Temperature analysis for the horizontal target cooling with non-confined and inclined air jet

Abstract

Keywords: Jet Impingement; Inclined Jet; Comprehensive Cooling; Cold Spot

Introduction

Thermal cooling management is one of the most engrossed and significant region in various applications due to requirement of massive cooling. Cooling systems are integral part of all applications in electronics, machining, metals forming, etc. All such and similar uses of cooling requirements are having prominent importance as the overheating may cause the entire system to fail with major

damage and non-achievement of objectives. The hardware failure of systems will result in damaging components and may even lead to burning of system due to excessive temperature, by creating severe effects on entire plant and operations. Following are the challenges in front of investigators of thermal management systems.

*Corresponding author. *E-mail address: sbingole1@rediffmail.com This paper was recommended for publication in revised form by Editor Dr. Muslum Arici Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

1. Making compact cooling solution with high heat

dissipation capacity system is required as space is constraint for many applications such as space, electronics, etc.

2. Making lower cost effective solutions are always

desirable as manufacturing and operative costs will effect on economics in overall working of systems.

3. Making cooling systems which will not fail during

operation this indicates with high reliability. For this typically active and passive cooling systems are used in combination.

4. Inventing low cost, high conductive adhesive material

by composites or such methods as to use in components hardware. By these improvements in hardware will take place and it improves material properties related to thermal performance.

5. Use of hybrid system, which combines more cooling

techniques, as sometimes use of one system and its failure may be dangerous.

6. Application of direct liquid cooling which might be

effective but for typical electronics applications it is an important challenge.

7. Investigating and manufacturing high performance

air cooling / air movers with higher heat carrying capacity.

9. Minimize impact on environment by using minimum

power or use of solar power for cooling equipment. Jet cooling technique review is conducted. This includes confined jet impingement [1], jets by impinging normally upward on the target surface [2], multi air jet array [3], staggered arrays [4], are explored experimentally for jet cooling analysis. Jets can be classified based on different factors associated with jet generating system, fluid used, position of jet placement, number of jets used, jet geometry, etc. Also inclination and orientation of jet will effect on its performance [5] Twisted tapes are used for making swirling jet [6] flow to make more turbulence. A jet in jet, called coaxial jet mixing with swirled inner jet [7] is also used. A jet can be rotated instead of fluid rotation, called rotating jet [8]. As reported, jet geometry plays vital role in flow pattern generation. In this line, slot jet of air [9], spent air exits [10], have proved variations in physical structure / method of jet used for improving effectiveness. Various types of fluids are used to carry heat from hot surface. De-ionized water [11], air and water jets [12], mist jet impingement cooling using air-liquid mist [13], fluids with nanoparticles [14], electrically charged micro droplets [15] are the variations detected in jet fluid. Even, oil is also [16] observed to be one of the fluid experimented. If also fluid is one of the important selection parameter for jet, for economic production of jet, the method of producing jets is also considered to be the important. Synthetic jet [17], synthetic pulsating jet [18], high formation

