Surface junction thermal probe for transient measurements – conceptual design to field applications

Usually, the transient measurement of temperatures is performed by mounting the thermal sensors embedded on the surface of the heated material. In extension, X-ray diffraction (XRD) analysis has been attempted to study the deformation characteristics of the surface compound.

CONTENTS

Preface

The field of heat transfer is characterized as representing a good deal of both the art and science that goes into measurement; which requires a good understanding of flow physics. All heat transfer methods include some classical modeling methods with temperature measurement as one of the essential criteria for all modeling techniques which ultimately predicts the heat flow.

Heat Transfer

Transient Heat Transfer

• Transient Heat Conduction in Semi-Infinite Solids
• One-Dimensional Heat Conduction

Radiation occurs when heat energy is transferred through electromagnetic phenomena, the sun being one of the common important sources. In most heat conduction situations, the temperature variation is only significant in one direction, while its effect in other two Cartesian coordinates is negligible.

1.2) Considering the substrate to be a semi-infinite solid maintained at an initial temperature T i and is

Transient Temperature Measuring Device

Infrared thermography helps one to measure and interpret the transient temperature field prevailing over the surface of the body under observation. Usually the thermocouple is fixed in the wall by gluing to the back part of the element.

Fig. 1.5: Thin-skin calorimeter

Delineation of the Dissertation Work

The surface junction coaxial thermocouple is more robust than the thin film, but is less sensitive compared to the thin film meters. Finally, the review focuses on the work done in IIT Guwahati regarding the advancement of surface junction coaxial thermocouple.

Mathematical Modelling for Transient Heat Transfer Measurement

In addition, various techniques for measuring the heat flux were discussed, with special attention to the design aspects. Four different data reduction methods were used to obtain the heat flux data from the surface temperature.

Construction and Calibration of Coaxial Surface Junction Thermocouple

The developed sensor has a response time of the order of 1 µs and was suitable for measuring large transient heat fluxes in hyperspeed wind tunnels. To demonstrate the performance of the meter, a measurement of the heat flow around the front body of a cylindrical body in a hypervelocity flow was performed.

Estimation of Thermal Product

They used sandpaper and scalpel blades to form the connections on the temperature sensors. Furthermore, the use of different scratch techniques normally results in different thermal product values ​​of sensors; the exact TP value depended on whether the connection was on the positive or negative element or on both, and on the proximity of the connection to the thin insulating layer between the two elements.

Utilization of Shock tube as an Impulsive Experimental Tool

The facility was developed at the Shock Wave Laboratory (SWL) of the Aerospace department at IISc Bangalore. In short, the work revealed the viability of the high-speed forming of AA5086 aluminum alloy with a shock wave.

Heat Transfer Measuring Gauges

Robert and East (1996) highlighted the use of thermochromic liquid crystals to measure heat flux in transient hypersonic facilities. 2016) realized the simultaneous measurement of transient flow using fast-response pressure- and temperature-sensitive dyes in a long-duration hypersonic tunnel.

Application of Coaxial Surface Junction Thermocouple

• Internal Combustion Engine
• Hypersonic Facilities
• Gas Turbine

Furthermore, the engine speed has very little impact on the cylinder head's peak heat flux;. the heat flow of the exhaust manifold continues to rise with the increasing speed of the exhaust gases. This was observed for front body heat flux for a range of the angle of attack (0-20°).

Simulation Based Study

In addition, the surface temperature of the mica and thermocouple materials rises faster than that of the dural, due to the lower values ​​of thermal product for the mica and thermocouple materials. 2013) developed a 2-D finite volume-based in-house CFD solver to study the shock wave boundary layer interaction (SWBLI) and its associated changes in wall properties for ramp-induced flow breakdown. From the investigation, it was observed that the ratio of wall temperature to free stream stagnation temperature was the controlling parameter for SWBLI instead of the individual temperatures.

Research Undertaken at IIT Guwahati

It was found that there were no significant changes in the measured surface heat flux between the inverse and thin-layer analysis. In addition, the convoluted integral of the one-dimensional heat transfer equation was used to predict the surface heat flux and compare it with the input heat loads.

