R. A. Nelson

Liquid-solid contact measurements using a surface thermocouple temperature probe in atmospheric pool boiling water☆

Abstract The objective of this investigation was to apply the technique of using a microthermocouple flushmounted at the boiling surface for the measurement of the local surface-temperature history in film and transition boiling on high temperature surfaces. From this measurement direct liquid-solid contact in film and transition boiling regimes was observed. In pool boiling of saturated, distilled, deionized water on an aluminum-coated copper surface, the time-averaged, local liquid-contact fraction increased with decreasing surface superheat. Average contact duration increased monotonieally with decreasing surface superheat, while frequency of liquid contact reached a maximum of ~ 50 contacts s −1 at a surface superheat of ~100 K and decreased gradually to 30 contacts s −1 near the critical heat flux. The liquid-solid contact duration distribution was dominated by short contacts 4 ms at low surface superheats, passing through a relatively flat contact duration distribution at about 80 K. The results of this paper indicate that liquid-solid contacts may be the dominant mechanism for energy transfer in the transition boiling process.

Nonlinear Aspects of High Heat Flux Nucleate Boiling Heat Transfer

This paper outlines the essential details of the formulation and numerical implementation of a model used to study nonlinear aspects of the macrolayer-controlled heat transfer process associated with high heat flux nucleate boiling and the critical heat flux. The model addresses the three-dimensional transient conduction heat transfer process within the problem domain comprised of the macrolayer and heater. Heat dissipation from the heater is modeled as the sum of transient transport into the macrolayer, and the heat loss resulting from evaporation of menisci associated with vapor stems.

Possible mechanisms of macrolayer formation

Abstract This paper critically compares the mechanisms proposed for the formation of the liquid-rich macrolayer on heater surfaces during nucleate boiling. These mechanisms include Helmholtz instability analysis applied to vapor stems above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initally form during the mushroom bubbles waiting period, and the limitation of liquid resupply after mushroom departure as a result of vapor flow from active sites.

Saturated pool nucleate boiling mechanisms at high heat fluxes

Abstract A new model has been developed where the coupled transient two-dimensional conduction equation is solved for the heater and the liquid macrolayer, while allowing for the time-wise thinning of the macrolayer. The major conclusions are: (1) dominant evaporation occurs at the liquid-vapor-solid contact point (triple point) and is required to match boiling curve behavior quantitatively; (2) evaporation at the stem interface and bubble-macrolayer interface is negligible (except near critical heat flux); and (3) the results are sensitive to closure relationships, especially to the active site-density correlation.

Modeling of reflooding

Abstract The state of the art in modeling reflooding situations, mainly with the two-fluid system analysis codes, is reviewed; certain related general code development issues are included. Our current modeling of reflooding is reasonable and can be made sufficiently conservative for safety assessments, but it is not outstanding. Fundamental understanding of the detailed two-phase flow and heat transfer mechanisms has not progressed significantly over the state already available several years ago. The better understanding of system behavior achieved by the coordinated program of large-scale experiments is summarized and its impact on the modeling work discussed. In the future, factors such as the additional accident scenaria now considered, the new and advanced reactor types being analyzed, and the geometric growth of computing capacity are likely to drive our efforts. The new requirements and challenges can be met best by building into the codes pieces of understanding of the actual physical processes at the most fundamental level practicable. The discussion focuses on the existing codes and their successes and shortcomings; both certain specialized and the more complex general-purpose system codes are considered. The aim is not to conduct an exhaustive review of all aspects of the problem, but rather to reach consensus on certain issues.

A phenomenological model of the thermal hydraulics of convective boiling during the quenching of hot rod bundles Part I: Thermal hydraulic model

