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Chapter 2. Literature Review

2.7 Experimental Procedures

There are four different types of experimental setup found in the literature [1, 13], which are used to measure the combustion data of HFO;

1. Fuel Ignition Analyser 2. Visual Combustion Chamber 3. Visual Engines and

4. Thermogravimetry differential thermal analysis (TG-DTA).

2.7.1 Fuel Ignition Analyser (FIA)

The FIA is an instrument developed in Norway by joint effort from the Norwegian Institute of Technology and the Norwegian Marine Technology Research Institute – MARINTEK [132]. The FIA measures the ignition quality of fuels based on ignition delay. It works on the same principle as the constant volume combustion rig (discussed in section 2.7.2). In the FIA, the fuel sample is injected from the top into the highly pressurised and heated air constant volume chamber. The ignition delay of the fuel provides the measurement about the quality of fuel.

Generally, injection in FIA is attained by plunger piston with the help of springs in an injection pump. At the pre-defined chamber temperature and pressure, fuel is injected through a 0.35 mm diameter nozzle. The ignition delay of the fuel is defined as the time between start of injection (injection needle lift detected) to the time of first combustion. The first combustion is generally measured as the increase in 0.2 bar pressure from the initial chamber pressure. The ignition delay is measured in milliseconds. The chamber condition can be set at any chamber condition (temperature and pressure) but for marine and/or heavy fuel oil it is set as 450 oC and 45 bar, whereas for the light and distillate fuel it should be set as 450 oC and 20 bar. A schematic of the apparatus of FIA version 4 obtained from Takeda et al.[133] is shown in Figure 2-10. The complete testing procedure is controlled by computer, which allows the required parameters to be set, depending upon testing fuels.

Figure 2-10: FIA version 4 (from Takeda et al.[133]).

2.7.2 Visual Combustion Chamber

The visual combustion chamber is a large cylindrical chamber developed by Takasaki et al. [1] at Kyushu University in Japan. It is used to visualise the spray/flame and study the combustion. A typical setup of the visual combustion chamber along with the upper and lower windows is shown in Figure 2-11.

Figure 2-11: Visual combustion chamber along with configuration of windows (courtesy of Takasaki et al.[1]).

As shown in Figure 2-11, the visual chamber has 150 mm bore, greater than 275 mm length and it is electrically heated. The fuel spray is injected through an injector into the chamber, where air is kept at 2.5 MPa pressure and 600 oC. Windows of the chamber allow optical access to the full length of the spray. Since the distance between nozzle and the bottom of the chamber is around 300 mm, the interaction between wall and spray can be avoided. Hence, relatively long free sprays can be observed. The special design fuel injector system for bunker fuel oil is electronically controlled. In CVCC, injection proceeds beyond the ignition, while in FIA injection ceases before the start of the combustion.

The results obtained from the FIA and visual CVCC measurements of heavy fuel oil are given in Chapter 7 (section 7.2) where the actual comparison between experiments and simulations are illustrated.

2.7.3 Visual Engine

As described by Takasaki et al.[1], visual engine is used to visualise the combustion in the combustion chamber. A visual engine is two-stroke single cylinder engine with a 190 mm bore. It has 15-16 bar IMEP (Indicated Mean Effective Pressure) and 400 rpm engine speed. More detail about the visual engine setup can be found in Takasaki et al.[134].

2.7.4 Thermogravimetry Differential Thermal Analysis (TG-DTA)

Thermogravimetry Differential Thermal Analysis (TG-DTA) is a commercial instrument which is used to measured the combustion performances of fuel [13]. TG-DTA combines the high flexibility of the differential temperature analysis (DTA) feature with proven capabilities of the Thermogravimetry (TG) measurement technology. In TG-DTA experiment, a fuel sample is heated at a constant rate in oxidising environment.

In TG, change in the mass of the sample is continuously recorded with respect to time or temperature. In DTA, the sample and an inert reference are made to undergo identical thermal cycles, difference between sample and inert reference temperature are recorded.

This differential temperature is then plotted against time. Changes in the sample, either exothermic or endothermic, can be detected relative to the inert reference.

Uehara et al.[13] applied the TG-DTA technique to 28 samples of marine fuel oil by using the Rigaku TAS-300 thermal analysis system. In their experiment, temperatures ranged from room temp to 1000 oC and carrier gases were nitrogen and air. Moreover, air flow rate used was 100 mL/min and heating rate was 100 oC/min which is close to real combustion heating rate. Typical outcome of the experiment is shown in Figure 2-12.

Figure 2-12: TG-DTA results of marine fuel oil (from Uehara et al.[13]).

It can be observed in Figure 2-12 that mass loss due to evaporation and combustion (TG curve) begins at around 150-200 oC and continuously decreases till the sample reaches burnout point. The increase in DTA curve shows the exothermic reaction. Two DTA peaks are apparent in above figure, the first peak appeared at around 300 oC is due to the combustion of cutter stock and thermal cracking gases, and the second peak appeared at around 600 oC is due to the combustion of carbonaceous residue (polymer).

The ignitability and combustibility of the fuel is estimated from the ignition and the burnout points respectively. Uehara et al.[13] studied the ignition and combustion quality of the good (normal) and the poor fuels. They found that the ignition and burnout points of the poor fuel are higher than the good fuel. Completion of combustion of poor fuel occurs

at a higher temperature compared to good fuel. They [13] also pointed out that high asphaltene content fuel has more combustion failure tendency.