MSc Project: Experimental study using PIV of Flame-Wall Interaction of a Flame Jet Impinging on a Cooled Cylinder

 

The interaction between a flame and a wall is important topic both in heat transfer research and in combustion research.

A turbulent flame jet impinging normally on a surface is a flow configuration that is extensively used in process industries that aim to achieve intense heating. The Nusselt number characterizing the heat transfer rate is highest in the vicinity of the stagnation point. In order to be able to design and control the heat transfer process one has to understand the highly complex flow and the flame-wall interaction.  

Flame-wall interaction in combustion equipment often is an unwanted effect. Flame-wall interaction can lead to various effects on the overall conversion and the pollutant formation of the flame. It can also lead to an undesirable decrease of the lifetime of the wall.

In fact, flames usually do not actually touch the walls because the rapid loss of thermal energy of the flame to a relatively cold wall causes the temperature to drop, leading to quenching of the flame. Burnt gases can reach a temperature up to 2500 K while the wall temperature in technical applications is in the order of 500 K due to cooling. The temperature decrease from burning layer to wall, takes place in a layer less than 1 mm thick.

In general one has to distinguish between impingement on flat walls and curved walls. The present study concerns the case of a curved wall. To study the flame-wall interaction, a configuration has been designed with a premixed flame jet impinging perpendicularly on the sidewall of a cylinder. The cylinder can be cooled on the inside to investigate the effect of wall temperature on quenching. Other important parameters that can be varied are the air-fuel ratio, the distance between nozzle and wall to nozzle diameter ratio and the jet Reynolds number.

Particle Image Velocimetry (PIV) was be used to measure the flow velocities. The wall temperature profile was measured by ten miniature high temperature thermocouples placed carefully inside the cylinder.

This project was concluded by the presentation of the MSc thesis
Flame-wall interaction of a flame jet impinging normally on a cooled cylinder
by
Maikel van der Steen on June 3, 2014.

On October 9, 2014, at the Combura Symposium in Soesterberg, this thesis was given the NVV award for the best MSc Thesis on combustion of the year 2014 (in The Netherlands and Belgium)

Summary of the MSc Thesis:

The main objective of the present experimental investigation is to study the turbulent reacting flow field in the region where a stable premixed flame jet (Re = 3250 and ϕ= 1.3) impinges normally on a cooled cylinder and gain a better understanding of the effect of the cylinder wall temperature. PIV measurements were done in a plane normal to the cylinder axis for both a cold wall (100 °C) and a hot wall (500 °C). The effect of wall temperature on mean flow velocities and Reynolds stresses were investigated by comparing the results of detailed PIV measurements for both cases.

The effect of wall temperature on the mean flow velocities is minimal. In the impinging jet region, small differences in the mean velocities only exist near the wall due to thermal expansion. In the wall jet region, small differences in the mean velocities arise further away from the wall caused by the mean position where large scale vortices are created. It was found that the position where these vortices form are generally further upstream for a hot wall than for a cold wall. Large Reynolds stresses are found at two locations: (i) At the location where the inner flame bends around the wall, large Reynolds stresses are present due to the wavy motion of the inner flame as a result of shear between the fast unreacted cold core and the slow diffusion flame. (ii) At the outer edges of the diffusion flame, large Reynolds stresses are present due to the periodic passing of large scale vortices, altering the velocity flow field in the wall jet region. At both locations the Reynolds stresses increase with increasing cylinder wall temperature, which is related to the position where vortices form.

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