Jaideep Pandit

Combustor Heat Shield Impingement Cooling and its Effect on Liner Convective Heat Transfer for a Model Annular Combustor With Radial Swirlers

Increasing pressure to reduce pollutant emissions such as NOx and CO, while simultaneously increasing the efficiency of gas turbines, has led to the development of modern gas turbine combustors operating at lean equivalence ratios and high compression ratios. These modern combustors use a large portion of the compressor air in the combustion process and hence efficient use of cooling air is critical. Backside impingement cooling is one alternative for advanced cooling in gas turbine combustors. The dome of the combustor is a primary example where backside impingement cooling is extensively used. The dome directly interacts with the flame and hence represents a limiting factor for combustor durability. The present paper studies two aspects of dome cooling: the impingement heat transfer on the dome heat shield of an annular combustor and the effect of the outflow from the spent air on the liner heat transfer. A transient measurement technique using Thermochromic Liquid Crystals (TLCs) was used to characterize the convective heat transfer coefficient on the backside of an industrial heat shield design provided by Solar Turbines, Inc. for Reynolds numbers (with respect to the hole diameter) of ∼ 1500 and ∼ 2500. Reynolds-Averaged Navier Stokes (RANS) calculations using the k-ω SST turbulence model were found to be in good agreement with the experiment. A standard heat transfer correlation for impingement hole arrays overestimated the mean heat transfer coefficient compared to the experiment and computations, however this could be explained by low biases in the results.Steady state IR measurements were performed to study the effects that the spent air from the heat shield impingement cooling had on the liner convective heat transfer. Measurements were taken for three Reynolds numbers (with respect to the hydraulic diameter of the combustor annulus) including 50000, 90000, and 130000. A downstream shift in the flow features was observed due to the secondary flow introduced by the outflow, as well as a significant increase in the convective heat transfer close to the dome wall.Copyright

Evaluation of Multilouvered-Fin-Based Heat Exchangers for Automobile Exhaust Energy Harvesting Systems

Heat exchanger modules with thermoelectric generators are being used to harness energy from automobile exhaust flows. This paper focuses on increasing the hot-side heat transfer for improved performance of the thermoelectric generators using internal louvered fins. The flow and heat transfer behavior inside the exhaust-pipe test section are modeled using computational fluid dynamics. The multilouvered fins basically have multi-flat-plate behavior and enhance the heat transfer by deflecting the air from its original path and aligning it with the plane of the louvers. The heat transfer and pressure drop characteristics are compared with the baseline flow in a channel without louvered fins. Optimization of the louvered fin geometry is performed to determine the configuration that provides highest heat transfer while providing least pressure drop across the pipe length.

Experimental Investigation of Heat Transfer Across a Thermoelectric Generator for Waste Heat Recovery From Automobile Exhaust

The study investigates the temperature gradients achieved across a thermoelectric generator by using the exhaust gases from a vehicle as a heat source and the radiator coolant as the cold sink. Various heat transfer enhancement features are employed in order to achieve as high a temperature gradient as possible. Effect of flow Reynolds numbers and inlet temperatures are examined to create a body of data predicting total power output from the TEG. Data is normalized against results from baseline heat exchanger designs investigated in the past. The experiments are carried out at 1/5th scale of the previously examined geometry. Impingement geometry is employed on the coolant side to enhance heat transfer. The experimental test sections are fabricated using metal 3D printing. Water is used instead of radiator coolant and heated air is used for exhaust gases. The results from the experiments provide valuable data which can be used for system level optimization.Copyright

Experimental Study of Heat Transfer Augmentation in High Aspect-Ratio Channels Featuring Different Dimple Configurations

Transient liquid crystal technique was employed for experimental investigation of heat transfer distribution in high aspect ratio rectangular ducts featuring different dimple configurations. Four different dimple configuration shapes, viz. diamond, square, triangular and cylindrical, have been studied. This study also attempts to understand the dimple side wall heat transfer and correlate to the mechanisms of the flow inside the dimple. A brief numerical simulation study was also conducted to understand the vortex formation inside the dimples and how they affect the heat transfer on the dimple walls. Experiments were carried out at three different Reynolds number of 10,000, 16,000 and 21,000. Detailed normalized Nusselt number plots for different configurations has been presented. Pressure drop across each geometry was also recorded and a comparison of thermal–hydraulic performance is given. It was observed that heat transfer enhancement due to dimples was maximum for Re =10,000 case when compared to a flat plate scenario at the same Reynolds number. Thermal–hydraulic performance comparison showed that diamond and triangular dimples performed better due to lower pressure drop penalty.Copyright

Effect of Longitudinal Vortex Generator Location on Thermoelectric-Hydraulic Performance of a Single Stage Integrated Thermoelectric Power Generator

Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators: Transverse vortex generators (TVGs) and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (ITED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of, LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the ITED with a single module is examined. By fixing the material size D, the fluid inlet temperature Tin, and attack angle β; the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold’s number and best performance is obtained at Reynold number of Re = 500.Copyright

Experimental Validation of Temperature Distributions Across a Heat Exchanger for a Thermoelectric Generator

Thermoelectric generators (TEGs) are currently a topic of interest in the field of energy harvesting for automobiles. In applying TEGs to the outside of the exhaust tailpipe of a vehicle, the difference in temperature between the hot exhaust gases and the automobile coolant can be used to generate a small amount of electrical power to be used in the vehicle. The amount of power is anticipated to be a few hundred watts based on the temperatures expected and the properties of the materials for the TEG. This study focuses on developing efficient heat exchanger modules in order to maximize the power generation for a given vehicle and TEG. A computational fluid dynamics (CFD) model run by the authors has provided performance predictions for various cases on the cooling side of the heat exchanger. This paper discusses the setup and results of the experimental validation for the CFD model for the proposed TEG heat exchanger module.Copyright

Effect of Partial 3-Dimensional Pin Fin Geometry for Heat Transfer Enhancement in High Aspect Ratio Channels

The efficiency of a TEG (Thermoelectric Generator) may be thought of as the ratio of the power output to the heat input at the hot junction. This ratio is governed by the laws of thermodynamics and cannot thus exceed the Carnot efficiency. It follows that the greater the difference between the hot and cold side temperatures, the greater the efficiency of the TEG. Heat transfer enhancement measurements with 3-Dimensional partial pin-fin arrays of varying shapes on a flat plate are presented. The fin height is fixed at 15% of channel height. The hydraulic diameter and configuration of the fins are chosen based on existing literature. The study is carried out at various Reynolds numbers based on full channel height. The shapes studied are circular, semi-circular, triangular, hexagonal and diamond shaped. These shapes are compared against a baseline case without fins. The experiment uses the transient liquid crystal (TLC) method to calculate the heat transfer coefficient on the test surface. The data shows that diamond shaped fins provide the highest turbulent mixing downstream of the pins leading to the highest heat transfer coefficients.© 2012 ASME