G.P. Celata

Rationalization of existing mechanistic models for the prediction of water subcooled flow boiling critical heat flux

Abstract This paper presents an analysis of the critical heat flux (CHF) of subcooled flow boiling based on the liquid sublayer dryout mechanism, i.e. the dryout of a thin liquid layer beneath an intermittent vapour blanket due to the coalescence of small bubbles. Starting from the same basic mechanism adopted in earlier models, a new model is derived for the analysis of the CHF in subcooled flow boiling under conditions of very high mass flux and liquid subcooling, typical of fusion reactors thermal hydraulic design. The model is characterized by the absence of empirical constants always present in earlier models. Predicted CHF values are compared with data of 1888 data points for water, showing a good agreement both in precision and in accuracy.

Critical heat flux prediction for saturated flow boiling of water in vertical tubes

Abstract This study presents a new analytical model for the prediction of the critical heat flux (CHF) in water saturated flow boiling in round vertical and uniformly heated pipes. The CHF is assumed to occur in annular flow when the liquid film vanishes at the exit section of the heated channel. Channel pressure drop is calculated using the Friedel correlation. Liquid film flow rate is obtained by a balance of liquid entrainment and droplet deposition. Two mechanisms are considered in the calculation of the entrainment: liquid–vapour interfacial waves and boiling in the liquid film. The model CHF prediction values are compared with a data set of 5159 selected experimental points for water, showing a good agreement both in precision and in accuracy for liquid quality in the range 0.2–1.0.

Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcooling

The knowledge of the onset of subcooled boiling in water forced convective flow at high liquid velocity and subcooling is of importance in thermal hydraulic studies of high heat flux components in fusion reactors. The present paper reports the results of an experimental research on the onset of subcooled boiling in water forced convective flow. From the measurement of the pressure drop along the heated test channel (D = 8 mm, L = 100 mm) it is possible to evaluate the heat flux at which the subcooled boiling occurs. As far as the coolant is in single-phase flow, it is possible to simply calculate its pressure drop, also taking into account the temperature effect on the friction factor, for the temperature variation along the heated channel and in the cross section (due to the steep thermal gradients). The classical corrections available in the literature have been used. Once bubble formation is established, the additional, significant contribution of the bubbles presence to the pressure drop leads to a deviation of the experimental pressure drop curve from the single-phase theoretical line. This latter identifies the onset of subcooled boiling. A comparison of the experimental heat flux at the subcooled boiling incipience with that provided by the major correlations available in the literature is given. A further comparison with measurements performed with an accelerometer device is provided in the paper. The accelerometer detects the additional noise due to bubbles formation close to the heated wall and subsequent collapse in the subcooled bulk of the liquid. The results given by the accelerometer and those obtained with the present evaluation method are in close agreement.

The prediction of the critical heat flux in water-subcooled flow boiling

Abstract The prediction of water-subcooled flow boiling critical heat flux (CHF) in peripherally non-uniform heated tubes with or without swirl flow promoters is accomplished using a model based on the liquid sublayer dryout mechanism recently proposed by the authors. Peripheral nonuniform heating and/or twisted-tape inserts are properly and simply accounted for in the model, originally developed for uniform heating and straight flow. 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 proved to give a satisfactory answer for the prediction of the CHF under more general conditions, provided local thermodynamic conditions of the bulk flow at the CHF are sufficiently far from the saturated state.

Thermal hydraulic characterization of stainless steel wicks for heat pipe applications

Heat pipe design and manufacturing require the knowledge of the thermal hydraulic performance of the wicks. The aim of the present work is the thermal hydraulic characterization of stainless steel wicks (sintered porous media and gauzes) to be employed in our experimental water heat pipe. Commercial sintered porous media (able to capture 90 % of 90 μm particles and 99.9 % of 130 μm particles) and gauzes (nominal wire size 0.11 mm, square mesh opening 0.209 mm) have been used. Thermal hydraulic characterization of the wicks is obtained through the experimental measurement of: capillary height (through which the equivalent porous radius can be evaluated), liquid hydraulic head (through which the liquid pressure drop in the wick is evaluated) wick permeability is also evaluated from the hydraulic head (through Darcys law), heat flux, wick mass flow rate during the evaporation (through which, from the knowledge of other measured wick parameters, the wick two-phase pressure drop is calculated) and wick porosity (through which the thermal conductivity of the wick saturated with liquid can be determined). Concerning the heat flux, it is found to be dependent on the distance between the liquid level and the evaporating zone, the evaporating zone length, the wall superheat and the water subcooling, the contact between the heater and the wick and the superficial boundary conditions of the wick.

