M. Cumo

EXPERIMENTAL INVESTIGATION OF HYDRAULIC AND SINGLE-PHASE HEAT TRANSFER IN 0.130-MM CAPILLARY TUBE

The objective of the present study is to investigate the hydraulic characteristics and single-phase thermal behaviour of a capillary tube with internal diameter of 130 w m. As the Reynolds number varies in the range from 100 up to 8,000 in the experiments, and flow regimes from laminar to turbulent are thoroughly investigated. The laminar-to-turbulent flow transition is studied in depth. Experiments show that laminar-to-turbulent flow transition occurs for Reynolds number in the range 1,880-2,480, while heat transfer correlations in laminar and turbulent regimes, developed for conventional tubes, are not adequate for calculation of heat transfer coefficient in microtubes.

Burnout in highly subcooled water flow boiling in small diameter tubes

Abstract The present work deals with the critical heat flux (CHF) in subcooled flow boiling in short tubes. The field of application is in the very high heat flux region (up to 60 MW m−2) of interest to fusion technology (heat removal from divertors), which calls for a knowledge of the heat transfer under very high heat loading conditions. The experimental work was carried out with water at pressures ranging from 0.1 to 2.5 MPa and water velocities from 10 to 40 m s−1, employing stainless steel 2.5 mm i.d. tubes. The heated length was 0.1 m (L/D = 40) and the wall thickness was 0.25 mm. The effects due to variation of thermal hydraulic parameters (velocity, subcooling, pressure) on the heat transfer are presented together with a comparison of the experimental data with existing correlations and theoretical models. The main result achieved in the experiment is the possibility of reaching such high values of the CHF using water in subcooled flow boiling inside smooth tubes. The parameters that seem to be determinant are the level of subcooling of the coolant and its velocity. Considering that other parameters, such as the tube diameter not investigated here, may have an influence on the CHF, it would also seem possible to come to the right compromise—with an optimized choice of parameters—between high values of the CHF and pressure loss involved (high with high velocity and small tube diameter) using this simple cooling technique.

Assessment of correlations and models for the prediction of CHF in water subcooled flow boiling

Abstract The present paper provides an analysis of available correlations and models for the prediction of Critical Heat Flux (CHF) in subcooled flow boiling in the range of interest of fusion reactors thermalhydraulic conditions, i.e. high inlet liquid subcooling and velocity and small channel diameter and length. The aim of the study was to establish the limits of validity of present predictive tools (most of them were proposed with reference to LWR thermal-hydraulic studies) in the above conditions. The reference dataset represents almost all available data (1865 data points) covering wide ranges of operating conditions in the frame of present interest (0.1

Forced convective boiling in binary mixtures

ENEA started in 1990 a new research programme on ‘Thermal Hydraulics Of Mixtures’ (THOM) focused on the heat transfer in forced convective boiling of refrigerant mixtures. The main aim of the research is to accomplish new experimental data on binary mixtures in forced convective boiling and particularly on three main topics: convective boiling heat transfer, hysteresis in nucleate boiling incipience and critical heat flux. A first preliminary step of the research, started in the middle of 1990, consisted in the thermal hydraulic characterization of the pure components of the binary mixture chosen, namely R 12 (CCl2F2) and R 114 (C2Cl2F4). At the end of 1991 (and still in progress) the main investigation on the heat transfer performance of different compositions of an R 12/R 114 mixture started. In the present paper, a review of previous works, together with a description of the first experimental results on heat transfer coefficients is given. Finally a comparison of experimental data with the available correlations, is accomplished.

Enhancement of CHF water subcooled flow boiling in tubes using helically coiled wires

Abstract The present paper reports the results of an experimental investigation about the occurrence of the critical heat flux (CHF) in subcooled flow boiling of water, carried out to ascertain the influence of thermal hydraulic parameters on CHF under conditions typical of thermonuclear fusion divertor thermal hydraulic design. Helically coiled wires were used as turbulence promoters to enhance the CHF with respect to the smooth channel. Geometric characteristics of stainless steel 304 Type test sections were: 6.0 and 8.0 mm i.d., 0.25 mm wall thickness, 0.1 and 0.15 m heated length, horizontal and vertical (upflow) position. Test sections were uniformly heated using d.c. current. A maximum CHF of about 30 MW m −2 was reached with smooth tubes under the following conditions: T in = 30 C, p = 4.6 MPa, u = 10 ms −1 , D = 8.0 mm , L = 0.1 m. Helically coiled wires ( d = 1.0 mm , pitch = 20.0 mm ) allowed an increase of the CHF up to 50%, with reference to smooth channels, coupled with a moderate increase of pressure drop (down to 25%). Pressure revealed a negative effect on the efficiency of turbulence promoters. No observable influence of the channel orientation was detected.

Hypervapotron technique in subcooled flow boiling CHF

Abstract Fusion reactor thermal hydraulics requires suitable techniques for the removal of extremely high heat fluxes, of the order of some tens of megawatts per square meter. One possible technique, the hypervapotron, to enhance the critical heat flux (CHF) in subcooled flow boiling (already characterized by high values of CHF) was studied using water flowing in a horizontal annular test section designed for visualization. A full characterization of the hypervapotron effect as a function of geometry and fluid thermal hydraulic conditions was accomplished by making use of a high-speed videotape. The hypervapotron technique is suitable for the removal of high heat fluxes (up to about 30 MW/m 2 ) wherever high values of fluid velocity and subcooling are not practical. In fact, it is typically employed at low values of liquid velocity and subcooling that in turn directly affect enhancement of the CHF in subcooled flow boiling.

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.

Geometrical effects on the subcooled flow boiling critical heat flux

Abstract The present work deals with the critical heat flux (CHF) under subcooled flow boiling conditions in short tubes. The field of application is in the very high heat flux region (up to 70 MW.m−2), which is of interest in fusion technology. The experimental work was carried out with water at pressures ranging from 0.1 to 0.8 MPa and water velocities from 5 to 50 m.s−1; employing stainless steel tubes with diameters ranging from 0.25 to 4 mm. The heated length varied from 10 to 160 mm (L/D varied from 5 to 40) and the wall thickness varied from 0.125 to 1.195 mm. The effects due to the variation of geometrical parameters (diameter, wall thickness and heated length) on the CHF are presented. The main results achieved in the experiment are: the CHF increases as the diameter decreases until a defined value, after this value the CHF is constant and independent of the channel diameter; the CHF is almost independent of the wall thickness; the CHF increases as the channel diameter decreases, a threshold exists above which such a functional dependence does not appear any longer and the CHF is independent of L/D.

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.