Summary

The obtained results showed encouraging trends and ranges of all methods for smooth or noiseless temperature signals. In addition, for trend prediction and heat flux quantification, splicing-based fitting techniques were found to be superior to all temperature data.

Objective of the Thesis

CSJT real-time application-based study of heat flux measurement: Sensors are used to capture transient heat flux in real-time experiments. Estimation of surface heat flux for short duration transient study is the core of all these research requirements.

Figure 2.1 throws basic insights into the work carried during the entire curriculum. The flowchart highlights the path followed for achieving the desired objective

Introduction

Considering the temperature sensitive paints (TSPs), work on the principle of light intensity emitted from paint, which is captured by a photodetector and correlates with the transient temperature variation. On the other hand, coaxial thermocouples are considered to be most effective for day-to-day transient measurements.

Numerical Simulation

A transient simulation was performed for 1 second with a constant heat flux of 1000 W/m2 applied to the top surface of the thermal probe. 3.4(a), which shows the variation of temperature along the length of the substrate at time 1s.

Fig. 3.1: Schematic shows the geometry of the coaxial surface junction thermocouple

Theoretical Modelling of Heat Flux Measurement

3.2), considering the thermal properties of the coaxial sensors as constant, the heat flux qS t passing through the surface xl1 is calculated using Duhamel's superposition integral as given below [Taler, 1996; Carslaw and Jaeger, 1959]. For use of the above equation. 3.4), it is desirable to have a closed solution of transient temperature data and estimation of the value of the thermal product.

Fig. 3.8: Variation of heat flux extracted from the CFD simulation using the in-house developed code Furthermore, the obtained temperature signal from the CFD simulation as shown in section 3.2, i.e

Design of Coaxial Surface Junction Thermocouple

• Selection of Materials
• Surface Junction Formation

Chromel is considered the positive material and Alumel is considered the negative, but surface junction thermocouples can be formed with an inner wire of the positive or negative thermocouple element and an outer ring of the other thermocouple material. Further, Table 3.2 highlights the essential material properties of coaxial thermal sensors [Mohammed et al., 2007].

Table 3.1: Properties of thermocouple materials [Caldwell, 1962]

Summary

Thus, one-dimensional heat transfer theory with a semi-infinite substrate can be applied to the probe for short-time experiments up to 1 s. This chapter highlights a comprehensive and systematic methodology for the fabrication of a coaxial surface-joint thermocouple, along with its characterization to qualitatively account for the plastic deformation occurring at the surface of the thermocouple.

Introduction

Before using the sensor on the surface of interest, it is important to calibrate the thermal probe with the same nature of heat load to account for the error in the case of the actual experiment. The method includes subjecting the thermal probe to a known input power using a laser source in the range of 1.5-6 W respectively.

Fabrication of Coaxial Thermal Sensors

The resistance of the sensor is continuously monitored using a multimeter, which typically falls in the order of 1 Ω. Figure 4.1(a-d) shows the fabricated coaxial thermal sensors along with the schematic of the thermocouple assembly.

Characterization of Coaxial Surface Junction Thermocouple

• Electron Discharge X-ray (EDX) methodology

It is believed that a junction on the surface of the thermal sensor is created due to the deformation of one thermocouple over the other. Special care must be taken to form the surface junction as the size of the sensor is quite small.

Fig. 4.2: Schematic of X-ray diffraction mechanism 4.3.2 EDX on test sample

Calibration of Thermal Sensors

• Constant source of temperature
• Constant source of heat flux
• Radiation Based Calibration

The study of the variation of output with respect to inputs is known as sensitivity. In the present experiment, the entire procedure was repeated three times to verify the reproducibility of the thermocouple.

Fig. 4.6: Schematic of oil-bath based calibration set-up with known temperature

Summary

After characterization of the thermal sensor, calibration was performed using the constant temperature and constant heat flux methodology. Such prediction of surface heat flux depends on the correct estimation of thermal properties, known as “thermal product (TP)” of the substrate, and also on the surface temperature history.

