Boiling

 

BOILING

Evaporation is one of the major heat transfer operations for concentrating solutions by boiling off the solvent. So, boiling can be considered as the backbone for evaporation. Boiling is a phenomenon where the liquid is transitioned to its vapor phase at its boiling point, the temperature at which the vapor pressure is equal to the pressure exerted on the liquid by the surrounding atmosphere.

Boiling occurs at the solid-liquid interface where the liquid in contact with a surface whose temperature is greater than the saturation temperature of the liquid itself. The substantial amount of heat/energy that is required to vaporize 1kg of a liquid is called the latent heat of vaporization.

When spoken of boiling, a simple household setup of water on a pan over a stove is what we generally live up to. This is a great example to understand pool boiling, where the heating surface is submerged in a pool of liquid. Considering the above setup, now the heat flux for boiling will depend on the temperature difference between the heating surface and the liquid.

AB - Initially, heat is added at a low rate and vapor begins to rise from the free surface marking free convection heat transfer.

BC - Later as the ΔT increases bubbles are formed but as they detach they collapse just above the superheated liquid region.

 Now as the addition of heat increases the bubbles detach and are propelled to rise to the free surface thereby increasing the circulation velocity of the convective currents. At this regime, the heat transfer coefficient is higher than the one observed at the free convection zone. This regime is called the Nucleate Boiling regime where multiple bubbles are formed and rise. The end of this region is marked by the maximum heat flux that the system will now obtain. This point marks critical temperature drop and peak flux.

CD - Now rapidly bubbles form at the heating surface, rise as jets, and collapse at the free surface causing an unstable stream of vapor to blanket the free liquid surface. This vapor film starts to reduce the heat flux and heat transfer coefficient at this regime, the Transition Boiling regime. The end of this regime is the Leidenfrost point (D).

At the leidenfrost point, the heat transfer rate and the heat flux reach a minimum point. This is because a stable film of vapor forms over the heating surface through which heat transfer occurs by convection and radiation.

The vigorous streams of bubbles and vapors are replaced by the organized movement of steady forming bubbles at the liquid and hot vapor film interface, thereby restricting the bulk of the heat transfer resistance to the vapor region.

DE - As the temperature drops and the heat flux increases the boiling enters the stable Film Boiling regime where radiation through the film becomes significant and the heat transfer rate increases with ΔT. Film boiling is not preferred because of the slow heat transfer and large ΔT while higher heat transfer rates are obtained by the turbulence caused by the bubbles at the nucleate boiling regime. It also provides higher heat flux at smaller temperature differences.

LEIDENFROST EFFECT

The leidenfrost effect is the phenomenon that is observed when a liquid comes in contact with a surface whose temperature is significantly higher than the boiling point of the liquid itself, thereby producing a vapor cushion layer to protect from rapidly boiling away. 

This is the effect that is observed when water is sprinkled over a hot pan. The water clusters as a droplet and it appears as though the droplets are skittering and dancing over the hot pan. When the temperature of the pan is less than the leidenfrost point the water will spread over it and vaporize immediately as it conducts heat.  At or above the leidenfrost point when the water drop falls on the pan the bottom of the drop evaporates as soon as it comes in contact thereby creating a buffer vapor layer that protects the rest of the drop from vaporizing immediately. This layer gets recharged as more of the drop comes in contact. 

The vapor now supports the droplet and the heat transfer happens through radiation thereby slowing down the rate of vaporization. This significantly increases the residence time of the droplet over the hot pan before it vaporizes.

 

References:

Fundamentals of Heat and Mass Transfer by Dr. G.K. Roy.

GATEway to Chemical Engineering by M. Subbu and K Nagarajan.