International Journal of Thermal Sciences 44 (2005) 75–87 www.elsevier.com/locate/ijts Effect of submergence on boiling incipience in a vertical thermosiphon reboiler M. Kamil ∗ , M. Shamsuzzoha, S.S. Alam 1 Department of Petroleum Studies, AMU, Aligarh 202 002 (UP), India Received 22 May 2003; received in revised form 26 March 2004; accepted 26 March 2004 Available online 6 August 2004 Abstract An analysis of the incipience of nucleate boiling has been developed as a modification and extension of the previous analyses for the wall superheat including the effect of submergence. The minimum degree of wall superheat required for the onset of fully developed boiling of liquid was related to the thermophysical properties of test liquids. The model prediction was made with the experimental data available in literature for nine organic liquids including water. The results predicted from the theoretical model were found to be consistent with the data. An effort was also made to obtain a unified correlation for nine different fluids, together covering wide ranges of heat flux, submergence and inlet liquid subcooling. It was observed that the majority of the data points lie within a maximum error of ±22% and mean absolute deviation of 16 percent.  2004 Elsevier SAS. All rights reserved. Keywords: Incipient boiling; Submergence; Wall superheat; Thermosiphon reboiler; Heat flux 1. Introduction The vertical tube closed loop thermosiphon reboiler is a simple but effective heat transfer device. It is a one- pass heat exchanger in which boiling occurs inside vertical tubes. When vapor forms in the tubes of such an exchanger, the specific volume is increased and the weight of liquid in the return line causes circulation of liquid through the reboiler. The subcooled liquid entering the tube gets heated by single-phase convection and moves upwards. Depending upon wall temperature condition, subcooled boiling may set-in at the surface. When the liquid temperature attains saturation value; saturated boiling begins with the existence of net vapor, which increases resulting in bubbly to mist flow. Thus the heat transfer to liquid in the reboiler tube generates a changing two-phase flow with various flow regimes spread along the test section. The point at which the two-phase begins is known as incipient point of boiling (IPB), which corresponds to the conditions of minimum degree of wall superheat or heat flux required for the * Corresponding author. E-mail address: sm_kamil@rediffmail.com (M. Kamil). 1 Ex-Professor of Chemical Engineering AMU Aligarh. formation and detachment of first vapor bubble from the heated surface. Therefore, information on the conditions required for the onset of nucleate boiling are of paramount importance in the design of two phase flow heat transfer equipments. The point of onset of nucleation along the tube length and its required wall superheat depend upon a number of operating parameters. Numerous studies on predicting wall superheat through semi-empirical approach for forced convection boiling systems have been studied extensively during the last few years. These studies include the effects of various physical parameters, the size and geometry of nucleation sites, surface roughness, role of surface tension and wettability [1–5]. The widely accepted approach for the prediction of incipient boiling is based on the Gibb’s equilibrium theory of bubble in the uniformly superheated liquid and the one-dimensional steady or transient heat conduction equation. It was postulated that in the liquid film adjacent to the heating surface the superheated layer δ ∗ , must attain a threshold value so that the critical bubble nuclei with radius r c can further grow to the point of detachment. Zuber [6] was probably the first to analyze the inter- relationship between the local heat flux, the superheated layer and the diameter of the surface cavity. He employed 1290-0729/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2004.03.007