Experimental Investigations on the Local Distribution of wall static pressure coefficient Due To an Impinging Slot Air Jet on a Confined Rough Surface

Experimental Investigations on the Local Distribution of wall static pressure coefficient Due To an Impinging Slot Air Jet on a Confined Rough Surface 1 Adimurthy. M 1 BLDEA s VP DR. P G Halakatti college of Engineering and Technology, Vijayapura, Karnataka, India 2 Vadiraj V Katti 2 Principal, KLS Vishwanathrao Deshpande Rural Institute of Technology, Haliyala, Karnataka, India, ABSTRACT Experimental investigations on the local distribution of wall static pressure coefficient due to an impinging slot air jet on a confined rough surface is performed with the parameters: Reynolds number (2500-20000), jet to plate spacing (0.5 to 10), confinement ratio (6.8, 10.6, 13.2), size of the ribs (4mm, 6mm) and the distance of the point from the stagnation point in the lateral direction on the plate. Static pressure probe and a differential pressure transducer is used to record the pressure on the target plate. It is clearly identified that the presence of confinement and roughness on the surface changes the flow behaviour and an enhancement in the coefficient of pressure for the confinement ratio of 6.8 and rib size of 4mm at the range of Reynolds numbers studied. The results can help in design of a better cooling device for industrial applications. Keywords Confinement ratio, surface roughness, Reynolds Number, coefficient of pressure, jet to plate spacing, stagnation point pressure coefficient. 1.INTRODUCTION Jet impingement heat transfer received considerable research attention due to their inherent characteristics of high rates of heat transfer. Such impinging flow devices allow for short flow paths on the surface with relatively high heat transfer rate. The industrial processes which employ impinging jets are drying of textiles, film and paper; processing of some metals and glass; gas turbine blades and outer wall of combustors; cooling of electronic equipment The Impinging jet can be defined as a high velocity coolant mass ejected from a hole or slot that impinges on the heated plate. A characteristic feature of this flow arrangement is an intensive heat transfer between the wall and fluid. Various researchers have worked on impinging jets of different configurations in the past. A.H.Beitelmal et al [1] studied the effect of surface roughness on average heat transfer at on impinging air jet. The temperature was measured over a Reynolds number ranging from 9600 to 38500. Nozzle to plate spacing (z/d) ranging from 1 to 10. They observed up to 6%increase in average Nusselt number due to surface roughness that disturbs the boundary layer and promotes the turbulence of wall jet. The correlation between average Nusselt number, Reynolds number and nozzle to plate spacing was proposed. V.A.Chirac and A.Ortega [2] used the numerical finite difference method to determine steady and unsteady flow and heat transfer due to a continued slot jet impingement on isothermal plate. The jet Reynolds number ranging from 250 to 750 of Prandtl number 0.7 and jet to plate spacing (H/W) of 5 is studied. They observed that at Reynolds number 585-610 the flow become unsteady. In the steady regime stagnation Nusselt number is directly proportional to the Reynolds number and distribution of heat Transfer in the wall jet region was influenced by re-entrainment of the flow. X.Gao and B.Sunden [3] studied the heat transfer characteristics of confined single and multiple impinging slot jet experimentally. The liquid thermography was used map the temperature distribution on the plate. The effects of nozzle to plate spacing, Reynolds numbers, and width of jet and presence of the exhaust 69 Adimurthy. M, Vadiraj V Katti

