In the present study the optimum heat exchanger inventory allocation to maximize the thermal performance of a two-stage vapor compression system with two evaporators has been investigated. Both the cooling (A/C) and heating (H/P) Carnot and non-Carnot non-isentropic cycles have been considered. The optimum operating ranges of cycle parameters that maximize both the coefficient of performance (COP) and exergetic efficiency (η 2 ) of the cycles for both cooling and heating purposes are discussed. The research upon which this paper partly reports covered all possible ranges of cycle parameters using the Monte-Carlo method. For the Carnot cycle, maximum values of the cooling coefficient of performance (COP C ), cooling exergetic efficiency (η IIC ), heating coefficient of performance (COP H ), and heating exergetic efficiency (η IIH ) were found to be 9.6, 0.47, 10.7 and 0.87, respectively. The low-pressure (LP) thermal load and temperature difference in the condenser were found to critically affect both the A/C and H/P performance, while the heat conductance ratio and the mass flow rate ratio were found to have a pronounced effect on only the H/P performance. The best A/C and H/P cycle performance may be achieved by having the two evaporators with both the thermal load and mass flow rate in the high-pressure loop to be 20% less than that in the low-pressure loop. The analysis performed on the non-Carnot two-compressor, two-evaporator A/C and H/P non-isentropic cycles determined both the feasible and optimal ranges of variations of the controlling parameters. The combined maximum values of the low- and high-pressure evaporator thermal loads was found to be 10–15% lower than the maximum value of the condenser heat rejection rate, thus reflecting the relative sizes of these units as heat exchangers. Other factors that may help provide guidance for utilizing the system for cooling and heating purposes include the values of the COP C and COP H , the relative amounts of the mass flow rates in the low-pressure and high-pressure loops of the cycle, and the values of the low-pressure and high-pressure compressor powers.

A bubble pump is a key component for diffusion-absorption refrigeration systems operating at a single pressure. Nevertheless, research focusing on bubble pumps is not widely found in the literature. In this study, a bubble pump model with a shape of vertical tube subjected to a uniform heat flux from the tube outer surface is numerically simulated. A saturated ammonia-water solution enters the bubble pump inlet and receives heat from the pump wall along the entire pump length. During the process, most of the ammonia in the solution vaporizes, and the remaining solution is lifted by the buoyant force created by the ammonia vapor. A numerical model was implemented by employing a commercial CFD code. The applicability of the numerical model implemented in the code to the present numerical simulations was validated through a comparative study referring to experimental data of a boiling phase change flow of water in a vertical pipe being subjected to a uniform heat flux. To investigate the influence of the bubble pump’s geometrical dimension and the heat input on the operating status and performance, numerical simulations were performed for four bubble pumps with different diameters subjected to five amounts of heat flux. Simulation results are provided in terms of the flow parameters including void fraction, and vapor and liquid velocities. The simulated spatial distributions of the flow parameters were found to have steep gradients in the radial direction near the pump wall due to the heating from the pump wall. In addition, simulated flow parameters were compared to those in previous one-dimensional (1-D) work for the same problem. It was found that the void fraction profiles along the pump length simulated in this study seem to be similar to those in 1-D models, but somewhat different quantitatively. Based on the results, the present numerical simulation model of the bubble pump is considered to be useful for certain industrial applications.

High purity hydrogen is produced through a thermochemical water splitting process that utilizes iron reduction-oxidation (redox) reactions. An iron powder bed is fluidized to improve heat and mass transfer thus improving the reaction kinetics. Inert additives which act as sintering inhibitors, such as silica (SiO 2 ) and zirconia (ZrO 2 ), are added to the iron powder, and their effectiveness in inhibiting sintering in the oxidation step is evaluated. The influence of particle size, composition, mass fraction and bed temperature on reaction kinetics is investigated. Incorporation of zirconia in the powder bed is done by mixing it with iron powder or by coating the iron particles with a mixture of 1–3 μm and 44 μm zirconia particles. Two different batches of silica are used for blending with iron powder. The silica powder batches include particle diameters ranging from 0–45 μm and 200–300 μm. The mixing ratios of silica to iron are 0.33, 0.5, 0.67 and 0.75 by apparent volume. Experimental studies are conducted in a bench scale experimental fluidized bed reactor at bed temperatures of 450, 550, 650, 750 and 850 °C. It is verified that increasing the bed temperature and the steam residence time increases the hydrogen yield. Increasing the iron particle size reduces the specific surface area and reduces the hydrogen yield. It has been found that sintering can be completely inhibited by mixing iron with 0–45 μm silica powder and maintaining the reaction temperature below 650 °C.

