Determining the temperatures of non-combusting as well as combusting flows is indispensable for a variety of problems of heat transfer in aero-engines, combustion engines and for studies of spacecraft entry into atmospheres. A big concern is determining temperature gradients near surfaces exposed to cold or hot gas flows. The accuracy and precision of the measurements is also imperative. Owing to the fact that physical probes are usually perturbing and invasive, attention has recently focused on optical approaches that are non-invasive.
Most optical methods, however, suffer from limitations, including calibration issues, low signals, and interference from particulate matter, arising from reliance on measuring light intensity. Professor Paul Ewart and Dr. Christopher Willman have developed a thermometry technique using laser-induced thermal grating scattering (LIGS) that measures a frequency rather than intensity. The method works by generating a sound wave that modulates laser light scattered off a grating-like structure induced by two interfering pulses of laser light. The temperature can be derived from the modulation frequency, related to the speed of sound, that can be measured very precisely. In a proof-of-principle experiment, signals from four separate points along a line in toluene-seeded nitrogen flows were recorded from which the temperature at each point could be determined. Their work is published in journal, Experiments in Fluids.
The authors determined temperatures for two flow situations. The first setup consisted of two adjacent flows at different temperature to give a stable gradient at the boundary. The second setup involved uniform temperature flows over surfaces where the temperature gradient was established by the difference between the temperature of the surface and that of the flow. This is a common situation in boundary layer problems.
The flows were seeded by passing nitrogen over a toluene reservoir on a digital balance to determine the rate of mass loss in order to estimate toluene concentration. A 2.3% concentration was found adequate to give strong LIGS signals.
Using fiber optic coupling, the signals imaged from each of the four points were directed onto separate photodiode detectors thus overcoming the limitation of expensive streak cameras, which do not provide the necessary combination of time-resolution and recording duration.
Thus the authors determined time and space resolved temperatures using inexpensive apparatus with a precision of better than 1%. Temperature gradients were resolved in flows and boundary layers with a spatial resolution of 1mm using 1:1 imaging of the interaction region, allowing small temperature differences of 5K mm-1 to be resolved. These results will be of importance in heat transfer studies in internal combustion engines and aerodynamics. Professor Ewart comments, “The technique can readily be extended to more points or used with different seed molecules such as water vapour with a suitable laser and work is planned to apply the method in flow experiments in wind tunnels and shock tubes.”
Christopher Willman and Paul Ewart. Multipoint temperature measurements in gas flows using 1–D laser–induced grating scattering. Exp. Fluids (2016) 57:191.
Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, UK.Go To Experiments in Fluids