Korelacija za faktor trenja v kapilarni cevi brez njene hrapavosti
Obstoječi modeli kapilarnih cevi se močno zanašajo na uporabo korelacij za faktor trenja, ki omogočajo vnos relativne hrapavosti notranje površine. Merjenje notranje hrapavosti kapilarnih cevi pa je zamuden proces, ki zahteva uporabo drage merilne opreme. V tej raziskavi smo razvili korelacijo za faktor trenja, ki ne potrebuje podatka o hrapavosti kapilarne cevi, zagotavlja pa primerljivo točnost, kot obstoječe korelacije.
Capillary tube models frequently rely on friction factor correlations, which are functions of the Reynolds number, and the tube’s relative surface roughness. However, this study shows that explicitly correlating the friction factor with the relative surface roughness is unnecessary, at least when the latter is lower than 0.0025. To test this a correlation was fitted directly onto friction factor measurements obtained in an adiabatic flow of R600a, through nine capillary tubes from three manufacturers. The friction factor was correlated exclusively as a function of the Reynolds number, meaning that the impacts of the relative surface roughnesses on the friction factor were averaged, and included only implicitly. This correlation, and an existing correlation, which allows the specification of the relative surface roughness, were then implemented in a capillary tube model. The accuracy of the capillary tube model, using either the developed or existing correlation was then evaluated by comparing the results to experimental data. No significant decrease in the accuracy of the model was observed when the relative surface roughnesses, ranging from 0.0012 to 0.0025, were not individually specified for each capillary tube. To further verify this, the procedure was repeated on a larger database of friction factor and mass flow rate measurements, which also consisted of data from independent studies. Similar conclusions were drawn, as the average accuracy of the model differed only by 1 to 3% among different studies, and not always in favor of the correlation where the relative surface roughnesses could be specified.