Giovanni Guglielmini

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.

Pressure drop and void fraction profiles during horizontal flow through thin and thick orifices

Abstract Two-phase flow pressure drop through singularities such as thin and thick orifices has been experimentally investigated. The study refers to air–water horizontal flows in 60 and 40 mm pipes. The operating conditions cover the gas and liquid superficial velocity ranges Vsg=0.3–4 m/s and Vsl=0.6–2 m/s, respectively. Intermittent flows have been observed. The local pressure drop has been obtained by means of extrapolation from upstream and downstream linearized pressure profiles. Single-phase measurements have been carried out for local liquid Reynolds values ranging from 3×104 to 2×105 to obtain the discharge coefficient and calculate the two-phase local multiplier. The effect of orifice geometry has been considered by selecting six different singularities with two different area contraction ratios (σ=0.73 and σ=0.54) and different thickness (from 0.025 to 0.59 restriction diameters). The analysis of the results and their comparison with available relationships revealed a lack in the prediction capability of literature correlations for high values of the flow area ratio. Finally the void fraction profiles obtained by means of ring impedance probes showed that generally the effect of the singularity is to increase the void fraction values just downstream the orifice with differences up to 50% with respect to the straight pipe values.

Intermittent flow parameters from void fraction analysis

Abstract Two-phase horizontal intermittent flow in straight pipes is experimentally investigated. A new procedure is proposed to characterize the flow through the statistical analysis of the instantaneous cross-sectional averaged void fraction obtained by means of ring impedance probes. The algorithm, based on the statistical analysis of the void fraction records, allows the main intermittent flow parameters, such as slug frequency and length, time average void fraction, minimum and average liquid film height to be evaluated. The procedure is validated through flow visualizations, as obtained from a fast digital video camera. Experiments on air-water horizontal flows in 40 and 60 mm inner diameter pipes are performed. The operating conditions cover the 0.3–4.0 and 0.6-3.0 m/s gas and liquid superficial velocity ranges, respectively. An extensive comparison with literature data shows a general agreement with present measurement. The reliability of both the instrumentation and the signal analysis procedures allows new correlations for minimum and average liquid film height in stratified regions to be proposed. Finally proper dimensionless numbers were applied to correlate frequency data in a wide range of superficial velocity values.

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.

Remarks on heat and work transfers in systems with uniform intensive properties

Abstract The differential forms of heat and work, owing to their dependence upon parameters characterizing the interaction between system and environment (and not only upon the state functions of the system), differ basically from the usual non-exact differentials. The main aim of this paper is to examine these differences considering quasi-static irreversible processes with the uniformity of internal temperature and pressure. Besides, the specific heat is defined referring only to the properties of the system and to the path of the process and not to the actual heat transfer

Experimental study on nucleate boiling of water in vertical upflow and downflow

Abstract An experimental study on nucleate boiling of water was carried out using an annular vertical channel both in upflow and downflow. Heat transfer data are given in different conditions of subcooling and fluid velocity. Photographs show different behaviour of heat transfer mechanism.

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.

CFD Simulations Devoted to the Study of Fitting Effects on the Phase Distribution in Parallel Vertical Channels

Abstract Uneven distribution of phases in plate heat exchangers is a cause of reduction in both thermal and fluid-dynamic performances. With respect to two-phase flows, phase separation in manifolds with several outlets is a complex phenomenon and no general rules are available for predicting the phase distribution at header–channel junctions. The design of compact heat exchangers and their distributors is still based on empirical approaches and both experimentation and numerical analyses are needed for defining the best geometries able to reduce the mass flow rate non-uniformities in parallel channels. In this paper, a series of CFD simulations are carried out to infer the effects of a protrusion fitting (inside the header) on the single-phase distribution in parallel upward vertical channels fed by a common horizontal distributor. The numerical results are compared with both experimental single-phase and two-phase (liquid/gas) experimental data. The effects of the operating conditions are investigated and general conclusions on the differences and analogies between single-phase and two-phase flows in the present problem are discussed.