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

Vacuum gauge from ultrathin MoS2 transistor

We fabricate monolayer MoS2 field effect transistors and study their electric characteristics from 10^-6 Torr to atmospheric air pressure. We show that the threshold voltage of the transistor increases with the growing pressure. Hence, we propose the device as an air pressure sensor, showing that it is particularly suitable as a low power consumption vacuum gauge. The device functions on pressure-dependent O2, N2 and H2O molecule adsorption that affect the n-doping of the MoS2 channel.Comment: 10 pages, 4 figure - conference pape

Gene pathway development in human epicardial adipose tissue during early life

Studies in rodents and newborn humans demonstrate the influence of brown adipose tissue (BAT) in temperature control and energy balance and a critical role in the regulation of body weight. Here, we obtained samples of epicardial adipose tissue (EAT) from neonates, infants, and children in order to evaluate changes in their transcriptional landscape by applying a systems biology approach. Surprisingly, these analyses revealed that the transition to infancy is a critical stage for changes in the morphology of EAT and is reflected in unique gene expression patterns of a substantial proportion of thermogenic gene transcripts (~10%). Our results also indicated that the pattern of gene expression represents a distinct developmental stage, even after the rebound in abundance of thermogenic genes in later childhood. Using weighted gene coexpression network analyses, we found precise anthropometric-specific correlations with changes in gene expression and the decline of thermogenic capacity within EAT. In addition, these results indicate a sequential order of transcriptional events affecting cellular pathways, which could potentially explain the variation in the amount, or activity, of BAT in adulthood. Together, these results provide a resource to elucidate gene regulatory mechanisms underlying the progressive development of BAT during early life

Notulae to the Italian alien vascular flora: 17

In this contribution, new data concerning the distribution of vascular flora alien to Italy are presented. It includes new records and status changes from casual to naturalized for Italy or for Italian administrative regions for taxa in the genera Callianthe, Chamaecyparis, Chamaeiris, Cotoneaster, Erigeron, Freesia, Hemerocallis, Juglans, Kalanchoë, Ludwigia, Nassella, Paulownia, Physocarpus, Pistia, Saccharum, Setaria, and Vachellia. Nomenclatural and distribution updates, published elsewhere, and corrections are provided as supplementary material

Combined effects of refrigerant leakages and fouling on air-source heat pump performances in cooling mode

For electric heat pump systems (EHP), soft faults such as refrigerant leakages and heat exchangers fouling can lead to significant performance degradations, which can remain unidentified for a long period. This paper, through a digital model, wants to simulate the cooling season behavior of a case study air-source heat pump for residential air-conditioning, analyzing the performance degradation in case of both standalone and simultaneous occurrence of soft faults. The study is carried-out in different scenarios of not planned and ordinary maintenance of the machine and in typical climate conditions of Naples, Miami and Shanghai. The impact of faults on seasonal performances (SCOP) and total equivalent warming impact (TEWI) has also been analyzed. Results show that refrigerant leakages can have an impact on system performance higher than 25%, whereas heat exchangers fouling can downgrade the efficiency of approximately 15%. It is also found that, for each climate condition, a different maintenance strategy can halve the cases with SCOP penalization higher than 10%. Since faults have not the same probability of occurrence, a smart monitoring and a fault detection system can be the solution to guarantee good performances avoiding at the same time excessive maintenance costs

Soft faults in residential heat pumps: Possibility of evaluation via on-field measurements and related degradation of performance

The growth of Internet of Things (IoT) technologies is leading to new opportunities in the heat pump sector for residential air conditioning, in which fault detection and diagnosis (FDD) techniques are being developed from measured data to better manage maintenance operations. This paper presents a physics-based digital model simulating the effects of soft faults (refrigerant leakages and condenser/evaporator fouling) concurring with different intensities on a residential heat pump system. Firstly, their effect on cooling capacity and COP has been analysed. Then, each thermodynamic cycle (as a sequence of measured pressures and temperatures) uniquely related to a peculiar combination of fault intensities is determined. In this way, the field of faults which can be uniquely diagnosed and evaluated has been calculated, and the interrelationship between this field and the uncertainty of the measurements has been demonstrated. The results are encouraging, since common instruments available on the market may detect penalizations of the energetic performance as small as 5%

