Development of a constant-volume combustion apparatus for fuels research

Miller, Geoff

Development of a constant-volume combustion apparatus for fuels research

Thesis

2007

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

This project, initiated by the Sasol Advanced Fuels Laboratory at UCT, was to develop a piece of apparatus in which single compression ignition events could be viewed and studied. The pressure and temperature at injection were to be independently variable. Following a literature survey into previous fuels research work and the associated equipment used, such as rapid compression machines and continuous flow combustion chambers, the decision was made to develop a constant volume combustion chamber (CVCC). This option provided the greatest potential for optical access. The development of the high pressure and temperature environment was to be achieved using a pre-charge combustion event as opposed to electrical heating. The combustion of a lean mixture of flammable gas resulted in the desired conditions being developed rapidly, avoiding the material strength limitations of sustained electrical heating. The CVCC has a cylindrically shaped chamber, 100mm in diameter and 50mm deep, which is formed by a martensitic tool steel vessel of 230mm nominal diameter. The chamber is optically accessible by means of sapphire windows on both ends as well as from a maximum of three 38mm diameter (medium-sized) circumferential ports. Flexibility of the experimental setup was ensured by the use of common porting, whereby the two ends, the four medium-sized ports and the four small circumferential ports were interchangeable with one another. All ports were sealed with Viton 0-rings that permitted a maximum operating temperature of 200°C. The CVCC could be electrically heated to 150°C to prevent condensation on the windows. The exhaust valve, which also served as a safety relief valve, was pneumatically powered and automatically controlled by a solenoid valve. The high pressure and temperature environment required for compression ignition combustion events were achieved by the introduction of predetermined amounts of Hydrogen and/or Acetylene, Nitrogen and Oxygen into the chamber. The gases were metered into the CVCC by means of a pressure transmitter feedback loop and a solenoid valve, both of which were controlled by the main computer that made use of a data acquisition card. The mixture was ignited by a spark plug, resulting in complete pre-charge combustion and a rapid pressure and temperature increase. Following pre-charge combustion, the temperature and pressure dropped relatively gradually due to heat lost to the walls of the vessel. At a pre-selected pressure, the fuel being tested was injected into the chamber by an electronically controlled high pressure injector via a common rail or accumulator system. The predetermined component of residual oxygen in the chamber, being dependent on the original constituents, could yield either an inert environment (for non-combustive fuel spray studies) or a range of environments supporting compression ignition (i.e. a range of oxygen concentrations including an equivalent atmospheric fresh charge). In this way, visual observations could be made of controlled compression-ignition combustion events. These events were also recorded in a high-speed pressure trace from the quartz pressure transducer within the chamber. A thermodynamic model was set up to determine the required fill pressure of each gas to be metered into the vessel. Testing up to 180bar was achieved and results showed that peak pressures were repeatable to within 4%. Up to 60 highquality image experiments were achieved in one day without significant fouling of the windows or degradation of the seals. A discrepancy of approximately 15% existed between predicted peak pressures and measured pressures (being lower). Several losses were known to cause this pressure shortfall, such as slight gas leakage from the exhaust valve, radiation losses through the windows and heat lost to, and reaction quenching at, the walls of the chamber. Recommendations for improvements to the CVCC included reducing the causes and rates of leakage and quantifying the impact of surface-area to volume ratio on losses at the internal surfaces.

Keywords

Mechanical Engineering

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