ORIGINAL Multi-walled carbon nano-tubes for performance enhancement of thin film heat flux sensors Akash Jadhav 1 & Ravi Peetala 1 & Vinayak Kulkarni 2 Received: 13 June 2019 /Accepted: 9 October 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Surface heat transfer measurement is an important aspect in many research problems. Thin film heat flux sensor (TFHFS) is mostly considered in such situations for heat flux measurement due to its quick response and high accuracy. In the present studies, multi-walled carbon nanotubes (MWCNTs) are mixed with platinum while making the TFHFSs. Such addition is noticed to increase the sensitivity and decrease the temperature coefficient of resistance (TCR) of the thin film sensors. Improved sensitivity by 151% and 119% for Macor and Quartz sensors has led to increase in strength of the temperature response of the sensors during dynamic calibration experiments. Though heat flux recovery is seen to have encouraging agreement for all the sensors, sensitivity enhancement is noticed to be more prominent and advantageous for Macor based sensors. Present studies recommend adequately finished substrate for better adhesion. Further, use of MWCNTs is advisable especially for low heat flux measurement and also for Macor substrate since either situation demands the higher sensitivity to increase the output response. Nomenclature k thermal conductivity (W/mK) c specific heat (J/kgK) ρ density (kg/m 3 ) ffiffiffiffiffiffiffi ρck p thermal effusivity (J/m 2 s 0.5 K) g grams T temperature (K or °C) R electrical resistance (Ω) S sensitivity (ohm/K) r coefficient of correlation α temperature coefficient of resistance (K -1 ) q heat flux (W/m 2 ) t test time (s) τ scaled time (s) x depth of substrate (m) Subscripts 0 room temperature s substrate h heating c cooling 1 Introduction Knowledge of surface heat flux is essential since it is a very important design parameter for many engineering systems or sub-systems like steam turbines, gas turbines, IC engines, high speed space vehicles etc. However, surface heat flux estima- tion is an inverse problem where temperature needs to be measured. Having these measurements as inputs, the local heat transfer rates are then obtained using the suitable numer- ical technique. An immediate requirement of fast acting ther- mal sensor can be gauged here for periodic, highly unsteady or short duration temperature measurements [1]. Therefore, heat flux prediction becomes very crucial due to high frequency changes in temperature or due to small experimental duration. With this stringent requirement, low response time thermal sensor remains the primary need. In general, TFHFSs [2–4], temperature sensitive paints [5, 6], calorimeter gauges [7] and co-axial thermocouples [8–10] are used for temperature and further for heat flux measurement. The TFHFSs are conven- tionally preferred as these sensors have very small response time and can be used for various applications. Such sensor was first developed and used to record the short duration heat transfer in a shock tube by Vidal in 1956. Then onwards many types of thin film heat flux sensors were reported in the liter- ature. The TFHFSs made by coating conducting film (plati- num/nickel/silver) on an insulating substrate (Macor/Quartz/ Pyrex) are extensively used. In some studies, enamel coated metal models were painted with conducting thin film for heat * Ravi Peetala rkpeetala@mec.vnit.ac.in 1 Department of Mechanical Engineering, VNIT, Nagpur, Maharashtra 440010, India 2 Department of Mechanical Engineering, IIT Guwahati, Guwahati, India Heat and Mass Transfer https://doi.org/10.1007/s00231-019-02765-0