Mario Misale

Boiling of saturated FC-72 on square pin fin arrays☆

Abstract Pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric fluid (Fluorinert FC-72) has been experimentally studied. Twelve extended surfaces with different geometrical configurations were tested. Each extended surface consisted of an array of pin fins with square cross-section. Fins were 3 or 6 mm long and their width varied from 0.4 to 1.0 mm. Fins were uniformly or non-uniformly spaced on the base surface: starting from the uniform configuration, in which the width and the spacing of fins were equal, non-uniform surfaces were obtained by regularly removing some rows of fins. For each extended surface, boiling curves were obtained at three different saturation pressures: 0.5, 1.0 and 2.0 bar. The effects of fin dimensions, spacing and pressure on heat transfer in saturated pool boiling were examined. In particular, the effect which the non-uniform distribution of fins produces on boiling behaviour was analysed. When fins thin out, the overall heat transfer coefficient based on the total area of the extended surface increases, but the heat transfer rate does not improve, even in the boiling region close to the maximum heat flux. The better wetting of the boiling surface by the liquid refrigerant, as a result of the sparser grouping of fins, is apparently offset by the reduction in the heat transfer area. If pressure is increased, the boiling curves of finned surfaces move towards lower wall superheats, as already observed in previous studies with regard to both plane and finned surfaces.

Analysis of single-phase natural circulation experiments by system codes

In this paper, the results of simulations of natural circulation loop performance, obtained by Cathare and Relap codes, are reported. Both series of results are analyzed and compared with experimental data gathered in the MTT-1 loop, a rectangular natural circulation loop realized by DITEC at the University of Genova. Both Cathare and Relap codes, in absolute terms, show poor agreement with experimental data. At low power, the Cathare code shows a good capability to predict the steady state quantities, after the initial transient. On the other hand, no unstable behavior is predicted at each analyzed power level. The Relap code is able to show oscillating quantities, but not at the same power levels as in the experiments.

Measurement of Total Hemispherical Emittance and Specific Heat of Aluminum and Inconel 718 by a Calorimetric Technique

An apparatus for the measurement of the total hemispherical emittance and specific heat of metals has been developed. The measurement principle is based on the calorimetric technique: the sample, heated by Joule effect and placed in a vacuum chamber; exchanges radiative heat transfer with the walls of the container, kept at a relatively low temperature. Emittance is deduced from the radiative heat transfer laws at the steady state. When the heating power is switched off the specific heat of the sample can be recovered from the time history of the sample temperature during the cooling transient. Measurements have been performed on samples of aluminum Anticorodal alloy and Inconel 718 alloy under different surface conditions in the 350-635 K range.

The influence of channel width on natural convection and boiling heat transfer from simulated microelectronic components

The thermal behavior of three vertical inline heaters placed in narrow channels and cooled by one of two dielectric fluids (FC-72 or Galden HT-55) was experimentally studied. The influence of the channel width on natural convection and boiling heat transfer is discussed. The experiments were carried out at saturation or under subcooled conditions, and the channel widths investigated were 33 mm, 2 mm, and 0.5 mm. The reduction of channel width decreases the heat transfer coefficient for natural convection, but it causes an increase in heat transfer performance for nucleate boiling. In particular, for the boiling data taken in saturation conditions, the better thermal behavior was noted for the 0.5-mm channel width; whereas, in the case of a subcooling of 10 K, the better thermal behavior was noted for the 33-mm channel width. In the latter test condition, on the heat transfer surface, nucleate boiling and natural convection probably occur at the same time. The data obtained are useful for the design of compact microelectronic packages.

Experiments on pool boiling of a dielectric fluid on extended surfaces

An experimental study on pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric liquid (Galden HT-55) is presented. Two extended surfaces of different dimensions were tested using vertical and horizontal orientations. The effects of nonboiling waiting period, pressure and spine dimensions on boiling behavior were examined. A marked enhancement of heat transfer performance was observed on passing from plane to extended surfaces. Increasing the pressure improved the heat transfer coefficient and critical heat flux. The nonboiling period, orientation and pressure significantly influenced the boiling incipience and the hysteresis phenomenon that accompanies increasing and decreasing heat fluxes. Experimental data are expressed in terms of enhancement ratios of extended surfaces as a function of base surface superheat and pressure.

