Experimental Investigation on Comparison of Local Nusselt Number Using Twin Jet ... ·...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 60

173604-8585-IJMME-IJENS © August 2017 IJENS I J E N S

Experimental Investigation on Comparison of Local

Nusselt Number Using Twin Jet Impingement

Mechanism Mahir Faris Abdullah*, Rozli Zulkifli*, Zambri Harun*, Shahrir Abdullah,

Wan Aizon W.Ghopa, Ashraf Amer Abbas Department of Mechanical Engineering and Materials, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor

*Corresponding Author [email protected], [email protected], [email protected]

Abstract-- Jet impingement is one of the best methods for

achieving high heat-transfer coefficient over a flat plate surface. It

has been an active research topic for several decades [1]. This

study performed experiments on various parameters, such as

nozzle-to-nozzle spacing (S/d = 1, 2, and 3 cm) and the distance

between the nozzle and the aluminum plate (H/d = 1, 6, and 11 cm),

to determine the effect of different Reynolds (Re) numbers using

the twin jet impingement mechanism on the local heat transfer of

an impinged flat aluminum plate. The same setup was used to

measure the heat flux of the jet impinging on a flat aluminum plate

surface. The heat flux of the heated air jet impinging on the plate

was measured using a heat flux micro-sensor at radial positions 0–

14 cm away from the stagnation point. The measurement of the

heat flux was used to calculate the local heat-transfer coefficient

and local Nusselt (Nu) number for steady air jet and air jet

impingement. The Re used were 17,000, 13,000. Results show that

the local Nu number was calculated at all measurement points.

Furthermore, the Nu number increases with the Re number in the

steady jet. The relationship between the results shows that higher

flow velocity results in the higher localized heat flux of the steadily

heated air jet impinging on the aluminum plate. In addition, the

best heat-transfer coefficient in the area near the nozzles and

aluminum plate and the nearest distance between the nozzles,

especially in the first five points at the plate, decrease away from

the center of the aluminum plate for all Re numbers used. Thermal

data were collected by Graphtec GL820 multichannel data logger

and Fluke Ti25 to capture the temperature distribution in front of

the aluminum foil.

Index Term-- Twin jet impingement; enhancement heat transfer;

Reynolds number, heat flux; Nusselt number; stagnation point

INTRODUCTION

Heat transfer is in the key to increasing the efficiency of

engineering applications. Jet impingement is utilized in many

industrial applications for its ability to produce high heat

transfer rates. It is used in inclined turbine blade, film cooling,

bearing cooling, electronics cooling, automobile windshield

deicing/defogging, drying of paper, and glass tempering [2-5].

Numerous studies on impingement heat transfer, both in

numerical and experimental aspects, have been published [6-8].

Most of the available information on the heat transfer

characteristics of impinging jets focus on normal jet

impingement on a flat surface.

An experimental investigation was conducted to study the

effect of different Reynolds (Re) numbers of air jets on the heat-

transfer rate number using the twin jet impingement mechanism

(TJIM). Impinging jets have wide industrial applications. They

are very important in the industry for heating and cooling. In

various applications, the thermal conductivity of fluids should

be enhanced for efficient heat transfer [9, 10]. The jet

impingement heat transfer technique has attracted considerable

research interest because of the high heat-transfer coefficients

produced by the forced convection action. Impinging jets are

increasingly used in industrial applications over a wide range of

disciplines and configurations, such as in textile drying, food

industry, turbine blade cooling, electronic chip cooling, metal

annealing, aircraft engine nacelle and blade, and glass

tempering. Extensive research has been conducted to study the

effects of applying multiple impinging steady jets on flow and

heat-transfer characteristics. Many studies discussed how to

enhance heat transfer using single and twin impingement jets

[11-13].

Unlike the review articles by Jambunathan [14], which

discussed steady impingement in remarkable detail, more

studies have begun to investigate the effect of flow pulsations

on heat-transfer enhancement experimentally and numerically.

Sheriff and Zumbrunnen [15] experimentally discussed the

effect of flow on cooling performance using jet arrays. The

presence of coherent structures was observed, but no significant

enhancement with respect to the heat-transfer characteristics

was recorded. Zulkifli and Sopian [16] presented the results of

two experimental studies on jet impingement heat transfer.

