Analysis of Flow Boiling in Vertical Tubes

1. Analysis of Flow Boiling in Vertical Tubes P M V Subbarao Professor Mechanical Engineering Department Selection of Optimal Parameters for Healthy and Safe Furnace Walls with Frictional Flow…..

2. Flow Boiling • Flow boiling occurs when all the phases are in bulk flow together in a channel; • e.g., vapor and liquid flow in a pipe. • The multiphase flow may be classified as adiabatic or diabatic, i.e., without or with heat addition at the channel wall. • Void fraction and Pressure drop are two important parameters in real flow boiling.

3. Adiabatic Flow Through A Pipe

4. Diabatic Flow Through A Pipe

5. Selection of Flow rate in Flow Boiling • This process may either be forced convection or gravity driven. • At relatively low flow rates at sufficient wall superheats, bubble nucleation at the wall occurs such that nucleate boiling is present within the liquid film. • At high qualities and mass flow rates, the flow regime is normally annular. • As the flow velocity increases, convection in the liquid film is augmented. • The wall is cooled below the minimum wall superheat necessary to sustain nucleation. • Nucleate boiling may thus be suppressed, in which case heat transfer is only by convection through the liquid film and evaporation occurs only at its interface.

6. Pressure drop: Religious to Secular Attitude Dphydro+Dpfriction Dphydro+Dpfriction

7. Pressure Drop in Tubes The pressure drop through a tube comprise several components: friciton, entrance loss, exit loss, fitting loss and hydrostatic. Exact prediction of wall temperature, it is important to know the pressure Variation along the flow

8. Circulation Vs Once Through

12. Auto Control Mechanism in Natural Circulation

13. Selection of Steam Mass Flow rate • A once-through forced circulation furnace with high mass flow: • If any tube receives more heat than the average, then it will accept receives less flow. • This can result in further increasing temperatures, potentially leading to failures. • Thus the mass flow per tube must start very high to ensure adequate remaining flow after the heat upsets. • Designs with medium mass flow: • These were attempted in once through forced circulation boilers with moderate success. • These exhibit worse consequences than the high mass flow designs. • When the mass flow is degraded during load reduction in a tube receiving more heat than the average, the remaining flow will have less margin to provide acceptable cooling. • Medium mass flow designs can experience heat upsets and/or flow excursions that result in flows in individual tubes that are lower than the low mass flow design.

14. Destructive Mechanisms in Forced Circulation/Onec through

17. Both the multi-pass and the spiral designs use high fluid mass flows. • High fluid mass flow rates result in high pressure losses as well as a “once-through” . • Means that strongly heated tubes have a reduction in fluid mass flow and a correspondingly high increase in fluid and therefore metal temperature which can result in excessive tube-to-tube temperature differentials. • This type of behavior is sometimes referred to as a "negative flow“ characteristic. • In the Vertical design, the furnace enclosure is formed from a single, upflow pass of vertical tubes.

18. The tube size and spacing is selected to provide a low fluid mass flow rate of approximately 1000 kg/m2-s or less. • With low mass flow rates, the frictional pressure loss is low compared to the gravitational head, and as a result, a tube that is heated strongly, i.e., absorbs more heat, draws more flow. • With an increase in flow to the strongly heated tube, the temperature rise at the outlet of the tube is reduced which limits the differential temperature between adjacent tubes. • This is known as the "natural circulation” or "positive flow" characteristic. • Minimize peak tube metal temperatures. • To minimize peak tube metal temperatures, multiple pass and spiral types designs use high fluid mass flow rates to achieve good tube cooling. • This results in the "once-through” characteristic noted above.

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