Bjornar Sandnes

A photovoltaic/thermal (PV/T) collector with a polymer absorber plate. Experimental study and analytical model ☆

A polymer solar heat collector was combined with single-crystal silicon PV cells in a hybrid energy-generating unit that simultaneously produced low temperature heat and electricity. The PV/T unit was tested experimentally to determine its thermal and photovoltaic performance, in addition to the interaction mechanisms between the PV and thermal energy systems. Thermal efficiency measurements for different collector configurations are compared, and PV performance and temperature readings are presented and discussed. An analytical model for the PV/T system simulated the temperature development and the performance of both the thermal and photovoltaic units.

Five-nanometer diamond with luminescent nitrogen-vacancy defect centers

A study was conducted to investigate the properties of weakly bound clusters of single-digit nanodiamonds (SND) using spectrally and temporally resolved luminescence detection, electron paramagnetic resonance (EPR) spectroscopy, and transmission electron microscopy (SEM). Nitrogen-vacancy (NV) centers were created in diamonds by high-energy proton irradiation followed by thermal annealing. Samples containing equal weights of 55-nm HTHP diamonds and SNDs were uniformly distributed on quartz substrates for luminescence measurements. It was observed that the SNDs exhibited significant luminescence in the red spectral region before irradiation. The emission was blue-shifted and the absence of the zero-phonon lines at 637 and 575 nm indicated that the luminescence did not originate from NV centers as compared with the NV spectrum.

Patterns and flow in frictional fluid dynamics

Pattern-forming processes in simple fluids and suspensions have been studied extensively, and the basic displacement structures, similar to viscous fingers and fractals in capillary dominated flows, have been identified. However, the fundamental displacement morphologies in frictional fluids and granular mixtures have not been mapped out. Here we consider Coulomb friction and compressibility in the fluid dynamics, and discover surprising responses including highly intermittent flow and a transition to quasi-continuodynamics. Moreover, by varying the injection rate over several orders of magnitude, we characterize new dynamic modes ranging from stick-slip bubbles at low rate to destabilized viscous fingers at high rate. We classify the fluid dynamics into frictional and viscous regimes, and present a unified description of emerging morphologies in granular mixtures in the form of extended phase diagrams.

Gas migration regimes and outgassing in particle-rich suspensions

Understanding how gases escape from particle-rich suspensions has important applications in nature and industry. Motivated by applications such as outgassing of crystal-rich magmas, we map gas migration patterns in experiments where we vary (1) particle fractions and liquid viscosity (10 Pa s – 500 Pa s), (2) container shape (horizontal parallel plates and upright cylinders), and (3) methods of bubble generation (single bubble injections, and multiple bubble generation with chemical reactions). We identify two successive changes in gas migration behavior that are determined by the normalized particle fraction (relative to random close packing), and are insensitive to liquid viscosity, bubble growth rate or container shape within the explored ranges. The first occurs at the random loose packing, when gas bubbles begin to deform; the second occurs near the random close packing, and is characterized by gas migration in a fracture-like manner. We suggest that changes in gas migration behavior are caused by dilation of the granular network, which locally resists bubble growth. The resulting bubble deformation increases the likelihood of bubble coalescence, and promotes the development of permeable pathways at low porosities. This behavior may explain the efficient loss of volatiles from viscous slurries such as crystal-rich magmas.

The physics and the chemistry of the heat pad

Flexing a metallic disk triggers the crystallization of the supercooled sodium acetate solution contained in commercial heat pads. Many mechanisms have been proposed to explain the apparent nucleation of crystalline material. In this paper a simple experiment is described that demonstrates that nucleation is triggered by preserving seed crystals clamped between opposing metal surfaces. An explanation for the retention of the crystalline particles is the elevated melting point caused by very high local pressures. A series of thermophysical properties of the sodium acetate solution is also measured, including the available enthalpy upon crystallization of the supercooled substance, and liquid and solid phase specific heat capacities.

Determination of the performance of solar systems with the calorimetric method

Abstract A calorimetric method is applied to determine the solar system performance by in-situ measured data. The heat store is interpreted as a calorimeter and the information on charge and discharge is extracted from the shape analysis of the heat store’s temperature profile. The method is applicable to any heating system which includes a heat store. In the present paper, this procedure is presented and applied to determine the solar gain for a large solar system for domestic hot water preparation and a solar combisystem. The uncertainty of this method is in the range of ±10%. The results obtained by the calorimetric method are compared to TRNSYS simulations.

Scar tissue classification using nonlinear optical microscopy and discriminant analysis

This paper addresses the scar tissue maturation process that occurs stepwise, and calls for reliable classification. The structure of collagen imaged by nonlinear optical microscopy (NLOM) in post-burn hypertrophic and mature scar, as well as in normal skin, appeared to distinguish these maturation steps. However, it was a discrimination analysis, demonstrated here, that automated and quantified the scar tissue maturation process. The achieved scar classification accuracy was as high as 96%. The combination of NLOM and discrimination analysis is believed to be instrumental in gaining insight into the scar formation, for express diagnosis of scar and surgery planning.

Retroemission by a glass bead monolayer for high-sensitivity, long-range imaging of upconverting phosphors

We introduce a retroemitter (REM) device comprising a planar glass bead set placed on a luminescent material substrate, which converges an excitation beam into a set of foci (voxels). The in-voxel emission is collimated by the beads, and propagates upstream over the long range, unlike the out-of-voxel emission spreading in all angles. The REM signal contrast is characterized as a function of incidence and observation angles and propagation distance. REM signal contrasts of approximately 20 and 1600 were found for the organic fluorescent dye and upconverting phosphor substrates, respectively. In the latter case, nonlinear optical signal enhancement plays a role in addition to the retroemission effect. This allows centimeter-scale REM patterns to be read out at the meter-scale distance using eye-safe sub-mW/cm(2) excitation intensities.

Frictional Fluid Dynamics and Plug Formation in Multiphase Millifluidic Flow.

We study experimentally the flow and patterning of a granular suspension displaced by air inside a narrow tube. The invading air-liquid interface accumulates a plug of granular material that clogs the tube due to friction with the confining walls. The gas percolates through the static plug once the gas pressure exceeds the pore capillary entry pressure of the packed grains, and a moving accumulation front is reestablished at the far side of the plug. The process repeats, such that the advancing interface leaves a trail of plugs in its wake. Further, we show that the system undergoes a fluidization transition-and complete evacuation of the granular suspension-when the liquid withdrawal rate increases beyond a critical value. An analytical model of the stability condition for the granular accumulation predicts the flow regime.

Pattern formation of frictional fingers in a gravitational potential

Aligned finger structures, with a characteristic width, emerge during the slow drainage of a liquid-granular mixture in a tilted Hele-Shaw cell. A transition from vertical to horizontal alignment of the finger structures is observed as the tilting angle and the granular density are varied.An analytical model is presented, demonstrating that the alignment properties are the result of the competition between fluctuating granular stresses and the hydrostatic pressure. The dynamics is reproduced in simulations.We also show how the system explains patterns observed in nature, created during the early stages of a dike formation.