Measurements were carried out with three Re numbers, namely,

16,000, 23,300, and 32,000. The local Nusselt (Nu) number of

an air jet impinging on a plate was calculated from the recorded

value of the heat flux. The heat flux was measured using a heat

flux sensor. Their results revealed that the calculated local Nu

numbers were higher in all radial positions away from the

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stagnation point. High instantaneous velocity can result in a

high Nu number at localized radial positions, as shown by the

velocity profile plotted in the first part of the experiment.

Zulkifli et al. [17] compared the local Nu numbers of the steady

and pulsating jets at various frequencies, different jet Re

numbers, and different radial positions away from the

stagnation point. Dobbertean and Rahman [18] analyzed the

steady-state heating of a patterned surface plate under free

liquid jet impingement. A constant heat flux was applied to the

cooled plate. Calculations were done for Re numbers ranging

from 500 to 1000 and depths from 0.000125 to 0.0005 m.

increasing the Renumber decreases the local heat-transfer

coefficient.

Wang et al. [19] studied the heat-transfer characteristics

through jet impingement at a high-temperature plate surface to

investigate the impact of initial surface temperature, water

temperature, and jet velocity on heat transfer characteristics for

various industrial applications. Heat flux maximum is

influenced by water temperature, jet velocity, and surface

temperature.

The heat-transfer characteristics were studied experimentally

[20] through high-velocity small-slot jet impingement boiling

on nanoscale modification surfaces to increase the critical heat

flux and to investigate the quantitative effects and the impact

mechanism of surface-distinguishing parameters. Furthermore,

[21] studied the jet impingement heat transfer at a concave

surface in a wing leading edge (experimental study and

correlation development). Heat-transfer increases at the

stagnation point with the Re and <alpha> numbers, and an

optimal H/d nozzle–plate distance exists to achieve the

preferable heat-transfer efficiency at the stagnation point that

conforms to specific operating parameters.

The transient heat-transfer characteristics on a flat plate using

circular air-jet impingement were studied by [6]. The local Nu

number rapidly increases when the air jet begins its

impingement. The increase in Nu speed slows down as the

impinging jet continues to cool down at the 50–80 s region.

Furthermore, [22] studied the heat transfer and fluid flow of a

slot jet impingement with a small nozzle-to-plate spacing in

which a secondary peak in the Nu number was observed. The

results showed that the mean velocity profile in the stagnation

point swerved from the standard law of the wall. The Nu

number was better than in the case with no perturbations, and

large-scale vertical structures were spotted near the location of

the secondary Nu number peak. [23] studied the influence of

nozzle-to-plate spacing on the fluid flow and heat transfer of

submerged jet impingement. The results revealed that the Nu

number and pressure are divided into three zones. In zone I, the

Nu number and pressure drastically increase with the

decreasing nozzle-to-plate spacing. In zone II, the effect of the

nozzle-to-plate spacing is negligible on the Nu number and

pressure. In zone III, the Nu number and pressure

monotonically decrease with the increasing nozzle-to-plate

spacing. Mladin and Zumbrunnen [24] theoretically

investigated the influence of pulse shape, frequency, and

amplitude on instantaneous and time-averaged convective heat

transfer in a planar stagnation region using a detailed boundary

layer model. They reported the existence of a threshold Strouhal

number, St > 0.26, below which no significant heat transfer

enhancement can be obtained.

However, comprehensive data on the effect of impingement jets

on local and average heat-transfer profiles at radial positions

from the stagnation point to the end of the plate surface are still

limited and require further investigation. The present study

aims to investigate the steady twin circular jet heat transfer

characteristics at different Re numbers and focuses on the local

heat transfer coefficient and Nu number. Furthermore, The Nu

numbers on the radial distance at the aluminum plate

impingement jet heat transfer were compared. The local Nu

number was assumed radially symmetrical on the stagnation

point. The total heat flux was proportional to the average Nu

number.

EXPERIMENTAL SETUP

Figure 1 shows a schematic of the experimental instruments.

