Resumo em Inglês:

The lecture presents a survey of results found by the author and his team during recent years. An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wettingfluids and fluids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean. However, even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run. The situation is different under transient conditions: heating and cooling transiens yield different curves even on clean surfaces. Measurements with microsensors give an insight in the two-phase dynamics above the heating surface and the temperature field dynamics above and beneath the surface. Microthermocouples (38 µm diameter) enbedded in the heater (distance to the surface 3.6 µm), a micro optical probe (tip diameter ~ 1.5 µm) and a microthermocouple probe (tip diameter ~ 16 µm), both moveable above the heater surface, are used for these studies. In nucleate boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching the critical heat flux (CHF), hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition boiling regime wetting events are observed, but no ones in film boiling. In low heat flux nucleate boiling very small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic change continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film boiling the bubbles leaving the vapor film can reach superheats of 30 K or more near the surface (e.g. for isopropanol). The optical probes confirm a liquid rich layer near the surface between nucleate boiling and high heat flux transition boiling. The void fraction in the layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter. Via the microsensor-data new approaches for heat transfer models on a mechanistic basis are proposed. An interfacial-area-density model enables the prediction of entire boiling curves. Furthermore the concept of a reaction-diffusion model is presented to predict CHF. Here the triggering of CHF is due to an instability of dry spots on the heating surface. Many aspects of the extremely complex mechanisms of boiling are, however, still not sufficiently understood. The problems should be tackled from both the experimental and the theoretical end and both approaches should be closely linked.

Resumo em Inglês:

Recent work on improving general thermal design methods for condensation inside plain, horizontal tubes is presented, summarizing primarily the advances proposed at the Laboratory of Heat and Mass Transfer at the EPFL in collaboration with the University of Padova and the University of Pretoria. This work has focused on the development of a unified flow pattern, two-phase flow structure model for describing local heat transfer coefficients for pure fluids, azeotropic mixtures and zeotropic mixtures. Such methods promise to be much more accurate and reliable than the old-style statistically-derived empirical design methods that completely ignore flow regime effects or simply treated flows as stratified (gravity-controlled) or non-stratified (shear-controlled) flows. To achieve these goals, first a new two-phase flow pattern map for condensing conditions was proposed, which has been partially verified by flow pattern observations. Secondly, a new condensation heat transfer model for pure fluids and azeotropic mixtures has been developed including not only flow pattern effects but also interfacial roughness effects. Finally, the widely used Silver-Bell-Ghaly condensation model for miscible vapor mixtures has been improved by including the effects of interfacial flow structure and roughness on vapor phase heat transfer and a new non-equilibrium effect added.

Resumo em Inglês:

This lecture addresses some recent developments in modelling of macroscopic thermodynamic and hydrodynamic non-equilibrium phenomena in convective phase change (boiling and condensation) of pure fluids and mixtures. Proper accounting of such phenomena may hold the key to explain and predict deviations from the classical (equilibrium) phase change convective heat transfer behaviour reported in the literature and yet not fully understood. In the first part of the paper, a detailed qualitative description of the classical heat transfer coefficient behaviour is presented together with two examples of departure from macroscopic equilibrium largely supported by experimental evidence. The second part of the paper reviews successful attempts to model the non-equilibrium phase change phenomena taking place in the two situations. The first example is a thermodynamic non-equilibrium slug flow model (one in which saturated Taylor bubbles become separated by slugs of subcooled liquid) that predicts the peaks in heat transfer coefficient at near-zero thermodynamic quality observed in forced convective boiling of some pure liquids. The occurrence of such peaks is typical of low latent heat, low thermal conductivity systems and of systems in which the vapour volume formation rate for a given heat flux is large. The second example is a comprehensive annular flow calculation methodology that predicts the decrease in the heat transfer coefficient with increasing quality observed in convective boiling of binary and multicomponent mixtures. In this case, as will be seen, coupled mass transfer resistance and hydrodynamic non-equilibrium effects generate concentration gradients between the liquid film and entrained droplets that are responsible for the heat transfer deterioration. In addition, it will be shown that for condensation of mixtures the methodology predicts a heat transfer intensification which has been subsequently confirmed by independent experimental results.

Resumo em Inglês:

In this tutorial the fundamentals of non-boiling heat transfer in two-phase two-component gas-liquid flow in pipes are presented. The techniques used for the determination of the different gas-liquid flow patterns (flow regimes) in vertical, horizontal, and inclined pipes are reviewed. The validity and limitations of the numerous heat transfer correlations that have been published in the literature over the past 50 years are discussed. The extensive results of the recent developments in the non-boiling two-phase heat transfer in air-water flow in horizontal and inclined pipes conducted at Oklahoma State University's Heat Transfer Laboratory are presented. Practical heat transfer correlations for a variety of gas-liquid flow patterns and pipe inclination angles are recommended.

Resumo em Inglês:

In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calculation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions) and those that are due to internal causes (sources of variable power, instabilities, turbulence), and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature) are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.