Flow boiling of ammonia in a plain and a low finned horizontal tube

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range -40 < T-sat < 4 degreesC, 0 < x < 0.9, 50 < (m) over dot < 150 kg/m(2) s and 2 < <Delta>T-w < 15 K with resulting heat fluxes of 17 < (q) over dot < 75 kW/m(2). The vapor quality is denoted as x, (m) over dot is the mass flux and <Delta>T-w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zurcher (Zurcher, O., Theme, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89-101), and with the correlations of Steiner (Steiner D. Stromungssieden gesattigter Flussigkeiten. VDI-Warmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Theme JR, Favrat D. Flow boiling in horizontal tubes: part 3 - development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation. (C) 2000 Elsevier Science Ltd and IIR. All rights reserved.