Page 415 - Practical Design Ships and Floating Structures

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condition and torch height are unchanged. Furthermore, these theories are applicable only for the cases
where torch travels at a constant speed in straight-line motion. These theories assume that the relative
distribution pattern of heat flux around the stagnation point remains unchanged in time. Strictly
speaking this assumption does not hold true, and this is the reason why the application of those theories
is narrow.
It is needed to develop a new theory with wide application range on the bases of the true nature of
thermal-flow filed within the gas flame and local heat transfer. In such study, it is necessary to
measure the transient temperature distribution of gas flame near the plate surface accurately. Such
measurement can be performed by laser induced fluorescence (L.I.F.) technique. Distribution of local
heat transfer can be estimated by inverse heat conduction analysis.
In this paper, the transient 3-dimensional temperature distribution of the spot heating gas flame is
measured in detail by a high performance L.I.F. measurement system. From the results of this
experiment, a hypothesis on the distributions of gas temperature and local heat transfer coefficient is
built up. The appropriateness of this hypothesis is proved by performing an inverse heat conduction
analysis of a spot heating experiment.

2 MEASUREMENT OF GAS TEMPERATURE DURING SPOT HEATING BY USING L.I.F.
TECHNIQUE
2.1 Esrperimental Apparatus

A methane gas is used as fuel gas. Table.1 shows the pressure and the flux of methane, oxygen and
NO. The heating power of this torch is,much lower than that of line heating torches used in shipyards.
A square mild steel plate with sides 500mm long and 16mm thickness is arranged horizontally and a
torch with a circle shaped nozzle of 0.9mm diameter is positioned above the plate. The center of the
plate is heated by a flame of premixed methane and oxygen. The distance Z between the nozzle and
the plate is arranged to be 12,20 and 28mm.
2.2 Laser Optical System

The thermal field within the combustion flame is measured by the laser induced fluorescence (L.I.F.)
technique and an optical measurement technique. Two wave-length L.I.F. technique, which utilizes
NO in the measurement of temperature, is used.
The absorption band of NO exists near the 225nm, and the fluorescence is emitted in the band of
23Onm-300nm. The characteristics of fluorescence and the wavelength of laser are selected so that
they are suitable for the temperature range of 1000K-2000K. The outline of L.I.F. optical system is
shown in Figure 1. Oscillated wavelength of YAG lasers and dye lasers is set to 10- which is the
absorbed wavelength of NO. Laser beams 1 and 2 are gathered together at a beam combiner, and then
they are irradiated to both a correction burner and an object flame of measurement.
The signals of fluorescence from target and correction burners are photographed on the upper and
lower part of camera pictures by using two ICCD cameras, and the strength distribution of fluorescence
NO molecules emits is measured. The measuring timing of the laser device and ICCD camera is
controlled with delay-generator. Measured pictures are forwarded to a computer to analyze flame
temperatures. The measured region is 20Omm in width and 20mm high. The space resolution of the
measured picture is 1.5 mm. From the instant when the torch is set up to home position laser beams
are irradiated for 0.4- by every 2 sec. Herder, t denotes the elapsed time since the measurement
starts. The total numbers of laser irradiation is 12 times per each measurement.