Thermocouple-based Measurements Research Articles

Liquid Crystal Thermography in Gas Turbine Heat Transfer: A Review on Measurement Techniques and Recent Investigations

Liquid Crystal Thermography is a widely used experimental technique in the gas turbine heat transfer community. In turbine heat transfer, determination of the convective heat transfer coefficient (h) and adiabatic film cooling effectiveness (η) is imperative in order to design hot gas path components that can meet the modern-day engine performance and emission goals. LCT provides valuable information on the local surface temperature, which is used in different experimental methods to arrive at the local h and η. The detailed nature of h and η through LCT sets it apart from conventional thermocouple-based measurements and provides valuable insights into cooling designers for concept development and its further iterations. This article presents a comprehensive review of the state-of-the-art experimental methods employing LCT, where a critical analysis is presented for each, as well as some recent investigations (2016–present) where LCT was used. The goal of this article is to familiarize researchers with the evolving nature of LCT given the advancements in instrumentation and computing capabilities, and its relevance in turbine heat transfer problems in current times.

Thermal Analysis of Paraffin-Embedded Tissue Blocks for Anatomic Pathology Processes.

We analyze temperature dynamics in anatomic pathology samples to identify the most efficient refrigeration method and to predict the time available for optimal sectioning before sample heating, thus getting appropriate information for a correct diagnosis by anatomopathologists. A thermal finite element (FE) analysis was carried out with comsolmultiphysics to evaluate temperature variations in paraffin-embedded tissues, i.e., muscle, bone and fat, and the corresponding thermal stresses. Experiments with different tissues and thermocouple-based measurements allowed validating the FE simulations. Simulations allowed to estimate the time needed to bring the sample at the optimal temperature for sectioning (-8 to -4 °C) in different conditions: refrigeration on a cold plate, refrigeration in a cooled environment, and refrigeration in an environment with forced convection. Among the three cooling methods tested, the forced convection at -20 °C and with an air-flow speed of 5 m/s resulted in the shortest cooling time. As compared to the other methods, thermal stresses can be modulated by varying the air-flow speed. For the different conditions, the time needed for the surface of the tissue block to exit from a temperature corresponding to an optimal cutting, when leaving the sample exposed to room temperature after refrigeration, ranged from 12 to 310 s. We quantify the time needed to adequately refrigerate paraffin-embedded tissue samples and the time available before they leave the optimal temperature window for sectioning. We also evaluate the maximum stress attained in the paraffin block during the cooling and the heating transients. This information will help optimize anatomic pathology processes.

Determination of the core temperature of a Li-ion cell during thermal runaway

Safety and performance of Li-ion cells is severely affected by thermal runaway where exothermic processes within the cell cause uncontrolled temperature rise, eventually leading to catastrophic failure. Most past experimental papers on thermal runaway only report surface temperature measurement, while the core temperature of the cell remains largely unknown. This paper presents an experimentally validated method based on thermal conduction analysis to determine the core temperature of a Li-ion cell during thermal runaway using surface temperature and chemical kinetics data. Experiments conducted on a thermal test cell show that core temperature computed using this method is in good agreement with independent thermocouple-based measurements in a wide range of experimental conditions. The validated method is used to predict core temperature as a function of time for several previously reported thermal runaway tests. In each case, the predicted peak core temperature is found to be several hundreds of degrees Celsius higher than the measured surface temperature. This shows that surface temperature alone is not sufficient for thermally characterizing the cell during thermal runaway. Besides providing key insights into the fundamental nature of thermal runaway, the ability to determine the core temperature shown here may lead to practical tools for characterizing and mitigating thermal runaway.

Fast-response temperature-sensitive-paint measurements on a hypersonic transition cone

Experiments are performed using a fast-response temperature-sensitive-paint (TSP) technique to measure the heat-flux distribution on a slender cone in a hypersonic shock tunnel under both laminar and transitional conditions. The millisecond-order test duration together with the self-luminosity of shock layers place stringent conditions on the choice of TSP luminophore and the TSP-layer thickness that can be employed. The luminosity and dimming from particulates in the free-stream cause additional problems in interpreting the obtained intensity profiles. Nevertheless, favorable agreement with thermocouple-based measurements show that it is possible to derive quantitatively accurate heat-flux distributions with the TSP technique for temperature rises of up to approximately 40 K above room temperature. The technique accuracy is adversely affected at higher temperatures, which is thought to result from non-constant thermal properties of the insulating base layer. At high unit Reynolds number conditions, time-resolved heat-flux distributions show large-scale unsteadiness in the boundary-layer transition location and reveal transient streamwise streaks developing in the transitional region.

