David R. Buttsworth

Assessment of effective thermal product of surface junction thermocouples on millisecond and microsecond time scales

Surface junction thermocouples are used extensively for transient heat flux measurements, but their accuracy is dependent on the effective thermal product (TP) of the gauge and this can be a function of the time scale of interest. In the present work the response of surface junction k-type thermocouples was investigated experimentally using a water droplet calibration technique (for millisecond times scales) and a small shock tube (for microsecond time scales). Different junctions formed by scalpel blade scratches and abrasive paper were investigated. When scratches from scalpel blades were used to form the junction, the TP identified from the water droplet calibrations consistently differs by approximately 20% depending on whether the junction was made on the chromel or alumel substrate, in accord with existing thermal properties data. However, the shock tube calibrations indicate that for scalpel-scratched junctions there is considerable variability in thermocouple response time due to effective junction depth variations produced during construction. In contrast, junctions formed with abrasive paper produced rise times consistently less than 1s, but the water droplet and shock tube experiments both indicated significant variability in the effective TP for these gauges. The consistency in TP for scalpel-scratched junctions for millisecond time scales and the variability for junctions created with abrasive grit for both the millisecond and microsecond time scales is attributed to the differences in the effective proximity of the junction to the insulation between chromel and alumel substrates. For junctions created with abrasive grit, the effective TP is approximately 30% smaller for microsecond time scales than it is for millisecond time scales.

Radial conduction effects in transient heat transfer experiments

[Introduction]: Convective heat transfer data is frequently obtained from transient surface temperature measurements. Thin film resistance gauges, thermocouples, and thermochromic liquid crystals, are used in various situations to measure the surface temperature history. By assuming that uniform semi-infinite flat plate conditions apply, it is possible to express the instantaneous surface heat flux as an analytical function of the transient surface temperature[ll. Various approaches can be used to account for the presence of multi-layered substrates and finite thickness substrate effects (Schultz and Jones; Doorly and Oldfield; Guo et al), however, the effects of surface curvature are usually, neglected. - If the heat transfer data is obtained on the premise that flat plate conditions apply. then error will be introduced if the surface is actually curved. Intuitively, the magnitude of such errors will depend on how far the heat penetrates into the substrate relative to the radius of curvature of the surface. Maulardc4) derived expressions to evaluate the accuracy of the flat plate assumption in cases where the surface under consideration is curved. However, his theoretical results do not provide a convenient means of accurately accounting for surface curvature effects in the routine analysis of experimental data. The current work presents simple analytical curvature corrections for heat transfer results inferred on the assumption that flat plate conditions prevailed during the experiment. Conditions of arbitrary surface heat flux are easily accommodated with the present analysis. The accuracy of the first-order correction analysis is demonstrated by comparing results from the approximate curvature analysis with exact results for a variety of configurations under constant convective heat transfer coefficient conditions. The practical utility of the radial heat conduction modelling is evident from a number of studies in which it has already been employed (Buttsworth and Jones); Hoffs et a1; Fletcher). Nevertheless, data from a recent heat transfer probe experiment is also considered as a further practical demonstration of the present analysis.

Eroding ribbon thermocouples : impulse response and transient heat flux analysis

We have investigated a particular type of fast-response surface thermocouple to determine if it is appropriate to use a one-dimensional transient heat conduction model to derive the transient surface heat flux from the measurements of surface temperature. With these sensors, low thermal inertia thermocouple junctions are formed near the surface by abrasive wear. Using laser excitation, we obtained the impulse response of these commercially available devices. The response of particular sensors can vary if new junctions are created by abrasive wear. Furthermore, the response of these sensors was found to deviate substantially from the one-dimensional model and varied from sensor to sensor. The impulse response was simulated with greater fidelity using a two-dimensional finite element model, but three-dimensional effects also appear to be significant. The impact of these variations on the derived heat flux is assessed for the case of measurements in an internal combustion engine. When the measured impulse response is used to derive the surface heat flux, the apparent reversal of heat flux during the expansion stroke does not occur.

Unsteady total temperature measurements downstream of a high-pressure turbine

An experimental technique for the measurement of flow total temperature in a turbine facility is demonstrated. Two thin film heat transfer gages located at the stagnation point of fused quartz substrates are operated at different temperatures in order to determine the flow total temperature. With this technique, no assumptions regarding the magnitude of the convective heat transfer coefficient are made. Thus, the probe can operate successfully in unsteady compressible flows of arbitrary composition and high free-stream turbulence levels without a heat transfer law calibration. The operation of the total temperature probe is first demonstrated using a small wind tunnel facility. Based on results from the small wind tunnel tests, it appears that the probe total temperature measurements are accurate to within ±1 K. Experiments using the probe downstream of a high-pressure turbine stage are then described. Both high and low-frequency components of the flow total temperature can be accurately resolved with the present technique. The probe measures a time-averaged flow total temperature that is in good agreement with thermocouple measurements made downstream of the rotor. Frequencies as high as 182 kHz have been detected in the spectral analysis of the heat flux signals from the total temperature probe. Through comparison with fast-response aerodynamic probe measurements, it is demonstrated that at the current measurement location, the total temperature fluctuations arise mainly due to the isentropic extraction of work by the turbine. The present total temperature probe is demonstrated to be an accurate, robust, fast-response device that is suitable for operation in a turbomachinery environment.

