In order to protect gas turbine blades from hot gases exiting the combustor, several intricate external and internal cooling concepts are employed. High pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. Present study is targeted towards these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg., V-shaped, inverse V-shaped, W-shaped and M-shaped have been investigated for Reynolds numbers ranging from 150,000 to 400,000. The ribs were installed on two opposite walls of a straight duct with aspect ratio of unity. For very high Reynolds numbers, the heat transfer enhancement levels for different rib shapes varied between 1.3 and 1.7 and the thermal hydraulic performance was found to be less than unity. / Master of Science

Complex flow structures consisting of branching, multi-scale, hierarchically arranged flow paths can be a beneficial in certain applications by providing lower hydraulic and thermal resistances than conventional flow arrangements. In this study, an experimental approach was used to investigate the hydrodynamic and thermal effects of the complexity, or degree of branching, in microscale complex flow structures. The primary focus of this work was to develop new concepts to advance the current capabilities of complex flow structures through management of complexity. The effects of complexity were determined from experiments performed on a set of microfluidic test sections which were identical except for the complexity of the underlying microchannel configuration. Comparison of the relative hydrodynamic and thermal performance indicates that complexity has a strong effect on both the pressure drop and heat transfer. When the pumping power is taken into account, the results suggest that higher complexity arrangements improve the overall thermal-hydraulic performance. This conclusion was confirmed by the trends observed in the coefficient of performance, a measure of the device thermal efficiency. To address the limitations of conventional fixed-complexity designs, the concept of a variable-complexity flow structure is developed. With a variable-complexity design, the configuration of a branched flow structure can be dynamically controlled to improve performance as operational conditions vary. This concept was successfully demonstrated by developing and testing an active variable-complexity microfluidic device in which pneumatically controlled microvalves were used to create different flow channel configurations. The variable-complexity concept was further refined by developing a microfluidic device with a passive variable-complexity design in which the flow channel configuration changed autonomously based on local temperatures. By using microvalves containing a temperature sensitive polymer, the flow configuration of the device was made thermally adaptive. Experiments were performed to characterize the behavior of the polymer microvalves and the overall device performance. The results showed that the device was capable of tracking changes in external heat sources by adapting and reconfiguring its internal flow structure. The experiments also showed how this variable-complexity design can reduce the pumping power expenditure by automatically directing flow only to areas where it is required. / text

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This study focuses on the investigation of flow behavior at low Reynolds numbers by the experimental and numerical performance testing of micro-channel heat exchangers. An experimental study of the heat transfers and pressure drop of compact heat exchangers with louvered fins and flat tubes was conducted within a low air-side Reynolds number range of 20 < ReLp < 225. Using an existing low-speed wind tunnel, 26 sample heat exchangers of corrugated louver fin type, were tested. New correlations for Colburn j and Fanning friction f factor have been developed in terms of non-dimensional parameters. Within the investigated parameter ranges, it seems that both the j and f factors are better represented by two correlations in two flow regimes (one for ReLp = 20 – 80 and one for ReLp = 80 – 200) than a single regime correlation in the power-law format. The results support the conclusion that airflow and heat transfer at very low Reynolds numbers behaves differently from that at higher Reynolds numbers. The effect of the geometrical parameters on the heat exchanger performance was investigated. The numerical investigation was conducted for further understanding of the flow behavior at the range of experimentally tested Reynolds number. Ten different heat exchanger geometries with varied geometrical parameters obtained for the experimental studies were considered for the numerical investigation. The variations in the louver angle were the basis of the selection. The heat transfer and pressure drop performance was numerically investigated and the effect of the geometrical parameters was evaluated. Numerical results were compared against the experimental results. From the comparison, it is found that the current numerical viscous laminar models do not reflect experimentally observed transitional two regime flow behavior from fin directed to the louver directed at very low Reynolds number ranging from 20 to 200. The flow distribution through the fin and the louver region was quantified in terms of flow efficiency. The flow regime change was observed at very low Reynolds number similar to the experimental observations. However, the effect of two regime flow change does not reflect on the thermal hydraulic performance of numerical models. New correlations for the flow efficiency � have developed in terms of non-dimensional parameters.