In this study the mechanisms of heat transfer associated with laser-target plume formation are being investigated. The primary objective of this study is to develop techniques that can be used to determine the relative importance of plume absorption, scattering, and emission in coupling of laser energy into targets. Experimental methods include high-speed photography, target temperature measurement, plume light scattering and transmission measurements, calibrated plume emission spectroscopy, and microforce target recoil measurements.

Radiative heat transfer plays an important role in high-temperature combustion. The optical properties of the materials involved are highly dependent on composition and temperature. Particulate radiation is a key factor and scattering of radiation is often important. The combination of nonhomogeneous gas and particle radiation makes the problem particularly challenging to model analytically. Various computational techniques are being investigated for facilitating incorporation of complicated radiative transfer in CFD codes. Line-by-line techniques for computing molecular gas radiation, including the K-distribution technique, are also being investigated.

Radiation heat transfer in scattering media including arbitrarily rough surfaces and general multidimensional participating media is under consideration. The bidirectional reflectivity function and the directional hemispherical emissivity for random rough surfaces and microcontoured surfaces are predicted from electromagnetic scattering theory. General and approximate results for all orders of roughness, as well as for structured surfaces, have been obtained. An experimental program to measure these quantities has been developed. A general energy transfer formulation for participating media using the characteristic lengths in media, which include nongray gaseous absorption and anisotropic scattering, has been developed. The correlated-k method is being developed for wide band structures in nonhomogeneous media.

Thermal processes generate irreversibilities, i.e., entropy. In many processes, the optimum performance on operating limits can be estimated by finding the minimum rate of irreversibility, i.e., entropy production. Such a technique is often considerably simpler than more conventional ones. The method has been successfully applied to cooled shields in insulations, to heat exchangers, and to heat exchanger networks.

A technique is investigated for noninvasive measurement of the temperature field inside three-dimensional bodies, particularly living tissues, using tomographic methods based on electron spin resonance (ESR) spectroscopy of nitroxide stable, free radicals. Initial studies on temperature-dependent spectra of several nitroxides in model systems, such as solutions and blood, indicate that there are several temperature-dependent signals, such as magnetic bandwidth, which have the potential of forming the basis for such a thermal tomographic method. In addition to ESR, nuclear magnetic resonance techniques are also explored.

Various aspects of the thermal behavior of biological materials, particularly the human body, are studied. The work ranges from the morphological studies of the blood vessels which affect heat transfer to computer modeling of various organs as well as the entire thermoregulatory system. One application is the prediction of the rise of the deep-body temperature in a hot bath; another is the determination of safe surface temperatures for various equipment.

A second-order scheme is developed and applied in the study of advection-diffusion problems in particular media. Lattice-Boltzmann models are discrete systems mimicking the hydrodynamics of continua. Given that the computational cost of such schemes grows only linearly with the number of particles, direct simulations of heterogeneous systems with lattice-Boltzmann codes will ultimately out distance conventional schemes.

The objective of this project is to quantify the two stages of frost growth (early and mature) by using two new techniques: scanning confocal microscopy (SCM) and magnetic resonance imaging (MRI). The SCM technique will be used during the early stages of frost growth and the MRI technique during the mature growth phase. SCM provides video-rate imaging in terms of optical slices with near- or submicron spatial resolution.

This is a fundamental investigation of complex systems characterized by two-phase media separated by a complicated interface. Currently, natural convection in porous media and freezing in water-saturated packed beds are investigated with magnetic resonance imaging and novel parallel computational schemes.

A numerical model of boiling heat transfer in heterogeneous porous layers with and without chimneys has been conducted. Experimental observations have provided qualitative modeling information and model refinements. 1-D and 2-D models have been evaluated numerically with nonlinear coupling between mass, momentum, energy, capillary pressure, and evaporation rate. Good agreement with published data has been obtained. Examination of artificially created layer performance suggests broad potential applications for controlled boiling heat transfer, such as computer chip cooling via commercial refrigerants with heat fluxes in excess of 100 W/cm² and for performance evaluation of steam generators with porous layers from corrosion and scale deposits.

The effect of interfacial mixing and contact area between two liquids of differing densities and temperatures which result from a high-density, high-temperature liquid passing through a lower density, low-temperature liquid has been studied. Heat transfer effects, including the effects of vapor generation as well as break-up and solidification, were modeled. Analytical modeling was carried out at UIUC, while experimental studies using simulant materials of both single and multiple injected columns were conducted at Argonne National Laboratory. Good agreement between model predictions and experimental data was found.

A half-acre solar pond has been constructed in the agriculture section of campus. Continuing research investigates the feasibility of solar ponds for low-temperature heating processes.

Laser-induced plasmas from solid targets are often characterized by the emission spectra and excitation tempera tures. Spatial results of spectral intensity recorded over the duration of the plasma emission show that the peak intensities occur away from the surface. Moreover, the temperature peaks do not coincide with the peak intensities. This project investigates how the transient plasma expansion during time-integrated intensity measurements affects both the excitation temperature and atomic number density of the emitting species. The effect of heat and mass diffusion during the plasma expansion is studied to better understand how measured plasma intensities and temperatures are related.

During short-pulse, high-power laser irradiation of solids, electrons are ejected from the surface in large numbers and with high energy, which creates very high transient electromagnetic fields near the surface. Radiant thermal emission from the surface may change nonlinearly due to the presence of this induced field. The purpose of this project is to measure the distribution of thermal radiation with wavelength under applied electric fields to determine the relative importance of electromagnetic fields on thermal emission at different temperatures.

During high-power laser irradiation of solids, the amount of mass removed per unit of incident laser energy declines after a certain power density is reached. This roll-off with incident laser power density is observed in the intensity of inductively coupled plasmas (ICP) which use lasers to sample solid targets. This project's purpose is to understand the mechanisms for the reduction in ablated mass per unit of laser energy, such as shielding of the target from the incident laser energy by plasma formation, changes in particle size distribution of the ablated species, or mass transport affects.

The emission intensity from a plasma depends on the excitation temperature and the total number density of the emitting species. In principle, the amount of mass ablated from a laser-irradiated target can be estimated from the spectral emission intensity of the laser-induced plasma (LIP) if the spatial temperature distribution is known. To verify whether this can be done in practice, the correlation between the mass emitted as vapor and the emission intensity of the LIP is being investigated.