2007, Piran, Slovenia

RADIANT FLOW THROUGH BICYCLE HELMETS:
A THERMAL MANIKIN STUDY
Paul A. Brühwiler
Laboratory for Protection and Physiology, Empa—Swiss Federal Laboratories for Materials
Testing and Research, Lerchenfeldstrasse 5,CH- 9014 St. Gallen, Switzerland
Contact person: Paul.Bruehwiler@empa.ch
INTRODUCTION
The head is well known to be important for thermal comfort (Cotter and Taylor, 2005).
Radiant heat exchange values in the order of 20 W have been reported (Buyan et al., 2006) for
a typical sunny day in temperate zones such as central Europe. Most studies of radiant flow in
headgear have focussed on industrial-type helmets, for which natural convection is an
important aspect of the heat exchange. Two recent studies have examined the role of radiant
flow in helmets, illustrating effects on comfort (Buyan et al. 2006), as well as considering
performance (Bogerd et al., 2005). For bicycle helmets, convection has been studied in some
detail (Ellis et al., 2000, Ellis, 2003, Brühwiler et al., 2004), but the effects of radiant flow
have yet to be evaluated.
This work reports the effects of directed radiant flow (e.g., sunlight) on heat transfer in
bicycle helmets. A total of 26 helmets were studied, with and without the visor. For two
helmets the visor was mounted at two angles. A single configuration for applied heat flow
was used to assess the roles of the forward and upper vents and the visor. Most, but not all,
visors were shown to accomplish a relevant shielding of the face from radiant flow, and there
were large variations among the helmets with regard to radiant flow to the scalp.
METHODS
The experimental arrangement, including thermal manikin, climate chamber, and wind tunnel,
is described elsewhere (Brühwiler, 2003). The climate conditions were set to 25.00 ± 0.05 °C
and 50% RH. The air speed was chosen to be 3.1 ± 0.1 ms -1 (11.2 ± 0.4 kmh -1 ), measured as
reported previously (Brühwiler, 2003), to achieve a moderate level of convective heat loss.
To study the effects of radiant flow, a 150 W heat lamp (T228, Osram, München, Germany)
was mounted above and directed at an angle of 55° downward onto the headform through a
plexiglass window, as shown schematically in Fig. 1(a). This was chosen to simulate an
intermediate situation, with the sun high in the sky, and to achieve a comparable radiant flow
to both the face and scalp sections. The data were acquired by switching the lamp on and off
for the given helmet-visor configuration, and taking the difference between two such
consecutive measurements. Helmets 16 and 28 had visors which could be swivelled up and
down through a small angle. To investigate the effects of this, and begin to gain an
appreciation of desirable modifications in visors relative to the present state-of-the-art, the
visors of these helmets were studied in the extreme positions: “d”, maximally down, and “u”,
maximally up, indicated with subscripts. 16i and 16s, where “i”, inferior, and “s”, superior,
refer to visors in the same positions as 16d and 16u, respectively, but with a single layer of Al
foil on the upper surface to study a possible role for visor heating in the overall heat transfer.
For comparisons of the radiant flow with and without the visor, the visor was added or
removed (without removing the helmet) to achieve two consecutive measurements, between
which the difference was taken; an example is given in Fig. 1(b, c), where the light intensity
and distribution on the face are seen to decrease with the visor. Measurements were carried
out three times on separate occasions for every helmet in each condition.
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