Surface Convective Heat Transfer Research Articles

Flow over an all-body hypersonic aircraft - Experiment and computation

The objective of the present investigation is to establish a benchmark experimental data base for a generic hypersonic vehicle shape for validation and/or calibration of advanced computational fluid dynamics computer codes. This paper includes results from the comprehensive test program conducted in the NASA Ames 3.5-ft Hypersonic Wind Tunnel for a generic all-body hypersonic aircraft model. Experimental and computational results on flow visualization, surface pressures, surface convective heat transfer, and pilot-pressure flowfield surveys are presented. Comparisons of the experimental results with computational results from an upwind parabolized Navier-Stokes code developed at NASA Ames demonstrate the capabilities of this code. HE advanced computational fluid dynamics (CFD) com- puter codes being developed for use in the design of such hypersonic aircraft as the National Aero-Space Plane (NASP) and other hypersonic vehicles require comparisons of the com- putational results with a broad spectrum of experimental data to fully assess the validity of the codes and to develop confi- dence in the numerical simulation procedures. This is particu- larly true for complex flowfields with such features as bound- ary-layer transition and turbulence, rapid flow expansions, and leeside flow with the attendant flow separation and vor- tices. Validated codes for such flowfields will be critical to the development of the NASP and other hypersonic vehicles. Therefore, the objective of the present investigation is to es- tablish a benchmark experimental data base for a generic hy- personic vehicle shape for validation and/or calibration of advanced CFD computer codes. This is being done by con- ducting a comprehensive test program for a generic all-body hypersonic aircraft model in the NASA Ames 3.5-ft Hyper- sonic Wind Tunnel to obtain pertinent surface and flowfield data over a broad range of test conditions. Experimental and computational results on flow visualization, surface pressures, surface convective heat transfer, and pitot-pressure flowfield surveys will be presented in this, paper. Of particular signifi- cance, comparisons of the experimental results with computa- tional results from the NASA Ames UPS code (an upwind parabolized Navier-Stokes solver) will be shown to demon- strate the capabilities of this code. Some comparisons of the data with computations from approximate inviscid methods will also be given.

Heat and Moisture with Thermodynamically Interactive Fluxes and Volumetric Changes. III. Computer Simulation

A generally applicable computer program was developed for estimating transient state moisture concentration and temperature distributions and volumetric shrinkage of food undergoing drying. The program used a modified Luikovs model and numerical solution algorithms previously developed. An overall shape of food was assumed to be a body of rotation or infinite column, both with any arbitrary cross-sectional contour. Heat and moisture transfer in a spherical sample was simulated using the developed computer program. The simulation results agreed well with published experimental data. According to additional sample simulations, convective surface mass transfer coefficient and moisture diffusivity influenced heat and moisture transfer in an oblate sample while convective surface heat transfer coefficient and Soret diffusivity did not.

Computer modelling for the control of particulate sterilization under dynamic flow conditions

A finite difference model was used for predicting the centre temperature of particulates during ultra-high temperature processing under continuous flow conditions. This model compared favourably with the analytical solutions. A sensitivity analysis was carried out on the input parameters to the numerical model, which indicated that particulate size and thermal diffusivity were the most critical parameters influencing process lethality and cook value. For example, a spherical particulate (thermal properties defined) of 20 mm diameter required a holding time greater than 5 min at 135°C to achieve a minimum safe cook ( F 0 of 3.0 min for Clostridium botulinum), compared to a 10 mm spherical particulate which would have received an F 0 value of 68 min after 5 min processing at 135°C. The centre temperatures predicted using the model were sensitive to changes in the convective surface heat transfer coefficient in the range 100–300 W m −2 K −1, but became less sensitive as the values increased up to 500 W m −2 K −1. An initial product temperature change from 20 to 60°C resulted in an 8 min increase in process lethality for a 15 mm spherical particulate processed for 5 min at 135°C. The determination of experimental results in particulates of less than 20 mm (the critical dimension) was found to be significantly affected by conduction errors through the thermocouple wires, and therefore the model presented will be a useful prediction tool in situations of experimental uncertainty.

Parametric Analysis for the Freezing of Spheroidal or Finitely Cylindrical Objects with Volumetric Changes

ABSTRACTParametric analyses were performed to examine heat transfer in freezing of spheroidal (including prolates and oblates) and finitely cylindrical foods through computerized simulation which considered volumetric changes. The boundary conditions included convective surface heat and moisture transfer and radiative surface heat transfer. Regression equations for freezing time estimation were developed through screening and central composite analyses and verified experimentally using a food simulator with physical properties similar to lean beef. Sensitivity analysis showed the convective surface heat transfer coefficient was the most significant factor affecting accuracy of calculated freezing time followed by operational temperatures, ther‐mophysical properties and food dimensions. Effect of volumetric expansion on freezing time estimation was not significant.

THERMOCAPILLARY CAVITY CONVECTION IN WETTING AND NONWETTING LIQUIDS

A numerical study has been conducted to examine the influence of free surface curvature, due to the presence of various contact angles, on thermocapillary convection within a cavity. The results indicate that the system hydrodynamics are changed by a combination of surface temperature gradient modification due to the presence of the adiabatic meniscus and variation in the length of the free surface, as well as viscous force dependence on the wetting angle. Local and overall heat transfer rates are, in turn, altered. Wetting liquids exhibit decreased surface fluid velocities and convective heat transfer rates, while non-wetting liquids are characterized by more complicated behavior with local increases and decreases in surface fluid velocities and an increase in local heat transfer near the free surface.

