Massachusetts Institute of Technology. Dept. of Mechanical Engineering. Thesis. 1965. Ph.D.

Microscale truss architectures provide high mechanical strength, light weight, and open porosity in polymer sheets. Liquid evaporation and transport of the resulting vapor through truss voids cool nearby surfaces. Thus, microtruss materials can simultaneously prevent mechanical and thermal damage. Assessment of promise requires quantitative understanding of vapor transport through microtruss pores for realistic heat loads and latent heat carriers. Pore size may complicate exegesis owing to vapor rarefaction or surface interactions. This paper quantifies the nonboiling evaporative cooling of a flat surface by water vapor transport through two different hydrophobic polymer membranes, 112–119μm (or 113–123μm) thick, with microtruss-like architectures, i.e., straight-through pores of average diameter of 1.0–1.4μm (or 12.6–14.2μm) and average overall porosity of 7.6% (or 9.9%). The surface, heated at 1350±20Wt∕m2 to mimic human thermal load in a desert (daytime solar plus metabolic), was the bottom of a 3.1cm inside diameter, 24.9cm3 cylindrical aluminum chamber capped by the membrane. Steady-state rates of water vapor transport through the membrane pores to ambient were measured by continuously weighing the evaporation chamber. The water vapor concentration at the membrane exit was maintained near zero by a cross flow of dry nitrogen (velocity=2.8m∕s). Each truss material enabled 13–14°C evaporative cooling of the surface, roughly 40% of the maximum evaporative cooling attainable, i.e., with an uncapped chamber. Intrinsic pore diffusion coefficients for dilute water vapor (<10.4mole%) in air (P total ∼112,000Pa) were deduced from the measured vapor fluxes by mathematically disaggregating the substantial mass transfer resistances of the boundary layers (∼50%) and correcting for radial variations in upstream water vapor concentration. The diffusion coefficients for the 1.0–1.4μm pores (Knudsen number ∼0.1) agree with literature for the water vapor-air mutual diffusion coefficient to within ±20%, but for the nominally 12.6–14.2μm pores (Kn ∼0.01), the diffusion coefficient values were smaller, possibly because considerable pore area resides in noncircular, i.e., narrow, wedge-shaped cross sections that impede diffusion owing to enhanced rarefaction. The present data, parameters, and mathematical models support the design and analysis of microtruss materials for thermal or simultaneous thermal-and-mechanical protection of microelectromechanical systems, nanoscale components, humans, and other macrosystems.

Since the 1950s, the research and industrial communities have developed a body of experimental data and set of analytical tools and correlations for two-phase flow and heat transfer in passages having a hydraulic diameter greater than about 6 mm. These tools include flow regime maps, pressure drop and heat transfer correlations, and critical heat flux limits, as well as strategies for robust thermal management of HVAC systems, electronics, and nuclear power plants. Designers of small systems with thermal management by phase change will need analogous tools to predict and optimize thermal behavior in the mesoscale and smaller sizes. Such systems include a wide range of devices for computation, measurement, and actuation in environments that range from office space to outer space as well as living systems. This paper examines important processes that must be considered when channel diameters decrease, including flow distribution issues in single, parallel, and split flows; flow instability in parallel passages; manufacturing tolerance effects; single-phase heat transfer; nucleation processes; boiling heat transfer and pressure drop; and wall conductance effects. The discussion focuses on engineering issues for the design of practical systems.

A model is developed for the rate of salt deposition by natural convection from aqueous salt solutions onto a horizontal cylinder heated beyond the solubility temperature for the dissolved salt. The model accounts for the deposition rate at the salt layer-solution interface (SLSI) formed on the cylinder, but it does not account for deposition which may occur inside the porous salt layer (PSL). Dissolved salt is transported to the SLSI by molecular diffusion (with advection) and subsequently nucleates heterogeneously there. The model is applied to the experimental deposition rate data acquired by Hodes et al. (1998, 2002) at conditions pertinent to Supercritical Water Oxidation (SWCO). The ratio of the predicted deposition rate to the measured one ranges from roughly 0.5 to 2 indicating that deposition inside the PSL can be considerable.

