A novel method for measuring heat flux of boiling water is designed and built to study critical heat flux (CHF) and observe the response of a heat flux sensor when CHF occurs. A high temperature heat flux sensor is embedded in the wall of a pipe to get a direct measurement of the surface heat flux and sensor temperature. By submerging the pipe in water and applying a controlled heat flux to the inside diameter over the area where the sensor is located, boiling is created on the outer surface while measuring the heat flux. The heat flux is gradually increased up to CHF and the heat flux response is observed to determine if the heat flux sensor could sense CHF when it occurred. The heat flux sensor is able to consistently measure the value for CHF, which is approximately 510 kW/m" for this system. It is also observed during the experiments that the heat flux response undergoes an inflection of the heat transfer coefficient at a consistent temperature just before reaching CHF. This observed inflection caused the heat flux response to deviate from its cubic relationship with the temperature and drastically increase for a very small change in temperature. This inflection response can be used as an indication for approaching CHF and can also be used to approximate its value without prior knowledge of when it occurs. / Master of Science

This work seeks to apply the novel heat flux sensor called as the Atomic Layer Thermopile to measure high frequency heat flux in high speed and high temperature flows found in Gas Turbine combustors. To achieve this the sensor must be able to survive the harsh environment of high temperature and high pressure. To have any confidence in our measurement, it is also imperative that there are tools available for precise estimation of the measurement uncertainty. This works strives to achieve these objectives by developing calibration techniques for uncertainty estimation using both exposure to radiation and in convective environments by calibrating against power input in steady state flow and transient heat flux calculated using wall temperature measurement. The response of the sensor is then investigated in high speed flows by measuring the heat flux inside a supersonic nozzle when exposed to shock waves. The shock waves are generated using a fast throttle valve located at the entrance of the supersonic nozzle by generating sudden rise in pressure. Lastly a numerical study is carried out to design a cooling system that will allow the sensor to survive in high temperature conditions of 1000°C while the sensor film is maintained at 50°C. A one-dimensional model is used to provide initial design parameters and then a two-dimensional axisymmetric conjugate CFD analysis is carried out to obtain the desired geometry that can meet the design conditions. A static structural analysis is also carried out on this geometry to ensure that it will be able to survive and avoid distortion under the operational pressure required for providing the desired coolant mass flow.

No description available.

This effort is focused on the design, calibration, and implementation of a high temperature heat flux sensor for thermal systems research and testing. The High Temperature Heat Flux Sensor (HTHFS) was designed to survive in the harsh thermal environments typically encountered in hypersonic flight, combustion and propulsion research, and large-scale fire testing. The sensor is capable of continuous use at temperatures up to 1000 â ¦C. Two methods for steady-state calibration of the HTHFS at elevated temperatures have been developed as a result of this research. The first method employs a water-cooled heat flux sensor as a reference standard for the calibration. The second method utilizes a blackbody radiant source and a NIST calibrated optical pyrometer as the calibration standard. The HTHFS calibration results obtained from both methods compare favorably with the theoretical sensitivity versus temperature model. Implementation of the HTHFS in several types of transient thermal testing scenarios is also demonstrated herein. A new data processing technique is used to interpret the measurements made by the HTHFS. The Hybrid Heat Flux (HHF) method accounts for the heat flow through the sensor and the heat storage in the sensor, and thus renders the HTHFS virtually insensitive to the material on which it is mounted. The calibrated output of the HTHFS versus temperature ensures accuracy in the measurements made by the sensor at high operating temperatures. / Ph. D.

