A numerical study on various pin–fin shaped surface air–oil heat exchangers for an aero gas-turbine engine

Highlights

• The performance of a surface air–oil heat exchanger is assessed numerically.

• Three types of pin–fin and plate-fin geometries are examined.

• The forward-slanted pin–fin shows accelerated flow near the bottom of the fin.

• Proposed pin–fin shows margin in the pressure drop performance.

• Detailed assessment using entropy generation is conducted.

Abstract

In this study, the performance of a surface air–oil heat exchanger for an aero gas-turbine engine having plate- and pin–fin shaped geometries was investigated numerically. Basic heat-transfer and pressure-drop characteristics were examined using a simplified channel model. Performance of pin-shaped fins is compared with that of the plate fin as a baseline. Using a parametric study, optimal fin pitches of the pin–fin geometries in stream- and span-wise directions were determined. Finally, the high-speed bypass effect of the surface air–oil heat exchanger was calculated using the geometry of a real engine. Aero-thermal performance for such as the heat transfer rate, pressure-drop along the bypass stream of the engine, and distorted velocity boundary layer profiles, were evaluated quantitatively. The entropy generation rate due to the surface air–oil heat exchanger is summarized to assess the irreversibility loss inside the bypass stream and the cooler region.

Introduction

The ever-increasing demand for environmentally friendly airplanes has created a demand for aero engines of sharply increased efficiency. The specific fuel consumption (SFC) of the gas-turbine engine is highly related to the operating pressure ratio (OPR) and the turbine inlet temperature (TIT). In order to achieve high efficiency using an advanced cycle, equipment with an ultra-light heat exchanger is essential. This is because high efficiency aero-engines require minimal weight penalty and high aero-thermal performance of the heat exchanger. Min et al. [1] reviewed existing and possible candidate heat-exchange technology for the use of recuperator, intercooler, and cooling-air cooler applications.

Increasing the bypass ratio (BPR) of the engine is another method by which to increase the efficiency of turbo-fan engines. In this case, the heat from the corresponding oil system, such as the gear box and generator in the transmission system, should be cooled using various oil coolers [2]. The surface air–oil heat exchanger (SAOHE) is located inside the engine fan casing, and dissipates the heat from the oil into the air stream in the bypass duct (BPD), as shown in Fig. 1. The main difficulty of SAOHE designs is the existence of the bypass stream, which affects the pressure drop inside the bypass duct. Kim et al. [3] studied the effect of cooler installation location, in a high-speed aero engine with a plate fin.

There have been a number of studies aimed at enhancing aero-thermal performance by varying the fin-shape of the heat exchanger. It was found that a modified fin surface may have a high heat transfer coefficient, but that its pressure drop is sometimes too great for wide applications. Yun and Lee [4] compared experimentally the performance of plain fins, louvered fins, and three types of slotted fins. The results showed that the slotted fins with protruding strips have high heat-transfer performance with an acceptable penalty in pressure drop. Kang et al. [5] compared four kinds of plate-fin surfaces (plain, corrugated with a triangular-cross-sectional channel, corrugated with a sinusoidal cross-sectional channel, and a slotted fin). They found that the slotted-fin surface could increase heat transfer rate about 30–40% compared to a plain plate-fin, using identical pumping power.

For pin–fin models, Shaukatullah et al. [6] measured the thermal performance of in-line square pin–fins and plate heat-sinks for different fin thickness, spacing, height, and angle of approach for velocities under 5 m/s, allowing flow to partially by-pass the exchanger. More recently, Jonsson and Moshfegh [7] experimentally studied characteristics of plate and circular, rectangular and strip pin fins, in both staggered and in-line configurations, for different dimensions, while allowing variations on tip and side by-pass. Computational fluid dynamics (CFD) approaches have been extensively applied to the study of flow and heat transfer in heat sinks, as can be seen from the works by Jonsson and Moshfegh [8] and Biber and Belady [9].

For the SAOHE application, Outirba and Hendrick [10] conducted an experimental study on surface-air-cooled oil coolers (SACOC), describing a new test rig that allowed complete tests of SACOC breadboards. Ko et al. [11] studied the effect of SAOHE installation using a numerical method. In this paper, an efficient numerical procedure for the study of cooler installation involving bypass ducts was proposed and successfully demonstrated and important design variables were clearly identified. Kim et al. [3] performed a detailed experimental study from which the results validated this numerical work.

In order to assess the performance of a heat exchanger, an appropriate performance metric should be used. This is because the pressure drop and heat-transfer characteristics represented by the friction factor and Colburn j-factor, usually vary in similar patterns, as can be seen in the Reynolds analogy for flat plate [12]. The concepts of volume and area goodness factors are good alternatives of such metrics to represent the compactness of a given geometry of heat exchanger [13]. Adams [14] and Doo et al. [15] used those factors to examine the aero-thermal performance of variously shaped, primary surface heat exchangers.

Although the loss of energy inside a heat exchanger usually arises near solid-wall regions such as the fins, losses due to complex flow mixing inside the flow passage are also important. Volumetric entropy generation [16] is a useful parameter to understand the loss mechanism in the heat exchanger. Doo et al. [17] assessed the pressure-loss mechanism inside a cross-corrugated plate-type heat exchanger using the volumetric entropy generation rate. This arrangement results in a strong mixing layer between the plates.

Although there have been various fundamental studies on pin–fin shaped surfaces for heat exchangers, usually those are done under the condition of idealized heat transfer, without considering the bypass effect, which makes the problem complex [18], [19]. Moreover, study of the bypass effect has usually involved an incompressible flow regime, such as heat sink applications for cooling electronic devices [20], [21], [22]. It is necessary to carry out studies on the high-speed bypass effect, in relation to the fin-shapes of various heat exchangers, in order to understand the advantages of enhanced heat-transfer characteristics for application to aero engines.

In the present study, the influence of various fin shapes of surface air–oil heat exchanger on the aero-thermal performance of the cooler, was investigated numerically. To improve the efficiency of the calculations, the fundamental pressure drop and heat-transfer characteristics were investigated using an idealized, channel-shaped computational domain without bypass effect. Corresponding design parameters such as fin pitches in stream- and span-wise directions were determined by means of a parametric study. Finally, the bypass effect due to the installation of the cooler inside an aero engine was calculated using a unitary-fin model, but also considering the rotational periodicity of the multiple fins. The loss due to irreversibility inside the bypass stream was assessed by considering the variation of entropy generation inside the engine.