1,721,040 research outputs found
Numerical modelling of a PV-T collector working as evaporator in a solar assisted heat pump
Full text linkA numerical study of a PV-T collector working as an evaporator for a solar assisted heat pump is presented. The work has been realized in the framework of the SolairHP project for the study of a reversible dual source heat pump. The main objective is to increase the seasonal coefficient of performance of the heat pump compared to an air source heat pump and reduce the overall electrical consumption by including a solar collector-evaporator. A mathematical steady state model of the solar evaporator device has been realized in Matlab environment: it considers the energy and mass fluxes in three dimensions by working a discretization of the main surfaces of the PV sandwich and of the absorber plate (roll-bond plate). The input of the model are the ambient conditions (irradiance components and air temperature), the thermodynamic conditions of the refrigerant at the inlet (enthalpy and mass flowrate) and the circuitation. The main results of the model are the electric and thermal efficiency, the conditions of the refrigerant all along the channels and the temperature distribution on the PV-T surfaces. The numerical model has been used in the design procedure of the solar evaporators to be coupled to a 8 kW heat pump working with R32. A preliminary seasonal analysis on the PV-T evaporator has been realized in order to evaluate the advantage of the dual source system
Modello Monodimensionale di Previsione delle Prestazioni Fuori Progetto di una Turbina Radiale
No full textExperimental and numerical study of an evaporator with integrated latent heat storage for a compressed air dryer
No full textIn the current work, a latent heat storage system with organic phase change material is integrated in a plate-and-fin evaporator for air dehumidification, embedded in a refrigerated compressed air dryer. The effect of the phase change material type, evaporator geometry and dew point range on the fluctuations of the air temperature at the evaporator outlet and on the compressor cycle duration is experimentally investigated at partial loads. The addition of a paraffin phase change material allows to keep the air dew point temperature variations within the desired range at partial loads and to cut the energy consumption at the compressor up to 40% at 30% partial load. A two-dimensional model for simulating the dynamic behaviour of the plate-and-fin evaporator with latent heat storage is developed in Matlab®. The model aims at predicting the temporal variations of the dew point and phase change material temperatures inside the evaporator and the compressor ON/OFF cycle durations. The numerical results are compared with the experimental data, showing that the model is able to reproduce the performance of the evaporator at partial loads. A parametric study is carried out to analize the effect of the evaporator geometry and type of phase change material on the system performance
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
Experimental measurements on the effect of a modulating ejector in a transcritical CO2 system
No full textCOMPARATIVE ANALYSIS OF TWO-PHASE THERMOSYPHONS SOLAR COLLECTORS
No full textThis paper deals with two-phase thermosyphons used in glazed flat plate solar collectors. It analyses the performance of thermosyphons solar collectors by means of a steady-state mathematical model based on the lumped capacitance method. The model simulates the thermal behaviour of a single tube and of the whole collector at various operating conditions.
The present mathematical model has been validated experimentally using data taken on a copper thermosyphon. A test section has been set up, which is made of a single ended copper thermosyphon, where the input power at the evaporator is supplied by an electrical heater.
The validated model is finally applied to an analysis of two-phase thermosyphons solar collectors with variable configuration
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
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