Applied Thermal Engineering 29 (2009) 966–972

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate /apthermeng

Cooling effectiveness of effusion walls with deflection hole angles measuredby infrared imaging

Chi Zhang a,*, Bo Song b, Yuzhen Lin a, Quanhong Xu a, Gaoen Liu a

a Institute of Thermal Power Engineering, School of Jet Propulsion, BeiHang University, National Key Laboratory on Aero-Engines, Beijing 100083, PR Chinab Gardner Denver, Inc., 100 Gardner Park, Peachtree City, GA 30269, USA

a r t i c l e i n f o

Article history:Received 1 July 2006Accepted 13 May 2008Available online 21 May 2008

Keywords:Gas turbine engineCombustorEffusion coolingCooling effectivenessInfrared imaging

1359-4311/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.applthermaleng.2008.05.011

* Corresponding author. Tel./fax: +86 10 8231 6518E-mail address: zhangchi@sjp.buaa.edu.cn (C. Zhan

a b s t r a c t

Experimental investigations were performed on the overall cooling effectiveness of three effusion coolingtest plates. The test plates had different hole-spacing (PS/d2 = 32.5 and 72), different deflection angle(b = 45� and 90�), and the same inclination angle (a = 30�). Although effusion cooling has been appliedto advanced gas turbine combustors, its fundamental study seen in the open literature is limited. Usingthe infrared (IR) imaging technique, the current study documented distribution of the overall coolingeffectiveness on the surfaces of above specified flat walls. Based on the IR measured cooling effectivenessdata, different effusion cooling configurations (different PS/d2 and b) were evaluated and the optimumconfiguration was determined. The current work demonstrated that the infrared imaging techniqueserved as an ideal method to detail cooling characteristics in the combustor heat transfer research.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

High-pressure ratio and high temperature rise are the necessaryfor improving performance of modern gas turbine engines. Thatimposes increasing challenge on the combustor design. One chal-lenge is the combustor liner durability due to increased coolanttemperature and decreased coolant flow rate. A possible solutionwould be an enhanced cooling technique that fully utilizes thecooling potential of the available cooling air. Compared to otheradvanced cooling techniques, effusion cooling is a competitivechoice. Effusion cooling can significantly enhance convective heattransfer (cooling) inside a large number of inclined holes beforethe cooling air forms a cooling film on the hot-side surface. Theadiabatic film cooling effectiveness from this cooling scheme israther impressive. Therefore, effusion cooling is favored in moderncombustor development and has attracted considerable researchinterest. Fig. 1 illustrates this cooling scheme.

The overall cooling effectiveness of the effusion wall is depen-dent on the adiabatic film cooling effectiveness, heat transfer onthe hot-side wall surface, and the cooling effect inside the inclinedholes. Song and Lin et al. [1–6] conducted a series of studies on theinfluencing factors of the effusion cooling scheme such as blowingratio. In their studies, the effects of the multi-hole pattern, holeinclination angle and hole deflection angle were also evaluated.They obtained a great deal of experimental data on adiabatic cool-ing effectiveness using a convenient heat/mass transfer analogy

ll rights reserved.

.g).

method [7], by which the concentration of CO2 was measuredusing a gas analyzer and a 3D traverse device. To study surface con-vective heat transfer, they applied a large number of thermocou-ples on the surface and used the constant heat transfer method.Their efforts led to important findings on the effusion coolingscheme with data obtained on discrete spatial points. The coolingand heat transfer characteristics had to be presented by means ofdata interpolation, not a strict 2D characterization on the wall sur-face. Some detailed information of the cooling or heat transfercould be missing, particularly in the near-hole region.

Goldstein and Jin [8] studied the film cooling effectiveness andmass/heat transfer coefficient downstream of only one row ofholes with the inclination angle of 35� deflection angle of 45�, usingthe naphthalene sublimation technique and the heat/mass transferanalogy. Ling et al. [9] obtained the film cooling effectiveness andheat transfer coefficient for a full coverage film cooling wall withthe inclination angle of 20�, deflection angle of 0�, and hole-spacingof 16d to 10d, using the transient liquid crystal technique. Expen-sive liquid crystal paint was used on the measured surface andcomplicated liquid crystal color analysis was done after the exper-iment. Probably due to the high expense and inconvenience of theexperiment, Ling’s research only dealt with the first several rows ofholes and more downstream rows, with quite a few rows betweenskipped.

