Critical flow of liquid in a long channel

The results of experimental investigation of critical flow of liquid nitrogen in a long adiabatic channel are presented. As the experimental specimen a channel with a length of 1646 mm, and an inner diameter of 4 mm was used. Pressure in the experiments at the inlet to the specimen was changed in the range $p_{\operatorname{in}}$ = (3.06–5.39) $\times$ 10$^5$ Pa, the outlet pressure was $p_{\operatorname{out}}$ = (1.26–4.44) $\times$ 10$^5$ Pa, and the volumetric flow rate was in the range $V$ = (0–0.042) $\times$ 10$^{-3}$ m$^3$/s. The calculation of the coordinate of the cross section, in which one should expect the emergence of a shock wave, has been conducted. The calculation of local values of speed of sound and the velocity of the mixture allowed us to determine the critical value of the volumetric vapor quality in the critical cross section. Quantitative comparison of the conditions of emergence and existence of the critical flow in the long channel in comparison with short channels is carried out. Fundamental differences are shown.