Beni Cukurel

Particle Image Velocity Investigation of a High Speed Centrifugal Compressor Diffuser: Spanwise and Loading Variations

An efficient diffuser is essential to a modern compressor stage due to its significance in stage performance, durability, and operability. To address the need for data that describe the complex, unsteady flow field in a vaned diffuser, particle image velocity is utilized to characterize the spanwise and circumferential variations in the flow features in the vaned diffuser passage of a transonic centrifugal compressor. The spanwise variation in the diffuser flow field is further investigated by the comparison of three different operating conditions representative of low, nominal, and high loading. These data demonstrate that not only the diffuser flow field is highly dependent on the operation conditions, e.g., hub-to-shroud variation increases with loading, but also the circumferential periodicity, created by the highly three dimensional impeller discharge flow, generates a larger unsteadiness toward the hub region of the vaned diffuser.

Local Heat Transfer Dependency on Thermal Boundary Condition in Ribbed Cooling Channel Geometries

The present study is geared toward quantifying the effects of imposed thermal boundary condition in cooling channel applications. In this regard, tests are conducted in a generic passage, with evenly distributed rib type perturbators at 90 deg, with a 30% passage blockage ratio and pitch-to-height ratio of 10. Uniform heat-flux is imposed on the external side of the slab which provides Biot number and solid-to-fluid thermal conductivity ratio around 1 and 600, respectively. Through infrared thermometry measurements over the wetted surface and via an energy balance within the solid, conjugate heat transfer coefficients are calculated over a single rib-pitch. The local heat extraction is demonstrated to be a strong function of the conduction effects, observed more dominantly in the rib vicinity. Moreover, the aero-thermal effects are investigated by comparing the findings with analogous aerodynamic literature, enabling heat transfer distributions to be associated with distinct flow structures. Furthermore, the results are contrasted with the iso-heat-flux wetted boundary condition test case. Neglecting the thermal boundary condition dependence, and thus the true thermal history of the boundary layer, is demonstrated to produce large errors in heat transfer predictions.

Conjugate Jet Impingement Heat Transfer Investigation via Transient Thermography Method

Investigating the conduction–convection coupling, the present study is focused upon measurement of conjugate heat transfer ensuing jet impingement on a 15-mm-thick metallic plate. Based on a rapid change in jet temperature and using time-accurate infrared thermography, a transient measurement methodology is developed toward acquisition of heat transfer coefficients. The new technique is shown to have comparable levels of Nusselt number and effectiveness accuracy, all while significantly reducing the number of consecutive measurements and their duration. To highlight the significance of the conjugate effect, different plate materials (copper, steel, and Inconel) are employed to differ the solid thermal conductivities, resulting in Biot-number variations. The plate surface heat transfer is studied at two injection Reynolds numbers (34,000 and 37,000) and for two nozzle-to-plate distances (two and five jet diameters). The changes in slab material conductivity reveal small but quantifiable differences in heat...

Micro-Turbojet to Turbofan Conversion Via Continuously Variable Transmission: Thermodynamic Performance Study

In this study, the viability, performance, and characteristics of a turbojet-to-turbofan conversion through the use of a continuously variable transmission (CVT) are investigated. By an in-house thermodynamic simulation code, the performance of the simple cycle turbojet, a direct shaft joined turbofan, and a CVT coupled turbofan with variable bypass are contrasted. The baseline turbojet and turbofan findings are validated against a commercial software. The comparison indicates high quantitative agreement.Analyzing the results of the turbofan engine equipped with a variable bypass and CVT, it is observed that both the thrust and the efficiency are increased. The augmented thrust is observed to be an artifact of enhanced component matching and wider operational range introduced by variable bypass capability. On the other hand, the introduction of CVT contributes to the reduction in fuel consumption. Therefore, the current research suggests that adaptation of a micro-turbojet into a variable cycle micro turbofan will greatly improve the performance and efficiency of existing engines, in addition to providing a pathway towards extended use in various applications.Copyright

Film Cooling Extraction Effects on the Aero-Thermal Characteristics of Rib Roughened Cooling Channel Flow

The present study is geared towards quantifying the effects of film cooling holes on turbine internal cooling passages. In this regard, tests are conducted in a generic stationary model, with evenly distributed rib type perturbators at 90°, constituting a passage blockage ratio of H/Dh = 0.3 and pitch-to-height ratio of P/H = 10. The 1/3H diameter surface-perpendicular film cooling holes are employed at a distance of 5/3H downstream of the preceding rib. Through liquid crystal thermometry measurements, the aero-thermal effects of a change in suction ratio are contrasted for various configurations (Re = 40,000 SR = 0–6), and compared with the analogous aerodynamic literature, enabling heat transfer distributions to be associated with distinct flow structures. At increased suction ratio, the size of the separation bubble downstream of the rib is observed to diminish, triggering globally an earlier reattachment, in addition to low-momentum hot fluid extraction via film cooling suction. Hence, in the presence of active flow extraction, higher overall heat transfer characteristics are observed throughout the channel. Moreover, the findings are generalized via friction factor and Nusselt number correlations, along with an analytical 20-pitch passage model. SR∼3.5 is observed to provide favorable characteristics of pitch-to-pitch uniform suction ratio, lack of hot fluid ingestion and to sustain the highest passage averaged heat transfer.Copyright

Experimental Transonic Centrifugal Compressor Investigation: Loading Effects on Deterministic Diffuser Velocity Fields

The future of small gas turbines relies on higher-efficiency and higher-pressure-ratio centrifugal compressors for lighter, more efficient, and reliable designs. An efficient diffuser is essential to the performance, durability, and operability of a modern compressor stage. The diffuser entry flow in a high-speed centrifugal compressor is highly unsteady and complex, featuring shocks, boundary-layer/shock interactions, and large incidence variations imposed on the diffuser vanes. The observedflow structures are not only unsteady, but they are also strongly dependent on the steady compressor-stage loading. To characterize the variation in diffuser flow at different operating points of a modern transonic centrifugal compressor, particle image velocimetry experiments are conducted in the Purdue University high-speed centrifugal compressor facility. The data at all loading conditions demonstrated that the flowfield in the diffuser is characterized by a much more complicated structure than that associated with steady uniform diffusion. The semivaneless-space acceleration region’s size is shown to be weakly dependent on loading, whereas the diffuser throat structures are strong functions of the loading condition.

