Heat transfer enhancement on a flat surface with axisymmetric detached ribs by normal impingement of circular air jet

An experimental investigation is carried out to study the heat transfer enhancement from a flat surface with axisymmetric detached rib-rougheners due to normal impingement of circular air jet. A single jet from nozzle of length-to-diameter ratio (l/d) of 83 is chosen. Effect of rib width (w), rib height (e), pitch between the ribs (p), location of the first rib from the stagnation point and clearance under the rib (c) on the local heat transfer distribution is studied. Local heat transfer distribution on the impingement surface is investigated for jet-to-plate distances (z/d) varying from 0.5 to 6 using thermal infrared camera. Turbulence intensity using hot-wire anemometer and wall static pressure measurements are reported for the rib configuration in which maximum heat transfer was observed. Contrary to the results of smooth surface, there is a continuous increase in the heat transfer coefficient from the stagnation point in the stagnation region. This trend is well substantiated by the flow distribution in this region. The ratio of average Nusselt numbers of ribbed and smooth surface is seen to increase with Reynolds number. Correlation is developed for Nusselt numbers averaged upto an r/d of 1.5. Enhancements in heat transfer decrease for higher z/d s.© Elsevie

INFLUENCE OF STREAMWISE PITCH ON LOCAL HEAT TRANSFER DISTRIBUTION FOR IN-LINE ARRAYS OF CIRCULAR JETS WITH SPENT AIR FLOW IN TWO OPPOSITE DIRECTIONS

The effect of streamwise jet-to-jet spacing on local heat transfer distribution due to an in-line rectangular array of confined multiple circular air jets impinging on a surface parallel to the jet plate are experimentally studied. The length-to-diameter ratio of nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in two opposite directions from the confined passage formed between the jet plate and target plate. Mean jet Reynolds numbers based on the nozzle exit diameter (d) covered are 3,000, 5,000, 7,500, and 10,000; jet-to-plate distances studied are d, 2d, and 3d. Streamwise jet-to-jet distances of 3d, 4d, and 5d, and a constant spanwise pitch of 4d, are considered. The jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless-steel metal foil. Local temperature distribution on a target plate is measured using a thermal infrared camera. Wall static pressures in the streamwise direction are measured midway between the spanwise jets to estimate crossflow velocities and individual jet velocities. The streamwise distribution of the jet flow and the cross flow is found to be least influenced by the streamwise pitch variation for the range of parameters considered during the present study. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on the strip-averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, indicates deterioration for lower streamwise pitch and higher jet-to-plate distance

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.© Elsevie

Influence of streamwise pitch on the local heat transfer characteristics for in-line arrays of circular jets with crossflow of spent air in one direction

Influence of the streamwise pitch on local heat transfer distribution due to a rectangular in-line array of circular air jets of length-to-diameter ratio (l/d) of 1.0 is studied experimentally. The flow, after the impingement, is constrained to exit in one direction. Mean jet Reynolds number is varied from 3000 to 10000 and jet-to-plate spacing from d to 3d. Streamwise jet-to-jet distances of 3d, 4d and 5d and a constant spanwise pitch of 4d are considered. A flat target surface is made of thin stainless steel metal foil. The local temperature distribution on a target plate is measured using thermal infrared camera. The jet exit pressures are measured to estimate the cross-flow velocities and individual jet velocities. The streamwise distribution of the jet-flow and the cross-flow is least influenced by the streamwise pitch variation for the range of parameters investigated. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on strip averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, deteriorates for lower streamwise pitch and higher jet-to-plate distance

Influence of Spanwise Pitch on Local Heat Transfer for Multiple Jets with Crossflow

The influence of span wise jet-to-jet spacing on local heat transfer distribution due to multiple impinging circular air jets from an in-line rectangular array on a surface parallel to the jet plate is studied experimentally. The length-to-diameter ratio of the nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in one direction from the confined passage formed between the jet plate and the target plate. Mean jet Reynolds numbers based on the nozzle-exit diameter d covered are 3000,5000,7500, and 10,000, and jet-to-plate spacings studied are d, 2d, and 3d. Spanwise pitches considered are 2d, 4d, and 6d, keeping the streamwise pitch at 5d. For all configurations, the jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless steel metal foil. Local temperature distribution on the target plate is measured using thermal infrared camera. Wall static pressure on the target plate is measured in the streamwise direction to estimate crossflow velocities and individual jet velocities. Heat transfer characteristics are explained on the basis of flow distribution. A simple correlation is developed to predict the strearnwise distribution of the Nusselt number averaged over each spanwise strip resolved to one jet hole as a function of jet-flow and crossflow distributions

