurements and the predictions varied between 17 and 61 percent in the temperature range of 300 to 500K. In order to determine the correct values of the effective thermal conductivity and determine which set of the measurements or predictions were most accurate, the combined radiation and conduction heat transfer in the honeycomb core panel was modeled using a finite volume numerical formulation. The transient radiant step heating measurements provided the best agreement with the numerical results. It was found that a modification of the Swann and Pittman semi-empirical relationship which incorporated the facesheets and adhesive layers in the thermal model provided satisfactory results. Finally, a parametric study was conducted to investigate the influence of adhesive thickness and thermal conductivity on the overall heat transfer through the panel.

Natural convection in a trapezoidal enclosure with offset baffles

Abstract: A numerical investigation has been made of natural convection heat transfer in a trapezoidal enclosure (representing attic spaces) with offset baffles. Two thermal boundary conditions representing summerlike conditions (upper surface heated) and winterlike conditions (upper surface cooled) and two baffle heights are studied. For each boundary condition and baffle height, two baffle positions are considered. In position I, the upper baffle is offset toward the heated vertical wall and the lower baffle is offset toward the symmetry plane of the enclosure, whereas in position II, the upper baffle is offset toward the symmetry plane and the lower baffle is offset toward the heated wall. Rayleigh number values range from 10 3 to 5 × 10 7 for summerlike conditions and from 10 3 to 10 6 for winterlike conditions. Predictions reveal a decrease in heat transfer in the presence of baffles. In winterlike conditions, convection starts to dominate at a Rayleigh number much lower than that in summerlike conditions. The maximum reduction in heat transfer is achieved with long baffles placed in position II for summerlike conditions and in position I for winterlike conditions. Average Nusselt number correlation for both boundary conditions are presented.

Entry Conditions in Planetary Atmospheres: Emission Spectroscopy of Molecular Plasma Arcjets

Abstract: A stationary arcjet, operating at low pressure (0.13 mbar in the vacuum chamber), is used for the simulation of the gas flow surrounding a vehicle during its entry into the atmosphere of Titan and Mars. For the Titan atmosphere the gas mixture 99% N 2 - 1% CH 4 is used and 97% CO 2 - 3% N 2 for the Martian atmosphere. The respective plasma arcjets are analyzed by optical emission spectroscopy for identifying the emitting molecules and atoms and also deducing the temperatures associated with their different internal modes. For the N 2 -CH 4 plasma the vibrational temperatures T v deduced from CN and NH spectra are found in concordance with the measured electron temperature, that is, 8000 K, whereas, from CH spectra, T v is obtained close to 3700 K. The rotational temperatures are found to be between 2500 and 2800 K for CH and NH and nearly 5000 K for both CN and N 2 + spectra. For the CO 2 -N 2 plasma no emission from the arcjet is detected in the UV-visible range; the feasibility of the infrared analysis of the Δν = 2 band of CO is demonstrated in a stationary plasma discharge experiment

Temperature dependence of thermophysical properties of disodium hydrogenphosphate dodecahydrate

Abstract: The temperature dependence of thermophysical properties was evaluated for disodium hydrogenphosphate dodecahydrate, which is used in long-term, supercooled thermal energy storage (Super-TES). Super-TES stores thermal energy at temperatures lower than the melting point of the phase-change material, which reduces heat loss from the storage system. Although the degree of supercooling depends not only on the intensive material properties, such as phase-change temperatures, but also on extensive properties, such as the total material volume, except for distilled water and metals there is insufficient information in the open literature on supercooling phenomena to develop heat storage devices. A promising material for Super-TES applications is disodium hydrogenphosphate dodecahydrate. The thermophysical properties of disodium hydrogenphosphate dodecahydrate are evaluated, and its suitability for Super-TES applications is clarified. We found that in the liquid phase the hydrate density and viscosity decrease monotonically with increasing temperature. We also found that the specific heat in the liquid phase (3.45 kJ/kg . K) and the thermal conductivity in the solid phase (1.01 W/m . K near the melting point) were twice as large as reported in the open literature. This indicates that this hydrate is useful for storing both latent and sensible heat, for reducing heat loss from energy storage systems, and for efficiently exchanging energy between heat sources and the hydrate.

