MODULE 3

Section 1:   

Boundary Layer and Turbulence

Section 2:   

Pressure Drop and Friction Factor

Section 3:   

Heat Transfer Coefficient

Boiling regimes happen if the heat transfer is accompanied by the formation of vapor bubbles, jets or films. The vapor and the heated liquid are carried away from the heated surface by the effect of buoyancy (in natural convection or pool boiling configurations) or by a combination of buoyancy and the forced flow of liquid (in mixed convection or flow boiling configurations).

Pool boiling is characterized by the presence of a heater surface (T_w) immersed in a pool of initially stagnant liquid (T_l)

As in one-phase convection the question here is to establish a relationship between surface heat flux q"_w and temperature differenceT_w -T_sat, whereT_sat is the saturation temperature of the liquid.

Figure 7 (a) shows the subcooled boiling regime where the bulk liquid temperature is lower than the saturation temperature (i.e., subcooled ). In this case boiling is confined to a layer in the immediate vicinity of the heater surface. The vapor bubbles collapse (condense) as they rise through the subcooled liquid. 

       a) Subcooled  Liquid      b) Saturated Liquid

Figure 7 - Pool Boiling Regimes

If the liquid is at the saturation temperature, figure 7 (b), the vapor generated at the heater surface reaches the free surface of the pool.

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