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Fazeres cotidianos na biblioteca escolar
No full textSILVA, R. J. da; BORTOLIN, S. (Orgs.). Fazeres cotidianos na biblioteca escolar. 2.ed. São Paulo: ABECIN Editora, 2018. 163p. (Coleção Estudos ABECIN; 03). ISBN: 978-85-98291-12-3
Convective condensation at low mass flux: Effect of turbulence and tube orientation on the heat transfer
No full textIt is well proved in the literature that gravity affects in-tube condensation heat transfer at low mass flux.
Nevertheless very limited data are taken at low mass flux when changing tube orientation, despite the
many practical applications. In this paper, convective condensation inside a 3.4 mm inner diameter tube
is investigated in horizontal and vertical downflow using R134a as the working fluid. The experiments are
performed at low mass flux, between 50 kg m-2 s-1 and 200 kg m-2 s-1, which are usually the less investigated
despite the relevance of gravity force at such low velocities. The condensation heat transfer coefficient
in vertical downflow can be as low as half the value in horizontal flow at the same operating
conditions, since gravity acts for the thinning of the liquid film in the horizontal tube. In vertical downflow,
the heat transfer coefficients show an early effect of turbulence, thus a new transition criterion is
here proposed. Criteria for predicting the relevance of channel orientation on the heat transfer coefficient
are also assessed
Heat transfer enhancement during dropwise condensation over wettability-controlled surfaces
No full textDropwise condensation (DWC) is a complex heat transfer process in which vapor phase changes to liquid phase forming discrete droplets on a surface whose temperature is below the dew temperature of the condensing fluid. DWC mode can strongly enhance the heat transfer compared to filmwise condensation (FWC) mode that usually takes place when a vapor condenses over a metallic surface. The wettability of the surface plays a crucial role on the promotion of DWC instead of FWC. This Chapter is focused on heat transfer measurements and modeling during DWC. The first two Sections are dedicated to a short literature review and to the description of the experimental procedures that can be used for the measurement of the heat transfer coefficient. DWC involves millions of droplets per square meter that form the so called droplet population. Section 3 is dedicated to the description of the droplet size distribution. Section 4 presents selected models that can be used for the prediction of the heat transfer during DWC. Formed droplets can be removed from the condensing surface by gravity or by other external forces. In the literature, most of the DWC experimental data are taken with quiescent vapor and very few works investigate the effect of the vapor drag force on the droplet departing radius and thus on the heat transfer during DWC. Furthermore, the effect of vapor velocity is not accounted for in available DWC models. Therefore, the last Section of this Chapter is focused on heat transfer modeling in presence of vapor velocity. A recent approach proposed by the present authors to account for the reduction of droplets departing diameter due to vapor velocity is here presented. The model is then used to show the effect of the main parameters on the DWC heat transfer coefficient
Condensation heat transfer of non-azeotropic mixtures inside channels
Full text linkBinary or ternary blends of hydroflourocarbons (HFCs) and hydrofluoroolefins (HFOs) are recently emerging as possible substitutes for the high GWP (Global Warming Potential) fluids currently employed in refrigeration and air-conditioning industry. In the present paper, heat transfer coefficients of a ternary mixture of R1234yf, R32 and CO2, ASHRAE designation R455A, 75.5/21.5/3.0 by mass composition, have been measured during condensation inside a minichannel having a 0.96 mm internal diameter and in a conventional tube with 8.0 mm diameter. Tests have been performed at 40°C mean saturation temperature. The present experimental database is used to assess available predicting correlations for condensation of mixtures, providing information on the applicability of available models
Condensation heat transfer in minichannels: A review of available correlations
No full textHeat exchangers with enhanced performance are demanded in various engineering
applications. Very often heat transfer devices are requested to guarantee not only high heat
transfer coefficients but also small size and weight, thus limiting the charge of the operative
fluid inside the heat exchanger. In order to increase the performance of condensers and to
properly design new heat exchangers, it is essential to have predictive tools that are validated
with experimental data. Sometimes, even well established semi-empirical correlations can be
inaccurate in some microscale flow conditions or with new refrigerants. The present paper
starts from the experimental database measured during condensation with different fluids (pure
fluids and refrigerant blends) inside small channels (hydraulic diameter around 1 mm) at the
Department of Industrial Engineering of the University of Padova. A critical review of
available correlations for heat transfer during condensation in minichannels is presented.
