Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

We describe the use of micro-thermocouples to estimate local temperature gradients in steady laminar boundary layer diffusion flames. By extension of the Reynolds Analogy, local temperature gradients can be further used to estimate the local mass burning rates and heat fluxes in such flames with high accuracy.

Transcript

The overall goal of this experiment is to describe the use of micro-thermocouples to estimate local temperature gradients at condensed fuel services for steady laminar boundary layer diffusion flames. This method can help answer key questions in the fire science field, such as exploring the dynamic relationship between a combustible condensed fuel surface and gas phase flames in laminar and turbulent boundary layers. The main advantage of this technique is that it can be used to accurately measure local mass burning rates in steady laminar boundary layer diffusion flames.

Demonstrating this procedure will be Colin Miller and Wei Tang. Both are PhD students in mechanical engineering. For liquid fuel experiments, prepare a fuel wick from porous noncombustible material, such as alkaline earth silicate wool.

Bake the wick for about 20 minutes using a diffusion flame from a propane torch. This will burn all the organic binders inside the wick. Next, apply a high temperature adhesive to aluminum foil and secure the aluminum foil to the wick.

The top must remain clean. For solid fuel experiments, prepare a sheet of the solid fuel with the same dimensions as the wick. To mount the solid fuel, cut a long slot into a sheet of ceramic fiber insulation board, or use a porous noncombustible material sealed with high temperature matte black paint.

For either free convection or forced flow experiments, position a digital SLR camera where it can view the central axis of the fuel and a full side of the flame. For forced convection flames, view the center of the fuel specimen in an area of 16 by eight centimeters, so the flame stand off distance in the pyrolysis zone can be calculated. Next, place the traverse mechanism above the fuel sample.

Then, carefully attach a 50 micron wire thermocouple to its horizontal axis. Then, turn on the programmable stepper motor controller and, for forced flow experiments, plug in the centrifugal blower of the wind tunnel. Next, set the PWM controller according to the blower speed.

Use a hot wire anemometer to verify the velocity at the wind tunnel outlet. Now, put on protective gear and make the final preparations to conduct the test. Soak the wick with liquid fuel up to saturation.

Then, carefully place the fuel-soaked wick or solid fuel plate into the fuel-wick holder. Now, critically verify the flatness of the fuel wick surface using an angle gauge. Then check the mass balance and note its reading before the test.

Before igniting the fuel, check the room ventilation. The exhaust should work but be set to minimally disturb the air flow around the experiment. Before a sample is ignited, calibrate the digital camera using a ruler where the sample will be placed.

Also, start the data acquisition from the mass balance software. Now, position the sample and then ignite the fuel with a propane torch. For wicks, momentarily touch the tip's surface with the flame.

For solid fuels, pass the flame uniformly over the surface for 50 to 60 seconds. After the burn period, extinguish the flame by blowing it out. In the case of solid fuels, let the burn go for a set time.

Then, repeat the burn process several times using the same time interval. Once all the burn data is collected, turn off the blower by setting the PWN controller to zero and unplugging the blower. Lastly, turn off the controller for the stepper motor.

Over several burns, the incremental regression of a solid fuel can be measured to verify other metrics. After each burn period, cut the burned solid along the center line. Then, document the burn regression with a side view photograph.

Analyze the side view photos in ImageJ. Open them via Select File and Open Image. Then open the image of the ruler using the Open Calibration Image option.

Next, stack the calibration image onto the solid fuel image. Now, set the measurement scale by drawing a line between two points of known distance on the ruler. Then, choose Set Scale under the Analyze menu.

In the Set Scale window, input the Known Distance of the line and click OK.Repeat this process with a new line to confirm that the measurement scale is correct. Now, measure distances in the sample photograph by drawing a line and then notice the angle and distance in the status bar. By pressing Control M, the data is saved to the data window.

The burn regression is calculated as the thickness of the sample subtracted from its initial thickness. Importantly, note the time interval over which the surface of the fuel remains approximately flat. Use the burn regression from this period to compare with temperature mapping or for adjustments of thermocouple positions made to compensate for surface regression.

To set up temperature mapping, align a micro thermocouple carefully with the surface of the fuel using an X-Y unislide. Place it at the center of the wick of a solid sample. For a fuel wick, use the X-Y unislide to position the micro thermocouple carefully along the leading edge.

It is critical that a thermocouple is positioned perfectly on the sample surface. If it is off, then all the measurements will be off by some degree. It must be as close as possible to the surface of the fuel for accurate measurements.

In the Velmex script, set the temperature mapping interval during steady burns to 150 seconds for solid PMMA or to 400 seconds for liquid-soaked wicks. Set the step size, measured at the sample's surface, to collect data every quarter millimeter. Set the sampling rate to between 100 and 500 hertz.

If the surface deforms within the errors of the experiment while mapping, then the results are no longer valid, so it is critical to set the measurement period and spacing correctly. Now, when conducting the burn experiment, run the data acquisition program so that it reads the grid scanning algorithm from a folder on the desktop. With the program running, the data will be recorded automatically when triggered by the data acquisition module in lab view.

Mass loss rates and temperature profiles over a forced convection boundary layer diffusion flame were measured for PMMA samples. The results show accurate temperatures can be captured close to the fuel surface with small errors, thanks to micro thermocouples and two thermocouple radiation correction method. Non-dimensional temperature gradients along the fuel surface of a PMMA boundary layer diffusion flame were measured at two air current speeds.

Variation of the local mass burning rates were also measured in different free stream conditions. Local mass burning rates obtained through non-dimensional temperature gradients were also compared against the experimental data obtained through regression of the PMMA surface. They matched very well.

Components of the flame heat fluxes were examined in greater detail at a forced flow of 2.06 meters per second. The detail of temperature measurements near the surface allowed for extraction of convective heat fluxes which, when combined with surface mass loss rates, provided all the components of burning for local analysis. Once mastered, this technique can be performed in a few minutes per fuel sample.

Following this procedure, analysis of local mass burning rates and heat transfer can be performed in order to further understand the burning behavior of materials and validate numerical codes. Don't forget that working with these flames can be extremely hazardous and precautions such as wearing protective eye wear, working in well-ventilated conditions, and ensuring burning materials are non-toxic, should always be taken while performing this procedure.

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Journal (Engineering)

Journal (Engineering)

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

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