Heat flux gages are important in applications where measurement of the transfer of energy is more important than measurement of the temperature itself. There is a need for a heat flux sensor that can perform reliably for long periods of time in high temperature and high heat flux environment. The primary objective is to design and build a heat flux sensor that is capable of operating for extended periods of time in a high heat flux and high temperature environment. A High Temperature Heat Flux Sensor (HTHFS) was made by connecting 10 brass and steel thermocouple junctions in a thermopile circuit. This gage does not have a separate thermal resistance layer making it easier to fabricate. The HTHFS was calibrated in a custom-made convection calibration facility using a commercial Heat Flux Microsensor (HFM) as the calibration standard. The measured sensitivity of the HTHFS was 20.4 ±2.0ìV/(W/cm2). The measured sensitivity value matched with the theoretically calculated value of 20.5 ìV/(W/cm2). The average sensitivity of the HTHFS prototype was one-fifth of the sensitivity of a commercially available HFM. Better ways of mounting the HTHFS in the calibration stand have been recommended for future tests on the HTHFS for better testing. The HTHFS has the potential to be made into a microsensor with thousands of junctions added together in a thermopile circuit. This could lead to a heat flux sensor that could generate large signals (~few mV) and also be capable of operating in high heat flux and high temperature conditions. / Master of Science

No description available.

Technical challenges for developing micro sensors for Ultra High Temperature and turbine applications lie in that the sensors have to survive extremely harsh working conditions that exist when converting fuel to energy. These conditions include high temperatures (500-1500°C), elevated pressures (200-400 psi), pressure oscillations, corrosive environments (oxidizing conditions, gaseous alkali, and water vapors), surface coating or fouling, and high particulate loading. Several technologies are currently underdeveloped for measuring these parameters in turbine engines. One of them is an optical-based non-contact technology. However, these nondirective measuring technologies lack the necessary accuracy, at least at present state. An alternative way to measure these parameters without disturbing the working environments is using MEMS type sensors. Currently, the techniques under development for such harsh environment applications are silicon carbide (SiC) and silicon nitrite (Si3N4) –based ceramic MEMS sensors. But those technologies present some limitation such as narrow processing method, high cost (materials and processing cost), and limited using temperatures (typically < 800 C). In this research we propose to develop two sensors based on recently developed polymer-derived ceramics (PDCs): Constant Temperature Hot wire Anemometer, temperature/heat-flux sensor for turbine applications. PDC is a new class of high temperature ceramics. As we shall describe below, many unique features of PDCs make them particularly suitable for the proposed sensors, including: excellent thermo-mechanical properties at high temperatures, enable high temperature operation of the devices; various well-developed processing technologies, such as injection molding,photolithography, embossing, DRIE etching and precise machining, can be used for the fabrication of the devices; and tunable electric conductivity, enable the proposed sensors fabricated from similar materials, thus reliability considerations associated with thermal mismatch, which is a big concern when using MEMS-based sensors at elevated temperatures, will be minimized.

The mechanism by which convective heat transfer is augmented by freestream turbulence in the stagnation region was studied experimentally. Previous work has suggested that the primary mechanism for the observed augmentation is the amplification of vorticity into strong vortices which dominate the flow field near the surface. Therefore, two separate experimental investigations were performed to further study this phenomenon. In the first, the spatiotemporal convection from a heated surface was measured during the normal collision of a vortex ring. The convection was observed to increase dramatically in areas where vortices forced outer fluid through the natural convection boundary layer to the surface. Regions where fluid was swept along the surface experienced much smaller increases in convection. These observations led to the development of a mechanistic model which predicted the heat transfer based on the amount of time that fluid remained within the thermal boundary layer prior to reaching the surface. In subsequent testing, the model was able to accurately predict the time-resolved convection based solely on the transient properties of the vortex present. In the second investigation, the model was applied to the vortices which form in a stagnating turbulent flow. Three turbulence conditions were tested which changed the properties of the vortices produced. Again, the model was successful in predicting the time-resolved convection over much of the experimental measurement time. The work of designing and calibrating the heat flux sensor used is also reported. A new sensor was developed specifically for the convection research performed herein as no existing sensor possessed the required spatiotemporal resolution and underwater capabilities. Utilizing spot-welded foils of thermoelectric alloys resulted in a very robust and sensitive sensing array which was thoroughly analyzed and calibrated. In the final section, the hybrid heat flux (HHF) method is presented which significantly increases the performance of existing heat flux sensors. It is shown (both numerically and experimentally) that by combining the spatial and temporal temperature measurements from a standard sensor, the time response increases by up to a factor of 28. Also, this method causes the sensor to be insensitive to the material to which it is mounted. / Ph. D.

