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Analysis of Flow Physic within Tip Clearance Gap of an Unshrouded high pressure Turbine Blade

FA20260604939

Hello everyone,

 I'm currently using Fidelity Turbo to generate a mesh for an unshrouded high-pressure turbine blade. This particular blade features a sharp pressure-side gap corner and a tip thickness that is five times the gap height (t > 4h). Before I run the simulation, I would appreciate it if someone could provide an explanation of the following question:

What is the most accurate description of the expected flow physics within the tip clearance gap, and what is the primary driving mechanism behind the resulting efficiency losses?

 The options are as follows:

 A) The flow will separate at the sharp corner and remain entirely detached across the gap, with the main loss being driven by centrifugal forces mixing at the hub.

 B) A separation bubble will form at the sharp corner, but the flow will reattach before exiting, with the primary loss being driven by the pressure differential moving flow from the pressure side to the suction side.

 C) The flow will remain perfectly attached due to the high thickness-to-gap ratio (t/h = 5), and the primary loss will be strictly due to over-tip thermal heat transfer penalties.

 D) A vena contracta will form, causing immediate supersonic choking, with the primary loss being driven by shockwave-boundary layer interaction near the stationary casing.

  • Gaurav
    0

    When flow enters a tip gap with a sharp pressure-side corner, it undergoes a sharp contraction, resulting in a flow restriction known as a vena contracta. This phenomenon causes a separation bubble to form immediately after the corner. Whether the flow reattaches to the blade surface depends largely on the ratio of the blade tip thickness (t) to the gap height (h).

    There are two distinct scenarios based on this ratio:

    For thin blades, where the thickness is less than four times the gap height (t < 4h), the separation bubble dominates the flow, often extending across the entire blade thickness and preventing the flow from reattaching.

    In contrast, for thicker blades, where the thickness exceeds four times the gap height (t > 4h), the physical surface area is sufficient for the flow to mix, recover, and reattach to the blade tip before exiting into the suction side. Specifically, given the blade's thickness being five times the gap height, reattachment of the flow is assured.

    In the context of unshrouded turbomachinery blades, the primary driving force behind leakage is the aerodynamic load on the blade, specifically the pressure difference between the high-pressure (pressure side) and low-pressure (suction side) surfaces. This pressure gradient effectively forces fluid through the clearance gap.

    The resulting over-tip leakage flow has two significant consequences:

    1. Aerodynamic losses: These losses account for approximately one-third of the total losses in a turbine stage. Even a relatively small gap height, equivalent to just 1% of the total blade span, can result in a stage efficiency penalty of 1 to 3 percent or more.

    2. Thermal losses: The high-velocity over-tip flow significantly enhances convective heat transfer coefficients, subjecting the blade tip region to extreme thermal loads.

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