Step-Like Jet Impingement Cooling

Sudden fluid jet impingement to TSP coated on a hot body, which produces a rapid decrease of the surface temperature, can be used for testing the time response of TSP. A lumped heat transfer model gives an approximate solution for a temporal evolution of the temperature on a paint layer during step jet impingement cooling

(8.37)

where Tin is the initial temperature of the paint and Tmin is the minimum

temperature of the paint that is asymptotically reached as t The timescale

for this cooling process is t3 = kh/( aThc), where hc is the average heat transfer coefficient of the impinging jet and h is the paint thickness.

Figure 8.18 shows an experimental setup for step jet impingement cooling. A 475-nm blue laser beam was used for illumination at the impingement point. The luminescent intensity was measured using a PMT and then was converted into temperature using a priori calibration relation. To achieve a small response time, a sub-zero temperature impinging Freon jet generated by a Freeze-it® sprayer was utilized, where a mechanical camera shutter was used as a valve to control issuing of the jet. After the shutter opened within 1 ms, the Freon jet impinged on the surface of a hot soldering iron (about 100oC) which was coated with a 19-|jm thick Ru(bpy)-Shellac TSP. Figure 8.19 shows a rapid decrease of the surface temperature on the thin paint coating to the minimum temperature of about 44oC. The measured timescale t3 of TSP for this cooling process was 1.4 ms. Cool air impingement jet was also tested; the measured timescales were 16 ms and 25 ms for 19 |jm and 38 |jm thick Ru(bpy)-Shellac TSP coatings, respectively.

Hot body

jet

Argon laser for illumination

Fig. 8.18. Schematic of a step-like jet impingement cooling setup for testing TSP time response

0 5 10

Time (ms)

Fig. 8.19. Temperature response of Ru(bpy)-Shellac TSP to step-like Freon jet impingement cooling