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.

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.

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.

Compact Carbon-Carbon Composite Heat Exchanger

Composite heat exchangers can provide significant reduction in the weight of a compact heat exchanger in comparison to metal heat transfer units. A Carbon-Carbon (C-C) composite heat exchanger core has been fabricated to study the heat transfer and friction characteristics of single layer plate-fin geometry. The plate and fins are both C-C composites, in which the carbon matrix is reinforced by high conductivity graphitic carbon fibers to improve the fin effectiveness of the heat exchanger. A heat flux is applied to the top and bottom of the heat exchanger core to heat an air stream passing through the channels. The heat transfer coefficient and the friction coefficient are determined and the experimental results are compared with data for a standard plate fin heat exchanger of similar geometry. It was determined that the C-C heat exchanger had lower Colburn heat transfer factor than a similar metal plate-fin heat exchanger; but had high surface temperature effectiveness. The friction factor of the C-C heat exchanger was slightly lower than the metal plate-fin heat exchanger.

Pressure Drop Characteristics of R134a Two-Phase Flow in a Horizontal Rectangular Microchannel

Adiabatic two-phase flow pressure drop of R134a have been investigated experimentally in a rectangular channel with hydraulic diameter of 0.14 mm. Single-phase flow experiments were also conducted with liquid and vapor R134a, and an empirical correlation was developed for the turbulent flow regime. The parameter ranges examined for two-phase flow are: mass flux from 158 to 785 kg m−2 s−1; vapor quality from 0.01 to 0.95; and saturation temperature at about 24 ~ 32°C. The experimental data were compared with twelve existing correlations. The homogeneous model and the Mishima and Hibiki (1996) correlation give better predictions than any other correlations.

Heat Transfer in Wavy Duct With Different Corrugation Angle

The present study investigates the effects of duct corrugation angle and flow velocity on the convective heat/mass transfer characteristics in wavy ducts applied in a primary surface heat exchanger. Local heat/mass transfer coefficients on the corrugated duct sidewall are determined using a naphthalene sublimation technique. The flow visualization technique is used to understand the overall flow structures inside the duct. The corrugation angles of the wavy ducts are 145° and 130°, and the duct aspect ratio is fixed at 7.3. The Reynolds numbers, based on the duct hydraulic diameter, vary from 1,000 to 5,000. The results show that secondary vortex flow cells, called Taylor-Go¨rtler vortices, exist periodically in the wavy duct. Therefore, non-uniform distributions of the heat/mass transfer coefficients are obtained on the duct walls. On the pressure-side wall, high heat/mass transfer cell-shaped regions appear due to the secondary vortex flows for both corrugation angles. On the suction-side wall, the heat transfer coefficients are lower than those on the pressure-side wall. The wavy duct with the corrugation angle of 130° has the stronger strength of the secondary vortex cells resulting in higher heat/mass transfer rates on the duct wall because the sharp turn enhances the development of the secondary flow cells.