1.2) Considering the substrate to be a semi-infinite solid maintained at an initial temperature T i and is
2.9 Research Undertaken at IIT Guwahati
Sahoo and Peetala (2010) successfully attempted to estimate heat flux out of the temperature data, obtained from a nickel film sensor for a supersonic flight test. Mainly three different curve fitting techniques were utilized to recover temperature data history of real-time flight using piecewise linear fit, polynomial fitting and cubic-spline method. A one-dimensional transient heat transfer modelling was employed to infer heat flux from the temperature history. The results had shown that the polynomial and cubic-spline technique matches very closely, with piecewise linear fitting
Kumar et al. (2010) described the methods of designing and fabricating a fast response thermocouples and a thin film gauges (TFG) for transient measurement. Both the thermocouple and thin film gauge were calibrated under same experimental conditions using the oil-bath technique. The calibration results proved that the hand-made sensors could be utilized to accurately measure the surface temperature and heat transfer rates in transient facilities.
Peetala and Sahoo (2011) carried out an investigation on the effect of sensor thickness on surface heat flux measurement during flight measurement. The measurements were estimated considering the temperature histories obtained from the nickel thin film sensor, mounted on quartz crystal, during a flight test. Further, heat flux was estimated from inverse methods using the analytical solution as well as control volume approximation. Later, the experimental data were discretized using the cubic spline method. The results were validated with the standard benchmark data using the thin-film gauge analysis based on a semi-infinite assumption for one-dimensional medium. It was observed that there were no significant changes in the measured surface heat flux between the inverse and thin-film analysis. Furthermore, it was noticed that when the thickness of the thin-film was increased 100 times during numerical simulation of inverse methods, the peak surface heat flux increased by 20%. It was observed that the inverse methods heat balance equations based on control volume were relatively easy and reduces computational time compared to that of analytical solutions.
Kumar et al. (2011) fabricated platinum based thin film sensors and further calibrated by applying step heat load using the laser light of known wattage. Four different step heat load were applied to the TFG and subsequent transient data were captured. Numerical simulation were performed to support the experimental investigation. The estimated results have shown the satisfactory demonstration of the developed calibration set-up and cost effective means of in-house fabrication of thin-film sensors.
Kumar and Sahoo (2012) proposed fabrication of different types on thin film gauge (TFG) using high conducting platinum and nanomaterials. The sensors were prepared by depositing high conducting gauge material on the insulating surface such as Pyrex, Macor and Quartz. Oil-bath based calibration set-up were utilized to obtain the sensitivity of all the TFGs and a comparison
study were drawn among the sensors. The obtain results suggested that the use of carbon nanomaterial and graphene enhanced the sensitivity as compared to only platinum based sensors.
Kumar et al. (2012) explored the possibility of using thin film gauges for short duration transient measurements with pure conduction mode of heat transfer. A simple calibration set-up was used to supply known heat flux of different magnitudes to the thin film gauges that were fabricated in- house with platinum as sensing element and Pyrex as an insulating substrate. The experimentally recorded temperature signals from the gauges were compared with simulated temperature histories obtained through finite element analysis, performed using ANSYS. The transient temperature data were discretized using the cubic-spline technique. Furthermore, convoluted integral of one- dimensional heat conduction equation was used to predict the surface heat flux and comparison of it with input heat loads. The developed calibration setup was seen to be very useful for conduction based measurements of thin film gauges. The obtained signals for temperatures and surface heating rates were seen to match well within a reasonable uncertainty of ±2%. In general, conduction based calibration explored the feasibility of using thin film gauges in various inter-disciplinary applications.
Kumar and Sahoo (2013) worked on the design and in-house fabrication of a K-type coaxial thermocouple and thereafter performed dynamic calibration using simple laboratory instruments.
There were two methods, where the known step load loads were applied through radiation and conduction modes of heat transfer. The radiation and conduction based experiments for step- heating loads were almost analogous to the heating environment experienced by any aerodynamic model in short duration impulse facilities and in flights. The surface heat fluxes (measured temperature histories) have been successfully predicted, and compared with input loads using one- dimensional heat conduction modelling. The recovered value for the heat flux from the laser-based and conduction based calibration experiments was comparable within 4% and 6% respectively from its true input heat load. In addition, finite element based numerical study was performed to compare the experimental results. The obtained simulation results were within an accuracy of
±0.3 % from the experimental value.
Peetala et al. (2013) predicted heat flux from the temperature signals obtained from the in-house built finite volume computational solver, and experimental data (shock tunnel and flight testing)
using one-dimensional heat conduction modelling and Laplace transform techniques. Three different discretization techniques were used namely piecewise linear fitting, polynomial fitting and cubic spline for the temperature signal. The obtained results have shown encouraging trends and magnitude from all the methods for smooth or non-noisy temperature signals. The applicability of polynomial based fitting was seen to be restricted only to the smooth temperature signals due to the qualitative prediction of heat flux or discontinuous or noisy temperature signals. Further, for trend based prediction and quantification of heat flux, spline based fitting techniques were found to have upper hand for all temperature data. Additionally, lower order (linear spline) were equally effective to higher order spline but were limited to short duration measurement.
Sahoo and Kumar (2016) successfully fabricated three types of thermal sensors namely TFG made out of platinum, TFG made of platinum mixed with CNT and chromel-alumel coaxial surface junction thermocouple. The sensors were calibrated in-house using an oil-bath calibration set-up.
A comparison study was shown using a sudden step heat load in convection mode for a 200 ms time duration. All the sensors predicted the surface heat flux well within the range of ±4%.