Crazy School

Sound Emitting Flame and Water Droplet Formation

by T. Takács

AEKI School

Tata, March 26-28, 2008

Contents

·       prelude: Structure of Sound Emitting Flames, MSc thesis, 1987

·       experiments objective: optical and acoustic study of steady and quasiperiodic premixed flame fronts on slot burner with changing gas mixing ratios

·       results

·       fire, sound and water droplet: from similarity to analogy

·        

The Experiment

·       experimental setup is shown below

·       its elements:

o   burner head with a 1x100 mm horizontal slot,

o   a gas pipe and an air pipe was connected to the burner head in which gases have been naturally mixed,

o   volumetric flow rate of town gas (V_g) was controlled by a tap and volumetric flow rate of air (V_l) was changed by a tiristor controller (air was supplied from vacuum hoover) and both measured by rotameters,

o   premixed free jet flame of the mixture is studied above the burner slot (blue arrow below),

·       devices:

o   optical part: a stroboscope, a fast camera and a schlieren device with color strip filter to make flame shape photos,

o   rotameters to measure volumetric flows of air and gas,

o   acoustic part: a microphone and a memory oscilloscope to registrate sound

Sketch-plan of the Experimental Setup

Results

·       method: air flow rate was set step by step and gas flow rate was changed to investigate flame behavior, sound and shape were detected

·       flame front: the surface where burning occurs, separating fresh unburnt gas from the burner head and burnt gas on the other side of the front (in red circles on photos afterwards), its position is determined by flow and burning velocity

·       observed flame fronts:

o   1 steady „bell” (regular flame front shape),

o   2 steady left-S* (see on photo) or right-S* (gas was ignited from left or right perpendicular to slot longer axis), when the burning speed was not able to close the front to steady bell having been less than the flow velocity

o   3 quasi-steady bell (sound emitting state)

(*: S-shapes rather stable at mixture ratio when flame produced sound)

·       sound observation: characteristics of flame sound 600-1100 Hz and 80-90 dB occured at certain gas-air mixture ratios

·       deformation and break of laminar flame front could be detected optically (below the longitudinal axis is perpendicular to the plain of the photo):

·       sound emission occured only when the mixture was burning

·       sound occured only when „bubbles” are separated from the flame front tip

·       hydrodynamically non-turbulent flame emits sound! (photo at right shows the sensitivity of schlieren method)

·       sound could not be tuned out by inserting several screw nuts in the burning head so resonance was excluded

·       flame shot in sound emitting state with a slightly rotated burner head (cylindrical structures on side view):

·       supposed reason for sound emission: burning of periodically separating gas bubbles

·       map of sound emission can be seen on figure to the right (x = Vl air flow rate, y = Vg gas flow rate):

·       scaled amplitude(t) and its Fourier transformation power(f) of measured sound

o   on the right figure the first hump at cca. 300 Hz is from vacuum hoover (air supply), the second one cca. 700 Hz is by the flame

·       main findings:

o   steady flame fronts: different stable shapes have been detected

o   at certain mixture ratios very intensive sound occured from the flame with around 90 dB and frequency between cca. 600 and 1100 Hz

§  flame shape was filmed when sound was produced

§  flame sound was recorded by memory oscilloscope

§  even non-turbulent flame can produce combustion sound

·       possible explanation: sound is produced by periodically detached flame front bubbles, the fresh gas burns inward the bubble generating sound – may be checked in gas boilers as well

·       where can we find similar „bubbling” in nature?

·       obvious visual similarity can be noticed with water droplet formation on bottom of horizontal plain or jet bridges in fluid splash

·       mathematical description of water droplet detachment was given in Science (X.D. Shi et al. 1994)

·       PDES to describe water breakup here is similar to flame front equation system so there is a mathematical analogy behind visual similarity!

·        See next photo. liquid splash form is compared with sound emitting flame front shape.

That’s all folks!