Combustion of Liquid Fuel Sprays (eBook)

Alan Williams (Autor)

eBook Download: EPUB

2013 | 1. Auflage
300 Seiten
Elsevier Science (Verlag)
978-1-4831-0158-3 (ISBN)

Infos

Combustion of Liquid Fuel Sprays outlines the fundamentals of the combustion of sprays in a unified way which may be applied to any technological application. The book begins with a discussion of the general nature of spray combustion, the sources of liquid fuels used in spray combustion, biomass sources of liquid fuels, and the nature and properties of fuel oils. Subsequent chapters focus on the properties of sprays, the atomization of liquid fuels, and the theoretical modeling of the behavior of a spray flame in a combustion chamber. The nature and control of pollutants from spray combustion, the formation of deposits in oil-fired systems, and the combustion of sprays in furnaces and engines are elucidated as well. The text is intended for students undertaking courses or research in fuel, combustion, and energy studies.

Textauszug

CHAPTER 2

THE PROPERTIES OF SPRAYS

Publisher Summary

This chapter focuses on the properties of sprays. The analysis of sprays of liquid fuels produced by oil burner atomizers in terms of droplet size distribution, spray angle and spray pattern is important in all applications of fuel spray combustion. Only a limited number of atomizers produce reasonably mono-sized droplets and atomizers in practical combustion chambers produce directly or indirectly a spray having a spectrum of droplet sizes. This varies throughout the spray and it is best when the experimental data on droplet sizes are obtained on a point-wise basis. In addition to the spray mass-flux distribution, it is necessary to have information on the gas phase velocity field and this is obtained in a spatially resolved way. Experimental determination of the droplet sizes in terms of numbers of droplets in a particular size range is important. The size distribution of a spray varies with distance from the atomizer because the acceleration (or deceleration) of each droplet is an inverse function of droplet diameter, and thus the spectrum of a poly-disperse spray varies with distance from the atomizer. The most commonly used sizing techniques are: the frozen drop and wax method, microscopic examination of collected liquid droplets, probe techniques, direct photography, and laser diffraction techniques.

The analysis of sprays of liquid fuels produced by oil burner atomisers in terms of droplet size distribution, spray angle and spray pattern is important in all applications of fuel spray combustion. In general, only a limited number of atomisers produce reasonably mono‐sized droplets and atomisers in practical combustion chambers produce directly or indirectly a spray having a spectrum of droplet sizes. Since this may vary throughout the spray it is best if experimental data on droplet sizes are obtained on a point‐wise basis although often integrated values are used. In addition to the spray mass‐flux distribution it is necessary to have information on the gas phase velocity field and this must be obtained in a spatially resolved way.

2.1 Experimental Methods for Determining Droplet Size Distributions

From the practical point of view it is important to be able to obtain an experimental determination of the droplet sizes in terms of numbers of droplets in a particular size range. However it should be noted that the size distribution of a spray will vary with distance from the atomiser because the acceleration (or deceleration) of each droplet is an inverse function of droplet diameter and thus the spectrum of a polydisperse spray will vary with distance from the atomiser. In addition droplet‐droplet collisions may occur, especially in dense sprays under turbulent combustion conditions and furthermore the droplet sizes may vary significantly across a spray because of the mechanics of the atomisation process.

The most commonly used sizing techniques are described in the following sections.

The Frozen Drop and Wax Method

These are older but still useful techniques in which the atomised spray is directly measured as solid particles. In the frozen droplet technique the spray is injected into an alcohol or acetone bath which is maintained at the temperature of dry ice; alternatively the spray is injected directly into liquid nitrogen. The frozen particles so produced can then be sized either simply by sieving, or by photographing the particles and counting them later using enlarged photographs, or by means of an automatic particle counting and sizing technique. Of the automatic techniques available one convenient method involves the use of a commercial image analyser (eg the Quantimet Image Analyser) in which the sample may be examined directly by a microscope system or photographs may be sized by an epidiascope arrangement. The image produced by the video scanner, which is of the type shown in Plate 2.1, can be analysed by the central processor to give the size distribution of the particles being studied.

Plate 2.1 Use of the Quantimet Image Analyser to size wax particles produced from an atomiser.

