Develop a correlation of convective coefficient in a vertical evaporator with boiling processes inside pipes where the flow is stagnant
Advancing the chemical engineering fundamentals
Thermodynamics (T2-1P)
Keywords: convective, boiling, heat
Actually there are many correlations to determinate the convective coefficient in boiling processes, condensation and processes without change phases. Nevertheless, a reliable correlation does not exist for vertical evaporators with boiling processes inside pipes where the flow is stagnant, reason which an average correlation of convective coefficient is require for processes inside pipes. This coefficient involve three processes: First, where there is a sensitive heat transference (Sub-cooling zone). Second there is latent heat transference (Stagnant boiling inside pipes) and third one there is sensitive heat transference (Overheating zone).
In order to determinate the number of experiment for the correlation was used a factorial design and them a statistical tool in order to reduce the number of experiment. On the other hand, the selection of the parameters involved in the correlation was relied in studies developed before where was determinated that the dimensionless parameters of a boiling processes and condensation are getting with utilization of Buckingham Theorem. For both processes the convection coefficient can depend for the difference between surface temperature and saturation, the body force that arises from the difference between density liquid-steam, latent heat, superficial tension, typical length and the properties thermo physical from the liquid or steam. Additional for these particular case was added the mass flow, internal diameter of pipes and the level of liquid in the same ones.
For the establishment of the define correlation was realize a lineal regression of the data, In which the exponents were modified for the diverse of dimensionless groups resting on existing correlations for reservoir boiling processes and forced convection for internal flow. Also, for every exponents supposition the quotient calculated between the experimental convective coefficient and the obtained one by the combination of π groups. (proportional constant) which must be a constant value for the diverse conditions, in such a way that having incorporated the above mentioned value in the correlation the equality was fulfilled to the diverse conditions. Nevertheless, for a relation of groups a particular behavior was observed. The quotient between the experimental convective coefficient and the obtained for the combinations of π groups in the differents conditions changes. However the above mentioned quotient keep constant value under conditions where the level of liquid inside pipes from the evaporator is equal, of such form an expression was realized where there is obtained a relation of this constant from the dimensionless group that involves water level in order to include this expression and obtain a definite correlation that represents the convective coefficient.
Finally, in order to assure the predictive power of the correlation there were realized a series of random experiments from which the convective coefficient was calculated, being the maximum deviation reported 1,70 %.
Presented Monday 17, 13:30 to 15:00, in session Thermodynamics (T2-1P).