Local heat transfer distribution on a smooth flat plate impinged by a slot jet

Experimental investigation of local heat transfer distribution on a smooth flat plate impinged by a normal slot jet is conducted. Present study concentrates on the influence of jet-to-plate spacing (z/b) and Reynolds number on the fluid flow and heat transfer distribution. A single slot jet with an aspect ratio (l/b) of about 50 is chosen to get the fully developed flow at the nozzle exit. Reynolds number based on slot width is varied from 4200 to 12,000 and jet-to-plate spacing (z/b) is varied from 0.5 to 12. The local heat transfer coefficients are estimated from the thermal images obtained from infrared thermal imaging camera. Measurement for the static wall pressure is carried out for various jet-to-plate spacings at a Reynolds number of 12,000. Normalized value of turbulence and velocity are measured using hot wire anemometer along the streamwise direction (x/b) for jet-to-plate spacings (z/b) of 1, 2, 4, 6, 8, 10 and 12. The entire flow field is divided into three regimes namely stagnation region (laminar boundary layer associated with favorable pressure gradient), transition region (associated with increase in turbulence intensities and heat transfer) and turbulent wall jet region. Semi-empirical correlation for the Nusselt number in the stagnation region is proposed. Heat transfer characteristics in the transition region are explained based on the fluid dynamic behavior from the hot wire measurements. Semi-empirical correlation for the Nusselt number in the wall jet region is presented using the velocity profile obtained from the hot wire measurements. (C) 2010 Elsevier Ltd. All rights reserved

Local heat transfer distribution between smooth flat surface and impinging air jet from a circular nozzle at low Reynolds numbers

An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6

Pressure distribution on a semi-circular concave surface impinged by a single row of circular jets

The higher gas turbine entry temperatures for increased gas turbine engine performance require active cooling of the turbine blade. Arrays of impinging jets are one of the potential methods used to reduce the blade temperature on the mid-chord and leading edge regions. The impingement cooling of turbine blade leading edge is modeled by considering impingement of a row of jets on semi-circular concave surface. The experimental model of the present study mimics the scaled-up gas-turbine blade cooled at the leading edge by a row of impinging jets. The local distribution of convective heat transfer rates depend on fluid flow characteristics. Hence, the present study focuses on the experimental investigations of the influence of curvature ratio (D/d = 4.28-8.6), jet-to-jet distance (s/d = 2.4-5.6) and jet-to-plate distance (z/d = 1.0-6.0) on the wall static pressure distribution of a semi-circular concave surface impinged by a single row of multiple jets at a Reynolds number of 20,000. Wall static pressure measurements are made along the spanwise line through the stagnation and two other circumferential locations. The wall static pressure coefficients are seen to decrease with higher jet-to-plate distances. A secondary peak in the distribution of wall static pressure coefficient is observed between the adjacent jets at larger pitches. These are up-washes which may occur due to the collision of wall jets along the longitudinal line of the cylindrical concave surface. (C) 2012 Elsevier Inc. All rights reserved

Heat Transfer Distribution of Semicylindrical Concave Surface Impinged by Circular Jet Rows

The impingement cooling of the leading edge of a gas turbine airfoil is modeled by considering impingement of three rows of jets on a semicylindrical concave surface. Experimental investigations are conducted to study the influence of jet-to-jet distance (s/d = 2.83, 4, and 6) and jet-to-plate distance (z/d = 2, 4, 6, and 8) on the local heat transfer of a semicylindrical concave surface impinged by three rows of multiple jets for different Reynolds numbers (12,000, 15,000, and 18,000). The local heat transfer coefficient is estimated using thermal images obtained by infrared thermal imaging technique. The local distribution of Nusselt numbers and the overall average Nusselt numbers were computed and compared. It was observed that the local heat transfer coefficients at theta = 0 degrees decrease with increase in z/d, whereas the heat transfer coefficients at theta = 60 and 80 degrees increase with an increase in z/d at all Reynolds numbers. The configuration with s/d = 2.83 and z/d = 4 is observed to have the maximum heat transfer distribution with minimum coefficient of variance compared to other configurations at all Reynolds numbers covered in this study