Thermodynamic Analysis of a Multi-Ejector, CO2, Air-To-Water Heat Pump System

Abstract Nowadays, air conditioning systems for residential and office buildings, contribute largely to the energy consumptions and to the direct and indirect emissions of greenhouse gases. Carbon dioxide (CO2) could be an interesting option to replace traditional HFCs in space heating applications, due to its environmentally friendly characteristics: zero ODP and extremely low GWP, but, in order to spread its use, improvements in performances are needed. In fact, CO2 requires transcritical cycles with high expansion losses. The use of an ejector can reduce these losses and improve the performances up to 30% (depending on the performances of the ejector itself and on the operating conditions). In the a/c applications, characterized by variable operating conditions, multi-ejector systems could be used, where some ejectors work in parallel, in different combination, varying the operating conditions. Currently, a project of DTE-PCU-SPCT Department of ENEA and Industrial Engineering Department of Federico II University of Naples, is in progress, in order to evaluate experimentally the effect of several ejectors geometries on the global performance of a CO2 heat pump working with a transcritical cycle. As a part of this project, a complete heat pump system for production of hot water for sanitary use and for space heating is tested to investigate the effect of the ejector size on the balancing of the global performance of the whole system

Experiments of convective evaporation of refrigerant R513A in a horizontal stainless-steel tube

Refrigerant R513A represents an interesting solution for the retrofit of conventional high-GWP fluorinated gases, such as R134a, R401A, R401B and R409A for low and medium temperature applications. R513A is an azeotropic mixture (almost zero-temperature glide at any operating pressure) made up of R134a and R1234yf (44% and 56% in mass, respectively), allowing at the same time a very low GWP of 580 and favourable safety characteristics such as no flammability and no toxicity (A1 ASHRAE class). The boiling performance of this blend is scarcely explored and studied in scientific literature, especially in case of commercial tubes typically adopted for refrigeration purposes. For this reason, this paper presents two-phase flow boiling experiments of refrigerant R513A in a 6.00 mm horizontal stainless-steel tube. Heat is provided by means of Joule effect directly on the tube surface, and the peripheral average heat transfer coefficients are obtained by measuring the temperatures at four sides (top, bottom, left and right) of the channel. The effect of the operating conditions is experimented and discussed, by varying the mass flux between 150 and 300 kg/m2 s, saturation temperature between 20 and 50°C and imposed heat flux between 5 and 20 kW/m2. Also, a comparison with the boiling performance of refrigerant R134a is proposed within the same operating conditions. Finally, the assessment of well-known flow boiling prediction methods is presented and discussed

Flow boiling of azeotropic and non-azeotropic mixtures. Effect of the glide temperature difference on the nucleate boiling contribution: Assessment of methods

Due to the increasing concern about the global warming caused by the use of conventional refrigerants, new HFC/HFO blends are currently proposed to replace high-GWP substances. Most of them, however, present a considerable temperature glide that may negatively affect the nucleate boiling contribution to the heat transfer during evaporation. In this paper, flow boiling data of the new non-azeotropic mixtures R452A and R448A (carrying a high temperature glide of almost 5 °C) and of the conventional quasi-azeotropic blend R404A are provided in a horizontal stainless-steel tube having an internal diameter of 6.0 mm. For all the investigated fluids, the operating conditions explore mass fluxes from 150 to 600 kg/m2s, saturation temperatures from 25 to 55 °C and imposed heat fluxes from 10 to 40 kW/m2, in the whole range of vapor qualities. The nucleative boiling contribution is then isolated from the overall heat transfer coefficient data at disposal and the effect of the heat flux is discussed for both types of blends. Finally, the experimental values and trends are compared to different nucleate boiling correlations taken from literature and conceived for pure fluids, by testing some correction factors explicitly developed for high temperature glide substances

Mosquito saliva enhances virus infection through sialokinin-dependent vascular leakage

Viruses transmitted by Aedes mosquitoes are an increasingly important global cause of disease. Defining common determinants of host susceptibility to this large group of het-erogenous pathogens is key for informing the rational design of panviral medicines. Infection of the vertebrate host with these viruses is enhanced by mosquito saliva, a complex mixture of salivary-gland-derived factors and microbiota. We show that the enhancement of infection by saliva was dependent on vascular function and was independent of most antisaliva immune responses, including salivary microbiota. Instead, the Aedes gene product sialokinin mediated the enhancement of virus infection through a rapid reduction in endothelial barrier integrity. Sialokinin is unique within the insect world as having a vertebrate-like tachykinin sequence and is absent from Anopheles mosquitoes, which are incompetent for most arthropod-borne viruses, whose saliva was not proviral and did not induce similar vascular permeability. Therapeutic strategies targeting sialokinin have the potential to limit disease severity following infection with Aedes mosquito-borne viruses.</p

Behaviour and performances of R502 alternative working fluids in refrigerating plants

This is an experimental study that aims to evaluate both general characteristics and system performances of the most credited fluids that are likely to substitute for R502 in a refrigeration plant. R402A, R402B, R403B and R408A (still containing HCFC) and R404A, R407A and FX40 (chlorine free) have been tested. Generally, both of them have showed performances very close to those of R502 except R403B, whose COP has been found to be about 8% lower than that of R502

The uncertainties in measuring vapour compression plant performances

The metrological features related to an experimental pilot vapour compression plant are presented. The COP (coefficient of performance) of the plant, accomplished to test working fluid candidates to R502 replacement, can be evaluated without employing the new refrigerant's thermodynamic properties, whose reliability is often inadequate. The coefficient of performance value represents the first step for every further evaluation about the suitability of the new substances, therefore in this paper its accuracy is presented. © 1998 Elsevier Science Ltd

Working Fluids Thermodynamic Behavior For Vapor Compression Cycles

A computer code has been employed to evaluate some thermodynamic properties of the most common working fluids for vapor compression plants and to analyze a standard reversed Rankine cycle. A study of the plant irreversibilities has been carried out. The thermodynamic behavior of the following substances: R503, R23, R13, R13B1, R115, R502, R22, R500, R134a, R12, R152a, RC318, R124a, R717, R142b, R114, R21, R11, R113, R114B2, R718 is outlined, for different working conditions, by means of tables and charts

Exergetic and Thermodynamic Comparison of R12 and R134a As Vapor Compression Refrigeration Working Fluids

A software package for both the computation of thermodynamic properties and the analysis of the usual vapour compression plant schemes is a useful tool for air conditioning and refrigeration researchers and manufacturers. At present, it could make an important contribution to the search for CFC substitutes; in fact, comparisons between two or more working fluids could be accomplished more easily. A program code carried out by the authors is presented by means or some demonstrative diagrams referring to a comparison between the 'ozone killer' R12 and its substitute R134a. An R134a exergy-enthalpy chart is also provided