-Impinging synthetic jets have excellent potential for energy-efficient local cooling in confined geometries. For a given geometry, synthetic jet flows are mainly characterised by the Reynolds number and the ratio of stroke length to a geometric length scale. The flow field of an impinging synthetic jet and the corresponding surface heat transfer distribution are strongly dependent on the dimensionless stroke length, yet few studies have investigated the flow field dependence for a wide range of stroke lengths. Therefore, the aim of this paper is to identify the various flow regimes as a function of stroke length. The experimental approach combines high speed particle image velocimetry and single point hot wire anemometry, and investigates an 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

5A measurement system has been designed to characterize the radiant energy efficiency of infrared heating 6 elements. The system also allows for measurement of the radiant heat flux distribution emitted from radiant heater 7 assemblies. To facilitate these, a 6-axis robotic arm is fitted with a Schmidt-Boelter radiant heat flux gauge. A 8 LabVIEW interface operates the robot and positions the sensor in the desired location and subsequently acquires 9 the desired radiant heat flux measurement. To illustrate the functionality of the measurement system and 10 methodology, radiant heat flux distributions and efficiency calculations are performed for a commercially 11 available ceramic heater element for two cases. In the first, a spherical surface is traced around the entire heater 12 assembly and the total radiant power and net radiant efficiency is computed. In the second, 50 cm x 50 cm vertical 13 planes are traced parallel to the front face of the heater assembly at distances between 10 cm and 50 cm and the 14 in-plane power and efficiencies computed. The results indicate that the radiant efficiencies are strongly dependant 15 on the input power to the element and, for the in-plane efficiencies, depend on the distance from the heater.

A study has been carried out to compare steady jet and synthetic jet heat transfer distributions at low Reynolds numbers. Both jets issued from a 5mm diameter orifice plate with air for the steady jet being supplied by a compressor via a plenum chamber. Tests were conducted for Reynolds numbers ranging from 1000 to 4000, and for non-dimensional surface to jet exit spacings (H/D) from 1 to 6. Dimensionless stroke length (Lo/D) for the synthetic jet was held constant at 8. A significant difference was observed between the steady and synthetic jet Nusselt numbers at low Reynolds numbers and low H/D. In comparison to steady jets, the stronger entrainment of surrounding fluid and the vigorous mixing near the impingement surface are characteristics of synthetic jets that are beneficial to heat transfer. Nonetheless, the steady jet yields higher Nusselt numbers for all test conditions.

Impinging synthetic jets are considered as a potential solution for convective cooling, in applications that match their main characteristics (high local heat transfer rates, zero net mass flux, scalability, active control). Nevertheless the understanding of heat transfer to synthetic jets falls short of that available for steady jets. To address this, this paper uses detailed flow field measurements to help identify the main heat transfer mechanisms in impinging synthetic jets.Local heat transfer measurements have been performed for an impinging round synthetic jet at a range of Reynolds numbers between 1000 and 3000, nozzle to plate spacings between 4D and 16D and stroke lengths (L 0 ) between 2D and 32D. The heat transfer results show evidence of distinct regimes in terms of L 0 /D and L 0 /H ratios. Based on appropriate scaling, four heat transfer regimes are identified which justifies a detailed study of the flow field characteristics.High speed particle image velocimetry (PIV) has been employed to measure the time-resolved velocity flow fields of the synthetic jet to identify the flow structures at selected L 0 /H values corresponding to the identified heat transfer regimes. The flow measurements support the same regimes as identified from the heat transfer measurements and provide physical insight for the heat transfer behaviour.