Experimental study of premixed flame propagation over various solid obstructions

Abstract

Accounting for the probability of an explosion is essential in the design of plants and onshore or offshore gas or oil exploration platforms. This, of course, will have major implications on the safety of personnel both in terms of potential loss of life and the possibilities of escalation processes, which could lead to catastrophic consequences. The mechanisms which enhance explosion overpressures, therefore, need to be established with some certainty in order to ensure that all aspects of the safe design of structures and processes and the safe protection of personnel are taken into account. This paper presents an experimental investigation of the interaction of propagating premixed flames with various solid obstructing bodies to quantify the role of generated turbulence in flame acceleration and structure. Three different obstructions with circular, triangular and square cross-sections are studied here covering blockage ratios ranging from about 10% to 78%. High-speed video images were used to track the propagating flame front and the volume of trapped mixture behind the obstacle downstream from the ignition point. Images taken at different times after ignition are presented and discussed in terms of flame structure and acceleration. It is found that obstructions with square cross-sections result in the fastest flame acceleration followed by triangular and circular cross-sections. Also, flame speed is found to increase with increasing area blockage ratio. The volume of trapped unburned mixture is found to be high when square and triangular obstacles are used.

Introduction

The probability of an explosion has major implications on the safety of personnel on a platform both in terms of potential loss of life and the possibility of escalation of the process, which could lead to a domino effect and more serious consequences. The mechanism which enhance explosion overpressure, therefore, need to be established with some certainty in order to ensure that all aspects of safe design (structure and processes) and the safe protection of personnel are taken into account.

The role of turbulence is well-established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the flame surface area and, hence burning rate. Abdel-Gayed and Bradely [1] have produced a range of experimental data which allow the turbulence enhancement of burning velocity to be predicted. However, in real situations, such as those found in complex process plants, the acceleration of flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. Such blockage causes a complex interaction between the flame and the flow fields around these obstacles and leads to a local acceleration of the flame front in the form of jetting. This interaction between the gas movement and the obstacles create both turbulence by vortex shedding and local wake/recirculation whereby the flame can be wrapped in on itself, increasing the surface area available for combustion and potentially producing localised flame quenching. The influence of such local events on the overall explosion process and overpressure is not understood. It is clear, however, that any localised increase in overpressure will have important implications on any adjacent plant and equipment and may lead to an escalation process internal to the overall event. The long-term objective of the current work is to establish the influence and importance of combustion in the wake of obstacles representative of onshore/offshore plant and process equipment (i.e. pipe-work, pressure vessels, walls, etc.).

There have been several comprehensive studies of flame acceleration due to the presence of circular or flat obstacles in the path of premixed propagating flame, including Moen et al. [2], Starke and Roth [3], [4], Teodorczyk et al. [5] and Phylaktou and Andrews [6]. These studies provided evidence that the presence of obstacles increases flame speed and explosion overpressure. Hjertager et al. [7] presented results from a series of large-scale obstacle accelerated explosions which studied the influence of fuel concentration on both flame speed and overpressures. These workers also provided hot wire anemometry measurements of the local flow velocity inside a flame propagation tube and attempted to relate velocities to flame speed. To demonstrate the advantage of laboratory studies of explosions Samuels [8] carried out a series of experiments in the 1/12th scale models of offshore modules. More detailed experimental investigations were carried out, recently, by Lindstedt and Sakthitharan [9] and Fairweather et al. [10] on the interaction of flames with baffle type obstacles in laboratory scale explosion vessels. These investigations provided, in addition to pressure time histories, high quality flame shape information as well as mean and fluctuating velocity data.

In real explosion situation, following ignition, the flame spreads and interacts with obstruction of various sizes and cross-section such as cylinders, squares, flat walls and sharp edges. The propagating flame front is likely to interact differently with these obstacles. The flame acceleration and the amount of unreacted mixture trapped behind various obstacles are also likely to be different. The nature of such interaction is poorly understood.

In this paper we report preliminary results for the effects of various obstacle geometry and blockage ratio on the shape and speed of a premixed propagating flame front.