Time Constant Of Thermocouple Research Articles

Determination of Time Constant of Temperature Sensors and its Application in Aero Gas Turbine Engines

Temperature sensors are widely used in aero gas turbine engines for measurement of air and gas temperature at various locations starting from inlet of fan to exhaust gas from the nozzle. Thermocouples are one of the most widely used sensors for the above purpose. Time constant of thermocouple is a key parameter in the selection of sensor for a particular application. Even though time constant is available for standard stand alone thermocouple configurations, it varies widely based on many physical parameters and inlet flow conditions inside a gas turbine engine. Experimental determination of time constant of stand alone thermocouples with step input is carried out for various thermocouple configurations and a Fibre Bragg Grating sensor using a dry block calibrator with hot air as medium. Two different methods are used to determine the time constant of thermocouple sensors. Further determination of time constant of thermocouple assembled in a rake and tested in real time situation in aero gas turbine engine is carried out. A comparative study reveals that many parameters are involved which influence the value of time constant under different input conditions. Finally the selection of sensors is carried out based on the measurement requirements.

Dynamic Characterization of Thermocouples under Double-Pulse Laser-Induced Thermal Excitation.

This study investigated the dynamic characteristics of thermocouples by using double-pulse laser excitation for dynamic temperature calibration under extreme conditions. An experimental device was constructed for double-pulse laser calibration; the device uses a digital pulse delay trigger to precisely control the double-pulse laser to achieve sub-microsecond dual temperature excitation with adjustable time intervals. The time constants of thermocouples under single-pulse laser excitation and double-pulse laser excitation were evaluated. In addition, the variation trends of thermocouple time constants under different double-pulse laser time intervals were analyzed. The experimental results indicated that the time constant increases and then decreases with the decrease in the time interval of the double-pulse laser. A method for dynamic temperature calibration was established for the evaluation of the dynamic characteristics of temperature sensors.

Prediction of Dynamic Characteristics of Thermocouples with Thin-Wire Sensing Elements

Thermocouples dynamic characteristicsʼ prediction is one of the relevant directions in the field of dynamic measurements of non-stationary temperatures of liquid and gaseous media. Thermocouples dynamic characteristicsʼ prediction makes it possible to provide effective continuous correction in automatic control systems for non-stationary temperatures. The purpose of this paper was to develop a theoretically justified relation linking the current or expected time constant of fine-wire thermocouples with the known time constant established at known parameters of liquid and gaseous media.A formula linking the time constant of fine-wire thermocouples with the conditions of heat exchange with the measured medium and the thermophysical characteristics of the thermocouple sensing elements has been deducted. An approximate formula is also given for calculating the internal resistance of wire sensing elements of thermocouples, which must be considered when calculating the time constant of a thermocouple. In consideration of the obtained formulas, a multi-parameter relation linking the current or expected time constant of fine-wire thermocouples with the known time constant set at the known parameters of the measured media has been formed.It is suggested to simplify the formed multi-parameter relation and make it dependent, for example, on the “expected velocity of the measured medium × expected density of the measured medium” complex (Vm2 ρm2 ). Simplified relations in the form of hyperbolic functions with constant parameters and argument in the form of Vm2 ρm2 complex were obtained for airflowat different temperatures, pressures, and velocities.On the example of airflow, it is shown that the complex multi-parametric relation linking the expectedand known time constants of thermocouples can be simplified to a hyperbolic dependence, where the argument can be the Vm2 ρm2 complex. Moreover, the degree of approximation of hyperbolic dependencies to the exact values of the multi-parametric relation can reach the R-square = 0.9592 criterion.A multi-parametric relation has been proposed. That relates the known time constant of a thermocouple to the expected or current time constant of the same thermocouple at other parameters of the measured medium from the point of view of the heat exchange and thermal conduction theory. The proposed relation can be used in automatic control systems of non-stationary temperature of various liquid or gaseous media to provide continuous correction of thermocouples dynamic characteristics. Depending on the number of measured medium parameters, the suggested multi-parameter relation can be replaced by simplified relations with other complexes containing, for example, density, velocity, flow rate and pressure of the measured medium.