The infrared imaging technique has been widely used for thenon-intrusive surface temperature measurements. A lot of usefulexperiences have been made available from previous research[10–15], which greatly helped later practitioners of this techniqueto facilitate the application and reduce the experimental

Nomenclature

d diameter, mmG flow flux = mass flow rate per unit area and pressure,

kg/s m2 Pal turbulence length scale, mmL length, mm_m mass flow rate, kg/s

N numberP spanwise hole pitch, mm and pressure, PaRe Reynolds numberS streamwise hole-spacing, mmT temperature, KTu turbulence intensityu velocity, m/sx streamwise geometrical location, mm

y spanwise geometrical location, mma inclination angleb deflection angleg cooling effectivenessk thermal conductivity, W/m Kq density of air flow, kg/m3

Subscriptsan annulus coolantav averagec coolantg mainstream gasw wall surface

C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972 967

uncertainty. Quan [10] and Sweeney et al. [11] made detailed two-dimensional steady-state measurements of flat surface tempera-ture for the LamilloyTM double wall cooling design, and obtainedthe overall cooling effectiveness for different hole angles and spac-ing. Gustafsson et al. [12] investigated the temperature distribu-tion on effusion-cooled plates with different temperature ratiosand velocity ratios between hot gas and coolant, and differentinjection hole-spacing, inclination angle, and thermal conductivityof the test plates. Harrington et al. [13] performed an experimentalinvestigation on the adiabatic effectiveness of large-scaled full cov-erage film cooling plates, considering the effect of the mainstreamturbulence. Ekkad et al. [14] obtained both film cooling effective-ness and heat transfer coefficient from a single test by means ofthe transient infrared thermography technique. The advantage ofIR technique was exhibited adequately in these previous researchefforts, in terms of the wider temperature range handled thanthe liquid crystal technique, and more detailed 2D temperaturefield information with better accuracy. It is seen that these re-search with successful employment of the IR technique enriched

Fig. 1. Effusion co

high-quality data in the literature, but the effect of the deflectionangle of the injection holes was not attempted.

The current paper details an experimental investigation of theeffusion cooling with the focus on assessing the effect of the deflec-tion angle. Due to the deflection angle, the coolant jets have strongspanwise momentum, resulting in quite different cooling charac-teristics from what is seen on the case without the deflection angle,particularly in the near-hole region. The infrared imaging tech-nique was chosen to ensure extensive 2D temperature mappingand detailed cooling characteristics of the effusion walls. The IRexperimental results presented can be used as reference to checkother published data by means of discrete point measurementmethods and validate CFD simulation.

2. Experimental apparatus and conditions

The overall cooling effectiveness is defined as a normalizedparameter, involving temperature of the hot mainstream, coolantair, and wall surface [11,13], as shown in Eq. (1). It addresses the

oling scheme.

968 C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972

cooling effect that the coolant provides to isolate the wall surfacefrom the hot mainstream. Obviously three temperatures neededin Eq. (1) can be directly measured by the heat transfer experiment.

g ¼ Tg � Tw

Tg � Tc: ð1Þ

In order to obtain the cooling variation trend in the streamwisedirection, the spanwise averaged cooling effectiveness at eachstreamwise location (x/d) is calculated as Eq. (2):

gav ¼XN

i¼1

gi=N; ð2Þ

where N is the number of data points in the spanwise direction.The coolant flux Gc for the effusion cooling can be defined as the

coolant mass flow rate per unit area and pressure differential. Theequation is

Gc ¼_mc

AcPg: ð3Þ

This term addresses the effect of the cooling surface area andoperating pressure. It helps assess different cooling schemes. Spe-cifically, if two cooling schemes #1 and #2 have the same coolingeffectiveness, but #1 has lower Gc than #2, then #1 is superior to#2. If #1 and #2 have the same Gc, but #1 has higher cooling effec-tiveness than #2, then #1 is superior to #2. It can be seen that thesuperiority of one cooling scheme to another needs to be assessedby both the cooling effectiveness and coolant flux.

In the present experiments, temperature of the hot mainstreamand coolant air was measured by thermocouple probes. Tempera-ture distribution on the wall surface was measured by an infrared(IR) imaging camera in steady-state and calibrated by referencethermocouples embedded in the test effusion plates. The measuredsurface was painted with a high-emissivity matt black paint, withthickness less than 50 lm to ensure uniform heat emission.

