Rajendra P. Vedula

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.

Blowoff Stability of Methane-Air Premixed Flame on Tube Burners

Stability of premixed flames is important in applications involving industrial and domestic impingement heat transfer processes, gas turbine combustion chamber and others. Blowoff limits of premixed methane-air flames stabilised on an uncooled Bunsen type burner is considered in the present experimental work. The effect of burner material, wall thickness and burner exit shape on the blowoff limits is presented. Burner materials covered in this study are stainless steel, brass and pyrex. Wall thicknesses considered are 1 mm, 2 mm and 3 mm for pyrex tubes of 10 mm inside diameter. The burner exit shapes covered in this study are circle, triangle, square and hexagon. The operating mixture Reynolds number range is 800 - 4000. It is found that the burners with low thermal conductivity, larger wall thickness and minimal sided polygon shapes provide better lean blowoff stability. Critical velocity gradient parameter defined on the basis of hydraulic diameter collapses the blowoff limits for all shapes covered in...

Local Heat Transfer Distribution in a Rectangular Pin Channel With and Without Vortex Generators

Short pin fins are used to enhance heat transfer rates by increasing the level of turbulence in the trailing edge of gas turbine blades. Experiments are conducted to investigate the local Nusselt number distributions in a staggered pin-fin array using the infrared thermal imaging technique. The pin fins are arranged in a rectangular channel with an aspect ratio of 9. The pins have streamwise pitch-to-diameter (XS/D) and spanwise pitch-to-diameter (XT/D) ratios of 2 with a pin height-to-diameter (H/D) ratio of 2. Ejection holes of 5-mm diameter with a pitch of 12.7 mm are used to study the effects of lateral ejection. Both one-wall and two-wall heating situations are studied for straight-flow and lateral-ejection cases. It is found that the local Nusselt numbers are highest below the horseshoe vortices just upstream of the individual pin fins. For the straight-flow case, the Nusselt numbers for the two-wall heating case are observed to be 15–20% higher than those of the one-wall heating case. Lateral ejection causes a decrease of about 1–10% for the one-wall heating case, while there is an increase of about 10% for the two-wall heating case. Experiments are also carried out with vortex generators between individual pin fins. Vortex generators cause an increase in heat transfer by about 50% compared to the straight-flow cases.

Effect of Channel Orientation and Rib Pitch-to-Height Ratio on Pressure Drop in a Rotating Square Channel with Ribs on Two Opposite Surfaces

The effect of channel orientation and rib pitch-to-height ratio on the pressure drop distribution in a rib-roughened channel is an important issue in turbine blade cooling. The present investigation is a study of the overall pressure drop distribution in a square cross-sectioned channel, with rib turbulators, rotating about an axis normal to the free stream. The ribs are configured in a symmetric arrangement on two opposite surfaces with a rib angle of 90∘ to the mainstream flow. The study has been conducted for three Reynolds numbers, namely, 13 000, 17 000, and 22 000 with the rotation number varying from 0–0.38. Experiments have been carried out for various rib pitch-to-height ratios (P/e) with a constant rib height-to-hydraulic diameter ratio (e/D) of 0.1. The test section in which the ribs are placed on the leading and trailing surfaces is considered as the base case (orientation angle=0∘, Coriolis force vector normal to the ribbed surfaces). The channel is turned about its axis in steps of 15∘ to vary the orientation angle from 0∘ to 90∘. The overall pressure drop does not change considerably under conditions of rotation for the base case. However, for the other cases tested, it is observed that the overall pressure drop increases with an increase in the rotation number for a given orientation angle and also increases with an increase in the orientation angle for a given rotation number. This change is attributed to the variation in the separation zone downstream of the ribs due to the presence of the Coriolis force—local pressure drop data is presented which supports this idea. At an orientation angle of 90∘ (ribs on the top and bottom surfaces, Coriolis force vector normal to the smooth surfaces), the overall pressure drop is observed to be maximum during rotation. The overall pressure drop for a case with a rib pitch-to-height ratio of 5 on both surfaces is found to be the highest among all the rib pitch-to-height ratios covered in this study with the maximum increase in the overall pressure drop being as high as five times the corresponding no-rotation case at the highest rotation number of 0.38 and 90∘ orientation angle.

