169 research outputs found
Flow pattern during condensation of three refrigerants: microfin vs. smooth tube
No full textThere is agreement in the open literature that the mechanisms of heat transfer and pressure drop are intimately
linked with the prevailing two-phase flow regime.
During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity
forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear
when gravity is the controlling force.
Very poor evidence about the effect of microfins on the flow patterns during condensation is given in the open
literature. Thus, to investigate the two phase flow pattern during condensation a specific test section was built
up; the fluid outlet pattern can be analysed and recorded.
The heat transfer and pressure drop performances of microfin tubes during condensation of refrigerants have
been studied experimentally and theoretically.
In previous papers the present authors reported their own data condensing R134a, R410A and R236ea inside two
different tubes: a 9.50 mm outer diameter microfin tube (7.69 mm inside diameter at the fin tip and 60 fins with
0.23 mm fin height and 13° helix angle) and a plain 8.00 mm inner diameter tube.
The same operative conditions (saturation temperature, vapour quality and mass flux) used for heat transfer
measurements are reproduced in the visualisation section in order to investigate the specific flow pattern for
three different refrigerants with high (R410A), intermediate (R134a) and low (R236ea) saturation pressure.
For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the
visualisation experimental data are analysed with reference to parameters like dimensionless vapour velocity,
Martinelli parameter and void fraction, that are adopted in the main available flow pattern maps.
Since the available maps are designed only for smooth tubes, the results of this wide experimental work can be
used in order to define a new flow pattern map specifically dedicated to microfin and enhanced tubes
Experimental investigation into two-phase flow patterns inside a herringbone microfin tube
No full textDuring in-tube microfinned enhanced tubes show a heat transfer enhancement, as compared to
equivalent smooth tubes under the same operating conditions, that is partly due to the mere increase
in the effective exchange area and additionally to the turbulence induced in the liquid film by the
micro fins and to the surface tension effect on the liquid drainage.
Furthermore there is agreement in the literature that the mechanisms of heat transfer and pressure
drop are intimately linked with the prevailing two-phase flow regime.
In the recent open literature evidence is given to the effect of fins orientation on flow patterns in
herringbone tubes (Miyara et al., 2003). In particular, at the same operating conditions, it was
pointed out that when the fins convergences are positioned at the top and bottom of the tube
(dubbed here as Position-I), the occurring flow pattern can be completely different from the tube
arrangement with the fins convergences at both sides (Position-II). In a previous paper by the
present authors, the “Position-II” arrangement was investigated with three different refrigerants for
a saturation temperature of 40°C and mass velocities 100÷800 kg m-2s-1. In this paper the “Position-
I” arrangement is now investigated for the fluid R134a and a comparison with the available
visualization data for “Position-II” is presented.
In order to investigate the two phase flow pattern during condensation a specific test section was
built up. For the study of the main flow patterns, in particular focusing on the stratified/annular
mode transition, the visualisation experimental data are analysed with reference to the
dimensionless vapour velocity and the Martinelli parameter
Numerical simulation of latent and sensible concrete thermal energy storage system
No full textA new type of concrete with PCM (Phase Change Material) thermal energy storage system is presented. The system, developed for industrial applications, is supposed to operate with temperature up to 400 °C and the PCM added mixture presents enhanced performances. A cylindric concrete module was numerically simulated, with the CFD software ANSYS Fluent, and the results were compared with previous experimental tests. A stainless-steel pipe is inserted in the center of the cylinder and the whole system is insulated. The charging stage is obtained by heating the pipe’s surfaces by Joule effect and the module is then cooled with compressed air through the pipe. Two concrete mixtures were tested, with and without PCM addition, in the same conditions, to evaluate their differences. A binary mixture of salts was used as PCMs, composed of 40% of KNO3 and 60% of NaNO3, and they were absorbed by diatomite, a porous fossil flour
Scambio termico e gradiente di pressione nella condensazione entro microcanali
No full textL’utilizzo di microcanali nei condensatori e negli evaporatori di impianti di condizionamento dell’aria, specialmente
nell’industria automobilistica, è ormai piuttosto diffuso, dati i vantaggi dal punto di vista sia energetico che ambientale. Infatti,
data la loro forma e le ridotte dimensioni esterne, questi condotti oppongono una minore resistenza all’aria, nonché realizzano
migliori prestazioni di scambio termico lato interno, se paragonati ai tubi tradizionali di dimensioni maggiori; inoltre,
permettono di contenere la carica di refrigerante, limitando così le emissioni nell’ambiente. Tuttavia, dal punto di vista termico
l’utilizzo di questi canali richiede uno studio specifico poichè molti fenomeni, che sappiamo essere presenti durante la
condensazione in tubi di dimensioni tradizionali (diametro interno 7-10 mm), vengono soppressi o ridotti, mentre tendono a
diventare importanti nuovi fattori, come ad esempio la tensione superficiale, che influenza in maniera rilevante il regime di
deflusso.
