Flow boiling critical heat flux in microgravity

Hui Zhang, Purdue University

Abstract

This study examines flow boiling critical heat flux (CHF) in microgravity that was achieved in parabolic flight experiments with FC-72 onboard NASA's KC-135 turbojet and Zero-G Corporation's Boeing 727-200. At high heat fluxes, bubbles quickly coalesced into fairly large vapor patches along the heated wall. As CHF was approached, these patches grew in length and formed a wavy vapor layer that propagated along the wall, permitting liquid access only in the wave troughs. CHF was triggered by separation of the liquid-vapor interface from the wall due to intense vapor effusion in the troughs. This behavior is consistent with, and accurately predicted by the Interfacial Lift-off CHF Model. It is shown that at low velocities CHF in microgravity is significantly smaller than in horizontal flow on earth. CHF differences between the two environments decreased with increasing velocity, culminating in virtual convergence at about 1.5 m/s. This proves it is possible to design inertia-dominated systems by maintaining flow velocities above the convergence limit. Such systems allow data, correlations, and/or models developed on earth to be safely implemented in space systems. This study is also the first attempt at extending the Interfacial Lift-off CHF Model to subcooled flow boiling conditions. A new CHF database was generated for FC-72 from ground tests as well as from microgravity tests that were performed in parabolic flight trajectory. These tests also included high-speed video imaging and analysis of the liquid-vapor interface during the CHF transient. Both the CHF data and the video records played a vital role in constructing and validating the extended CHF model. The fundamental difference between the original Interfacial Lift-off Model, which was developed for saturated flow boiling, and the newly extended model is the partitioning of wall energy between sensible and latent heat for subcooled flow boiling. This partitioning is modeled with the aid of a new "heat utility ratio." Using this ratio, the extended Interfacial Lift-off Model is shown to effectively predict both saturated and subcooled flow boiling CHF in Earth gravity and in microgravity.

Degree

Ph.D.

Advisors

Mudawar, Purdue University.

Subject Area

Mechanical engineering

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