Compressed air was supplied from the main compressor of 4

psi (0.275 bar). The compressed air was stored in air reservoir

and controlled by a ball valve. We used a refrigerated air dryer

to remove moisture from the compressed air. A pressure gauge

and a regulator were respectively installed to control air

pressure and avoid fluctuation from the cyclic on/off of the

main compressor. On the contrary, the air flow rate was

measured using a digital air flow meter (VA 420, CS

Instruments). The air entered the twin jet impingement

mechanism (TJIM) through two identical pipe lines. Each line

was controlled by a ball valve to ensure identical flow

characteristics for the twin jets.

http://www.sciencedirect.com/science/article/pii/S1359431116305579

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Fig. 1. Schematic of heat transfer tests and thermal imaging setup

The aluminum foil was held tightly to ensure a flat

impingement surface. A square aluminum foil of 30 cm × 30

cm × 0.4 cm and a heat flux-temperature foil sensor were fixed

on the front surface of the aluminum foil using a high-

conductivity heat sink compound and Kapton tape to reduce the

effect of air gaps between the sensor and the aluminum surface

(Figure 2).

Fig. 2. The locations of the thermocouples and

Heat flux sensor on the flat impingement surface.

Figure 3 shows the arrangement of nozzles for all models. A

square aluminum foil of surface dimensions and thickness (L)

was used as a jet impingement target. The locations of the heat

flux sensor and thermocouples on the aluminum plate surface

are shown in Figure 2.



Fig. 3. Arrange of nozzles for the 9 models

In this study, we used two K-type thermocouples that were 120

mm apart and attached to an aluminum plate to monitor the

plate temperature. Data from all sensors, such as temperature,

static pressure (pitot tube), room humidity, atmospheric

pressure, and due point, were collected by the comet model

H7331 [25].

The high-thermal conductivity (k) and small thickness of the

aluminum foil ensured uniform temperature distribution

through the foil thickness for obtaining accurate temperature

measurement at the surface [18].

Thermal data were collected by the Graphtec GL820

multichannel data logger. The Fluke Ti25 Infrared thermal

imager was used to capture the temperature distribution at the

front of the aluminum foil. It is suitable for different kinds of

thermocouples (i.e., J, K, T, E, R, S, and B types) [17].

EXPERIMENTAL PROCEDURES AND METHODOLOGY

Figure 4 shows the schematic diagram of the experimental

setup. The experimental procedures were performed as follows.

First, the air flow was set to achieve a Re number of 17,000 and

13,000 for each jet in the steady jet case by measuring the

velocity of the twin jet center point at the nozzle exit using a

pitot tube. Second, the digital airflow meter was installed in the

TJIM to measure the flow rate and velocity of the steady jet

flow in constant temperature mode at 100 °C. The flow meter

anemometer used in this experimental setup was purchased

from Dantec Dynamics. This flow meter was placed between

the refrigerated air dryer and the PJIM pipes passing the twin

jets. Meanwhile, the run in the twin impingement jets was

executed by obtaining the velocity obtained from the pitot tube,

and this velocity was verified by the flow meter. Then, the

highest Re number=17000 was obtained and also 13000 to

capture the heat transfer per unit time (q) from the data logger

and to calculate the convective heat transfer coefficient (h) by

units (W/(m²K)).



Fig. 4. Schematic of twin impingement jets tests setup.

Subsequently, the localized Nu number was calculated for the

15 points at a radial distance from the stagnation point on the

surface measured. Third, the differential pressure provided the

pressure difference as an analog used as input to data

acquisition Ni 6008, converted to a signal, then convert to a

value using the Scilab code developed to carry out the results.

This differential pressure was set up between the pitot tube and

the Ni 6008 data acquisition. Fourth, the aluminum foil was

installed at 1, 6, and 11 cm away from the nozzle exit to the

surface measured, and the space between the twin nozzles was

1, 2, and 3 cm, which means that nine models were built for the

experimental test. This preparation was carried out to start

measuring the heat flux and the surface temperature on the

impingement surface. Fifth, the Fluke Ti25 Infrared thermal

imager was used to capture the thermal images and temperature

distribution at the surface simultaneously until the heat transfer

reached the steady state. The steady heat transfer was achieved

when the heat inlet to the aluminum foil by the jets equaled the

heat lost by natural convection. A total of 540 samples were

recorded to reduce the experimental error in heat flux–

temperature sensor measurements, and the average value was

considered (Figure 5).