A Measurement System for Time Constant of Thermocouple Sensor Based on High Temperature Furnace

In dynamic temperature measurement, thermocouple sensors are widely used, and their dynamic characteristics directly affect the accuracy of the test results. So before applying thermocouple sensors to dynamic temperature measurement, their dynamic characteristics should be obtained, and their dynamic performance parameters should be analyzed. The time constant is the most important dynamic parameter, which reflects the response speed of a thermocouple sensor. Therefore, it is necessary to measure the time constant. The time constant is closely related to the heat transfer mode, so the heat transfer environment of the time constant measurement system should be similar to the application environment of thermocouple sensor. When using the thermocouple to measure the temperature in various kilns, the heat transfer is mainly through the radiative mode, and existing equipment, such as constant temperature water/oil tank, shock wave tube and other devices, cannot be used to measure the time constant to reflect scenarios in actual measurement applications. Therefore, this paper proposed a new method to measure the time constant of the thermocouple by improved high temperature furnace. In the system, the high temperature furnace was used to generate the stable temperature field, and the fast feed device was used to insert the thermocouple into the high temperature furnace and generates the ramp temperature excitation. The temperature can reach 1500 °C in the furnace, and the temperature error in uniform temperature field is ±1 °C. Finally, the time constant of a K-type thermocouple was measured, and the uncertainty of the measurement result was analyzed.

In-situ measurement and dynamic compensation of thermocouple time constant in nuclear reactors

In-situ measurement and dynamic compensation of thermocouple time constant in nuclear reactors

Conjugate heat transfer investigation of core damage propagation during total instantaneous blockage in SFR fuel subassembly

Total instantaneous blockage at the inlet of a fuel subassembly (SA) is considered as a beyond design basis event in sodium cooled fast reactors. It is the bounding case of various types of local blockages and to cater for this event, a core catcher is provided at the bottom of the core. The extent of core damage propagation, before reactor trips, forms an important input for thermal design of the core catcher. Neutronics of the core is influenced by various phenomena that take place within the blocked SA, viz., (i) sodium boiling and clad/fuel melting, (ii) location within the blocked SA where these phenomena initiate and (iii) their rate of propagation inside the SA. Reactor trip by core temperature monitoring system of the neighboring SA is affected by (i) heat transfer between the molten fuel pool and the neighboring SA, (ii) number of fuel pins in the SA, (iii) rate of melting of the hexcan and (iv) thermocouple time constant. Towards understanding the thermal hydraulics of damage propagation, a coupled 2-dimensional transient thermal hydraulic model has been proposed. The model considers explicit representation of all the rows of fuel pins within the blocked SA and axial power distribution in the pins. For a realistic estimate of heat flux on the neighboring SA hexcan, natural convection in the heat generating molten fuel pool is considered. The damage propagation to the hexcan is determined by an enthalpy based energy equation, to model moving interface with melting/solidification. The models for turbulent natural convection in the fuel pool, and thermocouple response of neighboring SA are validated against published data in open literature. By detailed parametric studies, it is established that the damage is restricted to seven SA, in the event of TIB for a medium size fast reactor. The proposed 2-D model predicts a faster damage propagation than the 1-D models. However, residual thickness of the hexcan at the time reactor trip is 51%, which is nearly same as that predicted by the 1-D model. Thermocouple time constant is found to be a sensitive parameter for early detection of the TIB. When the number of pins in a SA is reduced, the residual thickness of the neighboring SA hexcan increases.

Physical Insight into System Identification Parameters Applied to Inverse Heat Conduction Problems

This paper compares the noninteger system identification method to an analytic formulation of transient inverse heat conduction containing a spatially lumped temperature sensor model for extracting physical meaning to noninteger system identification parameters. The two-sided noninteger system identification expansion coefficients are compared term-by-term to the resulting analytic model in the semi-infinite geometry. This physically motivated equivalence suggests which noninteger system identification expansion coefficients are dominant and should be retained for producing the best identification from a physical point of view. The analytical results are compared to an idealized one-dimensional semi-infinite noninteger system identification case with the goal of correlating parameters and obtaining their physical meaning. It is shown that one parameter is connected to the effusivity of the system in use, which substantiates that there is a physical meaning for the noninteger system identification parameters. The parameters on the temperature side are connected to the thermocouple time constant and thermocouple lead losses. In this study, the effusivity value has been examined. With a numerical simulation, the sensitivity is analyzed, showing that the comparison gives reasonable effusivity values. In a laboratory experiment using copper, the measured effusivity is , which is in favorable agreement with the nominal value .

