Experiments on parallel connected loops in single phase natural circutation: preliminary results

Natural circulation is the most important heat removal mechanism for passive protection systems in a lot of industrial applications, such as nuclear power plants, solar energy systems, reboilers and cooling of electronic systems. The aim of the present work is to investigate the flow and heat transfer characteristics of parallel loops, connected in the lower heated sections, in single-phase natural circulation. The test facility was composed by 2 vertical circuits connected in parallel; each of them was rectangular in geometry, the aspect ratio (defined as the height to width ratio) was 1.63, with circular copper tube of 4 mm (I.D.). An upper cold heat exchanger provided the heat sink, while the heat source at the bottom was a power supply system. Several calibrated thermocouples (T-type) placed in the fluid along the vertical tubes allowed the evaluation of the hot and cold legs average fluid temperature differences. Tests were carried out imposing 3 different heat sink temperatures (10, 20, 30\ub0C); for each of these temperatures the power supply at the lower heater was increased from 20 to 90 W. The fluid investigated was distillate water. The experimental results have been analysed in terms of thermal performance of the single or connected loops. Collected data have also been compared with Vijaiyan\u2019s correlation

Performance assessment and LCA of a PCM-based coating for residential buildings of the north-west Mediterranean region

The paper focuses on the thermo-economic and life cycle assessment of three different Phase-Change Materials (PCM) for use in residential buildings on the North-West Italian coast. For the purpose of this work, we considered the climatic conditions of the city of Genoa, Italy, and used publicly available weather data from year 2020. We numerically assessed three PCMs against conventional thermal insulating materials, on three different flat wall geometries, using a one-dimensional heat transfer model, implemented in MATLAB. The most relevant characteristic of PCMs is their phase transition condition. Our model is based on the assumption that PCMs transition occur in a specific temperature range, and this yields to an instantaneous increase of their specific heat. Subsequently, based on a 25-year PCM life cycle assumption, we carried out a thermo-economic analysis based on the Net Present Value (NVP) index, a life cycle assessment (LCA) and a carbon dioxide (CO2) saving estimation. Linear regression was used to predict the future economic and environmental scenarios. Simulation results showed that PCM performance is not as high as expected when benchmarked against a conventional insulating material. Specifically, PCMs do not reduce winter thermal demand and CO2 emissions over their life cycle are twice those of the classical insulator taken as a reference. We then numerically evaluated their performance in a warmer climate, corresponding to a South Mediterranean region, and under these conditions PCMs outperformed against conventional insulators, thus justifying their current higher cost

Enhancing the phase distribution in parallel vertical channels with single and double chamber coaxial headers

In this paper, in order to maximize the degree of flow rate uniformity of the two phase flow distribution into parallel vertical channels, a number of distributor fittings have been tested with respect to a test section characterized by a horizontal header and a series of vertical upward channels. The fluids are air and water and the flow regimes at the header cover the intermittent and annular flow pattern conditions. The ranges of liquid and gas superficial velocities are 0.45\u20131.2 and 1.5\u201316.5\u202fm/s, respectively. A header geometry without any inserts, such as the smooth reference header, is taken as a reference case. Data from a number of inserts to enhance the uniformity of distribution are compared to the reference case, such as a series of cylindrical distributors with in-line holes along their length (\u201cflutes\u201d). New data in particular are presented for the \u201cdouble chamber flute\u201d fitting. As in previous studies by the Authors, extracted mass flow rates of liquid and gas from parallel channels are recast in terms of normalized flow rates and normalized standard deviation of phases. A new overall performance parameter is used for data reduction and for summarizing the comparison between the flute fitting configurations

PHASE SPLIT IN PARALLEL VERTICAL CHANNELS IN PRESENCE OF A VARIABLE DEPTH PROTRUSION HEADER

The air and water flow distribution are experimentally studied in a test section simulating a heat exchanger composed by a round header and 16 parallel upward channels. The effects of the tube protrusion depth as well as the gas and liquid superficial velocities are investigated and the results are compared with previous data. A new fitting solution to be inserted in the header has been developed based on previous findings by the Authors. The fitting geometry belongs to the family of the protruding pipes but the protruding depth has been varied along the header in order to cope with expected liquid and gas mass flow rates in the parallel channels. The flow at the header inlet is intermittent and annular and water and air have been employed as two phase mixture. The new header fitting demonstrated to yield meaningful improvements in phase distribution in terms of either dimensionless liquid and gas flow ratios or standard deviation of phase flow ratio (the overall performance parameter STD2 here presented decreased by 64%) when compared to the previous configurations

Effects of The Presence of Protrusions on The Air-Water Distribution in Parallel Vertical Channels

Uniform fluid and phase distribution is essential for efficient operation of engineering equipment such as plate heat exchangers, reactors, mixers where two phases are flowing together. Even though the principles of single phase distribution in parallel channels have been studied for more than three decades, further work is needed when a mixture is present, since to date many process equipment still suffer from uneven distribution of phases inside them. In this paper an experimental investigation is devoted to establish the influence of the operating conditions and of the header geometry on the phase/mass distribution into parallel vertical channels. The study is carried out with air-water mixtures and it is based on the measurement of gas and liquid flow rates in individual channels. The liquid and gas superficial velocities range from 0.2-1.2 and 1.5-16.5 m/s, respectively. In order to control the flow distribution in the channels, the header has been modified by introducing a number of protrusion pipes, whose protrusion height can be varied. The effects of presence of the protruded pipes into the header are analysed in terms of liquid and gas flow ratios and normalised standard deviation of phases and the present results are compared with previous ones obtained in different conditions of header geometry

