A wind tunnel for the calibration of Mars wind sensors

A major limitation in the development of wind sensors for use on Mars is the lack of suitable testing and calibration facilities. A low-density wind tunnel has been developed at Oxford University for calibration of wind sensors for Mars landers, capable of providing stable or dynamically varying winds, of air or carbon dioxide, at Martian pressures (5-10 mbar) and speeds (0.5-30 m/s), and temperatures of 200-300 K. The flow field in the test section was calculated using analytical and computational modelling techniques, and validated experimentally using a pitot probe. This facility's stability and accuracy offer significant advantages with respect to previous calibration facilities. © 2008 Elsevier Ltd. All rights reserved

Comparison of Heat Transfer Measurement Techniques on a Transonic Turbine Blade Tip

The present study considers spatially resolved surface heat transfer coefficients and adiabatic wall temperatures on a turbine blade tip in a linear cascade under transonic conditions. Five different measurement and processing techniques using infrared thermography are considered and compared. Three transient methods use the same experimental setup, using a heater mesh to provide a near-instantaneous step-change in mainstream temperature, employing an infrared camera to measure surface temperature. These three methods use the same data but different processing techniques to determine the heat transfer coefficients and adiabatic wall temperatures. Two of these methods use different processing techniques to reconstruct heat flux from the temperature time trace measured. A plot of the heat flux versus temperature is used to determine the heat transfer coefficients and adiabatic wall temperatures. The third uses the classical solution to the 1D nonsteady Fourier equation to determine heat transfer coefficients and adiabatic wall temperatures. The fourth method uses regression analysis to calculate detailed heat transfer coefficients for a quasi-steady-state condition using a thin-foil heater on the tip surface. Finally, the fifth method uses the infrared camera to measure the adiabatic wall temperature surface distribution of a blade tip after a quasi-steady-state condition is present. Overall, the present study shows that the infrared thermography technique with heat flux reconstruction using the impulse method is the most accurate, computationally efficient, and reliable method to obtain detailed, spatially resolved heat transfer coefficients and adiabatic wall temperatures on a transonic turbine blade tip in a linear cascade. © 2011 American Society of Mechanical Engineers

TRANSONIC TURBINE BLADE TIP AERO-THERMAL PERFORMANCE WITH DIFFERENT TIP GAPS: PART II-TIP AERODYNAMIC LOSS

Blade tip aerodynamic loss results from experimental and numerical investigations are presented for engine representative conditions downstream of a blade row with an exit Mach number Mexit of 1.0, and an exit Reynolds number Reexit of 1.27x106 (based on axial chord). These results are presented for three different tip gaps of 0.5, 1.0, and 1.5 percent relative to engine-equivalent blade span. Experimental data are obtained by traversing a specially-made and calibrated three-hole pressure probe as well as a single-hole probe one axial chord downstream of the blade within the Oxford High Speed Linear Cascade research facility. Three-dimensional RANS CFD numerical predictions are conducted using the Rolls-Royce HYDRA numerical prediction code for steady flow with the Spalart-Allmaras (SA) turbulence model. Included are detailed distributions of stagnation pressure losses, and pitch-wise flow angle. Local total pressure data and mass-averaged total pressure loss coefficients show that the strength of the tip leakage vortex decreases as the tip gap decreases. Magnitudes of the pitch-wise flow angle increase within over-tip leakage vortices, as these vortices become stronger and the tip leakage flow increases. The most important difference between experimental and numerical results is in relation to the passage vortex signatures, which are more apparent for all three tip gap values within the numerical results. The effects of relative casing motion and tip clearance are also examined and discussed, and show that the relative casing movement has a relatively small impact on the size of the over-tip leakage vortex at the medium (1.0% of span) and large (1.5% of span) tip gaps, with more noticeable impact at the smallest tip gap (0.5% of span). Copyright © 2010 by ASME

Transonic Turbine Blade Tip Aerothermal Performance With Different Tip Gaps-Part I: Tip Heat Transfer

A closely combined experimental and computational fluid dynamics (CFD) study on a transonic blade tip aerothermal performance at engine representative Mach and Reynolds numbers (Mexit=1,Reexit=1.27×106) is presented here and its companion paper (Part II). The present paper considers surface heat-transfer distributions on tip surfaces and on suction and pressure-side surfaces (near-tip region). Spatially resolved surface heat-transfer data are measured using infrared thermography and transient techniques within the Oxford University high speed linear cascade research facility. The Rolls-Royce PLC HYDRA suite is employed for numerical predictions for the same tip configuration and flow conditions. The CFD results are generally in good agreement with experimental data and show that the flow over a large portion of the blade tip is supersonic for all three tip gaps investigated. Mach numbers within the tip gap become lower as the tip gap decreases. For the flow regions near the leading edge of the tip gap, surface Nusselt numbers decrease as the tip gap decreases. Opposite trends are observed for the trailing edge region. Several "hot spot" features on blade tip surfaces are attributed to enhanced turbulence thermal diffusion in local regions. Other surface heat-transfer variations are attributed to flow variations induced by shock waves. Flow structure and surface heat-transfer variations are also investigated numerically when a moving casing is present. The inclusion of moving casing leads to notable changes to flow structural characteristics and associated surface heat-transfer variations. However, significant portions of the tip leakage flow remain transonic with clearly identifiable shock wave structures. © 2011 American Society of Mechanical Engineers

A time-resolved hot-wire shear stress probe for turbulent flow: use of laminar flow calibration

10.1007/BF02412806Experiments in Fluids171-275-83EXFL

Dean Flow Dynamics in Low-Aspect Ratio Spiral Microchannels

A wide range of microfluidic cell-sorting devices has emerged in recent years, based on both passive and active methods of separation. Curvilinear channel geometries are often used in these systems due to presence of secondary flows, which can provide high throughput and sorting efficiency. Most of these devices are designed on the assumption of two counter rotating Dean vortices present in the curved rectangular channels and existing in the state of steady rotation and amplitude. In this work, we investigate these secondary flows in low aspect ratio spiral rectangular microchannels and define their development with respect to the channel aspect ratio and Dean number. This work is the first to experimentally and numerically investigate Dean flows in microchannels for Re > 100, and show presence of secondary Dean vortices beyond a critical Dean number. We further demonstrate the impact of these multiple vortices on particle and cell focusing. Ultimately, this work offers new insights into secondary flow instabilities for low-aspect ratio, spiral microchannels, with improved flow models for design of more precise and efficient microfluidic devices for applications such as cell sorting and micromixing