Exploring the use of nanofluids in pump-free systems for solar thermal applications.

By using nanofluids as a working fluid in pump-free designs, thermal energy systems can become more efficient and have reduced maintenance costs, ultimately extending the system's lifespan. In this paper, our goal is to investigate unsteady phenomena in the irradiation process and highlight their significance. To accomplish this, we conducted a series of experiments using a square loop of glass pipes filled with carbon black nanofluids and irradiated with a halogen lamp to simulate solar irradiation. The resulting convective motion of the nanofluids allowed us to observe the performance of different concentrations of carbon black, with 0.005-0.01 wt.% proving to be the most effective. Additionally, we identified unsteady processes that occur at the beginning of the process or when the irradiation changes. Finally, we employed computational fluid dynamics simulations to gain further insight into these phenomena.

Photothermal conversion of biodegradable fluids and carbon black nanofluids.

The paper is devoted to the topic of direct absorption solar collectors (DASCs). Various kinds of fluids can be used as heat transfer fluid in DASCs, and the main focus of our paper is on comparing nanofluids (water with carbon black nanoparticles, concentrations between 0.25 and 1.00% weight) and biodegradable coffee colloids. At first, these fluids were tested by exposing them to irradiation caused by artificial light in indoor experiments, and the corresponding temperature increase was recorded. The fluids were placed in a beaker with a relatively large size so that most of the fluid was not directly irradiated. In these experiments, the performance of the two studied fluids was similar: the resulting temperature increase varied between 46 and 50 °C. Our next experiments involved a smaller system subjected to irradiation obtained by using a solar collector. As a result, we detected an intense absorption on the nanoparticle surface so that the temperature rise in the nanofluid was higher than in the coffee colloids. Next, the process was analysed using a theoretical analysis that gave good correspondence with the experiments. Finally, we extended the theoretical analysis to a DASC with a flowing fluid. The model was validated against results from the literature, but it also supported our experimental findings.

Extension of the hard-sphere model for particle-flow simulations.

Discrete element methods require appropriate models for particle-particle collisions. Usually, researchers use soft-sphere types of models where the collision dynamics is solved numerically. This makes the simulation computationally expensive. In this paper, however, we show a hard-sphere model that uses ready analytic formulas that relate the pre- and postcollisional velocities of the particles in contact. This hard-sphere model is an extension of an existing model that uses three input parameters. For this, we applied the linear-spring soft-sphere model, where analytic relations can be found. These relations were implemented into the standard hard-sphere model. As a result, we obtain a robust hard-sphere model that is more accurate than the standard one and is still computationally cheap.

Extension of the hard-sphere particle-wall collision model to account for particle deposition.

Numerical simulations of flows of fluids with granular materials using the Eulerian-Lagrangian approach involve the problem of modeling of collisions: both between the particles and particles with walls. One of the most popular techniques is the hard-sphere model. This model, however, has a major drawback in that it does not take into account cohesive or adhesive forces. In this paper we develop an extension to a well-known hard-sphere model for modeling particle-wall interactions, making it possible to account for adhesion. The model is able to account for virtually any physical interaction, such as van der Waals forces or liquid bridging. In this paper we focus on the derivation of the new model and we show some computational results.

Numerical investigation of explosion suppression by inert particles in straight ducts.

This paper is devoted to the mitigation of explosions in long galleries by means of an inert dust cloud. In practice, this technique bases on mounting shelves under the roof, on which the inert dust is distributed. This issue was numerically investigated in this research. The medium was assumed to be a two-phase mixture consisting of a fast flowing gas (representing the explosion) and a cloud of solid particles (representing the dust phase). The model makes use of the Eulerian-Lagrangian approach, where the solid particles are modelled as moving points, interacting with the gas flow. The objective was to analyse the suppression process and compare with experimental findings.

Aerodynamic evaluation of the empty nose syndrome by means of computational fluid dynamics.

The potential outcome of a surgical enlargement of internal nasal channels may be a complication of nasal breathing termed the Empty Nose Syndrome (ENS). ENS pathophysiology is not entirely understood because the expansion of air pathways would in theory ease inhalation. The present contribution is aimed at defining the biophysical markers responsible for ENS. Our study, conducted in silico, compares nasal aerodynamics in pre- and post-operative geometries acquired by means of computer tomography from the same individual. In this article, we elucidate and analyse the deviation of airflow patterns and nasal microclimate from the healthy benchmarks. The analysis reveals 53% reduction in flow resistance, radical re-distribution of nasal airflow, as well as dryer and colder nasal microclimate for the post-operative case.

An investigation of the consequences of primary dust explosions in interconnected vessels.

In this article Eulerian-Lagrangian 2D computer simulations of consequences of primary dust explosions in two vessels connected by a duct are described. After an explosion in the primary vessel a propagation of hot pressurised gases to the secondary vessel, initially uniformly filled with dust particles, is simulated. The gas phase is described by the standard equations and it is coupled with the particulate phase through the drag force and the convective heat transfer. No chemical reaction is considered in the model since the objective was to model the system up to the time of ignition. The computation was performed for different lengths and diameters (heights) of the linking duct. Having analysed the results, it was concluded that the simulations agree with experimental observations in that the probability of transmission of an explosion from the primary to the secondary vessel decreases with decreasing diameter (height) and increasing length of the connecting pipeline. Snapshots of particle positions for different times are presented. The work illustrates the behaviour of the mixture in the secondary vessel: the particles tend to concentrate in clouds, and domains with no particles are observed. This may influence the explosion characteristics of the system.

Extended hard-sphere model and collisions of cohesive particles.

In two earlier papers the present authors modified a standard hard-sphere particle-wall and particle-particle collision model to account for the presence of adhesive or cohesive interaction between the colliding particles: the problem is of importance for modeling particle-fluid flow using the Lagrangian approach. This technique, which involves a direct numerical simulation of such flows, is gaining increasing popularity for simulating, e.g., dust transport, flows of nanofluids and grains in planetary rings. The main objective of the previous papers was to formally extend the impulse-based hard-sphere model, while suggestions for quantifications of the adhesive or cohesive interaction were made. This present paper gives an improved quantification of the adhesive and cohesive interactions for use in the extended hard-sphere model for cases where the surfaces of the colliding bodies are "dry," e.g., there is no liquid-bridge formation between the colliding bodies. This quantification is based on the Johnson-Kendall-Roberts (JKR) analysis of collision dynamics but includes, in addition, dissipative forces using a soft-sphere modeling technique. In this way the cohesive impulse, required for the hard-sphere model, is calculated together with other parameters, namely the collision duration and the restitution coefficient. Finally a dimensional analysis technique is applied to fit an analytical expression to the results for the cohesive impulse that can be used in the extended hard-sphere model. At the end of the paper we show some simulation results in order to illustrate the model.