I think I read somewhere that the heat transfer coefficient increases over the evaporator length until it reaches the dry out point. If this is true, I think it only holds for a specific flow regime, however, maybe there is a general rule?
Thank you for your reply!
Let say if in the length of evaporator flow regime changes from liquid only to annular, in that case there is axial increase in heat transfer even if you have only the annular regime in the evaporator, then also due to evaporation from the thin liquid film heat transfer coefficient can increase axially.
Thank you for your answer, Balkrishna. Do you know any references regarding specifically this topic?
The heat transfer coefficient in two-phase flow is highly dependent on the local conditions. In general, the heat transfer coefficient increases with increasing vapor quality (i.e. mass fraction in vapor phase) due to the increase in flow velocity. Also, as the vapor quality increases, the nucleate boiling contribution usually decreases and the two-phase evaporation part increases (in sum leading to an increased heat transfer coefficient). As a further complication, the nucleate boiling contribution to the heat transfer coefficient is a strong function of the local heat flux. In order to quantify this behavior you may check out e.g. the Chen correlation for vertical tubes or the Shah correlation for horizontal tubes. Both of hese include the comined effect of the nucleate boiling and two-phase flow evaoration contributions to the heat transfer coefficient.
Cristoph you can see the book titled as flow boiling and condensation authored by J G Collier and J Thome. It might be helpful in understanding you problem
There is a very useful book (writen by John R. Thome) on the Wolverine tube web page (http://www.wlv.com/heat-transfer-databook/). I am sure you will find the answer there.
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