Noninvasive platinum thin-film microheater/temperature sensor array for predicting and controlling flow boiling in microchannels

Abstract

Thermal sensors and actuators are usually introduced at/in micro-channels in two ways: first, in an invasive manner through drilled holes which can lead to leakage problems and a change in channel dimensions and second, mounted in a non-invasive manner outside the channel which reduces heat transfer, accuracy, precision and response time. These problems are multiplied as soon as microchannel boiling with associated rapid thermal effects is investigated. Therefore, in this work, a platinum microheater/temperature sensor array has been developed to provide non-invasive, but nevertheless direct heating and temperature measurement at several locations along microchannels. This array comes without the mentioned disadvantages, resulting in ultra-fast response times that are limited only by the measurement speed of the sensor resistance and linear temperature coefficient of resistance. The working platinum area of each element in the array is only 238 nm thick, 0.1 mm wide and 0.5–2 mm long. The structures can be operated in heating and temperature measurement mode simultaneously. They are integrated into microreactors with the working surface as a side wall of a microchannel and in direct contact with the flowing fluid. Since the structures are deposited on transparent Pyrex glass in a cleanroom process, optical observation of two-phase flow boiling processes is possible for flow regime identification. The maximum operating temperature of the Pt microstructures is 450 °C and the linear temperature coefficient of resistance is about 2.98 × 10−3 °C−1. The relative measurement error of temperature measurements is less than 0.05% due to 2.9 µm thick highly conductive electroplated gold pads which form the electrical contact to the Pt microheaters/T-sensors. The platinum heater structures are capable of generating stationary vapor bubbles with a defined size proportional to the applied heating power. They can also predict two-phase flow boiling regimes (e.g. slug flow), vapor bubble length and frequency via the periodic signal of the measured temperature. Furthermore, they allow accurate calculation of heat transfer coefficients using the exact fluid temperatures measured with the Pt structures at the inlet and outlet of the microchannel, rather than analytically calculated values from temperature measurements outside the channel.