The experimental work involves calculation of radial distribution of heat transfer coefficient at the surface of a flat Aluminium plate being impinged by a turbulent flame jet. Heat transfer coefficient distribution at the surface is computed from the measured heat flux and temperature data using a reference method and a slope method. The heat transfer coefficient (h) has a nearly bell shaped radial distribution at the plate surface for H/d =3.3. The value of h drops by 37 % from r/d =0 to r/d= 2. Upon increasing the axial distance to H/d = 5, the stagnation point h decreased by 15%. Adiabatic surface temperature (AST) distribution at the plate surface was computed from the measured heat flux and temperature. AST values were found to be lower than the measured gas temperature values at the stagnation point. Radial distribution of gas temperature at the surface was estimated by least squares linear curve fitting through the convection dominated region of net heat flux data and was validated by experimental measurements with an aspirated thermocouple. For low axial distances (H/d =3.3), the gas temperature dropped by only 15 % from r/d = 0 to r/d = 2. Total heat flux distribution is separated into radiative and convective components with the use of calculated heat transfer coefficient and estimated gas temperatures. At H/d = 3.3, the radiation was found to be less than 25 % of the net heat flux for r/d ≤ 2. / Master of Science

En vue d'applications futures dans le domaine de la récupération d'énergie à basse température à partir de microsystèmes, la connaissance des transferts thermiques aux parois est d'une grande importance. Ces mesures sont réalisées à l'aide de capteurs spécifiques qui permettent de connaître le flux thermique total échangé entre la paroi sur laquelle ils sont installés et le milieu environnant. Cette étude se situe dans le cadre du développement de fluxmètres thermiques a gradient répondant aux contraintes liées aux machines thermiques de type micro-moteur Stirling. Six types de fluxmètres thermiques à sondes à résistance ont été développés à partir de différents matériaux, géométries et formes de capteurs. Une étude électrothermique, prenant en compte l'effet d'auto-échauffement, a été menée et validée expérimentalement. Les techniques de fabrication et de caractérisation, opérés majoritairement en salle blanche, ont permis de réaliser de nombreux capteurs à couches minces. Des supports spécifiques pour ces capteurs ont été développés. Ces capteurs de flux thermique, dont les résistances ont été étalonnées par rapport à une sonde de référence de haute précision, permettent également la mesure de la température. Pour mesurer la densité de flux thermique, deux méthodes ont été testées. La première méthode est indirecte : elle consiste à mesurer le gradient de température à l'aide de deux sondes à résistance en platine. La deuxième méthode est directe : elle repose sur un étalonnage en flux. Enfin, l'influence de l'intrusion des capteurs sur le répartition des flux thermiques dans la pièce à mesurer, avec ou sans support spécifique, a été étudiée. / For future applications in the field of low-temperature energy have sting in microsystems, knowledge of wall heat transfer is of great importance. These measurements are carried out using specific sensors which makes possible to know the total heat flux exchanged between walls and the surrounding environment. This study concerns the development of a gradient heat flux sensor compliant with their requirements associated with thermal machines such as micrometric Stirling engines. Six types of heat flux sensor with resistance temperature detector have been developed from different materials, with various geometries and shapes of sensing elements. An electro thermal study, taking into account the self-heating effect, was carried out and experimentally validated. The fabrication and characterization techniques, mainly carried out in clean rooms, allowed to produce numerous thin-film sensors. Specific housings for these sensors have been developed. These heat flux sensors, whose resistances have been calibrated with a highly accurate reference probe, also allow measuring the temperature. To measure the heat flux density, two methods were tested. The first method is indirect : it consists in measuring the temperature gradient using two platinum resistance probes. The second method is direct : it is based on heat flux calibration. The calibration and measurement benches have been modeled. Then, the experimental and numerical results have been compared. Finally, the influence of the sensor intrusion on the thermal fluxes distribution in the investigated sample, with or without a specific housing, has been studied.