The ‘wax method’ is dependent upon the fact that paraffin wax, when heated to an appropriate temperature above its melting point, has properties such as viscosity, surface tension, etc. which are similar to many liquid fuels, in particular the aviation turbo‐fuels. In the application of this method molten paraffin wax at the appropriate temperature is pumped to the spray nozzle and the wax spray so produced is directed into water in order to solidify the droplets produced. The solid wax particles are again collected and sized by passing them through a series of graded gauge sieves.

Whether sieving or image sizing techniques are used both methods result in the formation of a drop size histogram of the type shown in Figure 2.1.

Figure 2.1 Typical drop‐size histogram.

Microscopic Examination of Collected Liquid Droplets

One of the simplest techniques involves collecting the sample of a spray on a glass microscope slide and making a microscopic examination of some 500 to 1000 droplets. There are many variants of this technique which have been described by Putnam and Thomas (1957) and by Giffen and Muraszew (1953). One convenient form, particularly for volatile liquids, involves using a microscope slide coated with a film of magnesium oxide on to which the droplets may impinge. These cause indentations in the MgO film as illustrated in Plate 2.2. The sizes of the indentations may be measured automatically as previously indicated but they have to be corrected for the effect of flattening on impaction, the factor of 0.74 being commonly employed. If direct impingement is made on to a microscope slide (without a MgO film) then in the case of heavy fuel oils the factor is 0.5. In all cases care must be taken that not more than 1% of the slide is covered with droplets so that the chances of multiple impactions are reduced and a photographic shutter device to give a short exposure is a convenient way of achieving that.

Plate 2.2 A magnesium oxide coated slide used for droplet measurements (larger droplet sizes in the region of 100 μm).

Probe Techniques

Another method of directly measuring the sizes and distribution of droplets in a spray is by means of direct electronic counting using a probe inserted into the spray. Such methods have the additional advantage of providing information on the spatial distribution of droplets, and can, at least in principle, be used to obtain results in high density spray regions which occur, for example, near atomisers. A typical example of this technique is the pulse counting technique which measures the droplet size spectrum by depending upon the individual droplets completing an electrical circuit as they pass between two needles positioned an appropriate distance apart (Pye, 1970). In such probes it is thus necessary to use a conducting liquid as the atomised fluid, that is, they are conductivity probes. By varying the spacing of the needles over the size range of interest and counting the number of droplets at each separation the size spectrum may be obtained. Usually an aqueous solution of a salt is used and care obviously has to be taken so that its viscosity is equivalent to the corresponding oil. Other forms of these probes can be operated as fast response pitot tubes containing pressure transducers or capacitance probes. Information on the local momentum flux or local liquid phase concentration can be determined as a function of time. Light guides can also be used to obtain similar data with large scale atomisers using an opaque oil such as heavy fuel oil (Yule et al, 1988).

Direct Photography

A further direct method of sizing droplets, but this time in situ and non‐intrusive, is by direct photography as illustrated in Figure 2.2. This technique is an extremely valuable and convenient method of recording the size distribution of droplets in a liquid spray since the collection and need for introducing and obstruction in the path of the spray are avoided. Essentially the technique involves a short duration light source, less than 1 μsec and a conventional, but high magnification, camera arrangement is used as shown in Figure 2.2. However great care must be taken in applying this technique to avoid distortion of the size distribution obtained. The technique gives the size distribution in a given volume, i.e. the spatial distribution, whilst as far as spray combustion is concerned the need is for the temporal distribution, that is the size distribution passing through a particular plane over a period of time. Spatial distributions can be converted to temporal distributions if the velocity of each drop is known. A development of this technique is laser holography but this involves a lengthy analytical procedure.

Figure 2.2 Typical arrangement for the spark‐illuminated photography of a spray. By means of a double spark system it is possible to obtain details of size, velocity and trajectory of the droplets.

Laser Diffraction Techniques

A number of optical and thus non‐intrusive techniques were developed in the...

Produkt-Details

• Erscheint lt. Verlag: 22.10.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 1-4831-0158-4 / 1483101584
• ISBN-13: 978-1-4831-0158-3 / 9781483101583