During the last decades, the combustion engines have become lighter, more powerful, less fuel consuming while emissions have been reduced. However, the thermal and mechanical stress in engine parts increase and make the later more prone to aging damage. This results in a growing necessity to understand aging damage and to reduce it. A typical aging symptom is the fuel based deposit formation. Unburnt hydrocarbons oxidize under thermal stress and polymerize at walls of the combustion chamber. Deposits cause an affected spray cone angle and flow losses at fuel injectors. The combustion process gets uncontrollable which results in reduction of torque and emissions increase. Fluiddynamics, thermodynamics and chemistry interact at the combustion chamber walls, in the gas and the liquid phase. The observed physical phenomena occur in different spatial and temporal scales. Single phenomena can be described only under strong limitations and a big lack of knowledge exists in the understanding of the interactions of the physicochemical processes.

This work is a contribution to a fundamental understanding of the fuel-based deposit formation. By means of two newly designed generic experimental test rigs the interactions of wetting,evaporation and deposit formation are investigated under practical and controllable conditions. Within Screening experiments single drops of various test liquids are evaporated on hot aluminum substrates. The process is recorded with a sideways camera to detect drop geometry and wetting state. The method is a convenient way to identify liquids with a high tendency for deposit formation. The second facility has been designed for the flow channel experiments. A thin liquid film of hydrocarbons is sheared by a gas flow and spreads on a heated stainless steel foil inside a flow channel. The flowdynamics of the liquid phase are recorded by a camera above the foil whereas the wall temperature at the lower side of the foil is measured by a pixel-wise calibrated infrared camera.

The Screening experiments show that the deposit formation strongly depends on the molecular structure of the hydrocarbons and the wall temperature. The drop evaporation of the double aromatic hydrocarbon methylnaphthalene results in strong formation of deposits whereas single aromatic compounds like benzene, toluene and xylenes form less deposits. The wall temperature strongly influences deposit morphology and spreading of deposits. For higher wall temperatures solid, insoluble areal and particulate deposits are formed. Deposit formation takes place more concentrated on the substrate. For lower wall temperatures deposit formation is less noticeable and leads to high viscous soluble residuals remaining widely spread on the substrate surface. Since the evaporation period decreases with increasing wall temperatures, temperature must have a stronger effect on deposit Formation than the retention time of the liquid. It has been shown, that the liquid’s state of oxidation strongly effects deposit formation. Preoxidized methylnaphtalene is turby and contains chemical groups which are known to be precursors of deposit formation. The Evaporation of only few drops already leads to the Formation of an insoluble, areal, connected shiny deposit network, which is concentrated near the impingement area.

A foil heater designed for rivulet flows enables reproducible liquid film development inside the flow channel. The spreading of the liquid film is influenced by the foil temperature, the mean gas velocity and the volumetric flow of the liquid. Increasing foil temperatures and higher mean gas flows lead to decreasing film width. High mean gas velocities result also in increased waviness of the liquid film. A quasi-steady state has been found, in which the mass flows of evaporating liquid, liquid supply and the polymerizing mass of high boiling compounds are balanced. The adhesion of polymerized high boiling compounds is mainly observed near the quasi-steady contact line and for the case of a receding contact line. The strongest deposit formation occurs on a periodically wetted foil heater. A less strong deposit formation is observed for the case of a quasisteady contact line. Porous deposit layers get preferably wetted. A dryout takes longer on deposit layers than on clean foil. This circumstance promotes further formation of deposits on already deposit-covered surfaces.

With the designed experimental test rigs, deposit formation processes have been investigated under controllable conditions. Under clean wall conditions, methylnaphthalene and o-xylene drops evaporate in constant contact angle mode. On deposit-covered surfaces, break-ups and local pinning of the liquid Phase change evaporation mode to either constant contact radius or mixed modes. This work emphasises, that the knowledge of the local wetting state and wall temperature is crucial for predicting fuel-based deposit formation. The initial wetting of walls and withthat wall Surface and material characteristics determine the later deposit formation.