The critical heat flux look-up table was applied to a large diameter tube, namely 67？mm inside diameter tube, to predict the occurrence of the phenomenon for both vertical and horizontal uniformly heated tubes. Water was considered as coolant. For the vertical tube, a diameter correction factor was directly applied to the 1995 critical heat flux look-up table. To predict the occurrence of critical heat flux in horizontal tube, an extra correction factor to account for flow stratification was applied. Both derived tables were used to predict the effect of high heat flux and tube blockage on critical heat flux occurrence in boiler tubes. Moreover, the horizontal tube look-up table was used to predict the safety limits of the operation of boiler for 50% allowable heat flux. 1. Introduction Critical heat flux (CHF) is a phenomenon corresponding to the point where a continuous liquid contact cannot be maintained at the heated surface. Strictly speaking, this particular term refers to the heat flux corresponding to the occurrence of the phenomenon. Other terms often used are burnout, dryout, boiling crisis, and departure from nucleate boiling (DNB). CHF results in sudden drop in heat transfer rate between the heated surface and the coolant. Beyond CHF, a small increase in heat flux leads to large increase in surface temperature for a heat-flux-controlled surface (e.g., electric heaters), and a small increase in surface temperature leads to decrease in heat flux for a temperature-controlled surface (e.g., steam condensers). This could lead to overheating damaging of the surface, corrosion in the CHF region, and reduction in the operating efficiency. Various prediction methods for CHF have been proposed during the past 60 years. The earliest prediction methods were primarily empirical [1, 2]. These crude empirical correlations lacked any physical basis and had a limited range of application. Subsequently, a large number of phenomenological equations or physical models for CHF were developed; many of these models were subsequently used in the safety analysis of nuclear reactors, boilers, and steam generators. Physical models, however, depend on the mechanisms controlling the CHF, which are flow-regime dependent. Flow regimes change significantly during a typical transient, and this necessitates the use of a combination of different models, equations, or correlations for CHF in safety analysis codes. Since most empirical CHF correlations and models have a limited range of application, the need for a more generalized technique is obvious. Hence, look-up tables
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