Local heat transfer for jet impingement onto a concave surface including injection nozzle length to diameter and curvature ratio effects

The effect of jet nozzle length to diameter ratio on local heat transfer coefficient measurements for a row of circular jets impinging onto a concave surface for varying target spacing are reported here. The nozzle length to diameter ratio (L/d) and the nozzle to target spacing (H/d) were varied from 0.2 to 6 and 0.67 to 8 respectively. Three curvature ratios, defined as the ratio of jet diameter to target surface diameter (d/D), equal to 0.1, 0.2 and 0.3 were studied and the jet to jet pitch to diameter ratio (P/d) was kept constant at 4.0. The Reynolds number was varied between 10,000 and 50,000 and wall static pressures for some cases were measured for obtaining a better understanding of the heat transfer coefficient variations. Higher stagnation zone Nusselt numbers were observed for the jet nozzles with small L/d at small H/d values whereas at larger H/d values nozzle length was observed to affect the data only marginally. The difference between Nusselt number values, between H/d = 2 and H/d = 8, was observed to progressively reduce with increasing L/d ratio and this was true for both the local stagnation point as well as the spanwise averaged stagnation line values. The maximum local Nusselt number values were observed to shift from the geometric impingement location to nearby locations for very small nozzle diameter to target distance ratios (H/d 1) for larger curvature ratios. A constant heat transfer impingement area with constant pumping power criterion is presented to compare the performance of the different configurations studied. The longer nozzle lengths and smaller curvature ratios are observed to perform better based on this criterion. A correlation is presented for the overall averaged Nusselt number, with a validity within the parameter range studied, with a 10% error band

Heat transfer and pressure drop measurements in a square cross-section converging channel with V and W rib turbulators

Ribs on the opposite walls of internal cooling passages of gas turbine blades are often used for heat transfer enhancement. These passages can be straight, converging or diverging. In the present study, experimental data for local heat transfer coefficients are presented for a converging channel with rib roughening elements with the cross-section being maintained square from inlet to exit. The local heat transfer coefficient distribution shows the same qualitative behavior observed for the straight channel. The overall averaged heat transfer coefficient along with the measured pressure drop across the test channel is used to calculate thermal performance based on constant pumping power and constant heat transfer area criterion. Data are presented for straight ribs normal to the flow, V ribs with included apex angle of 45 degrees and with apex pointing in the upstream and downstream directions, W ribs with all included angles of 45 degrees and with central apex pointing in the upstream and downstream directions. The inlet Reynolds number (Re-in) was varied from 5000 to 35,000. The rib height (e) was maintained constant throughout the length of the channel. The rib height to mean duct hydraulic diameter ratio (e/D-h,D-m) was kept constant at 0.08. Data for three pitch to height ratios (P/e) equal to 6, 10 and 17.5 are reported for straight and V ribs. The optimum P/e ratio based on constant pumping power thermal performance criterion was observed to be equal to 10 and this was the only configuration studied for W ribs. The difference in thermal performance between the V and W ribs was within uncertainty limits. However, the local variation in heat transfer coefficient in the cross stream direction on the ribbed wall was noticed to be very high for the V ribs compared to that for the W ribs making the W ribs a better choice as enhancement device. (C) 2015 Elsevier Inc. All rights reserved

Effectiveness distribution measurements for a row of heated circular jets impinging on a cylindrical convex surface at different inclinations

The spatially resolved effectiveness distributions for a single jet and row of circular jets impinging on a convex surface are reported in the present study. The impinging surface was inclined at 0, 15, 30 and 45 to the jet axis. Studies were conducted for a single curvature ratio equal to 0.05 at a constant Reynolds number equal to 40,000 for non-dimensional jet-to-target distances, L/d equal to 2, 4, 6, 8 and 10. Two non-dimensional jet-to-jet spacings, S/d, equal to 4 and 8 were studied. The effectiveness distribution for multiple jet impingement was noticed to be different from that for a single jet impingement. The entrainment from the surrounding was mitigated for the inner jets by the outer jets. The interaction of adjacent walljets forms a 'barrier' against the percolation of entrained ambient from the outer jet region towards the inner region. The zone of walljet interaction and region near to the inner jets were therefore observed to result in high effectiveness values. The inclined impingement of the jet reduces the strength of interaction of the walljets on up and downhill sides and thereby reduces the 'barrier effect' against the entrainment of ambient, which causes similar variation of effectiveness for all the jets in a row at high inclinations

