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In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO2) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same time, there is a large spike in specific heat which can yield high heat transfer coefficients and heat capacity rates. These unique properties have made sCO2 an attractive working fluid in next generation power and HVAC&R technologies. Microchannel heat exchangers are being used to safely and efficiently utilize the high pressure fluid in these applications. However, prior investigation of heating of supercritical CO2 has primarily focused on circular, uniformly heated channels at relatively low heat flux for nuclear power applications. Thus, it is unclear if models and correlations developed from large circular tube data can be scaled down to the smaller, non-circular channels, with non-uniform heating. In the present work, a methodology is developed to experimentally characterize heat transfer for multiple parallel microchannels with a hydraulic diameter of 0.75 mm and aspect ratio of 1. Experiments are conducted over a range of heat fluxes (20 ≤ q” ≤ 40 W cm−2), mass fluxes (500 ≤ G ≤ 1000 kg m−2 s−1), reduced pressure (1.03 ≤ P[subscript R] ≤ 1.1), and inlet temperatures (20 ≤ T[subscript in] ≤ 100°C) in a parallel square microchannel test article with a single-wall constant heat flux boundary condition. Local and average heat transfer coefficients are experimentally measured and the results are compared to previously developed correlations. The predictive capabilities for the supercritical models were poor, with the lowest mean absolute percent error (MAPE) of 55.3% for the range of bulk fluid temperatures, heat fluxes, and mass fluxes. Interestingly, subcritical correlations were also investigated and yielded much lower MAPE than 80% of the supercritical correlations even though the effects of variable fluid properties were not taken into account. The subcritical correlations did not incorporate property ratios to account for the variability in fluid properties; in some supercritical correlations it was found to add additional uncertainty for the case of the present study. The effects of buoyancy and flow acceleration were also evaluated. Based on dimensionless criteria, buoyancy was expected to play a role in heat transfer, especially when the bulk fluid temperature is below the pseudocritical temperature. However, the relative importance of flow acceleration was inconclusive. Despite the apparent importance of buoyancy effects, heat transfer was not degraded, as would be expected in larger, circular, uniformly heated tubes. The mixed convection could be inducing a density driven swirling with the stratification of low-density fluid near the top (unheated). This would ultimately improve the heat transfer at the bottom portion of the test section channels. Therefore, the flow geometry and the non-conventional heated boundary could be improving the heat transfer even with buoyancy driven effects under supercritical conditions.
An experimental analysis was conducted on a single circular tube heat exchanger using supercritical carbon dioxide as the working fluid. The heat exchanger was operated in two different orientations: vertically upward and downward. The experimental facility utilized two different mass flow rates: a low flow rate of 0.0183 kg/s and a high mass flow rate 0.03 kg/s, three system pressures: 7.5, 8.1 and 10.2 MPa and two different heat inputs: a low heat input of 540 W and a high heat input of 955 W. Inlet temperatures to the test section were varied from 20-55 °C. Thermocouples on the surface of the test section recorded the wall temperatures. Then, a one dimensional heat transfer analysis was conducted to calculate inner wall temperatures. Afterwards, the bulk temperature was calculated using a constant heat flux approximation and an energy balance on a differential control volume. Finally, the local heat transfer coefficient between the bulk and inner wall was calculated. Results showed that typically, for the 7.5 and 8.1 MPa cases, as the temperature reached the pseudocritical point, there was a heat transfer deterioration followed immediately by a substantially large heat transfer enhancement. After the critical temperature was reached, however, the heat transfer coefficient decreased. The results showed that the heat transfer coefficient, deterioration and enhancement were the greatest with the 7.5 MPa case and the downward orientation. Buoyancy effects seem to be present and have a significant impact on the heat transfer coefficient. In general, if heat exchangers are to be designed to be used with supercritical fluids, they should be designed, along with other important components, to be operated as close to the critical point as possible as well as have a downward flowing orientation to maximize heat transfer potential. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152835
Heat transfer inside microscale geometries is a complex and a challenging phenomenon. As supercritical fluids display large variations in their properties in the vicinity of the critical point, their usage could be more beneficial than traditional coolants. This numerical study, in two parts, primarily focuses on the physics that drives the enhanced heat transfer characteristics of carbon dioxide (CO2) near its critical state and in its supercritical state. In the first part of the study, the flow of supercritical Carbon Dioxide (sCO2) over a heated surface inside a microchannel of hydraulic diameter 0.3 mm was studied using three-dimensional computational fluid dynamics (CFD) model. The temperature of the heated surface was then compared and validated with available experimental results. Also, the heat transfer coefficients were predicted and compared with experiments. Additionally, the acceleration and pressure drop of the fluid were estimated and it was found that the available correlations for conventional fluids failed to predict the flow characteristics of the CO2 due to its supercritical nature. In the second part of the analysis, a relatively new phenomenon known as the Piston Effect (PE), also known as the fourth mode of heat transfer, was studied numerically inside a microchannel of depth 0.1 mm using a two-dimensional CFD model, and it was found that the adiabatic thermalization caused by PE was significant in microgravity and terrestrial conditions and that the time scales associated with the PE are faster than the diffusion time scales by a factor of 5 to 6400. In addition, this study revealed the presence of PE in laminar forced convective conditions. A new correlation was developed to predict the temperature raise of the bulk fluid that is farthest from the heated surface.
Broad coverage of buoyancy effects on convective heat transfer in duct flows. Provides an immense quantity of experimental data deriving from active and excellent research in the USSR. Acidic paper. Annotation copyright Book News Inc. Portland, Or.
With the continuous miniaturization of integrated circuit chips over the last decade, there has been a steady increase in power density of electronic devices giving rise to the need for aggressive and effective cooling systems. The rapid growth of microfabrication technology has led to the development of microelectromechanical system (MEMS) based microchannels, which can be used as miniaturized heat exchangers capable of cooling high power electronic devices. Simultaneously, studies show that carbon dioxide in supercritical state (sCO2) has an excellent ability for cooling applications due to its exceptional thermophysical properties near critical point. Implementation of pin fins in heat transfer systems requires a thorough understanding of both fluid dynamics and heat transfer mechanisms. Thus, the aim of this research is to extend the current fundamental knowledge about both thermal and hydraulic performance of supercritical carbon dioxide (sCO2) in microchannel pin fin heat sinks.
Within this project, the heat transfer and pressure drop characteristics of CO2 (R-744) were investigated at supercritical pressures (cooling) and subcritical pressures (evaporation). The measurements were carried out with a microchannel type (MPE) tube with 25 parallel ports and a port diameter of 0.787 mm. The experimental data collected confirm that CO2 offers high heat transfer coefficients at supercritical pressures. The comparison of these data with common correlations shows good correspondence. The comparison of the measured pressure drop data with calculation models is satisfactory as well. At evaporation the situation is not as clear. The experiments at high mass fluxes have shown a strongly decreasing heat transfer coefficient from a certain vapour fraction upwards. A comparison with a calculation model for dry-out has shown that this drop has to be expected. On the contrary, at low mass fluxes no influence of the velocity was detected. None of the heat transfer calculation models investigated takes this phenomenon into account. The two-phase pressure drop correlations yield too low values in general, but for a first estimation models from the literature can be taken.