port on the local and average Nusselt number is studied. Comparing the single and multiple jets at two nozzles to plate spacing (H/B) 8 and 24, all the local Nusselt number distribution shows same behaviour for all Reynolds number from 1150-7550, which confirms that large slot to slot distance in multiple jets prevents from jet interaction with each other. The effects of the nozzle to plate spacings on stagnation Nusselt number of central jet of the three slot system and single slot jet at different Reynolds numbers observed and stagnation Nusselt number is maximum at H/B=8 and both jets have same value up to H/B=8, increasing in H/B leads to decrease in Nu 0 values of multiple jet. The effect of slot width shows that higher width has higher local Nusselt number and lower width have higher average Nusselt number, it occurs due to increase in turbulence intensity as the slot width increases. V.Narayan et al [4] experimentally studied the flow field surface pressure and heat transfer of turbulent slot jet. Two nozzle to surface spacing at 3.5 (transitional jet) and 0.5 (potential core jets) nozzle exit hydraulic diameter are considered for exist Reynolds number at 23000. They reported high heat transfer rate in the transitional jet impingement and non-monotonic decay in heat transfer coefficient. For potential core jet impingement and generation of turbulence near the surface prior to impingement and pressure at span wise vertices in the stagnation region with increase in near wall responsible for increased in heat transfer in transition jet impingement. E.Bayadar and Y. Ozmen [5] conducted experimental and numerical investigation on impinging air jet at Reynolds number ranging from 30000 to 50000. The mean velocity, turbulence intensity and pressure distribution were obtained for given Reynolds number and a nozzle to plate spacing ranging between 0.2-6. They observed pressure distribution on the impingement plate was independent of the Reynolds number. Sub atmospheric region occurs on impinging plate for nozzle to plate spacing up to 2 and it become stronger and more deductible with decreasing the nozzle to plate spacing. They reported maximum turbulence intensity and secondary peak in Nusselt number occurred in the sub atmospheric region. V.Katti and S.V.Prabhu [6] studied enhancement of heat transfer on a flat plate surface using axis symmetrical ribs by normal impingement of circular jet. Effect of ribs width (w) ribs height (e) pitch between the ribs (p) location of first rib from stagnation point and clearance under the rib (c) on local heat transfer distribution is studied jet to plate distance varying from 0.5 to 0.6. They observed that the enhancement in heat transfer due to acceleration of fluid in that region created by clearance under the rib and Nusselt number of stagnation point increases by shifting rib nearer to the stagnation. With increasing in width of rib major flow impinges on top surface of rib and may not reach to target surface it may lead to decrease in heat transfer and increasing rib height flow velocity along the target surface decaying at faster rate. They also observed wall static pressure gradient of stagnation point is higher for surface with detached ribs than smooth surface. Puneet Gulati et al [7] studied the effects of shape of nozzle, jet to plate spacing and Reynolds number on impinging air jet. Reynolds number varied between 5000 to 15000 and jet to plate spacing from 0.5 to 12 Nozzle diameters. Length to equivalent diameter ratio of 50 is chosen. The local heat transfer is determined using thermal images obtained from IR thermal imaging method. They observed that rectangular nozzles show high heat transfer rate due to high turbulence intensity at nozzle exit and it requires more pumping power due to larger pressure loss coefficient. Pressure drop measurements across the nozzle are made and pressure loss coefficient for all nozzle configurations reported. Nirmalkumar et al [8] conducted experimental investigation of local heat transfer distribution and fluid flow properties on a smooth plate by impinging slot air jet. Reynolds number based on slot width varied from 0.5 to 12. They identified three regimes on the flat surface viz. stagnation region (0 x 2), transition region (2 x/b 5) and wall jet region (x/b 5). Semi empirical correlation for Nusselt number in the stagnation region and wall jet region is suggested. Katti, Adimurthy and Ashwini [9] experimentally studied the local wall static pressure distribution on smooth and rough surface by impinging slot jet. They observed that wall static pressure is independent of Reynolds number and its strength decreases with increase in nozzle to plate spacing, they also observed enhancement in heat transfer rate at rough surface using detached ribs compared to smooth surface. Adimurthy and Katti [10] investigated the local distribution of wall static pressure and Heat transfer on a smooth plate impinged by a slot air jet. They adopted Infrared thermographs to study the heat transfer distribution and static pressure probe to analyze the flow distribution and behaviour over the plate. They presented a correlation to estimate the Normalized value of the Nusselt number in particular to the stagnation region. Adimurthy, Katti and Venkatesha [11] experimentally studied the fluid flow and heat transfer characteristics of a rough surface, impinged by a confined laminar slot air jet. Heat transfer distribution was 70 Adimurthy. M, Vadiraj V Katti

analyzed with the IR images and static pressure probe is used to capture the pressure distribution over the rough surface in presence of confinement. They correlated flow pattern and heat distribution in their study. They also presented an empirical formula to estimate the Stagnation point Nusselt number in the potential core as well as beyond the potential core region separately. Present work is intended, to study the influence of Reynolds number, jet to plate spacing, surface roughness and confinement of the jet on the local distribution of wall static pressure coefficient due to impingement of a confined slot air jet on the rough surface. 3. EXPERIMENTAL SETUP AND METHODOLOGY The experimental set-up for wall static pressure distribution study is as shown in 1 and Air is supplied by a blower through a well calibrated orifice meter and venturimeter. The flow rate is controlled by a flow control valve, to achieve the uniform flow rate at the exit of the nozzle. 1.Traversing table, 2.Target plate, 3.Detached rib, 4.Nozzle, 5.Plenum chamber, 6.Diffuser, 7.Control valve, 8.Orifice meter, 9.Air blower, 10.Venturimeter, 11.U-Tube manometer, 12.Differential pressure transducer, 13.Confinement plate Figure.1 Experimental setup for wall static pressure distribution with rough Surface The airflow is directed to the plenum through a diffuser and two meshes in the plenum. Velocity profile remains uniform because of the diffuser upstream of the plenum. Nozzle is made of an acrylic sheet with 45 mm height and 4 mm wide. The aspect ratio of the nozzle cross section is maintained very high around 24 so that the effect of nozzle height is neglected. Jet air temperature is measured using a Chromel-Alumel Thermocouple (K-type) positioned at the inlet of the nozzle. The output of the thermocouple is measured by milli voltmeter. The target plate is kept on 2-D traverse table, so it can be easily moved for various jet to plate spacings by operating the traversing system. Orifice meter and venturimeter are used to measure the rate of air flow through the jet. The target plate made up of acrylic sheet of 10mm thickness. A static pressure tap approximately 0.5mm in diameter is drilled in 10mm thick acrylic plate for 3mm deep from impingement surface and then counter bored to 3mm diameter for the remaining depth. The jet-to-plate distances (Z/Dh) of 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10 are considered in the present study. The ribs are fabricated from the thermally non active material Plexiglas (K=0.23w/mk). Two ribs of size 4mm and 6mm width, 45mm height and thickness is 2mm are chosen for the study. The spacer of thickness 1mm is used to mount the rib over the target plate to maintain uniform gap between the plate and the rib. 71 Adimurthy. M, Vadiraj V Katti