Influence of pressure drops on the behavior of a single-phase natural circulation loop: Preliminary results

Single phase natural circulation is of interest in various energy systems, including solar heaters, nuclear reactors, geothermal power production, engine and computer cooling. The present paper deals with an experimental study on the influence of pressure drops on the behavior of a single-phase natural circulation loop. In a simple rectangular loop (MTT-1) located at DITEC, three series of tests have been carried out, with localized pressure drops (orifices of 6 mm, 10 mm and 14 mm diameter in the vertical legs) and different power levels. The experimental data are analyzed and compared with previous results obtained with smooth pipes, demonstrating the stabilizing effect of the pressure drops on the overall behavior of the loop.

Experiments in Single-Phase Natural Circulation Miniloops With Different Working Fluids and Geometries

The aim of this work is to analyze experimentally the influence of geometrical parameters and fluid properties on the thermal performances of rectangular single-phase natural circulation miniloops, which could be used for cooling of electronic devices. The present paper analyzes two experimental campaigns performed on two rectangular miniloops (ML1 and ML2), characterized by different heights, when two working fluids (water and FC43) are employed. The temperature trends are measured for different combinations of miniloop inclination and power, and the associated fluid velocities are calculated by means of an enthalpy balance. The experimental data are compared with Vijayans model, developed for large scale loops in steady-state conditions, corrected with a parameter that takes into account the loop inclination. The dynamical behavior is always stable. The time of the initial transient is long at high miniloop inclination (close to horizontal) and at low power while the temperature overshoot grows up with increasing power and inclination. Results show that at the same power the velocity of FC43 is almost twice than that of water, but the thermal performances are worse because FC43 is characterized by low specific heat. Moreover, the velocities of the tallest miniloop are the lowest, probably because the enhancement of shear stresses overcomes the increase in buoyancy forces. For both fluids, the velocity grows almost linearly with power Experimental data show a good agreement with the modified Vijayans model.

Overview of single-phase natural circulation loops

Analytical and experimental studies on singlephase natural circulation loops (thermosyphons NCL ) have been performed in the past years by several authors,. However the problem of the stability of the loops, that is of interest in many industrial applications, cannot be considered solved. The present paper is focused on the experimental aspects that could be interesting in the study of these thermal systems. In particular, the following NCLs aspects will be discussed: applications, models, thermo-hydraulic behaviour, and influence of loop size. Finally a broad reference list is cited. Keyword – natural circulation, single-phase, instabilities, nanofluids)

Pool boiling heat transfer of dielectric fluids for immersion electronic cooling: effects of pressure

Experiments on nucleate saturated pool boiling were performed on dielectric fluids suitable for direct-immersion cooling of electronic devices. Four different fluids of three families were investigated: perfluoropolyetheres HT-55 and HT-70, perfluorocarbon FC-72, and chlorofluorocarbon R-113. Experiments were carried out on a plane horizontal surface made of copper. The effects of pressure on boiling curves and heat transfer coefficients were studied. Increasing pressure enhanced boiling performance and affected the nucleate boiling hysteresis for increasing and decreasing heat fluxes.

Effects of a Lateral Confinement on FC-72 Pool Boiling from a Plane Surface

An experimental study was carried out on pool boiling from a plane surface. The influences of two different periphery conditions (pool boiling surface confinement) were investigated. One condition involved the use of a 200 mm diameter glass cylinder (single confinement); the other condition was achieved by inserting a 70 mm diameter cylinder inside the first one (double confinement). The experiments were performed at atmospheric pressure in a saturated pool of FC-72 (dielectric fluid). The experimental data were compared with those obtained in the case of an unconfined pool boiling surface. The presence of the confinement seems to improve heat transfer performance at low and moderate heat fluxes, whereas it has no effect at the maximum heat flux. For the latter situation, the images taken during the experiments show different flow patterns on the boiling surface.