Jökull - 01.12.1973, Síða 55

Freezing on a Rotary Drill in Temperate Glacier Ice
HELGI BJÖRNSSON
SCIENCE INSTITUTE, UNIVERSITY OF 'ICELAND
During the summer in 1972 deep core drill-
lng by a rotary drill was carried out in the
temperate accumulation area in Vatnajökull
(Arnason, Björnsson and Theódórsson, 1974).
The most serious difficulty was found to be
freezing on the cutting tools, which hindered
further drilling until antifreezing mixture was
added to the hole. The cause of this pheno-
mena is described in the present note.
The rotary drill worked in a hole filled with
water, which hindered closure of the hole and
helped the ice chips to be removed from the
cutting bits. The ice chips were furthermore
transported along a helical thread on the core
barrel up to a storage department, which was
emptied after each run.
During drilling the pressure melting point of
the ice on the outside of the tip of the cutting
bit is lowered due to high pressure on the tip.
This causes melting of ice in contact with the
ttp, the temperature of the metal tip decreases
and flow of heat is set up inside the cutting
tool. This flow is maintained by freezing of
water and release of latent heat of melting at
other parts of the cutting tool, where the pres-
sure is lower. Additionally ice chips come in
contact with subcooled metal and freeze on
u' ^fom Fig. 1 it can be seen that freezing of
a relatively thin ice layer and freezing of ice
land glaciers in the last 250 years. Vatna-
jökull. Scientific results of the Swedish-Ice-
landic investigations 1936—37—38. Geogr.
Annal. 1—2. Stockholm.
Thoroddsen, Th. 1892: Islands Jökler i Fordd
og Nutid. Geogr. Tidsskrift, Bd. II, K0ben-
havn.
0strem, G. og T. Ziegler. 1969: Atlas over breer
í Sör-Norge. Norges Vassdrags- og Elektrisi-
tetsvesen. Meddelese Nr. 20 fra Hydrolog-
isk Avdeling (207 p.)
chips at the rear and beneath the cutting bit
will lift the bit and finally cover the edge and
hinder further drilling. Some simple calcula-
tions illustrate the effect of this process.
The power output (P) of the motor on the
drill shaft equals the product of momentum (M)
and angular velocity (oj). M is the momentum
of the force F which acts on the arm r. Then
r' 0)
The motor used was 1471 W (2 hp) and the
drill rotated 2 rps. Assuming P = 600 W and
inserting r = 5 • 10-2 m and a = 4tt rad/s we
1 cm
FI G. 1 Q
cutting
Fig. 1. The cutting bit. a) View from aside.
b) View from above.
Mynd 1. Bortönn. a) Séð frá hlið. b) Séð að
ofan.
JÖKULL 23. ÁR 53