Figure 3 illustrates another heater assembly which was used for observing the behavior of liquid droplets on the heater surface. A Pyrex block shaped into an anomalous prism configuration was substituted for the copper block. An 18 mm * 18 mm area in the top surface was plated with a thin transparent ITO (indium tin oxide) film which could serve as the source of Joule-heat generation. Since the electric resistance of the ITO film linearly increases with an increase in temperature, the heater surface temperature could be evaluated by measuring the resistance of the ITO film. Every experiment with this glass-prism heater was performed under a steady thermal condition in contrast with the experiments with the copper-block heater.
　　The whole experimental apparatus was installed in three standard racks (700 mm * 450 mm * 900 mm) for parabolic flights in the MU-300 aircraft. The spray chamber and the main part of the experimental hardware were mounted on one rack (main rack). The LDV and the optical signal processing systems were arranged in the second rack next to the main rack. The data acquisition and control systems were held in the third rack.

Procedure 　　
　　The spray volume flux at the heater surface was calibrated, on the ground, against the liquid pressure above the nozzle. In the calibration, droplets sprayed onto the surface were collected by use of a honeycomb-structured receiver(3) so that local liquid-volume flux could be evaluated. 　　　　　　　

　　The volume flux data thus obtained are plotted in Fig. 4 against the radial distance from the center of the heater surface. It is found that the volume flux under each prescribed condition is nearly uniform within a radius of some 15 mm of the center but significantly radius-dependent over a peripheral region. Thus, the volume flux averaged over the area within 15-mm radius is used as Dm, the nominal spray volume flux, in this paper.
　　The diameters and velocities of droplets falling onto, or rebounding from, the heater surface were measured with a PDPA (phase doppler particle analyzer) at a plane 100 mm below the nozzle. The PDPA measurements were carried out exclusively on the ground. The results of the measurements of spray-related parameters are summarized in Table 1.
　　A series of spray cooling experiments were conducted under a reduced gravity condition (0.01 times the terrestrial gravity ge) and an elevated gravity condition (2 ge) in each parabolic flight maneuver. Although each reduced or elevated gravity period lasts some 15 seconds, the only data obtained in the last 6 seconds in that period were employed for use in evaluating the heat transfer because of the higher stability of the gravity level in the later part of the period. The data-sampling time was set at 1 s while the heater surface was in the film-boiling regime. The sampling time was shortened to 0.1 s when the surface fell in the transition- or nucleate-boiling regime.