Horacio Perez-Blanco

Combined heat and mass transfer during bubble absorption in binary solutions

Bubble injection is an effective method for vapor absorption. This analytical study describes bubble behavior from inception to collapse as it translates inside a subcooled liquid binary solution. A finite difference method is employed to solve the governing equations and their associated boundary conditions. Special consideration is given to the complex interface mass and heat transport process. Subcooled liquid temperature and concentration fields surrounding a collapsing bubble are described. Bubble heat and mass transport variables are presented. Bubble diameter and mass are also described over the bubble life span.

Roll waves in falling films : an approximate treatment of the velocity field

Abstract Wavy flows, an important aspect of falling film absorption in refrigeration systems, are difficult to describe analytically because of their transitional regime. Within the wavy-laminar flow regime, high-frequency capillary waves are known to exist below a Reynolds (Re) number of 200. Above the critical Re, inertial waves driven by gravity, known as roll waves, can exist. These low-frequency waves, observed in an experimental absorber, were identified via image-processing studies of the falling film. A frequency analysis (fast Fourier transformation — FFT) of the film thickness trace in time yielded a wave frequency corresponding to roll waves at the given Re. A hydrodynamic description of this flow regime was then obtained from the literature. The flow equations provided by this model were solved for various wave parameters at different Reynolds numbers. The results from the solution were then extended using Fourier series expansions and continuity at each point to yield the complete periodic velocity field.

A study of absorption enhancement by wavy film flows

The mass transfer rates into wavy falling films are larger than those predicted by smooth-film theory. Inertial, roll waves in the falling film flowing over a vertical tube are responsible for the largest enhancement. Transient, two-dimensional (2-D) governing equations were formulated for simultaneous heat and mass transfer in the film, with roll wave hydrodynamics as input. The equations, coupled nonlinearly at the vapor-liquid interface, were solved by an iterative finite-difference scheme. Average heat and mass transfer coefficients were extracted from the results and compared to experimental as well as theoretical data from the literature. Excellent agreement with penetration theory was obtained for the smooth film, but in the roll wave regime, the model predicted much higher transport rates than those possible with a smooth film. The model results indicate that the normal convective flux attributable to the transverse velocity in its inward phase, coupled with the effect of the corresponding streamwise velocity, is responsible for transport enhancement in such wavy films.

Conceptual design of a high-efficiency absorption cooling cycle

Abstract The search for high-efficiency, gas-fired cooling cycles has led to the development of dual-loop absorption machines with cooling coefficients of performance (COPs) in the 1.2 to 1.7 range. This increased performance may call for high generator temperatures, new working fluids or new materials of construction. In most cases, two different sets of working fluids are required. The conceptual design presented here is aimed at obtaining high efficiencies with relatively low temperatures, employing only one set of fluids. The concept consists of two loops coupled in a configuration aimed at minimizing the loss of thermodynamic availability incurred when transferring refrigerant between the loops. The working fluid pair is a solution of lithium bromide-water. The calculated COPs are of the order of 1.8. The cycle relies on an elaborate evaporator-absorber combination. The paper presents the conceptual design, the critical assumptions, and the performance calculations for the concept.

Exergy analysis of gas-turbine systems with high fogging compression

Gas-turbine cycles with high fogging compression could offer enhanced efficiency and low complexity. Whereas compression power and heat rates decrease when small amounts of water (up to 2% of air flow by mass) are injected at the machine inlet, increased injection amounts could allow implementation of regeneration, further decreasing the heat rate. In this study, an exergy analysis is carried out for two high-fogging cycles. The theoretical projections show that high-injection improves the efficiency from 47% at a pressure ratio of 6-55.5% at a pressure ratio of 22.

