Porous coating influence on heat exchange crisis In circular cooling channels at one-sided loading


Investigation aims:

- Porous coating performing test data (obtained before on small-scale mock-ups at uniform one-sided heat loading) verification by test on medium-scale mock-ups.
- Research of heat loading non-uniform along channel influence on heat exchange crisis development at 3-D mock-up temperature distribution.
- Research of crisis development during process time.
- Test data base adding for correction calculated model at boiling on non-isothermal wall.


Calculation modelling pre-crisis temperature state of mock-up (with non-isothermal cooling surface at one-sided heat loading) is not developed perfectly up to present. The main difficult for boundary condition setting is absent at present time authentic mathematical correlation for boiling curve q = q(DT sat, DT sub, rў, w, P, d) in after-crisis area (transient and film boiling). There are separate tests [3] for boiling curves (in this after-crisis area) for low-temperature boiling coolant (Freon-113, liquid N2). Moreover, there is important fact that heat flux value in film boiling area increasing significantly in according with coolant velocity increasing and obtaining commensurable value heat flux value in point of first crisis (CHF Shl).

For calculation of the expected results we use two alternative conceptions (because of there is not correct correlation of boiling curve for water in transient and film boiling area) in this non-predicted area:

conception A: q = 0 (i.e. h = q/DT w=0) at Tw > T CHF I

conception B: q = CHFI = const (i.e. h = const /DT w) at Tw > TCHF I

Conception B does not leads to well impressed crisis (temperature avalanche-like increasing) at unlimited increasing of loading power. This conception is optimistic estimation. Conception A gives pessimistic estimation (moreover significant decreased). Reasonable result is between A and B conceptions.

Expected temperature of loaded by e-beam surface as a function of Incident Heat Flux calculated by these two conceptions for different inlet water temperature are shown on Fig. 6.
It will be seen that coolant subcooling significantly increases UHF value. Following correlation determined by previous test on small-scale mock-ups at one-sided loading was used for calculation medium-scale mock-ups temperature state:

h PC = 1.4 h smooth and (CHF Shl) PC = 1.4(CHF Shl)smooth

Point of nucleate boiling appearing may be schematically determined as a cross point of the single-phase heat transfer curve

q conv = h conv (T w - T inlet)

and nucleate boiling curve (Thom correlation).



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