Recent studies have examined the rate of salt deposition by natural convection on a cylinder heated above the solubility temperature corresponding to the concentration of salt in the surrounding solution at conditions typical of the Supercritical Water Oxidation (SCWO) process (Hodes et al. [1,2], Hodes [3]). The total deposition rate of salt on the cylinder is the sum of the rate of deposition at the salt layer-solution interface (SLSI) formed on the cylinder and that within the porous salt layer. The rate of deposition at the SLSI cannot be computed without determining whether or not salt nucleates homogeneously in the adjacent (natural convection) boundary layer. A methodology to determine whether or not homogeneous nucleation in the boundary layer is possible is presented here. Temperature and concentration profiles in the boundary layer are computed under the assumption that homogeneous nucleation does not occur. If, under this assumption, supersaturation does not occur, homogeneous nucleation is impossible. If supersaturation is present, homogeneous nucleation may or may not occur depending on the amount of metastability the solution can tolerate. It is shown that the Lewis number is the critical solution property in determining whether or not homogeneous nucleation is possible and a simple formula is developed to predict the Lewis number below which homogeneous nucleation is impossible for a given solubility boundary and set of operating conditions. Finally, the theory is shown to be consistent with experimental observations for which homogeneous nucleation is absent or present.

A flow cell fitted with large diameter optical ports suitable for visual observation and Raman spectroscopic studies of aqueous solutions to temperatures of 500°C and pressures to 25 MPa is described. The cell uses commercially available compression fittings that have proven to be reliable and robust to cycles of assembly, temperature exposure, and disassembly. A unique design is given for construction of the optical ports. The design of a heated cylindrical insert for salt solubility, salt deposition and salt nucleation studies in near-critical aqueous solutions is described. A method for in situ detection of the crystalline phase of salt deposited from aqueous solution at near-critical conditions and then surrounded by pure supercritical water is demonstrated for Na2SO4, using Raman spectroscopic observation.

Since the 1950’s, the research and industrial communities have developed a body of experimental data and set of analytical tools and correlations for two-phase flow and heat transfer in passages having hydraulic diameter greater than 6 mm or so. These tools include flow regime maps, pressure drop and heat transfer correlations, and critical heat flux limits, as well as strategies for robust thermal management of HVAC systems, electronics, and nuclear power plants. Designers of small systems with thermal management by phase change will need analogous tools to predict and optimize thermal behavior in the mesoscale and smaller sizes. Such systems include a wide range of devices for computation, measurement, and actuation in environments that range from office space to outer space and living systems. This paper examines important proceses that must be considered when channel diameters decrease, including flow distribution issues in single, parallel, and split flows; flow instability in parallel passages; manufacturing tolerances effects; nucleation processes; and wall conductance effects. The discussion focuses on engineering issues for the design of practical systems.

Recent studies have experimentally and theoretically examined the rate of salt deposition by natural convection on a cylinder heated above the solubility temperature corresponding to the concentration of salt in the surrounding solution at conditions typical of the Supercritical Water Oxidation (SCWO) process (Hodes et al., 2000A; Hodes, 1998). The total deposition rate of salt on the cylinder is the sum of the rates of deposition at the salt layer-solution interface (SLSI) formed on the cylinder and within the porous salt layer. The rate of deposition at the SLSI can not be accurately computed without determining whether or not salt nucleates homogeneously in the adjacent (natural convective) boundary layer. A methodology to determine whether or not homogeneous nucleation in the boundary layer is possible is presented here. This is accomplished by computing the temperature and concentration profiles in the boundary layer under the assumption that homogeneous nucleation does not occur. If, under this assumption, supersaturation does not occur, homogeneous nucleation is impossible. If supersaturation is present, homogeneous nucleation may or may not occur depending on the amount of metastability the solution can tolerate. It is shown that the Lewis number is the critical property in determining whether or not homogeneous nucleation is possible and a simple formula is developed to predict the Lewis number below which homogeneous nucleation is impossible for a given solubility boundary and set of operating conditions.

