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The evaluation of the liquid evaporating flow rate in heat pipe arteries (i.e.Transfer heat flux) can be improved if the flow behavior inside these channels of very small sizes is better studied. In this paper, experimental tests performed in small square section open channel (4 mm side), in horizontal configuration, aiming at visualizing and analyzing the steady evaporating flow are presented. The channel size is not as small as the usual size used for heat pipe, and the characteristic length (width of the groove) is then larger than the water capillary length (about 2.7 mm), thus gravity force being predominant on capillary force. For this reason, although present results will not add specific and newer details about the liquid flow in the arteries (microchannels) of the heat pipes, they in any case extend the already available experimental data concerning the performance evaluation in small open ducts. In the initial cold conditions of the tests, before heating, no driving force acts and liquid fills the channel at a certain level without flowing in it. When evaporation starts, the liquid begins to flow both by driving capillary and gravity pressure axial gradient inside the channel. Then, according to the established steady evaporation rate, the liquid differently flows and fills the channel sections. The apparatus and the visualization devices used in these tests allow to take pictures of the gas-liquid interface (meniscus) at several positions along the channel for different steady evaporating rate (supplied heat flux). Images are processed using an appropriate software, ĝ€ImageJĝ€and the interfacial meniscus radius and liquid height along the channel are measured. The values of these two parameters allow the estimation of the liquid filling rate (fluid saturation), the capillary and gravity pressure gradients and the mean axial liquid speed in the channel. So, although gravity effects are present in these experiences, the above measured values and the maximum sustainable heat flux, are compared to the predictions of the analytical and numerical solutions performed in the study of R.H. Nilson, S.W. Tchikanda et al., in the case of no driven gravity pressure gradient (Gĝ- Combining double low line 0). © 2016 Elsevier Masson SAS. Visualization with high-speed camera is used to analyze liquid flow behavior in an open channel.Liquid meniscus is obtained from image processing of visualization.Evaporation rate in the evaporator is evaluated.Comparison between experimental data and available models is accomplished. © 2016 Elsevier Masson SAS.

Due to the important role of microscale heat transfer, an analysis of the heat transfer coefficient for a 1 mm inner diameter tube, with FC-72 as working fluid, has been performed. The study is aimed to identify the best correlation or model to predict the available experimental database. A comparison of several models and correlations, available in the literature for micro- and macroscale, has been performed, focusing the present preliminary analysis to the saturated boiling conditions. Experimental data have been provided by ENEA through the facility BO.E.MI.A. (BOiling Experiments in MIcrochannel Apparatus), in the pressure range from 3 to 5 bar, with a mass flux from 800 to 1200 kg/m 2 s and thermal fluxes from 1.6 to 181 KW/m 2 . The best results, in this preliminary analysis of the saturated boiling points, were obtained for microscale correlations of Li and Wu, with more than 91% of data within ± 30% error and a mean absolute percent error (MAPE) of 13.4%. Among the macroscale correlations only the Chen correlation provides good results, but with a lower agreement with the experimental data. Theoretical models are very promising but need further works to find the appropriate parameters valid for the specific fluid. A more detailed analysis including subcooling conditions will be performed in a future work.

In the present work an analysis of experimental data obtained exposing metallic targets to the flow of a number of nanofluids has been carried out. The investigated suspensions were composed of typical nanometer-sized solid particles, such as TiO2, Al2O3, SiC, and ZrO2 at different concentrations, suspended in different base fluids. The effects of the flow of these nano-materials on three reference targets made of commercial metals (aluminum, copper, and stainless steel) have been assessed. An accurate analysis of the results showed that, where present, the observed damages are caused by chemical corrosion rather than by mechanical erosion, the pH of the suspension being the most important parameter. Conversely, no influence of the particles' nature and content was found, even at high concentrations of nanoparticles (up to 20% by weight). This is believed to be a useful information about the practical feasibility of advanced systems, like in heat transfer applications, based on the use of these relatively new fluids. © 2015 The Institution of Chemical Engineers.