frequency synthetic jets [19], micro-jet [20], verified and found as improved effective. The physical exit paths in a compact system gives diverse flow patterns of jet fluid. Accordingly, elliptical shape [21], cusped elliptical [22], typical variations in inlet and outlet geometries of liquid jet [23], straight, chamfer inlet, chamfer outlet, chamfer in and outlet, countered inlet, and countered in and outlet, are studied. Triangular, square, pentagonal, and hexagonal shapes [24] are testified but slot jet and circular jets are used frequently due to their easiness in making and installation and application. Frequency of jet indicates magnitude of pulsation of jet and such pulsating jets are analyzed [25]. Flow currents, eddies and ultimately heat transfer also depends on the surface on which jet is to be impinged. Jet on a pin fin heat sink [26], jet on and around a central pedestal [27], jet on micro channel heat sink [28], jet on single spherical cavities [29], are considered. The cooling effectiveness of jet depends on criterions like method used for jet production, fluid used in jet, orientation of jet, application specific, direction of jet impingement, physical structure like shape, size, of jet etc. By seeing detailed study under all such type of vast jet configurations from literature, it can be concluded that; there is no specific diameter, shape, fluid, and Reynolds number are used for the best recommended jet cooling applications, but it differs from case to case – application to application. Air is used as main fluid for jet cooling. Slot jets, circular jets, elliptical jets are investigated by many ways. Essentially, hydraulic diameter is the leading factor for altogether study. To practice a circular jet will help to make system easily, with compactness, which is one of the prime requirement. To make compact systems, compact jets are to be thought, inclined jet is one of the promising area for analysis. Heat transfer analysis is investigated for understanding effect of exit air, in which the target plate is inclined at an angle to base reference [30]. Inclined vertical surface characteristics by using horizontal air jet [31], are observed in literature. Jets are to be used for heating as well as cooling applications [32]. As stated earlier, cross-section of jet in inclined jet also plays role and variations are observed in literature [33]. Local convective heat transfers from a vertical heated surface to an inclination (90 to 45°) of circular freesurface jet is investigated [34]. Two inclined jets for different geometry [35] and four jets [36] numerical analysis is presented to increase effective cooling. Jet Correlations are available in the form of summarized table, which are represented by various authors [37]. The heat transfer characteristics for an inclined jet with cross flow [38], is given and even 2D jets [39] are also considered for analysis for simplicity. A spray cooling effect on cooling performance for electronics applications with (0, 20, 40, 50, and 60°) are analyzed [40]. Convection between a downward facing inclined wall [41], the hot object which

is to be cooled is placed in moving position [42], and it is impinged with inclined jet, such compound techniques are also used. By compared with perpendicular jet, inclined jet is significantly neglected area, because of which probably perpendicular jet arrangements and applications for cooling are recognized generally [43]. The review of experimental jet study is presented [44] [45]and the inclined jet compressive review in table form is summarized as in Table 1. In spite of all above challenges, research takes place for many equipment, applications and used many methods. In overall, the heat produced by individual component can be dissipated by using spot cooling methods. But challenge can be to dissipate it for bigger volume or surface. This requisite can be focused and called as ‘comprehensive cooling’. This can be attained by inclined jet impingement methods,

which moves its fluid flow volume with high velocity impact on target surface, and inclination gives overall cooling solution. The clear temperature variation analysis of inclined jet cooling is not being observed. Also its position and analysis of cooling spots with reference to inclinations are required to study for cooling solution design. The consideration of cooling of entire object surface, called as comprehensive / collective cooling approach the experiment is designed. In such configuration, jet is not impinged at the center of the target object, but it is located at the vertical plane, which is in line with edge of the target plate i.e. on leading edge. It is also called as ‘offset cooling’, and same is used in current study. To examine and understand effect of offset jet impingement cooling on typically temperature by use of inclined air jet, at Reynolds numbers up to 20000 is the objective. If careful analysis is performed,

Ali A. Al Mubarak, Syed M. Shaahid, Luai M. Al-Hadhrami [30]

Hakan F. Oztop a,b, Yasin Varol, Ahmet Koca, 4mm Mujdat Firat, Betul Turan, Ilhan Metin [48]

Jiwoon Song, Jang Woo Lee,Man Sun Yu,Sangwoo Shin,Beom Seok Kim, Hyung Hee Cho [32]

M. Zunaid, Afzal Husain, Bhupendra Singh Chauhan, Rohit Sahu [55]

Experimentation

Experimental work is proposed for investigation as per parameters decided. The setup has the air jet impingement arrangement with required inclination on object to be cooled. The object target plate is to be heated by providing electrical supply and supply is recorded by measuring Voltage and Current. This energy is supplied to the target plate for heating, which is to be dissipated out by fluid flow of impinged jet. Following assumptions are made during calculations and experimentation.

1. The supply of heat is fixed – As the target plate is

heated by using electric supply to simulate the heating condition, constant supply is assumed to be given to plate in spite of minor variations, as it does not affect majorly on heating system.

