An experimental study of flow and heat transfer of supercritical carbon dioxide in multi-port mini channels under cooling conditions (original) (raw)
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2014
Forced convection heat transfer in supercritical carbon dioxide (SCO2) was investigated experimentally in a horizontal circular tube with an inner diameter of 8.7 mm. The experiments were performed by varying the inlet fluid temperature, system pressure, wall heat flux, and mass flow rate. The corresponding Reynolds number at the inlet was between 20000 and 50000. Nusselt number at each section in the tube was obtained to investigate the influence of the experimental parameters on the forced convection heat transfer in the testing tube. The obtained heat transfer results were then compared with widely used empirical correlations to show their prediction accuracy for the experimental conditions tested.
An Overview of Heat Transfer Correlations for Supercritical CO2 Cooling in Macro-channels
DEStech Transactions on Engineering and Technology Research
Recently, due to the rapid development of the supercritical water-cooled reactor, trans-critical CO 2 heat pump and other technologies, there has been an increasing interest in accurate prediction of convective heat transfer coefficient of supercritical CO 2 under cooling conditions. The main aim of this paper is to give a concise review of heat transfer correlations for supercritical CO 2 cooling in macro-channels (inner diameter larger than 3 mm). Before that the fundamental knowledge about convective heat transfer is first addressed, with an emphasis on heat transfer coefficient.
Finned compact gascoolers made of flat extruded aluminium tubes with internal mini/micro channels constitute a promising technology and the optimisation of this type of heat exchangers is therefore one of the main research goal for the development of refrigerating systems operated with carbon dioxide transcritical cycles. Unfortunately relevant discrepancies exist among different phenomenological correlations which have been suggested throughout the last years for predicting the convective heat transfer in mini/micro channels of carbon dioxide under cooling conditions close to the critical point. This work aims to compare different phenomenological correlations with the numerical results due to some turbulent closure models, which have been adopted in order to properly take into account the effects due to density fluctuations appearing when the thermo-physical properties strongly depend on temperature. The numerical predictions show that the effects due to density fluctuations are smaller than it could have been initially supposed and that the heat transfer impairment for mini/micro channels, which some experiments seem to highlight, is not completely explained by conventional models. The calculations are not exhaustive because of the high scattering of the numerical results, but a moderate preference for a specific phenomenological correlation has been outlined.
An experimental study of flow boiling characteristics of carbon dioxide in multiport mini channels
Applied Thermal Engineering, 2004
Due to its unique combination of ecological and personal safety, carbon dioxide (CO 2 ), a natural fluid, is becoming one of the most promising alternative refrigerants for air-conditioning and refrigeration systems. This paper presents an experimental study of boiling heat transfer and pressure drop of CO 2 flowing in a multi-port extruded aluminum test section, which had 10 circular channels, each with an inner diameter of 1.31 mm. CO 2 was heated by hot water flowing inside copper blocks that were attached at both sides of the test section. Temperatures at the outer surface of the test section were measured using 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured using twelve heat flux sensors to obtain the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO 2 at the inlet and outlet of the test section were measured using two K-type thermocouples. The measurements were performed for pressures ranging from 3.99 to 5.38 MPa, inlet temperatures of CO 2 from )3.08 to 16.96°C, heat fluxes from 10.1 to 20.1 kW/m 2 , mass velocities from 131.4 to 399.0 kg/m 2 s, and vapor quality from 0.0 to 1.0. The results indicate that pressure drop along the test section is very small, two-phase CO 2 flow exhibits a higher heat transfer coefficient than that of the single-phase liquid or vapor flow. It is also shown that the mass velocity and the applied heat flux have significant effects on flow boiling heat transfer characteristics. The measured heat transfer coefficients were compared with correlations reported in the literature and large discrepancies are observed.
2021 20th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (iTherm)
Water is a common coolant for many heat transfer applications, yet, it has many disadvantages relative to other liquid coolants, such as high corrosiveness, high electrical conductance, and high pressure drop characteristics. Carbon dioxide slightly above its supercritical pointknown as supercritical carbon dioxide (sCO2)exhibits excellent thermo-physical properties, such as high specific heat, thermal conductivity, and high heat capacity than many other gases, such as air. Also, sCO2 is less corrosive and experience less pressure drop than water. In this study, heat transfer characteristics of sCO2 and water were compared for a 300-μm square microchannel. Computational studies were made for both laminar and turbulent flow regimes, and it was found that at a mass flux of 5,555 kg/m 2 s, sCO2 experienced a pressure drop of 27.81 kPa compared to 51.77 kPa for water. Nusselt numbers were compared with available empirical correlations, such as Dittus-Boelter and Gnielinski, for both water and sCO2, and it was found that they failed to accurately predict the Nusselt number for sCO2 obtained through CFD simulation. At higher mass fluxes the local Nusselt number for sCO2 were higher than those for water as the flow became turbulent for sCO2 due to its low dynamic viscosity.
This paper investigated the flow and heat transfer characteristics of supercritical carbon dioxide (SC-CO2) and supercritical water (SC-H2O) in horizontal microchannels using a CFD approach. Model of a straight circular pipe of stainless steel with internal and external radii of, and , respectively and a heated length of 55 mm were considered. For the simulation, carbon dioxide and water at supercritical pressures of 9.5 MPa and 22.07 MPa respectively were used, while uniform heat was applied on the outer surface of the tube. The thermodynamic properties for both fluids were obtained from the NIST Chemistry Web book. The simulated temperature and heat transfer coefficient variation were compared with experimental results from literature. In general, the simulation results were close to the experiment. Both the simulation and experimental results showed that the wall temperature increased along the tube length. As expected, the heat transfer coefficient values for both supercritical fluids decreased as the length of the tube. This was due to the reason that a maximum and dominant convection heat transfer occurred at the entrance of the heated section of the pipe. The results from this study could assist in decisions regarding the use of supercritical fluids in industries which involve heat transfer.
Heat and Mass Transfer, 2015
The heat transfer characteristics of supercritical carbon dioxide (S-CO 2) turbulent flow were investigated experimentally in a horizontal circular pipe with an inner diameter of 8.7 mm. Local convection coefficients and Nusselt numbers of the flow were obtained at different locations along the pipe with a constant heat flux ranging from 16 to 64 kW/m 2. Experiments were performed for fluid mass flow rate ranging from 0.011 to 0.017 kg/s, an inlet fluid temperature ranging from 24 to 28 °C, and a flow pressure ranging from 7.5 to 9.0 MPa to investigate their effects on the convection heat transfer in the pipe. Both enhancement as well as deterioration in the heat transfer coefficient was observed for the flow conditions examined in this work. Experimental results were then compared with the widely used empirical correlation for pipe flow. Three commonly used buoyancy parameters were utilized to investigate their applicability in the present test conditions. Results indicate that all the parameters show a strong presence of buoyancy effects in the present test conditions. The trend and magnitude of these parameters, however, do not agree with the trend and magnitude of heat transfer enhancement and deterioration along the pipe. List of symbols A Area (m 2) Bu Buoyancy parameter c p Specific heat (kJ/kg K) D Diameter (m) g Gravity (m/s 2) Gr Grashof number; gβ(T w −T b)D 3 v 2 b * Reza Sadr
Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels
International Journal of Refrigeration, 2007
Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO 2. The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20e40 kW m À2 , a mass flux range of 200e600 kg m À2 s À1 , saturation temperatures of 10, 0, À5, and À10 C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO 2 is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO 2 was developed with 8.41% mean deviation.