Abstract In this paper, a phenomenological model of the thermal hydraulics of convective boiling in the post-critical-heat-flux (post-CHF) regime is developed and discussed. The model was implemented in the TRAC-PF1/MOD2 computer code (an advanced best-estimate computer program written for the analysis of pressurized water reactor systems). The model was built around the determination of flow regimes downstream of the quench front. The regimes were determined from the flow-regime map suggested by Ishii and his coworkers. Heat transfer in the transition boiling region was formulated as a position-dependent model. The propagation of the CHF point was strongly dependent on the length of the transition boiling region. Wall-to-fluid film boiling heat transfer was considered to consist of two components: first, a wall-to-vapor convective heat-transfer portion and, second, a wall-to-liquid heat transfer representing near-wall effects. Each contribution was considered separately in each of the inverted annular flow (IAF) regimes. The interfacial heat transfer was also formulated as flow-regime dependent. The interfacial drag coefficient model upstream of the CHF point was considered to be similar to flow through a roughened pipe. A free-stream contribution was calculated using Ishiis bubbly flow model for either fully developed subcooled or saturated nucleate boiling. For the drag in the smooth IAF region, a simple smooth-tube correlation for the interfacial friction factor was used. The drag coefficient for the rough-wavy IAF was formulated in the same way as for the smooth IAF model except that the roughness parameter was assumed to be proportional to liquid droplet diameter entrained from the wavy interface. The drag coefficient in the highly dispersed flow regime considered the combined effects of the liquid droplets within the channel and a liquid film on wet unheated walls. The heat-transfer and interfacial drag models used were based on the flow-regime map noted above with length averaging of the flow-regime length if more than one regime existed in a give hydraulic cell.

The effect of Helmholtz instability on the macrolayer thickness in vapor mushroom region of nucleate pool boiling

Abstract In the previously postulated relationship between the macrolayer thickness in saturated pool boiling and the Helmholtz instability wavelength is further investigated in the ligth of experimental data that recently appeared in the open literature. The study shows that the Helmholtz wavelength in the vapor stems is strongly dependent on parameters affected by surface chemistry. These parameters same order of magnitude as the ones reported in the literature, the Helmholtz instability model was able to successfully predict the macrolayer thickness data. This result suggests that the Helmholtz instability model must not be ruled out, unless further experimental research proves otherwise.

A phenomenological model of the thermal-hydraulics of convective boiling during the quenching of hot rod bundles Part II: Assessment of the model with steady-state and transient post-CHF data

Abstract After completion of the thermal-hydraulic model developed in a companion paper, we performed developmental assessment calculations of the model using steady-state and transient post-critical heat flux (CHF) data. This paper discusses the results of those calculations. The overall interfacial drag model predicted reasonable drag coefficients for both the nucleate boiling and the inverted annular flow (IAF) regimes. The predicted pressure drops agreed reasonably well with the measured data of two transient experiments, CCTF Run 14 and a Lehigh reflood test. The thermal-hydraulic model for post-CHF convective heat transfer predicted the rewetting velocities reasonably well for both experiments. The predicted average slope of the wall temperature traces for these tests showed reasonable agreement with the measured data, indicating that the transient-calculated precursory cooling rates agreed with measured data. The hot-patch model, in conjunction with the other thermal-hydraulic models, was capable of modeling the Winfrith post CHF hot-patch experiments. The hot-patch model kept the wall temperatures at the specified levels in the hot-patch regions and did not allow any quench-front propagation from either the bottom or the top of the test section. The interfacial heat-transfer model tended to slightly underpredict the vapor temperatures. The maximum difference between calculated and measured vapor temperatures was 20%, with a 10% difference for the remainder of the runs considered. The wall-to-fluid heat transfer was predicted reasonably well, and the predicted wall temperatures were in reasonable agreement with measured data with a maximum relative error of less than 13%.

Do we doubt too little? Examples from the thermal sciences

Abstract Has the thermal science community continued to question itself? While I do not address this question for all areas of the thermal sciences, I do consider our status in the area of boiling and in particular nucleate boiling. I believe our ability to model through correlation has been very fruitful and will continue to be the basis by which we design and build system into the future. I believe our ability to mechanistically model the nucleate boiling process is only now at a point where we might begin building simulations that can represent the temporal and spatial variations we observe experimentally. To begin what I hope will be a continuing discussion into the future, I propose a framework for modeling and understanding why boiling behaves as it does. Your debate of issues and elements within the framework (doubts about it) is not just expected, it is required if we are to make progress.

On the Hot-Spot-Controlled Critical Heat Flux Mechanism in Saturated Pool Boiling: Part II—The Influence of Contact Angle and Nucleation Site Density

In that paper, we investigated the hypothesis that CHF is the result not only of the complete evaporation of the macrolayer, but also of the the surface temperature at some point on the heater surface becoming high enough to prevent liquid rewetting it. The second objective of this note is to test the hot-spot hypothesis further for the lower contact angle to confirm the functional variation of the critical rewetting temperature with the contact angle