The conventional refrigerants have considerable ozone depleting effect (CFC/HCFC) and global warming impact (HFC). Carbon Dioxide (CO2) is being investigated as an alternative refrigerant for vapor compression systems. In addition to its environmental benefits, Carbon Dioxide offers certain attractive thermal characteristics such as small surface tension, small liquid viscosity and large refrigerant capacity. Furthermore, when used with micro channels CO2 heat exchangers provide additional advantage of high compaction, low weight/low volume, while yielding excellent thermal performance. The objective of the present work was to study the heat transfer and pressure drop characteristics of supercritical CO2 gas cooling process in microchannels. A 10 ports microchannels tube with ID of 0.79mm was tested for the pressure range of 8 to 10MPa and mass flux range of 300 to 1200 kg/m2 s. As expected, mass flux has a significant influence both on the supercritical heat transfer and pressure drop coefficients. Pseudo-critical temperature (temperature at which the specific heat has maximum value for the given pressure) is found to play an important role in the CO2 heat exchanging process as well. Conventional forced convection heat transfer correlations fail to accurately predict the heat transfer coefficients of supercritical CO2 with deviations as much as 50% from experimental data, especially near pseudo-critical temperature. As the gas cooling pressure increases, the pressure drop decreases, which is due to the lower viscosity & higher density. Employing average specific heat along the entire tube length, a semi-empirical correlation was developed to predict the supercritical gas cooling process of CO2 in microchannels, within an error of 20%.

The performance of a 300 kW organic Rankine cycle (ORC) prototype was experimentally investigated for low-grade waste heat recovery in industry. The prototype employed a specially developed single-stage radial turbine that was integrated with a semi-hermetic three-phase asynchronous generator. R245fa was selected as the working fluid and hot water was adopted to imitate the low-grade waste heat source. Under approximately constant cooling source operating conditions, variations of the ORC performance with diverse operating parameters of the heat source (including temperature and volume flow rate) were evaluated. Results revealed that the gross generating efficiency and electric power output could be improved by using a higher heat source temperature and volume flow rate. In the present experimental research, the maximum electric power output of 301 kW was achieved when the heat source temperature was 121 °C. The corresponding turbine isentropic efficiency and gross generating efficiency were up to 88.6% and 9.4%, respectively. Furthermore, the gross generating efficiency accounted for 40% of the ideal Carnot efficiency. The maximum electric power output yielded the optimum gross generating efficiency.

This paper provides a comprehensive review of existing correlations for supercritical heat transfer of CO 2 in microchannels, as well as a comparison of these correlations with experimentally measured data. Based on the experimental data, a new semi-empirical correlation is developed to predict the gas cooling heat transfer coefficient of supercritical CO 2 in microchannels, within an error of 15% for most (91%) of the presented experimental data that were obtained in an 11-port microchannel tube with an internal diameter of 0.79 mm and with a pressure range of 8 to 10 MPa and mass flux range of 300 to 1200 kg/m 2 s.

Carbon Dioxide (CO2) is being investigated as an alternative refrigerant for vapor compression systems. In addition to its environmental benefits, Carbon Dioxide offers certain attractive thermal characteristics such as small surface tension, small liquid viscosity and large refrigerant capacity. Furthermore, combination with microchannels provides CO2 heat exchangers that have low weight, high compaction and high heat transfer coefficient. But certain oil (e.g., lubricate oil for compressor) will be carried into the vapor compression system, which usually has negative effect on heat transfer and pressure drop. The objective of the present paper is to study the effect of oil addition on heat transfer coefficient and pressure drop in supercritical gas cooling process in microchannels. Experiments addressed effect of three different types of oil (two immiscible and one miscible) at various oil concentrations ranging from 0% (no oil) to 5% by weight. As expected, oil addition has significant negative effect on heat transfer coefficients. At higher oil concentrations the heat transfer coefficients are substantially lower and the pressure drops are higher. As far the type of oil is concerned, the immiscible oil demonstrated more negative influence on the heat transfer and pressure drops than the miscible oil.

Since conventional refrigerants have considerable ozone depleting and global warming effects (CFC/HCFC; HFC), Carbon Dioxide (CO2) is being investigated as an alternative refrigerant in vapor compression systems. Due to favorable thermo-physical properties, CO2 when utilized in microchannel heat exchangers (HXs), can result in heat exchangers that have a low weight, a high compaction, and a high heat transfer coefficient. However, CO2 vapor compression systems need to be used in transcritical and supercritical regions, where the pressures range up to 10 MPa (1,400 Psi). Combination of the high pressure requirement, the low stiffness of the microchannel tubes, and the design requirements of compact size and low weight, one will need to carry out a mechanical design and pay attention to system vibratory characteristics such as natural frequencies. To this end, in the present study, a finite-element method (FEM)-based analysis of tube-fin heat exchangers has been carried out to determine guidelines for HX design. This analysis includes fin approximation and solid modeling. Two methods were used to approximate the fins. A dynamic analysis of a full scale HX was also carried out by using a sub-structuring methodology. Predictions from numerical simulations are found to be in good agreement with the results of experimental modal analysis. The present work can serve as a guide for structural design of heat exchangers for a variety of applications.