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In the paper a method developed earlier by authors is applied to calculations of pressure drop and heat transfer coefficient for flow boiling and also flow condensation for some recent data collected from literature for such fluids as R404a, R600a, R290, R32,R134a, R1234yf and other. The modification of interface shear stresses between flow boiling and flow condensation in annular flow structure are considered through incorporation of the so called blowing parameter. The shear stress between vapor phase and liquid phase is generally a function of nonisothermal effects. The mechanism of modification of shear stresses at the vapor-liquid interface has been presented in detail. In case of annular flow it contributes to thickening and thinning of the liquid film, which corresponds to condensation and boiling respectively. There is also a different influence of heat flux on the modification of shear stress in the bubbly flow structure, where it affects bubble nucleation. In that case the effect of applied heat flux is considered. As a result a modified form of the two-phase flow multiplier is obtained, in which the nonadiabatic effect is clearly pronounced. Nomenclature A -cross section area, m 2 B -blowing parameter Bo -boiling number, B0 = q Gh LG C -mass concentration of droplets in two phase core C f -friction factor Con -confinement number cp -specific heat, J/kg K d -diameter, m D -deposition term, kg/ms; channel inner diameter, m E -entrainment term, energy dissipation, kg/ms G -mass flux, kg/m 2 s g -gravitational acceleration, m/s 2 h -enthalpy, J/kg hLG -specific enthalpy of vaporization, J/kg h lv -specific enthalpy of vaporization,J/kg mG -mass of vapour phase mL -mass of liquid phasė m -mass flux,kg/s P -perimeter, m p -pressure, Pa Pr -Prandtl number q -density of heat flux, W/m 2 qw -wall heat flux, W/m 2 Re -Reynolds number, Re = Gd µ L ReL -Reynolds number liquid film only ReL = G d (1−x) µ L s = uG/uL -slip ratio u -velocity, m/s u + = u/u h -reduced speed w -velocity, m/s v0 -transverse velocity, m/s x = m G m G +m L

In the paper presented are the results of calculations using authors own model to predict heat transfer coefficient during flow boiling of carbon dioxide. The experimental data from various researches were collected. Calculations were conducted for a full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house model. The results show the importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels as well as the influence of reduced pressure. The calculations were accomplished to test the sensitivity of the heat transfer model with respect to selection of the appropriate two-phase flow multiplier, which is one of the elements of the heat transfer model. For that purpose correlations due to Müller-Steinhagen and Heck as well as the one due to Friedel were considered. Obtained results show a good consistency with experimental results, however the selection of two-phase flow multiplier does not significantly influence the consistency of calculations.Keywords: Heat transfer coefficient; Flow boiling; Carbon dioxide; Minichannels Nomenclature B-blowing parameter Bo -boiling number, q/GhLG

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