Search

Experimental work was carried out to determine the flow pattern map in vertical heated pipes under steady-state and transient conditions, using Freon 12 in forced convective flow as working fluid and optical probes for the measurements Existing maps are based on adiabatic tests, steady-state conditions, and fluids different from Freon 12. Signals from optical probes (whose response is based on the variations in fluid refractive index) are analyzed in terms oflocal void fraction, using either the probability density function (PDF) or the ratio between the average and maximum values of the signal. From the analysis of the experimental measurements the definition of a map for annular and intermittent flow regimes was achieved. The map turned out to be in good agreement with the Weisman and Kang map developed in adiabatic, steady-state conditions Qualitative results for the transient conditions are also presented.

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.

The experimental tests were carried out at atmospheric pressure with an air-water loop, named FLEX. The test section made of Plexiglas. The full vertical test section lengths are 500 and 405 for inner test channel diameters of 20 and 16.2 mm, respectively. The hysteresis effect was studied keeping the gas flow rate constant during the tests, whilst the liquid flow rate was varied

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.

Some heat-transfer characteristics of Freon-12 have been experimentally studied in an electrically heated vertical tube. The 316 stainless steel tube was 0.475 cm i.d. by 0.635 cm o.d. and 197 cm in length. The observations made in this study cover both the wall-temperature behavior in the vicinity of the critical point (π = P/P c ≈ 1, T/T c ≈ 1) and the behavior of the wall-temperature jump through the boiling crises ΔT c as a function of reduced pressure.

The flooding phenomenon limits the stability of liquid-gas countercurrent flow inside a channel. As is known, this entrainment effect can completely prevent the liquid from falling down. A local flow channel cross section reduction, e.g. due to the presence of an obstruction, will affect the flooding parameters, depending on the obstruction location and on the obstruction flow cross section. The present work deals with air-water experiments performed with a transparent circular duct test section, inside which it is possible to insert orifices of different diameters, to test the effect of obstructions on the flooding phenomenon. Predictions obtained by using correlations available in the literature are compared with the experimental data and a method to evaluate the influence of such obstructions is proposed.

A study of direct contact condensation of stagnant saturated and superheated steam on slowly moving subcooled water has been performed with reference to a horizontal flat geometry. Inlet water mass flowrate and temperature together with inlet steam temperature and superheating have been investigated as experimental variables. Their influence on direct contact condensation heat transfer coefficient is shown. A theoretical model for direct contact condensation prediction is proposed, in the frame of the present geometry.

An experiment on direct contact condensation of saturated steam on subcooled liquid jets is performed together with a theoretical analysis of the experimental data. Correlations available in the literature are generally unable to predict both the local conditions of the liquid jet and the total heat transfer. From the analysis of the local and global fluid-dynamics effects, a calculation method is proposed based on an equivalent thermal conductivity of the liquid jet. Comparison with the experimental data looks acceptable and well within the experimental accuracy.

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.

The swirl flow of Freon 12 in a tubular heat exchanger, indirectly heated by forced convection of water in an annular gap, is studied. Two identical situations are compared, with and without a twisted tape, to deduce the influence of the swirl flow in the particularly interesting boundary condition of indirect heating by another fluid (and not simply of uniform Joule heating). The heat transfer is greatly increased by the swirl flow, up to a factor of two at the burnout or dryout point.

Experimental research on highly dispersed, two-phase, turbulent flows was performed employing Freon 12 at different pressures up to the critical value. By means of visualization techniques it was possible to study the droplet size distribution, their spatial concentration, and the relationships among the deduced law of distribution and the main thermo-hydraulic parameters for different thermo-hydraulic conditions (pressure, mass velocity, and quality). Simple relations were developed which make it possible to determine, at least for some reference thermo-hydraulic situations, all of the characteristics of the liquid dispersed phase.

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.

With reference to the interaction between saturated steam and horizontal, subcooled, slowly moving water, a theoretical model of direct contact condensation is presented, for a cylindrical geometry. A good agreement appears from the comparison of predictions with experimental data, as far as both heat transfer coefficient and outlet water temperature are concerned.

A study of direct contact condensation of stagnant saturated steam on slowly moving subcooled water has been performed with reference to a horizontal flat geometry. Inlet water mass flowrate and temperature together with inlet steam temperature have been investigated, as experimental variables, in the following ranges: 1. (a) pressure up to 6 bar, 2. (b) inlet steam temperature up to 160°C 3. (c) inlet water mass flowrate up to 120 kg/h, 4. (d) inlet water temperature up to 70°C, 5. (e) available steam mass flowrate up to 20 kg/h. Condensation heat transfer coefficients have been determined as functions of inlet water mass flowrate, inlet water and steam temperature. Heat transfer coefficient does not show, practically, dependence either on inlet water temperature or inlet steam temperature but only on inlet water mass flowrate. Correlations are given for the Nusselt number, as a function of Reynolds and Prandtl numbers. An evaluation of thermal non-equilibrium degree between the phases is also presented, together with a correlation for its prediction.

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: • - jet's external shape, obtained by means of photographic pictures; • - pressure profiles inside the jet, obtained by means of a movable “Pitot” gauge; • - jet phase's distribution information, obtained by means of X-ray pictures. Jet's 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.

The interaction between saturated or superheated steam in quasi-stagnant conditions and subcooled water in horizontal flow within a rectangular-duct test section has been experimentally investigated and the results are presented. A comparison between experimental results and predictions obtained using available correlations showed their inadequacy for a full description of the phenomenon. A laminar-turbulent mathematical model has been developed for the macroscopic description of the phenomenon and tested with the experimental data. The agreement is acceptable and well within the uncertainty band of the experimental measurements.

Referring to the interaction of superheated steam (practically stagnant) with subcooled water (slowly moving) the results of an experimental research are presented and discussed. As foreseen in a theoretical investigation, the total thermal power (and consequently the total heat transfer coefficient) does not show an appreciable dependence on superheated steam temperature, so it can be practically evaluated by means of available correlations for saturated steam conditions. The direct contact condensation heat transfer coefficient is linked to the overall one and is slightly dependent on the degree of steam superheating, as experimentally confirmed.

Post burnout heat transfer has been investigated comparatively in straight and coiled tubes, in order to study the influence of the geometric parameter. Freon 12 has been employed, up to supercritical pressures. An improved two-phase flow heat transfer has been discovered in coiled than in straight ducts: higher burn-out heat fluxes, smoother and lower wall temperature rises at the dry-out point. In post burnout heat transfer a wall temperature difference between the internal and the external side of the coil (with respect to the helix axis) is set up. This temperature difference remains almost constant with quality downward from the dry-out point, depends on flow centrifugal acceleration and pressure, and remains at moderate values up to supercritical pressures.

Two-phase cross flows have received minor consideration in thermal-hydraulics research, notwithstanding their applications in chemical engineering systems. Many types of heat exchangers and steam g...