Effect of thermocouple time constant on sensing of temperature fluctuations in a fast reactor subassembly

Abstract. Knowledge of temperature fluctuations in fast reactor subassembly is very important from a safety point of view. The time constant of thermocouples which are used for measuring coolant temperature in a fast reactor varies owing to various factors. Hence, it becomes necessary to investigate the effect of change in the time constant on sensed fluctuations. This paper investigates the dependence of temperature fluctuations on thermocouple time constants. A Scilab model consisting of source temperature profile, second-order thermocouple and histogram calculation is designed. Simulation is performed for various levels of fluctuations, fixed and variable thermocouple time constants. Kurtosis for each condition is calculated with the help of a histogram. It is found that the effect of true source fluctuations on sensor output is very large compared to that of a similar percentage of time-constant variations. Hence in systems like fast reactors, where the degree of source fluctuations (fluid enthalpy) is large in comparison to that of time-constant variations, the overall effect can be considered with great confidence to be the outcome of coolant temperature rather than thermocouple time-constant variations.

A robust thermal model to investigate radial propagation of core damage due to total instantaneous blockage in SFR fuel subassembly

A robust enthalpy based thermal model has been developed to investigate the sequence of various phenomena that take place during total instantaneous blockage (TIB) in a fuel subassembly (SA) of a sodium cooled fast reactor. The (liquid–solid) interface movement in the multiphase and multi-material heat transfer is predicted by the Voller’s algorithm employing a dynamic node deletion procedure for realistic simulation of density difference between immiscible fuel and steel. Employing this model, the scenario of TIB in the fuel SA of Indian Prototype Fast Breeder Reactor (PFBR) has been analyzed. The focus of the investigation has been, (i) detection of the event by online monitoring of sodium outlet temperature from the neighboring subassembly and (ii) determination of number of subassemblies that are likely to get damaged severely before reactor shut down, which is an important parameter to define thermal load on core-catcher. Detailed parametric studies have been performed to bring out the effects of various parameters that influence inter-subassembly heat transfer and damage propagation. The parameters include, subassembly power (and hence reactor power), hexcan wall thickness, thermal conductivity of hexcan material and thermocouple time constant. It is found that during nominal power condition of the reactor, thermal damage propagates to only one row of SA suggesting that the thermal load on core-catcher during the event of TIB in one fuel SA can be that corresponding to decay power of seven SA. Damage of the blocked SA is completed within ∼20s. Reactor SCRAM from neighboring SA thermocouple takes place at 55s after the TIB and the residual thickness of hexcan wall at the time of reactor SCRAM is 55%. At low power conditions of the reactor (and hence at low SA power ratings) detection capability improves and the residual thickness of neighboring hexcan during reactor SCRAM increases. Thicker hexcan does not enhance detection. Increase in thermal conductivity of hexcan material enhances early detection of the event and improves the residual thickness. Reduction in time constant of thermocouple does not significantly help in early detection of TIB event. The results predicted for PFBR fuel SA are found to agree qualitatively with that reported for other reactors.

Thermocouple thermal inertia effects on impingement heat transfer experiments using the transient liquid crystal technique

The transient liquid crystal technique is widely used for impingement heat transfer experiments. Additionally, due to the difficulty of producing pure temperature steps in the flow, many authors assumed the fluid temperature evolution as a series of step changes using Duhamel's superposition theorem. However, for small impingement configurations where the jets are fed from the same plenum chamber, and hence flow velocities are relatively small, thermal inertia of commercial thermocouples causes a delay, lagging from the real plenum temperature history. This paper investigates thermal inertia characteristics of thermocouples and their effect on the calculation of impingement heat transfer coefficient. Several thermocouples with exposed junction and different wire diameter were considered over a range of plenum flow conditions typically found in impingement heat transfer experiments. The effect of thermocouple time constant on the evaluation of the heat transfer rate was investigated in a narrow channel consisting of five inline impingement jets. The results indicated a significant effect of thermocouple response on the stagnation point region heat transfer, while lower local heat transfer rates are negligibly affected as liquid crystal signals appear later in time and the driving gas temperature history has a smaller influence on the evaluated data.

