Gas Turbine Fuel Nozzle Thermal Expansion Control
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Solution Overview
Problem
Existing fuel nozzles in gas turbine engines face issues with thermal expansion differences between the external heat shield and inner fuel tube, leading to increased fuel-derived deposits, flow restriction, and auto-ignition risks due to temperature differentials and stress concentrations at joints.
Innovation Solution
A fuel nozzle design featuring a stem with a central passageway and a fuel tube having an outlet end portion with a cavity defined by a fixed aft face, where an outer heat shield tube is concentrically aligned and configured to thermally expand by sliding against the fuel tube, preventing expansion into the combustor and including an insulator to prevent fuel ingestion and reduce thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the external heat shield is rigidly connected to the inner fuel tube, then structural strength is improved, but thermal stress concentrations develop at the joint due to differential thermal expansion
Solution Approach 1:
The fuel nozzle is divided into separate segments: the inner fuel tube and the outer heat shield tube are disconnected and can move independently. This segmentation allows each component to expand or contract freely according to its own thermal characteristics without generating stress concentrations at rigid joints.
Solution Approach 2:
The design transitions from a static rigid connection to a dynamic configuration where the heat shield tube can slide axially along the fuel tube. This dynamic arrangement accommodates differential thermal expansion by allowing relative motion between components, thereby preventing thermal stress buildup.
2Stress or pressure
If the inner fuel tube is completely detached from the external heat shield, then thermal stress is reduced, but a variable size pocket forms at the combustor interface creating auto-ignition risk
Solution Approach 1:
A fitting acts as an intermediary component between the fuel tube and combustor. This fitting serves multiple functions: it limits the axial movement of the heat shield tube to prevent pocket formation, maintains proper spacing, and ensures consistent fuel delivery geometry without requiring rigid connection between the detached components.
3Temperature
If the external heat shield extends substantially the length of the nozzle into the combustor, then thermal protection is improved, but fuel-derived deposits increase due to elevated fuel temperatures
Solution Approach 1:
The heat shield is segmented into a controlled section that does not extend fully into the combustor. This segmentation allows the heat shield to provide thermal protection in the critical zones while maintaining fuel temperature control in the combustion region, thereby reducing deposit formation.
Solution Approach 2:
The heat shield configuration is optimized locally: it provides extensive coverage in the cooler sections for thermal protection but is deliberately limited in the combustor region where fuel temperature must be controlled to prevent deposits. This local quality variation addresses different thermal requirements in different zones.
4Stress or pressure
If the heat shield tube is allowed to thermally expand freely, then thermal stress is reduced, but the tube may expand past the fixed aft face into the combustor creating harmful effects
Solution Approach 1:
The fitting is positioned in advance to preemptively limit the thermal expansion of the heat shield tube. By providing this mechanical constraint before excessive expansion can occur, the design prevents the tube from intruding into the combustor while still allowing sufficient expansion to accommodate thermal stress.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design reduces fuel-derived deposits, auto-ignition risk, and thermal stress, maintaining nozzle integrity by controlling thermal expansion and preventing fuel ingestion, while ensuring efficient fuel delivery to the combustion chamber.
Implementation Method 1
the circumferential outer wall of the heat shield tube is configured to thermally expand by sliding against the outer wall of the fuel tube
Implementation Method 2
including an insulator to prevent fuel ingestion and reduce thermal stress
Data Source
AI summary
A fuel nozzle configured to channel fluid towards a combustion chamber is provided. The fuel nozzle includes a stem having a central passageway and at least one fuel tube disposed within the passageway. The fuel tube includes an outlet end portion having inner and outer walls separated by a cavity defined by a fixed aft face. The inner wall defines a central bore for delivering fuel to the combustion chamber. Further, the fuel nozzle includes an outer heat shield tube concentrically aligned with the outlet end portion of the fuel tube. The heat shield tube includes a circumferential outer wall having an aft face that stops upstream of the fixed aft face of the fuel tube. Thus, during operation, the heat shield tube is configured to thermally expand by sliding against the outer wall of the fuel tube.