Knowledge about CHF is very important to deal with high heat flux for nuclear engineering and electronic equipment. Larger diameter and length of the channels are used for nuclear engineering because of large mass inventory. However, in recent times, the microchannels have been developed and the mass inventory inside the channels is usually very small. Single channel is seldom used, whereas multiple numbers of parallel channels are more often used. CHF increases with mass flux, channel hydraulic diameter for saturated boiling (and inversely for subcooled boiling), length-to-diameter ratio of the channel, saturation temperature and degree of subcooling at channel inlet. Modern machining techniques are now-a-days being routinely used for the fabrication of microchannels. Research with microchannels are using high speed photography with relatively high magnification to capture the details of flow regimes, bubble growth and measuring liquid film thickness. However, widely applicable correlations are needed to predict data accurately. The microchannel heat transport phenomena are still far from being clearly understood. More accurate measuring techniques are necessary. More and more finer models are necessary to predict CHF accurately. © 2015, Springer International Publishing Switzerland.

The continuously increasing power involved in many applications, coupled with the very small size of a number of component devices, is pushing the technical community to look for more efficient heat transfer systems, to remove the heat generated and keep the system under controlled operating conditions. In particular, significant interest has been devoted to the use of the so-called nanofluids, obtained by suspending nano-sized particles in conventional heat transfer liquids. According to some literature, these suspensions present enhanced heat transfer capabilities, without the inconveniencies of particles settlement and clogging of the channels encountered using larger particles. However, other results show that the actual improvement in the heat transfer efficiency may depend on the adopted working conditions and on the reference parameters (fluid velocity, Reynolds number, pressure drop, etc.) assumed to compare the performances of the nanoparticles suspensions with those of the clear thermal fluid. In the present work heat transfer experiments were carried out on a number of nanofluids systems, varying the type and the concentration of the nanoparticles, and the fluid dynamic regime. The investigated suspensions gave rise to heat transfer coefficients different from those of their respective clear thermal fluid, the thermal efficiency being higher or lower, depending on the fluid dynamic parameter used as a base for comparing the systems. Generally speaking, in most cases nanofluids may give an advantage from the heat transfer point of view only when the conditions are unfavorable for the traditional thermal fluids. Copyright © 2015, AIDIC Servizi S.r.l.

[No abstract available]

The critical heat flux (CHF) condition is characterized by a sharp reduction of the local heat transfer coefficient as a result of the replacement of liquid by vapor adjacent to the heat transfer surface [1]. The CHF condition in flow boiling can be of different nature [1–5]. At low vapor quality, it is associated with subcooled boiling or saturated boiling and high heat. However, at medium or high quality, it is the dryout and there is no liquid film on the tube wall. Usually this is in case of annular flow and due to surface wave instabilities or entrainment and vaporization. © 2015, Springer International Publishing Switzerland.

The paper deals with the cooling of batteries. The first step was the thermal characterization of a single cell of the module, which consists in the detection of the thermal field by means of thermographic tests during electric charging and discharging. The purpose was to identify possible critical hot points and to evaluate the cooling demand during the normal operation of an electric car. After that, a study on the optimal configuration to obtain the flattening of the temperature profile and to avoid hot points was executed. An experimental plant for cooling capacity evaluation of the batteries, using air as cooling fluid, was realized in our laboratory in ENEA Casaccia. The plant is designed to allow testing at different flow rate and temperatures of the cooling air, useful for the assessment of operative thermal limits in different working conditions. Another experimental facility was built to evaluate the thermal behaviour changes with water as cooling fluid. Experimental tests were carried out on the LiFePO4 batteries, under different electric working conditions using the two loops. In the future, different type of batteries will be tested and the influence of various parameters on the heat transfer will be assessed for possible optimal operative solutions.