In-tube condensation of a zeotropic mixture : Impact of uncertainties in the experimental design and in mixture properties on heat transfer coefficients

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Berger, Clemens ; Schnelting, Leonard ; Raabe, Gabriele ; Welzl, Matthias ; Heberle, Florian ; Brüggemann, Dieter:
In-tube condensation of a zeotropic mixture : Impact of uncertainties in the experimental design and in mixture properties on heat transfer coefficients.
In: Experimental Thermal and Fluid Science. Bd. 176 (2026) . - 111760.
ISSN 1879-2286
DOI: https://doi.org/10.1016/j.expthermflusci.2026.111760

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Abstract

Zeotropic mixtures are of growing interest for thermodynamic cycle applications due to an efficiency improvement by glide matching. One key factor in the development of mixture applications is research on heat transfer characteristics as generally lower heat transfer coefficients in comparison to pure fluids could counteract higher efficiencies. Due to the complex fluid behaviour of mixtures simplifications are applied in the evaluation of measurements, affecting the resulting heat transfer coefficients. This study investigates the condensation heat transfer for a zeotropic mixture R134a/R1233zd(E) with focus on the impact of different methodologies for the evaluation of measurement data. The main aspect is the fluid temperature definition by either measurement or calculation, which has a great influence on the determination of heat transfer coefficients. Additionally, the impact of available mixture properties is investigated by molecular simulations and fitting of binary interaction parameters. A comparison with the standard approximated interaction parameters is made with respect to composition and state point calculations. Experiments are conducted in a horizontal tube with an inner diameter of 32 mm, where a partial condensation in stratified flow occurs. The results challenge the common assumption of thermodynamic equilibrium as the measured local saturation temperature deviates from its calculated value. Heat transfer coefficients for two different compositions are reported in a range of 500 W/(m^2 K) to 1100 W/(m^2 K), showing clear trends against vapour quality, mass flux and pressure. The differentiation between temperature measurement and calculation has a large impact that clearly outweighs reported measurement uncertainties from the sensor equipment. Differences in the resulting heat transfer coefficients partly exceed 20 %, depending on the operating conditions. The deviation is further increased under consideration of fitted interaction parameters, even though the impact is comparable smaller and lies in the range of measurement uncertainties. The obtained results clarify possible sources of deviations between different experimental studies.