Stagnation Point Heat Flux Research Articles

Temperature distribution and heat transfer characteristics of an inverse diffusion flame with circumferentially arranged fuel ports

Experiments were conducted on IDFs burning butane. The flame holder has a central air jet surrounded by 12 circumferentially arranged fuel jets. The flame consisted of a short entrainment zone and a long mixing and combustion zone with intense combustion. The temperature profiles of the flame showed a cool core at low flame heights, which disappeared at high flame heights. The stagnation point heat flux and radial heat flux distribution of the same flame, upon impinging on a flat plate, also indicated a cool core at low nozzle-to-plate distance but disappeared at high nozzle-to-plate distance.

Influence of molecular vibration and transport model on computation of the wall heat flux

Zuppardi and Verde (Improved Fay–Riddell prcoedure to compute the stagnation point heat flux, Journal of Spacecraft and Rockets 35(3) (1998) 403–405) introduced some operating improvements in the Fay–Riddell computing procedure for the solution of the laminar boundary layer and for the computation of the heat flux at the stagnation point of spherical bodies in high speed, non-equilibrium dissociating air. Zuppardi and Verde pointed out that the most important improvements were related to the computation both of vibrational temperatures and of Prandtl and Lewis numbers. Vibrational temperatures were computed only as functions of the free stream thermodynamic parameters. Prandtl and Lewis numbers were computed as functions of both the free stream and the local thermodynamic parameters in the boundary layer. The Chapman–Enskog theory with the Lennard–Jones collision integrals were used to compute the transport coefficients. In the present paper the effects of the variability of the vibrational temperatures also in the boundary layer, and of using the more updated collision integrals by Yun and Mason (Collision integrals for the transport properties of dissociating air at high temperature, The Physics of Fluid 5(4) (1962) 380–386) have been evaluated. The results showed that the influence of these factors on the profile of the thermo-fluid–dynamic parameters in the boundary layer was negligible, but it was important on the wall heat flux. The inclusion of the variability of molecular vibration improved the matching both with Navier–Stokes results and experimental data.

Improved Fay-Riddell Procedure to Compute the Stagnation Point Heat Flux

Improved Fay-Riddell Procedure to Compute the Stagnation Point Heat Flux

Heat flux on partially catalytic surfaces in hypersonic flows

The combined effects of the chemical nonequilibrium and the surface catalysis on the stagnation-point heat flux of a blunt body are investigated by Navier-Stokes calculations on a spherical model under Scirocco Plasma Wind Tunnel conditions. The freestream properties in the test section are found by means of a preliminary nonequilibrium, full Navier-Stokes computation of the nozzle flow expansion. All available numerical heat flux results are correlated as a function of a dimensionless parameter, including the simultaneous effects of the finite-gas and the surface-phase reactions. The proposed parameter, which takes into account simultaneously the nonequilibrium effects in the bulk and surface phases, correlates well with the numerical results over a blunt-body stagnation region. The final correlation formula for the stagnation heat flux can be used to correlate experimental results on different materials with known catalytic coefficients or, conversely, to determine the surface catalytic efficiency of different thermal protection materials. Nomenclature a = speed of sound, m/s cp =

Influence of the velocity gradient on the stagnation point heating in hypersonic flow

In a number of experimental and numerical publications a deviation has been found between the measured or computed stagnation point heat flux and that given by the theory of Fay and Riddell. Since the formula of Fay and Riddell is used in many applications to yield a reference heat flux for experiments performed in wind tunnels, for flight testing and numerical simulations, it is important that this reference heat flux is as accurate as possible. There are some shortcomings in experiments and numerical simulations which are responsible in some part for the deviations observed. But, as will be shown in the present paper, there is also a shortcoming on the theoretical side which plays a major role in the deviation between the theoretical and experimental/numerical stagnation point heat fluxes. This is caused by the method used so far to determine the tangential velocity gradient at the stagnation point. This value is important for the stagnation point heat flux, which so far has been determined by a simple Newtonian flow model. In the present paper a new expression for the tangential velocity gradient is derived, which is based on a more realistic flow model. An integral method is used to solve the conservation equations and, for the stagnation point, yields an explicit solution of the tangential velocity gradient. The solution achieved is also valid for high temperature flows with real gas effects. A comparison of numerical and experimental results shows good agreement with the stagnation point heat flux according to the theory of Fay and Riddell, if the tangential velocity gradient is determined by the new theory presented in this paper.