Investigation of the Temperature Distribution on Radiator Fins with Micro Heat Pipes

Abstract: A e exible micro-heat-pipe panel, fabricated by sintering an array of aluminum wires between two thin aluminum sheets, was developed as part of a program to develop lightweight, e exible radiator e n structures for use on long-term spacecraft missions. A numerical model, which combined both conduction and radiation effects, was established to predict the heat-transfer performance and temperature distribution of the radiator e n in a simulated spaceenvironment. Three differentconceptsarepresented, evaluated,and discussed. Comparison ofthe predicted and experimental results indicated that the model developed herein can beused to accurately predict the temperature distribution and heat-transferperformance occurring in micro-heat-pipe radiators. Thiscomparison further indicates that the e exible radiator with the array of micro heat pipes has an effective thermal conductivity of more than 20 times that of the uncharged version and 10 times that of a solid material. This results in a more uniform temperaturedistribution, which could signie cantly improve the overall radiation effectiveness, reducethe overall size, and meet or exceed the baseline design requirements for long-term manned missions to Mars.

Thermal Spreading Resistances in Compound Annular Sectors

Abstract: Ageneralsolution,basedontheseparationofvariablesmethod,forthethermalspreadingresistanceincompound annular sectors, is presented. Results are given for three heat e ux distributions: inverse parabolic, uniform, and parabolic proe les. The general solution can be used to model any number of equally spaced heat sources on a compound or isotropic annulus. Graphical results are presented for a variety of parameter combinations. Finally, it is shown that the general solution for a thin annulus reduces to that of a e ux channel when curvature effects are negligible.

Inertia and thermal convection during crystal growth with a steady magnetic field

Abstract: The steady axisymmetric buoyant convection in an electrically conducting liquid in a cylinder with a uniform axial magnetic field is examined. The geometry and boundary conditions are chosen to model the liquid-encapsulated Czochralski growth of compound semiconductor crystals. The objective is to determine the errors associated with the neglect of inertial effects or of convective heat transfer for a series of magnetic field strengths

Effective Thermal Conductivity and Thermal Contact Conductance of Graphite Fiber Composites

Abstract: The transverse and longitudinal effective thermal conductivity and contact conductance of discontinuous and misoriented graphite fiber-reinforced composites has been studied over a range of temperatures (20-200°C) and pressures (172-1720 kPa). Three different fiber types (DKE X, DKA X, and K22XX) and three fiber volume fractions (55, 65, and 75%) in a cyanate ester matrix were studied. The addition of fibers to the matrix resulted in an increase in effective thermal conductivity, but appears to level off at fiber volume fractions of 65%. Furthermore, the effective thermal conductivity in the longitudinal direction was significantly greater than in the transverse direction and was more dependent on temperature. These data were used to develop an equation relating the thermal contact conductance to the harmonic mean thermal conductivity of the fiber and matrix material, fiber volume fraction, sample thickness, and microhardness

Computation of Hypersonic Radiating Flowfield over a Blunt Body

Abstract: A numerical method for calculating a strongly radiating axisymmetric flowfield is developed. Radiative transfer is treated one, two, and three dimensionally. Radiative heat flux is evaluated using a model developed earlier that combines Planck, Rosseland, and gray-gas models. The flow solution is obtained with a fully implicit time-marching method using a full block matrix inversion. To show its robustness the method is applied to calculate the environment of a blunt body flying at the velocity of 16 km/s