Flow Boiling Heat Transfer in Microgravity: Recent Results

Flow boiling heat transfer (FBHT) allows high performance heat transfer due to latent heat transportation and its use is therefore important to reduce size and weight of space platforms and satellites. Its knowledge is also important for the safe operation of existing single-phase systems in case of accidental increase of heat generation rate.The number of existing researches on flow boiling in reduced gravity is very small due to both larger heat loads and available room in a 0-g apparatus for experiments. Consequently, coherence in existing data is somewhat missing.This lecture will summarize the results of the few research carried out on FBHT in microgravity, with special emphasis to the recent research carried out at ENEA, in the frame of an ESA (European Space Agency) project.

Physical insight in the burnout region of water-subcooled flow boiling

Abstract The present paper reports the results of a visualization study of the burnout in subcooled flow boiling of water, with square cross-section annular geometry (formed by a central heater rod contained in a duct characterised by a square cross-section). In order to obtain clear pictures of the flow phenomena, the coolant velocity is in the range 3–9 m·s−1 and the resulting heat flux is in the range 7–13 MW·m−2. From video images (single frames were taken with a light exposure of 1 μs) the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the superheated layer close to the heating wall, their coalescence produces a sort of elongated bubble called a vapour blanket. One of the main features of the vapour blanket is that it is rooted to the nucleation site on the heated surface. Bubble dimensions, as well as those of the hot spots, are given as a function of thermal-hydraulic tested conditions.

Direct contact evaporation of nearly saturated R 114 in water

Abstract This paper reports the results of an experimental investigation on direct contact boiling of immiscible liquids. The continuous phase, water, is under stagnant conditions, while the dispersed one, Freon 114, is injected in the test section with different velocity and thermodynamic conditions through a nozzle. The injection system has been designed to vary the quantity of injected refrigerant and/or the liquid injection velocity. The test section is a Plexiglas vertical cylinder 72 mm i.d., 2.0 m long. Experimental data are obtained from high-speed movies of the continuous phase level during and after the Freon 114 injection, as well as from the movies of the rising boiling dispersed phase (injected under nearly saturation conditions). Vaporization rate has been characterized as a function of thermal hydraulic conditions (i.e. water temperature, system pressure and Freon mass flow rate). Direct contact boiling efficiency was derived by the evaluation of the fraction of Freon that did not undergo the boiling process during the transit in the test section.

Thermal hydraulic characterization of a heat pipe with extracapillary circulation

Abstract Heat pipe devices, for their typical working mechanisms, are particularly suitable for zero gravity applications, and have also been considered for applications in space satellites with nuclear propulsors, thanks to the absence of mobile systems for the coolant fluid circulation. The present work reports the results of experimental tests carried out on a heat pipe facility designed to investigate the thermal–hydraulic performance of a water heat pipe. The device layout, configuration and geometry, allow to simulate a heat pipe working utilizable in space applications and so under zero gravity conditions. The evaporating section, completely lined by wicks (sintered stainless steel), and nearly plane shaped, is housed in a cylindrical container. Two test series were performed with reference to different heater sizes (heated plate—wick contact surface size), obtaining for the evaporating heat flux q ″ ev a value of 13 W·cm −2 in the 1st series and of 20 W·cm −2 in the 2nd one, using a smaller heater size. This result means a potential possibility to use more compact evaporators and to work at higher system operating pressure values. It was possible to establish the increase of the limit heat flux q ″ evl versus the operating pressure in a range of 50–400xa0kPa. The condensate subcooling, due to the subcooler in the external channel, sensibly affects the boiling and the wick dispriming. In conclusion, it is necessary to point out that the obtained results are only preliminary and relative to a pilot test section. Further tests will be carried out with appropriate changes in the configuration, in the geometry and in the operating conditions.

Selected Papers on Advances in Heat Transfer

Advancement of technology in virtually all fields, in one way or another, has been due to simultaneous advances in thermal engineering. Design objectives of space stations, hazardouswaste destruction, high-speed transport, electronics, materials processing and manufacturing, etc. are fulfilled by proper energy management, heat-flow control, and temperature control. The scale of thermal engineering ranges from the very large to the near-molecular level and from very high temperatures of thousands of degrees to very low ones approaching absolute zero. Thermal energy transport occurs in every walk of human existence. In the recent past and of course with the passage of time, tremendous progress has been made in the field of thermal science and engineering. It is a continuous process and the quest of knowledge is truly unending. In this special issue of the journal, an attempt has been made to collate some scattered knowledge in the advances in heat transfer. Leading international heat transfer experts from the diverse fields of heat transfer have contributed in this special issue. It should be useful to the readers of the journal. This special issue is a compendium of the following 10 papers.