Investigation of tube fouling was carried out employing the heated nozzle module of Aviation Fuel Thermal Stability Test Unit (AFTSTU). This includes straight, thick-walled stainless steel tube, exposed to a constant heat flux provided by a Radio Frequency (RF) heating unit that is coiled around the tube. In this situation, deposits are allowed to form on the inner surface of the tube. Due to the considerably lower thermal conductivity of the deposit compared to the stainless steel, temperature at the tube external wall increases with time. This was recorded by the application of three pairs of thermocouple situated at different axial locations along the tube. Each pair of thermocouple monitored the data from two different radial positions. The fuel temperature at the tube exit was recorded by the use of another thermocouple. The recorded results for each thermocouple indicate that three distinct stages can be observed as function of time. The first stage represents a temperature invariant zone, known as induction period, followed by a remarkable temperature rise. The third stage represents a level off zone. Experiments were carried out for two different baselines fuels each experiment consists of four stages. In each baseline test, four different thermal powers were supplied by RF heater. Consequently, four initial tube wall temperatures were set. Then surface deposition was investigated during four different exposure time. The second baseline test is different from the first test in that the fuel inlet was kept at higher temperature. In order to imitate the temperature rise across the heated tube, a two dimensional, axis symmetric, time dependent Computational Fluid Dynamic (CFD) model was developed. In this model dynamic mesh technique was employed to predict the temperature rise in a changing boundary. This technique was also used to imitate the surface deposition in Near Isothermal Flowing Tube Reactor (NIFTR) where the tube is subject to the constant wall temperature. The experimental results of the NIFTR for code validation were taken from the literature.
A number of preliminary investigations were performed prior to the CFD modelling on the chemistry of autoxidation, the process which is responsible for the formation of species precursor. Pseudo-detailed mechanism of liquid phase autoxidation which is widely used in most of the investigations was used for in this thesis.
In order to develop an integrator code for the investigation of rate equations, two different approaches were applied. Firstly an integration based on the Taylor series was applied. In spite of the applicability of this method for the reduced chemical schemes (3-4 reaction steps), it proved to be computationally very expensive for the integration of pseudo-detailed mechanism. Hence, it was not included for the further applications. Secondly, an integrated program was developed in MATLAB to integrate the system of non linear ordinary differential equations. In order to overcome the stiffness caused by different time scales, Gear's method was used. Furthermore, a function was developed to calculate the sensitivity of dissolved oxygen profile as well as the hydroperoxide species to the rate constants of each reaction steps. It was proved that the reaction steps can be classified as high sensitive, low sensitive and no sensitive. As a result, the chemical scheme was reduced and applied in the CFD calculations.
It is very important to take in consideration the fact that fuel sample faces different temperature regimes through the various compartments of the AFTSTU. Unfortunately, due to the experimental constraints it was impractical to have an online measurement of jet fuel chemical compositions at tube inlet. Therefore, by the application of perfectly stirred reactor based on the residence time and temperature of each compartment, the species boundary condition for the tube inlet was calculated
