Ceiling Jets Assessment

Objective:

To investigate the properties of an unconfined ceiling jet flow involving a pool fire.

Apparatus:

The apparatus consists of an unconfined ceiling jet flow apparatus with an adjustable floor allowing the height of the ceiling to be easily altered (See figure 1). Three type K thermocouples are employed as shown in figure one. A Computerized data logging system is employed to provide results. Three sizes of metal fuel trays are provided.

Theory:

For a ceiling jet situation it has been suggested that the maximum temperature at ceilings of height H along the fire axis is given by.

Where Q is the rate of heat output from the pool fire.

For the radial temperature variation (Figure 2) the following expression is valid.

Examining the expression for r/H <=0.18 and simplifying by assuming quasi steady state conditions then the equation may be simplified to:

Where K is a constant or:

Employing Logs:

Where N is a constant and may be determined graphically in the form of the gradient of a plotted straight line of Log of T versus Log H. Log K is the Log of the intercept. Note employ temperatures at approx steady state.

Procedure:

• Locate the thermocouples and ensure they are correctly connected to the data logging equipment carefully noting the identity of each channel.
• Set the height to an initial distance e.g. 1.4 m.
• Set in a tray of methylated spirits (300ml approx.. for each tray size).
• Initialise the data recording devices,
• Light taper provided and carefully ignite the fuel whilst simultaneously starting the recording devices,
• Pay close attention to the flames and make relevant observations
• Repeat the experiment for four more heights and each tray size (heights of 1.2m, 1.0m, 0.8m and 0.6m).
• From the resulting data test the correlation suggested employing thermocouple two to prove the relationship i.e. N = -5/3.

Results and Observations:

Record clearly any observations made.

Analysis of Results:

• Using the data from the logging equipment plot graphs of the Log of ?T versus Log H and determine Both K and N.
• Make pertinent comments regarding the shape of the graphs,
• Plot temperature versus time graphs for the three thermocouple readings for each test and analyse.
• Make any relevant comments.

Conclusions:

Record any relevant conclusions you have made as a result of carrying out these experiments.

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