열전대 구조변화에 의한 열시정수 특성 변화

Thermal time constant measurement system was designed and fabricated to measure the thermal time constant, which shows a dynamic property of the thermocouple. Type K thermocouple samples were fabricated with variable shape each other and the thermal time constants of thermocouples were measured using the home-made thermal time constant measurement system. Thermal time constants of 12 type K thermocouple samples were distributed from 0.03 s to 8.2 s. It showed experimentally that the thermal time constant of thermocouple was increased linearly for the increase of the sheath diameter of thermocouple.

Problems Encountered in Fluctuating Flame Temperature Measurements by Thermocouple.

Some thermocouple experiments were carried out in order to obtain sensitivity of thermocouple readings to fluctuations in flames and to determine if the average thermocouple reading was representative of the local volume temperature for fluctuating flames. The thermocouples considered were an exposed junction thermocouple and a fully sheathed thermocouple with comparable time constants. Either the voltage signal or indicated temperature for each test was recorded at sampling rates between 300-4,096 Hz. The trace was then plotted with respect to time or sample number so that time variation in voltage or temperature could be visualized and the average indicated temperature could be determined. For experiments where high sampling rates were used, the signal was analyzed using Fast Fourier Transforms (FFT) to determine the frequencies present in the thermocouple signal. This provided a basic observable as to whether or not the probe was able to follow flame oscillations. To enhance oscillations, for some experiments, the flame was forced. An analysis based on thermocouple time constant, coupled with the transfer function for a sinusoidal input was tested against the experimental results.

Measurement of Time Constant of Wet-Bulb Thermocouple in Air

An experimental apparatus was built to measure the time constant of fine wet-bulb thermocouples in air. Rapid response solenoid valves (15 msec response time) were used to control airflow through tubing into which wet-bulb thermocouples were placed. Wet-bulb thermocouples (type T, 0.005 cm diameter) with the tip (bead) covered with a moistened wick were tested. Experiments were performed for air velocities ranging from 1.50 to 2.5 m sec−1 (Reynolds number of 2,500 to 4,500) and wet-bulb temperature ranging from 12.5 to 17.4°C. Experimental conditions were selected to simulate human respiratory conditions. Correlation between airflow velocity and wet-bulb thermocouple time constants was not significant at all levels.

Simultaneous measurement of velocity and temperature in high-temperature turbulent flows: a combination of LDV and a three-wire temperature probe

This paper deals with a simple and reliable technique for simultaneous measurement of velocity and temperature in high-temperature turbulent flows, including combustion. The technique is based on the combination of laser Doppler velocimetry and a digitally compensated fine-wire thermocouple. For temperature measurement, a two-thermocouple probe with a fine cold wire [Tagawa et al. (1998) Rev Sci Instrum 69: 3370–3378] is used, which enables in situ measurement of thermocouple time constants and accurate compensation of the thermocouple response. When tested in a turbulent wake behind a heated cylinder, the technique proves to be highly reliable and effective for investigating heat transport processes in various non-isothermal turbulent flows.

Simultaneous Measurement of Velocity and Temperature in High-Temperature Turbulent Flows.

This paper deals with a simple and reliable technique for simultaneous measurement of velocity and temperature in high-temperature turbulent flows, including combustion. The technique is based on the combination of laser Doppler velocimetry (LDV) and a digitally compensated fine-wire thermocouple. A central issue of this study is to find the applicability and reliability of the technique proposed. For this purpose, a two-thermocouple probe with a fine cold wire (Tagawa, Shimoji and Ohta, 1998), which enables in situ measurement of thermocouple time constants and accurate compensation of the thermocouple response, is combined with LDV. The technique is tested in a turbulent wake behind a heated cylinder, whose thermal field is of ordinary temperature and fluctuates with relatively small amplitude. This enables critical assessment of the measurement accuracy. The results show that the technique is highly reliable and effective for investigating heat transport processes in various non-isothermal turbulent flows.

