S. I. Abdel-Khalik

An experimental investigation of single-phase forced convection in microchannels

Turbulent, single-phase forced convection of water in circular microchannels with diameters of 0.76 and 1.09 mm has been investigated. The data show that the Nusselt numbers for the microchannels are higher than those predicted by traditional large channel correlations. Based on the data obtained in this investigation, along with earlier data for smaller diameter channels, a generalized correlation for the Nusselt number for turbulent, single-phase, forced convection in circular microchannels has been developed. The diameter, Reynolds number, and Prandtl number ranges are 0.102–1.09 mm, 2.6 × 103−2.3 × 104, and 1.53–6.43, respectively. With a confidence level of greater than 95%, differences between experimental and predicted Nusselt number values are less than ± 18.6%.

Two-phase flow in microchannels

Publisher Summary This chapter reviews recently published research dealing with gas-liquid two-phase flow in microchannels. Only microchannels with hydraulic diameters of the order of 0.1 to 1 mm and with long length-to-hydraulic diameter ratios are considered here. The hydrodynamic phenomena reviewed includes the two-phase flow regimes, void fraction, and frictional pressure drop in narrow rectangular and annular passages, a micro-rod bundle, and microchannels under conditions where surface tension and inertial forces are both significant. In these systems the channel characteristic dimension is of the same order of magnitude, or smaller than, the neutral interfacial wavelengths predicted by the Taylor stability analysis. The review is also restricted to situations where the fluid inertia is significant in comparison with surface tension. Such microchannels and flow conditions are encountered in miniature heat exchangers, research nuclear reactors, biotechnology systems, the cooling of high-power electronic systems, the cooling of the plasma-facing components in fusion reactors, and the heat rejection systems of spacecraft, to name a few. The flow through cracks and slits when such cracks develop in vessels and piping systems containing high-pressure liquids is another application of two-phase flow in microchannels of interest here.

A statistical analysis of saturated nucleate boiling along a heated wire

Abstract Bubble dynamics for saturated nucleate boiling of water on an electrically heated platinum wire at atmospheric pressure have been photographed using a high-speed movie camera. The average number density of active nucleation sites has been found to increase linearly with the boiling heat flux. In addition, the frequency distribution of bubble departure diameters has been found to be well represented by an asymptotic expansion of the normal frequency function. These data have been used to determine the relative contributions to the boiling heat flux of latent heat transport by vapor bubbles, natural convection, and ‘enhanced convection’ heat transfer.

THE ARIES-CS COMPACT STELLARATOR FUSION POWER PLANT

Abstract An integrated study of compact stellarator power plants, ARIES-CS, has been conducted to explore attractive compact stellarator configurations and to define key research and development (R&D) areas. The large size and mass predicted by earlier stellarator power plant studies had led to cost projections much higher than those of the advanced tokamak power plant. As such, the first major goal of the ARIES-CS research was to investigate if stellarator power plants can be made to be comparable in size to advanced tokamak variants while maintaining desirable stellarator properties. As stellarator fusion core components would have complex shapes and geometry, the second major goal of the ARIES-CS study was to understand and quantify, as much as possible, the impact of the complex shape and geometry of fusion core components. This paper focuses on the directions we pursued to optimize the compact stellarator as a fusion power plant, summarizes the major findings from the study, highlights the key design aspects and constraints associated with a compact stellarator, and identifies the major issues to help guide future R&D.

Two-fluid modeling of condensation in the presence of noncondensables in two-phase channel flows

Condensing two-phase channel flow occurs in many industrial applications, including heating and refrigeration systems. It can also occur in certain nuclear reactor accidents. For example, during a small-break loss-of-coolant accident in a pressurized water reactor, following the partial depletion of the primary coolant, condensation of steam on the primary side of the steam generator tubes can provide a heat sink for disposal of the decay heat generated in the reactor core. Condensing two-phase flow can also play an important role in the operation of the passive emergency cooling system in the advanced simplified boiling water reactor. Here, steady-state condensation in the presence of a noncondensable in a concurrent two-phase channel flow is analyzed using a two-fluid model. The effect of noncondensables on the combined heat transfer at the liquid-gas mixture interphase is accounted for by using the stagnant film model, and closure relations relevant to the annular-dispersed two-phase flow regime are applied. The conservation equations are cast into a system of coupled ordinary differential equations, which are numerically integrated. Model predictions are compared with published experimental data, with satisfactory results. It is shown that the two-fluid model can correctly predict all major data trends and is preferable to empirical methods.