THERMAL PRODUCT DETERMINATION – EXPERIMENTAL METHOD

• Water Droplet Technique
• Water Plunging Technique
• Comparative Assessment of Thermal Products

For each temperature differenceTs Tw, the TP values ​​of each CSJT (E-type; J-type) are calculated by Eq. For each temperature differenceTwTs, the TP values ​​of each CSJT (E-type; J-type) are calculated using Eq.

Table 5.1: Thermal properties of junction material properties for theoretical determination of TP values [Caldwell, 1962]

Determination of Instantaneous Surface Heat Flux

For E-type CSJTs, the maximum heat flux is found to be about 11 W/cm2 with experimental and theoretical values ​​of TP. However, a significant peak surface heat flux deviation of 4 W/cm2 is seen for J-type CSJT, which is mainly due to underprediction of TP values ​​(29%) during "water diving" experiments.

Fig. 5.5: Comparative assessment of surface heat flux histories for CSJTs: (a) water droplet technique; (b) water plunging method

Summary

Additionally, the shock tube is used to evaluate the heat flux in the supersonic environment using in-house developed coaxial surface junction thermocouples (CSJT). Next, prepared coaxial surface junction thermocouples (CSJT) namely, types E, T and J (as elaborated in previous chapters) are used to calibrate the installed shock tube.

Introduction

Thus, the gas from the high-pressure (drive) section expands into the low-pressure (drive) section of the tube, which contains the test gas. Upon reaching the end of the tube, the incident primary shock wave reflects and travels backward.

Fig. 6.1: Schematic representation of a conventional shock tube and its working principle as per the data obtained in the calibration run [Takayama et al., 2014]

Installation of Shock Tube

In the first pipe of the drive part, two openings are provided, one for connection to the vacuum pump, so that low pressures can be created inside the drive part, and the other is connected to the vacuum gauge and the U-. To check repeatability, the vacuum gauge is connected to a U-tube manometer (via a T-piece).

Fig. 6.3: Design and geometrical parameters for shock tube assembly

Instrumentation for the Shock Tube

6.9: (a) Design and manufacture of thermocouple inserts and their mounting on the end plate of the shock tube; (b) Schematic of the hemispherical model mounted in the end flange of the shock tube; (c) End flange of the shock tube showing the assembled model. The schematic and mounting assembly for the thermal sensors in the end section of the shock tube is shown in Fig.

Fig. 6.8: Design, fabrication of pressure sensor holder and its mountings in the driven section of the shock tube

Calibration of Shock Tube

• Shock tube relations
• Experimental procedure for shock tube operation
• Stagnation heat flux measurement in the shock tube .1 Signal Processing
• Heat Flux Estimation

Through the experiment an attempt was made to measure the transient heat flux in the highly transient environment of the shock tube (Fig. 6.16). The performance of the fabricated sensors is tested by measuring the heat flux at the stagnation point in the shock tube facility.

Fig. 6.11: Diaphragm rupture process in the shock tube

Summary

Due to this fact, the nature of the heat flux signal is found to be similar in all experiments. The jump in the heat flux value can be expressed in terms of the thermal product of the CSJT.

Introduction

In the present work, an attempt has been made to delve into different types of surface junction probes and address their comparative performance in shock tunnel experiments. Before the shock tunnel tests, the sensitivity of each thermocouple (i.e. the variation of the voltage versus temperature) is determined via the calibration experiments (chapter 4).

Experiments in Shock Tunnel

• Test Facility
• Test Model and Instrumentation
• Experimental Data Interpretation .1 Hemispherical Body
• Wedge Body Experiments

The entire probe sees a Mach flow of 8.2 in the test section of the hypersonic shock tunnel, with a test time of almost 1 ms. Furthermore, the dimensions of the grate are chosen so that all the measurements are carried out in the core flow regions of the test gas.