Nomenclature symbol Description A area of jet (m 2 ) a 1 cross sectional area of inlet pipe (m 2 ) a 2 cross sectional area of throat (m 2 ) B breadth of nozzle (m) C d coefficient of discharge C p co-efficient of pressure D h hydraulic diameter of nozzle in (m) g acceleration due to gravity 9.81 m/sec 2 L length of nozzle (m) Qact actual discharge of air (m 3 /sec) V j velocity of jet (m/s) x manometer deflection (m) μ dynamic viscosity of air in (kg/m sec) ρ m density of manometer fluid (kg/m 3 ) p Recorded value of pressure from DP 4. DATA REDUCTION The important parameters of the study are computed using the various equations listed below. The description for the notations is given in the nomenclature. Discharge of venturimeter (Qact), is calculated using the equation, a Q act = C d 1 a2 2gH (1) 2 2 ( a1 a2 ) Equivalent head for the flow measurement is computed m using, H= p 1 x (2) Hydraulic diameter (Dh) for the slot jet is computed using 2 LB L the equation, D = h B (3) Reynolds number of jet (Re jet ) is given by, Velocity of jet (V j ) can be measured using the formula, Re jet = Q V j = A V D act j j h (4) (5) Co-efficient of wall static pressure(c p ) is calculated from 2 p equation, C p = 2 V a j (6) 72 Adimurthy. M, Vadiraj V Katti

5 RESULTS AND DISCUSSION The results of an investigation to determine the local wall static pressure distribution of a normally impinging confined slot air jet on flat rough surface for Reynolds number ranging from 2500 to 20000 and for the jet to plate spacing (Z/Dh) of 0.5 to 10 are presented. Pressure distribution is measured using Differential Pressure (DP) transducer. 5.1 Influence surface roughness and confinement on wall static pressure distribution at Re=2500 Fig 4.2.1(a) to Fig 4.2.1(f) shows variation of local wall static pressure along the stream wise direction for particular Z/D h values at Reynolds number 2500 of different confinement ratios of 4mm and 6mm rib. From fig 4mm rib with confinement ratio of 6.8 shows maximum stagnation wall static pressure and sub atmospheric region at Z/D h =0.5, sub atmospheric region and secondary peaks are depend on the Z/D h value. (a) (b) (c) (d) Fig 4.2.1 Comparison of wall static pressure of different rib width and confinement ratios 73 Adimurthy. M, Vadiraj V Katti

5.2 Influence of Confinement on smooth and rough surface (4mm rib) From Fig 4.2.2(a) to Fig 4.2.2(f) shows effect of confinement ratios on variations of local wall static pressure along stream-wise direction of smooth and rough surface (4mm rib) for different nozzle to plate spacing (a) (b) (c) (d) 74 Adimurthy. M, Vadiraj V Katti

(e) (f) Fig 4.2.2 effect of confinement on smooth and rough surface (4mm rib) The maximum stagnation wall static pressure occurred for confined ribbed surface compare confined smooth surface. The sub-atmospheric region occurred up to Z/D h =4 for confined ribbed surface, Z/D h =0.5 for confined smooth surface. It clear that strength of sub-atmospheric region depend on ribbed configuration at Z/D h >0.5, and secondary peaks observed for confined ribbed configuration and peaks height increases with increase in Z/D h. The spreading rate of confined smooth jet is more than ribbed and confined. 5.3 Influence of confinement ratio on stagnation point wall static pressure Fig 4.2.3(a) to Fig 4.2.3(e) shows variation of stagnation point wall static pressure with Z/D h for different confinement ratio of 4mm rib. At lower Reynolds number L c /D h =6.8 shows better values. (a) (b) 75 Adimurthy. M, Vadiraj V Katti

