1. Background

Thermal radiation is the main mechanism for transferring heat from flames and combustion products to the load in a furnace or to the steam or hot water within a boiler. The IFRF has developed a number of instruments that allow radiant heat fluxes to be measured in different ways and at different locations. Such measurements may be used directly for the design of furnaces and boilers. These days, however, they are increasingly used as a means of validating and calibrating mathematical models that often offer a more versatile tool for furnace design.

2. What parameters can be used to characterise radiant heat transfer?

Within the IFRF, the most common parameters used to characterise radiant heat transfer are:

• Unidirectional heat flux – the amount of total radiation (over all wavelengths) incident at a given position from a given direction
  (Wm^-2 steradians^-1)
• Unidirectional flame [GLOSS]emissivity[/GLOSS] measured by the [GLOSS]Schmidt method[/GLOSS]. This gives a measure of the emissivity of a flame, usually along a radius through the flame axis, and as a function of axial distance from the burner
• Local unidirectional flame emissivity, a measure of the total emissivity of a small volume of gases within a flame or a furnace
• Hemispherical heat flux – the thermal radiation incident at a given position from a hemisphere centred on that position
  (Wm^-2)
• Total heat flux – the total heat incident on a plane at a given position by radiation and by convection
  (Wm^-2)

In addition, it is also useful to determine the temperature and composition of the gases within the flame and the furnace, both of which have a major influence on radiant heat exchange.

3. How can these parameters be measured?

At the IFRF, a number of probes to measure the above parameters have been developed and proven in furnace use over several decades. The probes used are identified below. Although alternatives to the IFRF probes are available, including many laser based methods, the IFRF probes still offer a low cost, reliable and proven method of measuring flame radiation parameters.

Unidirectional heat flux

Measured with a Narrow Angle Radiometer (CF141). The probe comprises a radiation detector viewing the flame or furnace through a long narrow tube, which acts as a collimator. Requires calibration.

Flame emissivity by the Schmidt method

Calculated from radiation measured with a narrow angle radiometer (CF141) against hot and cold targets introduced into the furnace. Requires the probe to be traversed through the flame/furnace.

Local flame emissivity

Measured by differentiating the curve of incident unidirectional radiation obtained as a narrow angle radiometer
(CF141) is traversed across a furnace or through a flame. The accuracy of these measurements is sometimes compromised by the small differences that exist between successive readings of a fluctuating value. A special version of the IFRF narrow angle radiometer was therefore developed to measure this value directly by creating a well-defined column of local combustion gases within a small extension to the narrow angle radiometer.

Hemispherical heat flux

Measured using an ellipsoidal radiometer (CF136). This instrument collects all the radiation incident at a given point and passing through a small orifice at one focus of an ellipsoidal cavity. A detector at the other focus picks up the radiation, but is protected from convection caused by direct contact with the flame or furnace gases. Requires calibration.

Total heat flux

Measured using a small heat flux meter whose flat receptor surface is exposed to both radiation and convection from flame gases
(CF138). Requires calibration.

Gas temperatures

Measured using a suction pyrometer (CF145). An appropriate thermocouple (for the temperature being measured), is placed within a system of refractory shields. The hot gases under study are sucked through the shields at a rate that ensures that the thermocouple measures the true gas temperature without the need for calibration or radiation corrections.

Local gas compositions

Various methods exist to determine the local composition of gases within a furnace, including loading in solids and liquids. These vary from probes, which aspire samples for remote analysis, to optical methods (often laser based). A general description of the probe-based methods is given in [1].

4. Where can I find out more?

IFRF members can find further information on these measuring techniques in additional Combustion Files that are directly accessible from the links below.

Advice is also available from the IFRF Research Station, which can supply calibrated probes to meet specific requirements.

Reference [1] contains a more detailed description of these and other techniques for in flame measurements.

Sources

[1] Chedaille J and Braud Y, Measurement in Flames, London, E Arnold, (1972)