The paper deals with an experimental study of flow boiling heat transfer coefficients in horizontal tubes. The experiments are performed in an electrically heated thermosiphon loop with two different evaporator tube diameters. Measurements are performed with constant heat flux along the tube and at relatively low heat and mass fluxes. The gravitational forces will influence the flow pattern and incomplete wetting of the tube walls will occur in a great part of the measurements. The tests are performed with the pure substances R22, R142b, R32 and R134a as well as several mixtures of R22/R142b and R32/R134a. The two tube dimensions tested have quite different behavior in spite of having the same length and refrigerant distribution system.
A Chen-type correlation is chosen to predict the local heat transfer coefficients. The influences on the heat transfer coefficient of incomplete wetting are correlated by correcting the enhancement factor by a function of the Froude number. The correlations obtained for the two tube diameters and with pure refrigerants are then used as basis for the evaluation of the refrigerant mixture tests.
The degradation of the heat transfer coefficient for mixtures is discussed. The measured mixture heat transfer coefficients are well estimated by using Jung’s correlation for mixtures, if Jung’s enhancement factor is replaced by the enhancement factors evaluated from the tests and the real mixture transport properties are used or by the theoretically approached correlation by Granryd.