1.2) Considering the substrate to be a semi-infinite solid maintained at an initial temperature T i and is
2.4 Estimation of Thermal Product
the helium-air gas combination used in the shock-tube gave the highest temperature rise compared to other working fluids.
D’Aleo and Prasser (2012) described the construction, calibration and testing of a thin film platinum RTD arrays on glass substrates developed using the photolithographic technique. The fabrication involved an array of resistive temperature detectors (RTDs), based on a micropatterned thin film platinum resistor has been built. The sensors were located on a glass substrate (40×53 mm2), which has the platinum resistors and the electrical contact leads are embedded. The operational temperature range was below 0 °C up to 200 °C. The temperature coefficient of resistivity (TCR) was found to be 0.0011 °C-1 with a high correlation coefficient. The thermal time constant of the sensor has been verified at about 30 ms. The sensors can be utilized to measure and characterize in terms of frequency and amplitude of thermal fluctuations at the inner wall of T- junction pipelines to evaluate the heat flux through the wall and thus the heat transfer coefficient.
sensor substrate. The tested sensors were the eroding and vacuum-deposited metal surface thermocouples, platinum thin-film resistance thermometers and spot-welded surface thermocouples. In addition, Spot-welded surface thermocouples give erroneous results for convective heat transfer because of heat-conduction effects from the bare lead wires extending into the boundary layer. The comparison of the heat flux histories measured in the rapid compression machine by a platinum thin-film resistance thermometer on a ceramic substrate to those measured by an eroding surface thermocouple on a cast-iron substrate suggested that significant errors in the heat flux measurements. The reason could be because of material mismatch between the sensor body and the walls of the test section. In nutshell, systematic error leads one to conclude that the use of ceramic-substrate probes in heat transfer studies with an otherwise metal test section was undesirable, despite the superior signal-to-noise characteristics they possess.
Buttsworth (2001) experimentally determined the response of K-type surface junction thermocouples using a water droplet calibration technique (for millisecond times scales) and a shock tube (for microsecond time scales). Different junctions formed by scalpel blade scratches and abrasive paper were investigated. When scratches from scalpel blades were used to form the junction, the thermal product (TP) identified from the water droplet calibrations consistently differs by approximately 20% depending on whether the junction was made on the chromel or alumel substrate, in accord with existing thermal properties data. However, the shock tube calibrations indicate that for scalpel-scratched junctions, there was considerable variability in thermocouple response time due to effective junction depth variations produced during construction. On the other hand, junctions formed with abrasive paper produced rise times consistently less than 1 µs, but the water droplet and shock tube experiments both indicated significant variability in the effective TP of the made gauges. Moreover, the consistency in TP for scalpel-scratched junctions for millisecond timescales and the capriciousness for junctions created with abrasive grit for both the millisecond and microsecond timescales were attributed to the differences in the effective proximity of the junction to the insulation between chromel and alumel substrates. For junctions created with abrasive grit, the effective TP was approximately 30%
smaller for microsecond timescales than it was for a millisecond. Further, the insulation was likely to have a more significant influence for junctions created with abrasive grit than for scalpel scratched junctions; because the effective junction location would be much closer to the insulation
in the abrasive grit case because the physical scale of the plastic deformation was much finer when abrasive grit was used.
1Mohammed et al. (2010) presented the dynamic calibration technique for evaluating the thermal product values of different scratched temperature sensors. Mainly, two types of scratch such as abrasive papers with different grit sizes and scalpel blades with different thicknesses to form the sensor junction were used. The sensor was tested in a shock tube facility operating under different conditions; the result shows that the TP of sensors depends the Mach number, surface junction scratch technique, junction location as well as on the enthalpy conditions. Furthermore, it was noticed that using scalpel blade technique with a particular blade size gives consistent thermal product values and so it does not require an individual calibration. However, for sensors whose junction created using the abrasive paper technique with different grit sizes, a calibration for each sensor is likely to be needed and hence, the effects of thermophysical properties on the TP were greatly examined. The sensor performance was influenced by the way of forming the surface junction; the TP for alumel is larger than that of chromel by approximately 17.33%. The results obtained have provided useful and practical data for TP values for different scratched temperature sensors and were beneficial to the experimentalists in the field and can be used for accurate transient heat transfer rate determination. In addition, the calibration technique used shows that the response time of these sensors is on the order of microseconds (less than 50 μs) and it has a rise time less than 0.3 μs. The literature further includes a numerical technique for evaluating the transient heat flux history from the measured temperature-time signal.
2Mohammed et al. (2010) obtained practical data for thermal product values of E-type CSJT for accurate transient heat transfer measurements under hypersonic flow conditions. The data were evaluated using different scratched temperature sensors. Further, the effect of using different scratch techniques (abrasive papers and scalpel blades) to form the sensor’s junction were also investigated. It was observed that the TP of a particular sensor depends on the Mach number, junction scratch technique, junction location and enthalpy conditions. The results demonstrated that using different scratched technique would produce different TP values. The accurate TP value depends on whether the junction formed where actually located on the positive, or the negative or on both and on its proximity to the thin insulating layer. It was observed that there were apparent differences between the thermal product for junctions formed on chromel element and those
formed on constantan element; the TP value for constantan element was larger than that of chromel element by approximately 15.1%.
2Mohammed et al. (2011) reported the effect of different scratch techniques on the thermal- product (TP) value of temperature sensors using a dynamic calibration technique in a shock tube facility. They utilized abrasive papers and scalpel blades to form the junctions on the temperature sensors. It was outlined that TP of a particular sensor was found to be dependent on the flow Mach number, junction scratch technique, junction location, and also on the enthalpy conditions. Further, using different scratch techniques normally results in different thermal-product values of sensors;
the exact TP value depended upon whether the junction was located on the positive or negative element or on both, and on the proximity to the junction to the thin insulating layer between the two elements. The temperature sensors scratched with scalpel blades of appropriate thickness showed consistent thermal-product values, requiring no individual calibration. However, calibration for each temperature sensor whose junction was prepared using abrasive papers with various grit sizes is likely to be needed. The thermal product for alumel was larger than that of chromel by approximately 17.33%. The experimental procedure used in the present study has yielded practical data on characteristics of scratched temperature sensors; these data can be utilized for accurate measurement of transient heat transfer under hypersonic flow conditions.
3Mohammed et al. (2011) depicted the experimental technique to evaluate the thermal product (TP) values of rugged and fast response temperature probes for hypersonic aerodynamic experiments mainly by using abrasive papers with different grit sizes and scalpel blades with different thicknesses, to form the probe junction. A shock Tube facility was utilized for testing and calibrating the developed probe. The outcome of the experimental results showed that TP of a particular sensor depends on Mach number, junction scratch techniques, junction location as well as on enthalpy condition. The effect of the thermophysical properties of different substrates on the temperature sensor thermal product was examined, and some new correlation equations for evaluating the temperature sensor thermophysical properties were derived for getting accurate results. It was identified that depending on the scratch techniques, individual calibration is not required for sensors whose junction are made from scalpel blade, but individual calibration is necessary for junction formed with abrasive grits. A significant difference between the thermal
technique ranged up to about 17.33%. The accurate thermal product value of a particular TP depends upon junction location on the positive/negative element or on both and upon its proximity to the thin insulating layer between the two elements.