Pressure drop characteristics in a rib roughened rotating square duct with a sharp 180 degrees bend

An experimental investigation was carried out to study the local pressure drop distribution in a square cross-sectioned channel with a sharp 180degrees bend rotating about an axis normal to the free-stream direction in the presence of rib turbulators. The sharp 180degrees turn was obtained by dividing a rectangular passage into two channels using a divider wall with a rounded tip where the flow negotiates the turn. The study was conducted for three ratios of divider wall thickness to hydraulic diameter (W/D), 0.24,0.37, and 0.73. The rib height-to-hydraulic diameter ratio (e/D) was maintained constant at 0.1 with a constant pitch-to-rib height ratio (P/e) of 10. The ribs are configured in a staggered arrangement with angles of 60degrees and 90degrees to the main-stream flow, with ribbed surfaces forming the leading and trailing surface during rotation. The Reynolds number was varied from 10,000 to 17,000, with the rotation number varying from 0-0.38. The pressure drop distribution normalized with the mainstream pressure head being presented for the leading surface, trailing surface, and outer surface. The friction factor is less sensitive to the variations in the W/D ratio for a given rib angle, both in the stationary and rotating conditions. The pressure drop distribution for the 90degrees ribs remains practically unchanged under conditions of rotation. However, the pressure drops for the 60degrees ribs are smaller than without rotation. At the highest rotation number, the pressure drop obtained by both 60degrees and 90degrees ribs are comparable

Local effectiveness and Nusselt number distributions for a rectangular jet impinging on a cylindrical convex surface at different angles

Local effectiveness and Nusselt number distribution measurements for heated rectangular jets impinging perpendicularly and obliquely on a cylindrical convex surface are reported here. The jets exit from a rectangular nozzle of height 'H' and width 'W' with aspect ratios, H/W, equal to 10 and 5 and the corresponding the curvature ratios (ratio of hydraulic diameter of nozzle to diameter of the impinging convex surface), b/D, were equal to 0.083 and 0.076. Studies were conducted for non dimensional jet-to-target distances, L/b, equal to 2, 3, 4 and 5, and for inclination of jet axis with the convex target surface, 0, equal to 0, 30 and 45. The Reynolds numbers, Re (based on average exit velocity of jet and hydraulic diameter of nozzle) was kept constant at 17000. The entrainment of the fluid from the surroundings into the walljet from the top surface and edge regions, which is symmetric on the either side of the impingement zone for perpendicular impingement, gives a 'rhombus like' shape for the effectiveness contours. However, the entrainment is asymmetric for oblique impingement, with the drop in effectiveness rapid in the uphill side and gradual in the downhill side, giving the effectiveness contours a 'kite like' shape. The fluid entrainment from the top surface and edge regions influence the Nusselt number distribution also. The relatively higher mixing of the jet fluid with the ambient at the edge regions increases the turbulence and thereby increases the Nusselt number at these locations, resulting in contour shapes different from those observed for the effectiveness

Effectiveness and heat transfer characteristics for a single heated rectangular jet with different aspect ratios impinging perpendicularly on a flat surface

Results from an experimental investigation to study the distribution of local effectiveness and Nusselt number for a heated rectangular jet with different aspect ratios impinging perpendicularly on a flat surface are reported here. The jet exits from a rectangular nozzle of height 'H' and width 'W' with a developing length equal to 50 times the hydraulic diameter of the nozzle geometry, b. Nozzle aspect ratios, H/W, equal to 10, 5 and 1, Reynolds numbers (based on average exit velocity of jet and hydraulic diameter of nozzle) equal to 11,000 and 17,000 and non dimensional jet-to-plate distances, L/b, equal to 2, 3, 4 and 5 were studied. The contours of effectiveness and Nusselt number are initially similar to the exit shape of the nozzle but very quickly take the shape similar to that of a rhombus with rounded corners with diagonals along the major and minor axes of the rectangular jet. The effectiveness variations show that after impingement, the jet fluid entrains the surrounding fluid from the edges as well as the top surface. The edge entrainments progress towards the center, giving the contours the characteristic 'rhombus like' shape. The Nusselt number is also influenced by the fluid entrainment, as indicated by the contours. Regions of high heat transfer coefficients are observed due to high turbulence generated by mixing with the stagnant ambient, especially at the low L/b ratios. (C) 2016 Elsevier Ltd. All rights reserved

Prediction of air leakage and aerosol transport through concrete cracks with a fractal based crack morphology model

