A new contact time model for the mechanistic assessment of local heat transfer coefficients in bubble column using both the four-optical fiber probe and the fast heat transfer probe-simultaneously

Abstract

The purpose of this study is to analytically assess the local heat transfer coefficients by using the local bubble properties, which include the local gas holdup, bubble pass frequency, bubble chord length, and axial bubble velocity in a bubble column reactor. Therefore, for the first time, a combined probe that consists of a fast-response heat transfer coefficients probe and an advanced four-point optical probe was used for simultaneously measuring the heat transfer coefficients and the bubble properties, respectively. A new model, which has been developed, applied to estimate the contact time (τ) between the thin liquid film on the heating surface and the bulk liquid, which is one of the two parameters required in the mechanistic equation for determining the heat transfer coefficients. The experiments were conducted using a Plexiglas bubble column of 0.44 m diameter and 3.66 m height. Analytically, the consecutive film and the renewal mechanistic model of the unsteady-state surface have been used to calculate the rate and coefficients of the heat transfer. Results illustrate that the heat transfer coefficients is significantly affected by the local bubble properties and their distributions over the surface of the heat sensor. However, the contact time (τ) is a function for the local gas holdup and the bubble pass frequency. Thus, the variation in the local heat transfer coefficients with the contact time is due to the bubble pass frequency and the local gas holdup. A very good agreement, within 13%, was found between the predicted and the experimental values of the heat transfer coefficients, even though the model overpredicts the heat transfer coefficients at all the evaluated conditions.