Computation of nonequilibrium hypersonic flowfields around hemisphere cylinders

Hypersonic flows past hemisphere cylinders at zero incidence in chemical and thermal nonequilibrium are investigated for a range of Mach numbers from 10 to 18. The numerical code shows excellent comparison for surface pressure and heat transfer prediction with recent experiments conducted in a shock tunnel. The numerical code also compares well for stagnation point heat flux predictions at altitudes of 22 and 37 km with a set of earlier experiments. Numerical solutions with the vibrational equilibrium model are compared with those of multitemperature nonequilibrium. The stagnation point heat transfer is 10-23% higher for the nonequilibrium solutions in the Mach number range of 12-18. The importance of a multitemperature model for accurate prediction of stagnation properties, particularly the heat transfer, is noted. The variation in computed shockstandoff distance substantiates that the Mach number independence principle applicable to ideal gases does not hold for dissociating flows. Over the range of Mach numbers, the noticeable influence of vibrational relaxation on the temperature distributions and mass concentrations in the vicinity of shocks is shown in the present study.

An improved method to determine free stream conditions in hypersonic facilities

For determing pressure coefficients and Stanton numbers from the measured surface pressures and heat fluxes at a model surface, the dynamic pressure, mass flux and the total enthalpy of the free stream have to be known. Usually these values are determined by computing the wind tunnel nozzle flow. But a lot of uncertainties enter the computation which may lead to unreliable results. Therefore, a simple method was developed which yields the desired free stream conditions with high accuracy. This could be achieved by using mainly values which are measured within the test section. The method requires the measurement of the Pilot pressure, the stagnation point heat flux on a sphere and the static pressure of the free stream. For the static pressure an estimated value can also be used, because it has no large influence on the result. Some simple considerations show that the derived method is also valid for nonequilibrium free stream conditions. With the procedure presented the accuracy of the pressure coefficients and Stanton numbers could be increased significantly. Further, it improved the repeatability of these test results. This is very important for fundamental research, for the design of hypersonic vehicles as well as for CFD-validation with experimental data. The application of the method presented is not limited to short duration facilities, it can also be used for continuously working wind tunnels.

Two-phase mass injection from the stagnation zone of a blunt body in a hypersonic flow

A mathematical model of the hypersonic steady gas flow over the stagnation zone of an axisymmetric blunt body ~Mth given twophase injection from the surface is proposed. The two-continuum model of a dusty gas [3] is used for describing the flow in the region of the walt. The problem is solved in the boundary layer and thin viscous shock layer approximations. On the basis of the numerical calculations the distribution of the parameters of the carrier and dispersed phases near the axis of symmetry is obtained. The similarity parameters determining the convective heat transfer are found. The stagnation point heat fluxes w~th and without particles are compared. The range of parameters on which particles can significantly reduce the heat transfer is determined. Research into the thermal erosion behavior of various materials in high-enthalpy gas flows and, moreover, the development of new active thermal protection techniques [1--3] make it necessary to investigate the effect on the thermal load of dispersed particles introduced into the flow from the body surface. In the above-mentioned applications a mixture of gas and disperse particles, which then move in a narrow region near the wall, is injected from the surface. Two-phase injection into a supersonic flow without allowance for the viscosity and thermal conductivity of the carder phase was previously considered in [4]. The influence of particles on the flow in the boundary layer without injection effects was investigated in [5, 6].

Stagnation point heat flux in hypersonic high enthalpy flow

The paper describes stagnation point heat flux measurements at a range of enthalpies relevant to re-entry speeds of aero-assisted space transfer vehicles (ASTVs) and proposed space planes, using the Australian National University Free Piston Shock Tunnel T3. The unique feature of these experiments is that they were conducted in the ‘straight through’ (reflectionless) mode which enabled higher enthalpies and densities hitherto unattained.