2. Insignificant heat loss by heat conduction through the

object is assumed as thickness of plate is very small and lower side is insulated for radiation heat loss by applying color. Convective heat loss is neglected as air flow is prevented from bottom side of the plate.

3. The plate material used is consistent as it is taken

from one bigger size plate, and minor material variations can be neglected.

4. Air is used as a working fluid that will lead to assume

constant Pr with value 1. As we change air Pr, it will majorly add multiplying factor to equations. If fluid is changed for cooling applications, then Pr is to be considered as it effects strongly on heat transfer. The heated object will dissipate heat by both surfaces. The heat transfer takes place by majority by convection, and partly by conduction and radiation. The heat transfer through conduction by thin object is very minor and hence to be neglected. The radiation heat loss by the thin foiled target from bottom is neglected as it is coated with black color and from top surface it is neglected. Heat transfer by natural convection from bottom side is minor and neglected. The energy coming out of plate is written as: Eout = Qconv.jet + Qconv.bottom + Qrad(both surface) + Qcond

The experimental setup is as shown in Figure 1 , the blower is used to supply air (Powerica Ltd., Centrifugal Type, Rated RPM 1500, 500 CFM, with motor 0.5 HP) with the arrangement of generation of air jet and its impingement on the target is made. The initial temperature of target is room temperature. The object target surface is placed horizontally [47]. The test object is hot flat surface. It is manufactured by thin foil of stainless steel 0.05mm thick and heated by Joule’s effect. The one side surface area of target plate is 264mm x 108mm. The flat plate is fixed in between two copper bus bars in close-fitting plane position. The housing is made ready with the purpose of placing the target plate as per essential location. Also it is easy to adjustment height distance of jet from target (H). This height ‘H’ is verified with use of height blocks of standard sizes. Copper bus bars are fixed in combination with heater plate foil [57]. The supply of 230V, 5Amp is given to a transformer (Make – Super transformers, input 440/230V, Max DC voltage 60 V, 50 Hz, Insulation class – A, Max Current 300Aand converted to required ratings) and the supply given to target surface is of 3V and 50Amp AC. This heated target plate is dissipating heat by impinging air by circular cross-section jet. The width of target plate (shorter side) is considered as the reference Y axis for tracing the jet position. The jet is placed at the center location along the width of target surface. The longer side of target plate (length) is considered as X reference axis. For cooling of target plate, air is used as a fluid in present study. This will also satisfy requirements of air at Reynolds number varying from 2000 to 20000. The jet to target plate distance is one of the significant parameter to examine. To understand the effect of change in height H, it is wide-ranging from 10 mm to 55 mm. The axis of jet can make an angle of 0 to 90° with horizontal target plate as shown in block diagram. The angle of impinged jet is measured by angle measuring unit which is placed at the exit end of the jet. An infrared thermometer (Kusam-Meco

Temperature Study – Center LINE Of Target Plate Analysis

The jet cooling gives heat transfer enhancement [59], but in presented case only temperature as the parameter used for analysis. The average cooling is analyzed related to temperature profile after cooling steady state. After data collection and plotting, results are noted and presented. Center line of target temperature analysis, temperature ratio analysis, its variation related to height H, variation of temperature ratio with inclination, effect of Reynolds number, and minimum temperature are presented. Also to understand cold spots, temperature profiles of target at various experimental combinations are submitted with reference to jet impingement point. During initial study, it is observed that cooling takes place in negligible quantity for Re 2000 and for (H ⁄ D) =6.8, hence not considered in this presentation. The temperature at center line of target plate is noted. The temperature analysis gives entire view of cooling

Figure 3. (Tmin–T0) ⁄ (Tc–T0), for different inclinations at H = 40mm.

