Marco Fossa

Study of free convection frost formation on a vertical plate

Abstract Processes involving heat transfer from a humid air stream to a vertical plate, with simultaneous deposition of frost, are of great importance in a variety of refrigeration equipment. In this work, frost growth on a vertical plate in free convection has been experimentally investigated. The plate, cooled by the internal circulation of glycol, was placed in a vertical channel open at the top and bottom in order to permit the natural circulation of ambient air. The cold plate temperatures were varied in the −13 to −4 °C range, while the relative humidity and temperature of the ambient air were taken in the 31–58% and 26–28 °C ranges, respectively. Measured quantities (frost thickness, frost surface temperature, deposited mass of frost, heat flux at the plate/frost interface) were compared with results given by a simple model based on standard relationships (for heat and mass transfer coefficients, frost density and thermal conductivity) reported in the literature.

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.

Design and optimisation of impedance probes for void fraction measurements

Abstract The performance of impedance probes for measuring the conductance of gas–liquid mixtures in horizontal pipes is here studied by means of the numerical solution of Laplace problem. In particular the work is aimed at optimising the probe geometry in order to improve the probe response both in terms of linearity and in terms of spatial resolution to step changes in phase distribution. Starting from basic shapes employed in literature (ring and half-ring electrodes), the adopted approach allowed new probe geometries (characterised by non-uniform electrode distance and/or electrode width) to be found. The analysis is performed with reference to annular, stratified and dispersed distributions and the results are compared with available theoretical models. Measurements have been carried out to verify the predicted sensor response and to ascertain the effect of the particle size on the probe response under dispersed flow conditions. The analysis demonstrates the behaviour of different probe arrangements with respect to different flow patterns and shows the possibility to obtain optimised geometries matching the desired features of linear response and of enhanced spatial resolution.

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.

Dynamic void fraction measurements in horizontal ducts with sudden area contraction

Abstract Void fraction distributions in horizontal pipes with sudden area contraction have been experimentally investigated. The experiments are aimed at analysing the effect of the singularity characteristics on void fraction profiles and phase distribution. The technique employed to obtain the cross-sectional average void fraction along the test pipes is based on the instantaneous measurement of the electrical impedance of the air–water mixture. The instrumentation set up consists of ring electrode pairs placed on the internal wall of the cylindrical test duct, flush to the pipe surface. Two different test sections, a 70\60 mm i.d. pipe and a 70\36 mm i.d. pipe have been employed. Plug and slug flow regimes were investigated in order to obtain the mean void fraction along the channel in five different locations upstream and downstream the singularity. The analysis of the results, in terms of mean values and probability distributions, shows that the characteristics of the flow restriction deeply modify the flow structure upstream and downstream the discontinuity.

A fast method for evaluating the performance of complex arrangements of borehole heat exchangers

Ground coupled heat pumps are systems combining a heat pump with a ground heat exchanger for building heating and cooling purposes. Borehole heat exchangers (BHE) are the most frequently adopted solution for ground-assisted heat pump applications, and in most installations, they represent the most important cost item. For this reasons, a careful design analysis is needed to either assure long-time performance or reduce the payback period, both parameters related to overall BHE length. The required approach for such a problem is a transient analysis of ground response to different heat loads at different time scales. A computationally efficient way to tackle the problem is the recursive calculation of a basic thermal response factor for given different heat pulses representing the building energy demand. In this article, a review of the existing response factor models for BHE analysis is performed, and the finite line source model is employed to develop and test new simplified solutions suitable for fast spatial and temporal superposition and for generating thermal response factors for arbitrary BHE arrangements.

A simple model to evaluate direct contact heat transfer and flow characteristics in annular two-phase flow

Abstract Calculations are performed to study the behavior of air-water mixtures in vertical flows employing a simple model for the description of mass, momentum, and energy transfer in annular gas-liquid flows. The model is applied to various flow conditions including thermal nonequilibrium and variable cross section ducts. Theoretical results agree reasonably well with experimental data regarding well-known isothermal flows. A contribution is provided to the understanding of the two-phase mixtures interactions for gas-liquid internal heat transfer in two-phase, annular, nonisothermal flows.

Experimental heat transfer of drag-reducing polymer solutions in enhanced surface heat exchangers

Abstract A set of experimental tests on the effect of polymeric additives in water are presented from the point of view of friction losses and heat transfer performance variations, in single-passage countercurrent straight-pipe heat exchangers. The tests were performed on smooth, finned, and grooved pipes of different diameters and show that the use of polymers in augmented surface heat exchangers is generally not advisable. However, they also confirm that thermal bus systems as a whole can take advantage of drag-reducing polymer solutions, provided that accurate trade-off analyses are performed to get the best compromise between pressure losses and global heat transfer performance of the thermal loop.

Experimental direct contact heat tranfser in upward air-water developing annular flow

Abstract Some experimental results are presented in order to quantify the impact of several operating parameters on the direct contact heat and mass transfer processes between hot water and cooling air, flowing upward inside a vertical, adiabatic, circular tube (12 mm i.d.), in the developing co-current annular flow regime. The proposed temperature measurement technique for the gas phase is based on the energy and mass balance equations applied to proper control volumes. The data processing method seems to be particularly suitable for the performance evaluation of the direct contact heat transfer occurring between the phases during the flow. Further developments are expected in the characterisation of different injection systems used for the mixing of two fluids in thermodynamic non-equilibrium conditions.

Dynamic measurements of particle concentration in solid-liquid slugging suspensions

Abstract Experiments have been carried out to analyse the characteristics of liquid suspensions which exhibit a non-uniform solid concentration. In order to investigate the bed behaviour, a new probe was developed and tested to measure the instantaneous concentration of solid particles. Attention is focused on slugging solid-liquid fluidized beds, where regions of low solid concentration alternate with regions of high solid concentration. The analysis of the results allowed some relevant flow characteristics (slug frequencies and slug velocities) to be recognized.