Multiwalled Carbon Nanotubes Films for Sensing Purpose

Films of multiwalled carbon nanotubes (MWCNTs) are proposed as the sensing element of low-cost sensors for the detection of temperature, pressure and water droplets. Liquid solutions of functionalized MWCNTs are vacuum filtered to produce freestanding films of randomly oriented MWCNTs, known as buckypapers, which are patterned in strips of several mm size. The electrical conduction of the buckypaper is highly sensitive to the environmental conditions. It increases for rising temperature or when a pressure is applied; conversely, it is decreased under tensile strain or by exposure to water droplets. The experimental data presented in this work confirm the suitability of buckypapers for multipurpose sensors able to detect different physical quantities simultaneously

Flow boiling heat transfer and pressure drop data of non-azeotropic mixture R455A in a horizontal 6.0 mm stainless-steel tube

This paper carries out an experimental investigation on R455A, a low GWP (148) non azeotropic mixture, to determine its performance in terms of flow boiling heat transfer coefficient and pressure drop in a smooth circular horizontal stainless-steel tube, with an internal diameter of 6 mm. The effect of the operating parameters, such as mass flux (from 300 to 500 kg‧m−2‧s−1), bubble saturation temperature (from 10 to 40 °C), heat flux (from 5 to 40 kW‧m−2) and vapor quality (between 0 and 1) is analyzed. Results show an increase of the heat transfer coefficient with mass and heat flux, while it decreases with saturation temperature. On the other hand, pressure drop increases with mass flux, and decreases with bubble saturation temperature. An assessment of several predictive methods from literature is finally conducted. Two-phase heat transfer methods based on an exclusive convective contribution can predict quite well the experimental results, while all other methods improve their statistics once a glide-related modification on the nucleate boiling contribution is assumed. The fictional pressure gradient results are well fitted with the separated flow model of Müller Steinhagen and Heck and with the Cicchitti two-phase viscosity expression considering a homogeneous flow approach

Flow pattern, condensation and boiling inside and outside smooth and enhanced surfaces of propane (R290). State of the art review

A comprehensive review of two-phase flow characteristics, boiling heat transfer and condensation outside and inside tubes of propane (R290) is presented in this paper. The first section focuses on the global heat transfer performances in heat exchangers, with a collection of 11 works dealing with boiling and condensation of propane in fin and tubes, shell and tubes, plate heat exchangers and complex geometries. The second section reviews local analyses on fluid flow (8 works) and heat transfer characteristics of propane during flow boiling (23 works), pool boiling (10 works), condensation inside (13 works) and outside (2 works) single and multi-channel geometries. The critical analysis of the collected papers has the objective to provide useful information regarding the components performance for those involved in the assembly of refrigeration plants, and also to offer a critical summary on local heat transfer and fluid flow behavior of propane, particularly useful during the processes of heat exchangers design and manufacturing

Experimental thermal and hydraulic characterization of R448A and comparison with R404A during flow boiling

Refrigerant R404A is going to be banned in Europe for different applications due to its very high GWP of 3943. R448A is a suitable alternative (GWP of 1390) among other new mixtures, and is thus receiving considerable attention in literature. However, despite several overall system studies, there is a lack of flow boiling data for this new blend. This paper therefore presents an experimental investigation on two-phase heat transfer and pressure drop of R448A in a single horizontal stainless steel (AISI 316) tube with an internal diameter of 6.0 mm. The effect of the operating parameters is firstly analyzed for the whole range of vapor qualities, by changing the mass flux from 146 to 601 kg/m2 s and the bubble saturation temperature from 23.3 to 56 °C (reduced pressures from 0.266 to 0.554). For the heat transfer coefficient measurements, the imposed heat flux is also varied from 2.5 to 40 kW/m2, leading to a total amount of 460 data points. Several pressure drop and heat transfer coefficient data are then compared to those of conventional refrigerant mixture R404A at the same operating conditions, showing different relative importance of nucleate and convective boiling contributions. Finally, the agreement between the R448A experimental database and some two-phase boiling heat transfer coefficient and frictional pressure drop prediction methods is evaluated with a statistical analysis. The best results are respectively obtained with the correlations of Gungor and Winterton and of Friedel