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Abstract: Carbon dioxide is again becoming an important refrigerant. While the thermophysical properties are well known there is a lack of data on its heat transfer characteristics. In this study, heat transfer coefficients for nucleate boiling of carbon dioxide are determined using a standard apparatus for the investigation of pool boiling based on a set-up from Karlsruhe [D. Gorenflo, J. Goetz, K. Bier. Vorschlag für eine Standard-Apparatur zur Messung des Wärmeübergangs beim Blasensieden. Wärme-und Stoffübertragung 16 (1982), 69–78; J. Goetz, Entwicklung und Erprobung einer Normapparatur zur Messung des Wärmeübergangs beim Blasensieden. Dissertation Universität Karlsruhe (1980).] and built at our institute. Electrically heated horizontal cylinders with an outer diameter of 16mm and a length of 100mm are used as heating elements. Measurements with constant heat flux are performed for different wall materials and surface roughnesses. The heat transfer is investigated within the pressure range of 0.53≤ p ≤1.43MPa (0.072≤ p/p c ≤0.190) and a temperature range of −56≤ t ≤−30°C, respectively. Heat fluxes of up to 80,000Wm−2 are applied. The influences of wall material and roughness on the heat transfer coefficient are evaluated separately. The obtained coefficients are compared to generally accepted correlations and to experimental results of other authors, who used similar configurations with copper tubes and carbon dioxide. These are the only previous experimental data, which could be found. Results for copper, stainless steel and aluminium as wall materials are presented. [Copyright &y& Elsevier]

Abstract: The inclusion of an expander with work recovery provides two advantages for transcritical CO2 refrigeration cycles: the COP is improved and the exhaust pressure of the main compressor is lowered. Several designs of expanders have been proposed for this application and some prototypes have been tested already. In our laboratory a three-stage expander has been developed, which replaces the throttle valve of the normal refrigeration cycle and expands into the two-phase region. For optimum integration into the overall system it is proposed to install a vapour-liquid separator between the second and third stage of expansion. The vapour is guided back to the third expander stage whereas the liquid is supplied to the cooling stations via thermostatic or electronic expansion valves. [Copyright &y& Elsevier]