Heat Transfer Performance for an Obliquely Impinging Slot Jet on a Convex Surface

Experimental measurements for the local variation of effectiveness, pressure and Nusselt number for straight and inclined slot jets impinging on a convex cylindrical surface are reported in this study. The curvature ratio, defined as the ratio of slot width to diameter of impingement target surface (b/D) was kept constant at 0.045. Data are reported for Reynolds number, based on the velocity of the jet and width of the slot, equal to 8500 for jets inclined at 0°, 30° and 45° to the jet axis. The non dimensional distances between jet exit and convex surface (H/b) equal to 4, 6, 8 and 10 were studied. The results show that the effectiveness increases on the downhill side but exhibits a rapidly decreasing trend on the uphill side with increase in inclination angle. The maximum pressure coefficient and Nusselt number shifts towards the uphill side for obliquely impinging jets. Detailed distributions of effectiveness and Nusselt number show that the entrainment from the top and bottom edges of the jet after impingement penetrates inwards causing an effectiveness variation along slot height direction at large circumferential distance from impingement point and increases heat transfer at those positions by increasing the turbulence.

Influence of the Shape of the Orifice on the Local Heat Transfer Distribution of a Flat Plate Impinged by Under-Expanded Sonic Jets

Experiments are carried out for the circular, square, and equilateral triangular orifices of the same contraction ratio in order to explore the heat transfer characteristics for nozzle pressure ratios of 2.36, 3.04, 3.72, 4.4, and 5.08 at z/d = 2, 4, 6, and 8. The presence of vena contracta and shock cells and its locations are estimated from the impact pressure distribution along the axial direction. The pressure drop during the initial expansion is minimal for the equilateral triangular orifice. However, this pressure drop is comparable for the square and circular orifices. The heat transfer characteristics of the circular orifice are nearly axisymmetric, but that for non-circular jets are asymmetric, and the three dimensional effects are clearly observed. This is inferred through the spatial gradients of the Nusselt number and adiabatic wall temperature distributions. The linear correlations for the average Nusselt number are proposed for the circular, square, and equilateral triangular orifices.

Effect of asymmetric heating on local heat transfer distributions in a pin fin channel

High performance gas turbines require an efficient blade cooling system to allow for high rotor inlet temperatures. This work focuses on the pin fin cooling technique, where short pin fins are used to enhance the heat transfer rates by increasing the level of turbulence in the trailing edge of the blade. Experimental investigations are performed using infrared thermal imaging to determine local Nusselt number distributions in a staggered pin fin array. The pins have streamwise pitch-to-diameter ( XS/D) and spanwise pitch-to-diameter ( XT/D) ratios of 2 with a pin height-to-diameter ( H/D) ratio of 2. Both one wall and two wall heating cases are carried out using a constant heat flux boundary condition. It is found that the local Nusselt numbers are highest below the horseshoe vortices just upstream of the individual pin fins. The Nusselt numbers for the two wall heating case are observed to be 15-20% higher than those of the one wall case.

Local heat transfer distribution in a rib roughened rotating square duct with ribs on two opposite surfaces

An experimental investigation has been carried out to study the local heat transfer distribution in a square channel rotating about an axis normal to the free-stream direction in the presence of rib turbulators. The ribs are configured in a symmetric arrangement on two opposite surfaces with a rib angle of 90” to the mainstream flow. Experiments have been carried out for pitch-to-rib height ratios of 5 and 10 for a constant rib height-tohydraulic diameter ratio of 0.1. Experiments have been conducted for Reynolds numbers varying from 23000 to 41000 with the rotation number varying from 0 0.38 with water as the working medium. The test section in which the ribs are placed on the leading and trailing surfaces is considered as the base case (Orientation angle = O”, Coriolis force vector normal to the ribbed surfaces). The channel is turned in steps of 30” to vary the orientation angle from 0” to 90”. At all orientation angles other than zero, the heat transfer observed in case of pitch-to-rib height ratio of 5 is comparatively highter than that of a pitch-to-rib height ratio of 10. Hence, the pitch-to-rib height ratio of 10 configuration that is most preferred for the stationary case no longer appears to yield high heat transfer in the presence of rotation. Nomenclature A Area of cross section of copper cylinder ( m2 ) CP Specific heat of water ( kJ / kg K ) D Channel hydraulic diameter ( m ) e Rib height (m) h Heat transfer coefficient ( W / m2 K ) kf Thermal conductivity of water (W/m K) I Length of the copper cylinder ( m ) Nu(st) Nusselt number under stationary conditions Nu(rot) Nusselt number under conditions of rotation *Assistant Professor, Department of Mechanical Engineering **Associate Professor, Department of Mechanical Engineering Copyright