Scaling has been identified as a particularly important element of the Severe Accident Research Program because of its relevance not only to experimentation, but also to analyses based on code calculations or special models. Recognizing the central importance of severe accident scaling issues, the United States Regulatory Commission implemented a Severe Accident Scaling Methodology (SASM) development program involving a lead laboratory contractor and a Technical Program Group to guide the development and to demonstrate its practicality via a challenging application. The Technical Program Group recognized that the Severe Accident Scaling Methodology was an integral part of a larger structure for technical issue resolution and, therefore, found the need to define and document this larger structure, the Integrated Structure for Technical Issue Resolution (ISTIR). The larger part of the efforts have been devoted to the development and demonstration of the Severe Accident Scaling Methodology, which is Component II of the ISTIR. The ISTIR and the SASM have been tested and demonstrated, by their application to a postulated direct containment heating scenario. The ISTIR objectives and process are summarized in this paper, as is its demonstration associated directly with the SASM. The objectives, processes and demonstration for the SASM are also summarized in the paper. The full body of work is referenced.

A review is presented of the selection, design and operation of steam-water separation equipment. The criteria for choosing a separation system are first discussed and typical applications are described. Then, each of the major types of separator (gravity separators, drop inertia separators and cyclone separators) are reviewed in detail and the principal problems in design and operation for each respective type presented. Finally, procedures for testing separators are reviewed and overall conclusions drawn.

Water hammer due to steam bubble collapse when cold water is admitted to vertical upward flowing, vertical downward flowing, and nominally horizontal pipes has been studied both experimentally and analytically. The work in horizontal pipes included a study of the effect of a slight downward inclination, a slight upward inclination, and the length of the pipe on the initiation of water hammer. Stability maps showing the combinations of filling velocities and liquid subcooling that cause water hammer and those which do not for each flow geometry were obtained from experiments. Analytical models were developed to predict those stability boundaries in the stability maps. All these models were tested with experimental data. Based on the verified models, a step-by-step approach for each flow geometry is presented for plant engineers and designers to follow in avoiding water hammer induced by steam bubble collapse.

Experiments showing the frequency and amplitude of the flow induced motion of the gate for a 2- and a 4-in. swing check valve have been performed. The gate motion is due to turbulence in approach flow. We have found the dominant turbulent frequency of the approach flow is about half the natural frequency of the valves. The valves appear to be almost critically damped. Because of this, the valves respond almost as they would to a static force of the magnitude characteristic of the turbulent fluctuation in the flow. Both the dimensionless exciting force and the damping ratio have been found to be independent of valve size so the above statements are true for larger valves also. The recommended valve oscillation amplitudes and frequencies are used to calculate the wear at the shaft and at the stop. For an unpegged check valve, such as one of the 10-in. valves which was used at the San Onofre Nuclear Generation Station, it was found that shaft bearing wear would amount to 0.27 in.3/year and stop wear to 0.03 in.3/year.

The dryout zone is that region between the calculated pool liquid level and the point where a rod or tube submerged in the pool dries out. A method of calculating the extent of the dryout zone is presented. Recommended values for the drift flux model constants are given for rod bundles. These are needed to make these calculations.

Experiments were performed using air and water on three types of centrifugal separators plus gravity and secondary separators. Experiments were also done in the MIT blowdown rig, with and without a centrifugal separator, using steam and water. Appreciable carry-over from the steam generator occurs when the drain lines from the three stages of separation (centrifugal, gravity, and secondary) are unable to carry-off the liquid flow due to the high downcomer water level. Failure scenarios of the separator for conditions ranging from quasi-steady state to fast transients are presented. Separators, in general, in fast (blowdown) transients were found to increase the carry-over slightly. A module showing the general separator model structure is provided and recommendations are made for modeling the separator.