analysis and pattern. Hence temperature at central line of the target plate (line C-C in Figure 2) is considered for analysis in this section. As cooling takes place symmetrically about this line, it is the best possible way to analyze data by investigations of non-dimensional temperature ratio (Tmin–T0 ⁄ Tc–T0). This nullifies room temperature effect on calculations and is frequently used in heat transfer analysis. The above stated ratio, in which Tc indicates temperature at section C-C, can be understood by considering as Tmin equal to Tc. This indicates that ratio is 1, or the point where Tc is measured is coolest point of section. Temperature Ratio for Different Jet Angles with H=40mm Figure 3 shows the variation of (Tmin–T0 ⁄ Tc–T0), along X axis of target plate. It is plotted at target to jet height as 40mm. By this it is possible to locate the minimum temperature / cold spot conditions during cooling by jet impingement. Its location goes to downhill side for jet impingement with an angle of 75, 60, 45, 30 and 15°. The phenomenon of temperature jump is observed at 30° inclination at beyond X ⁄ D of 13. It is because of fluid jump. 75 and 60° jet inclinations found to follow nearly same path after X ⁄ D of 3 above. The cold spot location is going to change according to inclination changes. It can be observed that at 15° jet inclination, at X ⁄ D 13 peak is observed indicating coolest point and it is related to geometrical inclination line (explained further). For 30°- angle trend line takes a deep at X ⁄ D 13 as jet flow after impact will divert and mixes with surrounding

as momentum force and does not have any more direction control after impact. Temperature Ratio for All Heights with respect to Jet Impingement Angles Also geometrically center line of jet intersects at the end of the target / downhill side end in case of jet of 15°, which is obviously as shown in Figure 4. Similarly, performance of analogous parameters for all target to jet heights from 10mm to 55mm are represented in Figure 5(a). It is strongly indicated that variations of temperatures are seen clearly as the function of impingement angle and angle is the predominant factor which influence on cooling. It is observed that trends are similar for 75, 60 and 45° of jet inclination, but it does show variations in pattern for 30 and

15° jet inclination. It is because of jet angle at 15 and 30° are making flow of air nearly parallel to plate and giving lesser cooling effectiveness. Further Fig 5 (b) shows normalized data indicating highest cold spot locations are at (X / D) of 2, 4, 6,8 and 15 for jet inclinations of 75, 60, 45, 30 and 15° inclination. Temperature Ratio for All Angles with respect to Variation in Target to Jet Height (H) The average of the non-dimensional temperature ratio (Tmin–T0 ⁄ Tc–T0) is plotted for all angles and all range of Reynolds number with respect to different target to height distances H in Figure 6(a). It is observed that all trend lines are giving similar trends. Cooling is better at leading edge for H=10mm up to X / D = 5, then it decreases. Cooling appears to be increasing up to X / D = 5, and then it declines. The cooling intensity depends upon turbulence intensity of fluctuations near wall [60]. Hence turbulence created due to impact of jet gives good cooling results, but it vanishes in lesser X / D distances. It may be because of lesser turbulence intensity after specific X. At H as 40 and 55mm, the trend line gives nearly parallel and overlying pattern, because jet might get stabilize till it reaches the target. In the view of observing all graph points together, a common configuration is considered. The plot for all variations of inclinations (Θa), and target to jet height (H) is shown in Figure 6(b). It shows that better cooling takes place at X / D distance of 5 to 7.

(Tmin–T0 ⁄ Tc–T0) Variation Related to Reynolds Number The variations of (Tmin–T0 ⁄ Tc–T0), dimensionless temperature ratio and dimensionless (X/D) is plotted for Reynolds number ranging from 4000 to 20000, i.e. turbulent inclined air jet as shown in Figure 7. It is giving almost

similar profiles for all range of Reynolds number. This will be helpful to know temperature at a particular location. The ratio (Tmin–T0 ⁄ Tc–T0)* is normalized data plotted in Figure 7(b). In this figure, 1 on Y axis is indicating location of minimum temperature, or can be also called as cold spot

Figure 7. Temperature ratio variation related to Reynolds number.