Fig. 2 shows a schematic of the experimental setup. The size ofthe test section is 240 � 200 � 100 mm (length �width � height),and the opposite side to the test surface has a 240 � 50 mm(length �width) rectangular window opening for the infraredimaging measurement. As no glass was used, it is entirely transpar-ent to infrared radiation. The airflow leakage through the windowopening was checked by silk threads and found to be very weakcompared to the mainstream. CFD simulation was also conductedto inspect the effect of the leakage on the mainstream. It was foundthat less than 5% of the total flow fled through the window openingand the mainstream flow field was hardly affected by the leakageflow.

Flowmeter

RotameterAir Tank

Electrical Heater

Air Blower

Turbulence Generator

Fig. 2. Schematic of the

The mainstream air was supplied by an air-blower at atmo-spheric pressure, and heated to 100 �C by an electrical heater, withReynolds number being 57000. In order to simulate actual combus-tor turbulence conditions [13], a grid turbulence generator wasplaced 240 mm upstream of the test plate edge, which led to themainstream turbulence intensity Tu = 14.7% and length scalel = 9 mm [16]. An orifice flowmeter was used to measure the main-stream flow rate.

Coolant air was provided by a high-pressure air tank, led into achamber assembled on the test plate, and was kept at atmospherictemperature. To eliminate the impingement effect of the coolantair at the chamber intake, multiple layers of grids were placed inthe chamber. A thermocouple probe was put at the entrance ofthe chamber to measure the coolant temperature. The coolantflowrate was measured by a rotameter.

A ThermaCAM P60 infrared camera made by the FLIR SystemsCompany was used to measure the effusion wall surface tempera-ture, with the thermal sensitivity of 0.08 �C at 30 �C, accuracy of±2 �C, 7.5 lm to 13 lm spectral range and 320 � 240 pixels imageoutput [17]. Given the 1.3 mrad spatial resolution of the infraredcamera and the measurement distance, one pixel in the infraredimage corresponded to 0.68 mm approximately.

Table 1 shows the geometries of three test effusion flat plates.They were made of Bakelite composite material with low heat con-ductivity (k = 0.453 W/m K). The plate size was 150 � 240 mm,with a thickness of 6 mm. For all test plates, multi-holes with an30� inclination angle were arranged in a staggered pattern (P/S = 1:1), but varying in spacing (PS/d2 = 32.5 and 72) and deflectionangle (45� and 90�).

The experimental conditions are shown in Table 2. Themainstream to coolant temperature ratio (Tg/Tc) was kept at1.33 ± 2.26%. It did not represent the real temperature ratio ingas turbine combustors where Tg/Tc is usually up to 2.5. The exper-imental results by Gustafsson et al. [12] suggested that Tw/Tc

increased linearly with Tg/Tc, so the relationship between Tw/Tc

and Tg/Tc can be represented by Eq. (4), with g being a constantto maintain the linear relationship of the two ratios. Eq. (4) canbe rewritten as Eq. (1), which means by looking at Eq. (1), the cool-ing effectiveness (g) is independent of the hot mainstream temper-ature within a certain range (Tg/Tc < 1.9 [12]). Or at least one canconclude from findings of the Ref. [12] that g is hardly responsiveto Tg/Tc.

Tw

Tc¼ ð1� gÞ Tg

Tc� 1

� �þ 1: ð4Þ

The infrared images were adjusted according to the experimentalparameters, including wall surface emissivity, atmospheric

ThermocoupleMeasurement

Infrared Camera

Window

Test Plate

Multiple layers of grids

experimental setup.

Table 1Test plate geometries

Test plate L/d P/d S/d a (�) b (�)

No. 1 3.75 5.7 5.7 30 45No. 2 3.75 5.7 5.7 30 90No. 3 5 8.5 8.5 30 45

Table 2Operating conditions of experiment

Atmosphere pressure (Pa) 101,604Atmosphere temperature (�C) 10–17Mainstream velocity ug (m/s) 10Mainstream Reynolds number Reg 57,000Mainstream temperature Tg (�C) 105–110Coolant temperature Tc (�C) 8.5–15Temperature ratio Tg/Tc 1.33 ± 0.03Effusion jet Reynolds number Rej 500–4000Coolant flux Gc (�10�6 kg/s m2 Pa) 0.5–3.5

