Film boiling heat transfer and vapour film collapse on spheres, cylinders and plane surfaces

Copyright @ 2009 Elsevier B.V. The final version of this article may be viewed at the link below.An experimental study of transient film boiling was conducted, with different coolant velocities, on two spheres with different diameters, two cylindrical specimens of different lengths in parallel flow, a cylinder in cross flow and two flat plates with different lengths. A frame by frame photographic study on the nature of the vapour/liquid interface and the collapse modes has revealed a new mode for film collapse, in which an explosive liquid–solid contact is followed by film re-formation and the motion of a quench front over the hot surface. Steady state tests were carried out on a plate similar to the short plate used in the transient experiments and the heat transfer, film stability and collapse results are compared with those of the transient investigation. Heat transfer coefficients and heat fluxes during film boiling were found essentially to depend on specimen temperature and water subcooling. In contrast, the influences on heat transfer of specimen size and water velocity were relatively small for the ranges studied. A theoretical model predicted heat transfer coefficients to within 10% of experimental values for water subcoolings above 10 K and within 30% in all cases

Experimental and PIV-method studies of lean mixture flames, focused on extinction during its propagation in a standard tube resulting from flame stretching

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.The main goal of this work is to determine experimentally local stretch rate distribution along limit methane/air and propane/air flames – using Particle Image Velocimetry (PIV) method. It allows to obtain necessary moving flame velocity fields in a standard flammability column and, also, to recognize the flame structures. For this purpose each mixture was seeded with MgO particles (of known size) before entering the tube (column) – using a special system. The amount of seeds in the mixture, their dispersion system and the laser power producing a sheet of light penetrating the column – were carefully chosen (not to disrupt the combustion or flame propagation in it). After learning process, finally it allowed to obtain the good quality velocity field images in the region concern – acceptable for further processing. The methodology developed for these experiments proved to be reliable and able to supply analyses with repeatable data. On the basis of performed experiments it was possible to derive the flame stretch rate which causes its extinction for both mixtures.vk201

Computer simulation study of high-pressure gas quenching of a steel element within vacuum furnace batch for different working conditions

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.The paper presents analysis of the high-pressure gas quenching of metal elements, after their vacuum carbonisation process, based on the computer simulation. The ANSYS-CFX code has been used for this purpose. This transient quenching process includes challenging problems, which requires deep knowledge and application of heat transfer, fluid mechanics and thermodynamics. The numerical simulation analysis method focuses on a single element contained in an elementary, repeatable section (usually a cuboid) of the whole batch. In this study quenching of a steel cylinder of diameter d = 20mm and length L = 150 mm is analysed. This approach allows defining precisely: (i) the geometries of the metal element and of the elementary cubical section associated with it and, also, (ii) flow and thermal boundary conditions on the walls of this elementary section. Above definition of the elementary section (a computing domain) allows using the whole available computing power for the quenching process simulation in it. The ratio of volumes mentioned in (i) defines a “porosity of batch” and the analysis covers the range from 7 to 70%. The number of grid points used for the elementary section varies from 55000 to 240000. The influence of the pressure gradient value (a flow “driving force” through the elementary section) and its direction – on temperature and quenching rate time distributions in a steel element – is analysed and presented. The use of periodicity and symmetry conditions, for the velocity field, on the elementary section walls, allows simulating different single element quenching conditions – reflecting its position in the batch. The transient quenching has been usually analysed for a fixed velocity field. However, the steel element body thermal properties in this transient process varied – appropriately to its time dependent temperature distribution. The initial element temperature is assumed to be equal 1300 K. The influence of different gases, i.e. argon, helium, nitrogen and hydrogen, and their static pressures (up to 30 bar), on the steel element quenching process, is analysed and presented.cs201

Corrugated channels heat transfer efficiency analysis based on velocity fields resulting from computer simulation and PIV measurements

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Numerical and experimental studies of flow and heat transfer, in corrugated channels, are presented. Such channels are representative of compact heat exchangers − for example air or water pre-heaters. The most important characteristic parameter of these channels, apart from the channel wall shape, is the angle between two corrugated sheets. Paper presents measurement results, related to velocity field in such channels – by the PIV-method. The efficiency analysis, based on the irreversible entropy generation, takes account of two processes: flow resulting from the pressure gradient and heat transfer delivered from solid walls. This approach is checked initially, in details, for arrangements related to two values of the angle mentioned, i.e. 0 and 90° : it allows comparing velocity fields obtained from the computer simulation and PIV measurement (the latter in special corrugated sheets). More extensive computer simulation results, for different wall shapes (sinuses and semi-circles) and for angle value mentioned and equal to 90°, are also presented.mp201

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described