Investigation on CHF of saturated liquid nitrogen flow boiling in a horizontal small channel

The critical heat flux (CHF) characteristics of saturated liquid nitrogen flow boiling in a horizontal small channel were experimentally investigated over a wide range of heat flux, mass flow flux and inlet pressure. The results show that the departure from nucleate boiling (DNB) type CHF and the dry-out type CHF dominate the heat transfer separately depending on the vapor quality. The mass flow flux and the axial position along the channel affect the DNB type CHF, which occurs at the end of the channel with a vapor quality of 0.4. The higher mass flow flux and inlet pressure increase the dry-out type CHF as well as the corresponding vapor quality. These two types of CHF are both significantly affected by the gravity effect. By comparing the dry-out CHF with six existing correlations, it is found that almost all the correlations overpredict the CHF. This is because the flow boiling instability is relatively evident in small channels, which leads to the periodical vibration of the mass flow flux and consequently the earlier dryout in the lower mass flow flux region. In comparison, Qu and Mudawar's and Zhang's correlations match better with the dry-out type CHF under the present operating conditions.