Fig. 5. Twin impingement jets effect and original image of the setup

Prior to the experiments, several parameters related to the TJIM and thermal imager were kept constant as listed in Table 1.



Table I

Fixed value parameters

Constant parameter Value

Nozzle to target distance 1 to 11 cm

Nozzle to nozzle distance 1 to 3 cm

Reynolds number 17000,13000

Ambient temperature 24oC

Aluminium plate temperature 100oC

Emissivity of foil aluminium 0.97

Background temperature 25oC

Transmission 100%

Jet impingement heat-transfer problems require fluid

mechanics and heat-transfer considerations. Consequently,

related dimensionless numbers should be determined.

The Re number of the air jet, which relates the inertial forces

due to the viscous forces of the fluid, is computed as follows

[19]:

Revd

(1)

where μ is the dynamic viscosity of the fluid (Pa·s or N·s/m2 or

kg/(s)), ν is the velocity of the fluid (m/s), and ρ is

the density of the fluid (kg/m3).

In jet impingement heat transfer, forced convection is

dominated. The heat-transfer coefficient (h) could be obtained

from Newton’s law equation, [26] ( )s jQ h T T , which

results in

s j

qh

T T

(2)

where sT is the surface temperature, jT is the jet temperature,

and q is the amount of heat transferred (heat flux), W/m2.

The ratio of convective to conductive heat transfer can be

calculated by the Nu number equation as follows [27]:

hdNu

k (3)

Where h is the convective heat transfer coefficient, d is the pipe

diameter, and k is the thermal conductivity of the fluid.

RESULT AND DISCUSION

Heat-transfer enhancement tests were carried out at nozzle-to-

nozzle spacing (S/d=1, 2, and 3 cm) and nozzle to plate distance

(H/d=1 to 11 cm) to obtain the Nu numbers shown in Figures

6–14. These results confirm that Nu number improved using the

TJIM. Moreover, steady jets produce different Nu numbers

using the nine models with different velocities.

https://en.wikipedia.org/wiki/Mu_(letter)

https://en.wikipedia.org/wiki/Dynamic_viscosity

https://en.wikipedia.org/wiki/Fluid

https://en.wikipedia.org/wiki/Nu_(letter)

https://en.wikipedia.org/wiki/Rho_(letter)

https://en.wikipedia.org/wiki/Density

https://en.wikipedia.org/wiki/Heat_flux

https://en.wikipedia.org/wiki/Convective

https://en.wikipedia.org/wiki/Heat_transfer_coefficient

https://en.wikipedia.org/wiki/Hydraulic_diameter

https://en.wikipedia.org/wiki/Hydraulic_diameter

https://en.wikipedia.org/wiki/Thermal_conductivity



Fig. 6. Nusselt number values at Model (1)

Figure 6 presents the first model with the spacing between

nozzles of S/d=1 cm and the distance between the nozzles and

the aluminum plate surface of H/d=1 cm. The heat-transfer

enhancement in the first points decreased gradually when the

heat flux sensor moved away to the end of the aluminum plate

until it reached around 44 at Re = 17,000 and around 41 at Re

= 13,000, and the maximum Nu numbers were approximately

136 at Re = 17,000 and 122 at Re = 13,000.




Figure 7 presents the second model when the spacing between nozzles was S/d = 1 cm and the distance between the nozzles and

the aluminum plate surface was H/d = 6 cm. The heat-transfer enhancement in the first four points decreased gradually when the

movement of the twin jets was in the opposite horizontal direction to the end of the plate surface until it reached around 48 when

Re = 17,000 and 44 when Re = 13000, and the maximum Nu numbers were approximately 136 at Re = 17,000 and 127 at Re =

13,000.


