Revue Générale de Thermique最新文献

The air heater studied in this paper is made of an external transparent plastic cover, such as plexiglas, the role of which is to protect the collector from accidentally thrown stones and an internal transparent glass cover which produces the necessary greenhouse effect for heating the absorber. An analysis of the unsteady state heat exchanges in such a collector is presented. It is shown that at quasi-steady state the energy balance equations of the components of the heater cascade into a single first order differential equation, which is able to predict the thermal behaviour of the collector. The solution of this differential equation is written down as an explicit expression of the local temperature of the fluid flowing in the collector in terms of the time-dependent incident solar intensity. The effect of various parameters such as the inlet fluid temperature, the mass flow rate and the depth of the air channel on the thermal performances of the combined plastic-glass air collector is also studied.

A set of experiments was carried out with propane pool boiling on the smooth and porous surfaces of horizontal tubes in the saturation pressure ranges p = 3.45 to 13.8 bar ($\text{p}\text{p}{}_{\mathrm{c}}\text{= 0.081}\text{to}\text{0.323}$), temperature intervals − 10 to + 40 °C, and heat flux ranges 10² to 10⁵ W·m⁻². Three modes of heat transfer were determined: free convection, a transitional mode and developed boiling. Good correlation between the experimental data on smooth tubes and the data published in other references was observed. The dependence of boiling process on saturation pressure and the influence of porous coating parameters on heat transfer intensity were studied. Samples of smooth tubes, tubes with small roughness and tubes with gas-thermal metal sintering on the surface were used for the experiments. Remarkable heat transfer enhancement of up to 2.5–3 times was observed for propane pool boiling on porous horizontal tubes, in comparison with that on smooth and small roughness tubes over the heat flux intervals tested.

The initial liquid charge of a vertically orientated two-phase closed thermosyphon for adequate thermal performance as determined theoretically by assuming that the condensate is in the form of a relatively thin film underestimates the amount determined by using experimental correlations. Knowing the physical details of the two-phase flow within the thermosyphon could explain this discrepancy. Because, however, of the difficulty of directly observing two-phase flow in an actual metal thermosyphon it was decided to investigate the two-phase flow by using air and water in a transparent tube. The tube that was used is closed at the lower end and open at the top end, was partially charged with water, and air was introduced into the closed end at increasing air flow rates until water droplets were just about to be expelled from the open end. The flow patterns occurring as a function of air flow rate were identified. The average liquid fraction in four sections of the tube was determined for different initial charge fractions and inclinations. It was observed that even at low air flow rates significant quantities of liquid were propelled up into the tube and that the flow is oscillatory. It was concluded that care would have to be taken in assuming a relatively thin and uniform liquid film in theoretically modelling a thermosyphon.

A model, based on semi-empirical correlations, was developed to simulate the behavior of a pressurized fluidized bed combustor (PFBC) under various operating conditions. This paper presents the results of a comprehensive survey of one-dimensional empirical correlations applicable to a PFBC and which were used to assemble a 1-D predictive model of the complete performance of a pressurized fluidized bed combustor. The model was adjusted in order to reproduce process and in-bed data taken at Escatrón PFBC power station (Spain) under a wide range of operating conditions. The novelty of this approach lies in the use of full-scale information to validate empirical correlations, the objective being to draw specific conclusions on the reliability of empirical PFBC modeling. The model was validated using experimental power plant data. Good agreement was found between the computed results and actual plant data of bed temperature profiles, bed density, char and sorbent elutriation, fuel feed rate, in-bed and entrainment particle size distributions, oxygen and carbon dioxide concentrations, heat transfer coefficients along the combustor height and heat transfer to the tube bundles under different operational regimes. This paper describes the model, as well as its results, validation and prospects for future work.

Introduction of a thermal field as a passive scalar seeding has been made in a channel with significant wall-injection convection and presence of a predominant obstacle. In such a complex flow, the study of the temperature field provides a good description of the whole dynamic organization of the flow: development of a highly sheared flow, reversal region back to the obstacle… The scalar and the dynamic fields are close together, i.e. the study of the mean temperature and of its fluctuations allows analysis of the scales of flow turbulence and description of the creation of vortices in this sheared flow.

In this work, the laminar mixed convection heat transfer in a ventilated rectangular cavity submitted to uniform heat flux from the side is studied numerically. Flow fields, temperature fields and heat transfer rates are examined for values of Rayleigh and Reynolds numbers such as: 10³ ≤ Ra ≤ 10⁶ and 5 ≤ Re ≤ 5·10³. Useful correlations are proposed by considering air as a working fluid (Pr = 0.72) and a constant relative height of the openings ($\text{B =}\text{1}\text{4}$).

The results of the investigation of the reactive flow in a scaled-down (≅1:3) model of a gas turbine combustion chamber are presented. The lean-premix combustion chamber model was tested at atmospheric pressure; measurements in the reactive flow were taken by using a two-channel fibre-optic laser Doppler anemometer with Al₂O₃ seeding on the air flow. Measurements cover a wide region of the axial-symmetric combustion chamber at points characterised by different seeding conditions and data rates, including the flame development and recirculating zones. This work presents a detailed analysis of the fluctuating components of flow velocity; mean flow properties are presented as a prerequisite. Frequency and time domain characteristics of turbulence including power density spectra, autocorrelation coefficients and time integral scales are reported and discussed, and a physical description of the reacting turbulent flow field is attempted.