Limits of mass transfer enhancement in lithium bromide-water absorbers by active techniques

Abstract A model to predict the theoretical limits of mass transfer enhancement in a falling film absorber using LiBr aqueous solution has been developed and it provides a means of comparative absorber performance evaluation. During vapor absorption, the vapor- liquid interface becomes saturated immediately after it is exposed to vapor. Further absorption can only be sustained by mass diffusion and heat conduction into the fluid bulk, assuming a stationary interface. Due to the relatively small heat and mass diffusivities of LiBr aqueous solution, the mass absorption rates sustained by pure diffusion and conduction processes are small. To increase the mass absorption rates, mixing of the solution is favored. Therefore, every mass transfer enhancement technique, passive or active, consists of basically disturbing the film and causing mixing of the solution near the interface. The higher the mixing rate, the higher the mass absorption rate. To assess the effect of a mixed interface, a mathematical model is formulated. This model solves one-dimensional heat and mass differential equations coupled at the interface. An asymptote of the mass transfer rate as the mixing rate increases is derived. The model, which takes into account all relevant parameters such as temperatures, pressures, concentrations, mass flow rates, and geometry, is an ideal tool for rating absorber performance. Results show that, under typical operating conditions found in commercial chillers, the theoretically possible maximum mass absorption rate is 0.049 kg m2 · s−1, and that, at a mechanically feasible mixing frequency of 1000 Hz, a mass absorption rate of 0.0256 kg m2·s−1 is possible. The latter rate is about an order of magnitude larger than that found in commercial chillers.

Experimental characterization of mass, work and heat flows in an air cooled, single cylinder engine

Small air cooled engines, although large in numbers, receive scant attention in the literature. Experimental data for a four stroke, air cooled, single cylinder engine are presented in this report. Air to fuel ratios, indicated and output power, exhaust composition and heat loss are determined to result in suitable thermal and mechanical efficiencies. The data obtained are discussed with the perspective obtained from other literature references. Exhaust composition figures appear reasonable, but the measurement of the transient exhaust flows is still a concern. Based on the measurements, a graph illustrating the different energy transformations in the engine is produced. Undergraduate students in the curriculum routinely use the engine and the present work allows one to conclude that the measurement approach produces reasonable results. These results could be used by engine modelers and others interested in this wide field of technology.

A study of solution properties to optimize absorption cycle COP

Abstract This paper presents the theoretical study of a cooling absorption cycle with working fluids exhibiting phase separation. The cycle performance can be calculated if the working fluid Gibbs excess energy, as a function of temperature, pressure and condensation, is known. The Gibbs excess energy function was cast in such a way that the interaction between the two components of the working solution was characterized either by four or by six parameters when the statistical-mechanical-local-composition (SMLC) model was used. The cycle performance was optimized by combination of steepest ascent and random search methods. The results are dependent on the equation of state used. It is shown that coefficients of performance (COP) of 0.6–1.65 may be reachable if suitable fluids can be found.

An Assessment of High-Fogging Potential for Enhanced Compressor Performance

Humidified gas turbines have the potential of enhanced cycle efficiencies with moderate initial cost. Evaporatively-cooled air compression is of importance to the power generation industry. The present work is aimed at contributing to a number of unanswered questions concerning the wet-compression process. Current operational margins limit the vapor mass fraction to 1∼2% by mass of the inlet flow. Yet, machines specifically designed to accommodate higher mass fractions are conceivable. Our aim is to explain the theoretical limits of those machines via a heat and mass transfer model. Continuous compression cooling via evaporation is modeled numerically based on droplet evaporation analysis. Parametric studies show the effect of variables such as droplet size, water injection ratio or compression ratio on transient behavior. Wet compression parameters such as evaporation time, compressor outlet temperature and compression work are estimated.Copyright

Handling Wind Variability Using Gas Turbines

A significant amount of energy is expected to come from wind in the upcoming years. The variability and uncertainty of this power source needs to be managed by the grid operator. Electricity networks with wind energy need extra reserves to deal with the extra uncertainty associated with the presence of wind. This paper evaluates the possibility to couple a 1000 MW wind farm with gas turbines (GTs) to provide firm capacity to the grid with a reasonable investment. Taking into account two different days of wind production with one minute data, the study analyzes the possibility of integrating the wind power output with two different types of GTs (heavy duty and aeroderivative). GTs operational constrains are included in the model in order to correctly demonstrate how the wind variability stresses turbine performance, as it probably would in extreme cases. Limitations on GTs ramps rates and start–up time are considered for both, heavy duty and aeroderivatives. GTs power output profiles, ramp rates and fuel consumption for the selected days of analysis are shown. The results show that the integration between wind and gas turbines could be a viable solution to compensate wind variability and to accommodate the increasing wind penetration into the electrical grid.Copyright