Study of an annular two-phase thermosyphon used as an isothermal source in thermometry/Studija obrocastega dvofaznega termosifona kot izotermnega vira pri merjenju temperature

The feasibility of using an annular two-phase thermosyphon (TPT) in thermometry was studied experimentally; the main results are presented in this paper The TPT was conceived as an isothermal source for calibration purposes, using the comparison method. For this purpose, an annular TPT was designed, built and characterized, using water as a working fluid. In order to determine experimentally how isothermal the TPT annular surface can be, eight thermocouples were installed axially on its surface. Tests were performed for two different cooling water flows 16.67 [cm.sup.3]/s and 4.17 [cm.sup.3]/s. For each cooling water flow, four heat power levels were supplied in the evaporation zone: (38, 154, 350, 1,385) W. The experimental results have shown that the thermal device works isothermal for power levels higher than 350 W, where the so-called bar effect is not present and the temperature measurement error is reduced. Key words: thermometry, two-phase thermosyphons, isothermal, annular, calibration Opravljena je bila studija uporabnosti obrocastega dvofaznega termosifona (TPT) pri merjenju temperature in glavni rezultati toplotne zmogljivosti so predstavljeni v tem clanku. Obrocasti TPT je zasnovan kot izotermen vir za umerjanje merilnikov temperature po metodi primerjave. V ta namen je bil zasnovan, zgrajen in eksperimentalno okarakteriziran obrocast TPT z vodo kot delovnim fluidom. Za eksperimentalno opredelitev delovanja obrocaste povrsine TPT je bilo aksialno po notranji obrocasti povrsini v obmocju kondenzatorja namescenih osem termoparov. Toplotno moc je zagotavljal elektricni upornik v obmocju uparjalnika in toploto je zbiral pretocni sistem hladilne vode okrog kondenzatorja. Toplota je bila merjena po nacelih kalorimetrije. Preizkusi so bili opravljeni pri dveh razlicnih pretokih hladilne vode: 16,67 cm3/s in 4,17 cm3/s. Pri vsakem pretoku hladilne vode so bile vzpostavljene stiri razlicne ravni toplotne moci v obmocju uparjanja: (38, 154, 350, 1385) W. Rezultati eksperimentov so pokazali, da je delovanje toplotne naprave izotermno pri ravneh moci nad 350 W, kjer se aksialni temperaturni gradient zmanjsa na zelo nizko raven in torej ni vec vedenja kot pri palici, ki je na enem koncu v stiku s toplotnim virom in oddaja toploto kot rebro. Manjsa je tudi napaka meritev temperature. Izotermni pogoji se po zaslugi visoke prevodnosti naprave TPT vzpostavijo ze v par minutah in tako je mogoce v kratkem casu opraviti vec umerjanj. Pri manjsih toplotnih moceh (3