Agitation of the Surface Sub-layer

It can be seen that the large impedance offered by the mixed conduction/convection of the heat flow across region 2, only 0.4 mm thick, is the dominant term. 

0-0-A depicts equilibrium-equilibrium—Mode 1 rollover mass fluxes in Fig. 5.7. 0-A depicts equilibrium—Mode 2 rollover mass fluxes in Fig. 5.8 

It is this thin high-impedance region which is separating the bulk superheated liquid from the surface molecular evaporation region, and is preventing a much larger evaporation mass flux from taking place. 

Any disturbance or agitation of this thin conduction/convection region, whereby motion of bulk superheated liquid penetrates through it, or replaces it, will result in an immediate, rapid and large, increase in evaporation rate. The surface impedance drops to that of the molecular evaporation impedance and assuming that the evaporation coefficient remains at 10 the evaporation rate rises 23 fold for LIN, and some 19 fold for LCH4, as indicated in Fig. 4.8 [15]. 

When the disturbance ceases, the surface sub-layer regions re-estabUsh themselves, the high thermal impedance reappears and the evaporation rate falls. Since region 2, the conduction/convection layer, is so thin, the self-repair takes place in a few seconds or minutes and the evaporation rate consequently recovers its previous normal value in the same time. 

---

The three mechanisms appear to be self-repairing, within seconds after the disturbance, or agitation of the surface sub-layer, ceases. 

It is possible for agitation of the surface sub-layer to be maintained continuously over a longer period of time, several seconds, or minutes, or hours in length, so as to prevent the self-repairing mechanism re-establishing the equilibrium sub-layer structure. 

Examples of continuous agitation of the surface sub-layer include ... 

A number of phenomena have been observed in which agitation modifies the structure of the surface sub-layer and thereby changes the evaporation rate. 

---