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nu number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.

The Power, Water Extraction, and Refrigeration (PoWER) engine has been investigated for several years as a distributed energy system, among other applications, for civilian or military use. Previous literature describing its modeling and experimental demonstration have indicated several benefits, especially when the underlying semi-closed cycle gas turbine is combined with a vapor absorption refrigeration system, the PoWER system described herein. The benefits include increased efficiency, high part-power efficiency, small lapse rate, compactness, less emission, air, and exhaust flows (which decrease filtration and duct size) and condensation of fresh water. The present paper describes the preliminary design and modeling of a modified version of this system as applied to distributed energy, especially useful in regions which are prone to major grid interruptions due to hurricanes, under - capacity, or terrorism. In such cases, the distributed energy system should support most or all services within an isolated service island, including ice production, so that the influence of the power outage is limited in scope. This paper describes the rather straightforward system modifications necessary for ice production. The primary focus of the paper is the use of this ice-making capacity to achieve significant load-leveling during the summer utility peak, hence reducing the electrical capacity requirements for the grid as well as load-leveling strategies.

Despite of the fact that numerical simulation of two-phase flows in microchannels has been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble (sometimes called gas pocket or gas slug) may be a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field. Thus, there is a strong motivation to employ numerical simulation methods in order to avoid some of the experimental difficulties. In this paper, the characteristics of two-phase slug flows in microchannels are calculated with the help of the Volume-of-Fluid (VOF) method. Formation of the slugs for different superficial velocities, Capillary numbers, and gas volume fractions are investigated. The minimum mesh resolution required to capture the liquid film surrounding the gas bubble is reported employing a dynamic mesh adaption methodology with interface tracking. Results are shown to be in good agreement with experimental data and empirical correlations.

The effect of swirl on laminar buoyant jets with low Reynolds numbers is explored. Three dimensional direct numerical simulations are performed to solve the time-dependent, incompressible Navier-Stokes equations. We use a body fitted grid system and employ the finite volume method to discretize the governing equations. A second-order central difference scheme is employed for all spatial derivative terms. The numerical simulation is advanced in time by a fractional step method with the second-order Adams-Bashforth scheme for explicit-convection terms and the Crank-Nicholson scheme for implicit-diffusion terms. The amount of swirl and buoyancy is varied from zero to very large values and the effect on the velocity field, jet width, entrainment and vortex are examined. Comparisons with analytical and experimental models are discussed.

Rotary-vane positive displacement machines have played an important role for many years in refrigeration and air compression applications because of their inherent simplicity and reliability. This paper details the development and testing of a rotary-vane two-phase expander for the purpose of work recovery in an air-cooled chiller. The paper describes the methodology used to design an expander for two-phase flow from a single-phase compressor and an air-motor. Input from detailed analytical models developed by the authors are used to improve the expander design with respect to internal leakage losses and friction. Controls and performance testing are also presented. Directions for future research are highlighted.

In this paper the Navier-stokes equations for a single liquid slug have been solved in order to predict the circulation patterns within the slug. Surface tension effects on the air-water interface have been investigated by solving the Young–Laplace equation. The calculated interface shape has been utilized to define the liquid slug geometry at the front and tail interfaces of the slug. Then the effects of the surface tension on the hydrodynamics of the two-phase slug flow have been compared to those where no surface tension forces exist. The importance of the complex flow field features in the vicinity of the two interfaces has been investigated by defining a non-dimensional form of the wall shear stress. The latter quantity has been formulated based on non-dimensional parameters in order to define a general Moody friction factor for typical two-phase slug flows in microchannels. Moreover, the hydrodynamics of slug flow formation has been examined using computational fluid dynamics (CFD). The volume-of-fluid (VOF) method has been applied to monitor the growth of the instability at the air-water interface. The lengths of the slugs have been correlated to the pressure fluctuations in the mixing region of the air and water streams at an axisymmetric T-junction. The main frequencies of the pressure fluctuations have been investigated using the Fast Fourier Transform (FFT) method.