SWISS HAIL TUNNEL 
tially reduced for evaporation measurements, 
with the vaporizer elements then serving as an 
air dryer. The moisture content in the measuring 
section corresponds in this case to the moisture 
content of the atmosphere at saturation point 
relative to the vaporizer temperature as com- 
pared with ice. Suitable diagrams could easily be 
worked out from the data given earlier. 
The behavior of the tunnel during unstable opera- 
tion—In nature relatively rapid changes in the 
conditions of growth play an important part in 
the way hail forms, as is seen from hailstones 
having a layered, or ‘onion-coat,’ structure. In 
order to clarify the possibility of effecting some 
equivalent imitation, the flexibility of the atmos- 
pheric conditions in the measuring section of the 
tunnel was also investigated. 
The greatest obstacle in the way of rapid alter- 
ations in the operational conditions, and particu- 
larly in temperature, lies in the large mass of the 
air cooler. As a result of its large heat capacity 
(corresponding to 1200 kg of iron) the quickest 
changes of temperature that can be achieved in 
either direction are in the order of 1.5 to 2°C /min. 
At first, these values appeared to be inadequate. 
They can, however, be very considerably im- 
proved by the use of the 19-kw heater, although 
the temperature changes thus produced are al- 
ways upwards, making the tunnel air warmer. 
In creating periodic variations the basic temper- 
ature ty is arranged so that it remains constant, 
with the cooling compressor set at a fixed out- 
put-level. The maximum rise in temperature 
with the heating switched fully on is accordingly 
dependent only on the air speed, and it goes up 
as the volume of air put through decreases. 
The rapidity with which this rise in temperature 
shows up in the measuring section is dependent 
on the heat inertia of the tunnel between the 
a =F 
=50) 
-10 -20 -40 -50 a 
Fic. 9—The maximum possible water injection 
Wym as a function of the measuring-section tem- 
perature ty and at various air speeds vy 
917 
old 
4ty 10 
1S 
20 
20 
10 
a 
2 4 6 8 10 min 
Fic. 10—The maximum increase of temperature 
At which can be produced in the measuring section 
by switching the heater full on, expressed as a 
function of time and for various air speeds vi 
(showing the heat inertia of the hail tunnel at a 
constant vaporizer temperature fy) 
He 20} 
10 
| 
5 
20 a | 
| | 
2 
10 ! 
Tamin | 
| | | | 
| eee ee ta | | VM 
5 ite) 15 20 20 m/s 
Fie. 11—Behavior of the hail tunnel under 
periodic changes of temperature: maximum double 
temperature amplitude 2A as a function of the air 
speed vy, and for different period times 7’ 4 
heater and the measuring section. Figure 10 shows 
these relationships, observed for various air 
speeds as a function of time with maximum heat- 
ing. If, moreover, the refrigerating compressor 
is stopped at the moment when the heating 
is switched on, increases in temperature result 
which are on an average 20 to 40% higher than 
the values shown in Figure 10. 
Since we have been able to observe that heat- 
ing takes as long as cooling, we are in a position 
to estimate the double periodic amplitude 2A 
(which equals the maximum temperature increase) 
as a function of the period time; (this is shown 
in Figure 11). These periodically induced changes 
of temperature are, however, subject to some 
limitation from the capacity of the refrigerating 
plant in the lower temperature ranges. Here care 
must be taken that the heat setting for the lowest 
temperature wanted does not exceed the requisite 
cold output, otherwise the compressor cannot 
adjust itself to this operational point. Similar 
