• Energy Research

Abstract A robust, high-performance, cascaded double-effect absorption/vapor-compression cycle with a high temperature lift was conceptualized and analyzed for a naval ship application. A double-effect LiBr–H2O absorption cycle driven by high-temperature (>275 °C) exhaust heat was coupled to a subcritical CO2 vapor-compression cycle to provide low temperature refrigerant (-40 °C) for high-heat flux electronics applications and medium temperature refrigerant (5 °C) for space conditioning and other low-heat flux applications. The double-effect absorption/vapor-compression cycle outperformed the corresponding single-effect cycle, providing 74.4% higher cooling capacity for the same waste heat input; however, it required more heat exchange area. The improved performance of the double-effect configuration was much higher at lower heat rejection temperatures. A parametric study on the exhaust and heat rejection temperatures showed that the cycle exhibited high COPs over a wide range of operating conditions, indicating the robustness of the cycle.

Abstract This paper provides a critical review of ejector technology for chiller applications, combining an understanding of ejector fluid flow fundamentals with cycle applications. An ejector is a passive momentum-transfer device that requires no external mechanical input or moving parts. The progression of studies on ejectors from the early 1940s to the present from analytical and numerical modeling to visualization studies of the ejector itself is discussed. Included is an assessment of the most recent computational models. Suggestions for future research include improved computational modeling of shock phenomena and the effects of two-phase flow in ejectors. Application of ejectors in chillers is also reviewed, with an emphasis on the basic ejector-based chiller cycle and the development of passive systems that require zero mechanical input for operation. Important connections are made between ejector component- and system-level studies that would together lead to improved overall system performance.

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.

Abstract Experiments and models for in-tube pressure drop and heat transfer characteristics during condensation of propane at low mass fluxes (G = 75–150 kg m−2 s−1) in vertical minichannels (D = 1.93 mm) for two saturation temperatures (Tsat = 47 °C and 74 °C) are presented. Trends in heat transfer coefficients and frictional pressure gradients with mass flux and quality are reported and discussed. For the conditions under consideration in this study, heat transfer coefficient appears to increase with an increase in saturation temperature, while the frictional pressure gradient trend becomes relatively insensitive to an increase in saturation temperature. No correlation from the literature adequately predicted the results from the present study. Two models developed here for frictional pressure drop and condensation heat transfer coefficient demonstrate significant improvements in predicting the observed trends.

Abstract Visualization of evaporating water films falling over flat horizontal tubes, representative of the external surfaces of rectangular microchannel tubes, is presented using high-speed video. Experiments were conducted with a bank of three tubes at a saturation temperature of 17 °C. In addition to a qualitative description of the flow mechanisms, this work quantifies key droplet and wave characteristics using a semi-autonomous image analysis technique that develops a mathematical description of the droplets and waves. This allows the surface area, volume, velocity, and impact frequency of the droplets, as well as the width, surface area, and velocity of the waves to be measured. It was observed that droplet diameters, surface areas, and volumes are smaller than those measured in flow over round tubes, but were not influenced significantly by Reynolds number. The observed roll waves demonstrated similar surface area growth rates throughout their development, with stretched profiles relative to those described in flow over round tubes.

Abstract not Available.

Abstract A wearable cooling system was developed for use in elevated temperature environments by military, fire-fighting, chemical-response, and other hazardous duty personnel. The cooling system consists of an engine-driven R134a vapor compression system assembled in a backpack configuration, coupled with a cooling garment containing refrigerant lines. A 2.0 L fuel tank powers a small-scale engine that runs a compressor fabricated in house. The overall cooling system, including the wearable evaporator, had a mass of 5.31 kg and measured 0.318 × 0.273 × 0.152 m. Controlled environment tests determined system performance over a range of ambient temperatures (37.7–47.5 °C), evaporator refrigerant temperatures (22.2–26.1 °C), and engine speeds (10,500–13,300 RPM). Heat removal rates of up to 300 W, which is the cooling rate for maintaining comfort at an activity level comparable to calisthenics or moderate exercise, were demonstrated at an ambient temperature of 43.3 °C. The system consumed 1750 W at a fuel flow rate of 0.316 kg h −1 to provide a 178 W of cooling for 5.7 h.

Abstract Improvements to the energy efficiency of open-cycle tumble dryers have the potential to substantially reduce CO2 emissions. A novel adsorption-based thermal energy storage system is integrated into a gas-fired dryer here. An adsorbent bed is used to capture waste heat from the exhaust stream, store, and reuse it in the current and subsequent drying cycles. A heat and mass transfer model is developed to capture the dynamics of the thermal storage system, and validated experimentally on a 11.33-kg capacity gas-fired tumble dryer. The model predicts the inlet and exit temperatures of the drum with average absolute deviations of 7.1% and 8.4%, respectively. The analysis indicates that an 8.5-kg silica gel adsorption bed can yield a specific moisture extraction ratio of 1.166 k W h k g w − 1 , a 22% reduction over the energy consumption of the conventional gas-fired tumble dryer. In addition, drying time is reduced by 19%. This technology can be implemented in a variety of dryers to take advantage of the waste heat in the exhaust stream.

Abstract A novel miniaturization technology for binary-fluid heat and mass exchange was developed and numerous components were fabricated for demonstration as different parts of an ammonia/water absorption heat pump. Short lengths of microchannel tubes are placed in an array, with several such arrays stacked vertically. The ammonia/water solution flows in falling film/droplet mode on the outside of the tubes while coupling fluid flows through the microchannels. Coupling fluid heat transfer coefficients are extremely high due to the use of microchannel tubes. Effective vapor–solution contact on the absorption side minimizes heat and mass transfer resistances. This concept addresses all the requirements for absorber design in an extremely compact geometry. The technology is suitable for almost all absorption heat pump components (absorbers, desorbers, condensers, rectifiers, and evaporators) and for a wide range of binary-fluid processes. The development of several components for absorption and desorption at different capacities using this technology is reported here.