Prediction of flow boiling heat transfer of carbon dioxide inside small diameter tubes

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.This paper reviews recent experimental work on intube flow boiling of carbon dioxide and describes the updated flow boiling heat transfer model and flow pattern map proposed by Cheng, Ribatski, Wojtan and Thome [1] for evaporation of carbon dioxide in small diameter horizontal tubes. This new flow boiling heat transfer model predicted 76% of the CO2 database taken from the literature within ±30%. The new model and map are applicable to: tube diameters from 0.8 to 10 mm, mass velocities from 170 to 570 kg/m2s, heat fluxes from 5 to 32 kW/m2 and saturation temperatures from -28°C to 25°C (reduced pressures from 0.21 to 0.87). Simulations of the flow pattern map and heat transfer model are also shown.cs201

Fundamental issues, technology development and challenges of boiling heat transfer, critical heat flux and two-phase flow phenomena with nanofluids

This paper presents a comprehensive and critical review of studies on nucleate pool boiling heat transfer, flow boiling heat transfer, critical heat flux (CHF) and two-phase flow phenomena with nanofluids. First, general analysis of the available studies on the relevant topics is presented. Then, studies of physical properties of nanofluids are discussed. Next, boiling heat transfer, CHF phenomena and the relevant physical mechanisms are explored. Finally, future research needs have been identified according to the review and analysis. As the first priority, the physical properties of nanofluids have a significant effect on the boiling and CHF characteristics but the lack of the accurate knowledge of the physical properties has greatly limited the studies. Fundamentals of boiling heat transfer and CHF phenomena with Nanofluids have not yet been well understood. Flow regimes are important in understanding the boiling and CHF phenomena and should be focused on. Two phase pressure drops of nanofluids should also be studies. Furthermore, economic evaluation of the enhancement technology with nanofluid should be considered for the new heat transfer enhancement technology with nanofluids. Finally, applied research should be targeted to achieve an enabling practical heat transfer and CHF enhancement technology for engineering application with nanofluids

Micro-channel flow boiling heat transfer of R-134a, R-236fa, and R-245fa

The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 mu m, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000 kg/m(2) s, and heat fluxes up to 200 kW/m(2). Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator : preliminary analysis and performance comparisons

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.No abstract available, please open full text articlevk201

Two-phase flow boiling in microchannels for cooling of microelectronics

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Recent progress in the study of two-phase flow boiling in single microchannels and multi-microchannels and the challenges posed by microelectronics cooling are reviewed. Experimental investigations have shown that uniform and transient heat fluxes and local hot-spots can be well handled by micro-evaporators. Furthermore, advances in the prediction of two-phase pressure drops and flow pattern transitions in microchannels are presented. The development of mechanistic models specific to the important flow regimes (elongated bubble flow and annular flow) opens the possibility for integrated flow pattern-based prediction methods. Presently two such methods are combined together to capture the transition between these two regimes and thus better predict the trends in the local flow boiling heat transfer process.pm201

Two-phase flow boiling in microchannels for cooling of microelectronics

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Recent progress in the study of two-phase flow boiling in single microchannels and multi-microchannels and the challenges posed by microelectronics cooling are reviewed. Experimental investigations have shown that uniform and transient heat fluxes and local hot-spots can be well handled by micro-evaporators. Furthermore, advances in the prediction of two-phase pressure drops and flow pattern transitions in microchannels are presented. The development of mechanistic models specific to the important flow regimes (elongated bubble flow and annular flow) opens the possibility for integrated flow pattern-based prediction methods. Presently two such methods are combined together to capture the transition between these two regimes and thus better predict the trends in the local flow boiling heat transfer process.pm201

Boiling of Two Zeotrope Mixtures and R–502 Inside a Plain Horizontal Tube.

Intube flow boiling experiments for two new zeotropic refrigerant mixtures and R-502 have been carried out at three saturation temperatures over a wide range of local vapor qualities. The zeotropic mixtures tested were HP80 and HP62. Local flow boiling coefficients for HP62 are slightly larger than those for R-502 under the same test conditions while those for HP80 are slightly smaller. The heat transfer performances of the three fluids are generally quite accurately predicted by an existing flow boiling correlation modified here to include a nucleate boiling mixutre equation. However, in the post dryout regime at high vapor qualities the measured coefficients are much lower than those predicted

Comparisons of experimental results and prediction methods of supercritical CO2 cooling heat transfer and pressure drop in macro- and micro-scale channels

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Comparisons of heat transfer and pressure drop experimental data and correlations for supercritical CO2 cooling are presented in this article. First, the physical and transport properties of CO2 at supercritical conditions are discussed and then their influence on heat transfer and pressure drop. Then, comparison and analysis relative to the available heat transfer and pressure drop correlations for supercritical CO2 cooling were done where possible. Noting the lack of all pertinent experimental details required to use the data published in many of the available studies, comments are given on how to reduce and present supercritical CO2 experimental data properly in the future. Simulations by the available heat transfer correlations were performed and the predicted results were compared with each other. Based on the comparisons and analysis, it is recommended that further efforts be made to develop improved heat transfer methods for supercritical CO2 cooling based on a more accurate database in the future. To achieve this, more careful experiments should be done in both macro- and microchannels over a wide range of test parameters, including the effect of oil. In addition, several experimental studies show that the Blasius equation works well for pressure drop of CO2 cooling in the supercritical region. More careful experimental data are still needed to further validate this conclusion.vk201

R–502 and Two Near-Azeotropic Alternatives. Part I: Intube Flow Boiling Tests.

Flow boiling experiments for two new non-azeotropic, three-component refrigerant mixtures (HP80 and HP62) and R-502, the azeotropic refrigerant binary mixture they are replacing, have been carried. Comparative results show that local flow boiling heat transfer coefficients for HP62 are slightly larger that those for R-502 under the same test conditions while those for HP80 are on average slightly smaller. The heat transfer performances of the three fluids are accurately predicted by an existing flow boiling correlation modified here to include a nucleate boiling mixture correlation

Online Measurement of Oil Concentrations of R134a–Oil Mixtures with a Density Flowmeter

A very high accuracy, straight vibrating tube type of density flow meter has been used online to measure oil concentraiton of flowing R-134a/oil mixtures. The calibrations covered oil concentrations from 0-6 wt.% oil over the temperature range from -9.4 to 5.9°C. The oil concentrations were correlated statistically as a function of density, temperature and liquid compressibility to an average error of 0.09 wt.% oil with a 95% confidence limit of 0.21 wt.% oil. In addition, a simplified method not requiring calibration tests was developed for general industrial application of the density flowmeter to any refrigerant-oil mixture combination, with an average error of 0.22 wt.% oil and 95% confidence limit of 0.67 wt.% oil for the present data set