Heat transfer coefficients are very important for the design of the various flow paths found in turbomachinery. An accurate measurement of heat transfer is difficult considering gaseous flow in combination with good thermal conductivity of the boundaries along the flow path. The majority of the measurement methods applied frequently have at least one of the following problems. The measurement setup as for instance a heat flux sensor is a thermal barrier or introduces, for measurement reasons, a lot of heat into the object of interest. In both cases the main error results from the modification of the system, which is critical for the investigation of any sensitive flow condition. Furthermore, insufficient fluid reference temperature and/or heat flux with changing sign corrupts any attempt to calculate reasonably heat transfer coefficients.
Recent research and development activities are often focused on non-stationary effects as for instance caused by blade passing and combustor noise or any other source of transient or non-stationary flow in turbomachinery. This is not limited to the main gas path. A lot of attention is also paid to these effects for the internal air system for example the intensification of heat transfer by usage of non-stationary effects is very interesting for efficient cooling along the hot gas path. Therefore, the measurement of heat transfer coefficients becomes even more important for transient/non-stationary flow conditions.
This contribution presents a new test rig and an experimental investigation of local heat transfer coefficients in oscillating and superposed stationary and oscillating gaseous flow with metallic boundaries. The measurements presented are based on a novel measurement/sensor concept tested first time in non-stationary flow. The measurement setup features miniaturized sensor dimensions, low energy consumption, low backlash of the measurement on the flow and improved resolution compared to other concepts for the conditions addressed.
Furthermore, measurements of the radial distribution of the heat transfer coefficient on a flat plate in front of an oscillating and superposed stationary and oscillating free jet are presented. For the continuous jet, the air flows through a cavity and a nozzle for stationary conditions. The oscillating or so called synthetic jet is a zero mass flow jet through a nozzle generated by a pulsating diaphragm. Additionally, the superposition of continuous and synthetic jet results in the pulsed jet. All jets described are examined in an orthogonal impingement setup for Reynolds between 1000 and 9000. The nozzle-to-plate distances are varied between 0.5 and 7 nozzle diameters covering a flow region from stagnation point up to five nozzle diameters off the jet axis. Additionally a comparison with correlations found in literature as well as a discussion of the results is included.