Thermal-performance evaluation of bicycle helmets for convective and evaporative heat loss at low and moderate cycling speeds

The main objective of the study was to investigate the thermal performance of five (open and closed) bicycle helmets for convective and evaporative heat transfer using a nine-zone thermal manikin. The shape of the thermal manikin was obtained by averaging the 3D-point coordinates of the head over a sample of 85 head scans of human subjects, obtained through magnetic resonance imaging (MRI) and 3D-printed. Experiments were carried out in two stages, (i) a convective test and (ii) an evaporative test, with ambient temperature maintained at 20.5 ± 0.5 °C and manikin skin temperature at 30.5 - 0.5 °C for both the tests. Results showed that the evaporative heat transfer contributed up to 51%-53% of the total heat loss from the nude head. For the convective tests, the open helmet A1 having the highest number of vents among tested helmets showed the highest cooling efficiency at 3 m/s (100.9%) and at 6 m/s (101.6%) and the closed helmet (A2) with fewer inlets and outlets and limited internal channels showed the lowest cooling efficiency at 3 m/s (75.6%) and at 6 m/s (84.4%). For the evaporative tests, the open helmet A1 showed the highest cooling efficiency at 3 m/s (97.8%), the open helmet A4 showed the highest cooling efficiency at 6 m/s (96.7%) and the closed helmet A2 showed the lowest cooling efficiency at 3 m/s (79.8%) and at 6 m/s (89.9%). Two-way analysis of variance (ANOVA) showed that the zonal heat-flux values for the two tested velocities were significantly different (p < 0.05) for both the modes of heat transfer. For the convective tests, at 3 m/s, the frontal zone (256-283 W/m2) recorded the highest heat flux for open helmets, the facial zone (210-212 W/m2) recorded the highest heat flux for closed helmets and the parietal zone (54-123W/m2) recorded the lowest heat flux values for all helmets. At 6 m/s, the frontal zone (233-310 W/m2) recorded the highest heat flux for open helmets and the closed helmet H1, the facial zone (266W/m2) recorded the highest heat flux for the closed helmet A2 and the parietal zone (65-123 W/m2) recorded the lowest heat flux for all the helmets. For evaporative tests, at 3 m/s, the frontal zone (547-615W/m2) recorded the highest heat flux for all open helmets and the closed helmet H1, the facial zone (469W/m2) recorded the highest heat flux for the closed helmet A2 and the parietal zone (61-204 W/m2) recorded the lowest heat flux for all helmets. At 6 m/s, the frontal zone (564-621W/m2) recorded highest heat flux for all the helmets and the parietal zone (97-260W/m2) recorded the lowest heat flux for all helmets