F. D'Annibale

Direct contact condensation of steam on droplets

Abstract An experiment of direct contact condensation of saturated steam on subcooled water sprays characterized by droplets of uniform size has been carried out with the aim of testing the influence of droplet diameter and velocity on the heat transfer rate, up to a pressure of 0.6 MPa. Liquid sprays with a uniform distribution of droplet diameters (in the range 0.3–2.8 mm) were obtained by means of an ad hoc injection system based on the superposition of a high frequency acoustic vibration in the liquid. Continuous measurements of the average droplet temperature along the axis of the spray jet were performed. The condensation efficiency and local heat transfer coefficient were calculated as functions of the main parameters involved (droplet diameter and velocity, thermodynamic condition of the fluids). Comparisons of experimental results with predictions obtained using available models are reported. A method allowing a better data reduction, based on consideration of the turbulence inside the droplet, is proposed.

CHF behaviour during pressure, power and/or flow rate simultaneous variations

Abstract The results of an experimental investigation are presented on critical heat flux in forced convective flow boiling during transients caused by simultaneous variations of either two or three parameters among pressure, flow rate and thermal power. The three parameters are varied according to an exponential law for the flow rate and the pressure decrease, and to a ramp and a step law for the input power increase. Experiments are carried out employing a tubular test section which is electrically and uniformly heated. Test parameters include the flow rate half-flow decay time, several values of the initial power (before the transient) and the final power (at the end of the transient) in the case of step transients, and the slope of the ramp in the case of ramp transients, and the depressurization rate. An analysis of the experimental data is performed using the local conditions approach, and applying the quasi-steady-state method. The effect of the simultaneous variation of either two or three main parameters on the time-to-crisis is also analysed for transients in which only one of the parameters is varied.

Critical heat flux in transient flow boiling during simultaneous variations in flow rate and thermal power

Abstract The results of an experimental investigation on critical heat flux in forced convective flow during transients caused by simultaneous variations of flow rate and thermal power are reported. The two parameters were varied according to an exponential law for the flow rate decrease and according to a ramp and a step law for the input power increase. Experiments were carried out with a tubular test section that was electrically and uniformly heated. Test parameters included the flow rate, half-flow decay time, several values of the initial power (before the transient) and final power (at the end of the transient) in the case of step transients, and the slope of the ramp in the case of ramp transients. The pressure was kept constant during the transients. An analysis of the experimental data was conducted on the basis of the local conditions approach and applying the quasi-steady-state method. The effect of the simultaneous variation of mass flow rate and power on the time to crisis was also analyzed with respect to transients in which only one of the two parameters was varied.

A data set of critical heat flux of boiling R-12 in uniformly heated vertical tubes under transient conditions

Abstract A critical heat flux (CHF) data set under various transient conditions employing R-12 (Freon 12 or CCl2F2) as the test fluid is provided. Transients were carried out by varying one, two, or three parameters among flow rate, pressure, and thermal power, taking care to investigate all possible combinations. Other parameters were kept constant during the test. Reference steady-state CHF tests were conducted over a wide experimental range, and results are presented here. The employed test section is a stainless steel tube (7.72 mm I.D., 8.52 mm O.D., 2.3 m long) heated by direct passage of a dc current.

The influence of non-condensible gas on two-phase critical flow

The present paper deals with a two-phase steam-water critical flow experiment in long tubes, in which known air flow rates are injected into the stagnation region. The aim of the experiment is to detect the influence of non-condensible gas on the two-phase critical mass flux as well as to establish the limit, in terms of air concentration, beyond which the critical flow is affected by the presence of the gas. The test section is a vertical, circular channel with i.d. 4.6 mm and a length of 1500 mm (L/D = 325). Results of experiments with initially subcooled liquid (together with some data from saturated liquid discharges), up to pressures of 1.5 MPa are reported together with the analysis of the effects of the non-condensibles under the different stagnation conditions.

Two-phase flow models in unbounded two-phase critical flows

Abstract Referring to a Loss-of-Coolant Accident situation in LWRs, an analysis of the two-phase region just downstream from the broken pipe, in which a two-phase critical flow takes place, has been performed. A characterization of the flow pattern inside the unbounded two-phase jet is given considering: • - jets external shape, obtained by means of photographic pictures; • - pressure profiles inside the jet, obtained by means of a movable “Pitot” gauge; • - jet phases distribution information, obtained by means of X-ray pictures. Jets X-ray pictures show the existence of a central high-density “core” gradually evaporating all around, which gives place to a characteristic “dart flow” the length of which depends on stagnation thermodynamic conditions.

Development and realization of lithium-ion battery modules for starting applications and traction of off-road electric vehicles

The paper describes the development and realization of standard battery modules 12 V, made by LiFePO4 cells selected in a previous study by ENEA and the University of Pisa. Module means the group of four cells series connected, the electronic battery management system, the thermal management system and the mechanical case. Standard means that the same battery module can be used for different applications: in fact, the previous study showed that three standard battery modules, 30 Ah (little size), 60 Ah (medium size) and 100 Ah (large size), are sufficient to reach the levels of voltage/capacity requested by the most part of the applications in the field of the starting/auxiliary supply batteries (also for the nautical sector) and traction of off-road electric vehicles. More units of standard modules can be series/parallel connected to build complete battery systems able to satisfy the required performances. The development and realization of the modules mostly consisted of testing the selected cells to verify their suitability for the above mentioned applications, to make a thermal battery characterization so to define the thermal management system, to develop an electronic battery management system and to build a mechanical case. The paper shows all these aspects in detail.

Qualitative thermal characterization and cooling of lithium batteries for electric vehicles

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.