C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972 969

temperature and humidity, and distance between the camera andwall surface, in the software named ThermaCAM Researcher pro-vided by FLIR Systems. Then the surface temperature data in theinfrared images was reduced to the local and averaged overall cool-ing effectiveness for each test plate. In order to obtain accurate sur-face temperature, the emissivity of the test wall surface wascalibrated by reference temperature readings from three thermo-couples embedded in a flat plate calibration specimen without cool-ing. Fig. 3 shows the comparison between the infrared temperaturemeasurement (with the emissivity set to be an optimum value of0.86) and the thermocouple readings. 91% out of all data points fallin the variation band of ±5 �C. So the emissivity of the measuredsurface for three plates in the experiments was set to 0.86. Basedon the experimental conditions and the accuracy of the instru-ments, the experimental data of the overall cooling effectivenesswas analyzed to have an uncertainty of ±11% in the presentresearch.

3. Experimental results and discussion

There are eight columns of holes (making a spanwise coverageof 7P) on each test plate. The measurement area was chosen to cov-er the middle region between the third and forth column.

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

-5o C

Ther

moc

oupl

e Te

mpe

ratu

re (º

C)

Infrared Thermography (ºC)

+5o C

Fig. 3. Comparison between infrared thermography and thermocouple temperature.

4. Cooling effectiveness distribution

The test plates were made of low heat conductivity material toallow close-to-adiabatic conditions in the experiments. Hence, thedetailed cooling features, such as the overall cooling effectiveness gdistribution and its gradient, could be distinct enough and exhib-ited by the IR imaging. Fig. 4 summarizes the measured g distribu-tion at different coolant flux Gc for each plates, respectively. Thebeginning of the streamwise location (x/d = 0) is set at the exit cen-ter of the first row holes.

Cooling effectiveness distribution for No. 1 test plate is shownin Fig. 4a. Stripes of relatively high cooling effectiveness down-stream of the injection holes are visible. The coolant jets with a45� deflection angle are attached to the wall surface under a cer-tain Gc range [9,13], but they have strong interaction with themainstream due to the strong spanwise velocity component. Assuch, the high effectiveness stripes with a deflection angle areshorter than those without a deflection angle at similar conditions[12]. There is a small region of high cooling effectiveness aroundthe exit of the injection hole, as the low heat conductivity materialof the test plate reduces the cooling effect of convective heat trans-fer inside the effusion hole. Otherwise the cooling effect from theinside-hole cooling will be extended to a wider range to over-shadow the film cooling characteristics, which is not desired ofcourse. The effect of the coolant flux Gc is clearly exhibited in thatthe overall cooling effectiveness increases with the coolant flux Gc.On one hand, the stripes downstream of the injection hole underlow Gc are more obvious and longer than those under high Gc, asa result of the coolant separation under high Gc. On the other hand,there is not sufficient coolant air mixed with hot mainstream un-der low Gc, so the low cooling effectiveness regions exist on bothsides downstream of the holes. The attached cooling film has theeffect of superposition such that cooling effectiveness increaseswith the streamwise location x/d at a given Gc. Comparing the cur-rent results with the cooling effectiveness of the undeflected effu-sion cooling wall at similar conditions [2,12], one can tell that thedeflection angle helps strengthen cooling protection in the area be-tween the streamwise columns of holes.

No. 1 and 2 plates have the same hole spacing but differentdeflection angles of 45� and 90�, respectively. In Fig. 4b, the higheffectiveness stripes of 90� deflection angle downstream of theinjection holes are very short, especially under low Gc. Further-more, the low cooling effectiveness regions obviously exist on bothsides of the spanwise coolant jet and between the hole-rows, mak-ing a rather uneven cooling effectiveness distribution in thestreamwise direction, i.e., larger temperature gradient in thestreamwise direction.

The deflection angle of No. 3 plate is also 45�, but the hole-spac-ing is 1.5 times of No. 1. As shown in Fig. 4c, due to the wider hole-spacing, the coolant jet film starts to lose continuous coverage onthe wall surface [12], so the high cooling effectiveness stripes arevery short and the low effectiveness regions are wider with largetemperature gradient in the two-dimensional distribution.

5. Spanwise averaged cooling effectiveness

Fig. 5 shows the spanwise averaged cooling effectiveness gav ofthree effusion plates at different Gc, in the streamwise direction.

