I want to measure boiling heat transfer of water from a hot solid surface at atmospheric conditions (exposed to surrounding air at constant pressure). Right now I'm determining which parameters to control in the experiment to obtain reproducible results.
For evaporation of water below its boiling point, it's important to take into account the relative humidity and temperature of the surrounding air. These determine the driving force for evaporation.
But for water above it's boiling point, the atmospheric conditions seem irrelevant, because water can continue to evaporate from the hot solid surface, even against its own pure vapor. It seems that whether the surrounding gas would be dry air, saturated air, or helium for that matter, the same results will be obtained. Is this reasoning correct?
With fast heat transfer from the solid surface to the boiling water, we may possibly expect the actual heat transferred to the liquid to be limited by the atmosphere capacity to uptake heat, both as sensible and latent heat. If the surrounding incoming humid air ― that approaches the boiling liquid surface ― has higher relative humidity, it also has a lower capacity to uptake latent heat, compared with dry air. Regardless of the details of the highly complex transfer phenomena above the liquid surface, some compensation could occur if either the outcoming air from the boiling surface region could increase its sensible heat content, and/or, if the displaced air flow could be enhanced. Nevertheless, some of the overall heat uptaking capacity could still be lost; what would consequently limit the mass flow of vapour released.
Dear Lendrer,
your reasoning is correct. if I well understood, you are doing pool boiling maybe to determine the Nukiyama curve. the relative humidity doesn't influence or has a very limited effect on the boiling process. it has an effect on the evaporation process with air removal at the water/air interface. is this type (evaporation), the mass transfer dominates and it is affected by the relative humidity. in the case of boiling the heat transfer in mainly dominated by the temperature difference between the hot wall and the saturation temperature and the geometry of the solid hot surface.
Good luck,
Kind regards,
Kifah SARRAF
Dear Leander,
boiling temperature at a given pressure doesn't depend on surrounding gas composition. Yet heat transfer would depend on it and surrounding pressure might depend on vapour and gas flow rate. So you should define heat transfer modes in your experimental setup and take trace of those measures that would be useful to evaluate actual heat transfer. If all the water is at boiling temperature, as usual, relative humidity is not relevant to your experiment.