(c) (d) (e) (f) (g) (h) 76 Adimurthy. M, Vadiraj V Katti

(i) (j) Fig 4.2.3 Influence of confinement ratio on stagnation point wall static pressure At Re =5000 to 20000 The L c /D h =13.6 shows higher stagnation wall static pressure within potential core region and L c /D h =10.2 shows better values behind potential core region. Fig 4.2.3(f) to Fig 4.2.3(j) shows variation of Cp 0 with Z/D h for different confinement ratio of 6mm rib. The L c /D h =13.6 shows better Cp 0 values at Z/D h <4. After potential core the Cp 0 values of all configuration merge with smooth plate values 6. CONCLUSION An experimental study is carried out to investigate local wall static pressure distribution and local heat transfer distribution between the normally impinging confined slot air jet and flat rough surface. A single fully developed jet issued from a slot jet is chosen. Reynolds number based on nozzle exit condition is varied from 2500 to 20000. Different configurations of detached ribs and confinement plates are chosen. Sequential parametric study is carried out to investigate the influence confinement on the local wall static pressure distribution on rough surface. The jet to plate distance is varied from 0.5 to 10 times the Hydraulic diameter of the jet. The conclusions drawn from the investigation are as follows: Maximum wall static pressure occurs at stagnation point and its value decreases with increase in nozzle to plate spacing for all the configurations investigated. Sub-atmospheric region is observed up to Z/D h =4 for all rough surface configurations studied. Effect of confinement is pronounced more at Z/D h =0.5 for all configurations since at lower jet to plate spacing, more recirculation of the jet happens. Secondary peaks in wall static pressure are visible invariably at all jet to plate spacings and Reynolds numbers for rough surface due to the effect of flow acceleration under the detached ribs. Increase in confinement length increases the potential core length with increasing Reynolds number. 7. REFERENCES [1] Beitelmal, Abdlmonem H., Michel A. Saad, and Chandrakant D. Patel. "Effects of surface roughness on the average heat transfer of an impinging air jet." International Communications in Heat and Mass Transfer 27.1 (2000): 1-12. [2] Chiriac, Victor A., and Alfonso Ortega. "A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface." International Journal of Heat and Mass Transfer 45.6 (2002): 1237-1248. 77 Adimurthy. M, Vadiraj V Katti

[3] Gao, Xiufang, and B. Sunden. "Experimental investigation of the heat transfer characteristics of confined impinging slot jets." Experimental heat transfer 16.1 (2003): 1-18. [4] Narayanan, V., J. Seyed-Yagoobi, and R. H. Page. "An experimental study of fluid mechanics and heat transfer in an impinging slot jet flow." International Journal of Heat and Mass Transfer 47.8 (2004): 1827-1845. [5] Baydar, E., and Y. Ozmen. "An experimental and numerical investigation on a confined impinging air jet at high Reynolds numbers." Applied thermal engineering 25.2 (2005): 409-421. [6] Katti Vadiraj, and S. V. Prabhu. "Heat transfer enhancement on a flat surface with axi-symmetric detached ribs by normal impingement of circular air jet." International Journal of Heat and Fluid Flow 29.5 (2008): 1279-1294. [7] Gulati, Puneet, Vadiraj Katti, and S. V. Prabhu. "Influence of the shape of the nozzle on local heat transfer distribution between smooth flat surface and impinging air jet." International Journal of Thermal Sciences 48.3 (2009): 602-617. [8] Nirmalkumar, M., Vadiraj Katti, and S. V. Prabhu. "Local heat transfer distribution on a smooth flat plate impinged by a slot jet." International journal of heat and mass transfer 54.1 (2011): 727-738. [9] V.V.katti, Adimurthy.M. & Ashwini..M. Tamagond, Experimental Investigation on Local Distribution of Wall Static Pressure Coefficient due to Impinging Slot Air Jet on Smooth and Rough Surface. Journal of Mechanical Engineering Research and Technology, Volume2, Number 1, (2014) pp500-508. [10] Adimurthy M and Katti V.V Local distribution of wall static pressure and heat transfer on a smooth flat plate impinged by a slot air jet. Heat and Mass Transfer [2016], DOI 10.1007/s00231-016-1847-9 [11] Adimurthy M, Katti V.V and Venkatesha, Experimental Study of Fluid Flow and Heat Transfer Characteristics of Rough Surface, Impinged by a Confined Laminar Slot Air Jet. International Journal of Engineering Research & Technology (IJERT), ISSN: 2278-0181, Vol. 5 Issue 06, (2016), JERTV5IS060418 78 Adimurthy. M, Vadiraj V Katti