Fig. 7.1: Hypersonic shock tunnel experimental facility

Computations of Surface Heat Flux

• Prediction of Stagnation Heat Flux Based on Correlation for the Stagnation Point The test gases in the shock tunnel have very high stagnation enthalpies. At the same time, during
• Numerical computations for surface heat flux prediction .1 Numerical Methodology
• Numerical Computation

It can be observed that the initial pressure (before the arrival of the hypersonic flow) in the test section of the shock tunnel is of the order of 0.1 Pa. The results from the internal solver are compared with the experiment result.

Fig 7.7: Computational domain and associated boundary conditions for flow over thermal probe

Recovery of Surface Heat Flux during Experiment

All the acquired temperature signals from the surface junction probes during shock tunnel experiments are analyzed to infer heat flux at stagnation point of hemisphere as well as over the perimeter of the wedge body. As mentioned, the acquired temporal signals of all the CSJTs over the wedge model are analyzed during the shock tunnel experiments to infer surface heat flux at the leading edge.

Fig. 7.15: Comparison of experimental signals from surface junction probes with discretized temperature data.

Performance Assessment of Surface Junction Probes

In addition, Fig 7.17 shows the typical pitot signal representing the usual characteristics of the shock tunnel. When the similar comparison is made for the J-type probe at stagnation point and leading edge on the plate, there is a significant overprediction of mean surface heat flux during the shock tunnel experimental time scale for both experimental models.

Summary

It essentially means that the “surface junction thermal product” does not fall along the shock tunnel time scale of experiments. Furthermore, it has higher sensitivity values ​​and an allowable “thermal product” for the ultra-short time durations common in shock tunnel experiments.

Introduction

Alkidas and Myers (1982) measured the transient heat flow in the cylinder head of a spark-ignition engine. The proposed work involves capturing the instantaneous surface heat flux from transient temperature data through one-dimensional heat conduction modeling in the combustion chamber and exhaust gases of the spark ignition engine, and in turn testing its applicability.

Experiment of Thermal Sensor in the Combustion Chamber

• Fabrication of Coaxial Surface Junction Thermocouple
• Experimental Facility
• Results and Discussion

Once the calibration and fabrication of the coaxial thermal sensor is over, the sensor is examined to measure the instantaneous heat flux into the combustion chamber. The thermal sensor is mounted flush with the inner surface of the engine head facing the combustion chamber.

Figure 8.1 shows the schematic of the fabricated coaxial surface junction thermocouple

Experiment in the Exhaust of the Internal Combustion Engine

• Test Facilities
• Signal Processing

The fraction of the voltage and temperature signal for the duration of 5 ms is shown in figure. The thermal sensor performance is only accessible once the heat flux has been predicted.

Fig. 8.7: Typical voltage-time signal captured from CSJT for internal combustion engine at various load conditions (a) 0%, (b) 20%, (c) 40%, (d) 60%, (e) 80%, (f) 100%

Summary

Focus tends to mount the thermo sensor in the "jet tube section" of the large gas turbine engine where the expected temperature is more than 1000 C. The recovered signals from both experiments have shown some interesting results that demonstrate the usefulness of the thermal sensor to capture the transient phenomenon in the jet pipe.

Introduction

During rewarming evolution, one of the most persistent problems is the development of high frequency screeching [ Ashirvadam , 2009 ]. The calibration of the sensors and their efficiency in terms of high response time (few microseconds) to obtain temperature rise in the combustion chamber of an internal combustion engine (IC) have been successfully investigated [Chapter 8].

Fabrication of Coaxial Surface Junction Thermocouple

• Packaging of Surface Junction Thermocouple

The entire additional exercise using MI cable is the need for K-type CSJT for exploring the high-temperature environment of the gas turbine engine. First, once the ceramic cement has been applied to the outer circumference of the sensor, followed by natural drying at room temperature for about 30 min.

Fig. 9.3: Fabrication procedure of specially designed K-type CSJT: (a) surface junction assembly; (b) preparation of M-type MI cable using sheath cutter; (c) MI cable connection to CSJT; (d) Ceramic bead separating chromel and alumel wire of M

Static Calibration of surface Junction Probe