A two-thermocouple probe technique for estimating thermocouple time constants in flows with combustion: In situ parameter identification of a first-order lag system

A two-thermocouple probe, composed of two fine-wire thermocouples of unequal diameters, is a novel technique for estimating thermocouple time constants without any dynamic calibration of the thermocouple response. This technique is most suitable for measuring fluctuating temperatures in turbulent combustion. In the present study, the reliability and applicability of this technique are appraised in a turbulent wake of a heated cylinder (without combustion). A fine-wire resistance thermometer (cold wire) of fast response is simultaneously used to provide a reference temperature. A quantitative and detailed comparison between the cold-wire measurement and the compensated thermocouple ones shows that a previous estimation scheme gives thermocouple time constants smaller than appropriate values, unless the noise in the thermocouple signals is negligible and/or the spatial resolution of the two-thermocouple probe is sufficiently high. The scheme has been improved so as to maximize the correlation coefficient between the two compensated-thermocouple outputs. The improved scheme offers better compensation of the thermocouple response. The present approach is generally applicable to in situ parameter identification of a first-order lag system.

Effect of Swirl on Combustion Characteristics in Premixed Flames

A double concentric premixed swirl burner is used to examine the structure of two different methane-air premixed flames. Direct flame photography together with local temperature data provides an opportunity to investigate the effects of swirl number distribution in each annulus on the global and local flame structure, flame stability and local distribution of thermal signatures. An R-type thermocouple compensated for high-frequency response is used to measure the local distribution of thermal signatures in two different flames, each of which represents a different of thermal signatures in two different flames, each of which represents a different of thermal signatures in two combination of swirl number in the swirl burner. In order to improve the accuracy of the temperature data at high-frequency conditions, information on the thermocouple time constant are also obtained under prevailing conditions of local temperature and velocity by compensating the heat loss from the thermocouple sensor bead. These results assist in quantifying the degree of thermal nonuniformities in the flame signatures as affected by the distribution of swirl and to develop strategies for achievinguniform distribution of temperatures in flames.

Improvement of a Two-Thermocouple Probe Technique for Fluctuating Temperature Measurement.

The reliability of a two-thermocouple probe, which is composed of two fine-wire thermocouples of unequal diameters and provides a technique for estimating thermocouple time constants without any dynamic calibration, is appraised in a turbulent wake of a heated cylinder. A reference temperature was obtained simultaneously using a fine-wire resistive thermometer (cold-wire) with a fast response. A comparison between the cold-wire measurement and the compensated thermocouple ones shows that a previous scheme underestimate the thermocouple time constants, unless the noise in the thermocouple signals is negligible and/or the spatial resolution of the probe is sufficiently high. The scheme is improved so as to maximize the correlation coefficient between the two compensated thermocouple outputs, and it offers better compensation of the thermocouple response.

Two-thermocouple probe for fluctuating temperature measurement in combustion—Rational estimation of mean and fluctuating time constants

A theoretical basis for the application of a two-thermocouple probe, comprised of two fine-wire thermocouples of different diameters, to carry out fluctuating temperature measurements is established with the aid of the method of least squares. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures has been realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, there is no need to introduce a so-called time-constant ratio into the reconstruction scheme. Consequently, the essential drawbacks in the two-thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method of correcting for heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the original reconstruction scheme with little modification. The scheme developed has been tested in a fluctuating temperature field with a diffusion flame formed in a two-dimensional wind tunnel. It is shown that the present scheme is successful and even allows study of the characteristics of time-constant fluctuations in high-temperature turbulent flows.

Two-Wire Thermocouple for Fluctuating Temperature Measurements in Combustion. Rational Reconstruction of Mean and Fluctuating Time Constants.

A theoretical basis for the application of a comprised of two fine thermocouples of different diameters to fluctuating temperature measurements is established with the aid of the least squares method. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures is realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, introduction of a 'time-constant ratio' into the reconstruction scheme is unnecessary. Consequently, problems in two-wire thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method for correcting heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the reconstruction scheme with little modification. The developed scheme is tested in a fluctuating temperature field with a diffusion flame formed in a two-dimensional wind tunnel, It is shown that the present technique is successful and allows us to investigate the characteristics of time-constant fluctuations in high-temperature turbulent flows.