Experiments on the fundamental mechanisms of boiling heat transfer

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

Numerical characterization of transient two-phase flow of nozzle under the impact of diesel fuel temperature and injector backpressure

“…For the liquid phase, k = 2 as continuous phase and k = 1 for the dispersed or gas phase is considered. Next, ρ k and υ k represent density and velocity of corresponding phase k, and Γkl and M kl symbols denote the interfacial mass exchange and momentum exchange between two phases of k and l. Meanwhile, τ k and T k t are shear stress and Reynolds stress given as [21,22]:…”

Numerical simulation of immersion quenching process of an engine cylinder head

“…However, it is to be noted that transient heat flux is significantly fluctuating in comparison to the steady state heat flux [3,[34][35][36] …”

Local data of heat flux, wall temperature and the void phase along the boiling curve during vertical subcooled flow boiling of refrigerant Novec 649 at a copper wall

“…A value of corresponds to the liquid phase and a value of to the void phase. This approach has been used similarly in other works, for example [9] , [10] , [11] as well as for the well-known experimental database by Garnier et al [12] . A moving average approach was used to calculate mean values for a given time interval containing a number of samples .…”