Long-term experimental study on gravitational sedimentation of water aluminum oxide nanofluid at different volumetric concentrations

The stability of nanofluids is critical in engineering applications. The sedimentation of the nanoparticles in the base fluid limits the stability of the nanofluid. By measuring the absorbance of a visible laser through water aluminum oxide nanofluids it is possible to observe the evolution of the sedimentation process. A simple experimental setup consisting of a thin closed test cavity (0.8 mm or 1.5 mm thickness) filled with nanofluid and a set of laser diodes-photodiodes pairs was used in the experiments to determine the absorbance and the local volumetric concentration. The evolution of the local volumetric concentration of the nanofluid was measured at 13 height positions along the test cavity. Five initial volumetric concentrations of aluminum oxide (Al2O3; 0.5%, 1.0%, 1.5%, 2.0%, 3.0% and 4.0%) were considered. Data and digital photos were acquired for a total time lapse of 260 days. The results were independent of the container thickness. The same general trend was consistent for all the initial volumetric concentrations, i.e., the sedimentation rate decreases in time. The time evolution of the volumetric concentration was compared with results from a modified Mason-Weaver model. The comparison was possible by setting a variable sedimentation velocity in the model, suggesting that the sedimentation is affected by the dynamics of nanoparticle clusters with diverse sizes produced by agglomeration

Study of a Square Single-Phase Natural Circulation Loop Using the Lattice Boltzmann Method

Natural circulation loops are thermohydraulic circuits used to transport heat from a source to a sink in the absence of a pump, using the forces induced by the thermal expansion of a working fluid to circulate it. Natural circulation loops have a wide range of engineering applications such as in nuclear power plants, solar systems, and geothermic and electronic cooling. The Lattice Boltzmann Method was applied to the simulation of this thermohydraulic system. This numerical method has several interesting features for engineering applications, such as parallelization capabilities or direct temporal convergence. A 2D model of a single-phase natural circulation mini-loop with a small inner diameter was implemented and tested under different operation conditions following a double distribution function approach (coupling a lattice for the fluid and a secondary lattice for the thermal field). An analytical relationship between the Reynolds number and the modified Grashof number was used to validate the numerical model. Two regimes were found for the circulation, a laminar regime for low Reynolds numbers and a non-laminar regime characterized by a traveling vortex near the heater and cooler’s walls. Both regimes did not present flux inversion and are considered stable. The recirculation of the fluid can explain some of the heat transfer characteristics in each regime. Changing the Prandtl number to a higher value affects the transient response, increasing the temperature and velocity oscillations before reaching the steady state

TWO-PHASE FLOW DISTRIBUTION IN PARALLEL VERTICAL CHANNELS FED BY A VARIABLE DEPTH PROTRUSION HEADER

In the present study phase distributions in parallel upward channels fed by a cylindrical horizontal header is experimentally investigated. A new fitting solution to be inserted in the header are been developed based on previous experiments by the Authors. The fitting geometry belongs to the family of the protruding pipes but the protruding depth has been varied along the header to cope with expected liquid and gas mass flow rate in the parallel channels. In this study the operating conditions, evaluated at the distributor inlet, covered the following ranges of gas and liquid (air and water close to atmospheric pressure) superficial velocity: Vsg=1.50\uf716.50 m/s and Vsl=0.20\uf71.20 m/s, respectively. Intermittent flows (plug, slug) and annular flow were visually observed. The new header fitting demonstrated to yield meaningful improvements in phase distribution in terms of either dimensionless liquid and gas flow ratios or standard deviation of phase flow ratio, when compared to constant depth protruding pipes

CFD initial assessment of a protrusions based experimental facility

The design of compact heat exchangers and their mass flow distributors is still based on empirical approaches and both experimentations and numerical analyses are needed for defining the best geometries able to reduce the mass flow rate non uniformities in parallel channels. This is a cause of reduction in both thermal and fluid-dynamic performances. In this paper, a series of single-phase and two-phase CFD simulations on water and water with air injection are carried out in order to estimate the capabilities of the solvers implemented in the OpenFOAM code to reproduce (in comparison with experimental data) such kind of configurations and phenomena. The effects of different turbulence models implemented in OpenFOAM are investigated; additionally, some general considerations on the differences and analogies among different Reynolds numbers flow and turbulence model effects applied to the present configuration are discussed. Finally, by the point of view of two-phase flow, the capability of the code to reproduce the intermittent behaviour is investigated, with the aim of obtaining an acceptable simulation of the non-uniform mass flow distribution in each protrusion; the obtained results are also compared with both ANSYS-FLUENT and STARCCM+ commercial codes

CFD preliminary assessment of a protrusions based facility

The design of compact heat exchangers and their mass flow distributors is still based on empirical approaches and both numerical analyses and experimentations are needed for designing the best geometries useful to reduce the mass flow rate non uniformities in parallel channels. As known, this is indeed a cause of reduction in both thermal and fluid-dynamic performances. In this paper a series of single-phase CFD simulations on water and water with air injection are carried out in order to estimate the capabilities of the solvers implemented in the OpenFOAM code to reproduce (in comparison with experimental data) such kind of configurations and phenomena. The effects of different turbulence models (both RANS and LES) implemented in OpenFOAM are investigated; additionally some general considerations on the differences and analogies among different Reynolds numbers flow and turbulence model effects applied to the present configuration are discussed. Finally, the capability of the code to reproduce the peculiar behaviour of a protrusions based experimental facility is investigated, with the aim of obtaining an acceptable simulation of the non-uniform mass flow distribution in each protrusion. The present paper is an extended version including the main conclusions and observations emerged in the 2nd AIGE-IIETA International Conference in Genoa