Cracks may appear in pressurized concrete containment of a nuclear power plant during a severe accident and provide leak paths for release of radioactive aerosols dispersed in the contained air. In this paper, a fractal based crack morphology model is presented for prediction of air leakage and aerosol transport through cracks in concrete. Airflow field generated in air leakage studies is used for aerosol transport studies with the Lagrangian discrete phase model using CFD code FLUENT. Computational studies conducted with the fractal based model are compared with the experimental data as well as the predictions from empirical relations available in open literature. The comparative studies demonstrate effectiveness of the proposed fractal based model and its versatility for practical applications. (C) 2013 Elsevier B.V. All rights reserved

Influence of rib height on the local heat transfer distribution and pressure drop in a square channel with 90 degrees continuous and 60 degrees V-broken ribs

Internal channel cooling is employed in advanced gas turbines blade to allow high inlet temperatures so as to achieve high thrust/weight ratios and low specific fuel consumption. The objective of the present work is to study the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 90 degrees continuous attached and 60 degrees V-broken ribs. The effect of detachment of the rib in case of broken ribs on the heat transfer characteristics is also presented. Reynolds number based on duct hydraulic diameter is ranging from 10,000 to 30,000. A thin stainless steel foil of 0.05 mm thickness is used as heater and infrared thermography technique is used to obtain the local temperature distribution on the surface. The images are captured in the periodically fully developed region of the channel. It is observed that the heat transfer augmentations in the channel with 90 degrees continuous attached ribs increase with increase in the rib height to hydraulic diameter ratio (e/D) but only at the cost of the pressure drop across the test section. The enhancements caused by 60 degrees V-broken ribs are higher than those of 90 degrees continuous attached ribs and also result in lower pressure drops. But, with an increase in the rib height, the enhancements are found to decrease in channel with broken ribs. The effect of detachment incase of broken ribs is not distinctly observed. The heat transfer characteristics degraded with increase in the rib height in both attached and detached broken ribbed cases. (C) 200

Thermocouple error correction for measuring the flame temperature with determination of emissivity and heat transfer coefficient

Temperature measurement by thermocouples is prone to errors due to conduction and radiation losses and therefore has to be corrected for precise measurement. The temperature dependent emissivity of the thermocouple wires is measured by the use of thermal infrared camera. The measured emissivities are found to be 20%-40% lower than the theoretical values predicted from theory of electromagnetism. A transient technique is employed for finding the heat transfer coefficients for the lead wire and the bead of the thermocouple. This method does not require the data of thermal properties and velocity of the burnt gases. The heat transfer coefficients obtained from the present method have an average deviation of 20% from the available heat transfer correlations in literature for non-reacting convective flow over cylinders and spheres. The parametric study of thermocouple error using the numerical code confirmed the existence of a minimum wire length beyond which the conduction loss is a constant minimal. Temperature of premixed methane-air flames stabilised on 16 mm diameter tube burner is measured by three B-type thermocouples of wire diameters: 0.15 mm, 0.30 mm, and 0.60 mm. The measurements are made at three distances from the burner tip (thermocouple tip to burner tip/burner diameter = 2, 4, and 6) at an equivalence ratio of 1 for the tube Reynolds number varying from 1000 to 2200. These measured flame temperatures are corrected by the present numerical procedure, the multi-element method, and the extrapolation method. The flame temperatures estimated by the two-element method and extrapolation method deviate from numerical results within 2.5% and 4%, respectively. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790471

Comparison of heat transfer distributions on a flat plate impinged by under-expanded jets from a convergent nozzle and a circular orifice

Experiments are carried out for a circular orifice and a nozzle for the same contraction ratio to explore the heat transfer characteristics. The pressure ratios covered in this study are 2.36, 3.04, 3.72, 4.4 and 5.08 for jet to plate distances (z/d) of 2, 4, 6 and 8. The presence of vena contracta and absence of the stagnation bubble in the orifice flow are confirmed from the surface pressure distributions. It is found that higher Nusselt number for the orifice than the nozzle are due to different shock structures and shear layer dynamics. Peak Nusselt number is found as high as 84 % than that for the nozzle. In the wall jet region, the heat transfer rates for the orifice and nozzle are almost of the same order, thus producing steeper temperature gradients under similar operating conditions. The average heat transfer rates are almost 25 % higher for the orifice than that of the nozzle. The recovery factors are in general higher in case of orifice than the nozzle. However, this has not resulted in decreasing the heat transfer rates due to shear layer dynamics