OPTIMAL NOZZLE-TO-PLATE SPACINE FOR CONVECTIVE HEAT TRANSFER IN NONISOTHERMAL, VARIABLE-DENSITY IMPINGING JETS

Abstract The optimal nozzle-to-plate spacing is experimentally studied for the case when a free jet of high temperature gas impinges normally on a large flat plate. The stagnation-point heat transfer becomes maximal when the flat plate is placed at the end of the potential core region. A generalized model is proposed which permits prediction of the optimal nozzle-to-plate spacing and stagnation-point heat flux over a wide range of the initial density ratio.

Stagnation point heat transfer from laminar, high temperature methane flames

Combustion of methane-rich fuels frequently provides forced convective heating in industry, and the ability to predict the rate of heat transfer from such flames to solid surfaces is often desirable. Mathematical modelling of stagnation point heat flux has been achieved by numerical solution of the boundary layer equations, and by an analytical equation modified to include the effects of chemical reaction in the free stream flow and to allow for the enhancement in heat flux caused by the diffusion and exothermic recombination of reactive species in the boundary layer surrounding the heat receiving body. Predictions from these models have been compared with experimental data obtained in high temperature methane flames of various equivalence ratios. Within the equilibrium region of these flames, predictions from the modified analytical equation based on total Lewis numbers equal to and greater than one form a tight envelope around the experimental results, and hence provide a relatively simple method of predicting heat flux. Numerical solutions tend to slightly underestimate predictions from the analytical equation and experimental data, although agreement with the alternative prediction method increases with the surface temperature of the heat receiving body

Stagnation Point Heat Transfer from Turbulent Methane-Air Flames

Abstract Mathematical models for the prediction of stagnation point heat flux in the chemical equilibrium region of turbulent flames are presented. These models include a numerical solution of the relevant boundary layer equations, and modified empirical correlations originally derived for the case of uniform, turbulent air flows. Experimental measurements of the stagnation point heat flux received by a hemisphere-cylinder probe placed in methane-air flames are included, and free stream temperatures, mean velocities and turbulence intensities were measured for comparisons between theory and experiment. Predictions of the mathematical models show that the influence of free stream turbulence on heat transfer from these names is relatively small for the geometry in question. Comparisons with experimental data confirm this finding. Good quantitative agreement occurs between numerical solutions and experimental data for flames which only entrain small amounts of atmospheric air. Conservative estimates of heat transfer rates are obtained from the modified empirical correlations proposed.

Ceiling heat transfer during fire plume and fire impingement

AbstractThe impingement of turbulent fires and fire plumes on a horizontal ceiling was considered. Free flame heights, impinging flame lengths along the ceiling and ceiling heat fluxes were measured for both unconfined and confined ceilings. The study was limited to the initial stage of ceiling heating by fire under conditions where convection dominates the flow. Fire sources were simulated by burning liquid methanol, ethanol, 1‐propanol or n‐pentane from the top surface of a cylindrical wick. Test variables include fire heat release rate of 50–7890 W, ceiling diameters of 610 and 660 mm, ceiling heights of 58–940 mm, wick diameters of 10–107 mm and curtain wall lengths (for confined ceilings) as large as the ceiling height. Simplified models were employed to suggest expressions for data correlation. Flame lengths increased up to 40% when the ceiling was confined. Ceiling heat fluxes were relatively independent of position in the stagnation region (radius along ceiling <20% of the ceiling height). Heat fluxes in the stagnation region for plumes were 25–40% of those measured for impinging jets at comparable conditions. In the ceiling jet region, at larger distances from the point of impingement, the heat flux decreased with increasing radius, in agreement with other studies. Confinement tended to increase ceiling heat fluxes in both regions. Ceiling heat fluxes for impinging flames and plumes were approximately the same, for flame lengths along the ceiling up to 25% of the ceiling height; however, stagnation point heat fluxes decreased for longer flame lengths.