No abstract

Screening analyses based on thermodynamic and heat transfer principles are conducted for a single-effect absorption heat pump to enable the comparison of working pairs on the basis of performance and component size. After a broad survey of working pairs in the literature, in which 83 are discovered, forty-two are analyzed for cooling mode operation, and ten are analyzed for heating mode operation. Working pairs with water as the refrigerant are the most favorable thermodynamically in cooling mode, although some of them require more heat transfer area due to higher viscosity. Working pairs with alcohol refrigerants are most favorable thermodynamically in heating mode, although working pairs with ammonia as the refrigerant yield much more compact systems. The analysis methods developed here can also be applied to newly-characterized absorption working pairs to allow comparison with those previously investigated in the literature. the absorber, desorber, condenser, and evaporator and the SHX effectiveness. For this generalized screening analysis, the following assumptions are made: 1. The high-refrigerant-concentration solution leaving the absorber, low-refrigerantconcentration solution leaving the desorber, refrigerant leaving the condenser, and refrigerant leaving the evaporator are in saturated states at their respective component temperatures.2. There is no pressure drop between components.3. The vapor leaving the desorber is at the desorber outlet temperature and pressure. Saturated liquid reflux flows from the rectifier to the desorber at a temperature 10°Cbelow the desorber temperature.5. The solution pump is ideal, and the solution flowing through the pump is incompressible.6. The fluid flowing through the evaporator and condenser is pure refrigerant. 7. The throttling valves operate isenthalpically.The above assumptions place the thermodynamic analysis on the simpler end of the spectrum. However, the analysis developed here is not intended as a design tool but rather as a screening and comparison method, which will yield the correct relative merits of the various working pairs while retaining simplicity in the analysis.In the analysis, the rectifier (if necessary) and condenser are treated as a single component. This is primarily to allow a fair comparison between working pairs that require rectification and those that do not. Consider an NH 3 -H 2 O system that requires rectification and an NH 3 -LiNO 3 system that does not. If the two systems are operating at the same desorber temperature and the rectifier and condenser are treated separately, the refrigerant in the NH 3 -H 2 O system will leave the rectifier at a lower temperature than the refrigerant in the NH 3 -LiNO 3 1

Sorbent-loaded hollow fibers operating in a rapid temperature-swing adsorption cycle are a unique platform for the capture of CO2 from power plants. They are ideally suited for heat recovery strategies that will reduce the operating costs of capture facilities. Accurate estimates of the fiber-level heat- and mass-transfer kinetics are critical for the design and implementation of these systems. A detailed coupled heat- and mass-transfer model of the adsorption process in sorbent-loaded fibers is developed here. The effects of varying fiber geometry on the heat- and mass-transfer kinetics are presented. The rapid diffusion and adsorption in the fiber and the direct cooling of the fibers will enable efficient capture of CO2, as well as substantial recovery of the sensible heat capacity of the beds, thus reducing energy costs of the thermal-swing adsorption process.

No abstract

Multiphase flow phenomena in single micro- and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air-water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.3 mm hydraulic diameter is visually investigated using high speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.