Investigation on the Effects of Body Force Environment on Flat Heat Pipes

Abstract: This paper reports on the effects of body forces environment: gravitation, vibration, and acceleration forces using constant heat load on the thermal performance of a flat copper/water heat pipe. The effect of gravitation forces is studied by testing the heat pipe in different positions: horizontal, vertical with a heat source upwards (antigravity position), and vertical with a heat source downward (thermosyphon position). Transient accelerations and vibrations are generated using centrifuge and shaking tables, respectively, in order to simulate vibration and acceleration forces corresponding to aircraft maneuvering in frequency, amplitude, duration, and direction. The experimental results on the orientation effects show that the heat pipe is hardly affected by the gravitation forces and exhibits nearly the same thermal performance whatever the tilt angle for input heat powers lower than 20 W. For input heat powers higher than 20 W, there is a slight heat pipe thermal performance dependency on gravitation. For the vibration tests the heat pipe is mounted on a tri-axis shaking table and it is subjected to sinusoidal excitation. The heat-pipe thermal performance is hardly affected by vibration whatever the mounting direction on the shaking table. An investigation into the effects of transient acceleration forces with constant input heat loads on the heat-pipe thermal performance has been conducted. Pooling of the excess working fluid plays a significant role in the heat transport potential of the heat pipe subjected to accelerations. There is a decrease in the heat-pipe thermal performance with increasing acceleration as a result of partial dryout of the evaporator and pooling in the condenser section. Dryout, which is demonstrated as a result of increased acceleration, depends on the input heat power and the acceleration type. However, under certain acceleration tests the heat pipe successfully reprimed with a suppression of acceleration. In all cases the increase of the heat-pipe thermal resistance does not exceed 70%. The maximum heat-pipe thermal resistance obtained under 10-g acceleration level remains an acceptable value for the electronic package safety.

Effect of Film-Hole Shape on Turbine-Blade Heat-Transfer Coefficient Distribution

Abstract: Detailed heat-transfer coefe cient distributions on the suction side of a gas turbine blade were measured using a transient liquid crystal image method. The blade has only one row of e lm holes near the gill-hole portion on the suction side of the blade. Studies on three different kinds of e lm-cooling hole shapes were presented. The hole geometriesstudied includestandard cylindricalholesand holeswith adiffuser-shaped exitportion (i.e., fan-shaped holes and laidback fan-shaped holes ). Tests were performed on a e ve-blade linear cascade in a low-speed wind tunnel. The mainstream Reynolds number based on the cascade exit velocity was 5 :3 ££ 10 5 . Upstream unsteady wakes were simulated using a spoke-wheel-type wake generator. The wake Strouhal number was kept at 0 and 0.1. The coolant-to-mainstream blowing ratio was varied from 0.4 to 1.2. The results show that unsteady wake generally tends to induce earlier boundary-layer transition and enhance the surface heat-transfer coefe cients. When compared to the cylindrical hole case, both the expanded hole injections have much lower heat-transfer coefe cients over the surface downstream of the injection location, particularly at high blowing ratios. However, the expanded hole injections induce earlier boundary-layer transition to turbulence and enhance heat-transfer coefe cients at the latter part of the blade suction surface.

Visualization of Flow Boiling in an Annular Heat Exchanger Under Microgravity Conditions

Abstract: An experiment consisting of a glass annular heat exchanger was flown on NASA's KC-135 reduced gravity aircraft to study the effects of gravity on flow boiling. Visual data were taken to determine flow boiling regimes, which were then analyzed using five different flow regime maps. The flow regime maps enabled predictions of the quality, two-phase heat-transfer coefficients and wall temperatures in the axial direction during periods of reduced gravity. Results from this work illustrated the following trends: 1) less heat addition was needed to cause flow regime transitions in reduced gravity environments; 2) Earth-based flow regime maps did not correlate well with visual data or zero-g flow regime maps; 3) all of the zero-g flow regime maps produced similar results for calculations of quality, heat-transfer coefficient, and heat-exchanger temperature, indicating that all of them were acceptable for this application; and 4) that maximum heat transfer occurred at locations in the heat exchanger near the transition from bubble to slug flow.

