Accurate Short-Characteristics Radiative Transfer in A Numerical Tool for Astrophysical RESearch (ANTARES)

We aim to improve the accuracy of radiative energy transport in three-dimensional radiation hydrodynamical simulations in ANTARES (A Numerical Tool for Astrophysical RESearch). We implement in the ANTARES short-characteristics numerical schemes a modification of the Bézier interpolant solver. This method yields a smoother surface structure in simulations of solar convection and reduces the artifacts appearing due to the limited number of rays along which the integration is done. Reducing such artifacts leads to increased stability of the code. We show that our new implementation achieves a better agreement of the temperature structure and its gradient with a semi-empirical model derived from observations, as well as of synthetic spectral-line profiles with the observed solar spectrum.

Photospheric magnetoconvection

Magnetic fields, convective motions and radiative energy transport all strongly influence each other in the solar photosphere. Attempts to understand, at a quantitative level, the diverse phenomona seen in the photosphere have relied on increasingly powerful computers since the 1980's. This approach has been remarkably successful and some recent examples, particularly cases where all three ingredients are important, will be discussed.

Round Table Summary: Radiative Transfer Problems

This Round Table addresses various problems related to the radiative energy transport calculations and the challenges related to the introduction of 3D hydrodynamic models, models with pulsations, connecting stellar atmospheres to the circumstellar medium etc. We have discussed several examples of environment not properly treated by radiative transfer calculations and outlined future development directions in this field.

The “Porcupine”: A Novel High-Flux Absorber for Volumetric Solar Receivers

A new volumetric (directly irradiated) solar absorber, nicknamed Porcupine, is presented. It was tested over several hundreds of hours at the Weizmann Institute’s Solar Furnace, using several flow and geometric configurations, at various irradiation conditions. The experiments, which were conducted at a power level of about 10 kW, showed that the new absorber can accommodate different working conditions and provide a convective cooling pattern to match various irradiation flux distributions. The capability of the Porcupine to endure a concentrated solar flux of up to about 4 MW/m2, while producing working gas exit temperatures of up to 940°C, was demonstrated. In comparative tests, the Porcupine sustained an irradiation solar flux level about four times higher than that sustained by other volumetric absorbers (foam and honeycomb matrices). Due to its ability to sustain and transport a much higher energy fluxes, the Porcupine yielded twice the power output of the other absorbers while its exit gas temperature was 300–350°C higher. The Porcupine design is highly resistant to thermal stresses development; none of the Porcupine absorbers tested showed any sign of deterioration after hundreds of operating hours, although temperature gradients of several hundreds °C/cm developed in some experiments. The basic Porcupine structure provides convective and radiative energy transport between the matrix elements, therefore alleviating the development of flow instabilities; this phenomenon causes local overheating and restricts the operation of other volumetric matrices. A Porcupine absorber was subsequently incorporated into the directly irradiated annular pressurized receiver (DIAPR), where it has been operating flawlessly at an incident flux of several MW/m2 and temperatures of up to 1,700°C.

Buoyancy-driven flows of a radiatively participating fluid in a vertical cylinder heated from below

The effect of radiative energy transport on the onset and evolution of natural convective flows is studied in a Rayleigh–Bénard system. Steady, axisymmetric flows of a radiatively participating fluid contained in a rigid-walled, vertical cylinder which is heated on the base, cooled on top, and insulated on the side wall are calculated by using the Galerkin finite element method. Bifurcation analysis techniques are used to investigate the changes in the flow structure due to internal radiation. The results of this two-parameter study – where the Rayleigh number, Ra and optical thickness, ז , are varied – apply to fluids ranging from opaque to nearly transparent with respect to infrared radiation. For any non-opaque fluid, internal radiation eliminates the static state that, without radiation, exists for all values of the Rayleigh number. This heat transfer mechanism also destroys a symmetry of the system that relates clockwise and counter-clockwise flows. The connectivity between characteristic flow families and the range of Ra where families are stable are found to depend greatly on ז . Results demonstrate the inadequacy of characterizing the behaviour of this system using simple notions of radiative transfer in optically thick or thin media; the nonlinear interaction of radiation and flow are far more complicated than these asymptotic limits would imply.