scispace - formally typeset
Search or ask a question
Book ChapterDOI

Transient Pool Boiling Experiments

31 May 2007-pp 241-256
About: The article was published on 2007-05-31. It has received 4 citations till now. The article focuses on the topics: Boiling & Transient (oscillation).
Citations
More filters
Journal ArticleDOI
TL;DR: In this article, an experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed, where micro-optical probes give an insight in the two-phase dynamics above the heating surface in different boiling regimes.

73 citations

Journal ArticleDOI
TL;DR: In this paper, an experimental technique suitable for precise and systematic measurements of entire boiling curves under steady-state and transient conditions is described, and the experimental results for pool boiling of well wetting fluids and fluids with a larger contact angle (FC-72, isopropanol, water) show no evidence of a hysteresis in the transition region if the heater surface is clean.

54 citations

Journal ArticleDOI
TL;DR: A survey of results found by the author and his team during recent years can be found in this article. But the results are limited to the case when the system is clean and even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run.
Abstract: The lecture presents a survey of results found by the author and his team during recent years. An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wettingfluids and fluids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean. However, even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run. The situation is different under transient conditions: heating and cooling transiens yield different curves even on clean surfaces. Measurements with microsensors give an insight in the two-phase dynamics above the heating surface and the temperature field dynamics above and beneath the surface. Microthermocouples (38 µm diameter) enbedded in the heater (distance to the surface 3.6 µm), a micro optical probe (tip diameter ~ 1.5 µm) and a microthermocouple probe (tip diameter ~ 16 µm), both moveable above the heater surface, are used for these studies. In nucleate boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching the critical heat flux (CHF), hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition boiling regime wetting events are observed, but no ones in film boiling. In low heat flux nucleate boiling very small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic change continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film boiling the bubbles leaving the vapor film can reach superheats of 30 K or more near the surface (e.g. for isopropanol). The optical probes confirm a liquid rich layer near the surface between nucleate boiling and high heat flux transition boiling. The void fraction in the layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter. Via the microsensor-data new approaches for heat transfer models on a mechanistic basis are proposed. An interfacial-area-density model enables the prediction of entire boiling curves. Furthermore the concept of a reaction-diffusion model is presented to predict CHF. Here the triggering of CHF is due to an instability of dry spots on the heating surface. Many aspects of the extremely complex mechanisms of boiling are, however, still not sufficiently understood. The problems should be tackled from both the experimental and the theoretical end and both approaches should be closely linked.

28 citations

Journal ArticleDOI
TL;DR: In this article, the authors summarized power transient boiling heat transfer experiments to illustrate the thermal-hydraulics characteristics of power transients, including exponentially escalating, ramp/quadraticly increasing, Nordheim-Fuchs reactivity-initiated accidents, and surface temperature surging.

3 citations