Comparison of Thin Film Heat Flux Gauge Technologies Emphasizing Continuous-Duration Operation

Abstract

Thin film heat flux gauges (HFGs) have been used for decades to measure surface temperatures and heat flux in test turbines with the majority being used in facilities that are short-duration. These gauges are typically composed of two resistive temperature devices deposited on opposing sides of a dielectric. However, because these sensors have been traditionally applied for measurements in transient-type facilities, the challenges facing adaptation of this technology for a steady facility warrant investigation. Those challenges are highlighted, and solutions are presented throughout the paper. This paper describes the nanofabrication process for heat flux gauges and a new calibration method to address potential deterioration of gauges over long runtimes in continuous-duration facilities. Because a primary uncertainty of these sensors arises from the ambiguity of the thermal properties, emphasis is placed on the property determination. Also, this paper presents a discussion on the use of impulse response theory to process the data showing the feasibility of the method for steady-duration facilities after an initial settling time. The latter portion of the paper focuses on comparing well-established heat flux gauges developed for short-duration turbine test facilities to recently developed gauges fabricated using modern nanofabrication techniques for a continuous turbine test facility. The gauges were compared using the test case of an impinging jet over a range of Reynolds numbers. The comparison between the PSU gauge and the reference device indicated agreement within 14%, and similar results were achieved through comparison with established sensors from partner institutions.

Development of coated heat flux gauges for fast responding measurements

“…After establishing the end application criteria and material choices, several design parameters are also required: (a) substrate thickness, (b) coating thickness, (c) RTD shape, and (d) RTD material. In practice, the RTD material is constrained by the electro-thermal properties of known materials and is usually nickel [10,16] or platinum (used for current study) [11,17,18]. Moreover, the shape of the RTD for the application of HFGs is constrained in this study by the pursuit of in-situ calibration techniques [19], which requires a rectangular shape with large length-to-width ratios (L/2b = 10 for current study).…”

“…The HFGs used in this study were fabricated at the Penn State University Nanofabrication Laboratory using a custom procedure detailed by Siroka et al [11]. The fabrication process involves both additive and subtractive processes starting with a commercially-available Pyralux, which is a 50 µm thick polyimide cladded on both sides with 9 µm of copper.…”

“…The calibration curves presented in 5 provide a transformation from the measured voltage to the temperature of the RTDs. More information about the annealing and calibration processes can be found in previous works [11,19].…”

“…Note, these values represent the room temperature thermal and geometric properties. Heat flux errors can arise if the thermal and geometric property determination temperature is different from operating temperature of the HFG [11]. Therefore, it is imperative to accurately determine the thermal properties at the operating temperature of the HFG.…”

“…Traditionally, HFGs have been used in short-duration facilities with typical test durations on the order of seconds [7][8][9][10]. As these sensors transition from shortduration environments to more engine-representative testbeds, additional protections must be deployed to ensure the survivability of the gauges [11]. One such protection is the inclusion of a coating on the surface of the traditional sensor.…”