Gli studi presenti in letteratura sulla condensazione entro microcanali sono ancora pochi e incompleti, e nuove analisi e
ricerche sono necessarie per comprendere e descrivere appieno il fenomeno. In questo lavoro gli autori presentano in maniera
critica alcuni tra i più recenti studi sulla condensazione di fluidi frigorigeni entro microcanali, confrontando i risultati
sperimentali con modelli di previsione dei coefficienti di scambio termico e dei gradienti di pressione
Studio sperimentale sul deflusso di una soluzione LiBr-H2O irrorata su una colonna di tubi
No full textA simplified analytical approach for concrete sensible thermal energy storages simulation
No full textA simple lumped capacitance based computing model was developed and here presented. The code permits the thermal and energetic analysis of concrete thermal energy storages (TESs) during time. The simulated system consists of a parallelepiped concrete module that can be heated (charging phase) and cooled (discharging phase) by a single-phase working fluid flowing in a tube embedded in the concrete. The modules can be piled up in different configurations to build any desired TES. The new simulation code was validated against the experimental data carried out by ENEA with two different concrete mixtures, during both the heating and cooling processes using mineral oil as working fluid. Furthermore, on the basis of the energetic analysis, two different TES thermal efficiencies were proposed to evaluate the charge or discharge progresses over time. This simple and easy-to-use model allows for a drastic reduction of the computational time needed to simulate the TES and it can be easily integrated, in various arrangements, to any concentrated solar power plant (CSP) and associated energy conversion plant simulation models to have a quick evaluation of the whole system performance
Flow patterns during condensation of refrigerants inside enhanced tubes
No full textEnhanced tubes have already been widely used for air-conditioning and refrigeration applications as
they ensure a large heat transfer enhancement with a relatively low pressure drop increase. During
condensation enhanced tubes show a heat transfer enhancement, compared to equivalent smooth
tubes under the same operating conditions, that is partly due to the mere increase in the effective
exchange area, and additionally to the turbulence induced in the liquid film by the enhanced surface
(fins) and to the surface tension effect on the liquid drainage.
There is agreement in the literature that the mechanisms of heat transfer and pressure drop are
intimately linked with the prevailing two-phase flow regime.
During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear
or gravity forces. While annular flow pattern is associated with high vapour shear, stratified, wavy
and slug flows appear when gravity is the controlling force. In a fully developed annular flow
pattern, there is a thin condensate film on the entire tube wall, while the gas phase flows in the
central core, and heat transfer is governed by vapour shear and turbulence. Very poor evidence
about the effect of microfins both in helical and “herringbone” shapes on flow patterns during
condensation is given in the open literature. Thus, to investigate the two phase flow pattern during
condensation, a special test section was built. Experimental observations for herringbone tube with
three fluids (R236ea, R134a, R410A) in a wide range of operative conditions (mass flux and vapour
quality) are reported in this paper.
For the study of the main flow patterns, in particular focusing on the stratified/annular mode
transition, the visualisation experimental data are analysed with reference to parameters like
dimensionless vapour velocity and Martinelli parameter, that are commonly used in most available
flow pattern maps
3D CFD Simulation of a New Ventilated Roof
No full textIn the last decades, energy management and saving have become challenging issues. Considering the building sector (residential or industrial), different technologies have been developed in order to realize tangible energy savings, such as: ventilated roof, double facades, glazed surfaces, etc. Nonetheless, it is important for these new technologies to contemporary assure the human thermal comfort. Passive cooling (or heating) technologies are of actual interest. Low or near-zero energy buildings can only be realized as a result of the good design of all their components; specifically, the roofs call for particular attention as they take large parts of a building’s total surface area. This paper presents a comparison between an innovative ventilated roof, based on an original design of the support and a traditional one. A 3D numerical model is developed to analyze the air flow and to compute the achievable benefits in terms of reduction of the summer heating gains. The simulations were performed by varying the solar irradiance from 600 to 1000 W m2. The investigation is conducted comparing a ventilated roof assembly to the same traditional structure, assuming buoyancy-driven airflow. Two roof types are studied: an insulated roof and a non-insulated one. The results reveal that the ventilated roof leads to a great reduction of the total amount of solar heat gains for all the simulated scenarios
A new analytical model for concrete sensible thermal energy storages simulation
No full textA simple lumped capacitance based computing model was developed and here presented. The code permits the thermal and energetic analysis of concrete thermal energy storages (TESs) during time. The simulated system consists of a parallelepiped concrete module that can be heated (charging phase) and cooled (discharging phase) by a single-phase working fluid flowing in a tube embedded in the concrete. The new simulation code was validated against the experimental data carried out by ENEA with two different concrete mixtures, during both the heating and cooling processes using mineral oil as working fluid. This simple and easy-to-use model allows for a drastic reduction of the computational time needed to simulate the TES and it can be easily integrated, in various arrangements, to any concentrated solar power plant (CSP) and associated energy conversion plant simulation models to have a quick evaluation of the system performance
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