Time-accurate conjugate CFD analysis of a jet-impingement configuration with sudden changes in heating and cooling loads

Chien-Shing Lee, Purdue University

Abstract

When the operating condition of a gas-turbine engine changes from one steady state to another, the cooling must ensure that the material temperature and its gradient never exceed the maximum allowable throughout the transient process. The objective of this study is to understand how to cool a material subjected to a sudden increase in heating load by using the minimum cooling flow. The focus is on understanding the unsteady processes associated with the cooling process and the response of the material to sudden changes on its heated and cooled sides. The problem selected to generate the understanding is a flat plate heated on one side by a specified heat flux and cooled on the other side by an array of impinging air jets. The plate is made of either a Ni-based superalloy or a composite-matrix material. The composite matrix consists of a grid made of a material with high thermal conductivity and high thermal diffusivity (denoted as Mat-C) and a material with high heat capacity (denoted as Mat-S), where contact resistance between Mat-C and Mat-S were not considered. For this problem, unsteady conjugate analysis based on RANS with the SST turbulence model for the air and the Fourier Law for the plate was used to study the details of the unsteadiness in the fluid flow and solid and to determine the minimum cooling flow rate. Results are presented to show the details of the steady states and the transients. At steady state when the plate is made of superalloy, though the Biot number is much less than 0.1, there is considerable temperature variation in the plate because of the large variation in the heat-transfer coefficient on cooled side of the plate. The unsteadiness in the cooling jet involves the reflection and the interaction of finite-amplitude pressure waves, the generation of a starting vortex, and the formation of vortical structures from Helmholtz instability. For the conditions of the present study, the transients in the fluid - though highly complicated - occurs at orders of magnitude faster than the transients in the solid, and the distribution of the heat-transfer coefficient on the cooled side achieved steady state almost instantly when compared to the time scale of conduction in the solid. Though the maximum temperature in the solid at steady state is always just below the maximum allowable for all cases studied, results show that the temperature in a material could exceed the maximum allowable during transients when the heat load is increased despite a corresponding increase in cooling. Studies were also performed to examine what could be done to the plate to reduce cooling flow for a given heat load with and without pre-cooling. If the plate's material is a composite matrix made of copper (Mat-C) and ceramic (Mat-S) instead of a Ni-based superalloy and subjected to the same heating loads, then the required cooling and period of over temperature were found to reduce greatly. A parametric study on the material properties of Mat-C and Mat-S in the composite matrix was performed to explore the effectiveness of material's thermophysical properties. To guide designers, a model based on one-dimensional-time-accurate integral solution and volume weighted time constants was developed to estimate the temperature distribution in a flat plate of thickness L that is exposed to a "hot" convective environment on one side and a "cold" convective environment on the other, where the two convective environments can change suddenly. This model provides estimates on the maximum temperature that can occur in a plate during transients when convective environments suddenly change, when that maximum temperature will occur after a sudden change, and how long could the temperature in the material exceed the maximum allowable. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Shih, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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