Gian Piero Celata

Terminal velocity of single bubbles in surface tension force dominant regime

Abstract Terminal velocity V T of a single bubble rising through an infinite stagnant liquid in surface tension force dominant regime was investigated theoretically and experimentally. A theoretical V T model, which is applicable to a distorted spheroidal bubble with a high bubble Reynolds number, was deduced from a jump condition and a potential flow theory for a flow about an oblate spheroid. Experiments were conducted using air and water to measure bubble trajectories, shapes and velocities. As a result, it was confirmed that (1) the primal cause of widely scattered V T in this regime is not surfactant concentration but initial shape deformation, (2) small initial shape deformation results in a low V T and a high aspect ratio, whereas large initial shape deformation results in a high V T and a low aspect ratio, (3) the primal role of surfactants in this regime is to cause the damping of shape oscillation, by which a contaminated bubble behaves as if it were a clean bubble with low initial shape deformation, and (4) the proposed model gives good predictions of V T for single distorted bubbles.

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

Water Single-Phase Fluid Flow and Heat Transfer in Capillary Tubes

This paper reports the results of an experimental investigation of fluid flow and single-phase heat transfer of water in stainless steel capillary tubes. Three tube diameters are tested: 172 μm, 290 μm and 520 μm, while the Reynolds number varying from 200 up to 6000. Fluid flow experimental results indicate that in laminar flow regime the friction factor is in good agreement with the Hagen-Poiseuille theory for Reynolds number below 800–1000. For higher values of Reynolds number, experimental data depart from the Hagen-Poiseuille law to the side of higher f values. The transition from laminar to turbulent regime occurs for Reynolds number in the range 1800–3000. This transition is found in good agreement with the well known flow transition for rough commercial tubes. Heat transfer experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional size tubes, are not adequate for calculation of heat transfer coefficient in microtubes. In laminar flow the experimental values of heat transfer coefficient are generally higher than those calculated with the classical correlation, while in turbulent flow regime experimental data do not deviate significantly from classical heat transfer correlations. Deviation from classical heat transfer correlations increase as the channel diameter decrease.Copyright

Thermal-hydraulic characteristics of single-phase flow in capillary pipes

The objective of the present paper is to provide a general overview of the research carried out so far in single-phase heat transfer and flow in capillary (micro) pipes. Laminar flow and laminar-to-turbulent flow transition are analyzed in detail in order to clarify the discrepancies among the results obtained by different researchers. Experiments performed in the ENEA laboratory indicate that in laminar flow regime the friction factor is in good agreement with the Hagen–Poiseuille theory for Reynolds number below 600–800. For higher values of Reynolds number, experimental data depart from the Hagen–Poiseuille law to the side of higher f values. The transition from laminar to turbulent regime occurs for Reynolds number in the range 1800–2500. Diabatic experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional tubes, are not properly adequate for heat transfer coefficient prediction in microtubes.

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.

Prediction of the critical heat flux in water subcooled flow boiling using a new mechanistic approach

Abstract A thorough examination of the results of existing models based on the liquid sublayer dryout theory suggested the need to postulate a new mechanism to predict the CHF in subcooled water flow boiling. Considering that we have local boiling with bulk subcooled conditions, there will be a distance from the wall at which the fluid temperature is equal to the saturation value. This distance is called superheated layer, and is the only region where a bubble may exist. Because of the accumulation and condensation of the vapour generated from the heated wall, a thin elongated bubble, called a vapour blanket, is formed, rising along the near-wall region as vertical distorted vapour cylinders. The CHF is postulated to occur when the vapour blanket replenishes the superheated layer, coming into contact with the heated wall (superheated layer vapour replenishment model ) . The vapour blanket thickness, assumed to be equal to the bubble diameter at the wall detachment, is independent of the heat flux, depending on physical properties, thermal-hydraulic and geometric parameters. The superheated layer depends on the heat flux, physical properties, thermal-hydraulic and geometric parameters. The heat flux for which the superheated layer is equal to the vapour blanket thickness will be the CHF. The comparison of new model predictions with fusion reactor relevant data (0.1 ⩽ p ⩽ 8.4 MPa, 0.3 ⩽ D ⩽ 25.4 mm, 0.0025 ⩽ L ⩽ 0.61 m, 1 ⩽ G ⩽ 90 Mg m−2 s−1, 25 ⩽ ΔTsub,in ⩽ 255 K) is pretty good, as more than 85% of the 1968 data are predicted within ±25%, with a standard deviation of ±16.6%. Besides, because of its structure, based on the heat balance method, the model is applicable to both peripheral uniformly and non-uniformly heated channels.

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.