Nusselt number around tube

FIGURE 15.87 Variation of Nusselt number around tube periphery for film boiling of refrigerant 113 on a horizontal tube (from Montasser and Shoji [225], with permission from Taylor Francis, Washington, DC. All rights reserved). 

At 700°C and 1 atm this leads to a diffusion constant of 0.81 cm /s The flow field around a superheater tube is very complex involving both laminar and turbulent boundary layers and the estimation of the local boundary layer thicknesses (velocity, diffusion and thermal boundary layers) around the tube requires computer simulations with computational fluid dynamic (CFD) software packages. However, for this rough analysis an average value of the thermal boundary layer thickness around the tube is enough and can be estimated if the average Nusselt number around the tube is known... 

On the other hand, as shown on pages 143 to 151, in the flow of fluids across a cylindrical shape boundary-layer separation occurs, and a wake develops that causes form friction. No sharp distinction is found between laminar and turbulent flow, and a common correlation can be used for both low and high Reynolds numbers. Also, the local value of the heat-transfer coefficient varies from point to point around a circumference. In Fig. 12.5 the local value of the Nusselt number is plotted radially for all points around the circumference of the tube. At low Reynolds numbers, is a maximum at the front and back of the tube and a minimum at the sides. In practice, the variations in the local coefficient hg are often of no importance, and average values based on the entire circumference are used. 

Annular. When the vapor velocity is high enough (j > 1.5), gravitational effects can be neglected, and the condensate collects as a thin annular film around the inside of the tube walls, with no stratification. A significant portion of most condensers operate in this flow regime. There are numerous predictive models described in the literature for annular flow. Laminar flow models predict heat transfer coefficients that are too low, and turbulent models must be used. The most commonly used models are listed in Table 14.1. All models have a form for the local Nusselt number... 

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