Measurements and computations of ventilation efficiency and temperature efficiency in a ventilated room

In order to improve the indoor air quality in a room and to save energy, the air movement and contamination distributions in the room with ventilation have been studied experimentally and numerically. The experiment is carried out in a full-scale climate room with different air supply systems, heat gains from the venetian blinds and ventilation rates. The measurements concern room airflow patterns and air temperature, velocity and contamination concentration fields, etc. The airflow computer program PHOENICS and the cooling load program ACCURACY have been applied for the numerical simulations. PHOENICS solves the conservation equations of air mass, momentum, energy, concentration, kinetic energy and dissipation rate of kinetic energy. ACCURACY, which considers the influence of room air temperature distributions, is employed for the determination of cooling load, wall surface temperatures and convective heat transfer on room enclosure surfaces. These are the boundary conditions required by PHOENICS. The agreements between the computations and the measurements are good. The ventilation efficiency and temperature efficiency which are used for evaluation of indoor air quality and energy consumption are reported for each case. Additional application of these computations to annual energy analysis is also discussed.

Surface heat transfer in thawing by forced air convection

In thawing by forced air the effective surface heat transfer coefficient, h eff depends not only on convective sensible heat transfer but also on simultaneous mass transfer. It could be useful to distinguish a true convective surface heat transfer coefficient, h c , from h eff . Methods of measuring and calculating the surface heat transfer coefficient are discussed. An equation for calculating h eff is given. Experiments using a metal model have been carried out. Comparison between calculated and measured values of h c and especially of h eff shows good agreement.

Selfsimilar solutions of the problem of heat and mass transfer in a saturated porous medium with a volume heat source

Selfsimilar solutions of a system of stationary equations of heat condunction and filtration of molten material in the presence of a volume heat source generated by absorption of the energy of electromagnetic radiation, are considered. The possibility of the existence of a self-similar solution in the case of various (plane, cylindrical and spherical) spatial symmetries is studied. The existence of a selfsimilar solution is shown for the axisymmetric case when the radiation obeys a prescribed law. The influence of the surface volume heating and convective heat transfer due to filtration is studied. A solution for the case when the filtration of the molten phase is quasistationary is also investigated.

In Situ Determination of Apparent Thermophysical Properties of a Composite Slab Undergoing Transient State Heat Transfer

ABSTRACTA method was developed to determine the thermophysical properties of a two layered composite slab whose one exposed surface was subjected to a known temperature history and whose other surface to convective and radiative heat exchange. The physical properties to be determined were the thermal conductivity and diffusivity of one of the two layers and the contact conductance between the two layers, the coefficient of convective surface heat transfer, and the absorbtivity of thermal radiation energy applicable to one exposed surface of the slab. The developed method was used to determine property values of the insulated wall of a still retort.

Approximate solutions in the theory of thermal explosions for semi-infinite explosives

If the solution, of the heat conduction equation θ τ ( 0 ) = θ ξ ξ ( 0 ) , ξ > 0 , τ > 0 of a chemically ‘inert’ material is known, then an approximate formula for the explosion time, ד expl. , of an explosive satisfying the heat conduction equation with zero order reaction, θ ד = θ ξξ +exp(-1/θ), ξ > 0, ד 0, and the same initial and boundary conditions as the ‘inert’, is given by the root of the equation, − ∂ θ ( 0 ) ( ξ , τ expt . ) / ∂ ξ | ξ − 0 = ∫ 0 ∞ exp ⁡ [ − 1 / θ ( 0 ) ( ξ , τ expl . ) ] d ξ provided 1/θ (0) (ξ, ד) is suitably expanded about the surface ξ = 0 such that the integrand vanishes as ξ→∞. Similar results hold for one-dimensional cylindrically and spherically symmetric problems. The derivation of the explosion criterion is based on observation of existing numerical solutions where it is seen that (i) almost to the onset of explosion, the solution θ(ξ, ד )does not differ appreciably from θ (0) (ξ, ד ) (ii) the onset of explosion is indicated by the appearance of a temperature maximum at the surface. Simple formulas for ד expl. readily obtainable for a wide variety of boundary conditions, are given for seven sample problems. Among these are included a semi-infinite explosive with constant surface flux, convective surface heat transfer, and constant surface temperature with and without subsurface melting. The derived values of ד expl. are in satisfactory agreement with those obtained from finite-difference solutions for the problems that can be compared.

Transparency assumption in hypersonic radiative gas dynamics

A detailed analytical investigation is presented of the influence of radiant energy transport 011 the structure of the inviscid flow field in the stagnation region of a blunt body in hypersonic flight. It is shown that the classical condition of small characteristic optical depth is not sufficient to justify neglecting self-absorption in flows with strong radiation. The omission of self-absorpti on terms leads to a qualitatively incorrect description of inviscid flow temperature and density distributions in the strongly cooled region near the body streamline and results in a zero temperature at this streamline. The inclusion of self-absorption eliminates these anomalous behavior characteristics. A second-order approximate solution is given which includes at each point local self-absorption of radiation emitted by hotter gas in the neighborhood of the point. This solution exhibits a reasonable physical behavior throughout the flow field and predicts that gas temperature approaches a well-defined nonzero value at the body surface. This provides an enthalpy potential for convective surface heat transfer in flow regimes where viscous effects are confined to a thin boundary layer. Direct estimates are thus obtained of the influence of radiation energy transport on both radiative arid convective heat transfer.