For No. 1, the spanwise averaged cooling effectiveness gav

increases with Gc, more significantly at low Gc than at high Gc, asshown in Fig. 5a. Compared with the value of gav at the lowestGc = 0.7 � 10�6 kg/s m2 Pa, gav increases 24.5% at Gc = 2.30 �10�6 kg/s m2 Pa, but only 27.2% at Gc = 2.97 � 10�6 kg/s m2 Pa.The reason is that under low Gc the coolant jets are pushed downby the mainstream to stay attached to the wall surface but the

Gc=0.69×10-6kg/sm2Pa

x/d

y/d

-2.9 0 10 20 30 40 50 60 63

x/d-2.9 0 10 20 30 40 50 60 63

024

5.7

y/d

024

5.7Gc=1.15×10-6kg/sm2Pa

Gc=1.84×10-6kg/sm2Pa

Gc=2.30×10-6kg/sm2Pa

Gc=2.94×10-6kg/sm2Pa

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(a) NO.1 PLATE

x/d-2.9 0 10 20 30 40 50 60 63

y/d

024

5.7

x/d-2.9 0 10 20 30 40 50 60 63

y/d

024

5.7

x/d-2.9 0 10 20 30 40 50 60 63

y/d

024

5.7

Gc=0.70×10-6kg/sm2Pa

x/d

y/d

-2.4 0 10 20 30 40 50 60 64024

5.8

Gc =1.13×10-6kg/sm2Pa

Gc =1.86×10-6kg/sm2Pa

Gc =2.27×10-6kg/sm2Pa

Gc =3.43×10-6kg/sm2Pa

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(b) NO.2 PLATE

x/d

y/d

-2.4 0 10 20 30 40 50 60 64024

5.8

x/d

y/d

-2.4 0 10 20 30 40 50 60 64024

5.8

x/d

y/d

-2.4 0 10 20 30 40 50 60 64024

5.8

x/d

y/d

-2.4 0 10 20 30 40 50 60 64024

5.8

Gc =0.50×10-6kg/sm2Pa

x/d

y/d

-4.4 0 10 20 30 40 50 60 70 80 860246

8.5

Gc =1.03×10-6kg/sm2Pa

x/d

y/d

-4 0 10 20 30 40 50 60 70 80 860246

8.5

Gc =1.58×10-6kg/sm2Pa

x/d

y/d

-3.5 0 10 20 30 40 50 60 70 80 860246

8.5

Gc=2.10×10-6kg/sm2Pa

x/d

y/d

-3.5 0 10 20 30 40 50 60 70 80 860246

8.5

Gc=3.14×10-6kg/sm2Pa

x/d

y/d

-3 0 10 20 30 40 50 60 70 80 860246

8.5

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(c) NO.3 PLATE

Fig. 4. Cooling effectiveness distribution.

970 C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972

coolant air is not sufficient to mix with hot gas fully; with increas-ing Gc the mixing becomes better and the convective heat transferincreases in the effusion hole and on the wall surface, but when Gc

is up to a certain extent about 2 � 10�6 kg/s m2 Pa the coolant jetsmostly penetrate into the mainstream as what was described byLing and Harrington [9,13] and the film lifts off the wall surface.

0 10 20 30 40 50 600.3

0.4

0.5

0.6

0.7

0.8

0.9

Gc=0.69x10 kg/sm PaGc=1.15x10 kg/sm PaGc=1.84x10 kg/sm PaGc=2.30x10 kg/sm PaGc=2.94x10 kg/sm Pa

η av

x/d(a) NO.1 PALTE

0 10 20 30 40 50 600.3

0.4

0.5

0.6

0.7

0.8

0.9

η av

x/d

Gc=0.70x10 kg/sm PaGc=1.13x10 kg/sm PaGc=1.86x10 kg/sm PaGc=2.27x10 kg/sm PaGc=3.43x10 kg/sm Pa

(b) NO.2 PLATE

0 10 20 30 40 50 60 70 800.3

0.4

0.5

0.6

0.7

0.8

0.9

η av

x/d

Gc=0.50x10 kg/sm PaGc=1.03x10 kg/sm PaGc=1.58x10 kg/sm PaGc=2.10x10 kg/sm PaGc =3.14x10 kg/sm Pa

(c) NO.3 PLATE

Fig. 5. Spanwise averaged cooling effectiveness.