Mission Analysis and Operational Optimization of Adaptive Cycle Micro-Turbofan Engine in Surveillance and Firefighting Scenarios

The current work focuses on mission-based evaluation of a novel engine architecture arising from the conversion of a microturbojet to a microturbofan via introduction of a variable speed fan and bypass nozzle. The solution significantly improves maximum thrust by 260%, reduces fuel consumption by as much as 60% through maintaining the core independently running at its optimum, and enables a wider operational range, all the meanwhile preserving a simple single spool configuration. Particularly, the introduction of a variable-speed fan enables real-time optimization for both high-speed cruise and low-speed loitering. In order to characterize the performance of the adaptive cycle engine with increased number of controls (engine speed, gear ratio, bypass opening), a component map-based thermodynamic study is used to contrast it against other similar propulsion systems with incrementally reduced input variables. In the following, a shortest path-based optimization is conducted over the locally minimum fuel consumption operating points, based on a set of gradient driven connectivity constraints for changes in gear ratio and bypass nozzle area. The resultant state transition graphs provide global optimum for fuel consumption at a thrust range in a given altitude and Mach flight envelope. Then, the engine model is coupled to a flight mechanics solver supplied with a conceptual design for a representative multipurpose unmanned aerial vehicle (UAV). Finally, the associated mission benefits are demonstrated in surveillance and firefighting scenarios. [DOI: 10.1115/1.4040734]

Nonhomogeneous Dual-Phase-Lag Heat Conduction Problem: Analytical Solution and Select Case Studies

Finite integral transform techniques are applied to solve the one-dimensional (1D) dualphase heat conduction problem, and a comprehensive analysis is provided for general time-dependent heat generation and arbitrary combinations of various boundary conditions (Dirichlet, Neumann, and Robin). Through the dependence on the relative differences in heat flux and temperature relaxation times, this analytical solution effectively models both parabolic and hyperbolic heat conduction. In order to demonstrate several exemplary physical phenomena, four distinct cases that illustrate the wavelike heat conduction behavior are presented. In the first model, following an initial temperature spike in a slab, the thermal evolution portrays immediate dissipation in parabolic systems, whereas the dual-phase solution depicts wavelike temperature propagation—the intensity of which depends on the relaxation times. Next, the analysis of periodic surface heat flux at the slab boundaries provides evidence of interference patterns formed by temperature waves. In following, the study of Joule heating driven periodic generation inside the slab demonstrates that the steady-periodic parabolic temperature response depends on the ratio of pulsatile electrical excitation and the electrical resistivity of the slab. As for the dual-phase model, thermal resonance conditions are observed at distinct excitation frequencies. Building on findings of the other models, the case of moving constantamplitude heat generation is considered, and the occurrences of thermal shock and thermal expansion waves are demonstrated at particular conditions. [DOI: 10.1115/1.4037775]

CONTINUOUS CLOSED-LOOP TRANSONIC LINEAR CASCADE FOR AERO-THERMAL PERFORMANCE STUDIES IN MICRO-TURBOMACHINERY

The present work summarizes the design process of a new continuous closed-loop hot transonic linear cascade. The facility features fully modular design which is intended to serve as a test bench for axial microturbomachinery components in independently varying Mach and Reynolds numbers ranges of 0–1.3 and 2 10–6 10, respectively. Moreover, for preserving heat transfer characteristics of the hot gas section, the gas to solid temperature ratio (up to 2) is retained. This operational environment has not been sufficiently addressed in prior art, although it is critical for the future development of ultra-efficient high power or thrust devices. In order to alleviate the dimension specific challenges associated with microturbomachinery, the facility is designed in a highly versatile manner and can easily accommodate different geometric configurations (pitch, 620 deg stagger angle, and 620 deg incidence angle), absence of any alterations to the test section. Owing to the quick swap design, the vane geometry can be easily replaced without manufacturing or re-assembly of other components. Flow periodicity is achieved by the inlet boundary layer suction and independently adjustable tailboard mechanisms. Enabling test-aided design capability for microgas turbine manufacturers, aerothermal performance of various advanced geometries can be assessed in engine relevant environments. [DOI: 10.1115/1.4037611]

Empirical compensation of reciprocity failure and integration time nonlinearity in a mid-wave infrared camera

Thermal-infrared radiation measurements, conducted using an InSb camera, indicated a failure of the reciprocity law for a wide range of radiation intensities and integration times. When reciprocity between radiation flux and integration time was assumed, the radiation estimates, computed from different combinations of output signals and selected integration time values, suffered from imprecisions of up to 12%. Temperature errors of ~4% were predicted for low emissivity surfaces, at all temperatures. A novel empirical methodology, which compensates for multiple nonlinearity effects, is presented. Among different types of models, it is demonstrated that an equation, which represents a power-law dependence of the output signal on integration time best describes the physical system. Experimental procedures are suggested to avoid nonlinearity-related errors.