Direct Contact Condensation in Falkner-Skan Flows

In this paper we present a method for solving direct contact condensation problems involving Falkner-Skan flows. Using this method we then perform a parametric study for the problem of coupled two-fluid (saturated vapor on subcooled liquid) stagnation flows with direct contact condensation. Selected results and the resulting correlations are discussed.

Convective Heat Transfer in a Triple-Cavity Structure Near Turbine Blade Trailing Edge

The present study explores the internal heat transfer in a triple-cavity cooling structure with a ribbed lip for a turbine blade trailing edge. The design consists of two impingement cavities, two sets of crossover holes, a third cavity and an exit slot with eleven ribs attached to it. Local heat transfer in each subregion is determined. Results indicate that the highest heat transfer occurs in the second impingement cavity. The exit slot area between the ribs is identified as a region of low heat transfer in the overall design. A comparison with enhancement induced by arrays of pin fins and fins of other geometries reveals that the triple-cavity design represents a lesser quality cooling scheme in the range of Reynolds numbers tested. Further improvement of the convective heat transfer at the exit slot with either film cooling, or different rib geometries appears to be essential to make the triple-cavity strategy superior to those of the traditional approaches for cooling of blade trailing edge.

Evaporation, Condensation, and Flow in an Idealized Micro Heat Pipe

Micro heat pipes have been used in cooling micro electronic components. However their effective thermal conductivity is low compared with that of conventional heat pipes. Due to the complexity of the coupled heat and mass transport, and to the complicated three-dimensional bubble geometry inside micro heat pipes, there is a lack of rigorous analysis. As a result, the relatively low effective thermal conductivity remains unexplained. We have conceptualized an idealized micro heat pipe that eliminates the complicated geometry, but retains the essential physics. Given the simplified geometry, many effects can be studied, such as thermocapillary flow, and evaporation and condensation physics. In this talk, we will present the flow field induced by evaporation.

Numerical Simulation of Black Liquor Spray Characteristics

The performance and capacity of Kraft recovery boilers is sensitive to black liquor velocity, droplet size and flow distribution in the furnace. Studies have shown that controlling droplet size and flow distribution improves boiler efficiency while allowing increased flight drying and devolatilization, and decreased carryover. The purpose of this study is to develop a robust two-phase numerical model to predict black liquor splashplate nozzle spray characteristics. A three-dimensional time dependent numerical study of black liquor sheet formation and sheet breakup is described. The volume of fluid (VOF) model is used to simulate flow through the splashplate nozzle up to initial sheet breakup and droplet formation. The VOF model solves the conservation equations of volume fraction and momentum utilizing the finite volume technique. Black liquor velocity, droplet size and flow distribution over a range of operating parameters are simulated using scaled physical models of splashplate nozzles. The VOF model is compared to results from a flow visualization experiment and experimental data found in the literature. The details of the simulation and experimental results are presented.

Characteristics of Inclined Fin-Tube Heat Exchanger for Compact Air Conditioner

For saving space at an office or a clean room, it is needed to reduce the space of an air conditioner. It is effective to miniaturize a heat exchanger because it occupies the large space in the air conditioner. Three types of a heat exchanger that are an in-line tube and cut fins type, a staggered tube and cut fins type and a staggered tube and uncut fins type were investigated as four inclined angle tests of 0, 45, 60 and 80 degrees in a heat wind tunnel. The coefficients of flow friction and heat transfer rates were obtained from these experiments, and the characteristics of inclined heat exchanger were clarified by effects of tube arrangements, fin types and inclined angles against flow direction. As a numerical approach, two-dimensional steady models were applied on the staggered tube and the in-line tube by using BFC (Boundary-Fitted Coordinate Method); BFC is available to make grids for any install angle of the heat exchanger. The results of the numerical analysis visualized flow patterns and heat transfer in these heat exchangers. In case of 80-degrees angle, the flow makes dead area in a part of the heat exchanger, and it causes reducing performance of the heat exchanger. These results are available for improve a compact high performance heat exchanger.

Experimental Study of Direct Contact Condensation in the Presence of Noncondensable Gas

A series of direct contact condensation tests of mixture of saturated steam and nitrogen has been carried out in a subcooled pool of water. Nitrogen is used as the noncondensable gas. A mixture of nitrogen and steam is discharged into the subcooled water pool through a vertical nozzle. The apparatus is equipped with appropriate instruments and flow visualization. Pre-heaters and super-heaters are used to heat up nitrogen and steam before they are mixed. Tests have been preformed with variations of the liquid temperature, system pressure, nozzle size and the concentration of noncondensable gas. Images of bubble behaviors are captured with high speed video camera and downloaded into a PC in digital format. Information on bubble size variation and formation frequency is obtained from the image analysis. Detaching bubble size, surface area and frequency are correlated with the various dimensionless numbers.