x/d

y/d 0.55 0.5

0.550.6

0.6

0.650.65 0.6 0.65

0.70.7

0.650.7

0.7

0.65

0.7

0.70.750.75

0.8-2.9 0 10 20 30 40 50 60 63024

5.7

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(a) NO.1 PLATE

x/d

y/d 0.4

0.40.5 0.60.5 0.6

0.60.6

0.5

0.60.6 0.6

0.6 0.6 0.60.6

0.60.6 0.6

-2.4 0 10 20 30 40 50 60 64024

5.8

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(b) NO.2 PLATE

x/d

y/d 0.45

0.45

0.50.50.55 0.55

0.55

0.55

0.50.55 0.6 0.55

0.6 0.65

0.60.65

0.6

-3 0 10 20 30 40 50 60 70 80 860246

8.5

0.3 0.4 0.5 0.6 0.7 0.8 0.9

(c) NO.3 PLATE

Fig. 6. Cooling effectiveness contours at Gc = 3 � 10�6 kg/s m2 Pa.

0 10 20 30 40 50 600.3

0.4

0.5

0.6

0.7

0.8

0.9

η av

x/d

NO.1 PLATE NO.2 PLATE NO.3 PLATEGc=3x10-6kg/sm2Pa

Fig. 7. gav Comparison of test plates at Gc = 3 � 10�6 kg/s m2 Pa.

C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972 971

Fig. 5b shows that the cooling effectiveness of No. 2 increaseswith the streamwise location x/d, but it has reached a fully devel-oped level before x/d = 10 and changes a little when x/d > 10. Inaddition, the spanwise averaged cooling effectiveness gav has avery little increase when Gc > 1.86 � 10�6 kg/s m2 Pa, especially inthe region of x/d > 40.

Fig. 5c indicates that gav of No. 3 has the similar trend in thestreamwise direction to that of No. 1, but the gav value of No. 3is lower and it increases a little when Gc > 1.58 � 10�6 kg/s m2 Pa.

6. Evaluation of three test plates

In order to compare the overall cooling effectiveness of threetest plates, the comparison of g contour plots and gav curves, at agiven coolant flux Gc = 3 � 10�6 kg/s m2 Pa, are shown in Figs. 6and 7, respectively.

In Fig. 6, the regions of high cooling effectiveness (>0.6) for No. 1are larger than those for No. 2 and 3. Compared with No. 1, thecooling stripes of No. 2 are more extend in the spanwise direction,which implies that at a given streamwise location x/d the coolingeffectiveness distributes are more even in the spanwise direction.The high cooling effectiveness stripes of No. 3 cannot be jointeddue to enlarged hole-spacing, also shown in Fig. 4c.

Fig. 7 shows the spanwise averaged cooling effectiveness gav

curves of three configurations in the region of 0 < x/d < 60. ForNo. 1, gav keeps increasing from x/d = 0 to x/d = 60, so it is difficultto say that it has become fully developed in this region from thepresent results, but it has the highest value compared to No. 2

972 C. Zhang et al. / Applied Thermal Engineering 29 (2009) 966–972

and 3. The spanwise averaged cooling effectiveness for No. 2reaches the fully developed state in the region of x/d < 10 andhas large gradient in the streamwise direction. No. 3 has the lowestcooling effectiveness. Therefore, among three configurations stud-ied, No. 1 has the highest value and the most even distribution ofoverall cooling effectiveness.

7. Conclusions

The infrared imaging experiments were performed to investi-gate the overall cooling effectiveness for three effusion flat plateswith deflection angles. The following conclusions are obtained:

(1) The overall cooling effectiveness g increases with the cool-ant flux Gc, but the increase tends to slow down after Gc

exceeds a certain value, and the specific Gc value varies fromdifferent configurations of the effusion walls. It can beexpected that there should be an optimum Gc value for thebest cooling effectiveness.

(2) The coolant jet can attach to the wall surface in the spanwisedirection and improve the cooling effect between the hole-columns due to the effect of the deflection angle, but if theangle was too large it would be easily disturbed by the main-stream to increase the temperature gradient in the stream-wise direction.

(3) With an appropriate deflection angle, the denser effusionhole-spacing configuration could have higher overall coolingeffectiveness and more even profile given the same coolantflux.

Acknowledgements

Our sincere thanks to Mengmeng Zhao and Peihua Lin for theirsupport in experiments.

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