A critical heat flux correlation for droplet impact cooling

Abstract The characteristics of a single stream of monodispersed water droplets impacting a horizontal, upward facing flat surface have been investigated. The objective was to determine the effect of droplet diameter, impact frequency and impact velocity on the critical heat flux (CHF). A generalized correlation has been developed for the nondimensional CHF as a function of the Weber and Strouhal numbers of the impacting droplets. The Weber and Strouhal numbers ranged from 175 to 730 and 7.00 × 10 −3 −3.00 × 10 −2 , respectively. With a confidence level of greater than 95% differences between predicted and experimental CHF values were less than ±22%.

Low-Flow Critical Heat Flux in Heated Microchannels

Critical heat flux (CHF) associated with the flow of subcooled water in heated microchannels is experimentally investigated. Four different channels, all 16 cm in length, are used: two are circular and uniformly heated and have 1.17- and 1.45-mm diameters, and the other two represent flow channels in a microrod bundle with a triangular array and 1.131-mm hydraulic diameter, with one uniformly heated over its entire surface and the other heated only over the surfaces of the surrounding rods. The test section parameter ranges are as follows: 250 to 1000 kg/m 2 . S mass flux, 344- to 1043-kPa exit pressure, 407- to 1204-kPa inlet pressure, and 49 to 72.5°C inlet temperature. The effect of noncondensables (air) on CHF is also examined by repeating some of the experiments with degassed water and with water saturated with air at test section inlet pressure and temperature. Critical heat flux occurs at very high flow qualities (0.36 and higher) in all the tests and indicates the occurrence of dryout. Furthermore, the CHF appears to monotonically increase with increasing mass flux or pressure. The CHF depends on channel cross-section geometry, and unlike high mass flux data, it increases with increasing channel diameter, The dissolved air slightly increases the CHF for the smaller circular channel and reduces the CHF for the other test sections. The experimental data are compared with the predictions of three widely used empirical correlations. The Bowring-1972 correlation could predict the data with reasonable accuracy.

Properties optimization for phase-change energy storage in air-based solar heating systems

Abstract A parametric study has been conducted to determine the optimum physical properties of phase-change energy storage materials for solar air-heating systems. Simulation techniques are used to determine the system performance over the entire heating season. Variations of the solar fraction of the load with melting temperature, latent heat, load characteristics, and control strategy have been determined. Air-heating systems with a wide range of hot water and space heating loads have been examined. The effect of semicongruent melting of the phase change material on system performance has also been investigated.

Gas-liquid two-phase flow in narrow horizontal annuli

Abstract Experimental data associated with the two-phase flow regimes, void fraction and pressure drop in horizontal, narrow, concentric annuli are presented. Two transparent test sections, one with inner and outer diameters of 6.6 and 8.6 mm, and an overall length of 46.0 cm; the other with 33.2 and 35.2 mm diameters and 43.0 cm length, respectively, were used. Near-atmospheric air and water constituted the gas and liquid phases, respectively. The gas and liquid superficial velocities were varied in the 0.02–57 and 0.1–6.1 m s −1 ranges, respectively. The major two-phase flow patterns observed included bubbly, slug/plug, churn, stratified, and annular. Transitional regimes, where the characteristics of two distinct flow regimes could be observed in the test sections, included bubbly-plug, stratified-slug and annular-slug. The obtained flow regime maps were different than flow regime maps typical of large horizontal channels and microchannels with circular cross-sections. They were also different from the flow regimes in rectangular thin channels. The measured average void fractions for the two test sections were compared with predictions of several empirical correlations. Overall, a correlation proposed by Butterworth [Butterworth, D., 1975. A comparison of some void fraction relationships for co-current gas–liquid flow. Int. J. Multiphase Flow 1, 845–850] based on the results of Lockhart and Martinelli (1949) provided the most accurate prediction of the measured void fractions. The measured pressure drops were compared with predictions of several empirical correlations. The correlation of Friedel [Friedel, L., 1979. Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow. 3R Int. 18, 485–492] was found to provide the best overall agreement with the data.

On the validity of the adiabatic spreading assumption in droplet impact cooling

Abstract The effect of heat transfer on the spreading of a water droplet impacting a heated surface has been numerically investigated. The aim is to assess the validity of commonly made assumptions in spray cooling models which ignore heat transfer during the droplet spreading period. The results of this study indicate that while the amount of heat transfer during the droplet spreading period may, itself, be small, its impact on the liquid coverage area and the subsequent heat transfer to the resulting liquid film may be substantial.