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7.6. Prediction of CHF in Annular Flow

Critical heat flux in annular flow is one of the most important mechanisms from a practical point of view since, for channels of reasonable length, the first occurrence of CHF may likely occur in this region. Film flow measurements have been used to investigate CHF mechanisms in two different ways:

  1. Measurement of the film flow rate at the end of a heated channel as a function of power input to the channel. The film flow rate at the end of the channel decreases with increasing power and the onset of CHF corresponds quite closely to the point at which the film flow rate becomes zero. This confirms the film dryout (as distinct from a film boiling, for instance) mechanism for critical heat flux.
  2. Although measurements of film flow rate at the end of the test section are useful in demonstrating the dryout mechanism, they do not demonstrate the conditions along the test section which have led to the occurrence of CHF. To achieve this, measurements of film flow rate may be made along the test section for a heat flux corresponding to the critical heat flux and then, for constant inlet conditions, and for the known CHF value making measurements of the film flow rate at the end of channels of various heated lengths, less than the length of the channel for which CHF is known.

The results of such measurements indicate that CHF occurs when the processes of droplet entrainment, droplet deposition and evaporation lead to the condition in which the film flow rate becomes zero. The rate of evaporation can be calculated from the local heat flux, provided the latent heat is known; for adiabatic flows, the rates of entrainment and deposition can be calculated using the methods described for any annular flow. The important question which arises is whether the rate of entrainment and/or deposition is affected by the presence of a heat flux normal to the surface. The effects which could be caused by a heat flux include:

  1. The effect of nucleate boiling in the film giving rise to additional entrainment due to the bursting of bubbles through the film surface and;
  2. The effect of a vapor flux away from the surface changing the hydrodynamic boundary condition and possibly inhibiting drop deposition onto the surface in the presence of evaporation.

Over a range of conditions, the entrainment/deposition processes are relatively unaffected by the presence of a heat flux, thus, the annular flow prediction methods can be applied to CHF prediction in the annular flow regime. The procedure is to write a mass balance equation for the liquid film and to integrate this equation along the channel from known (or assumed) boundary conditions, and determine the point at which the film flow rate goes to zero.


next up previous contents
Next: Correlations for CHF With Up: BURNOUT AND THE CRITICAL Previous: Mechanisms of Critical Heat


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