The figure above displays the Nu number values when the

spacing between nozzles was S/d = 1 cm and the distance

between the nozzles and the aluminum plate surface was H/d =

11 cm. The heat-transfer enhancement in the first points

decreased gradually when the heat flux sensor moved away to

end of the aluminum plate until it reached 48 at Re = 17,000

and 41 at Re = 13,000, and the maximum Nu numbers were

approximately 149 at Re = 17,000 and 136 at Re = 13,000.




Figure 9 presents the fourth model when the spacing between

nozzles was S/d = 2 cm and the distance between nozzles and

the aluminum plate surface was H/d = 1 cm. The heat-transfer

enhancement in the first 4 points decreased gradually when the

twin jets moved in the opposite horizontal direction to the end

of the plate surface until it reached around 49 when Re = 17,000

and 42 when Re = 13,000 and the maximum Nu numbers were





Figure 10 presents the fifth model when the spacing between

nozzles was S/d=2 cm and the distance between the nozzles and

the aluminum plate surface was H/d = 6 cm. The heat-transfer

enhancement in the first 4 points decreased gradually when the

twin jets moved in the opposite horizontal direction to the end

of the plate surface until it reached 52 when Re = 17,000 and

46 when Re = 13,000 and the maximum Nu numbers were



Furthermore, the maximum Nu numbers of approximately 152

when Re = 17,000 and 136 when Re = 13,000 were obtained by

the TJIM in the first 4 points at a radial distance from the

stagnation point and decreased gradually to the end of the

aluminum plate surface until it reached less than 50 when Re =

17,000 and 42 when Re = 13,000 at the end of the aluminum

plate, when the spacing between nozzles was S/d=2 cm and the

distance between the nozzles and the aluminum plate surface

was H/d = 11 cm (Figure 11).




Figure 12 presents the seventh model when the spacing between

nozzles was S/d = 3 cm and the distance between the nozzles

and the aluminum plate surface was H/d = 1 cm. The

enhancement of heat transfer in the first 4 or 5 points decreased

gradually when the twin jets moved in the opposite horizontal

direction to the end of the plate surface until it reached 48 when

Re = 17,000 and 42 when Re = 13,000, and the maximum Nu

numbers were approximately 151 at Re = 17,000 and 133 at Re

= 13,000.




Figure 13 presents the eighth model when the spacing between

nozzles was S/d = 3 cm and the distance between the nozzles

and the aluminum plate surface was H/d = 6 cm. The heat-

transfer enhancement in the first points increased gradually then

decreased when the heat flux sensor moved away to the end of

the aluminum plate until it reached around 52 at Re = 17,000

and 48 at Re = 13,000, and the maximum Nu numbers were





Finally, the maximum Nu numbers of the ninth model when the

spacing between nozzles was S/d= 3 cm and the distance

between the nozzles and the aluminum plate surface was H/d =

11 cm were approximately 150 when Re = 17,000 and 133

when Re = 13,000 and obtained by the TJIM in the first 5 points

at an approximate radial distance from the stagnation point and

decreased gradually to the end of the aluminum plate surface

until it reached less than 50 when Re = 17,000 and 43 when Re

= 13,000 at the end of the aluminum plate. Figure 14 shows the

Nu numbers when the spacing between nozzles was S/d = 3 cm

and the distance between the nozzles and the aluminum plate

surface was H/d = 11 cm.

. These results are more logically on comparing with other

research work. likewise to confirm the accuracy of the present

work, the value of the steady jet nusselt number versus

Reynolds number was plotted and Compared with the results of

other previous researchers as reported by [17, 29]

In summary, we can understand that the Nusselt number value

presented a sensible change with increase in values of Reynolds

number [14, 28]. These results are more logically on comparing

with other research work. likewise to confirm the accuracy of

the present work, the value of the steady jet Nusselt number

versus Reynolds number Compared with the results of other

previous researchers as reported by [17, 28]

In summary, these results reflect the behavior of the Nu number

quantitatively and qualitatively when steady and when twin jets

impinge on a hot flat plate at the center line of interference zone

passing to all the holes of the twin jets to the end of the surface

plate. Results from the experimental data are presented in this

section. Figures 15 and 16 illustrate the TJIM effect on the

surface temperature measured by the heat flux-temperature

sensor on the front surface and the thermal image on the surface,

respectively. The surface temperature increases at the first 4 or

5 points on the plate surface and then starts to decrease after the

5-point distance on the aluminum surface plate.

