Pasquale M. Sforza

Mass, momentum, and energy transport in turbulent free jets

An experimental and analytic investigation of mass, momentum, and heat transfer in a free turbulent flow field is presented. In particular, the case of a jet issuing from a circular orifice into the free ambient atmosphere of a laboratory room is studied. Three basic flow conditions are considered: (1) isothermal mixing of an air jet with the surroundings, (2) isothermal mixing of a binary mixture (air/CO2) jet with the surroundings, (3) nonisothermal mixing of a heated air jet with the surroundings. Specially designed aspirating probes are used to measure the mean fluxes of mass, momentum and total enthalpy. These properties are considered to be the dependent variables defining the flow field. Accurate measurements of the more usual variables, such as mean species mole fraction, mass-averaged mean velocity and mean temperature are a by-product of the raw data from the aspirated probes. The normalized results indicate that, for identical initial conditions the flux of mass and the flux of total enthalpy behave identically and that both of these variables decay faster and have larger halfwidths than does the momentum flux. However, the radial coordinate at which the value of the flux variable is sensibly equal to zero, that is, the mean “edge” of the jet, is found to be the same for all flux variables. An extended version of the Reichardt inductive theory of free turbulence is found to adequately predict the entire flowfield for all of the conserved flux variables.

A genetic algorithm model for high heat flux flow boiling

Abstract A new genetic algorithm model is introduced in a recently developed turbulent-boundary-layer scheme for the calculation of heat transfer in high heat flux subcooled boiling flows. Such flows, often desired for cooling of rocket nozzles and nuclear components, are characterized by high fluid velocities and extremely small bubbles that exist in a thin layer adjacent to the heated wall. The current model is based on the solution of the momentum and energy equations with a turbulent diffusivity term based on bubble collapse and associated flow agitation. An important parameter arising in the analysis is the boiling intermittency factor, and the innovation in the present paper is the prediction of this factor through use of a genetic algorithm. The algorithm is employed to simulate the adaptation of a bubble population to the environment in a thermally developing flow. Fourier and Biot numbers for this problem are shown to introduce material and geometric coupling that make the analysis “semiconjugate” in nature. The model is used to predict the heat transfer for a high heat flux water flow typical of that found in the cooling passages of a hypersonic wind tunnel nozzle throat and is compared with some recent high heat flux experimental data.

Subsurface momentum coupling analysis for near-earth-object orbital management

Abstract Momentum coupling methods are analyzed for near-Earth object (NEO) orbit modification that can use either conventional explosives (HE) or nuclear explosives (NE) effectively. Enhancing momentum coupling reduces the explosive yield needed to achieve a particular orbital alteration, which, in turn, reduces the number of launch vehicles needed while also eliminating (for the HE case) or minimizing radioactive contamination and associated problems. A disadvantage is the additional mass required for the penetrator payload used to bury the explosive in the initial NEO interaction. In computing the momentum coupling necessary to provide reliable estimates of momentum coupling for various target materials (asteroids and comets) three analytic methods are presented for determining the position and yield of buried explosives that can produce a given NEO velocity change, Δ V The first method makes use of experimental data on crater ejecta characteristics and relates the momentum of the ejected mass to the yield and placement of the explosive. Ejecta velocities are strongly dependent on target density and scale depth. The second method is based on kinetic energy transfer and determines the change in NEO kinetic energy based on the energy partition from HE or NE detonations. Computational results from this method depend on NEO material properties such as the equation-of-state and mechanical structure. The third method relies on impulse momentum transfer and analyzes the impulsive force generated by the shock and ejecta formation process. Conditions behind the explosively generated shock are exploited to produce accurate computations yielding results comparable to those of the first two methods. Computational results for all three methods are presented to estimate the energy requirement needed to produce in a given NEO the velocity increment Δ V as a function of asteroid density, radius, ejecta velocity, and explosive depth. All three analytical methods yield consistent numerical results over the range of parameters selected, which includes asteroids with radii from 0.01 to 1 km, densities from 2.2 to 8 g/cm 3 , and explosive yields from 0.01 to 10 4 kt of TNT. These energy requirements are considerably less than those required to obtain the equivalent Δ V from stand-off NE devices and, in some cases, eliminate the need for NE entirely.

Flow and Separation Characteristics of Three-dimensional Negatively Buoyant Wall Jets

This paper reports a study of the behavior of cold air jets, specifically three-dimensional, incompressible, negatively buoyant, turbulent wall jets, and their separation from horizontal surfaces. With a cold-air distribution system, both the supply air temperature and flow rate are lower than in conventional systems for the same cooling load. The cold air is supplied to a zone at temperatures between 39°F and 49°F (3°C and 9°C) instead of the conventional 55°F (13°C). As the buildings ventilation and cooling airflow supply temperature and flow rate are reduced below the conventional values, indoor thermal environment considerations become increasingly important. An analysis using integral forms of the momentum and energy equations is presented for determination of the separation point of buoyant turbulent wall jets. The analysis shows that the separation point is a function of the outlet Richardson number in addition to the geometry of the outlet diffuser or nozzle. Also reported in this paper is an exp...

Vortex induced breakup of free surfaces in microgravity environments

Organized vorticity with circulation Γand length scale ι can move through a liquid otherwise at rest and approach a free surface. The dominant parameter for such interactions is the Froude number, Fr = Γ/(gl3)0.5. Earthbound experiments, which are limited to Fr<10, have shown that the ensuing collisions result in substantial deformations of the free surface. The same experiments, carried out under microgravity conditions, can reach values 1000 times larger, suggesting that such waves will be dramatically enhanced, and that surface breaking is likely. Computational studies show that the effect of surface tension σ, expressed through the Weber number We=VρΓ/σ, cannot provide sufficient attenuation of the vortex-induced waves to avoid wave breaking even when Fr is only O(10). This poses distinct problems for the processing of liquids in space manufacturing scenarios. Surface waves may be controlled, or at least suppressed, by means of a contaminant layer of an immiscible liquid thinly dispersed over the free surface. The resulting surface elasticity, which does not depend on gravity, may provide a mechanism for controlling surface waves in a microgravity environment. The present paper explores these aspects of the effects of the microgravity environment on space materials processing and suggests a self-contained experiment which can be carried out in a drop tower or in space.

A numerical model for scaling of subcooled boiling heat transfer

The development and application of a numerical model for scaling of boiling phenomena in a circular cooling passage subject to high heat flux is described. The model enables the solution of the conjugate conduction/convection heat transfer problem for a variety of fluid and geometric parameters in order to facilitate the search for similarity relationships. Analytical scaling parameters available in the literature are often limited in application for characterizing practical problems, particularly those which exhibit the non-linearity and overall complexity of many configurations of interest. The model results provide detailed information for a complex problem permitting extraction of the essential nature of the heat transfer process. This, along with modern observations of high heat flux boiling phenomena, is used to formulate a reasonable scaling law. In particular, a glass-refrigerant system is analyzed as a potential model for a copper-water system, facilitating laboratory-scale study. 15 refs.