Indicadores

A number of approaches have been used to study and to measure liquid films. Both temporal development and quantitative thickness information is required. The technique(s) used should

give good temporal resolution, allowing a number of measurements during and after the spray injection event.

A good spatial resolution is also desirable, allowing fine detail in the film to be discerned. For the small automotive type diesel spray application studied here, this spatial resolution should ideally be in the mm range or better. The range of film thickness to be measured was unknown before commencing work, but was expected to be a few tens of microns. High absolute thickness measurement accuracy is not essential, but good accuracy mapping temporal change is important.

Film thickness measurement techniques can be broken down into mechanical, electrical and optical methods.

Mechanical film thickness measurement

Mechanical techniques are generally simple. One method is to use a micrometer gauge which is screwed in so that the end of the measuring rod first approaches and then just touches the liquid surface. This technique gives a direct measure of the film thickness in microns.

However, potential accuracy is low as it is difficult to control the exact moment that the metal rod touches the liquid, and it may not be clear when this has occurred. Surface tension effects are important. Mechanical uncertainties tend to limit the technique to relatively thick films, i.e. >100 microns. This technique is adequate for static films on a test surface, but the inherent intrusiveness and slow speed means that it is unsuitable for dynamically measuring a real spray impingement.

Electrical film thickness measurement

Electrical techniques rely on a change in the electrical environment of a immersed sensor as a liquid film thickness changes. Several electrical characteristics can be studied, including conductance and capacitance.

Conductance sensors measure the change in resistance between two metallic contacts immersed in a liquid film. The thickness of the film determines the effective resistance seen by the junction; as the thickness changes, the resistance across the contacts should also change. The resistance of the sensor can be measured by an electrical bridge circuit. The suitability of

the conductance sensor is dependent on the conductivity of the liquid itself. Liquids with high conductivity allow high sensitivity and give good accuracy.

The sensor can be small as point metal contacts can be used. This gives a good spatial resolution as there is little averaging of thickness over the sensor area. However, little success in diesel film measurement has been reported using this technique [19]. The normal

conductivity of typical diesel fuel is very low, and this leads to unacceptably poor sensor sensitivity and accuracy. A number of additives had been tried, in an attempt to increase conductivity, but were found to be needed in high concentration, which suggests a significant modification of the pure fuel physical and chemical characteristics.

Capacitance sensors measure the capacitance of two metallic contacts, again immersed in the liquid film. The liquid forms the dielectric of the resulting capacitor and so should have a low conductance to minimise the leakage across the junction. As the film thickness increases, the capacitance also increases. Gover and Ereaut [20] describe measurements of cylinder bowl film thickness using a 4 mm square capacitance sensor as part of an electronic oscillator. As the capacitance measured by the sensors changes, so the frequency of the oscillators also changes. The authors report that this method allows rapid measurement of the small

capacitances involved, thereby giving a high temporal resolution. Five sensors and associated circuits were miniaturised and fitted inside a piston. Infra red links were made to an external system, allowing measurements under engine motoring conditions.

The sensitivity of the capacitance sensor is dependent on the dielectric strength of the liquid. The sensitivity can be improved by increasing the area of the sensor, thereby increasing its capacitance, although reducing the spatial resolution achievable.

The main disadvantages of the capacitance technique are the need to install sensor(s) on the surface of interest, and extract signals from them. Also, the spatial resolution is limited by the size of the sensor.

Optical film thickness measurement

Optically based film thickness measurement techniques offer the advantage of non-intrusive measurement and several approaches are possible.

Ozdemir [21] describes a film thickness measurement technique based on optical interference. A laser beam is focused onto the liquid film through a transparent window in the surface. Reflected light from the window/liquid interface and that passing through the film and reflected from the liquid/air interface interfere together and this interfering light can be imaged onto a detector and measured. If the film thickness changes, the path length of the light ray passing through the film also changes and thus the phase between the two interfering rays. This phase change causes a change in the intensity. Ozdemir describes using a standard LDA signal processor to measure this intensity variation which is similar to a Doppler burst signal. The LDA signal processor measures the frequency of variation of intensity, from which can be calculated the rate of change in thickness of the liquid film.

The main disadvantage of this interference technique is that the actual film thickness cannot be measured directly, but can only be inferred by integrating the rate of change of film thickness. This requires the assumption of no film being present at the start of the measurement interval. Clearly, knowledge of the film thickness itself becomes less accurate as time goes on, due to the accumulation of measurement error.

Another optical approach for film thickness measurement is to exploit laser induced

fluorescence. Diesel fuel is rich in aromatic organic compounds which readily fluoresce when excited by laser light. This fluorescence is generally in the 450 - 550 nm range (blue-green). If a laser is focused onto the diesel fuel through a window in the surface, the diesel will fluoresce and the intensity of this fluorescence light is dependent on the length of the path through the liquid. The film thickness can thus be determined by measuring the fluorescence intensity. This measurement is made by focusing the laser to a spot and then imaging this spot onto a detector such as a PMT. The technique has been applied mainly for in-cylinder lubricant film

measurement [22] [23] [24] [25] [26] [27].

The main disadvantage of the above LIF technique is that the intensity of fluorescence in diesel has a strong temperature dependence. This means that local film temperature and its thickness cannot be uncoupled from the results. Consequently, a new technique was developed, which did not use the intensity of the fluorescence. This will subsequently be referred to as the “length of line” technique. Further discussion of this technique can be found in chapter 4.

The existing LIF and new length of line techniques have a number of limitations, primarily that they only measure thickness at a single point, or at best along a line. Ideally, the film

measurement technique should provide global information on the entire spray “footprint” with high temporal and spatial resolution. In addition, the technique should be capable of being employed in high temperature and pressure environments, preferably inside a real engine with a curved impingement bowl. A new technique for global film thickness measurement based on total internal reflection is proposed in the next chapter.