Hydrodynamic Considerations for Optimal Thermal Compressor Design

The use of small-scale thermally driven heat pumps allows for the design of innovative and efficient thermal systems. Recent developments have shown the feasibility of the implementation of ammonia-water absorption systems for small capacity applications ( \u3c 14 kW cooling capacity). The use of low-grade thermal energy in thermally driven systems of that scale has the potential to achieve significant reduction of high grade electrical or mechanical energy consumption. However, further research needs exist for the development of highly compact components that optimize the efficiency of the overall system. Analogous to the mechanical compressor in a vapor compression cycle, the thermal compressor in a vapor absorption cycle where driving energy input to the cycle is supplied. It has been shown that an optimal thermal compressor component configuration exists. The requirement of high refrigerant purity in the ammonia-water system favors the implementation of the diabatic distillation principle. It can significantly reduce exergy destruction in the thermal compressor and provides the opportunity to develop highly compact component designs. In this work, two new design solutions for the desorption stage of the thermal compressor are presented. These designs incorporate the diabatic distillation concept for a direct-gas-coupled as well as a coupling-fluid-driven thermal compressor. The designs operate in a liquid-vapor countercurrent flow configuration and utilize favorable temperature and concentration profiles of specific flow patterns. Therefore, the proposed designs were visually investigated using an air-water mixture to simulate the working fluids. Air-water flow experiments at adiabatic conditions were designed and conducted to simulate component operation. Proper vapor-liquid interaction (termed tray activation) in the purification stages was investigated for a wide range of flow rates to simulate variation within the component as well as part load operation. Countercurrent flow limitations exist at high flow rates, which can cause component flooding leading to detrimental system performance. These limitations were investigated and flooding curves were established. The effects of minor geometry adjustments on tray activation and flooding were studied. Surface tension effects were investigated by use of an ethanol-water mixture. High speed video data were used to obtain quantitative results. The total heat transfer area for single-phase and two-phase flow regions, as well as vapor-liquid interfacial area are quantified based on the flow visualization studies. Both designs were validated as effective solutions for the implementation of the diabatic distillation concept for small-capacity thermal compressors. Tray activation was achieved for part load operation, and resilience to countercurrent flow limitations could be shown. Consequently, these results provide specific and quantitative heat and mass transfer design guidelines for the desorption stage of the thermal compressor. Results from this investigation will guide the development of new prototype desorbers

Investigation of Air-Cooled Condensers for Ammonia-Water Absorption Chillers

Absorption heat pumps are being considered as alternatives to vapor-compression systems in some applications. These systems utilize environmentally friendly working fluids and can be driven by a variety of heat sources, such as process waste heat, solar, and geothermal energy. In some implementations of absorption systems, the use of direct air coupling of the heat exchangers can reduce the system size, complexity, and inefficiency. However, issues of material incompatibility with the refrigerant mixture and poor heat transfer properties of air pose challenges for these heat exchangers. Prototype air-coupled condensers for use in an ammonia-water absorption chiller driven by waste heat from diesel engine exhaust are investigated experimentally and analytically.  A segmented model is developed to predict the performance of round-tube corrugated-fin and multi-pass tube-array condensers designed for a 2.64-kW cooling capacity absorption system operating at high ambient temperature (51.7°C) conditions. Results from heat transfer and pressure drop correlations based on the two-phase flow regimes for zeotropic mixture condensation are compared at the design conditions and the most appropriate correlation is applied. Air-side heat transfer coefficients and pressure drop are calculated taking the fin-geometry, the fin and tube spacing, and air properties into consideration. Additionally, maldistribution of vapor and liquid in the header affects the performance of the air-cooled condenser significantly. The segmented models quantify maldistribution in the header based on the applicable two-phase flow regimes and account for its effect on heat transfer. Two round-tube corrugated-fin condensers (9.525 mm and 15.875 mm tube OD) and two multi-pass tube-array designs are modeled, fabricated and tested. A single-pressure ammonia-water test facility is constructed and used in conjunction with a temperature- and humidity-controlled air-handling unit to evaluate the condensers at design and off-design operating conditions. Condenser performance is recorded over a range of air temperatures (35-55°C), refrigerant inlet temperatures (57-67°C), air volumetric flow rates (0.33-0.45 m3 s-1), and refrigerant mass flow rates (0.0019-0.0026 kg s-1). Furthermore, surface temperatures of tubes in the condenser are measured to understand vapor- and liquid- phase maldistribution. Measured heat transfer rates and pressure drop values are compared with the predictions of models from the literature. Results from this investigation guide the development of air-coupled zeotropic mixture condensers for compact absorption heat pumps