Directional sensitivity of skin friction measurements using nematic liquid crystal

The directional sensitivity of a new liquid crystal technique for aerodynamic skin friction measurement has been assessed. Experiments were performed using a turntable device installed in a laminar flow duct. Nematic liquid crystal was deposited onto the centre of the turntable. Various initial liquid crystal layer orientations (turntable angles) and magnitudes of applied skin friction were investigated. The dynamic and steady state response was measured using the transmission of polarized monochromatic light. Steady state predictions based on a simple director model are shown to be in reasonable agreement with the experimental results. The current results demonstrate the potential of the nematic technique in skin friction vector measurement applications.

Isokinetic total water content probe in a naturally aspirating configuration: initial aerodynamic design and testing

Recent measurements in wind tunnel and flight experiments have demonstrated that water loss prior to complete evaporation is possible for the hot-wire total water content probes. Other cloud water content probes similarly have their own operating problems. To enhance the efficiency of water particle capture and water mass retention, isokinetic sampling is appealing. The probe described in this paper represents an extension of a concept that was developed and tested through the then RAE Farnborough, UK. The intention is to quantify and minimize possible departures from isokinetic sampling during operation - something that is difficult to achieve in the original RAE configuration. This paper describes the design methodologies and the experiments that have been performed to characterize the aerodynamic performance of the prototype.

Interaction of oblique shock waves and planar mixing regions

An analysis for predicting the interaction of a steady oblique shock wave and a planar mixing region is presented. Specifically, an equation for the shock curvature was obtained from the shock wave and isentropic wave difference equations which govern the shock transmission within a region of varying Mach number. The effects of nonuniform gas composition within the mixing region were assessed using a similar treatment; however, the wave equations were expanded in terms of a varying ratio of specific heats instead of a varying Mach number. An expression for the shock-induced vorticity due to velocity and density gradients within the mixing region was also obtained. This expression provides a means of estimating the possible mixing augmentation induced in various shock wave-mixing region interactions. When the velocity and density gradients within the mixing region oppose each other, it is demonstrated that the pre-shock vorticity may be attenuated by the shock. Applications of the analysis are discussed with reference to specific examples involving mixing augmentation and shock oscillation.

Theoretical and experimental investigation of SI engine performance and exhaust emissions using ethanol-gasoline blended fuels

In this study, potato waste bioethanol was evaluated as an alternative fuel for gasoline engines. The pollutant emissions and performance of a four stroke SI engine operating on ethanol-gasoline blends has been investigated experimentally and theoretically. In the theoretical study, a quasi-dimensional SI engine cycle model has been adapted for spark ignition engines running on gasoline-ethanol blends. A mathematical model using Matlab software was developed using the first law of thermodynamics and conservation equations to predict the SI engine performance for different blend ratios. The model was also used to evaluate the engine emissions and the mechanical and heat losses in the engine which is not included in this study. Experiments were performed with the blends containing 5, 10, 15 and 20 vol% ethanol. The results show that increasing ethanol-gasoline blended will marginally increase the power and torque output of the engine. For ethanol blends it was found that the brake specific fuel consumption (bsfc) was decreased using 5% and 10% ethanol while the brake thermal efficiency and the volumetric efficiency were increased. Exhaust gas emissions were measured and analyzed for unburned hydrocarbons (UHC), carbon dioxide (CO2), carbon monoxide (CO), Oxygen (O2) and Oxide of Nitrogen NOx at engine speeds ranging from 1000 to 5000 rpm. The concentration of CO and UHC emissions in the exhaust pipe were found to be decreased when ethanol blends were introduced. The concentration of CO2 and NOx was found to be increased when ethanol is introduced. Results obtained from both theoretical and experimental studies were compared. The simulation results have been validated against data from experiments and it results to a good agreement between the trends in the predicted and experimental results.

Transient response of an erodable heat flux gauge using finite element analysis

Abstract The transient response of an erodable ribbon element heat flux gauge has been assessed using a two-dimensional finite element (FE) analysis. Such transient heat flux gauges have previously been used for measurements in internal combustion (IC) engines. To identify the heat flux from the measurements of surface temperature, it is commonly assumed that the heat transfer within these devices is one-dimensional. A corollary of the one-dimensional treatment is that only one value of the thermal product, , is needed for identification of the transient heat flux, even though erodable heat flux gauges are constructed from at least two different materials. The current results demonstrate that two-dimensional transient heat conduction effects have a significant influence on the surface temperature measurements made with these devices. For the ribbon element gauge and timescales of interest in IC engine studies, using a one-dimensional analysis (and hence a single value of ) will lead to substantial inaccuracy in the derived heat flux measurements.

A Gun Tunnel Investigation of Hypersonic Free Shear Layers in a Planar Duct

An experimental investigation of high Mach number free shear layers has been undertaken. The experiments were performed using a Mach 7 gun tunnel facility and a planar duct with injection from the base of a central strut producing a Mach 3 flow parallel to the gun tunnel stream. This configuration is relevant to the development of efficient scramjet propulsion, and the gun tunnel Mach number is significantly higher than the majority of previous supersonic turbulent mixing layer investigations reported in the open literature. Schlieren images and Pitot pressure measurements were obtained at four different convective Mach numbers ranging from 0 to 1.8. Only small differences between the four cases were detected, and the relatively large high-speed boundary layers at the trailing edge of the struct injector appear to strongly influence the shear layer development in each case. The Pitot pressure measurements indicated that, on average, the free shear layers all spread into the Mach 3 stream at an angle of approximately 1.4 degrees, while virtually no spreading into the Mach 7 stream was detected until all of the low-speed stream was entrained. The free shear layers were simulated using a PNS code; however, the experimentally observed degree of spreading rate asymmetry could not be fully predicted with the k-epsilon turbulence model, even when a recently proposed compressibility correction was applied.