Giuseppe Zummo

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.

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.

A Mechanistic Model for the Prediction of Water-Subcooled-Flow-Boiling Critical Heat Flux at High Liquid Velocity and Subcooling

A new model is presented for the prediction of the critical heat flux (CHF) of subcooled flow boiling based on a liquid-sublayer dryout mechanism, i.e., the dryout of a thin, liquid layer beneath an intermittent vapor blanket due to the coalescence of small bubbles. The model focuses on the analysis of the CHF in subcooled flow boiling under conditions of very high mass flux and liquid subcooling, typical of fusion reactor thermal-hydraulic design, and is characterized by the absence of empirical constants always present in earlier models. Peripheral nonuniform heating and/or twisted-tape inserts are accounted for in the model, which was originally developed for uniform heating and straight flow. The simultaneous occurrence of the two events is also well predicted by the model. Although initially formulated for operating conditions typical of the thermal-hydraulic design of fusion reactor high-heat-flux components, the model is proven to be able to satisfactorily predict the CHF under more general conditions, provided local thermodynamic conditions of the bulk flow at the CHF are sufficiently far from the saturated state. 60 refs., 11 figs.

Burnout in subcooled flow boiling of water. A visual experimental study

Abstract The objective of the present work is to perform a photographic study of the burnout in highly subcooled flow boiling, in order to provide a qualitative description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), subcooled flow boiling and thermal crisis. In particular, the flow visualisation is focused on the phenomena occurring on the heated wall during the thermal crisis up to the physical burnout of the heater. Vapour bubble parameters are measured from flow images recorded, while the wall temperature is measured with an indirect method, by recording the heater elongation during all flow regimes studied. The combination of bubble parameters and wall temperature measurements as well as direct observations of the flow pattern, for all flow regimes, are collected in graphs which provide a useful global point of view of boiling phenomena, especially during boiling crisis. Under these conditions, a detailed analysis of the mechanisms leading to the critical heat flux is reported, and the so called events sequence, from thermal crisis occurrence up to heater burnout, is illustrated.

Visual analysis of flow boiling at different gravity levels in 4.0 mm tube

The aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed during the parabolic flight campaign of October-November 2013. The paper will show the analysis of differences between the heat transfer coefficients and vapour bubble parameters at normal and at zero gravity. The results of 4.0 mm tube are presented and discussed. With respect to terrestrial gravity, heat transfer is systematically lower at microgravity in the range of the experimental conditions. Heat transfer differences for the two gravity conditions are related to the different bubble size in each of them. The size of a bubble in flow boiling is affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heated wall. Vapour bubble parameters (bubble diameter, bubble length, width, and nose velocity) have been measured.

Flow Boiling in a Microtube: Flow Pattern and Heat Transfer

This paper presents the results of the flow boiling patterns of FC-72 in a microtube. The internal diameter of the tube is 0.48 mm, with a heated length of 73 mm. The mass flow rate varies from 50 to 3000 kg/m2 -s. The microtube is made of Pyrex in order to obtain the visualisation of the flow pattern along the heated channel. Different types of flow pattern have been observed: bubbly flow, deformed bubbly flow, bubbly/slug flow, slug flow, slug/annular flow, and annular flow. The flow pattern map is compared with those obtained for larger tubes (2.0, 4.0, and 6.0 mm). Flow patterns in the microtube, present less chaotic behaviour and regular vapour-liquid interfaces. Besides, as the tube diameter decreases, the intermittent flow regime shifts from the saturated boiling region towards the subcooled boiling region. The experiments, in the microtube, show the presence of flow instabilities in a large portion of the tests at low mass flow rates and low subcooling. Flow patterns in presence of flow instabilities are mainly characterized by bubbly/slug flow and slug/annular flow. Heat transfer rates have been studied in all flow pattern conditions. The two groups of results, with flow instabilities and without flow instabilities, show similar heat transfer behaviour. The experimental results of flow pattern are compared with the flow pattern maps of McQuillan and Whalley (1985), Mishima and Ishii (1984), and Ong and Thome (2010).Copyright

Experimental study of thermal crisis in connection with Tokamak reactor high heat flux components

The results of an experimental research on high heat flux thermal crisis in forced convective subcooled water flow, under operative conditions of interest to the thermal-hydraulic design of TOKAMAK fusion reactors, are here reported. These experiments, carried out in the framework of a collaboration between the Nuclear Engineering Department of Palermo University and the National Institute of Thermal - Fluid Dynamics of the ENEA - Casaccia (Rome), were performed on the STAF (Scambio Termico Alti Flussi) water loop and consisted, essentially, in a high speed photographic study which enabled focusing several information on bubble characteristics and flow patterns taking place during the burnout phenomenology.

Visual Investigation of Boiling Phenomena in CHF Subcooled Flow Boiling

The present work collects the main results obtained in an experimental research carried out at the laboratories of the National Institute of Thermal-Fluid Dynamics of ENEA. A photographic study of the burnout in highly subcooled flow boiling has been performed, in order to get a detailed description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), subcooled flow boiling and thermal crisis. In particular, the flow visualisation is focused on the phenomena occurring on the heated wall during the thermal crisis up to the physical burnout of the heater.