Reynold's Number Effects on Deposition in Heated Flows, 03-9277

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Principal Investigator
Clifford A. Moses

Inclusive Dates: 10/01/01 - 10/01/02

Background - Advanced, high-efficiency aircraft gas turbines operate at higher pressure ratios and temperatures. As a result, the fuel is exposed to higher temperature environments. Jet fuel has a thermal stability limit beyond which the fuel begins to react with trace oxygen dissolved in the fuel to form varnish deposits. These deposits can foul heat transfer surfaces or block small fuel passages or both. Research has shown that deposition rates are affected by fluid mechanics and heat transfer, both of which are well-correlated with Reynold's number as a measure of turbulence in the flow. However, there are conflicts in the literature as to whether increasing the Reynold's number results in higher or lower deposition rates. These conflicts make it difficult for engineers to utilize the results of laboratory deposition experiments in optimizing heated flow systems to minimize deposition.

Approach - The objective of this research project is to determine if, and under what circumstances, Reynold's number is an effective correlating parameter for the effects of velocity and dimensional variables on the deposition rates in fuel flows through heated tubes. The approach incorporates both a heat transfer analysis and deposition experiments. The analytic approach is to first use solutions from the literature for the temperature and velocity fields to predict the formation rate of deposit precursors and then to numerically solve a diffusion equation for their subsequent transport to the wall to form deposits. The goal is to develop a model for deposition rate as a function of flow variables, for example, velocity and diameter, boundary conditions, and initial flow conditions. The deposition experiments will be conducted with an existing deposition rig both to provide data for validating the analytic model as discussed above and to develop an empirical model that will provide insight on the effects of wall and fuel temperatures as well as the correct correlating parameters for flow velocity and diameter.

The deposition experiments are being conducted in short heated tubes of varying diameters and lengths typical of flow passages in the tips of fuel nozzles, i.e., approximately 0.25-inch length by 0.020-inch diameter. The outside wall temperature is adjusted according to the flow velocity and wall thickness to maintain constant wetted wall temperature among experiments. Furthermore, the heated length is adjusted according to the flow velocity to keep residence time constant. In this manner, the Reynold's number is the only variable.

Accomplishments - In the analytical study, a partial differential equation has been derived to describe the diffusion of deposit precursors. This equation, along with the appropriate boundary conditions, is being numerically solved with MATLAB™ to develop a relationship between deposition rate and the flow and heat transfer conditions.

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