Showing papers by "Raymond Viskanta published in 1995"

Discrete ordinates quadrature schemes for multidimensional radiative transfer

Abstract: The fundamental problem of applying the method of discrete ordinates to radiative transfer predictions is the selection of the discrete directions and their associated weights. Both the accuracy of the solution and the computational effort depend on the angular discretization. This paper provides a sound mathematical methodology for the derivation of angular quadratures. By applying the collocation principle, the errors introduced by a quadrature are analysed and the constituting equations of angular quadratures are identified. Special emphasis is placed on the rotational invariance of the quadrature schemes. Multidimensional radiative transfer in participating media with isotropic and anisotropic scattering is accounted for throughout the analysis. A major goal of the present study is the construction of a new principle for multidimensional angular quadratures which is essentially a generalization of the principles employed for the well-known S n quadratures. The new construction principle has two major advantages. First, it enables a very flexible tailoring of quadratures according to the actual requirements. Second, compared to the S n quadratures, the new types of quadratures provide a higher accuracy while using the same number of nodal points.

Rewetting temperatures and velocity in a quenching experiment

Abstract: A convenient and accurate method has been developed to infer apparent rewetting and rewetting temperatures associated with quench front propagation in transient boiling processes. Results obtained from quenching experiments based on jet impingement cooling of a preheated test specimen indicate that the rewetting temperatures depend on the initial specimen temperature, as well as on the flow and surface conditions. Measurements also reveal strong effects of the initial test specimen temperature and the flow velocity and subcooling on the quench front velocity.

Quenching phenomena associated with a water wall jet: i. transient hydrodynamic and thermal conditions

Abstract: Transient boiling experiments have been performed using a large, preheated test specimen exposed to a water wall jet on its top surface. Temperatures measured at discrete locations on the surface have been used with a solution of the transient heat conduction equation to obtain the temperature field throughout the specimen, as well as the heat flux distribution at the surface. Together with visualization of related two-phase flow conditions, the results yield new insights concerning coexistence of nucleate, transition, and film boiling, as well as single-phase convection, on a surface. Results are presented for jet velocities and subcootings representative of conditions in high-temperature materials processing.

Turbulence Dissipation in a Free-Surface Jet of Water and Its Effect on Local Impingement Heat Transfer From a Heated Surface: Part 2—Local Heat Transfer

Abstract: This paper presents local heat transfer data for a planar, free-surface jet of water impinging normal on a uniformly heated surface. The hydrodynamic conditions of the jet were altered through the use of different nozzle types (parallel-plate and converging) and flow manipulators (wire grid and screens) to investigate the relationship between jet turbulence and local impingement heat transfer. The flow structures for each of the various nozzle conditions are reported in a companion paper (Wolf et al., 1995), and results are used in this paper to interpret their effect on local heat transfer. In addition to qualitative interpretations, correlations are developed for both the onset of transition to turbulence and the dimensionless convection coefficient at the stagnation point. Higher levels of jet turbulence are shown to induce transition to a turbulent boundary layer at smaller streamwise distances from the stagnation point. The effect of stream-wise turbulence intensity on the convection coefficient is shown to scale approximately as the one-quarter power.

Quenching phenomena associated with a water wall jet: ii. comparison of experimental and theoretical results for the film boiling region

Abstract: To establish confidence in existing theoretical models and to provide justification for their use in engineering practice, experiments have been performed to obtain heat fluxes for subcooled film boiling on the surface of a preheated test specimen exposed to a water wall jet. Comparisons between the experimental data and theoretical results are generally favorable, with the largest discrepancy corresponding to underprediction of the data by approximately 40% at locations close to the quench front, where some liquid-solid contact is likely. Much better agreement is achieved in zones of developed film boiling, where the model assumptions are more appropriate.