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TECHNICAL PAPERS

On the Momentum and Thermal Structures of Turbulent Spots in a Favorable Pressure Gradient

[+] Author and Article Information
T. P. Chong

School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, Lancashire M60 1QD, UK

S. Zhong

School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, Lancashire M60 1QD, UKshan.zhong@manchester.ac.uk

J. Turbomach 128(4), 689-698 (Feb 01, 2005) (10 pages) doi:10.1115/1.2218520 History: Received October 01, 2004; Revised February 01, 2005

This paper represents the results from an experimental investigation of the flow physics behind the difference in the transition zone length indicated by the momentum boundary layer and thermal boundary layer parameters observed on the suction surfaces of gas turbine blades. The experiments were carried out on turbulent spots created artificially in an otherwise laminar boundary layer developing over a heated flat plate in a zero pressure gradient and a favorable pressure gradient. A specially designed miniature triple wire probe was used to measure the streamwise velocity component U, transverse velocity component V and temperature T simultaneously during the passage of the spots. In this paper, the general characteristics of the ensemble-averaged velocity and temperature perturbations, rms fluctuations, and the second moment turbulent quantities are discussed and the influence of favorable pressure gradient on these parameters is examined. When a favorable pressure gradient is present, unlike in the velocity boundary layer where significant velocity fluctuations and Reynolds shear stress occur both on the plane of symmetry and the spanwise periphery, high temperature fluctuations (and turbulent heat fluxes) are confined in the plane of symmetry. The difference in the levels of velocity/temperature fluctuations at these two locations gives an indication of the effectiveness of momentum/heat transfer across the span of the spots. The results of this study indicate that the heat transfer within a spot is inhibited more than that of the momentum transfer at the presence of a favorable pressure gradient. This phenomenon is expected to slow down the development of a transitional thermal boundary layer, leading to a longer transitional zone length indicated by the heat transfer parameters as reported in the literature.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagram showing the engineering drawing of the heater plate and the ceiling wedge insert for generating a FPG. All units in mm.

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Figure 2

Distributions of (a) free stream velocity and K at FPG; (b) momentum and displacement thickness at ZPG and FPG

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Figure 3

Engineering drawings of the triple wire. All units in mm.

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Figure 4

U,V, and T signals acquired simultaneously on the plane of symmetry of the spots along with the trigger pulses

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Figure 5

Comparison of Cl and Ct deduced from the velocity and temperature data in the (a) zero and (b) favorable pressure gradient boundary layer

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Figure 6

Contours of (a)Ũ∕U∞; (b)Ṽ∕U∞;(c)T̃∕ΔT;(d)u′∕U∞; (e)v′∕U∞; (f)θ′∕ΔT; (g)⟨uv⟩∕U∞2; (h)⟨uθ⟩∕U∞ΔT; (i)⟨vθ⟩∕U∞ΔT on the plane of symmetry of spots in the ZPG case at xs=780mm

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Figure 7

Contours of (a)Ũ∕U∞;(b)Ṽ∕U∞;(c)T̃∕ΔT;(d)u′∕U∞;(e)v′∕U∞;(f)θ′∕ΔT;(g)⟨uv⟩∕U∞2;(h)⟨uθ⟩∕U∞ΔT; (i)⟨vθ⟩∕U∞ΔT on the plane of symmetry of spots in the FPG case at xs=780mm

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Figure 8

Schematic diagram showing the location of the sensors of the triple-wire probe relative to the hairpin vortices

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Figure 9

Ensemble-averaged (a1)–(a8), u′∕U∞ and (b1)–(b8). θ′∕ΔT contours in the y−z plane at xs=660mm, t0=90, 99, 109, 115, 120, 123, 127, and 130ms in the ZPG case.

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Figure 10

Ensemble-averaged (a1–a8)u′∕U∞ and (b1–b8). θ′∕ΔT contours in the y - z plane at xs=780mm, t0=88, 93, 97.8, 104, 110, 115, 121, and 124ms in the FPG case.

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Figure 12

Ensemble-averaged (a)u′∕U∞ and (b)θ′∕ΔT contours on the spanwise plane at xs=780mm, y=0.11δtur for the FPG case

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Figure 11

Ensemble-averaged (a)u′∕U∞ and (b)θ′∕ΔT contours on the spanwise plane at xs=660mm, y=0.07δtur for the ZPG case

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