Predictions of heat transfer coefficients obtained applying selected models are compared with
the experimental database that covers various refrigerants: hydrofluorocarbons (HFC, i.e. R32
and R134a), new hydrofluoroolefins (HFOs) with low global warming potential (R1234ze(E)),
natural refrigerants (hydrocarbons such as propane) and zeotropic refrigerant blends of HFCs
and HFOs (R32/R1234ze(E)). Refrigerant mixtures are studied because for some applications
they may be a proper solution. For instance in the air-conditioning industry there are not dropin
pure fluids to replace the high global warming potential (GWP) fluids currently employed
(e.g. R410A). Unfortunately, the design of condensers working with zeotropic mixtures poses
the additional problem to account for the mass transfer resistance that leads to a penalization of
the heat transfer coefficient. Experimental data are necessary for the assessment of predicting
correlations that can be used with these new refrigerants blends
Vaporization of binary and ternary non-azeotropic mixtures inside channels
No full textThis study is aimed at the experimental investigation of vaporization of zeotropic mixtures inside small channels.
In the recent years, the search for alternatives to high-GWP (Global Warming Potential) refrigerants is focused
primarily on the use of natural fluids (hydrocarbons, ammonia, carbon dioxide) and new synthetic refrigerants
having low-GWP. Unfortunately, single-component low-GWP refrigerants cannot cover all the applications
unless some drawbacks, such as flammability, are accepted. A solution may be found using blends of
refrigerants, to satisfy the demand for a wide range of working conditions. In the present paper, the experimental
heat transfer performance of binary and ternary non-azeotropic mixtures during flow boiling is investigated. The
adoption of zeotropic mixtures poses the problem of how to extend the correlations developed for pure fluids.
The additional mass transfer resistance, due to the zeotrope of the mixture, leads to a degradation of the heat
transfer performance, thus models developed for pure fluid vaporization cannot be directly applied. In the
present paper, the contribution of the additional mass transfer resistance is assessed and the corrections needed in
the model are discussed
Two-phase heat transfer performance of ternary mixtures of HFOs and HFCs inside channels
No full textSome blends of low-GWP refrigerants have been developed to satisfy the demand for a wide range of
working conditions and substitute R410A in air-conditioning applications and R404A in refrigeration
applications. The present paper investigates the two-phase heat transfer performance of R455A
(mixture of R1234yf, R32, CO2 at 75.5/21.5/3.0% by mass composition) and R452B (mixture of R32,
R1234yf, R125 at 67.0/26.0/7.0% by mass composition). New experimental heat transfer coefficients
obtained during convective condensation in 1 mm and 8 mm inner diameter channels have been
compared with those of the pure components R1234yf and R32. This allows to analyze the heat transfer
penalization due to the zeotrope of the mixture and to assess available predicting models for
condensation. In order to fully characterize the performance of the two blends, the pressure drop during
adiabatic two-phase flow are measured and compared to predicting models
Transient numerical simulations of condensation in a minichannel compared to flow visualizations
No full textNumerical simulations of vapour-liquid flow by means of the Volume of Fluid (VOF) method are more and more considered an interesting tool to study two-phase flow with and without mass transfer. When dealing with small geometries, experimental investigation can be invasive, affecting the phenomenon itself but it is also very expensive and time demanding. However, validation of the numerical simulations by experiments is still a key point to obtain reliable results. The present paper aims at providing an understanding of the annular condensation process inside minichannels at mass velocities around and below 200 kg m-2 s-1 by means of VOF simulations and flow visualizations. From the flow visualizations performed during downflow condensation of R134a inside a 3.4 mm diameter channel, it can be seen that at 100 kg m-2 s-1 mass velocity, the liquid interface is irregular with the presence of waves. Clearly, steady state simulations cannot account for the presence of waves at the vapour-liquid interface. For this reason, time-dependent numerical simulations have been performed. For computational time saving, a 2-D axisymmetric domain has been considered and the results of transient simulations have been compared with flow visualizations of vertical downflow. The present simulations allow to evaluate the effect of waves on the heat transfer coefficient. The influence of mass flux on the occurrence of interfacial waves has also been investigated
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