# Beiträge zur Hitzdrahtmethode

## Summary

1. (i)

The characteristics of hot-wires are calculated approximately, by using the experimental results of the heat loss of thin cylinders in a cross-stream. They are based upon the similarity law of heat transfer by forced convection.

The approximation lies in that the heat losses at the two ends of such probes are neglected, so that the results apply exactly only to wires of finite length as a part of infinitely long wires.

2. (ii)

Starting from an evaluation formula previously derived, a practical method is developed for determining the sensitivity of hot-wire probes. This method is based upon the complete semi-empirical equation byKing.

FromKing's linear equation a simple and universally valid expression for the sensitivity of hot-wires is derived.King's complete equation differs from the linear one by two correction terms, one of which is neglected as small. Using a result derived from the linear equation, the complete equation and its derivative are linearized. From this corrections are derived which are rather widely valid and which, when applied to the results obtained from the linear equation, lead to the more accurate ones corresponding toKing's complete equation. The measurements necessary to obtain the data for these corrections for a given kind of wire are easily made in still air without forced heat convection.

The relation between pressure and temperature which in air influence the measurements without flow are explained on the basis of experimental results in connection with the similarity law for heat transfer by free convection.

The curves given include the influence of the heating circuit upon the sensitivity and the thermal lag of the hot-wire.

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## Literaturverzeichnis

1. 1)

R. Betchov,Línfluence de la conduction thermique sur les anémométres á fils chauds, Proc. Konnkl. nederl. Akad. Wetensch.51, Nr. 6. 721–730 (1948).

2. 1)

J. Ulsamer,Die Wärmeabgabe eines Drahtes oder Rohres an einen senkrecht zur Achse strömenden Gas-oder Flüssigkeitsstrom, Forschung3, H. 2, 94–98 (März/April 1932).

3. 2)

Siehe z. B.J. M. Burgers,Hitzdrahtmessungen, in:Wien-Harms,Handbuch der Experimentalphysik, Bd. 4, 1. Teil (1931), S. 637–667.

4. 1)

G. Dätwyler,Détails théoriques et pratiques de la méthode du fil chaud, Congr. nat. Aviation franç. 1946, Rapport Nr. 42/500, S. 7.

5. 1)

G. Dätwyler,Zwei Beiträge zur Hitzdraht-Meßmethode, Helv. Phys. Acta15, H. 3, 266 bis 272 (1942).

6. 1)

Nach einem Vorschlag vonW. Amrein, Zürich.

7. 1)

R. Betchov,Théorie non-linéaire de l'anémomètre à fil chaud, Proc. Koninkl. nederl. Akad. Wetensch.52, Nr. 3, 195–207 (1941).

8. 1)

Ableitung durchW. Amrein, Zürich.

9. 1)

Hütte,Des Ingenieurs Taschenbuch, 1. Bd., 27. Aufl. (Wilhelm Ernst & Sohn, Berlin 1941), S. 594.

10. 2)

Hütte,l.c., Tafel 7.

11. 1)

Hütte,l. c., Tafel 8.

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