G.E. Farello

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.

Direct contact condensation of steam on slowly moving water

Abstract 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.

A comprehensive analysis of direct contact condensation of saturated steam on subcooled liquid jets

Abstract 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.

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.

HYSTERESIS EFFECT IN FLOODING

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

Flow pattern recognition in heated vertical channels: Steady and transient conditions

Abstract 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.

The influence of flow obstructions on the flooding phenomenon in vertical channels

Abstract 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 theoretical and experimental study of direct-contact condensation on water in turbulent flow

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.

Direct contact condensation of superheated steam on water

Abstract 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.