The purpose of this research was to provide further insights on bubble-induced agitation of heated bulk liquid. Fundamental studies on the bubble disturbance of a stratified thermal layer were carried out for a 6 mm sphere-volume equivalent diameter air bubble suspended in water (Eo=1.2). A video digital image system and thermochromic liquid crystals were used to reproduce the bubble movement as well as the wake drift of the liquid. A three-dimensional interface tracking simulation was used as a numerical tool. The results have revealed a long open wake region that is formed along the fluctuating bubble path. The amplitude of the bubble wake is much larger than that of the bubble path. In addition to longitudinal mixing, strong lateral mixing is also caused by the movement of vortices in the transverse direction. Details of numerical simulations have revealed a wake that tends to form a chain of Omega shaped vortex rings. These "rings" are connected to each other in the near wake region. The coherent effects further downstream lead to more complex vortex patterns in the far wake. The disintegration of the coherent chain of vortices due to bubble surface deformability is highly complex and not clear at this stage. A comparison with liquid crystal temperature response has revealed that the time scale of the mixing is much longer than the ascending bubble residence, approximately 8 s as obtained by several experimental runs. A transverse propagation of entrained cold water has given an estimate of the bubble-induced diffusion to be about 170 times larger than the diffusivity of momentum. The bulk liquid agitation cannot be correctly modeled without taking into account the bubble size dependent wake structure.
COBISS.SI-ID: 12220187
The research refers to the district heating (DH) system in Ljubljana, which includes 245 km of highly diversified pipelines. The purpose of this study was to determine the effect of the soil thermal conductivity coefficient (λs) on the heat loss from pre-insulated pipes during operation. Pipeline geometry, material properties (particularly insulation and soil thermal conductivity) and time-dependent data on supply, return and environs temperature were considered. Measurements of temperature, moisture, thermal conductivity of soil and heat flux through the soil were carried out at the chosen locations on the pre-insulated pipeline. In addition, laboratory measurements were conducted on the soil samples from this site in order to determine the soil density, specific heat and thermal diffusivity. For the evaluation of heat loss, transient and steady-state numerical simulations of the soil temperature field were performed. In transient simulations, in addition to the impact of environment temperature, the influence of supply and return temperatures was taken into account. A method for λs prediction during pipeline operation is presented. The algorithm is based on a comparison of the measured (Θexp) and simulated (Θsim) temperatures in the selected period of time with the same boundary conditions.
COBISS.SI-ID: 12406555
Emptying of an initially water-filled horizontal PVC pipeline driven by different upstream compressed air pressures and with different outflow restriction conditions, with motion of an air-water front through the pressurized pipeline, is investigated experimentally. Simple numerical modeling is used to interpret the results, especially the observed additional shortening of the moving full water column due to formation of a stratified water-air tail. Measured discharges, water-level changes, and pressure variations along the pipeline during emptying are compared using control volume (CV) model results. The CV model solutions for a nonstratified case are shown to be delayed as compared with the actual measured changes of flow rate, pressure, and water level. But by considering water-column mass loss due to the water-air tail and residual motion, the calibrated CV model yields solutions that are qualitatively in good agreement with the experimental results. A key interpretation is that the long air-cavity celerity is close to its critical value at the instant of minimum flow acceleration. The influences of driving pressure, inertia, and friction predominate, with the observed water hammer caused by the initiating downstream valve opening insignificantly influencing the water-air front propagation.
COBISS.SI-ID: 12581403
We present a mixing-time analysis for a double-disk turbine (DDT, SI Pat.No. 22243) and the well-known Rushton turbine (RuT) based on liquid stirring in a baffled vessel. The mixing time was measured locally based on the pulse/response technique. A small quantity of hot water, poured into the liquid bulk, just above the measurement location, was used as the pulse, while the change in the liquid temperature represented the system response. The results were obtained in two ways: (i) from measurements on the set-up and (ii) based on a CFD analysis. The pouring of the hot water was numerically simulated through the initialization of the scalar field. The duration of the temperature-pulse initialization around the measuring location corresponded to the pouring time in the experiment. All the energy introduced was freely swept away by the flow. The CFD-analyzed mixing times were consistently higher than the measured ones across the whole testing range, from 150 to 460 min1. When comparing our mixing-time results with those from the literature based on a dimensionless mixing time we found them to be in good agreement.
COBISS.SI-ID: 12578075
The aim of the present research work was to investigate the possibilities of designing surface texturing for different lubrication regimes and to evaluate its effectiveness, especially under starved, boundary and mixed lubrication regimes. This was achieved by combining an experimental tribological investigation with a surface-roughness analysis, a 2D FEM simulation and fluid dynamic modelling. The tribological investigation under unidirectional and reciprocating sliding was focused on the effect of the laser-texturing parameters—including the dimple depth and size, the dimple area density and the contact size—on the coefficient of friction under different lubrication regimes, achieved by varying the sliding speed, the normal load and the lubricant viscosity.
COBISS.SI-ID: 12287003