Calculation of Nonequilibrium Radiation from a Blunt-Body Shock Layer

Abstract: A numerical method is developed for calculating the radiating axisymmetric flowfield accounting for nonequilibrium thermochemistry. Solutions of the flow with radiation are obtained in a fully coupled manner using a fully implicit time-marching method through a full block-matrix inversion. The method is used in calculating the radiation from the blunt-body shock layer in both air and carbon-containing airflows. The calculated results are compared with the experimental data obtained in a ballistic range at flight speeds of up to 13.4 km/s

Prediction of Thermal Contact Conductance in Vacuum Using Monte Carlo Simulation

Abstract: Predictions of thermal contact conductance between two conforming e at surfaces in vacuum are carried out. Theprediction model considersheate owthrough thecontactzonesof asperitiesonthesurfacesby conduction.The probability of occurrence of contact between the surface asperities is evaluated using a Monte Carlo simulation. A Gaussian distribution of asperity heights on the surfaces, which has been validated earlier, is assumed. The distribution of these contact spots and their respective contact areas are determined in the model and used for evaluating the thermal contact conductance. Predictions are carried out for different materials at varying contact pressures in a vacuum environment. A wide range of surface characteristics is considered. The predictions show excellent agreement with the measured values of thermal contact conductance.

Gas-Surface Interaction Model Influence on Predicted Performance of Microelectromechanical System Resistojet

Abstract: The free molecule micro-resistojet was designed as a micropropulsion system capable of performing attitude controlandprimarymaneuversfornanospacecraftwithmasslessthan10kg.Thedetailsofgas ‐surfaceinteractions between propellant molecules and surfaces held at elevated temperature are critical in predicting the propulsion system’ s performance and efe ciency. The aim is to assess parametrically the performance of a typical thruster geometry using a general Maxwell scattering model and two versions of the Cercignani ‐Lampis‐Lord model (Lord, R. G., “ Some Extensions of the Cercignani ‐Lampis Gas‐Surface Scattering Kernal,” Physics of Fluids, A , Vol.3,No.4,1991,pp.706 ‐710 andLord, R.G., “ SomeFurtherExtensionsoftheCercignani ‐LampisGas‐Surface Interaction Model,” Physics of Fluids, A , Vol. 7, No. 5, 1995, pp. 1159 ‐1161). The models are incorporated into a direct simulation Monte Carlo numerical code and are used to bound the predicted performance characteristics of the thruster. The total specie c impulse varies by approximately 20% over range of accommodation coefe cients from specular to diffuse surface scattering. However, there was only a maximum difference of about 5% between the models for a given accommodation coefe cient. Other more microscopic parameters, such as axial velocity distribution functions, appear to depend more on the scattering model assumed.

Oxygen Atoms' Effect on Vibrational Relaxation of Nitrogen in Blunt-Body Flows

Abstract: Numerical simulations are presented of the steady-state aire ow over a hemisphere cylinder of 1-m radius having hypersonic Mach numbers, where vibrational relaxation is the dominant mechanism and the dissociation of oxygen is small. A Mach 6.5 e ow was analyzed at freestream pressure of 50 Pa with a nonequilibrium freestream translational temperature of 300 K and vibrational temperature of 4000 K; a Mach 1.5 e ow was also studied to delineate effects of vibration ‐translation (V‐T)energy losses due to N 2‐O collisions. The effects on the vibrational population distribution, temperature, and pressure in the e owe eld were studied for various media: pure nitrogen and air mixtures of 0.0001, 0.1, and 1% oxygen atoms. Code validation was performed with previously reported computational results and experimental data for equilibrium e ow in freestream, but nonequilibrium in the shock layer. An upwind difference numerical scheme was used to solve the inviscid Euler equations coupled to a vibrational kineticsmodel ofN 2,assumedasananharmonicoscillatorof40quantum levels. Theshock-standoffdistance comparison with experimental data for a Mach 7.7 and 8.6 aire ow past a blunt body showed good agreement. For the Mach 1.5 e ow at nonequilibrium freestream conditions, the high efe ciency of the V ‐T rates of N2‐O collisions introduces additional heating in the shock layer for 0.1% and higher atomic oxygen, thus increasing the shockstandoff distance; for the Mach 6.5 e ow, a 0.1% atomic oxygen in air decreases the translational temperature in air compared to that of pure nitrogen in the stagnation region.