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
The paper deals with the impact of inlet conditions on bubble to slug flow transition in mini systems. A new experimental test loop with a glass mini-tube (D=1.2 mm ID) has been constructed to assess the effects of inlet conditions on the two-phase flow pattern development in the spatial and temporal domains. The interchangeable inlet part of the test section allowed different geometrical combinations for the mixing of gas and liquid prior to it entering the mini-tube. Porous media mixer and cross-junction mixer were considered. High speed video recordings were taken of 70 combinations of flow rates, corresponding to superficial velocities ranging from 0.2 to 11 m/s and 0.25 to 3 m/s for air and water, respectively. The following discernible flow patterns are considered: bubbly, slug and semi-annular flow. No significant differences were found when comparing flow pattern maps for each mixer. Digital image post processing of high speed video recordings was used to estimate the equivalent diameter for every gas structure. A comparison of the distribution of bubbles with equivalent diameter revealed the inlet mixer's strong impact on bubble size and bubble distribution along the mini-tube and thus, the bubble to slug flow transition. The VOF method, implemented in ANSYS Fluent was used as the numerical tool to predict the flow patterns in a mini-tube of 1.2 mm ID. A novel approach to mimic continuous mixing is presented. By changing only the prescribed flow rates, different flow patterns can be simulated. Similar interfacial structures were obtained by numerical simulation and experiment for both mixers. Reasonable quantitative agreement was also achieved when analyzing bubble to slug flow transition.
COBISS.SI-ID: 13081371
The purpose of this research was to provide further insights on bubble-inducedagitation of heated bulk liquid. Fundamental studies on the bubble disturbance of a stratified thermal layer were carried out for a 6mm 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 8s 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
Delineation of mini and micro scale channels with respect to two-phase flow has been the subject of many research papers. There is no consensus on when the small channel can be characterized as a mini channel or micro channel. The idea proposed by this paper is to use the normalized bubble nose radius, liquid film thickness top over bottom ratio, and bubble shape contour, which are found under normal gravity conditions in slug flow through a horizontal adiabatic channel, as the delineation criteria. The input parameters are bubble nose radius and bubble nose velocity as the characteristic length scale and characteristic velocity scale respectively. 3D numerical simulation with ANSYS FLUENT was used to obtain the necessary data. Following CFD good practice, a mesh independence study and a numerical model validation against published experimental data were both conducted. Analysis of the numerical simulation results showed that channels with D 100 um can be characterized as a micro system, while channels with D 400 um belong to mini systems. The region 200 um D 300 um represents a transition from the micro scale to mini scale.
COBISS.SI-ID: 13258779
To better understand the underlying two-phase phenomena and thus better predict transitions between the various two-phase flow patterns, it is necessary to update our way of thinking from one-of-a-kind flow pattern maps of limited applicability to a generalized approach based on first principles, mechanistic analysis and multi-scale characterization and representation of the important features of these complex flows. While in macro-sized channels and pipes this need is typically addressed by the use of empirically validated flow regime maps, there is - as yet - no consensus on two-phase flow regime maps for microchannels and miniature pipes. This study presents a set of recommendations for the development of a new comprehensive type of flow pattern map that not only covers adiabatic, evaporating and condensing flows in one seamless flow pattern identification tool, but also includes multiscale information about the flow itself, and furthermore contains embedded mechanistic methods for the principal two-phase phenomena for use in developing unified models for pressure gradients, heat transfer, void fraction, CHF, etc., all in one coherent global method.
COBISS.SI-ID: 12534811