Film Cooling Patterns over an Aircraft Engine Turbine Endwall with Slot Leakage and Discrete Hole Injection

Highlights

• Separate and combined cooling patterns from discrete injection and purge flow were obtained on a high-pressure turbine endwall.

• Effects of turbine inlet Reynolds number on endwall film cooling effectiveness were highlighted.

• Experiments were performed in engine-representative oncoming flows based on the superposition principle.

• Overall film cooling performance by combining cooling effectiveness and heat transfer coefficients was evaluated to verify the endwall cooling design.

Abstract

Detailed characteristics of film cooling from discrete injection and purge flow, and associated coolant injection patterns are presented over a turbine endwall for wide engine-representative coolant flow rate ranges. Measurements, in combination with numerical simulations, are performed in a linear cascade that is geometrically and aerodynamically scaled up from the hub section of the turbine vane. Reynolds numbers at the vane cascade inlet are varied from 1.40 × 10⁵ to 4.20 × 10⁵, representing the variations of real engine operating conditions (from take-off to cruise conditions). In order to examine the isolated and combined cooling effects, discrete coolant injection and purge flow for the endwall cooling can be injected separately or simultaneously. Numerical simulations are conducted to gain further insight into interactions between endwall-nearby secondary flows and coolant injection. Additionally, a net heat flux reduction parameter is used to evaluate overall film cooling performance of the cooling scheme, that takes heat transfer enhancement by coolant injection into consideration. Results show that endwall-nearby flow structures dictate thermal protection patterns. Increasing coolant flow rates decreases effectiveness values of discrete film cooling but improves cooling effectiveness of purge flow. In spite of no direct interactions between discrete injection and purge flow, adding purge flow could help enhance discrete film cooling effectiveness. Higher passage inlet Reynolds number leads to reduced mixing of coolant injection and mainstream flows, resulting in improved effectiveness values. Changing trends of overall cooling performance for purge flow and discrete film cooling by increasing coolant flow rates are opposite but those are the same by increasing Reynold numbers.

Introduction

The durability and the ever-increasing specific thrust-to-weight ratio of an aircraft engine have higher requirements on effective cooling schemes for the hot turbine components, particularly the high-pressure (HP) turbine nozzle guide vane (NGV), since it must endure the hottest gases from the combustor. Moreover, the unpredictable oncoming flow distortions (hot streaks, swirl, and turbulence) exiting from the combustor further aggravate the difficulties of thermal protection for the HP turbine. In this section, one region, that is more sensitive to the oncoming flows, is the endwall area. Compared with the airfoil surfaces, the endwall features more unique flow and heat transfer patterns because of highly three-dimensional secondary flow topologies [1], [2]. These complicated flow structures dictate the endwall heat transfer patterns and nearby mixing and coverage of film coolant flows introduced for endwall external thermal protection, but the film coolant injection, in turn, disturbs the already-complex secondary flow structures. Therefore, sophisticated film cooling designs for the turbine NGV endwall region are a challenging task for thermal designers of aircraft engines.

Early work on endwall mainly focused on detection of the development of flows near the endwall throughout the vane passage, and multiple endwall secondary-flow models were developed [1], [2], [3]. Afterwards, endwall heat transfer patterns were concentrated on due to their strong links to the endwall-nearby complex flows [4], [5], [6]. With the requirement of cooling for the turbine endwall due to higher approaching temperatures, the introduce of aggressive film cooling that actively disturbs the secondary flows turns the turbine endwall into a “three-dimensional” region related to flow and film cooling issues. Turbine airfoil film cooling is generated by coolant injection from discrete film holes, while there are generally two coolant sources for the NGV endwall external film cooling: one is film coolant injection from discrete holes (discrete film cooling) and another is slot leakage injected from combustor-turbine interfacial cavity (purge flow cooling).

Regarding endwall discrete film cooling only or purge flow cooling only, numerous studies have been reported on the possible affecting parameters and thus, endwall typical film cooling patterns were found. In general, film coolant coverage is dictated by the endwall-nearby flow structures. Changes in injection shape [7], [8], [9] or injection position [12,13] have minimal opportunity to eliminate the dominant effects of the endwall-nearby flow structures. In vane leading-edge regions, the presence of horseshoe vortex makes this corner on the endwall difficult to cool due to its prevention against film coolant coverage [14]. As the coolant proceeds into the turbine passage, film coolant is directly dictated towards the low-pressure areas along the endwall suction side, leaving the pressure side uncovered [15]. With its further migration downstream, coolant is detached from the endwall due to passage vortex, cross flows, and corner vortex within the passage. In spite of this, cooling effectiveness by discrete coolant injection or upstream purge flow can be increased by increasing coolant mass flow rates [9,10] and by designing cooling schemes carefully [16]. However, there are some significant differences in cooling patterns between discrete coolant injection and purge flow. Purge flow has been shown to inhibit the secondary flows, somewhat. Moreover, thermal protection by purge flow can be continually improved by increasing coolant flow rates, while that by discrete hole injection has its optimal coolant blowing condition.

Currently, a cooling design by combining discrete film cooling with upstream purge flow is a better possibility to achieve a full coverage for the entire endwall surface. Nicklas [17] investigated the combined cooling effects from discrete hole injection and purge flow by positioning the discrete holes in the endwall fore part, resulting in the rear regions, particularly in the wake areas, of the endwall were rarely film-covered. To provide a full coverage for the endwall, Knost and Thole [13] placed discrete holes in the passage mid-chord region and Wright et al. [18] did this with discrete holes near and beyond the passage throat. Both their results showed that purge flow and discrete film cooling were complementary in coverage patterns, resulting in a full coverage for the entire endwall. In addition to purge flow upstream of the endwall, leakage from a slashface, which represents the mid-passage gap between adjacent vanes when airfoil units are assembled in an annular pattern, was introduced by Cardwell et al. [19], Hada and Thole [20], Chowdhury et al. [21], and Chen et al. [22]. In comparison with upstream purge flow, cooling effectiveness of the slashface leakage was highly reduced by turbulent flows within the vane passage.

As highlighted from above discussion, much efforts have been taken to provide adequate thermal protection for the endwall in recent years. This involves detailed studies on a variety of discrete film cooling or making the best use of endwall upstream purge flow. However, very few work deals with isolated and combined cooling effects from discrete injection and purge flow on a turbine endwall, especially in a HP turbine passage where secondary flows are much stronger. This gives us the motivation of studying the cooling and interacting effects of discrete injection and purge flow in the HP turbine endwall regions. Therefore, the current work focuses on further extending endwall external cooling studies in engine-representative flow conditions and aims at gaining valuable insight into the flow physics involved in purge flow and discrete hole injection via measurements and numerical simulations. In addition, since there is a gap in literature reporting film cooling relevant to variations of operating conditions in a turbine vane passage, a more special attention in this study is paid to detailed coolant coverage patterns on the turbine endwall for various passage Reynold numbers that represents the variation of operating conditions in a real engine. Finally, heat transfer coefficients measured in previous work [23] are integrated into cooling effectiveness values to comprehensively evaluate the overall cooling performance of the two cooling sources. This study is expected to provide a complete understanding of film cooling characteristics from discrete injection and purge flow cooling so that over-cooled or under-cooled areas can be identified on the endwall, with the purpose of saving coolant consumption, extending turbine lifetime as well as improving engine cycle thermal efficiency.