Dynamics of Thermal Energy Storage in Integrated Energy Systems With On-Site Power Generation

Integrated energy systems (IES) incorporating on-site power generation provide opportunities for improving reliability in energy supply, maximizing fuel efficiency, and enhancing environmental quality. To fully realize these attributes, optimum design and dynamic performance of integrated systems for a given application have to be pursued. Whether referred to as cogeneration, combined heat and power (CHP) or building cooling, heating, and power (BCHP), integrated energy systems manifest effective energy management aimed at closing spatial and temporal gaps between demand and supply of electrical and thermal energy. This is accomplished by on-site power production and utilization of the resulting thermal energy availability for thermally-driven technologies including desiccant dehumidification, absorption cooling, and space heating. The notion that the demands for thermal and electrical energy are not always congruent and in phase signifies the importance of considering thermal energy storage (TES) for integration. This paper explores the potential impact of implementing TES technology on the overall performance of integrated energy systems from the first- and second-law perspectives. In doing so, the dynamics of packed bed thermal energy storage systems for potential energy recovery from the exhaust gas of microturbines are investigated. Using a validated simulation model, the transient thermal response of these TES systems is examined via parametric analyses that allow variation in the thermal energy availability and physical characteristics of the packed beds. The parasitic electrical energy requirement associated with the pressure losses in the packed beds is included in the performance assessment. The results of this study are indicative of the promising role of TES in integrated energy systems.

Experimental Investigation of a Capillary Pumped Loop Towards Its Integration on a Scientific Microsatellite

This paper presents the experimental investigation of a capillary pumped loop (CPL) to be integrated on a scientific microsatellite. Tests in laboratory have been focused on the thermal behavior of a CPL on a reduced scale, using UHMW (Ultra High Molecular Weight) polyethylene as porous structure and anhydrous ammonia as working fluid. The experimental tests have shown that the proposed CPL presents reliable startups when operating on a heat load range between 20 and 50 W, also presenting very short transients when operating on different heat load profiles. Very fast responses of the CPL have been verified for sudden changes on the heat load applied to the capillary evaporator with reduced superheat. The proposed CPL will be part of a payload to be integrated on a Scientific Microsatellite scheduled to be launch in early 2004.

Investigation of Thin Film Evaporation Limit in Single Screen Mesh Layer

Thin film evaporation heat transfer plays an extremely important role in capillary microstructures of the type used extensively in micro heat pipes, loop heat pipes and high-flux film heat spreaders. Because the formation of the liquid meniscus in the pore cell has a significant effect on the evaporation process occurring at the interface of the liquid meniscus, it is necessary to investigate the mechanisms and limitations of the phase-change phenomena occurring in the thin layer. In the current study, an analytical model, which combines the heat conduction in the wick layer with bubble formation mechanisms in the capillary structure, has been developed to determine the evaporation heat transfer limit. Temperature distribution, superheat, and heat flux distribution in the liquid meniscus area are investigated for a single layer of metal screen mesh. The wire diameter, the space between the wires and the contact conditions between the solid wall and mesh layer is shown to have a significant effect on the evaporation limit and capillary force. Results indicated that evaporation takes place mainly in the thin film region, and the heat transfer coefficient is much higher in this area than in the intrinsic region. The evaporation limit is restrained by the formation of the liquid meniscus, and the higher the capillary pressure, the lower the evaporation heat transfer limit.

Thermochemical Design of a Micro Liquid Monopropellant Rocket With Catalytic Reaction of Hydrogen Peroxide

A theoretical and experimental investigation on a design of catalytic reactor of submilimeter scale to be used as a micro propulsion device is described. A micro reactor was fabricated on an aluminum plate and catalyst was prepared on the anodized internal surface of the reactor. The reactor has a height of 1mm and width of 10mm. The height of the reactor is the major constraint when a prototype device is to be fabricated on a wafer by MEMS processing. Thermodynamic properties of product gases from the decomposition process of hydrogen peroxide in contact with perovskite based redox cycling catalyst were measured. A theoretical model was developed to predict the heat and gaseous mass generated from the decomposition process of hydrogen peroxide by using asymptotic approximation of reacting flow in 1-D channel with height of 1mm or less in order to approximate the actual operating condition of propulsion device on a chip. The measured heat transfer coefficients and thermodynamic properties were used in the calculation. As the monopropellant decomposes into water and oxygen, the reaction products are heated. The enhanced heat loss due to the small size of the chamber, however, adversely affected the thermochemical process of decomposition.