Presenting how TJIM affects the Nu number in the midpoint or

center between the twin jets that pass to end of the interference

zone at the end of the aluminum plate surface is important. The

figures present the influence on the Nu number based on micro

foil sensor measurements. The Nu number was recorded and

then decreased away from the center of the aluminum plate

surface to the end of the surface (low rates at distant points from

the center of the surface). This result is logical because in the

current experiment the heat-transfer rate increases as long as the

twin jets are close to the surface and when the heat flux sensor

is under the direct impact of the twin jets air flow on the surface.

It gradually decreases as we move away from the center of the

interference zone.

Figures 15 and 16 show the images captured by the thermal

imager. These thermo-images represent the effect and

distributions of TJIM on the surface of the impinged target. The

center of the temperature values are labeled in the images. The

steady jet cases are illustrated below for the nine models,

respectively. Furthermore, we captured 18 pictures for all

models, one picture for every model at Re= 17000 and 13000.

Some observations can be recognized in these images. First, the

hottest spots due to the effect of twin jets impingement can be

clearly seen. Then, an elliptical temperature distribution after

the midlevel point of temperature can be observed. Moreover,

the steady jet case achieved higher temperature rates due to the

high flow rate of the jets. In contrast, the lower flow rate was

supplied in TJIM given its duty principles. Moreover, the higher

center temperature of 165.8 °C in Figure 15 was produced by

the fourth model when H/d = 1 and S/d = 2. Furthermore, twin

jets exchange their superiority in having the highest

temperature because of the high sensitivity of the thermal

imager to the minimal changes in the temperature between both

jets. See Figure 15 below.



Fig. 15. Thermographic temperature distribution on Aluminuim foils Model (1) to (9) at Re=17000

In Figure 16, the steady jet case shows the heat-transfer

behavior and thermal distributions. We have captured nine

pictures for all models. Some observations can be recognized in

these images. First, the hottest spots due to the effect of twin

jets impingement can be seen clearly. Then, an elliptical

temperature distribution after the midlevel point of temperature

can be observed. The steady jet case achieved higher

temperature rates because of the high flow rate of the jets. In

contrast, the lower flow rate was supplied in TJIM given its duty

principles. Moreover, the higher center temperature of 164.4 °C

in Figure 16 was produced by the fourth model when H/d = 1

and 6 and S/d = 2.



Fig. 16. Thermographic temperature distribution on Aluminuim foils Model (1) to (9) at Re=13000

CONCLUSION

The present study extensively investigated the impact of twin

jet impingement heat-transfer mechanism for heat-transfer

enhancement. The present investigation included heat flux–

temperature micro foil sensor measurements and IR thermal

imaging. The results revealed a significant enhancement in the

localized Nu number of the steady flow at positions of radial

distance on the aluminum measured surface 1–5 cm at Re

numbers of 17,000 and 13,000 and gradually decrease as we

move away from the center of the interference zone.

Subsequently, the thermography capturing process was carried

out on the surface of the aluminum foil flat target while the heat

flux–temperature data were collected for the nine models at

different Re on the impinged surface of the target. The results

revealed logical behavior for all parameters under

consideration. The identical effect of twin jets verifies the

performance of twin jets system which was designed to

generate identical two jets. Moreover, the distance between

nozzles and the spacing between jet and nozzles can be

considered the optimum condition for achieving higher heat

transfer rates for the present problem. In conclusion, the

variously presented results could describe the effect of the nine

models on heat-transfer characteristics of TJIM that may

contribute to the performance improvement of various

industrial and engineering applications.

ACKNOWLEDGEMENTS

We would like to thank the financial supports provided by

FRGS/1/2013/TK01/UKM/02/3,

FRGS/1/2016/TK03/UKM/02/1 and Prof. Dr. faris Abdullah

Aljanaby.

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