Interferometry based whole-field heat transfer measurements of an impinging turbulent synthetic jet

The present work is concerned with exploring the potential of refractive index-based imaging techniques for investigating the heat transfer characteristics of impinging turbulent synthetic jets. The line-of-sight images of the convective field have been recorded using a Mach Zehnder interferometer. Heat transfer experiments have been conducted in infinite fringe setting mode of the interferometer with air as the working fluid. The effect of the excitation frequency of the synthetic jet on the resultant temperature distribution and local heat transfer characteristics has been studied. The fringe patterns recorded in the form of interferograms have first been qualitatively discussed and thereafter, quantitatively analyzed to determine the two-dimensional temperature field. Local heat transfer coefficients along the width of the heated copper block have been determined from the temperature field distribution thus obtained from the interferograms. The results have been presented in the form of interferometric images recorded as a function of frequency of the synthetic jet, corresponding two-dimensional temperature distributions and local variation of heat transfer coefficients. Interferometric measurements predicted maxima of the heat transfer coefficient at the resonance frequency of the synthetic jet and at a jet-to-plate surface spacing (z/d) of 3. These observations correlate well with the thermocouple-based measurements of temperature and heat transfer coefficient performed simultaneously during the experiments. The interferometry-based study, as reported in the present work for the first time in the context of synthetic jets, highlights the importance of refractive index-based imaging techniques as a potential tool for understanding the local heat transfer characteristics of synthetic jets.

Measurement of Interface Thermal Resistance With Neutron Diffraction

A noncontact, nondestructive neutron diffraction technique for measuring thermal resistance of buried material interfaces in bulk samples, inaccessible to thermocouple measurements, is described. The technique uses spatially resolved neutron diffraction measurements to measure temperature, and analytical or numerical methods to calculate the corresponding thermal resistance. It was tested at the VULCAN instrument of the Spallation Neutron Source, Oak Ridge National Laboratories on a stack of three 6061 alloy aluminum plates (heat-source, middle-plate, and heat-sink), held in dry thermal contact, at low pressure, in ambient air. The results agreed with thermocouple-based measurements. This technique is applicable to all crystalline materials and most interface configurations, and it can be used for the characterization of thermal resistance across interfaces in actual engineering parts under nonambient conditions and/or in moving/rotating systems.

Digital Temperature Tracking in Porous Media Burners

Combustion in porous media burners presents considerable advantages over free flame burners due to several outstanding features inter alia clean and highly efficient combustion properties allowing a considerable amount of feedback energy from the flame area to preheat the mixture of fuel and air resulting in a considerable reduction of unavoidable pollutant formations appearing e.g. as the emission of CO and NOX. In addition, porous media burners are manufactured in highly compact small sizes suitable to industrial and household heating characteristic applications. Heat transfer between solid and gas depends mainly on the porous thermophysical properties of the component known as the solid matrix. These systems are characterized by the formation of a combustion flame pulse or wave which can travel inside the burner, depending on the operating conditions at velocities of about 0.1 mm/s. In this paper, a new temperature tracking scheme is proposed based on digital image processing to determine the position and the velocity of the thermal profile. Results showed reduced errors in the estimation of the peak temperature position using digital image analysis compared to conventional thermocouple-based measurements techniques.

Determination of temperature distribution in the cutting zone using hybrid analytical-FEM technique

In this study, the temperature distribution in the cutting zone was determined by integrating thermal analytical and simulation models of orthogonal cutting process with uncoated and coated carbide tools. Primarily, 2D FEM simulations were run to provide numerical solutions of temperatures occurring at different points through the chip/tool contact region and the coating/substrate boundary under defined cutting conditions. In addition, an analytical model for heat transfer in the cutting tool and its partitioning, proposed in References [W. Grzesik, P. Nieslony, Physics based modelling of interface temperatures in machining with multilayer coated tools at moderate cutting speeds, Int. J. Mach. Tools Manufact. 44 (2004) 889–901; W. Grzesik, P. Nieslony, A computational approach to evaluate temperature and heat partition in machining with multilayer coated tools, Int. J. Mach. Tools Manufact. 43 (2003) 1311–1317], was employed to generate the input data to computations of the tool–chip interface temperature. The changes of the temperature distribution fields resulting from varying heat flux transfer conditions are the main findings of the FEM simulations. Finally, the analytically and numerically predicted average temperatures were validated against the tool-work thermocouple-based measurements and discussed in terms of relevant literature data.