Showing papers by "S.V. Prabhu published in 2008"

Survey on measurement of tangential momentum accommodation coefficient

Abstract: The value of tangential momentum accommodation coefficient (TMAC) is required while prescribing the boundary condition for flow of gases in the slip and transition flow regimes. The precise determination of its value is important for several other applications as well. This article reviews the experimental techniques employed by researchers over the decades to measure this coefficient and the values reported in the literature, with relevance to calculation of the slip velocity. The review shows that the value of TMAC is dependent on a number of parameters including nature of the gas, pressure of the gas, material of the surface, surface cleanliness and roughness, and surface temperature. For monatomic gases, the TMAC at about 0.93 is almost constant with respect to the Knudsen number, and this value can be employed for most commonly available surface materials. However, for nonmonatomic gases, TMAC decreases with an increase in Knudsen number; a correlation of TMAC with Knudsen number for this class of ga...

Experimental investigations on single stage, two stage and three stage conventional savonius rotor

Abstract: The performance of single stage (rotor aspect ratio of 1.0), two stage Savonius rotor with rotor aspect ratios of 1.0 and 2.0 (stage aspect ratios of 0.50 and 1.0) and three stage Savonius rotor with rotor aspect ratios of 1.0 and 3.0 (stage aspect ratios of 0.33 and 1.0) are studied at different Reynolds numbers and compared at the same Reynolds number. The results show that the coefficient of power and the coefficient of torque increase with the increase in the Reynolds numbers for all the rotors tested. The coefficient of static torque is independent of the Reynolds number for all the rotors tested. The performance of two stage and three stage rotors remains the same even after increasing the stage aspect ratio and the rotor aspect ratio by a factor of two and three, respectively. For the same rotor aspect ratio of 1.0, by increasing the number of stages (stage aspect ratio decreases), the performance deteriorates in terms Cp and Ct. However, at the same stage aspect ratio of 1.0 and same Reynolds number, two and three stage rotors show the same performance in terms of coefficient of power and coefficient of torque. The variation in coefficient of static torque is lower for a three stage rotor when compared with the variation of coefficient of static torque for two stage or single stage rotor. Copyright © 2008 John Wiley & Sons, Ltd.

Experimental investigations on the effect of overlap ratio and blade edge conditions on the performance of conventional Savonius rotor

Abstract: The torque of a conventional Savonius rotor is studied experimentally for overlap ratio from 0.10 to 0.70, blade edge condition (flat and round) and the change in Reynolds number. The results indicate that the coefficient of static torque improves marginally for round edged rotors for rotor angles up to 45°. The coefficient of static torque decreases as the overlap ratio is increased to 0.70 from 0.50 and is independent of the Reynolds numbers studied in the range between 120000 to 200000. Wind tunnel blockage ratio is studied. The power coefficient increases with the increase in the Reynolds number, whereas the coefficient of static torque is independent of the Reynolds number in the range studied. The coefficient of power, torque coefficient and coefficient static torque are independent of blockage ratios at a given Reynolds number.

Influence of the Shape of the Nozzle on Local Heat Transfer Distribution Between Smooth Flat Surface and Impinging Air Jet

Abstract: An experimental investigation is performed to study the effects of the shape of the nozzle, jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged air jet on smooth and flat surface. Three different nozzle cross-sections, viz. circular, square, and rectangular, each with an equivalent diameter of around 20 mm are used during this study. Reynolds number based on hydraulic diameter (d) is varied between 5000 and 15000 and jet-to-plate spacing from 0.5 to 12 nozzle diameters. Length-to-equivalent diameter ratio (l/de) of 50 is chosen for each nozzle configuration. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Local and average Nusselt number on the impinged surface are presented for all the nozzle configurations investigated. Pressure drop measurements across nozzles are made and pressure loss coefficient for all nozzle configurations is reported. Average Nusselt numbers are found to be insensitive to the shape of the nozzle.

Method and apparatus for inverted vortex generator for enhanced cooling

Abstract: Some embodiments of a method, apparatus and computer system are described for inverted vortex generator enhanced cooling. In various embodiments an apparatus may comprise a first surface comprising at least one heated component, a second surface in proximity to the first surface, the second surface comprising a non-heated surface, and one or more inverted vortex generators attached to the non-heated surface, a portion of the one or more inverted vortex generators in proximity to and configured to dissipate heat from the at least one heated component. Other embodiments are described.

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

Abstract: The influence of spanwise 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 streamwise distribution of the Nusselt number averaged over each spanwise strip resolved to one jet hole as a function of jet-flow and crossflow distributions.

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

Abstract: 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.