1,721,014 research outputs found
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 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
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
Enhancing thermal comfort: a comprehensive review of wearable cooling systems
No full textExposure to hot environments can induce physiological thermal strain in the human body, leading to reduced working endurance, impaired performance, and an elevated risk of heat-related illnesses. Activities such as sports, military training, and physically demanding work like firefighting can worsen these conditions. The increasing demand for energy-efficient solutions and diverse application requirements has driven the development of wearable cooling systems. These systems offer a localized and efficient alternative to conditioning entire environments, especially for individuals working outdoors or in settings where traditional air conditioning is impractical. This review provides a comprehensive overview of wearable cooling systems, covering their operating principles, designs, testing methodologies, applications, benefits, challenges, and classifications. Wearable cooling systems have been categorized into active, passive, and hybrid types, employing various cooling mechanisms, including air cooling, liquid cooling, vapor-compression cycle cooling, thermoelectric cooling, gas cooling, vacuum desiccant cooling, evaporative cooling, phase change materials, and conductive and radiative textile-based cooling. The review assesses these technologies based on cooling capacity, weight, and operating time, offering a rationale for their selection. Additionally, insights into future research opportunities in wearable cooling systems are discussed, emphasizing the need for continued innovation to enhance thermal comfort and safety
Experimental study of low-GWP refrigerants in a chiller: system performance and compressor efficiencies
No full textCondensation 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
Experimental investigation of large scroll compressors working with six low-GWP refrigerants
Full text linkThis work presents an experimental study on the use of low global-warming-potential refrigerants as drop-in alternatives to R134a in a water-to-water vapor compression system. The refrigerants used in the present investigation are: R1234ze(E), R152a, R516A, R515B, R450A and R513A. The system is composed of two large scroll compressors (swept volume for each compressor equal to 222.5 m3/h) working in parallel. Experimental tests have been carried out at full and partial loads with fixed inlet/outlet water temperatures in the heat exchangers. The use of these new fluids implies the adoption of compressors with large flow rates (because of the low volumetric refrigerating effect), but in the case of scroll compressors very limited data is available for this size and these fluids. The experimental data allow to evaluate the cooling capacity and the energy efficiency ratio (EER) for each refrigerant and compare them to the ones of R134a: R516A and R513A display 3.0% and 4.4% higher cooling capacity, respectively; R152a and R450A can achieve 8.5% and 3.8% higher EER, respectively, with a limited reduction of the cooling capacity; R1234ze(E) and R515B display 22.9% and 24.0% lower cooling capacity, respectively, with a comparable EER. The present experimental data have also been used to investigate the efficiencies of a scroll compressor with large swept volume and to tune two predicting methods: the Pierre (1982) and Navarro et al. (2013) correlations. With regards to the compressor isentropic efficiency, those correlations have been modified from data regression analysis of the present database to provide a useful prediction tool for scroll compressors using low-GWP alternatives to R134a. This is of particular interest considering the lack of data on scroll compressors working with such medium-pressure fluids
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