André Bontemps

Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

Abstract The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m2 and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselts theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.

Enhancement of Cooling Rate by Means of High Frequency Ultrasound

Heat transfer in the presence of ultrasound has been investigated. Experiments were performed using a specific experimental setup involving a homemade, high-frequency, ultrasonic reactor equipped with a cooling helical coil. The aim of this study was to investigate the effect of ultrasound with different frequencies on heat transfer between water contained within the reactor and cooling water flowing through the coil. It has been shown that the presence of the ultrasonic field results in an increase of the cooling rate due to an enhancement of the overall heat transfer coefficient of the coil by up to 100%, depending on operating parameters.

Countercurrent gas-liquid flow in plate-fin heat exchangers with plain and perforated fins

Abstract Countercurrent gas–liquid flow in narrow rectangular channels simulated by plain and perforated fins is studied. Different flow patterns dependent on flow rates are observed and visualised in the channels. Flooding velocities and pressure drops are measured. Results are compared with previous experimental data obtained in rectangular channels. The present results focus on two particular types of flooding phenomena: that occurring at the column base and linked to plugging and that at the top, linked to reentrainment. A further set of experiments using perforated fins surprisingly shows a greater tendency to flooding of the perforated fins. However, there are marked differences for the two types of fins and an explanation may be the two distinct flooding occurrences.

Effects of Geometrical and Thermophysical Parameters on Heat Transfer Measurements in Small-Diameter Channels

An experimental investigation of the liquid flow friction factor and heat transfer coefficient in small diameter channels is presented. Rectangular and circular mini-channels with hydraulic diameters from 0.77 to 2.01 mm were used. Literature in the 1990s showed scattered experimental results and concluded that new physical phenomena in mini-channels could occur. However, the present experimental results show that three main causes can explain how different results were obtained in mini-channels compared to macro-tubes: uncertainties on the channel dimensions, inlet and outlet singular pressure losses, and longitudinal heat conduction. When these elements are taken into account, good agreement is found with standard correlations or theories.

Reflux condensation in narrow rectangular channels with perforated fins

Abstract Reflux condensation is an industrial process that aims to reduce the content of the less volatile component or to eliminate the non-condensable phase of a vapour mixture, by the means of separation. Separation consists in condensing the less volatile phase and to recover the condensate while simultaneously, the non-condensable species are recuperated at the top of the system. Compact plate-fin heat exchangers can be used in gas separation processes. The aim of this study is to test the process of reflux condensation of an air–steam mixture in the channels of a plate fin heat exchanger with a hydraulic diameter of 1.63 mm. The experimental study shows that reflux condensation occurs in specific parts of the heat exchanger, the other parts remaining dry. Moist air condensation is modelled by the film theory and the results show that the model is well adapted to simulating the heat and mass transfer.

Heat Transfer and Sustainable Energy Technologies

Energy and global warming are the two major challenges for the twenty-first century. To achieve a sustainable development, the origin and the use of energy have to be addressed, and advanced energy technologies for both fossil and renewable energy carriers have to be developed, requiring significant progress in research and technology. The recent rise of petrol price (100% during the last two years) and major natural catastrophes in 2005 have made the public aware and more sensitive to energy aspects and the impact of global warming. As the Kyoto protocol has entered into force in February 2005, this is the birth of a new hope for mankind and a visible signal for scientists to increase their efforts in research on safe, efficient, and sustainable systems. Therefore, the scientific and engineering communities have to develop and prove the technical and economical availability of innovative energy technologies. However, there is no unique solution—more likely a set or combinations of measures that have to be adapted at the local level: we should think globally but act locally. The scientific community has not only to think and develop advanced energy technologies, but also to contribute in improving the existing ones. Even if renewable energies and new energy carriers, such as hydrogen, are promising solutions, our society is still relying on fossil fuels as primary energy for many applications. Finally, energy conservation and energy efficiency has to be promoted, for the best energy is the one that is not wasted. It is only by re-enforcing collaboration between the various sectors and promoting an exchange of knowledge and

Experimental characterisation of the thermal behaviour of different materials submitted to ultrasound in an ultrasonic fountain

The purpose of this study was to characterize the thermal behaviour of different materials subjected to an ultrasonic fountain created by an ultrasonic piezoelectric transducer. Tests were conducted with an infrared camera to determine the surface temperature of the material samples. The main conclusion is that the plastics when subjected to the ultrasonic fountain tend to heat up strongly and can reach temperatures up to 200 °C in few seconds. The temperature rise depends on the nature of the plastic material and the experimental conditions namely the height of liquid above the transducer and also the distance between the liquid surface and the sample. The comparison with an aluminium plate and a Pyrex plate gives a better understanding of the phenomenon that is related to three characteristics of the experiment: the transmission of the incident wave, the ability of the material to absorb this energy as heat and finally, the thermal and thermodynamic properties. These three parameters determine the behaviour of the plate when it is subjected to the ultrasonic fountain. In order to achieve a performing active control system, the choice of the material that faces an ultrasound fountain is very important to avoid any damage.

A phenomenological approach of solidification of polymeric phase change materials

Phase Change Materials (PCMs) are widely used in thermal energy storage and thermal management systems due to their small volume for a given stored energy and their capability for maintaining nearly constant temperatures. However, their performance is limited by their low thermal conductivity and possible leaks while in the liquid phase. One solution is to imprison the PCM inside a polymer mesh to create a Polymeric Phase Change Material (PPCM). In this work, we have studied the cooling and solidification of five PPCMs with different PCMs and polymer fractions. To understand the heat transfer mechanisms involved, we have carried out micro- and macrorheological measurements in which Brownian motion of tracers embedded in PPCMs has been depicted and viscoelastic moduli have been measured, respectively. Beyond a given polymer concentration, it was shown that the Brownian motion of the tracers is limited by the polymeric chains and that the material exhibits an elastic behavior. This would suggest that heat t...

EXperimental study of convective heat transfer and pressure loss of SiO 2 /water nanofluids Part 2: Imposed uniform heat flux - Energetic performance criterion

This paper describes an experimental study on convective heat transfer and flow behaviour of SiO2/water nanofluids at different concentrations (5, 16 and 34 % in weight) inside a 1.3mm diameter tube heated at constant heat flux. The experimental test bench allowing us to measure local wall temperatures as well as fluid inlet/outlet temperatures is described. The flow regimes range from laminar to turbulent in the heated tube.

Flow Boiling in Minichannels

The use ofmini-channel heat exchangers (hydraulic diameter about 1 mm) in compact heat exchangers improves heat transfer coefficients, and thermal efficiency while requiring a lower fluid mass. They are widely used in condensers for automobile air-conditioning and are now being used in evaporators, as well as in other applications such as domestic air-conditioning systems. However, more general use requires a better understanding of boiling heat transfer in confined spaces. Many definitions of micro and minichannel hydraulic diameter are used throughout the literature. Kandlikar and Grande (2003) proposed the following classification: conventional channels (Dh > 3 mm), minichannels (200 μm < Dh < 3 mm), micro-channels (Dh < 200 μm), that will be used throughout this paper. These definitions rely upon the molecular mean free path in a single-phase flow, surface tension effectsand flow patterns in two-phase flow applications. In recent studies in minichannels the hydraulic diameter ranges from 100 μm to 2–3 mm. The channel cross sections were either circular or rectangular and much of the research concerned boiling. Commonly, classical correlations have been used with or without modifications to predict flow boiling results in minichannels. However agreement was poor and the need for new correlations was evident.