Figure 8. (Tmin/Tc) for a specific angle of 15o and 75o and its effect at variation in Reynolds number a) 15°, b) 75°. on target plate location. For jet diameter under study, best cooling is seen at (X/D) of 5 to 7. At higher Reynolds number better cooling is seen after cold spot. At the same time, lesser cooling is observed before cold spot. By this the trend on graph beyond X/D as 6, are shifting upward / higher side with high Reynolds number giving (Tmin–T0 ⁄ Tc–T0)* higher. This reverse change in direction of trendiness related to Reynolds number is called as ‘Reversal cooing effect’. Also temperature jump is seen beyond X/D of 13, giving hot spot at that location. The higher Reynolds number will produce massive scale vortex along the wall due to wall jet, and it sources difference in Nusselt number. The temperature jump may cause due to outer section turbulence by which thermal boundary layer gets disordered, particularly outside the impingement zone (Tmin/Tc) at Specific Jet Inclinations of 15, 45 and 75° The (Tmin ⁄ Tc) is examined separately for a specific angle and its effect by variation in Reynolds number. Figure 8(a) shows the variations for jet inclination of 15°. It is seen that, after being exit at low Reynolds number, the jet flow change is direction due to lower momentum. By this cold spot is seen at (X/D) of around 5 to 7. But at high Reynolds number, because of high momentum of flow, cold spot is at extreme end of target plate (X/D = 13 above). It is also observed that the cooling is uniform throughout the plate for 15°, shown by graph point locations in upper region on graph. Exactly reverse situation is seen for inclination of 75° as in Figure 8 (b). The cold spot is seen on / near leading

Figure 9. (Tmin / Tc) for a specific angle of 45o at variation in Reynolds number.

edge as expected. But in the downhill direction, the trend of increase in temperature is observed. For the third case of 45°, it is seen that results are as expected and gets highest cooling at (X/D) of 5 as shown in

Figure 11. Temperature profile Θa =45o, Re = 16000 with a) H=10mm, b) H=25mm, c) H=40mm, d) H=55mm.

Temperature Profile And COLD Spots

Figure 10. Temperature profile for Θa =45°, H/D=3.4 with a) Re = 4000, b) Re = 8000, c) Re = 12000, d) Re = 16000, e) Re = 20000.

Figure 9. Reynolds number is having lesser impact on cooling performance compared with cooling performance due to inclination effect. For 45° of jet impingement, the temperature variation is nearly similar for both Reynolds number of 12000 and 20000. But at Reynolds number of 4000 only, the momentum of jet plays important role related to gravity, giving little diverted line. The temperature ratio gives the wavy nature indicates coolest point is surrounded by temperature isotherm contours.

It is assumed for simplicity in analysis that the flow is of two dimensional. But as per understanding physics of flow, it is a complex and three dimensional phenomena [43]. Hence temperature variation on entire target surface is presented to understand the effect of inclined jet. Effect of Reynolds Number on Temperature Profile The temperature profile at different Reynolds number ranging from 4000 to 20000 as plotted and shown in Figure 10. In the plot X0 indicates position of jet. For understanding effect of jet cooling, and simplicity, target to jet height is kept constant as 55mm (H/D = 3.4) during the analysis in this section. As known, highest Reynolds number is giving lowest temperature zone. But the position of cold spot remains same (almost around X distance of 100mm) as the effect of inclination dominates other factors. The X0 is 66mm in

