Local heat transfer coefficient measurement through a visibly-transparent heater under jet-impingement cooling

The investigation of transient and steady jet-impingement requires highly-sensitive local and instantaneous temperature measurements in order to accurately establish the characteristically high heat transfer coefficients. A structured method is proposed, wherein IR thermography measurements are taken from a heater that is visibly-transparent yet infrared-opaque. The heater is realized as a transparent conductive coating, Indium Tin Oxide, applied to an IR window, and allows the application of simultaneous optical and thermal measurement. Here, high-speed photography is synchronized to the IR thermography for the evaluation of the temporal sensitivity and response of the heater. For increased accuracy, the IR emissivity of the heater is established at temperatures far above the ambient through a dedicated experimental setup, and thermal-emittance subtraction is employed.

Results are presented for the local temperature distribution in several steady and transient cases of water jet impingement in the transitional and weakly-turbulent region (Reynolds number 2400 to 3800): the start-up and steady state of a vertical submerged jet; as well as the steady-state, pulsed-flow and intermittent-flow of a horizontal free-jet. No time-delay was found in the heater’s thermal response, and even highly-transient phenomena, such as a droplet impinging on the liquid-film, a bubble flowing down the heater or large scale dryout/re-flooding can be clearly captured, optically and thermally. However, the thermal response is dampened by the substrate (IR window), and therefore the heat transfer coefficient is only calculated in steady state cases, where noise levels are further reduced by time-averaging. For the values of the local heat transfer coefficient, good agreement with the literature is found, in both free and submerged-jets.

Due to a high sensitivity of the method, evidence of a local heat transfer peak is found at a distance of around 0.5 nozzle diameters from the stagnation point and some asymmetry is found under the influence of gravity. In the case of a free jet impinging on a vertical heater the measurement method is extended to allow local temperature measurement from both sides of the liquid-film flowing down the heater, by employing an IR mirror. From these measurements, a truly localized heat transfer coefficient is found that accounts for the heating-up of the liquid as it flows down the heater and develops into a falling liquid film, resulting in higher local heat transfer coefficients, beyond 3 diameters downstream from the stagnation point.

Finally, an error analysis is conducted and some quantative recommendations are made regarding the range of validity of this method, in terms of the minimal temperature difference over which the heat transfer coefficient can reliably be found.