Experimental Evaluation of a Small-capacity, Direct-fired Ammonia-water Absorption Chiller

Vapor absorption heating and cooling systems, utilizing heat input from different sources such as waste heat or natural gas, are attracting increasing interest in commercial and residential applications. Residential heat pump applications require compact heat exchanger geometries to ensure a small system footprint. Compact microscale heat and mass exchangers are developed and implemented here. These novel heat and mass exchanger geometries for different components of the system require evaluation at design and off-design conditions to characterize the individual component and overall system performance. This study presents results from experimental investigations of a small-capacity ammonia-water absorption chiller. The chiller comprises discrete heat and mass exchangers with novel design features, and is installed on a breadboard test facility. The absorber and condenser are directly coupled to ambient air and designed to operate at extreme ambient temperatures as high as 51.67°C. The microchannel evaporator is hydronically coupled. The desorber is direct-fired and coupled to a hot air-stream simulating a waste heat source. The system is designed to deliver 2.64 kW of cooling at a coefficient of performance (COP) of 0.55 based on the cooling duty and the desorber heat input rate. A steady-state model to specify various state-points in the system is developed. The experimental setup and details of heat exchanger geometries, measurement and instrumentation, and data analysis are presented. The overall system and component performance is evaluated to determine system limitations. The evaluation is conducted at lower ambient conditions of 35-40°C and the system model is modified to account for the corresponding changes. Results from baseline system operation at design mass flow rates of concentrated solution and heat input rates are presented. The performance of individual components is analyzed and compared to design predictions. Various operating conditions in the system such as the flow rate of concentrated and refrigerant solution, the heat source temperature and flow rates, and ambient temperature are varied to study the effect on the overall system and component performance.   The results from this investigation demonstrate the potential of small-capacity absorption chillers using microchannel and direct air-coupled heat exchangers, and will guide the development of a packaged waste-heat-driven chiller operating at extreme ambient conditions

Beyond behaviour as individual choice: a call to expand understandings around social science in health research

The focus of behavioural sciences in shaping behaviour of individuals and populations is well documented. Research and practice insights from behavioural sciences improve our understanding of how people make choices that in turn determine their health, and in turn the health of the population. However, we argue that an isolated focus on behaviour - which is one link in a chain from macro to the micro interventions - is not in sync with the public health approach which per force includes a multi-level interest. The exclusive focus on behaviour manipulation then becomes a temporary solution at best and facilitator of reproduction of harmful structures at worst. Several researchers and policymakers have begun integrating insights from behavioural economics and related disciplines that explain individual choice, for example, by the establishment of Behavioural Insight Teams, or nudge units to inform the design and implementation of public health programs. In order to comprehensively improve public health, we discuss the limitations of an exclusive focus on behaviour change for public health advancement and call for an explicit integration of broader structural and population-level contexts, processes and factors that shape the lives of individuals and groups, health systems and differential health outcomes

Simultaneous chilled and hot water generation using smelter off-gas waste heat

Issued as final reportALCOA Technical Cente

Dominant Flow Mechanisms in Falling-Film and Droplet Mode Evaporation Over Horizontal Rectangular Tube Banks

Flow visualization with high-speed video of evaporating water films falling over flat horizontal tubes, representative of the external surfaces of microchannel tubes, is presented. Experiments were conducted with 1.4 mm thick and 27 mm tall tubes over a film Reynolds number range of 23 \u3c Re \u3c 126. In addition to a qualitative description of the flow mechanisms, this work quantifies key droplet and wave characteristics using image analysis techniques. A semi-autonomous edge-detection technique is used to develop a mathematical description of the droplets and waves, allowing the surface area, volume, velocity, and frequency of the droplets, as well as the width, surface area, and velocity of the waves, to be measured. The results are useful for developing accurate, phenomena-based models for falling-film evaporation over flat horizontal tube banks