this case, hence the uphill side flow in all the case is giving higher variation in temperature with smaller X distance, whereas for downhill side flow is showing lesser zones of temperature variation, as flow gets stabilize by creating boundary layer along surface. Influence of Target to Jet Height on Temperature Profile For understanding effect of variation in height on temperature profile of target, temperature contours are plotted. As H increases, cold zone gets shifted to downhill side, obviously the geometrical stagnation distance (the distance from leading edge, where axis of jet geometrically intersect on target surface) effects on location of cooling zone. In plot it can be observed that apart from geometrical stagnation distance (Xo) (shown by dark vertical line) and proximate area of that line, the temperature profile is showing large variations in temperature as in Figure 11. The Reynolds number is function of velocity and convective heat transfer coefficient increases as jet impinges on the surface [61], as a result of flow counters with velocity and momentum. Effect of Jet Inclination on Temperature Profile Temperature profile isotherms for 15°, 45° and 75° jet inclinations is shown in Figure 12 a), b), and c) respectively. The effect of temperature profile variation is to be seen during change in inclinations of jet. The location of minimum temperature during cooling by jet impingement, goes to downhill side (Figure 12) for jet impingement with an angle of 75, 60, 45, 30° and 15°, 75° and 60° jet inclinations found to follow nearly same path after X/D of 3 above and jet inclination angle is the predominant factor which influence on cooling. The variations of (Tmin ⁄ Tc), dimensionless temperature ratio and dimensionless (X/D) is giving almost similar trends for all range of Reynolds at (Tmin ⁄ Tc) jet inclinations of 15, 45 and 75°. Reynolds number is having lesser impact on cooling performance related to location/place. During analysis of temperature profile of a target, it can be observed that apart from stagnation distance and surrounding region, the temperature profile is showing large variations in temperature. It can be concluded that, for 15° inclined jet, it can be seen that lowest temperature zone is at the end of downstream boundary of target plate. For 45°, it shifts to center of X distance, whereas for 75° it is on the leading edge of the target plate. As stated in earlier sections, jet under study is considered as submerged jet of alike fluid. The aim in this article is investigate location of maximum cooling / heat transfer by identifying hot and cold spots. When air impingement jet angle increases, the three dimensional velocity gradient is decreased at the location of maximum cooling. [34] This happens because jet fluid entry near surface volume starts heat exchange between the jet and plate, typically on upstream side. This leads to growth in spread

Figure 12. Temperature profile for H = 55mm, Re = 16000 with jet inclination of a) 15°, b) 45°, c) 75°.

in flow on opposite i.e. downstream side. Whereas, on the contrary the flow reduces in upstream side. This leads to crowded temperature profile lines in upstream side leading to poorer heat transfer.

Conclusion

The inclined, non-confined jet of air is experimentally investigated for cooling the hot target plate. The conclusions are: • The Center line temperature analysis shows that the location of minimum temperature during cooling by jet impingement, goes to downhill side for jet impingement with an angle of 75, 60, 45, 30 and 15°. • The phenomenon of fluid jump is observed at 30° inclination at beyond of 13. • Cooling is observed to be increase up to X/D = 5, and then it declines. The variations of dimensionless temperature ratio and dimensionless (X/D) is giving almost similar trends for all range of Reynolds number at jet inclinations of 15, 45 and 75°. • Cold spot is seen at (X/D) of 5 to 7. But at high Reynolds number, because of high momentum of flow, cold spot is at extreme end of target plate (X/D = 13 above).

• It is also concluded that Reynolds number is having lesser impact on cooling performance related to cold spot analysis in inclined jet cooling. • During analysis of temperature profile of a target, it is concluded that apart from geometrical stagnation distance and nearby region, the temperature profile is showing large variations in temperature, which is difficult to predict owing to boundary effects. Analysis of combination of jet cooling or multiple inclined jet cooling, its variation with same and different inclination can be future scope for analysis for compact cooling devices.

Nomenclature

Area (m2) Diameter / Hydraulic diameter of Jet (m) Target to jet height (m) Convective heat transfer coefficient (W/m2K) Current (Amp) Characteristic length (m) Heat (W) Temperature (°C or K) Velocity (m/s) Distance, along X Axis (m) Geometrical Stagnation distance (m)

Symbols Angle (°) θ Density (kg/m3) ρ Non Dimensional terms AR Angle Ratio EF Enhancement Factor Nu Nusselt number Pr Prandtl number Re Reynolds number Subscripts / Superscript a, act Actual avg Average avg-y Average along Y axis c, c-c Along Centerline Conv Convection max Maximum min Minimum o Ambient Condition x,y,z Directions * Normalized

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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