Real heat transfer coefficient h during nucleate boiling process means a ratio h=q/(T-Ts). Effective heat transfer coefficient is calculated as h = q/(T-Tm). Where q is heat flux density; Ts is saturation temperature; Tm is medium temperature. A temperature difference T-Ts is responsible for bubble growth during boiling and in this case HTC h is a function of T-Ts . Difference T-Tm >>T-Ts and it can differ more than 10 times. It means that effective heat transfer coefficient (HTC) is 10 time less as compared with the real HTC. What do you think about It?
Effective HTC is usually used for calculating cooling time at the core of steel parts. And it cannot be used for accurate calculating temperature field in steel part during boiling process. Effective HTC doesn't consider self - regulated thermal process and cannot provide possibily of governing phase transformations during quenching just by adjusting pressure. This problem needs a wide discussion.
You are absolutely correct in that the nucleate boiling relates to the local tempeature difference (Tsurface-Tsaturation), as the bubble ebollution cycle is not related to the mixing-cup temperature Tm . Introducing Tm instead of Tsaturation makes no sense, and will only produce HTC results that are specific only to the experiment from where they were obtained..For calculations that are reasonably transferable to other situations, use the temperature difference (Tsurface-Tsaturation). For strongly subcooled boiling situations, there are specific correlations that implements the effects of subcooling.
Thanks for your support, Prof. Erling Næss ! In heat treating industry engineers use effective HTC and nobody use real HTCs. As a result, it is impossible to make correct calculations for surface temperature during transient nucleate boiling process. No one can observe self - regulated thermal process during transient nucleate boiling if effective HTCs are used. I think, thermal scientists, material scientists and metallurgists should discuss this problem at the special conference or special seminar. I appreciate your comments and let's continue our discussion.
The information may be useful for your research
To calculate correctly the self - regulated thermal process with an
immediate drop of the part surface temperature to the boiling point of liquid, the real HTC should be taken into account [5, 6, 7]. An effective HTCs cannot be used for calculation of the surface cooling curve during a self - regulated thermal process. As known, the surface cooling curve is responsible for delaying or accelerating the phase martensitic transformation during quenching. Depending on the part surface cooling curve, correspondently the current and residual stresses are formed. It means that residual stresses during the self - regulated thermal process and quench recipes can be calculated only using the real HTCs.
References:
5) Kobasko, N.I, Steel Quenching in Liquid Media Under Pressure, Kyiv, Naukova Dumka, 1980, 206 pages.
[6] Kobasko, N.I, Duration of the transient nucleate boiling process and its use for development of new technologies, In a Book "Film and Nucleate Boiling Processes, ASTM Stock #: STP1534", ASTM International, W. Conshohocken, 2012,
pp. 103 - 125.
[7] Kobasko, N. I., Transient Nucleate Boiling as a Law of Nature and a Basis for Designing of IQ technologies, Proceedings of the 7th ASME/WSEAS International Conference on Heat Transfer, Thermal Engineering and Environment (HTE ‘09), Moscow, August 20– 22, 2009, pp. 67–75.
Ref Paper: Verification of Presence of Direct Convection Mode of Heat Transfer
During Intensive Quenching in High Velocity IQ System
N.I.KOBASKO, , M.A.ARONOV, , J.A.POWELL and , B.L.FERGUSON
www.wseas.us/e-library/conferences/2014/.../ICMS/ICMS-19.pdf
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