When measuring the convective heat transfer coefficient inside a duct, one normally induces heat on a surface, and measures the temperature change at the wall of the surface compared to the bulk fluid temperature, thereby obtaining heat transfer coefficient.
I am curious about a certain situation when the bulk temperature of a fluid (liquid) is at saturated condition, say, 100C for water. In this situation, if I were to measure single-phase convective heat transfer coefficient by applying an appropriate heat that does not induce phase change (boiling), is the measured heat transfer coefficient same to the one that I would obtain at a subcooled condition? (assuming temperature-independent properties) Wouldn't the latent heat (of vaporization) affect the temperature change?
Or is this situation unacceptable to view as a genuine "single-phase" heat transfer experiment because latent heat is involved?
For sure if there is a mass transfer due to phase change, latent heat plays a role: both for boiling (When wall temperature is sufficiently larger than the saturation temperature), and also during evaporation.In the former case, heat transfer is the cause and mass transfer is the effect. For the latter, reverse is true.
For the boiling situation ( pool boiling) the latent heat is involved in heat transfer co-efficient expression ( See, for example, Rohsenhow correlation).In addition, if there is flow of bulk liquid, the effect of single phase convection is to be added.So for flow boiling ( boiling flow through pipe) you will have added effect of single phase convective effect + latent heat effect due to phase change.
I feel for a pool boiling with saturated condition of bulk liquid you will have effect of latent heat, but the role of liquid superheated layer cannot be ignored, hence one cannot ignore the convective effect.
Thank you Vinicius and Koushik for the inspiring answers.
Besides phase changing situations, I am wondering what will happen just before the phase change. (I believe this situation is when the bulk fluid temperature does not increase anymore but absorbs heat as a latent heat. Am I getting right?).
Since temperature did not change as much due to the latent heat, I suppose the measured HTC may have estimated larger than the true value, and I don't know whether this is correct or not. Can anyone please tell me more about this?
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Heat Transfer