Experimental studies on post dryout heat transfer are conducted in an inconel-600 tube (2.44 m long and 10.16 mm ID). A set of unusual quench data are presented in this paper in that liquid N2 is used as the fluid and the absolute temperature ratio, wall to saturation, is as large as about 3. Theoretical modelling is shown to give good prediction by considering the whole boiling curve for liquid N2 after a superheat correction for the vapour is made.

In some special boiler installations (for example, once-through boilers and fluidized-bed type boilers), horizontal or slightly inclined steam-generating tubes are used for technological or other reasons. One of the problems of these installations, leading to frequent failures, appears to be the circumferential anisothermality that occurs at certain operating and geometric conditions in the horizontal boiler tubes. This is a serious constraint for successful boiler design and maintenance.

A series of experiments have been performed which help to provide fundamental understanding of the phenomena which are important to the analysis of a PWR pressurizer. The transients considered include insurges to a partially-full tank, outsurges, insurges to a tank with hot walls, empty tank insurges, and combined insurges and outsurges. The experiments include the effects of noncondensable gases, and free surface heat transfer. These experiments provide a data base from which recommendations are made for calculating such phenomena, as: (i) stratification of the hot water and incoming cold water, (ii) wall condensation, (iii) flashing, (iv) rainout, (v) suppression of flashing, (vi) wall conduction, (vii) the effect of noncondensable gases on wall heat transfer, and (viii) free surface heat transfer. From these experiments a general model of a PWR pressurizer has been developed. It will predict the pressure-time behavior of a PWR pressurizer during a variety of transients. The model has been benchmarked against the low pressure experiments of this study and a single full scale pressurizer transient experiment. The most significant finding is the pool in the pressurizer remains stratified during an insurge transient so that practically no heat transfer to the cold insurge liquid occurs. Wall heat transfer can be significant for insurge transients, however.

Bubbly flow experiments are conducted in a 25.4 mm ID pipe to investigate the behaviour of friction factor, radial void fraction and the momentum flux. The purpose of this work is to present experimental data on bubbly flow which can serve as a standard against the prediction models. Despite the fact that the homogeneous model produces reasonably good predictions of pressure drop, improved models are required taking into account the radial distributions of bubble velocity and void fraction.

This paper presents the post dryout steam water data obtained in two set of steady state experiments by Hull (1982). As an important factor, the sizes of drops within the heated section are controlled by a nozzle and measured photographically. The effect of drop size is found not to be as prominent as that suggested by the theory for post-CHF heat transfer.

In a pressurized water reactor steam generator, a moisture separator is used to separate steam and liquid and to insure that essentially dry steam is supplied to the turbine. During a steam line break or combined steam line break plus tube rupture, a number of phenomena can occur in the separator which have no counterparts during steady-state operation. How the separator will perform under these circumstances is important for two reasons, it affects the carry-over of radioactive iodine and the water inventory in the secondary side. This study has as its goal the development of a simple separator model which can be applied to a variety of steam generator for off-design conditions. Experiments were performed using air and water on three different types of centrifugal separators: a cyclone as a generic separator, a Combustion Engineering type stationary swirl vane separator, and a Westinghouse type separator. The cyclone separator system has three stages of separation: first the cyclone, then a gravity separator, and finally a chevron plate separator. The other systems have only a centrifugal separator to isolate the effect of the primary separator. Experiments were also done in MIT blowdown rig, with and without a separator, using steam and water. The separators appear to perform well at flow rates well above the design values as long as the downcomer water level is not high. High downcomer water level rather than high flow rates appear to be the primary cause of degraded performance. Appreciable carry-over from the separator section of a steam generator occurs when the drain lines from three stages of separation are unable to carry off the liquid flow. Failure scenarios of the separator for extreme range of conditions from the quasi-steady state transient to the fast transients are presented. A general model structure and simple separator models are provided.