1,721,122 research outputs found
Carbon dioxide as a natural refrigerant
No full textIn the beginnings of mechanical refrigeration, at the end of the nineteenth century, carbon dioxide was one of the first refrigerants to be used in compression-type refrigerating machines, later gaining widespread application mainly onboard refrigerated ships, but common in other sectors of refrigeration as well. It was only immediately after World War II that CO2 was rapidly eclipsed as a refrigerant, due to the advent of the synthesised halogenated working fluids, addressed as safe and ideal refrigerants at that time. Because of the stratospheric Ozone depletion environmental issue, CFC and HCFC working fluids are now in the process of being phased out of use under the Montreal Protocol. The Global Warming environmental issue casts concern over the use of the new HFC fluids as substitute refrigerants, because of their high GWP values, which make them subject to regulations under the Kyoto Protocol. In this mixed situation, CO2 is being revisited as a fully environmentally friendly and safe refrigerant. An intense research activity on its prospective applications is underway in many research establishments in Europe, Japan and North America, and important results have already been reached in exploiting the peculiar characteristics of this high-pressure fluid operated with a transcritical cycle. In some applications CO2 systems have already been commercialised; this applies to heat pump water heaters, as a brine in indirect systems and in the low temperature stage of cascade systems. The paper critically analyses the prospects for the future return of CO2 as a working fluid, or sometimes as a brine with change of phase, in important application areas. These include air conditioning and heat pump systems in the residential and commercial sectors, commercial and transport refrigeration and mobile air conditioning
Heat transfer and pressure drop during condensation of the low GWP refrigerant R1234yf
No full textThe present paper reports local heat transfer coefficients measured during condensation of
R1234yf within a single circular 0.96 mm diameter minichannel and compares them to the
ones of R134a.
This experimental work is carried out in a unique test apparatus which allows to determine
the local heat flux extracted from the condensing fluid from the temperature profile of the
coolant. For this purpose, the temperatures of the coolant and of the wall are measured
along the test section. The saturation temperature is determined from the saturation
pressure which is measured at inlet and outlet of the test channel.
Condensation tests are carried out at mass fluxes ranging between 200 and 1000 kg m2 s1
and the heat transfer coefficients result to be lower as compared to the ones of R134a. Since
the saturation temperature drop directly affects the heat transfer rate, the pressure drop
during adiabatic two-phase flow of R1234yf is also measured and compared to R134a
Thermodynamic properties of eight fluorinated olefins
No full textGroup contribution methods are used to predict the critical temperatures, critical pressures, critical densities, acentric factors, and ideal gas specific heats at constant pressure for eight fluorinated olefins, namely: R-1225ye(E), R-1225ye(Z), R-1225zc, R-1234ye(E), R-1234yf, R-1234ze(E), R-1234ze(Z), and R-1243zf. For the same eight refrigerants, the Peng–Robinson equation of state is used to predict thermodynamic properties, which are presented in pressure-enthalpy and temperature-entropy state diagrams. To provide the reader with some sense of the predictive capability of the methodology, property predictions for R-134a are compared to known data. Property predictions for R-1234yf are compared, where possible, to the open literature
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
Heat transfer coefficient and pressure drop during condensation of the low-GWP refrigerant R1234yf
No full textR1234yf Flow Boiling Heat Transfer Inside a 2.4-mm Microfin Tube
No full textThis paper presents an experimental study on R1234yf flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 2.4 mm. R1234yf is a new refrigerant with an extremely low global warming potential (GWP <1), proposed as a possible substitute for the common R134a, whose GWP is about 1300. The mass flux was varied between 375 and 940 kg m−2 s−1, heat flux from 10 to 50 kW m−2, and vapor quality from 0.1 to 1. The saturation temperature at the inlet of the test section was kept constant and equal to 30°C. The wide range of operative test conditions permitted highlighting the effects of mass flux, heat flux, and vapor quality on the thermal and hydraulic behavior during the flow boiling mechanism inside such a min imicrofin tube. The results show that at low heat flux the phase-change process is mainly controlled by two-phase forced convection, and at high heat flux by nucleate boiling. The two-phase frictional pressure drop increases with increasing both mass velocity and vapor quality. Dry-out was observed only at the highest heat flux, at vapor qualities of around 0.94–0.95
Estimations of the thermodynamic and transport properties of R-1234yf using a cubic equation of state and group contribution methods.
No full textData are provided for temperature, pressure, density, enthalpy, entropy, specific heat at constant pressure, thermal conductivity, viscosity, and surface tension for saturated mixtures and slightly superheated vapors of R-1234yf. The thermodynamic property data are generated using the approach illustrated in Brown (2007a,b,c, 2008a,b) and the transport property data are estimated using the methodology described in Brown et al. (2009). To provide the reader with some sense of the predictive capability of these estimation techniques, data for R-134a generated by them are compared to those of REFPROP 8.0 (Lemmon et al. 2007). Moreover, data generated by these predictive methods for R-1234yf are compared, where possible, to the open literature (e.g., Spatz and Minor 2008)
- …
